Renewable Energy Guide for Highway Maintenance Facilities (2013)

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P A R T I I I Renewable Energy Technologies and Strategies

55 6.1 Overview Energy efficiency is a key piece of the puzzle when evaluating energy strategies and especially capital-intensive renewable energy projects. Having efficient and functional facilities can greatly reduce the power generation requirements, which will either reduce the quantity of renewable energy that needs to be generated or decrease the conventional energy that still needs to be purchased when renewables are installed. There are a few driving factors behind energy consump- tion in most commercial buildings, and this holds true for maintenance facilities of all types. According to the International Energy Agency, space heating is by far the most energy-intensive operation, accounting for 36% of total consumption, followed closely by lighting at 21%. Shop spaces will have higher heating, lighting, and ventilation loads compared to the industry average for commercial spaces, while buildings used for non-shop activities should follow much more closely. Demand reduction refers to the strategy of reducing electric power requirements during utility-defined peak demand periods as a means of reducing utility bills. This strategy is most beneficial when the electric rate schedule includes high demand charges for power use during the peak periods. Many utility rates, especially those for commercial buildings, include energy usage charges plus charges based on demand. Typically, demand is measured over a short duration— a 15-minute time period—which represents the instantaneous power draw of the facility. Understanding the facility’s electric use profile is the key to reducing peak demand. Note that reducing peak demand may not save much energy if the reduction is temporary or if it is done in a way that simply moves the energy use to another time period (referred to as load shifting). However, from the utility’s perspective, reducing peak demand is very beneficial since it reduces its requirement for investment in “peaking plants,” which are generators that are used for relatively short periods simply to meet spikes in demand resulting from extremes in weather (e.g., hot spells that increase air conditioning requirements and hence electric power generation requirements) or other situations. Facilities that are able to reduce their demand through strategic scheduling of equipment operation or other means—the use of on-site generation—are best able to benefit from reductions in demand charges. In much of the United States, utility peak demands are set in the summer, and high demand charges are established for summer peak periods. This is less beneficial for maintenance facilities since the maintenance areas are typically not air conditioned. Nonetheless, there are still some opportunities for demand reduction, depending on the mix of space types within the building or site. The use of building controls for implementing peak demand reduction is an effective strategy. C H A P T E R 6 Energy Efficiency and Demand Reduction

56 Renewable Energy Guide for Highway Maintenance Facilities 6.2 Types of Systems and Strategies 6.2.1 Energy Efficiency Building Envelope. The building envelope has the greatest impact on energy efficiency since each function is specific to the characteristics of the building. Fenestration, insulation, and air infiltration all have a huge impact on energy use. Site selection and building orientation have a large impact on building efficiency, as do external factors such as energy spent transporting materials, commuting, security, accessibility, energy consumption, and the impact on local eco- systems and the use/reuse of existing structures and infrastructures. Therefore, it is important to address site selection early in the project development process to ensure that issues like solar access are considered.35 An existing site with existing buildings represents a substantial amount of work and energy already invested in the site. By looking at the total energy expenditure to manufacture, transport, erect, and sustain buildings, the better options are usually reusing build- ings or parts of buildings, followed by using local or recycled/recyclable materials. The number, size, and orientation of windows will let in light that can reduce the lighting and cooling load, as discussed in Chapter 7. The color of the roof will have a large effect on solar heat gain;36 because many highway maintenance facilities have more roof area than wall area, cooling loads are driven by the roof heat gain, which is especially important for spaces cooled only by ventilation. There are a wide variety of high-albedo roofing products that minimize heat gain. Air movement in these facilities is very important, and when the ventilation supply is not enough to create air movement, high-capacity or high-velocity fans are an energy-efficient means to prevent air stratification and can increase both convective heating and cooling for workers at ground level. A well-sealed, well- insulated space will be more comfortable and require less energy. In some states, requirements for air sealing of buildings have been added to the building code. The idea behind a sealed building is to know where the air penetrates the building so it can be controlled. This allows the intake air to be reheated by the warm outlet air, especially with the use of a heat recovery ventilator (HRV) or energy recovery ventilator (ERV), instead of simply having arbitrary leaks in the building envelope.37 A building with low air infiltration must be designed in such a way that humidity is well controlled inside and the dew point is outside the vapor barrier. Free modeling software for building envelope design is available from Oak Ridge National Laboratory.38 Maintenance facilities can use this strategy for offices and non-vehicular spaces, but service bays prove a larger challenge, as discussed in Section 2.2.2, Building Functional Characteristics: Maintenance Bay. Cooling. In many parts of the country, vehicle maintenance bays are ventilated but not cooled; in most locations, administrative and office spaces are air conditioned. Larger spaces that are cooled with a chiller and cooling tower have a number of energy-efficient options, including modular chillers that operate at peak efficiency over a very wide range of loads, and rotary scroll chillers, the most efficient traditional type, rated up to 80 tons. For smaller spaces, high-efficiency direct exchange (DX) equipment is now available, up to SEER 21, as well as some alternative cooling strategies, such as hybrid direct–indirect evaporative coolers39 and air conditioners that use stored solar thermal energy to power an absorption cycle cooling system.40 Cooling and 35The Whole Building Design Guide provides a good discussion of the importance of site selection and the various consider- ations. http://www.wbdg.org/design/site_potential.php. 36U.S. EPA, Heat Island Effect. http://www.epa.gov/hiri/. 37BMT, Air Tightness of the Building Envelope in Practice. http://www.bmd.dk/uploads%5Cdocs%5Cair%20tighness%20 of%20buildings%20final%20version1.pdf. 38http://www.ornl.gov/sci/btc/apps/moisture/index.html. 39Federal Energy Management Program, Technology Installation Review. http://www1.eere.energy.gov/femp/pdfs/tir_ coolerado.pdf. 40U.S. Department of Energy, Heating & Cooling. http://www.energysavers.gov/your_home/space_heating_cooling/index. cfm/mytopic=12450.

Energy Efficiency and Demand Reduction 57 heating equipment is generally sized to meet peak demand and so will generally not need to operate at full capacity to maintain a comfortable environment. Including a variable frequency drive (VFD) can match system capacity to the actual load throughout the entire year, resulting in major savings in system motor energy use: 25% to 50% compared with constant-speed systems.41 Heating. There are many strategies that can be taken to reduce heating loads beyond maximizing insulation and minimizing air infiltration. Heat loss from the duct system or piping can contrib- ute up to 35% of the energy use of a heating system,42 so ensuring that there are no air leaks and providing duct insulation are two low-cost strategies to improve overall system performance. In high-bay spaces, heat tends to rise to the top of the building, resulting in air stratification with the colder layer on the bottom. To combat this tendency, some spaces use high-efficiency fans to distribute heat evenly throughout the entire volume of the space. A more efficient heating strategy in tall, large-volume spaces is to put the heat closest to where it’s needed: in the floor. This heats objects resting on the floor by conduction and other objects and surfaces by radiation. Radiant floor heating can be provided by a geothermal heat pump or a wide variety of boilers, some equipped to run on waste oil. The use of energy-efficient heating equipment can reduce operating costs and energy consumed. Modern condensing boilers and furnaces achieve efficiencies of greater than 90% by using primary and secondary heat exchangers that remove both sensible and latent heat from the flue gases. Highly efficient heat pumps and geothermal heat pumps (see Chapter 14) are also options for highway maintenance facilities. Lighting. A properly lit facility is necessary for safe, efficient operations. Often, past energy saving efforts resulted in de-lamping or simple removal of bulbs, which helps in over-lit areas, but when applied improperly can negatively affect facility operations or safety. Letting daylight into the building is the most effective way to save lighting energy but must be coupled with reducing the use of electric lighting and be planned carefully, as discussed in Chapter 7. Properly designed lighting systems can provide sufficient lighting for specific purposes, and using efficient fixtures and lamps with variable controls and occupancy sensors will reduce energy consump- tion. A number of energy-efficient lighting products have become more widely available in the last few years, including light-emitting diode (LED) lighting, induction lighting, high-intensity discharge (HID) lighting, and compact fluorescent and electronically ballasted fluorescent bulbs. The Department of Energy keeps updated information on lighting43 and provides energy saving estimations for commercial lighting projects.44 6.2.2 Demand Reduction 6.2.2.1 Direct Digital Controls Direct digital control (DDC) system is a term frequently found in building automation specifications. Over the last 60 years, control systems have evolved from pneumatic control to microprocessor-based controls, which have reduced costs and increased performance. Generally, DDC refers to the use of computers or microprocessors in concert with sensors and actuators to provide closed-loop control of building systems. DDC is not one controller but a combination of 41The energy savings result from the VFD reducing speed to match loads. Since the motor power is a function of the motor speed cubed, any reduction in speed results in a far greater reduction in power and energy required. http://www.facilitiesnet. com/hvac/article/The-Benefits-of-VFDs-In-HVAC-Systems--11278. 42“Furnaces and Boilers,” 2012. http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12530. 43http://www1.eere.energy.gov/buildings/lighting.html. 44https://www.lightingsolutions.energy.gov/comlighting/login.htm.

58 Renewable Energy Guide for Highway Maintenance Facilities controllers and other devices within a building, also known as distributed control systems, which may not be HVAC-specific but control other aspects of the building operations. 6.2.2.2 Building Monitoring Building monitoring systems provide real-time and historical data of building system opera- tions. This involves retrieving data from meters, submeters, and sensors on individual systems as well as key equipment, which is organized and displayed in a way that makes it most useful to the building operator. The types of information that may be gathered are energy use; building, ambient, and equipment temperatures; water flow rates; and any other characteristic deemed worthwhile to monitor. 6.2.2.3 Building Automation Building automation systems (BASs), also called energy management and control systems (EMCSs), control energy-consuming equipment in a building to reduce energy use while maintain- ing a comfortable environment. These systems may also include other features such as maintenance planning, fire- and physical-safety functions, and security services.45 The line between building monitoring systems and building automation systems depends on the definition that the product vendor uses to describe its systems. Generally, monitoring is restricted to the collection and display of data, which are then used to make decisions on operating the systems. Building automation systems can implement these decisions via computer control of the systems. Some of the most common strategies that building automation systems employ to cut energy use are: • Scheduling. Scheduling turns equipment on or off depending on time of day, day of the week, day type, or other variables such as outdoor air conditions. • Lockouts. Lockouts ensure that equipment does not turn on unless it is necessary. For example, a chiller and its associated pumps can be locked out according to calendar date, when the outdoor air falls below a certain temperature, or when building cooling requirements are below a minimum. • Resets. When equipment operates at greater capacity than necessary to meet building loads, it wastes energy. A BAS can ensure that equipment operates at the minimum needed capacity by automatically resetting operating parameters to match current weather conditions. For example, as the outdoor air temperature decreases, the chilled water temperature can be reset to a higher value. • Diagnostics. Building operators who use a BAS to monitor information such as temperatures, flows, pressures, and actuator positions may use these data to determine whether equipment is operating incorrectly or inefficiently and to troubleshoot problems. Some systems also use these data to automatically provide maintenance bulletins. BAS systems can be used to help integrate lighting controls with daylighting to maintain a prescribed lighting level for occupied spaces. Window shade controls can be used to modulate daylighting as well as to help control passive solar heating and cooling loads. This level of tailored and detailed control is part of what makes modern building automation an effective energy efficiency tool. 6.2.2.4 Occupancy Sensors Occupancy sensors detect human presence and prevent systems from running when they are not necessary. The most common type of occupancy sensors are lighting controls, which switch 45Kamm, K., 2007. “Achieving Energy Savings with Building Automation Systems.” http://www.automatedbuildings.com/news/ apr07/articles/esource/070322105430kamm.htm.

Energy Efficiency and Demand Reduction 59 off lights after a period of inactivity. These motion sensors use passive infrared, ultrasonic, or a combined multi-sensing technology, and some models can interface with building management or automation systems.46 Many applications can use motion sensors instead of switches, but for larger areas or where the switch location does not cover the active area, an array of detectors must be installed to prevent the unintentional cutoff of lighting or other building functions in occupied areas. 6.3 Applications for Energy Efficiency and Demand Reduction 6.3.1 Economics Energy efficiency strategies are often the most effective investments, with generally quicker payback and higher return on investment than renewable energy projects. The less efficient the building is, the greater the advantage of energy efficiency improvements. As energy efficiency improves, additional efficiency projects have less of an impact, and renewable energy systems become the more appealing investment. 6.3.2 Commissioning Total building commissioning is defined by the National Conference on Building Commission- ing as the “systematic process of assuring by verification and documentation, from the design phase to a minimum of one year after construction, that all facility systems perform interactively in accordance with the design documentation and intent, and in accordance with the owner’s operational needs, including preparation of operation personnel.”47 Energy, water, productivity, and operational savings resulting from commissioning offset the cost of implementing a building commissioning process. Recent studies indicate that on average, the operating costs of a commissioned building range from 8% to 20% below that of a non-commissioned building.48 The one-time investment in commissioning at the beginning of a project results in reduced operating costs that will last the life of the building. In general, the cost of commissioning is less than the cost of not commissioning. Continuous commissioning or periodic recommissioning of buildings is an enhancement to O&M that typically makes facility operations and management more efficient. The cost of commissioning is dependent on many factors, including a building’s size and complexity and whether the project consists of new con- struction or building renovation. In general, the costs of commissioning a new building range from 0.5% to 1.5% of the total construction cost. For an existing building, never before commissioned, the cost of retro-commissioning can range from 3% to 5% of total operating cost. 49 46Examples of the various types of sensors can be found at the Leviton product website: http://www.leviton.com/ OA_HTML/SectionDisplay.jsp?section=37707&minisite=10251. 47http://www.wbdg.org/ccb/GSAMAN/buildingcommissioningguide.pdf. 48U.S. Department of Energy, 1998. Building Commissioning Guide. 49U.S. Department of Energy, 1998. Building Commissioning Guide.

60 7.1 Overview Daylighting is the use of natural light, admitted into a building in a controlled manner, to illuminate the interior space. The intent of this strategy is to provide adequate lighting levels for the intended occupancy use while reducing electricity consumption for interior lighting. To do this, daylighting systems incorporate wall or roof apertures (windows, skylights, and so forth) to allow daylight into the building, shading and reflecting elements to control solar heat gain and decrease glare, and control systems to modulate the interior electrical lighting as necessary to maintain required lighting levels while reducing the energy consumed by electric lighting. Daylighting design requires balancing the reduction of energy loads for lighting provided by the daylighting system with the potential impacts on heating and cooling loads. For example, lights give off heat to the space, which can be detrimental during the cooling season but beneficial during the heating season. By reducing electric lighting, daylighting systems reduce these lighting- related heat gains. However, if the daylighting design introduces more direct sunlight to the space, this could increase solar heat gains, which could offset cooling load reductions. Daylighting also offers the opportunity for more building occupants to have views to the building exterior. 7.2 Types of Systems and Strategies Daylighting systems can include daylight admitted to the building interior by apertures on the building perimeter (wall or roof apertures) or by apertures to the building core areas (core daylighting). Solar heat gain can negate the energy savings of daylighting systems if not properly controlled. In general, east and west apertures will be energy liabilities while south- and north-facing apertures can be designed to allow daylighting of a space while not contributing an unacceptable amount of overheating. For this reason, most daylighting designs do not incorporate large east- or west-facing apertures. Perimeter Systems (Wall Apertures or Side Lighting). Windows are the most common daylight collectors. High windows allow light to penetrate deeper into the space, while low windows provide better views from the interior. Daylight penetration from low windows is limited to the build- ing perimeter. A system incorporating both high and low windows combined with glare control strategies such as overhangs and light shelves can provide daylight up to 30 ft into a space.50 Perimeter Systems (Roof Apertures for Top Lighting). Skylights are common daylight collectors on roofs. Roof monitors, which incorporate either vertical or sloped windows, are also consid- ered roof apertures, as are clerestory windows (see Figures 7-1 through 7-4). Roof apertures are C H A P T E R 7 Daylighting 50Ander and FAIA, 2011. http://www.wbdg.org/resources/daylighting.php.

Daylighting 61 Source: http://wiki.naturalfrequency.com/wiki/Daylight_Strategies. Figure 7-1. Roof monitor providing top lighting, EcoTect model. Source: http://wiki.naturalfrequency.com/wiki/Daylight_Strategies. Figure 7-2. Sawtooth roof monitor providing top lighting, EcoTect model. Figure 7-3. Roof monitor on West Ox Road Fairfax, VA, maintenance facility, Virginia Department of Transportation.

62 Renewable Energy Guide for Highway Maintenance Facilities an effective method of providing daylight to low-rise buildings that have interior spaces that are open to the ceiling. They do not work well in multistory applications without affecting the design of the floor plan significantly, except for top floor applications. Traditionally, skylights have been viewed by many facilities managers as opportunities for potential roof leaks. To counter this perception and respond to the increasing demand for viable daylighting components, many manufacturers of roofing materials have recently added roof aperture systems (various forms of skylights) to their offerings and have included these elements as a part of their roof system water-tightness warranties. Core Daylighting Systems (Light Wells, Atriums, and Courtyards). A courtyard or atrium uses a daylighting strategy that employs a large roof aperture combined with wall apertures located at one or more stories (see Figure 7-5). Many large, complex projects use this approach to provide daylighting to spaces remote from the building perimeter. Depending on location and building configuration, the strategies can employ either horizontal or sloped apertures or vertical aper- tures installed in light monitors. Core Daylighting Systems (Concentrators and Light Guides, Fiber Optics). Systems that use a concentrating collector to focus sunlight, combined with light pipes or fiber optic cables, have the potential to transport daylight over considerable distances within the building. These sys- tems are still largely experimental, but do hold promise for increasing daylight use to interior areas of taller and multistory buildings, including in retrofit situations. 7.3 Applications Daylighting works with most nonresidential building spaces, including those usually encoun- tered in a vehicle maintenance facility, such as office, storage, and shop areas. Daylighting of these spaces can reduce the energy required to operate electric lighting. Source: NREL. Figure 7-4. Daylighting with clerestory windows at Bighorn Home Improvement Center, Silverthorne, CO.

Daylighting 63 An office area at a vehicle maintenance facility may be of limited size and able to be adequately daylighted with wall apertures. A warehouse space or vehicle maintenance or storage area will normally be one or two stories in height and quite a bit larger in area than the supporting office space. Consequently, it may be best to provide daylighting using roof apertures or a combination of roof and wall apertures. Climate and Resource Availability. Properly designed daylighting systems will work in all climates in the United States, including regions that experience diffuse, overcast skies over prolonged periods of time. Because of the variations in climate, the amount of aperture, shading, and anti-glare strategies as well as the impact of the daylighting system on overall building energy consumption will vary with location. Building Occupancy and Use. Daylight alone can provide adequate lighting for many tasks. Most maintenance areas can benefit substantially from a properly designed daylighting system. The illumination needs of circulation areas and general office areas can also be reasonably met with daylight and task lighting. For specific activities, supplemental task lighting may be required. Building Type and Construction. Daylighting is applicable to most construction methods and building types. Wall apertures will easily work with any wall system. Roof apertures will work best with flat roofs. (Sloping roofs require daylight to penetrate through any space between the ceiling plane and the roof plane.) The best daylighting strategies are those that respond to the local climate conditions and spe- cific building use and building design. Understanding the interrelationship of these elements is essential to a successful design solution. 7.3.1 Economics Illumination of interior spaces represents 20% of all the site energy consumed and an average of 38% of the site electricity consumed in commercial buildings.51 This is a significant amount Figure 7-5. Atrium core daylighting in multistory application, EcoTect model. Source: http://wiki.naturalfrequency.com/wiki/Daylight_Strategies. 51Consumption & Efficiency, 2009.

64 Renewable Energy Guide for Highway Maintenance Facilities of both total building energy and electricity consumed. Daylighting is a strategy that can signifi- cantly reduce this energy consumption and the related carbon emissions resulting from reduced electrical consumption. Additionally, heat generated by lighting increases a building’s cooling loads and decreases heating loads. Well-designed daylighting systems have been shown to save the energy required to illuminate the building and provide additional savings (10% to 20%) in cooling loads, resulting in a total load reduction of up to 33%.52 The actual economic impact of a daylighting strategy will vary depending on time of occupancy of the building, size and shape of floor plate, type of system employed, and many other potential variables. However, the majority of the savings are due to the reduction of electric use for lighting. Control systems and controllable fixtures can add $0.50 to $0.75 per square foot to the overall building cost. Using these controls in concert with well-designed wall and roof apertures can result in potential savings of $0.05 to $0.20 per square foot annually.53 Daylighting of work areas can provide economic benefits to employers in addition to many benefits to building occupants, including better health, reduced absenteeism, increased produc- tivity, and an increase in general well-being.54 These benefits have proven significant enough that some building codes (mostly in Europe) require workers to be no more than a specified distance (usually about 25 ft) from a window. In addition to benefiting occupants, there are resulting economic benefits to employers from reduced absenteeism and increased productivity.55 7.3.2 Design Options Due to the complex nature of daylighting system design, it has become a specialty service offered by many consulting firms. Daylighting system design requires control of the quality of the daylight as much as the quantity. This requires strategies to control glare and respond to changes in ambient daylight. Additionally, the system needs to balance heat gain and heat loss with the lighting energy saved. Control of electric lighting in a daylighting system can either be manual or automatic. Automatic controls will respond more quickly to changes in ambient light. Dimming of lighting fixtures for daylighting purposes can be controlled by photosensors, timers, switching fixtures in groups, separately switching lamps within fixtures, or using continuous dimming fixtures. Controls need to be integrated with sensors to maintain necessary illumination levels, and the daylighting system should be integrated with the building energy management system to optimize performance. Daylighting design can have an impact on the building design and orientation, such as: • The shape of the building footprint may need to be modified to optimize daylighting. • The shading strategy may become an important part of the building’s exterior design. • The building may need to be reoriented on the site to optimize daylight exposure. • A light well or courtyard may be necessary to provide daylight access in larger buildings. • Perimeter offices may need to be relocated to permit daylight to reach open office areas. • Interior design may need to be modified to reflect daylight. Rules of Thumb. The following rules of thumb apply to daylighting design and can be used during the conceptual design phase of a project.56 52Ander and FAIA, 2011. http://www.wbdg.org/resources/daylighting.php. 53Ander and FAIA, 2011. http://www.wbdg.org/resources/daylighting.php. 54Ander and FAIA, 2011. http://www.wbdg.org/resources/daylighting.php. 55Davis et al., 2009. 56Torcellini et al., 2006. http://www.nrel.gov/docs/fy06osti/37542.pdf, p. 37.

Daylighting 65 • Vertical wall aperture: Penetration of useful daylight into the building is equal to approximately 2.5 times the window head height minus the windowsill height. Example: For a window head height of 10 ft with a sill height of 3 ft: 10 ft – 3 ft = 7 ft × 2.5 = useful daylight penetration of 17.5 ft. • An economical practical limit for high-quality daylight is from 25 ft to 30 ft into a space. Thus, for daylight admitted from north- and south-facing apertures, the maximum practical floor-plate dimension in the north–south direction is between 50 ft and 60 ft. • Interior surfaces should be highly reflective: • Ceiling reflectance >80% of light striking the surface is reflected. Wall reflectance >50%. Floor reflectance >20%; design should try to achieve twice the luminance of the electric lighting system. Predicting Performance. The interrelation of building design elements (facade and interior layout, proportions of rooms, and so forth) indicates that a thorough analysis is required for any daylighting design solution. Since light levels are not affected by scale, physical models (small-scale three-dimensional mock-ups of the space) incorporating actual reflectance levels can be used. In addition, there are over a dozen daylight simulation software programs available to predict system performance, with several available for download at no cost.57 Design Recommendations. Daylighting design should respond to the building program estab- lished prior to commencing the design process. The following recommendations are general in nature and may assist decision making at the conceptual level. • If incorporating daylighting into a design, it is recommended that a competent consultant who has successful experience with daylight technology and modeling software be engaged. • High-performance glazing allows more glazing area with less heat loss and allows more light and less heat into the space than standard glazing. Depending on the climate zone, glazing should be selected for proper visual light transmittance and solar heat gain factor to maximize daylighting performance. • When large glazed areas are anticipated, window assemblies with a low-emittance (low-E) coating and a gas-filled void can provide a higher level of comfort and better performance. • A successful design needs to balance heat gain in summer and heat loss in winter to provide the best overall annual performance. Codes, Standards, and Rating Systems. The International Green Construction Code was released in March of 2012 and has requirements for daylighting. Some local jurisdictions have similar requirements. ASHRAE Standard 90.1-2007 also limits the total area of fenestration and has a window-to-wall ratio. These limits apply to buildings that use the Prescriptive Building Envelope path to compliance with the standard. Daylighting of interior spaces is a requirement of many of the U.S. Green Building Council’s LEED rating systems. It is recommended that a thorough code and standards review be performed prior to commencing any project to identify applicable building code and rating system require- ments relative to daylighting, wall aperture limits, and other requirements. Occupancy issues: • Control of daylighting systems can be complicated; thus, commissioning of system performance to design requirements is critical, as is education and training of occupants and maintenance personnel. • Occupant interaction with daylighting systems can be an issue. 57U.S. Department of Energy, 2011. Building Energy Software Tools Directory.

66 Renewable Energy Guide for Highway Maintenance Facilities State of the Technology. Daylighting for buildings is a readily available and efficient strategy for incorporating renewable energy into vehicle maintenance facilities. Daylighting design has progressed rapidly with the development of high-performance glazing systems and the advent of sustainable building rating systems and BIM software. Improvements in glazing technology have allowed more glass to be incorporated into buildings with less thermal impact. Rating systems generally require daylighting and views for regularly occupied spaces, driving the demand for daylighting. BIM, when combined with other available software, provides accurate three- dimensional modeling of daylighting performance integrated with building energy requirements. Initial costs to include daylight strategies are minimal, and it may only require rearrangement of already planned glazing, revisions to glazing materials, or adding of shading elements to include daylight strategies. Trends in daylighting will see the continual development of new glazing materials that will improve system efficiencies. Highly insulating glazing materials such as nanogels (used as trans- lucent insulation in available glazing systems with R-values as high as 20) should continue to drop in price and will increase in use as production is scaled to meet demand. The availability of improved modeling tools will improve system design and reduce costs. As more climate-responsive buildings are constructed, the aesthetics of daylighting will become more visible and accepted. Automated shading systems that respond to daylight availability and demand will be integrated into lighting controls and building automation systems. One current trend in design is to use a pattern guide to predict daylighting performance. The pattern guide developed by the New Buildings Institute58 is available for free and includes analysis of 19 commonly occurring patterns in daylighting systems and recommendations for implementation. 7.4 Best Practices Rules of Thumb for Sizing and Performance • Daylighting systems with automatic controls and dimmable fixtures can reduce energy consumption for lighting between 35% and 60% [per the New Buildings Institute (www. newbuildings.org)]. • The effective depth of side-lighting strategies is limited to 2.5 times the window height. Minimum surface reflectance values of interior finishes: ceilings = 80%, walls = 60%, floors = 20%. Site Considerations • Site selection and planning should take daylighting basics into account. Daylight admitted through the south- and north-facing windows is easiest to control. Daylighting admitted by east and west windows is difficult to control and can overheat interior spaces. Therefore, orient the long dimension of building in an east/west direction for side-lighting strategies. Minimize east and west windows since it is difficult to control direct sunlight through the windows, particularly during the early morning or late afternoon when sun angles are low. • Avoid future shading of daylit facades from new building construction, growth of landscape elements, and so forth. System Selection • An integrated process is necessary in the design of daylighting systems. The inputs of the owner, occupants, architect, lighting designer, mechanical engineer, and energy modeler all need to be integrated into the final design. • Set a realistic daylighting goal for the project for the design team to achieve. 58New Buildings Institute, 2012. Daylighting Pattern Guide. http://patternguide.advancedbuildings.net/home/.

Daylighting 67 • One-story buildings with dimensions greater than 60 ft in total depth (from face to face) are candidates for top-lighting strategies. Side-lighting strategies require overall dimensions of less than 50 ft to 60 ft. • Daylighting requires supplemental electrical lighting. Lamp and luminaire selection should work with the daylighting, sensor, and control strategies and systems. Design Best Practices • Integrated daylighting strategy requires consideration of site, climate, and patterns of use, with integrated electric lighting designed to respond to daylight and provide the highest-quality and most visually appropriate and energy-efficient interior environment. • A qualified daylighting consultant should be engaged. • Early consideration and modeling of the impact of light levels, heat gain, and daylighting inputs are necessary. • Use software to model the impact of daylighting on whole building performance. This will be required when a building rating certification is pursued. • Strategies to control glare and excess heat from daylighting are critical to the success of the system. • Segregate view windows from daylighting apertures. Low windows may cause local glare at the building perimeter and require a different shading strategy than high windows that deliver light deeper into the space. • Locate open office areas at the building perimeter to allow daylight to penetrate into work spaces. Provide enclosed private offices with glazed walls to allow access to daylight. • Sunlight is not the same as daylight. Direct sun penetration should be avoided. Diffuse daylight should be the primary source of daylight in a space. • Since daylighting can be supplemented with task lighting for specific activities, acceptable task lighting strategies need to be a part of the overall lighting plan. • As standards are revised and adopted, lighting power densities, maximum fenestration area, light sensing and controls, and other elements of a daylighting system will be affected. Similarly, building rating systems are also being continually revised and updated. Review applicable standards prior to design to determine specific criteria. Construction • Confirm that glass products delivered for installation have the visible light transmittance specified. • Commissioning is applicable to daylighting, including lighting controls, occupancy sensors, movable shading, and other elements. • Controls and monitoring: – Use passive controls to tune the daylighting system to the project’s latitude and climate zone. Passive controls include light shelves, reflectors, shades, and sunscreens. – Dimming of lighting is most accepted by occupants when it is done slowly. – Match range of response of light-level sensors with anticipated light levels from daylighting strategies. Top lighting may require different sensors than side lighting or an atrium. Operations and Maintenance • As occupancy of spaces changes, changes to the spaces can affect performance of daylighting. Changes to colors can affect light reflectance, relocation of partitions can reduce access to daylight, and activities with differing light levels may replace the activities for which the space was designed. • Maintenance personnel should be familiar with the system concept and controls. Routine maintenance as simple as dusting may be required for reflective surfaces; more complicated recalibration of controls may be necessary for differing seasons.

68 Renewable Energy Guide for Highway Maintenance Facilities • Routine maintenance of components is required for optimum performance. This includes cleaning of glazing areas and any highly reflective surfaces as well as checking controls/sensors. • Include maintenance personnel in the commissioning process. Rules of Thumb for Cost/Performance • Since a properly designed daylighting system reduces daytime energy demand, cost calcula- tions should include demand pricing if applicable to the utility servicing the project. • Daylighting systems should provide a minimum savings of 10% on the building’s cooling load and electrical lighting load compared to baseline for the building type. • Do not dim lighting below 10% of rated output since energy savings do not increase below this level. • Refer to ASHRAE Standard 90.1 for lighting power densities for various occupancies and IESNA standards for lighting level for various tasks. Each of these should be integrated into the daylighting design.

69 8.1 Overview Passive solar heating is defined as using the various elements of a building to collect, store, and distribute heat collected from the sun. In these systems, thermal energy is moved by the naturally occurring forces of conduction, convection, and radiation rather than by fans, pumps, and other mechanical devices. In most latitudes in the United States, the low angle of the sun in winter means more solar energy is directed horizontally, toward the south-facing wall. In passive solar design, solar energy enters the building through south-facing windows, is absorbed by surfaces and mass within the building, and the heat is radiated into the space. During the summer, the high angle of the sun directs more energy vertically, onto the roof of the building, and the amount of sun striking the southern windows can be easily controlled with simple overhangs. The advantages of passive solar include relatively low initial cost and low operating cost. Depending on climate and passive solar strategy, elements of passive solar heating systems can also be a part of a passive or natural cooling system or the building’s daylighting strategy. 8.2 Types of Systems and Strategies Passive solar designs can include various strategies for collecting, storing, and distributing solar energy, but in all cases a properly sited and efficient floor plan coupled with a high-performance building envelope and energy-efficient building systems and appliances will yield the best overall performance. Passive systems are generally categorized as follows:59 Direct Gain and Sun Tempered. In this strategy, the sun is admitted to the space via south-facing windows. In a sun-tempered space, windows are generally clustered on the south facade, and the size of the solar aperture and the room area are balanced to maintain comfort levels during occupied periods. South window apertures are maximized, while windows on other orientations are minimized to reduce heat loss and control overheating. Sun-tempered spaces generally do not incorporate significant amounts of thermal mass. A direct-gain passive design incorporates mass in the space to store the solar energy. The mass is most effective when in the direct sun path and is usually sized to balance with the amount of energy admitted to the space by the solar aperture. As in all passive systems, performance is most effective when the solar aperture incorporates a movable insulation system to reduce heat loss through the aperture at night. Indirect Gain. Indirect gain incorporates placing a high-mass material between the occupied space and the solar aperture, with the mass usually in the form of a wall. This strategy is also known as a mass wall and is characterized by the close proximity of the mass to the aperture C H A P T E R 8 Passive Solar Heating 59 Mazria, 1979.

70 Renewable Energy Guide for Highway Maintenance Facilities (usually 3 in. to 6 in.). The side of the thermal mass behind the aperture is directly heated by the sun, and the stored solar heat is slowly released via radiation into the occupied space from the opposite side of the wall. The mass in an indirect gain system is typically made of concrete, masonry, or containers of water. The amount of mass is determined by the available solar radiation at the site, local climatic conditions, and the heating load of the building. The most well-known indirect gain system is the Trombe wall, which has been successfully incorporated into many residential and small commercial projects (see Figures 8-1 and 8-2). Performance of indirect gain systems can be tuned using a variety of strategies, including various wall coatings to control solar absorption and re-radiation, venting of the space between the glazing and the wall, and various exterior movable insulation systems. Isolated Gain. To accommodate the inherent overheating and large nighttime heat loss of direct-gain systems, isolated-gain designs arrange occupied spaces around a unoccupied (or minimally occupied) space that contains the solar aperture. The common form of this strategy is called a sun space. Heat from the sun space is admitted to adjacent spaces when needed, and when not needed the sun space is allowed to overheat or cool below acceptable indoor design temperatures. Because the sun space is normally not an occupied space, wider temperature ranges are more acceptable. Mass can be incorporated into the sun space to moderate temperature swings, or a mass wall can separate the sun space from the occupied portion of the building. Venting of the sun space can be via manually operated windows or doors. Performance of isolated-gain systems can be enhanced by night insulation of the aperture, shading strategies, venting of the space during overheating, and incorporation of thermal mass. Thermal Mass. A critical component of a passive solar design is the incorporation of thermal mass. Masonry and concrete are typical construction materials and are the most common materials used for thermal mass. Water is a good material for thermal mass and stores more energy per volume than concrete or masonry. However, water needs to be protected from freezing. A more Source: http://energy.gov/energysaver/articles/passive-solar-home-design. Figure 8-1. Trombe wall basics.

Passive Solar Heating 71 recent and more efficient technology uses materials that change phase, absorbing and releasing thermal energy as they melt or solidify. Phase change materials are available in some construction products, including concrete masonry units, gypsum wallboard, and floor and ceiling tiles. Though phase change materials offer enhanced performance, they are not widely available and are inherently expensive. Hybrid Systems. Many passive solar heating systems include a combination of direct gain, indirect gain, and isolated gain and may incorporate daylighting strategies as well. The choice of passive strategy should take into consideration the need for daylighting and time of occupancy. A building may have different passive strategies for different spaces, or one space may combine more than one passive strategy. 8.3 Applications Climate/Resource. In areas of the United States where heating is required, passive solar strategies will work well on residential and small commercial buildings. Successful systems have been built all over the country, from cloudy climates such as in Oregon to sunny climates such as in Arizona, New Mexico, and much of the Southwest. Building Occupancy and Use. In most commercial buildings, large internal heat-generating loads such as high numbers of people, office equipment, solar gains, and lighting result in total heating loads that are usually less of an annual energy impact than the building’s cooling loads. These buildings are load-dominated and require cooling in some portion of the building during most months. Small commercial and residential buildings are skin-dominated, losing most of their heat through their wall, window, and roof surfaces without the intensity of internal loads of larger commercial buildings. In most climates, they require heating during the day for the majority of the heating season. Thus, a skin-dominated building is a better match for passive solar heating. Building Type and Construction. Because of the need for building mass to store passive solar energy, buildings with inherent mass, such as masonry buildings and buildings built using concrete Source: NREL. Figure 8-2. Trombe wall at Zion National Park Visitor’s Center.

72 Renewable Energy Guide for Highway Maintenance Facilities slab-on-grade or precast concrete plank floor systems, are economical to incorporate passive strategies into. However, framed buildings with very little mass can incorporate a separate mass wall to store and distribute solar energy with a well-insulated, easy-to-construct, and economical building envelope. 8.3.1 Economics Passive solar can be a very economical solar strategy since the elements of the system are usually part of the building structure or skin and function as part of the building and part of the solar energy system. Incremental cost increases may include additional mass for thermal storage, solar shading systems, high-performance glass, and night insulation. Due to the wide variations in passive strategies, potential materials, and design possibilities, it is difficult to make a general statement relative to the costs of passive solar systems. There are accurate tools for predicting performance to allow analysis of proposed design solutions. 8.3.2 Rules of Thumb Site Considerations. To collect energy efficiently, the passive solar aperture should be oriented toward the south and be a maximum of 15 degrees to the east or west of solar south. Sun from the east and west is the most difficult to control with elements of the building and should be minimized. North-facing windows will not receive solar gain. Building Footprint. In general, the long axis of the building should be oriented east/west to provide the largest southern solar wall exposure. South-facing sun is the easiest to control with shading. Interior layout: Spaces that are occupied during the day should be located to the south, with buffer spaces, such as circulation areas, storage areas, and spaces with high internal gains, located to the north. Windows. East and west apertures are difficult to control with shading and should be kept to a minimum to prevent overheating. North apertures allow light but not solar heat since they do not receive significant amounts of direct solar radiation. Use high-performance windows for east, west, and north exposures. South-facing apertures should be shaded to allow solar gain in the winter but exclude it during the summer. Consider how glare will be controlled at large window areas. Window-to-Floor Ratio. For a passive heating system to function, there needs to be a balance between the window aperture, the floor area of the room/space, and the amount of mass avail- able for thermal storage. It is critical to consider latitude, climate conditions, and the area of the space when sizing windows and thermal mass. Sun-Tempered Spaces. Solar apertures for spaces with no or minimal thermal mass should be limited or daytime overheating of the space is likely to occur. Seasonal shading and night insulation will enhance overall performance. Direct-Gain Spaces. With appropriately sized thermal mass located in the floor and/or walls, larger solar apertures than in sun-tempered spaces are possible without overheating the space and with corresponding higher levels of performance. Indirect Gain – Mass Walls. In this strategy, the entire area of the mass has adjacent glazing. Interior temperature is controlled by the mass of the wall, and the amount of mass should be calculated for the climate conditions. For masonry, the energy absorbed by the exterior surface of the wall takes approximately 1 hour to reach the interior surface of the wall. Thus, for an 8-in. masonry wall, energy absorbed at noon will be radiated into the adjacent occupied space at 8:00 p.m. The thickness of the mass needs to correlate to the occupancy times of the space and the amount of available solar energy to make sure the mass can be heated to comfortable temperatures by the sun.

Passive Solar Heating 73 Isolated Gain – Sun Spaces. Properly designed sun spaces will collect enough heat to maintain reasonable temperatures in the space plus add a significant amount of heat to the adjacent occupied spaces. Modeling of performance, taking into consideration the latitude, climate, thermal mass, and spaces to be heated, is critical to a successful design. Predicting Performance. Many different currently available building energy modeling software programs can accurately predict passive solar performance. They take into account local weather data for heating and cooling loads, solar resource availability, building orientation, window size and location, glass characteristics, shading, building mass, and other variables to calculate performance. 8.3.3 Issues and Considerations Overheating. Overheating can be an issue in passively heated buildings. Care must be taken during the design to consider window shading and solar gains during the non-heating seasons. Glare. Because of the large glass areas associated with passive design, glare can make spaces uncomfortable. Care must be taken in the design to minimize the negative impacts of glare. 8.3.4 Codes and Standards Various codes and standards limit maximum window area to a percentage of wall area under their prescriptive compliance path. This can be an issue with passive solar buildings and may require thermal modeling, which can be quite expensive, to verify that the building’s energy performance falls within the intent of the code or standard. 8.4 Best Practices Rules of Thumb for Sizing Depending on climate zone and latitude, glass area will vary. In general, the south wall should have the highest window-to-wall area ratio, with other exposures having a low window-to-wall area ratio. General rules of thumb for sizing glazing are: • Sun-tempered. South-facing glass area equal to 5% to 7% of total floor area. • Direct-gain passive solar. South-facing glass area between 9% and 12% of total floor area. • Providing south-facing glass areas in excess of 12% can cause overheating and should only be done when additional thermal mass is incorporated into the design. • Adequate thermal mass is considered to be six times the area of the accompanying glazing (per the Whole Building Design Guide). • Shading. South-facing windows can be easily shaded to allow winter solar gain while prevent- ing summer solar access. Various design and analysis tools are available, including this free overhang annual analysis tool from Sustainable by Design: www.susdesign.com/overhang_ annual/index.php. • Comfort. In passive solar design, an understanding of thermal comfort of occupants is critical. Since many passive strategies incorporate thermal mass, the mean radiant temperature of the space can have more impact on comfort than relative humidity and air temperature and needs to be taken into account. Site Considerations • Site selection should take into consideration solar access when passive solar strategies are contemplated. • Use site-specific climatic and solar data in selection and design of passive systems.

74 Renewable Energy Guide for Highway Maintenance Facilities • Orientation of solar collection elevations of building should be within 15 degrees of south for maximum performance. If orientation cannot be directly to the south, rotation of building axis to the east is preferable to the west. • Use site and landscape strategies to allow solar heat gain during the winter and protect the building from unwanted winter wind exposure. • Distribution system: – Thermal mass that is directly exposed is preferred. If direct solar exposure during heating season is not possible, thermal mass should be exposed to the conditioned space. – The amount of thermal mass must be appropriate for solar resource, or mass may reduce mean radiant temperature and affect comfort. System Selection • Define acceptable thermal comfort ranges for the intended occupancy and select appropriate passive strategies. • Consider occupancy use when selecting a passive strategy: – Sun-tempered and direct-gain systems can contribute to glare in occupied areas and may not be acceptable. If these strategies are considered, incorporate them into the building’s daylighting strategy. – Indirect gain can be designed to delay the delivery of solar energy and may be appropriate for buildings that are occupied in the evening and at night. – Sun spaces, atria, and other isolated-gain spaces may have wide temperature swings and are not appropriate for all occupancies. – Building shape, massing, and layout can have significant impacts on passive solar strategies. Selection of passive strategies should be integrated into the development of the floor plan and building organization. Design Best Practices • A compact design incorporating a well-insulated, airtight envelope and high-performance windows is an essential element of a passive solar building. • During the schematic design phase, locate buffer spaces at the building perimeter, where appropriate to the program, to reduce loads and optimize comfort. • Locate spaces tolerant of temperature swings in sun spaces and other isolated-gain locations. • Locate spaces that do not require conditioning to design temperatures to the north, east, or west. • Whole building energy modeling is recommended to optimize performance and predict impacts of the passive design on both heating and cooling loads. • Coordinate passive heating strategies with daylighting and natural cooling strategies. • Each facade should address the solar exposure characteristics of its orientation to maximize performance. • Incorporate ERV strategies to minimize energy requirements and maintain indoor air quality. Although ERVs are not passive strategies, they are an essential requirement for a building with an airtight building envelope. • Avoid significant glass areas oriented to the west or east. Where east and west glass is included, consider the impact on heating loads and include either active or passive shading strategies. • Supplemental heating systems should be correctly sized to avoid overcapacity and inefficient operation. Construction • Many opportunities for proper insulation and air sealing can only be seized during the construction process. Close monitoring of critical installations is essential. • Provide commissioning of air-barrier performance to verify that designed infiltration levels are achieved.

Passive Solar Heating 75 • Controls and monitoring: – Provide ongoing monitoring of building energy use to verify performance and ensure that building occupancy activities are not in conflict with passive system design and operation. – Use automated or passive shading to optimize blocking direct solar radiation in the cooling season with allowing solar gains during the heating season. Operations and Maintenance • Use of movable insulation requires active participation of building occupants. Depending on occupancy use, this may not be reliable. • During continued occupancy, ensure that landscape elements do not alter solar access. • Provide training on the design and operation of the system to building maintenance staff, and document system functions in a building user guide. • Rules of thumb for cost/performance: – Limit window-to-wall ratio to a maximum of 50% unless utilizing a buffer space. – Sun tempering can reduce heating requirements from 5% to 25% at little or no additional cost. – Other passive solar strategies can reduce heating requirements from 25% to as much as 75%, depending on climate, latitude, and building occupancy, and can be cost-effective on a life-cycle basis. – Concrete slab floors are the least expensive method of incorporating thermal mass into a project. Do not cover with carpet.

76 9.1 Overview Like passive solar heating, most natural cooling (or passive) cooling strategies have a long history. Before air conditioning and motor-driven ventilation, all cooling of buildings was done by using the forces of nature. If passive solar heating is defined as using the design of the building to collect, store, and distribute solar energy, then the definition of natural or passive cooling is for the building’s design to control the heat gain of a building. Heat that does not get into the build- ing during the cooling season does not require energy or equipment to remove. Passive heating and natural cooling are interrelated and require thorough coordination to arrive at a successful design solution where both approaches are to be applied. The control of heat gain is critical to natural cooling strategies. When considerations for summer cooling are neglected in a passive solar design, the glazing and thermal mass can work to increase heat gain and storage at a time when it is not wanted, causing extremely uncomfortable interior conditions. Natural cooling strategies are most effective for buildings in dryer climates that experience relatively cool nighttime temperatures. A well-designed system can function in more temperate zones with higher humidity levels but must be carefully designed. Many of the same strategies to take advantage of passive solar heating are appropriate for natural or passive cooling. Proper building orientation, floor plan layout, internal mass, window location, window shading, site shading, and building envelope design can minimize the cooling load to maximize the impacts of natural ventilation and building mass strategies. In some situations, passive cooling strategies can be enhanced by the addition of mechanical ventilation and evaporative cooling, which can maintain comfort levels without the energy required to operate vapor-compression air-conditioning systems. 9.2 Types of Systems and Strategies Comfort during the cooling season is dependent on the humidity and temperature of the air. Climates with high temperature and high humidity may not be appropriate for natural cooling. However, many of the strategies may reduce the required mechanical cooling loads. Natural Ventilation. The building design, site location, and window arrangement all contribute to internal air movement. Because naturally ventilated spaces require operable windows to be open during occupied times, this strategy may not be acceptable for all types of buildings. There are three basic approaches to natural ventilation: single sided, cross-ventilated, and stack effect. Single-sided ventilation uses high and low operable windows located on the same side of the occupied space to provide ventilation. This strategy requires relatively tall ceilings and is effective for a limited depth of space. C H A P T E R 9 Natural or Passive Cooling

Natural or Passive Cooling 77 Cross-ventilation uses operable windows on a minimum of two walls of an occupied space. This approach can be very effective, provided that the occupied spaces are open, unobstructed, and of limited dimension. Stack effect can be created by a tower specifically designed to induce ventilation or an atrium or other architectural space. By providing low intake openings and high exhaust openings, natural ventilation can be enhanced as the buoyancy of the warm air creates a chimney effect within the space. High Mass. Buildings with high ratios of mass to floor area are able to absorb heat energy during the day and release it at night. A typical example of this type of strategy would be an adobe building in the southwestern United States. The high mass of the building is essentially a thermal flywheel that dampens wide temperature swings in the occupied portions of the building. High Mass with Nighttime Ventilation. The performance of a high-mass building can be enhanced by closing the building to the warm air of the day and opening the building to flush out the heat during the night. In some instances, naturally cooled buildings use mechanical ventilation to improve the heat removal during the night, but because vapor-compression air-conditioning equipment is not used, this type of system uses considerably less energy. Evaporative Cooling. There are a variety of passive evaporative cooling strategies that have long been employed on buildings. Fine water spray evaporative cooling can reduce the temperature of shaded outdoor spaces in dry climates. In more humid regions, roof ponds are an example of a passive evaporative cooling strategy. Even when supplemented with mechanical fans, these systems are still highly energy efficient. Shading. Temperatures in the shade can be significantly lower than in areas with full solar exposure. Natural cooling strategies incorporate shading of windows to reduce heat gain as well as shading of the site and building. Solar panels (e.g., photovoltaic modules) can serve as awnings to provide shading and solar power. 9.3 Applications Climate/Site. Climates with relatively low humidity and low nighttime temperatures during the cooling season will accommodate natural cooling systems best. Orientation of buildings should be to maximize impact of summer breezes. Shade trees or shade structures can help reduce the temperature of entering ventilation air. Building Occupancy and Use. Natural cooling strategies are applicable to buildings with occupancies that can tolerate some temperature swings and variable air movement. High-mass buildings would also be good candidates for passive solar heating. Building Type and Construction. As in passive solar-heated buildings, natural cooling strategies also incorporate thermal mass. Buildings with inherent mass are economical to incorporate natural cooling strategies. In ventilated buildings, cross-ventilation or stacked ventilation can be provided to enhance airflow. Limiting the sources of heat by shading windows and minimizing heat transfer and infiltration through the building envelope will enhance the performance of natural cooling systems. Careful integration of daylighting strategies will balance electric lighting energy savings with heat gains in a way that optimizes annual energy performance. Internal thermal building loads should also be minimized. Light-colored pavement adjacent to the building and light-colored siding and roofing can reduce cooling loads. Ventilation through a space works best when spaces are narrow (35 ft to 45 ft) with open interiors, which can affect the floor plan of larger buildings. Due to potential solar gain, windows at the east and west exposures should be avoided.

78 Renewable Energy Guide for Highway Maintenance Facilities 9.3.1 Economics Natural cooling strategies can be a part of the building and may have minimal impact on the overall cost of a project. Added costs include those from operable windows, increased quantity of windows, shading of windows with architectural elements, and landscape shading. Performance modeling of natural ventilation systems can be quite complex and can also add to the project cost. Color selection can have an impact on performance and usually does not represent a sig- nificant additional cost. The percentage of natural cooling will vary with building design and location and can vary widely. The San Francisco Federal Building is totally naturally ventilated. At 18 stories tall with over 600,000 ft2 of floor area, it is an extreme example. However, many employees find that the variations in temperature produce less-than-ideal working conditions. 9.3.2 Predicting Performance Rules of thumb do not apply except in the simplest cases. Due to variations in building design, climate, and latitude, natural cooling systems should be modeled on a case-by-case basis. Many projects now employ computational fluid dynamic simulations to fine-tune natural ventilation. Building energy modeling software programs should also be used to accurately predict natural cooling contribution to the building’s thermal performance. 9.4 Best Practices Rules of Thumb for Sizing • Natural cooling strategies can be very effective, but due to the variations in climate, building layout, building use, occupant preference, and many other factors, rules of thumb do not generally apply. Site Considerations • Climate will determine appropriate natural cooling strategies. Consideration of daily tem- perature variations and extremes, relative humidity, prevailing winds, and other site-related factors are critical in selecting natural cooling strategies. • Use of the prevailing wind during the cooling season can affect selection of building site and orientation of major activity areas. • Use site and landscape strategies to reduce cooling loads and protect the building from unwanted solar exposure and heat island impacts. System Selection • Include thermal mass within the insulated envelope of the building. • Integrate thermal mass for cooling with thermal mass that is part of a solar heating strategy to optimize annual energy performance. • Use high-performance windows. • If incorporating only natural cooling strategies, windows should have a low shading coefficient to minimize solar gains. • If natural cooling is combined with passive heating, clear windows in combination with low-E coatings and operable external shading designed to allow solar gains in the winter and block solar gains in the summer should be used. • Where west and east windows are included for natural cooling, include external shading to limit heat gain. • Coordinate natural cooling strategies with daylighting and passive heating strategies. • Coordinate passive evaporative cooling strategies (roof ponds and water spray) with building design and detailing.

Natural or Passive Cooling 79 • Thermal mass must be sized for daily temperature swings, or the temperature of the mass may become uncomfortably high and affect occupant comfort. • Supplemental ventilating and cooling systems should be correctly sized to avoid overcapacity and inefficient operation. • Controls and monitoring: – Automated controls to open and close windows are available to provide natural ventilation. More sophisticated systems are required for night-flushing strategies. – Integrate natural ventilation controls with building HVAC and lighting system controls to optimize building energy performance. Operation and Maintenance • During continued occupancy, ensure that changes to building layout and landscape elements do not alter breeze and ventilation access. • Maintain operable shading devices to perform as designed. • Provide training on the design and operation of the system to building maintenance staff, and document system functions in a building user guide. • Rules of thumb for cost/performance: – Because the primary methods of natural cooling are preventing heat gain and providing ventilation to remove unwanted heat, rules of thumb are not applicable.

80 10.1 Overview Active solar heating systems use solar collectors to convert sunlight into thermal energy that can be used for water heating, space heating, or heating ventilation air. The solar collectors are essentially heat exchangers that are designed to trap the solar heat and transfer it to either liquid or air. Depending on the application and the climate, certain types of solar collectors may be advantageous. Solar collectors are categorized as liquid or air, depending on the medium they are designed to heat (e.g., water, water/glycol mix, other heat transfer fluid, or air). Standardized testing and rating methods have been developed to enable comparison of products and ensure basic levels of equipment integrity [e.g., ability to withstand wind loads, stagnation temperatures (high temperatures associated with periods of sunshine and no flow through the collectors), and heavy rains]. 10.1.1 Flat Plate Liquid Collectors Flat plate solar collectors are the most widely used and consist of a metal absorber plate (the heat exchanger) through which liquid can flow—either directly through integral flow passages or through piping or channels in direct contact with the absorber plate. The absorber is housed in a box-like enclosure that has a transparent cover plate on one side to admit sunlight, with the other surfaces (sides and back) designed to protect the absorber and provide structural rigidity. Insulation is provided behind the absorber and on the edges of the collector box to reduce heat losses. When sunlight strikes the absorber plate, it heats the plate and the fluid that is in contact with it. The absorber plate typically has a dark coating that helps it to maximize the absorption of solar energy. Variations on the basic design include the use of different materials for the trans- parent cover plate, the number of cover plates, and the type of absorber coating. These variations represent trade-offs between improved solar transmission, heat retention, and overall efficiency. Flat plate liquid collectors (see Figure 10-1) provide temperatures that range from 120°F to 160°F and are well suited for applications such as water heating and space heating. 10.1.2 Evacuated-Tube Collectors Evacuated-tube solar collectors enclose a tubular absorber or combination fin and tubular absorber within a clear glass tube. The space between the absorber and the outer tube is under a vacuum. What the vacuum does is reduce the heat losses due to convection from the absorber. Multiple tubes are connected in closely spaced rows to form a collector or array. The reduced heat losses enable the collectors to achieve temperatures from 160°F to 300°F, depending on the design. In some cases, a reflector is used in conjunction with the tubes to increase the amount of solar energy reaching the absorber. An advantage of the evacuated-tube collector is its perfor- C H A P T E R 1 0 Active Solar Heating

Active Solar Heating 81 mance under very cold conditions. This type of collector is well suited for medium- to higher- temperature water heating, such as process water heating, or as a source for thermally activated cooling systems. The downside is that they cost more than flat plate collectors. Figure 10-2 shows an image of an evacuated-tube collector. The Coney Island, NY, train maintenance facility is an example of a facility using evacuated-tube collectors to provide solar-heated water for washing vehicles (see Case Studies, 22.4). 10.1.3 Flat Plate Air Collectors Flat plate air collectors heat room air or ventilation (outside) air that is in contact with the absorber. A fan is used to draw air through the collectors and into ductwork for distribution to storage or to the space. While they do not achieve temperatures quite as high as flat plate liquid collectors, they are well matched for ventilation air heating and for systems that use air-side Source: Florida Solar Energy Center. Figure 10-1. Flat plate liquid solar collector. Photo by Alan Ford, NREL/PIX 09501. Figure 10-2. Evacuated-tube collector.

82 Renewable Energy Guide for Highway Maintenance Facilities distribution. Unlike liquid solar collectors that heat air, no heat exchangers are needed between the collectors and the heating load, thereby reducing thermal losses. They also do not need freeze protection and are therefore well suited for colder climates. The St. Clair, MO, DOT facility case study is an example of a facility using solar air collectors for providing space heating (see Case Studies, 22.1). 10.1.4 Transpired Solar Collectors Transpired solar collectors (TSCs) use a perforated metal panel as the absorber plate, which is attached to the exterior of a building and acts as external cladding. They are typically installed on a wall that has a proper orientation for capturing solar energy (e.g., south-facing walls). The collectors are attached to the walls by framing materials, which are spaced to provide optimized channels for airflow between the collectors and the wall. Outside air is drawn across the collectors from the bottom to the top and into the building by ventilation fans. Since the collectors do not have a transparent cover plate to reduce heat loss, they provide lower temperatures than glazed collectors. However, they do provide sufficiently high temperatures for preheating ventilation air—typically 20°F to 60°F above the outside temperature—and achieve fairly high efficiencies. A variation on the design uses a combination of glazed and unglazed TSCs to increase the tem- peratures. A transpired solar collector is depicted in Figure 10-3, and transpired solar collectors on an NREL facility are depicted in Figure 10-4. 10.1.5 Solar Collector Mounting Solar collectors are typically mounted on the roof of a structure but can be mounted on the ground or on the building walls. Optimal year-round performance is achieved for collector arrays that are south oriented and tilted from the horizontal at angles nearly equal to the latitude. Depending on the slope of the roof, this will require variations in support structures and mounting strategies. Architectural integration with the structure—for example, mounting the solar col- lectors parallel to the roof slope—may be desirable for aesthetics and result in less-than-optimal tilt/orientation. Key considerations include the ability of the roof to take the additional loads Source: U.S. Department of Energy. Figure 10-3. Transpired solar collector.

Active Solar Heating 83 imposed by the weight of the collectors and support structures, as well as the associated wind loads. Minimizing roof penetrations and making allowances for access and maintenance of collectors and the roof are also important design considerations. 10.2 Types of Active Solar Systems and Strategies 10.2.1 Solar Water Heating Systems Solar water heating systems are categorized as direct if the service or domestic water is heated directly in the collectors, or indirect if there is a heat exchanger between the solar collector loop and the service or domestic hot water. Further distinctions are whether the system uses pumps or natural convection to circulate water through the solar collectors (thermosiphon systems). Tanks for storing solar-heated water are another major component of solar water heating systems, and distinctions are made between single-tank and two-tank systems. In a single-tank system, one tank is used for storing solar-heated water. Any heat required in addition to that supplied by the solar energy is added by an auxiliary heater (e.g., electric heating element) that is also in the tank. In a two-tank configuration, the heat from the solar collectors is stored in a preheat tank, and a second tank provides supplemental heat to boost temperatures, if needed. The designs typically have provisions for bypassing auxiliary heaters and tempering the water to maximize use of the solar energy while ensuring that supply temperatures do not exceed safe levels. The suitability of the systems depends on the climate and application. For climates where the possibility of freezing is prevalent, indirect systems that use antifreeze fluid in the solar collector loop or that allow the water to drain from the collector loop are generally used. For mild climates, with little possibility of freezing and with supply water that has low total dissolved solids (TDS), direct systems can be safely used. 10.2.1.1 Direct Solar Water Heating Systems In a direct solar water heating system, potable water is circulated through the collectors and then to the load (see Figure 10-5). Since these systems do not require heat exchangers, they can have lower cost and higher efficiency than indirect systems. If occasional freezing temperatures are encountered, the systems need to be drained. An alternative is to circulate warm water from Source: NREL (http://www.nrel.gov/news/features/feature_detail.cfm/ feature_id=1522). Figure 10-4. Transpired solar collectors on NREL facility.

84 Renewable Energy Guide for Highway Maintenance Facilities the storage tank through the collector loop for short periods of time. In areas that have local potable water with high TDS, excessive scaling can occur in direct systems. 10.2.1.2 Indirect Solar Water Heating Systems Indirect systems circulate fluid through the collectors and transfer the heat to water in the storage tank through a heat exchanger. The fluid in the collector loop is typically an antifreeze– water mix or other fluid that has a low freezing point (see Figure 10-6). However, it can also be water. When an antifreeze solution is used, the heat exchangers used typically must be of a double-wall construction. This design helps ensure that leaks in the heat exchanger do not result Source: Design Manual for Commercial/Large Active Solar Systems (http://www.solar-rating.org/ commercial/designmanual/ASHRAEDesignManualIntro.pdf). Figure 10-5. Direct system with recirculation freeze protection schematic. Source: Design Manual for Commercial/Large Active Solar Systems (http://www.solar- rating.org/commercial/designmanual/ASHRAEDesignManualIntro.pdf). Figure 10-6. Antifreeze system schematic.

Active Solar Heating 85 in the antifreeze solution mixing with potable water. When water is used, the designs incorporate a means of draining the water when needed. Drain-back systems drain the water into a tank or separate reservoir whenever the solar collector loop pump is off (see Figure 10-7). The case study of the St. Clair, MO, DOT facility includes drain-back solar water heating systems that provide domestic hot water and heated water for washing vehicles (see Case Studies, 22.1). Drain-down (also referred to as drain-out) systems drain the collector loop water whenever the temperature is low enough to make freezing a possibility. Special valves are used that can open or close in response to the temperature sensor/control to automate draining. The collector loop must be refilled with water after each drain-out event. For systems that rely on draining the water, the collector and associated piping must be installed with sufficient pitch to enable complete draining to occur. Recirculation of warm water from the storage tank through the collectors can also be used to prevent freezing. However, if this is more than an infrequent requirement, it can result in a significant penalty in terms of conventional energy use. 10.2.1.3 Other Types of Solar Water Heating Systems Integral collector storage (ICS) combines a small water-storage tank within a box-like collector enclosure so that the solar energy heats the storage water directly. These are best suited for regions that do not experience freezing temperatures. They are primarily for residential applications with small water heating requirements. Thermosiphon systems use a tank that is elevated above the top of the collectors. When the water in the collector loop is heated, it becomes less dense and rises naturally to the water in the tank. The colder water in the tank moves down to the collectors, and circulation is established. The benefit is that no pumps or controls are needed for circulation of the water. When the temperature of the water in the collectors is no longer higher than that in the tank, circulation stops. However, freezing is an issue, so these systems are best suited to climates where freezing does not occur. Provisions for automated drain-out can be added but add complexity. Indirect thermosiphon systems are also possible. Air-collector–based water heating systems use flat plate air collectors rather than liquid collectors and an air-to-water heat exchanger in the solar collector loop for heat transfer. This introduces some inefficiencies to the system. The storage tank side of this configuration is the Source: Design Manual for Commercial/Large Active Solar Systems (http://www.solar- rating.org/commercial/designmanual/ASHRAEDesignManualIntro.pdf). Figure 10-7. Drain-back system schematic.

86 Renewable Energy Guide for Highway Maintenance Facilities same as in the liquid collector systems. An advantage of the system is that freezing of the collectors is not an issue. However, provisions must be made to prevent circulation of air across the heat exchanger during periods when the system is not operating (e.g., unwanted thermosiphoning) and freezing temperatures are encountered. This is to prevent freezing of liquid in the water side of the heat exchanger.60 10.2.2 Space Heating and Combined Water and Space Heating Systems Space heating systems using liquid collectors are typically indirect systems. Water or an antifreeze solution is circulated through the solar collectors and then through a heat exchanger. The heat exchanger transfers the heat to one or more tanks for thermal energy storage. The water from the tanks is circulated through a water-to-air or water-to-water heat exchanger depend- ing on the distribution system (e.g., air handling units, fan coil units, convectors, radiators, or radiant heat systems). For smaller heating requirements, the use of multiple water heater tanks can be advantageous. For larger systems, single large tanks are more cost-effective. A key consid- eration is the temperature requirement of the building distribution system. Since the output of flat plate solar collectors is typically less than 140°F during heating season operation, this may not be high enough for certain distribution system configurations (e.g., baseboard convectors) but is adequate for others (e.g., radiant heating). In a combined system, water heating is accomplished by using a separate heat exchanger and pump to circulate water between the main storage tank and a separate solar water heater tank. The circulation occurs as long as the temperature of the main tank exceeds that of the solar water heater tank, subject to a high temperature limit. When there is a demand for water, the water from the solar water heater tank is used and (if needed) is boosted to the required temperature by auxiliary heat. 10.2.2.1 Solar Heat Pump Systems The thermal energy from a solar system can also be used as a heat source for the evaporator of a heat pump (series heat pump arrangement). Since the solar heat will be at a higher temperature than the outside air, this will require less work (energy) by the heat pump’s compressor, thereby improving its performance. In this arrangement, the heat source is the solar-heated water in the storage tank. A parallel solar heat pump system makes use of the solar energy directly by use of a heat exchanger upstream of the heat pump indoor coil. If the heat from the solar-heated air is adequate, the heat pump will not be needed to boost the temperatures. If it is not warm enough, then the heat pump and/or auxiliary heater will provide the additional heat. 10.2.2.2 Seasonal Storage Systems During the non-heating seasons, solar space heating systems are underutilized. This is true even for combined systems, where the water heating load is small relative to heating requirements. During the summer, operation of the system may only be required for a few hours per day, with the solar collectors being idle the rest of the time. One possibility to make use of the available heat in the summer is to store it for use during the heating season. This requires a considerable storage volume and significant insulation. While this is possible for an individual building, it is more cost-effective to do this on a larger or community scale. One such strategy uses boreholes in the ground through which solar-heated antifreeze solution circulates. Over time the boreholes and surrounding soil are heated to useful temperatures. During the heating season, the antifreeze solution is circulated through the boreholes and the heat is supplied to the buildings. Figure 10-8 shows an example in the form of a schematic of the Drakes Landing community-scale seasonal storage system at Okotoks, Alberta, Canada. These systems can benefit from higher temperature 60Hunn et al., 1987, p. 64.

Active Solar Heating 87 collectors since the higher temperatures reduce the size of the storage volume required to store the same amount of heat. At Drakes Landing, the earth reaches a temperature of nearly 180°F when fully charged. 10.2.3 Solar Ventilation Air Heating Systems Preheating of ventilation air is accomplished by the use of transpired solar collectors (also referred to as unglazed transpired collectors) or flat plate air collectors. The solar collectors heat the ventilation air, which is mixed with the room air. The ventilation air temperature is boosted by conventional heaters if needed, depending on the design requirements. In this system, there are no storage tanks or heat exchangers involved, which reduces complexity and costs. However, this also means that solar heat cannot be stored for use during periods when there is no sunshine. Figure 10-9 shows an application of the transpired solar collector (trade name “SolarWall”) on a fire station. The case studies of the vehicle maintenance facilities at Fort Drum, NY (Case Studies, 22.2) and the Plattsburgh, NY, hangar (Case Studies, 22.3) illustrate two different versions of the transpired solar collector ventilation air heating systems. Source: http://www.dlsc.ca/. Figure 10-8. Seasonal storage. Source: Conserval: http://solarwall.com/en/products/solarwall-photo-gallery. php?img_count=36. Figure 10-9. Willow Springs, NC, fire hall SolarWall system.

88 Renewable Energy Guide for Highway Maintenance Facilities 10.3 Applications 10.3.1 Active Solar Water Heating Screening Methods The following provides methods to help determine the performance and economic suitability of active solar technologies at a screening level. 10.3.1.1 Manual Methods Solar resource maps of available solar energy can be used in conjunction with assumptions about solar water heating system efficiency to estimate system output. The basic steps are: Step 1: Determine Solar Resource Obtain the solar radiation values per unit area for the location from the resource map (see http://www.nrel.gov/gis/solar.html). (Units are typically in kWh/m2/day or Btu/ft2/day.) Two other sources for solar radiation data that can be used for resource estimation are the Solar Radiation Data Manual for Flat Plate and Concentrating Collectors (http://rredc.nrel.gov/solar/ pubs/redbook/) and the PVWatts software (http://rredc.nrel.gov/solar/calculators/PVWATTS/ version1/). The data manual provides monthly and annual solar resource data for flat plate and concentrating solar collectors at several different tilt angles. The units are in kWh/m2/day, which can be converted to Btu/ft2/day by multiplying the values by 317.1. Use the tilt angle equal to the latitude for the first pass. The PVWatts software can be run for the specific location, and the solar resource appears on the results page [Solar Radiation (kWh/m2/day)] after “calculate” is selected. Step 2: Determine Solar Array Area This should be based on an initial estimate of available area and amount required to meet between 40% and 70% of the annual water heating load. As a starting point, assume that about 1 ft2 of solar collector is needed for each 2 gal of hot water used on a daily basis. For example, if the daily usage is 1000 gal of hot water, consider a 500 ft2 area as the initial estimate. Appendix A provides information for estimating hot water energy requirements. Step 3: Determine System Output Multiply the solar array area × solar energy available/unit area × efficiency of the solar system × number of days per year. Typical efficiencies are between 35% and 45%. Efficiencies from manufacturer’s equipment can be obtained from the Solar Rating and Certification Corpora- tion (SRCC). Step 4: Refine Solar Array Area Compare the annual output to the annual solar water heating load. If this is between 40% and 70%, this is a good starting point. If not, change the assumed area and redo the calculation so that it falls in this range. Note that a system should generally be sized to provide no more than 100% of the water heating energy required on the sunniest days of the year. This minimizes any excess energy (oversizing) of the system, strictly from an energy capture perspective. If the Solar Radiation Data Manual or PVWatts is used, then the average daily solar array output for the sunniest month can readily be calculated. In this situation, the average daily water heating load would need to be estimated for the sunniest month and compared to this solar resource data. The ASHRAE Active Solar Heating System Design Manual (http://www.solar-rating.org/ commercial/index.html) has tables of pre-calculated outputs for solar water heating systems assuming two different levels of solar collector efficiency (average and good). These were based on the use of solar water heating system performance estimation software. The maps in Figure 10-10 and Figure 10-11 have been generated from these tables by the Department of Energy. They can be used in a manner similar to the method outlined previously. In this case, simply use the values from the map and multiply by the solar array area to determine the annual output.

Active Solar Heating 89 Source: U.S. DOE FEMP: Federal Technology Alert: Solar Water Heating. Figure 10-11. Annual output of solar water heating system for high-efficiency flat plate solar collector. Source: U.S. DOE Federal Energy Management Program (FEMP): Federal Technology Alert: Solar Water Heating. Figure 10-10. Annual output of solar water heating system for average efficiency flat plate solar collector.

90 Renewable Energy Guide for Highway Maintenance Facilities Another manual performance estimating method is to use the SRCC collector rating to estimate output for solar water heating systems. The SRCC rates solar collectors (OG-100) and residential-size solar water heating systems (OG-300) and provides a method for comparing systems (http://www.solar-rating.org/facts/collector_ratings.html#EfficiencyVsCost). A benefit of this method is that actual manufacturer’s data are used. 10.3.1.2 Rules of Thumb for Sizing • Size systems to provide between 30% and 70% of water or space heating loads on an annual basis. • For solar water heating, size so that the system provides 90% to 100% of the load on the sunniest days of the summer. This maximizes the solar utilization. • Assume 1 gal to 2 gal of water per square foot of collector for tank sizing. Use the low end of the range for colder, cloudier climates, and the high end for warmer, sunnier climates. • The solar collector array should be south facing (true south) and tilted to an angle equal to the latitude plus or minus 10 degrees: – Solar water heating. A tilt angle equal to the latitude will provide the most output for systems with relatively uniform year-round loads, such as for water heating for domestic or process applications. – Solar space heating or combined water and space heating. A tilt of latitude plus 10 to 15 degrees will increase wintertime output. – Solar cooling/maximize summer output. Use a tilt of latitude minus 10 degrees. • Note that orientations within 30 degrees of true south and tilt within 10 degrees of latitude will still provide adequate solar energy capture. Architectural integration considerations and local solar conditions (e.g., presence of early morning fog) may dictate different orientation and tilt selection. • Assume collector loop pump flow rates of between 0.02 gal/min and 0.05 gal/min per square foot of collector area. 10.3.1.3 Software Tools Solar water heating system software includes: • RETScreen. This software is sponsored by Natural Resources of Canada (NRCAN). It is free and can be downloaded as a stand-alone package. It also includes an economic analysis module. http://www.retscreen.net/. • Federal Renewable Energy Screening Assessment (FRESA). This software is sponsored by the U.S. DOE Federal Energy Management Program (FEMP). It is an online screening tool that provides energy and economic analysis information, including life-cycle cost analysis. https://www3.eere.energy.gov/femp/fresa/. • FChart. This software provides a simple method for determining the performance and eco- nomics of selected solar system configurations. http://sel.me.wisc.edu/fchart/new_fchart.html. • TRNSYS. This software provides detailed analysis that is more appropriate for larger and more complex systems. http://sel.me.wisc.edu/trnsys/. 10.3.1.4 Economic Screening The economic screening requires knowledge of how much purchased fuel the solar water heating system has saved, the cost of the fuel, and the capital and any nonfuel operating and maintenance costs of the system: Typical assumptions and information sources are: • Solar domestic hot water system costs: $100/ft2 to $150/ft2, assuming systems between 40 ft2 and 80 ft2.

Active Solar Heating 91 • Solar domestic hot water system efficiency: 40% (output is 175,000 Btu/ft2/year to 300,000 Btu/ft2/year, depending on location). • Commercial-size solar water heater costs: $100/ft2. • Solar space heating efficiency: 30% (output is 125,000 Btu/ft2/year to 200,000 Btu/ft2/year). • Solar space heating system costs: $75/ft2 to $100/ft2. • Annual O&M costs: 2% of system costs. • Energy prices (per unit): depends on location [see Energy Information Administration (EIA) for statewide costs as a default: http://205.254.135.7/electricity/state/ and http://205.254.135.7/ dnav/ng/ng_pri_sum_dcu_nus_a.htm]. • Fuel savings: fuel savings = solar energy used/efficiency of conventional water heater. • Efficiency of water heater/boiler/furnace: electric resistance 100%, natural gas: 80%. • Energy operating cost savings: energy operating cost savings = fuel savings × unit energy price. 10.3.2 Transpired Solar Collector Ventilation Air Heating System Screening Methods 10.3.2.1 Manual Methods Solar resource information can be used in conjunction with assumptions about the transpired solar collector system efficiency and ventilation air operating hours to estimate system output. The basic steps are: Step 1: Determine Solar Resource Two sources for solar radiation data that can be used for resource estimation are the Solar Radiation Data Manual for Flat Plate and Concentrating Collectors (http://rredc.nrel.gov/solar/ pubs/redbook/) and the PVWatts software (http://rredc.nrel.gov/solar/calculators/PVWATTS/ version1/). The data manual provides monthly and annual solar resource data for flat plate and concentrating solar collectors at several different tilt angles, including vertical south-facing surfaces. The units are in kWh/m2/day, which can be converted to Btu/ft2/day by multiplying the values by 317.1. Use the tilt angle equal to vertical, assuming that the transpired solar collectors are to be wall mounted. The PVWatts software can be run for the specific location, and the solar resource appears on the results page [Solar Radiation (kWh/m2/day)] after “calculate” is selected. Input the tilt angle 90 degrees to model a vertical south-facing wall. Non-south-facing orientations can also be modeled if using PVWatts. Step 2: Determine Transpired Solar Collector Area This should be based on the building ventilation air requirements, available collector mount- ing area, and the flow rates that are suggested for transpired solar collectors. An airflow rate of 3 cfm/ft2 to 10 cfm/ft2 of collector is typical. As a starting point, assume 6 cfm/ft2 and divide this number into the building ventilation air requirements. For example, if the ventilation air requirements are 3,000 cfm, then the collector wall area would be 500 ft2 (3,000 cfm/6 cfm/ft2). The resulting area should be compared to the available mounting area. If the wall area is less than this value, then assume the wall area as the mounting area. If the wall area is greater than this value, then use the calculated value as the transpired solar collector area. Step 3: Determine Ventilation Usage Fraction Estimate the number of days per week the ventilation system will be on. Calculate a ventilation usage fraction by dividing the number of days the ventilation is on by the total number of days per week. For example, if the facility’s ventilation system is on 5 days per week, then the ventilation usage fraction is 5/7 or 0.714.

92 Renewable Energy Guide for Highway Maintenance Facilities Step 4: Determine Transpired Solar Collector System Output Determine which months of the year constitute the heating season for the facility. From the Solar Radiation Data Manual or the PVWatts output, for each of these months, multiply the daily solar radiation values by the number of days in that month (or if heating is only required for less than a full month, the number of days heating might be needed). Then sum these values to get the useful annual solar radiation. The units are in kWh/m2, which should be converted to Btu/ft2 by multiplying the values by 317.1. The annual output is calculated by multiplying the useful annual solar radiation by the efficiency of the transpired solar collector system by the transpired solar collector area (in ft2) by the ventilation usage fraction. A value of 50% should be used for the transpired solar collector efficiency. For example, a 500 ft2 array with a useful solar radiation of 250,000 Btu/ft2/year would provide 62.5 MMBtu/year (250,000 Btu/ft2/year × 50% × 500 ft2). Note that additional benefits from the use of transpired solar collectors, such as heat loss reduction and possibly destratification, are not included in this value. 10.3.2.2 Rules of Thumb for Sizing • Size systems based on ventilation air requirements assuming transpired solar collector flow rate of 3 cfm/ft2 to 10 cfm/ft2 of transpired solar collector. • The TSC should be south facing (true south), although orientations within 30 degrees of true south will still provide adequate solar energy capture. Architectural integration considerations and local solar conditions (e.g., presence of early morning fog) may dictate different orientations. • The color for the TSC should be dark, although it does not have to be black. The selection of colors other than black will reduce the amount of solar radiation that can be absorbed. The selection of the color will be a trade-off between architectural integration and performance. The manufacturer should specify the absorptivity of the paint. 10.3.2.3 Software Tools Transpired solar collector performance software includes: • RETScreen. This software is sponsored by NRCAN. It is free and can be downloaded as a stand-alone package. It also includes an economic analysis module. http://www.retscreen.net/. • FRESA. This software is sponsored by the U.S. DOE FEMP. It is an online screening tool that provides energy and economic analysis information, including life-cycle cost analysis. https:// www3.eere.energy.gov/femp/fresa/. 10.3.2.4 Economic Screening The economic screening requires knowledge of how much purchased fuel the transpired solar collector ventilation air heating system has saved, the cost of the fuel, and the capital and any nonfuel operating and maintenance costs of the system. Typical assumptions and information sources are: • TSC costs: $25/ft2 to $35/ft2. • TSC efficiency: 50% (100,000 Btu/ft2/year to 150,000 Btu/ft2/year), not including any heat loss recapture or destratification savings. • Annual O&M costs: 1% of system costs. • Energy prices (per unit): depends on location (see EIA for statewide costs as a default: http://205.254.135.7/electricity/state/ and http://205.254.135.7/dnav/ng/ng_pri_sum_dcu_ nus_a.htm). • Fuel savings: fuel savings = system output/efficiency of conventional heating system. • Efficiency of conventional heating system: electric resistance: 100%, natural gas: 80%, air-source electric heat pump: coefficient of performance (COP) = 2. • Energy operating cost savings: energy operating cost savings = fuel savings × unit energy price.

Active Solar Heating 93 10.4 Active Solar Best Practices 10.4.1 Solar Water and Space Heating Systems Best Practices Site Access/Array Mounting Best Practices • Ensure adequate roof or ground area is available, such that the solar collector array is not shaded between 9 a.m. and 4 p.m. Use sun charts (see http://solardat.uoregon.edu/SunChart Program.html to create sun charts for specific locations) or other tools to check for obstructions that could shade the solar collectors during the course of the year. Sun charts for June 21, December 21, and March 21 provide information for the sun at its highest position, lowest position, and average position. • Minimize roof penetrations. Consider ballasted mounting for flat roofs for rack mounting; clip- type connections for standing seam metal roofs. High wind areas need special consideration. • Allow clearance between roof and collectors of 1½ in. to 2 in. for stand-off or rack-mounted arrays unless using integrated or flush-mounted design. • Account for snow shedding/damming and potential for additional snow loads. • Leave room for accessing the collectors. This could be up to 20% of the collector array area. Piping/Distribution Systems • Solar-ready design. Install plumbing chases that can easily be accessed for solar thermal distribution. • Antifreeze systems. Use nontoxic (e.g., propylene glycol) heat transfer fluid in the solar collector loop. Always consult the local plumbing code. • Water treatment. For indirect water systems, ensure that measures are taken to prevent scaling, such as water treatment or the use of deionized water. • Make sure there is a process/system for overheat protection. Design for times when there is high solar radiation and low or minimal usage. Avoid overheating. • Backup freeze protection must allow complete draining of collector loop/piping exposed to freezing unless an antifreeze system is used. • Make sure adequate allowance has been made for thermal expansion of piping; use proper fittings/connectors to avoid leaks. • Drain-back systems require proper pitch to ensure that all the water can drain out of the collector loop and piping. • Generally use a double-wall heat exchanger between collector loop and potable water. Always consult the local plumbing code. System Selection • Match system type to climate. Use direct systems in non-freezing areas, glycol or drain-back systems elsewhere. The supply’s water hardness (TDS) needs to be taken into account when using direct systems. • For colder climates, use flat plate collectors with selective absorbers or collectors. Consult SRCC OG-100 list to review efficiencies/output of tested solar collectors (http://www.solar-rating.org/ facts/collector_ratings.html#EfficiencyVsCost). • Evacuated-tube collectors can be beneficial in colder climates or when higher hot water temperatures are required. A cost trade-off analysis should be used when considering more expensive evacuated-tube collectors. Construction • Acceptance testing/commissioning. Make sure start-up is with solar collectors still covered or occurs during early morning. • Consider using in-house staff to assist with construction only if they are trained or there is proper supervision.

94 Renewable Energy Guide for Highway Maintenance Facilities • Equipment and installer certifications: – Certifications. SRCC OG-100 (collectors). OG-300 for solar domestic hot water systems (smaller/residential-size systems). – Make sure that vendor products are readily available. – Make sure that firm has demonstrated experience installing this type of system. • Use ASHRAE Active Solar Heating Systems Design Manual for larger systems (http://www. solar-rating.org/commercial/designmanual/ASHRAEDesignManualIntro.pdf). • Consultant should document load estimation, solar contribution, and savings. Software tools such as RETScreen, FCHART, or another recognized tool should be used for the solar contribution estimates. Load estimates should be based on measurements or calculations according to ASHRAE or similar. • Consider contractors who have received training through organizations such as North American Board Certified Energy Practitioners: http://www.nabcep.org/. Controls and Monitoring • Include monitoring instrumentation. Temperatures and flows. • Be careful about integrating control with BAS or EMCS—likely better to leave controller independent. Operation and Maintenance • If in-house staff are to maintain, then make sure they are adequately trained, have O&M experience, and so forth. 10.4.2 Transpired Solar Collector Best Practices Site Access/Array Mounting Best Practices • Ensure that the selected area for mounting (e.g., wall) is not shaded between 9 a.m. and 4 p.m. Use sun charts or other tools to check for obstructions that could shade the solar modules during the months of the heating season. Sun charts for June 21, December 21, and March 21 provide information for the sun at its highest position, lowest position, and average position. • Make sure that the area selected for intake air does not have the possibility of drawing from contaminated air sources (e.g., vehicle exhaust). Construction • Select firms that have experience in the installation of TSC systems. • Consultant should document load estimation, TSC contribution, and savings. Software tools such as RETScreen or another recognized tool should be used for the TSC contribution esti- mates. Load estimates should be based on measurements or calculations according to ASHRAE or similar. Controls and Monitoring • Integrate the TSC operation with the building management system or energy management system, if available. Make sure that the fan operation is linked to the fire protection system such that it will shut off if a fire alarm is activated. • Use variable-speed drive fans to modulate airflow through the TSC system in order to optimize solar energy capture. This would control the temperature rise based on variations in ambient temperature and wind conditions and indoor space temperature requirements. • Install performance monitoring equipment to determine the amount of energy being supplied by the standard test condition (STC) system. Operation and Maintenance • Provide training in system operation for in-house maintenance personnel.

95 11.1 Overview PV systems consist of modules of solar cells that convert sunlight to electricity. The electricity generated is direct current (DC), which can be used to run DC-powered equipment or to charge batteries. In many cases, the DC power is converted to alternating current (AC) by use of an inverter so that it is compatible with AC-powered devices and the electric grid. PV systems can be tied to the electric grid (grid connected) or used in off-grid situations. The major benefit of being grid connected is that the utility is able to provide power during periods when the PV sys- tem output is not sufficient to meet the facility’s loads (e.g., periods when there is no sunshine). In an increasing number of jurisdictions, the utility allows electricity that is in excess of the facility’s requirements to be exported to the grid, and pays for this electricity via a net metering arrangement. Off-grid PV systems generally use batteries if there is a desire to use PV-generated electricity during periods when there is no sunshine. For PV water pumping, it may be possible to store the water during periods of sunshine for use during periods when there is no sunshine. 11.2 Types of Systems and Strategies 11.2.1 Photovoltaic Modules Photovoltaic modules consist of solar cells—semiconductor materials—that generate electric- ity when exposed to light energy. The dominant type of PV module is based on crystalline silicon (c-Si) materials—either monocrystalline or polycrystalline. The solar cells are encapsulated and housed in assemblies that provide structural rigidity and protection from the elements. Typical modules for use in building applications range from 125 W to 300 W in capacity, although some higher output modules are available (see Figure 11-1). Amorphous silicon (a-Si)-based PV mod- ules are also available that can be applied to flexible substrates (e.g., metal roofing materials). These tend to have lower efficiencies and somewhat lower costs per unit area. Other materials that are used include cadmium telluride (CdTe), cadmium sulfide (CdS), copper indium disel- enide (CIS), and copper indium gallium (di)selenide (CIGS). These are classified as thin-film PV modules because they require less semiconductor material than crystalline silicon PV modules. The principal advantages of thin-film PV modules are (potentially) lower cost than crystalline silicon modules and the ability to be applied to various substrates. In general, the efficiencies of thin-film PV modules (11%) are higher than those of amorphous crystalline PV modules (7%), but lower than for crystalline PV modules (14%).61 Module efficiencies and power ratings C H A P T E R 1 1 Photovoltaics 61U.S. Department of Energy, 2011. 2010 Solar Technologies Market Report, p. 58. http://www.nrel.gov/docs/fy12osti/51847. pdf. Note that efficiencies of 20% are now available in commercial monocrystalline silicon PV modules.

96 Renewable Energy Guide for Highway Maintenance Facilities provided by manufacturers are generally at STCs. These conditions are an incident solar radiation of 1,000 W/m2, module temperature of 25°C, and air mass of 1.5. Another rating condition is based on the PVUSA program and is called the PVUSA test condition (PTC) rat- ing. The rating conditions in this case are an incident solar radiation of 1,000 W/m2, ambient temperature of 20°C, and wind speed of 1 m/s. The PTC provides a more real-world rating condition (ambient temperature) and results in lower efficiencies and rated outputs than the STC. It is used by the California solar incentive programs for estimating PV module per- formance (see http://www.gosolarcalifornia.org/equipment/pv_modules.php). Typical PV system costs have been $6/watt to $8/watt installed for grid-connected systems (no batteries) for systems under about 250 kW. PV system costs have been decreasing over the past few years as PV module costs have declined, and costs below $6/watt are becoming more prevalent. Typical system output is 8 W/ft2 to 12 W/ft2 (86 W/m2 to 129 W/m2), accounting for inverter losses as well as other system losses. 11.2.1.1 Photovoltaic Module Mounting Photovoltaic modules are typically mounted on the roof of a structure (e.g., maintenance building or parking area shade structure) but can be mounted on the ground or on the build- ing walls. Optimal performance is achieved for PV arrays that are south oriented and tilted at an angle, with respect to the horizontal, about equal to the latitude. Depending on the slope of the roof, this will require variations in support structures and mounting strategies. Architec- tural integration with the structure—for example, mounting the modules parallel to the roof slope—may be desirable for aesthetics and result in less-than-optimal tilt/orientation. Key con- siderations include the ability of the roof to take the additional loads imposed by the weight of the modules and support structures as well as the associated wind loads. Minimizing roof penetrations and making allowances for access and maintenance of the PV modules and the roof are also important design considerations. Ballasted mounting, where weights are used to keep the PV modules and support structures in place, is a method for minimizing roof penetrations (see Figure 11-2). Shading of crystalline silicon PV modules from roof-mounted equipment, building parapets, and landscape elements should be analyzed and avoided because intermittent shading of one cell in a module causes the entire module to shut down. Intermittent shading of amorphous silicon PV reduces output only from the area of the module that is shaded. Figure 11-1. Crystalline silicon PV Modules on parking garage at NREL’s South Table Mountain facility. Source: Dennis Schroeder/NREL.

Photovoltaics 97 Building-integrated photovoltaics (BIPV) where the PV modules replace building envelope materials or components are also a possibility. For example, flexible thin-film amorphous silicon PV panels have been attached to metal roofing substrates to form an integral exterior roofing membrane (see Figure 11-3). The case study of the Kilauea Military Camp, HI, vehicle storage structure is an example of such an application (see Case Studies, 22.11). Translucent PV panels have been used for glazing in windows and skylights. PV tiles and shingles have also been used for roofing materials. 11.2.1.2 Tracking Systems Tracking systems can be used with flat plate modules to increase the output of the system. By tracking the movement of the sun through the sky, more direct-beam sunlight can be captured. This additional solar energy—as much as 40% more—translates into increased output (see Fig- ure 11-4). Trackers can be single axis or dual axis. Single-axis trackers typically have their axis in a north–south direction and track east–west. Dual-axis trackers track the sun’s position in the Figure 11-2. Ballasted mounting support structure. Source: Genmounts: http://www.genmounts.com/index.php/project-list/. Source: NREL. Figure 11-3. Amorphous silicon PV material and metal roofing application—Big Horn Home Improvement Center, Silverthorne, CO.

98 Renewable Energy Guide for Highway Maintenance Facilities sky hourly, daily, and seasonally. They are more typically used with concentrating PV systems.62 Trackers add cost and complexity to the systems and are most effective in regions where there is substantial direct-beam sunlight, such as the southwestern United States. They also provide more power in the morning and afternoon than non-tracking systems. Tracking systems tend to be used for larger installations serving a utility or community rather than for individual facilities. 11.2.2 Power Conditioning Equipment Power conditioning equipment includes components designed to help the DC power from the PV system conform to the requirements of the intended loads and applications. For grid- connected systems, inverters sized to meet the maximum output of the PV array are required. These are generally stand-alone devices, although some PV modules have been developed that include micro-inverters as part of the module. For off-grid systems or those that use batteries, charge controllers are the principal means of regulating the power from the PV modules. The charge controllers control the rate of charge and discharge to ensure that the batteries operate within their specified charging and discharging regimes. This helps to prevent damage to the bat- teries and premature failure. The batteries most commonly used with PV systems are deep-cycle lead acid batteries (either flooded or sealed/maintenance-free types). 11.3 Applications 11.3.1 Manual Screening Methods Solar resource maps of available solar energy on surfaces tilted at various latitudes can be used in conjunction with assumptions about photovoltaic system efficiency to estimate system output. The basic steps are: 1) Determine solar resource. Obtain the solar radiation values per unit area for the location from the resource map (see http://www.nrel.gov/gis/solar.html and Figure 11-5; units are typically in kWh/m2/day or Btu/ft2/day). Another source for solar radiation data that can be used for Figure 11-4. Sun-tracking PV systems on the roof of the Arizona National Guard’s Eco-Building at Papago Park Military Reservation. Source: NREL. 62The DOE article “Flat-Plate Photovoltaic Balance of System” describes the basics of different tracking options for PV systems: http://www.eere.energy.gov/basics/renewable_energy/flat_plate_pv_balance.html.

Photovoltaics 99 resource estimation is the Solar Radiation Data Manual for Flat Plate and Concentrating Col- lectors (http://rredc.nrel.gov/solar/pubs/redbook/). The data manual provides monthly and annual solar resource data for flat plate and concentrating solar collectors at several different tilt angles. Use the tilt angle equal to the latitude for the first pass. 2) Determine photovoltaic array area. This should be based on an initial estimate of available area and amount required to meet a portion of the facility’s electrical requirements. 3) Determine annual photovoltaic system output. Multiply the solar array area × solar energy available/unit area × efficiency of the solar system × number of days per year × loss factor. Typical efficiencies at standard test conditions (PV module DC rating) are:63 • Crystalline technologies: – Mono-Si: monocrystalline silicon, 14%–19%. – Poly-Si: polycrystalline silicon, 13%–15%. • Thin-film technologies: – a-Si: amorphous silicon, 6%–8.5%. – CdTe: cadmium telluride, 8%–11%. – CIGS: copper indium gallium diselenide, 8%–11%. Alternatively, the manufacturer’s efficiency value for the specific module of interest can be used. The loss factor accounts for reductions in system output due to losses in the inverter, wires, dirt, and other mechanisms. A value of 0.77 (23% loss) is a good value to use. If the system does not use an inverter, then the factor should be 0.84. 63Green Energy Life Cycle Assessment Tool User Manual, 2012, pp. 4–6. Source: NREL http://www.nrel.gov/gis/images/map_pv_national_lo-res.jpg. Figure 11-5. Solar resource map for photovoltaic systems.

100 Renewable Energy Guide for Highway Maintenance Facilities 4) Refine solar array area and other parameters. The array area can be varied along with as- sumptions on efficiency. If the Solar Radiation Data Manual for Flat Plate and Concentrat- ing Collectors is used (http://www.nrel.gov/docs/legosti/old/5607.pdf), then the influence of tilt angles as well as tracking can also be investigated. Note that in general, tilt angles of ± 10 degrees from latitude and orientations of ± 30 degrees from true south do not ap- preciably change performance. Another method that is sometimes used is based on sun hours. A sun hour is defined as 1000 watts (1 kW) per square meter of sunshine falling on a surface that is perpendicular to the sun’s rays. This also corresponds to the amount of solar radiation (peak sun) that is used to define standard test conditions. A table or map of sun hours gives the equivalent number of hours per day that an area receives the peak amount of sunshine. For example, if during the 10 hours of sunshine the total solar energy that is received on a 1-m2 area is 6 kWh, this is equivalent to 6 sun hours per day (daily solar energy divided by 1,000 W). In order to calculate the annual electricity production using sun hours, the following relationship is used: Annual output (kWh/year) = sun hours (kWh/m2/day) × 365 (days/year) × PV array area (m2) × efficiency × loss factor. 11.3.2 Software Tools Photovoltaic system performance software includes: • PVWatts. This software is available through NREL. It is an online screening tool. http://www. nrel.gov/rredc/pvwatts/. • In My Back Yard (IMBY). This software is available through NREL. It is an online screening tool that provides a geographic information system (GIS)-user interface that enables the PV system to be sketched on a photo of the location. http://www.nrel.gov/eis/imby/. • RETScreen. This software is sponsored by NRCAN. It is free and can be downloaded as a stand-alone package. It also includes an economic analysis module. http://www.retscreen.net/. • FRESA. This software is sponsored by the U.S. DOE FEMP. It is an online screening tool that provides energy and economic analysis information, including life-cycle cost analysis. https:// www3.eere.energy.gov/femp/fresa/. 11.3.3 Economic Screening The economic screening requires a knowledge of how much purchased electricity the photo- voltaic system has saved, the cost of the electricity, and the capital and any nonfuel operating and maintenance costs of the system. Typical assumptions and information sources are: • Photovoltaic system costs:64 – $8/watt ($8,000/kW) for systems under 10 kW. – $6.50/watt ($6,500/kW) for systems between 10 kW and 100 kW. – $6.00/watt ($6,000/kW) for systems between 100 to 250 kW. – $5.50/watt ($5,500/kW) for systems larger than 250 kW. Note that these are based on 2010 (see Appendix A, Figures A-2 through A-4) prices, and prices have been declining. However, the prices are strongly dependent on market factors such as silicon prices, as well as on technology improvements. • Annual O&M costs: $0.005–0.01/kWh. • Electricity prices (per unit): depends on location (see EIA for statewide costs as a default). http://205.254.135.7/electricity/state/. 64U.S. Department of Energy, 2011. 2010 Solar Technologies Market Report, p. 56. http://www.nrel.gov/docs/fy12osti/51847. pdf.

Photovoltaics 101 • Electricity savings: electricity savings = electricity generated by the photovoltaic system. • Electricity cost savings: electricity cost savings = electricity savings × unit electricity price. Note that if there is net metering, and the amount generated by the PV system exceeds the facility requirements, the energy cost savings would depend in part on what, if any, the utility paid for this excess. In this case the cost savings would be the sum of the savings associated with the electricity used on-site and the amount received for the excess. 11.4 Best Practices Rules of Thumb for Sizing • When sizing, make sure to calculate the amount of electricity that is in excess of the facility’s requirements (if any) over the annual billing period. Where net metering is allowed, deter- mine if there are any system size restrictions. If net metering is allowed, then determine if the utility will pay for the excess electricity, and how much. Use this information in the economic analysis to help determine the appropriately sized system. If net metering is not allowed, the system needs to be designed based on the hourly daytime load, or battery backup can be considered. Consider the trade-offs of the added cost and maintenance of battery storage. The PV array area needed will vary from about 80 to 120 ft2 per kW of capacity, depending on module efficiency. • Make sure to account for system losses when estimating the output of the system. A typical loss factor of 0.77 (23% losses) should be used for systems with inverters and applied to the rated output of the module or array. • Make sure to account for the likely temperatures to be experienced by the PV modules when estimating the system output. Most photovoltaic modules will have reduced outputs at tem- peratures above the standard rating condition (25oC). This is due to the reduction in voltage with increases in temperature. A typical reduction will be around 10%. Under these condi- tions, a module that has an STC rating of 200 W will only produce 180 W. Furthermore, most modules have a tolerance of about ±5%, so the actual output could be lower. An exception is amorphous silicon modules, which actually show some increase in output with increasing temperature. • When sizing inverters, account for temperature and solar radiation variations under extreme conditions, and include an oversize factor (e.g., 5%). When possible, install inverters in shaded or cooler areas. • The solar array should be south facing (true south) and tilted to an angle equal to the latitude. However, orientations within 30 degrees of true south and tilt within 10 degrees of latitude will still provide adequate solar energy capture. Consider mounting collectors at a small pitch, even if horizontal mounting is desired. This can help make rain more effective at keeping the collectors clean. Architectural integration considerations and local solar conditions (e.g., pres- ence of early morning fog) may dictate different orientation and tilt selection. Site Access/Array Mounting Best Practices • Ensure that adequate roof or ground area is available, such that the solar array is not shaded between 9 a.m. and 4 p.m. Use sun charts or other tools to check for obstructions that could shade the solar modules during the course of the year. Sun charts for June 21, December 21, and March 21 provide information for the sun at its highest position, lowest position, and average position (see http://solardat.uoregon.edu/SunChartProgram.html to create sun charts for spe- cific locations). Shading can be a significant problem for PV systems since the output of whole strings of panels can be nullified by partial shading. • Minimize roof penetrations. Consider ballasted mounting for flat roofs for rack mounting, and clip-type connections for standing seam metal roofs. Take into account high wind loads.

102 Renewable Energy Guide for Highway Maintenance Facilities • Allow clearance between roof and modules of 1½ in. to 2 in. for stand-off or rack-mounted arrays unless integrated or flush-mounted design. PV panels need cooling to perform well, so mounting arrangements that can increase heat transfer away from the modules are preferred. • Account for snow shedding/damming and potential for additional snow loads. • Leave room for accessing modules and for fire code compliance. For roof-mounted systems, this could require a buffer zone of 4 ft to 6 ft in from the perimeter.65 This could reduce the avail- able area by up to 20%. Electric Distribution Systems • Solar-ready design. Install conduit that can easily be accessed for connecting the PV array to the inverter and electric service panel. • Make sure the service panel is adequately sized for the PV system plus the other electrical loads. System Selection • Make selection based on cost-effectiveness and architectural integration requirements. • Make sure to specify the rating assumptions clearly—STC, PTC, or other—and the tools or methods used for energy output estimation. The Caltrans case study (see Case Studies, 22.7) provides a useful example of the importance of proper specification. • Make sure inverter requirements are specified clearly. Construction • Consider in-house staff to assist with construction only if they are trained or there is proper supervision. • Equipment and installation certifications: – Make sure to work with utility to understand interconnection requirements, net metering rules, and so forth. – Certifications: UL-approved PV modules (UL 1703) and inverters (UL 1741). Adherence to NEC (e.g., National Electric Code (NEC) Article 690: Solar Photovoltaic Systems). – Warranties: 5 years on the system, 20 years on the PV modules, and at least 5 years on the inverters. • Make sure that vendor products are readily available—ask for experience/history of meeting delivery times. • Make sure that firm has demonstrated experience installing this type of system. • Consultant should document the estimated PV system output and savings. Software tools such as RETScreen, PVWatts, or another recognized tool should be used for the estimates. Controls and Monitoring • Include monitoring instrumentation and metering in all projects. • Consider online data access and visual display. Operation and Maintenance • If in-house staff are to maintain, then make sure they are adequately trained, have O&M experi- ence, and so forth. 65This is based on the photovoltaic system provisions of the International Fire Code. See http://irecusa.org/wp-content/ uploads/2010/10/Brooks-Fire-Guidelines-Webinar-Nov2010.pdf.

103 12.1 Overview Concentrating solar power (CSP) systems use concentrated solar energy as a source for higher- temperature process needs and for generating electricity. The CSP systems use mirrors or lenses to focus the sun’s rays on a receiver or absorber, through which a fluid circulates. This fluid is then used to heat process water or another fluid in a power generation cycle (e.g., steam turbine/ generator/Rankine cycle) or a refrigeration cycle (e.g., absorption chiller). All these systems have mechanisms for tracking the sun. These are required in order to effectively focus the direct-beam component of solar radiation. There are several system types that are distinguished by the type of concentrator. These include parabolic trough systems, parabolic dish systems, and central receiver (heliostat/power tower) systems. Fresnel-lens–based concentrators have also been used. Since CSP systems can only use direct sunshine and not diffuse sunshine (e.g., sunlight that is scattered by clouds and the atmosphere), they are best suited to geographical regions that experi- ence many clear sunny days, such as the southwestern United States. Current CSP applications are targeted at utility bulk power generation markets. Utility-scale CSP systems (e.g., 50 MW or more) have installed costs of from $4,100/kW to $8,500/kW.66 12.2 Types of Systems and Strategies 12.2.1 Parabolic Trough Systems These systems use a parabolic-trough–shaped concentrator to focus the sun’s rays on a tube that is located at the focal line of the reflector (see Figure 12-1). The tube has a dark coating that maximizes absorption but also minimizes heat losses. Some designs surround the receiver tube with an evacuated glass tube (similar to an evacuated-tube solar collector) to further reduce heat losses and achieve higher temperatures. Typical concentration ratios are 30 to 100 suns, and temperatures generated are from 300°F to 800°F. The heat transfer fluid can be water or a fluid that does not boil over the normal operating temperature range. Thermal storage tanks can be used for storing heat to extend the system’s operating periods and to help modulate the system output. The systems have multiple rows of trough collectors connected to achieve the desired output. They are typically oriented on a north–south axis, with sun tracking from east to west throughout the day. An advantage of these systems is relatively lower costs. Parabolic trough systems have been installed for industrial process heat applications and for electric power applications. C H A P T E R 1 2 Concentrating Solar Power 66U.S. Department of Energy, 2012. SunShot Vision Study, p. 105. http://www1.eere.energy.gov/solar/pdfs/47927_chapter5.pdf.

104 Renewable Energy Guide for Highway Maintenance Facilities 12.2.2 Parabolic Dish Systems Parabolic dish systems use parabolic-dish–shaped concentrators with the receiver at the dish’s focal point (see Figure 12-2). The heated fluid in the receiver is used to drive a heat engine power cycle. Typical concentration ratios are 250 to 500 suns, and temperatures generated are from 700°F to 900°F. Parabolic dish Stirling engine systems have the engine generator integrated with the dish to make a compact power source. Systems have been developed that provide up to 30 kW of output, with an efficiency exceeding 30% (net power out divided by solar energy heat input). Dish Brayton engine systems are currently being developed. A major benefit of these systems is their modularity. 12.2.3 Central Receiver (Heliostats/Power Tower) Systems These systems use a field of many large mirrors to focus sunlight on a receiver that is located at the top of a tower (see Figure 12-3). Typical concentration ratios exceed 1000 suns, and tem- peratures generated exceed 1000°F. A molten salt is generally used as the heat transfer medium in the receiver due to the high temperatures. Thermal storage systems can be used to extend the system’s availability. Central receiver systems are utility-scale systems and are not suitable for smaller applications. 12.2.4 Fresnel Concentrator Systems A Fresnel lens focuses sunlight passing through it either at a point or on a line, depending on the lens design. The receiver is situated beneath the lens at the focal point or focal line of the lens assembly. An alternative approach uses a Fresnel design as a concentrator, with the receiver situ- ated above the concentrator at the focal point or focal line of the concentrator. A linear Fresnel concentrator with a tubular receiver has generated temperatures above 700°F. Figure 12-1. Parabolic trough concentrating solar power system at Kramer Junction, CO. Source: Kramer Junction Company.

Concentrating Solar Power 105 Figure 12-2. 25-kW solar dish Stirling engine system. Source: Science Applications International Corporation. Figure 12-3. Solar One central receiver system at Dagget, CA. Source: Sandia National Laboratories.

106 Renewable Energy Guide for Highway Maintenance Facilities 12.3 Applications Most CSP systems are targeted at large, multi-megawatt, utility-scale applications and require substantial flat land area (e.g., about 5 to 10 acres per MW). As a result, they are generally not suitable for typical maintenance facility applications. A possible exception is solar dish engine systems, which are currently under development. For general screening purposes, CSP systems have been able to achieve the following solar thermal to electricity conversion efficiencies:67 • Parabolic trough: 13% to 15%. • Fresnel lens: 15% to 25%. • Parabolic dish: 20% to 30%. • Central receiver (heliostat/power tower): 15% to 18%. In order to determine the annual output, the annual direct solar energy on a concentrating tracking collector should first be obtained either from the Solar Radiation Data Manual for Flat Plate and Concentrating Collectors (http://www.nrel.gov/docs/legosti/old/5607.pdf) or from the PVWatts software. This is available for parabolic trough collectors. This value should be multi- plied by the efficiency for the type of CSP system and the collector concentrator’s aperture area (projected area in the plane normal to the sun, not the surface area of the concentrator). 12.3.1 Economic Screening The economic screening requires a knowledge of how much purchased electricity the CSP system has saved, the cost of the electricity, and the capital and any nonfuel operating and main- tenance costs of the system: Typical assumptions and information sources are: • Capital costs: $4,000/kW to $7,000/kW. • Annual O&M costs: $0.02/kWh. • Electricity Prices (per unit): depends on location (see EIA for statewide costs as a default). http://205.254.135.7/electricity/state/. • Electricity savings: electricity generated by the CSP system. • Energy operating cost savings: Electricity savings × unit electricity price. Note that if there is net metering and the amount generated by the CSP system exceeds the facility requirements, the energy cost savings would depend in part on what, if any, the utility paid for this excess. In this case, the cost savings would be the sum of the savings associated with the electricity used on-site and the amount received for the excess. 67U.S. Department of Energy, 2011. 2010 Solar Technologies Market Report, pp. 75-76. http://www.nrel.gov/docs/fy12osti/ 51847.pdf.

107 13.1 Overview Wind turbines convert the kinetic energy in wind into mechanical power as the wind turns the rotor blades of the turbine, which in turn spins a shaft connected to either a transmission or directly to a generator that produces electricity. Power output from wind turbines increases exponentially with wind speed. (Power is proportional to average wind velocity cubed.) When analyzing wind resources, it is important to assess both the magnitude of the wind speeds and the frequency with which they typically occur during the day and year. Occurrence of sufficiently large combinations of both values is a primary factor for identifying economically feasible proj- ects, and sites should be chosen to optimize that combination. Wind turbines may be connected to the grid at transmission levels (typical for large, multi- turbine wind plants) or distribution levels. At the distribution level, they may be connected on the customer side or the utility side of the meter. This guidebook focuses on the customer-side application. 13.2 Types of Systems and Strategies Wind machines are characterized by the axis around which the rotor blades rotate, whether the blades are attached upwind or downwind of the tower, and the number rotor blades. The wind energy market has adopted the three-bladed, upwind, horizontal-axis turbine for the vast majority of installations, although alternative designs may be beneficial under certain circum- stances and for specific purposes.68 Figure 13-1 shows a small horizontal-axis wind turbine with a two-blade rotor and coupled generator mounted atop a stand-alone tower. As a general rule of thumb, towers should be con- structed at least 30 ft above and 300 ft away from any obstruction to avoid turbulent airflows.69 In addition, power output generally increases proportionally with increasing tower height. Small upwind machines place the rotor blades in front of the tower and use a tail feature to keep the tur- bine facing into the wind. The force of the wind on the tail moves (yaws) the turbine passively— that is, without the help of a motor. Larger systems usually employ an electric motor directed by wind sensors to yaw the turbine. Downwind machines, much less prevalent in the market, are noisier than upwind machines and suffer from blade flexing and fatigue issues and reduced power output as a result of the wind-shadowing effect from the tower on the rotor blades.70 C H A P T E R 1 3 Wind Energy 68Randolph and Masters, 2008, pp. 475, 464, 468. 69O’Dell, 2007, p. 7. 70Randolph and Masters, 2008, pp. 475, 464, 468.

108 Renewable Energy Guide for Highway Maintenance Facilities Vertical-axis machines are much less common than horizontal-axis turbines but have certain advantages that make them useful for low- to medium-power applications where cost and reli- ability outweigh efficiency (e.g., remote and unmanned locations such a ocean buoys). Vertical- axis machines are less efficient than horizontal-axis machines but accept wind forces equally from all directions, require no yaw control, and have no issues with tower shadow. Another advantage of vertical-axis machines is that the electric generator can be mounted at the ground for easier access and maintenance.71 Two common vertical-axis designs are the Savonius rotor and the Darrieus rotor, shown in Figure 13-2. These designs may be employed independently or combined, as also shown in Figure 13-2. The Savonius rotor is composed of two hollow, half cylinders mounted in the shape of an “S” allowing wind to blow through the cavity. Savonius rotors can operate in wind speeds as low as about 5 mph but have a peak efficiency of only about 30%.72 The Darrieus rotor design employs two or three blades bowed out from the center vertical axis with an aerofoil cross-section (similar to an airplane wing). The Darrieus rotor has a peak efficiency of 35% but is not self-starting like the Savonius rotor. This means that the rotor blades must be spinning before the wind will exert a driving force.73 A Darrieus rotor can overcome this issue by integrating a Savonius rotor into the design or using its generator as a starter motor with power from batteries or the grid. Figure 13-3 shows a Darrieus type wind turbine on a state transportation facility. Although there are no standard definitions for turbine size categories, turbines generally referred to as small and suitable for commercial use range in size from about 1 kW to 100 kW. Turbines rated between 100 kW and 800 kW to 900 kW are generally identified as medium sized. Many municipalities have completed projects with medium-sized turbines at schools or other government facilities or have located one or several turbines away from the load at a site with better winds. Some projects have involved the sharing of the energy among several facilities. Figure 13-1. Upwind, horizontal-axis wind turbine. 71Shepherd and Shepherd, 2003, p. 332. 72Shepherd and Shepherd, 2003, p. 332. 73Shepherd and Shepherd, 2003, p. 332.

Wind Energy 109 Figure 13-2. Vertical-axis wind turbines. Savonius rotor Sources: Shepherd and Shepherd, 2003, pp. 475, 464, 468; http://upload.wikimedia.org/wikipediia/commons/f/f7/Taiwan_2009_JinGuaShi_Historic__Gold_Mine _Combined_Darrieus_Savonius_Wind_Turbines_FRD_86638.jpg. Darrieus rotor Combined Darrieus- Savonius wind turbines at Jinguashi Historic Gold Mine in Taiwan Figure 13-3. Windspire Darrieus type wind turbine at Missouri Department of Transportation Conway Welcome Center. Source: Conway Welcome Center in Southwest Missouri, http://www.windspireenergy.com/case-studies/missouri-department- of-transportation/.

110 Renewable Energy Guide for Highway Maintenance Facilities The amount of energy a turbine produces annually is often expressed using a term called the capacity factor (CF). The CF represents the actual annual energy output of a turbine as a fraction (or percentage) of the energy that the turbine would produce if it could run at full-rated capacity every hour of the year. The CF is calculated using a distribution of wind speeds and the turbine’s power curve (a curve that represents the turbine’s power output at each increment of wind speed), which is a function of its design. A typical CF for a modern large wind turbine installa- tion in a site with good resource is about 30% to 40%.74 Small turbine projects typically, but not always, have a lower CF (in the 20% to 30% range) for two reasons. First, since the electricity is used on-site by the customer, it is valued at the retail rate rather than the wholesale rate that larger installations (wind farms) obtain. This means that customer-sited projects using smaller turbines connected to the distribution system can be cost-effective in lower wind resource sites typical of most customer locations, compared to the remote sites selected for large wind farms connected to the transmission system. Second, smaller turbines tend to be less efficient than the larger ones. As an example, a wind turbine with a rated power output of 10 kW and a CF of 30% will generate about 26,280 kWh per year.75 13.3 Applications 13.3.1 Applicability Climate/Resource Although attractive wind sites are most abundant in the middle of the country in states from Texas through North Dakota,76 and to a somewhat lesser degree in northern border and West Coast states, there are many other states that have potential sites. In general, less potential for wind energy exists east of the Mississippi River, with the Southeast part of the country having the lowest wind resources. However, local terrain features can heavily influence the resource level at any given location, either negatively or positively, making individual site evaluation a critical early step when considering wind energy. Table 13-1 shows the standard system used to classify wind resource levels. It lists the amount of available power in the wind for corresponding ranges of average annual wind speed.77 This system is used for portraying wind resource levels both on maps of any size area and at specific sites. As a general rule of thumb, the threshold for economic viability of large wind energy plants supplying the bulk power market is class 4 or higher.78 However, customer-sited projects can be cost-effective more often in class 3 sites, depending on the retail rates of electricity and other factors listed in the economics section that follows. Figure 13-4, produced by NREL, uses the power classification system in Table 3-1 to map U.S. wind resources on a very low level of special resolution. The figure shows that, on a regional level, the largest amount of opportunities for developing wind energy in the United States are found through Midwestern states such as Texas, Oklahoma, Kansas, Nebraska, Wyoming, Montana, Minnesota, Iowa, and the Dakotas. These and several neighboring states consistently experience class 3 wind conditions or higher in many locations. 74Randolph and Masters, 2008. pp. 475, 464, 468. 75Annual energy output (kWh/year) = 10 kW × 8,760 h/year × 30% = 26,280 kWh/year. 76Elliott and Schwartz, 1993. pp. 5-6. 77Randolph and Masters, 2008. pp. 475, 464, 468. 78Randolph and Masters, 2008. pp. 475, 464, 468.

Wind Energy 111 Wind Power Class Resource Potential Wind Power Density at 50 m (W/m2) Average Wind Speed at 50 m (mph) 2 Marginal 200–300 12.5–14.3 3 Fair 300–400 14.3–15.7 4 Good 400–500 15.7–16.8 5 Excellent 500–600 16.8–17.9 6 Outstanding 600–800 17.9–19.7 7 Superb >800 >19.7 Source: Randolph and Masters, 2008. Energy for Sustainability: Technology, Planning, Policy, Island Press, Washington, D.C., pp. 475, 464, 468. Table 13-1. Standard wind power classifications. Source: http://www.nrel.gov/gis/pdfs/windsmodel4pub1-1-9base200904enh.pdf. Figure 13-4. United States wind resource map ê class 3 at 50 meters.

112 Renewable Energy Guide for Highway Maintenance Facilities 13.3.2 Economics In general, grid-connected systems can be practical if favorable combinations of the following conditions exist: • Average annual wind speed exceeds 10 mph at the hub height of the turbine. • Purchased electricity exceeds 12 cents per kilowatt-hour. • Utility requirements for grid connection are not prohibitively expensive. • Good incentives exist for purchasing wind turbines and/or for the selling of electricity back to the grid. • Occurrence of wind is significantly correlated with the facility’s electric load. According to the American Wind Energy Association, small wind energy systems cost between $3,000 and $5,000 per kilowatt of generating capacity. Wind energy becomes more cost-effective for larger turbine rotor sizes.79 This is because the cost of a turbine rotor is approximately pro- portional to its diameter, while the power output is approximately proportional to the diameter squared. As a result, larger systems typically require a larger initial investment but have shorter payback periods. The intermittent nature of wind, and an often limited degree of coincidence between wind and peak electricity demand, present challenges to wind energy economics. Peak wind condi- tions often occur at night and during the winter months when electricity demand is relatively low in many locations.80 Batteries can help mitigate this issue but are currently very expensive. An alternative solution that is gaining popularity for customer-sited applications is to develop hybrid systems consisting of both solar PV panels and wind turbines. Solar PV systems produce peak power in the afternoon and during the summer months.81 When used in concert, hybrid systems can provide more consistent power throughout the year. 13.3.3 Rules of Thumb Site Selection • Many states have high-resolution wind resource maps available for them from the U.S. Department of Energy’s Wind Powering America Program.82 These are a good starting place for assessing the potential for viable projects at specific locations. • On a site-specific level, locations should be evaluated for potential benefits from terrain fea- tures such as hilltops and ridges that can accelerate wind speeds. • Sites should be as clear as possible of obstacles such as buildings or trees, which can decrease wind speeds and increase turbulence that leads to greater fatigue failure of turbines. Siting Issues • Turbine height. Some jurisdictions restrict the height of structures with towers in certain areas, in which case a variance would be required. However, this restriction is less likely at sites zoned for maintenance facilities. • Visual. Turbine owners should be conscious of neighbors that might object to a turbine that obstructs their view. • Noise. Unless the maintenance facility is very near other buildings, turbine noise should not be an issue. The noise level of most modern wind turbines is around 52 to 55 decibels, similar to a standard refrigerator.83 79O’Dell, 2007. p. 7. 80O’Dell, 2007. p. 7. 81O’Dell, 2007. p. 7. 82http://www.windpoweringamerica.gov/windmaps/resource_potential.asp. 83O’Dell, 2007. p. 7

Wind Energy 113 • Flicker. Placement of turbines should avoid the creation of shadows on buildings that can result from rotating turbine blades when the sun is at certain angles during the year. • Environmental. Care should be taken to make sure that the turbine is not located in the path of migratory birds. Placing turbines in areas with bat populations may require consultation with appropriate agencies or experts to determine level of risk and the potential for successful mitigation approaches. 13.3.4 Manual Screening Methods Unlike solar energy resource estimation, wind resource estimation is highly site-specific and usually requires a good local source of wind data to make an accurate prediction of energy production. Major advances in computational techniques for wind resource assessment over the past two decades have resulted in the development of a new generation of higher-resolution data for much of the country over the past several years. Under the sponsorship of the U.S. DOE, a set of state summary maps and associated GIS data files has been created and can be downloaded from DOE’s Wind Powering America website (http://www.windpoweringamerica. gov/windmaps/). In addition to the state maps on the DOE website, some states have sponsored additional analysis to produce larger-scale maps. In some instances, these state maps can produce a useful quick estimate of potential energy production. The calculation approach for such use is described in the following. However, this approach is not a substitute for sources of measured data from nearby locations or, finally, an actual site assessment. In some cases, local terrain features or obstructions have the potential to significantly change even these high-resolution data that have been estimated by analytic (mod- eling) techniques. The steps for a manual quick calculation of projected wind energy production are: Step 1 Determine the wind power density range (in units of watts per square meter, or W/m2) for your site using the largest-scale state map that you can find from the DOE Wind Powering America site or a state agency. Note that maps are shown for various heights above the ground, including 30 m, 50 m, and 80 m. You should select the map that most closely matches the hub height of the turbine you are using for your calculation. Also note that all maps show resource levels classified by average annual wind speed (meters per second, or m/s), but some do not show power density. If you cannot find a map with power density, this calculation approach will not work. Step 2 Select the swept area from the turbines listed by rated capacity in Table 13-2 for which you want to estimate energy production. You may use your own figure for swept area if you have Turbine Rating (kW) Rotor Diameter (m) Swept Area (m2) 2.4 3.7 10.9 10 7 38.5 20 9.5 71 50 19.2 289 100 21 346 850 52 2122 1,600 82 5278 2,500 100 7850 Table 13-2. Representative rotor diameter and swept area for selected turbine capacity.

114 Renewable Energy Guide for Highway Maintenance Facilities data for a specific turbine that varies from the data in the table. The swept area of the turbine is calculated with the following formula: Swept A/area (m2) = 3.14 × [rotor diameter (m)/2]2. Step 3 Determine annual energy output using the following formula: Annual output = swept area (m2) × wind power density (W/m2) × 1,000 W/kW × 8760 hours/ year. 13.3.5 Software Tools Whereas the maps referred to in the previous section showed wind resource data at a grid resolution of 2 km2, a tool newly developed under a U.S. DOE grant achieves a higher degree of accuracy by using data at a grid resolution of several hundred square meters. Therefore, this is the recommended tool to use for obtaining an estimate of the site wind resource. A free version of the tool requires establishing a login password, and the user is prompted to download and install a Microsoft browser add-in called Silverlight to enable web-based interactions. The tool is called the Distributed Wind Site Analysis Tool and is available at https://dsat.cadmusgroup. com/Default.aspx. All calculation tools combine distributions of wind speed with wind turbine power curves (plots of generated power versus wind speed) and assumptions about system losses to estimate system output. Several other tools with useful energy and financial calculation capabilities and turbine power curves and specification data sets are: • Windustry Wind Project Calculator: http://windustry.com/your-wind-project/community- wind/community-wind-toolbox/chapter-3-project-planning-and-management/wi. • Excel Wind Analysis Tool: http://www.inl.gov/wind/software/. • NREL Wind Energy Finance Calculator: http://analysis.nrel.gov/windfinance/login.asp. • FRESA: https://www3.eere.energy.gov/femp/fresa/. 13.3.6 Economic Screening The economic screening requires knowledge of how much purchased electricity the wind energy system has saved, the cost of the electricity, and the capital and any nonfuel operating and maintenance costs of the system. Rules of Thumb for Cost/Performance • Wind costs: $3,000 to $6,000/kW wind system output: Capacity factors can vary widely from much less than 0.20 to well over 0.30, depending on the wind resource and the design of the turbine. This is why accurate resource assessment and optimized siting are so critical. Table 13-3 provides typical wind energy system costs. • O&M costs: $0.01/kWh small turbine; $0.02/kWh for large turbine. • Electricity Prices (per unit): depends on location (see EIA for statewide costs as a default, http://205.254.135.7/electricity/state/). • Electricity savings = electricity generated by the wind energy system. • Electricity cost savings: electricity cost savings + electricity savings × unit electricity price. Note that if there is net metering, and the amount generated by the wind energy system exceeds the facility requirements, the energy cost savings would depend in part on what, if any, the utility paid for this excess. In this case the cost savings would be the sum of the savings associated with the electricity used on-site and the amount received for the excess.

Wind Energy 115 13.4 Best Practices Rules of Thumb for Sizing • When sizing, as with solar electric systems, make sure to calculate the amount of electricity that is in excess of the facility’s requirements (if any) over the annual billing period. Where net metering is allowed, determine if there are any system size restrictions. If net metering is allowed, then determine if the utility will pay for the excess electricity and how much. Use this information in the economic analysis to help determine the appropriately sized system. If net metering is not allowed, the system needs to be designed based on the hourly daytime load, or battery backup can be considered. Consider the trade-offs of the added cost and maintenance of battery storage. • The presence of a better wind resource or fewer impediments to a project (e.g., environmental concerns) at an off-site location could provide the impetus to explore a cooperative project within the department or with other departments or agencies. Installing a larger turbine could improve project economics. Siting/Regulatory • Research zoning and regulatory issues and discuss with zoning officials to ensure that turbine generator meets requirements for height, set back/distances from roads and buildings, noise, visibility, and, if near airports, FAA requirements (e.g., lighting). For large-scale installations (e.g., 5 MW or more, wind parks), additional regulations involving the state public utilities must be accounted for. • Confirm lack of potential for interference with radar at military installations and other airports. • Contact state/local environmental agencies such as wildlife agencies to discuss environmental review process and requirements to ensure compliance. This could include reviews of possible impacts on birds, bats, and other wildlife in proximity to the wind turbine generator. • For small wind turbines (e.g., under 100 kW), make sure that the tower height is such that the bottom tip of the rotor is at least 30 ft above the ground and the turbine is sited at least 300 ft from any structures, trees, or geologic formations.84 This is to minimize the possibility of wind turbulence that could affect the wind turbine performance and lifetime. • Make sure to check the permit requirements for temporary meteorological towers for obtain- ing wind data. Turbine Rating (kW) Installed Cost Range ($/kW) Installed Cost to Use with Default Power Curve ($/kW) 10 5,000–8,000 6,000 20 3,200–6,000 5,000 30 3,200–6,000 5,000 50 3,600–6,000 5,000 100 4,000–5,000 5,000 850 2,500–3,000 2,800 1,600 2,300–3,000 2,700 2,500 2,300–3,000 2,700 Source: Lorand et al., 2012.Green Energy Life Cycle Assessment Tool User Manual, Water Environment Research Foundation, 2012, pp. 5–6. Table 13-3. Wind energy system costs. 84U.S. Department of Energy, 2005. Small Wind Electric Systems: A Consumer’s Guide. http://www.windpoweringamerica.gov/ small_wind.asp.

116 Renewable Energy Guide for Highway Maintenance Facilities • Wind data should be collected for 1 year at the site if local data is not available. An anemometer can be used for this purpose. Sometimes this time period can be reduced if the data can be correlated with other data from nearby sites. Ideally, it should be mounted at a height equal to the approximate hub height of the rotor. Companies who specialize in this area should be considered for larger projects. System Selection • Wind energy systems should be selected on the basis of cost-effectiveness and track record of reliability and durability. • Make sure that the turbine is designed and, if possible, certified to withstand the turbulence levels and severe event loads for the site. • If utility or state incentive programs are available for wind energy systems, make sure that the wind turbines qualify for the incentives. Qualifying equipment may be listed on the program’s website. Qualified installers may also be listed. Examples are: – http://www.nyserda.ny.gov/en/Page-Sections/Renewables/Small-Wind/Eligible-Wind- Turbines.aspx, and – American Wind Energy Association (AWEA), which has a good list of sources (http://www. awea.org). Construction • Acceptance testing/commissioning. Make sure that the acceptance testing is done over a suf- ficient period of time to confirm proper operation. • Consider in-house staff to assist with construction only for small projects. For example, at the Milford, Utah, DOT facility (Case Studies, 22.8), the foundation work for the tower was done by maintenance facilities staff. • Make sure to work with the utility to understand interconnection requirements, net metering rules, and so forth. • Certifications. AWEA promulgated standards in 2009. They are intended to be in compli- ance with the American National Standards Institute (ANSI) Essential Requirements and be eligible for adoption as American National Standards. The International Electrotechni- cal Commission (IEC) started standardizing international certification of wind turbines in 1995, and the first standard appeared in 2001, with a number of additional standards appearing since then. The United States participates in this process, and its standards are developed such that they are compatible with the IEC standards. There are several UL and Canadian standards that apply to a number of small wind turbine components such as motors, generators, gear boxes, and controls. While most small wind turbines have not been certified by an independent testing organization, there has been some movement in this direc- tion. The Small Wind Certification Council (SWCC; http://www.smallwindcertification.org/) has begun certifying some equipment. In addition, NREL has been performing independent testing of a number of small wind turbines. The sites should be checked to see if wind tur- bine generators under consideration have been independently tested or certified by these organizations. • Warranties. Warranties of 5 years on the system, wind turbines and tower, and inverters is typical. Explore options beyond the standard warranty period. • Make sure that vendor products are readily available: ask for experience/history of meeting delivery times. This was an important lesson learned from the Ohio DOT Northwood Wind Turbine project (Case Studies, 22.9). • Make sure that the firm has demonstrated experience installing this type of system. • Consultant should document the estimated wind system output and savings. This should be based on the manufacturer’s power curves and local wind speed data, if available.

Wind Energy 117 Controls and Monitoring • Include monitoring instrumentation for measuring and recording wind speed and output, as well as any other metering needed per the interconnection requirements of the utility. Operation and Maintenance • O&M services should be contracted with firms experienced with the wind turbine equipment. While much of the maintenance is fairly routine—lubrication, checking and tightening bolts, and so forth—there are safety risks associated with performing these functions at the tower heights. • Proximity to repair personnel and parts can be a distinguishing factor to consider between turbine brands.

118 14.1 Overview The geothermal heat pump (GHP), also known as the ground-source heat pump, is a highly efficient renewable energy technology that is gaining wide acceptance for both residential and commercial buildings. GHPs are used for space heating and cooling as well as for water heating. They operate similarly to air-source heat pumps but use the heat in the ground, groundwater, or surface bodies of water (ponds, streams), rather than heat in the air, as the source for the heat pump. They overcome a key problem with air-source heat pumps—low performance in winter when air temperatures fall below about 40°F. Since the ground is at a relatively constant tem- perature throughout the year below a certain depth—higher than air temperatures in the winter and lower in the summer—it enables the GHP to operate more efficiently than an air-source heat pump. According to the EPA, geothermal heat pumps can reduce energy consumption up to 44% compared to air-source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment. GHPs also improve humidity control by maintaining about 50% relative indoor humidity, making GHPs very effective in humid areas. Geothermal heat pump systems allow for design flexibility and can be installed in both new and retrofit situations. Additional benefits include: • Hardware requires less space than that needed by conventional HVAC systems; • Systems provide excellent zone space conditioning; • They have relatively few moving parts, and all moving parts are indoors (compared to air- source heat pumps); and • Underground piping often carries warranties of 25 to 50 years, and the heat pumps often last 20 years or more. 14.1.1 Heating and Cooling Efficiency of Geothermal Heat Pumps The heating efficiency of GHPs is indicated by the COP, which is the ratio of heat provided to energy input. The cooling efficiency is indicated by the energy efficiency ratio (EER), which is the ratio of the heat removed (in Btu per hour) to the electricity required (in watts) to run the unit. Beginning in January of 2012, an Energy Star-qualified geothermal heat pump is required to have a COP of at least 3.0 and an EER of at least 14.1.85 A geothermal heat pump removes heat from the geothermal ground loop and transfers the heat to the space either through a water-to-air heat exchanger, if air-side distribution systems C H A P T E R 1 4 Geothermal Energy 85http://www.energystar.gov/index.cfm?c=geo_heat.pr_crit_geo_heat_pumps.

Geothermal Energy 119 are used, or water-to-water heat exchangers, if water-side distribution systems are used. Water- to-water GHPs coupled to radiant distribution systems can be an effective means of heating maintenance facility areas. Radiant heating is most commonly done by running pipes in a concrete floor slab, but can also include radiators or radiant panels mounted on the walls or overhead. When used for heating, this strategy de-couples the heating load from the ventila- tion load, which is particularly helpful in spaces with a large volume, high ceilings, or that have a high ventilation demand. Using the GHP for radiant cooling is possible but requires careful control of the humidity of the conditioned space to avoid condensation problems. A closed- loop, Energy Star-rated, Tier 3 water-to-water geothermal heat pump has a COP of at least 3.1 and an EER of at least 16.1. For GHPs that use air-side distribution (water-to-air GHP), cooling mode operation is the same as for standard cooling systems and does not introduce any special humidity control requirements, A closed-loop, Energy Star-rated, Tier 3 water-to-air geothermal heat pump has a COP of at least 3.6 and an EER of at least 17.1. 14.2 Types of Systems and Strategies There are four basic types of ground-loop systems. Three of these—horizontal, vertical, and pond/lake—are closed-loop systems. The fourth type of system is the open-loop option. Which one of these is best depends on the climate, soil conditions, available land, and local installation costs at the site. 14.2.1 Horizontal This type of installation is generally most cost-effective for new construction where sufficient land is available. It requires trenches at least 4 ft deep. The most common layouts either use two pipes, one buried at 6 ft and the other at 4 ft, or two pipes placed side-by-side at 5 ft in the ground in a 2-ft-wide trench. The “Slinky” method of looping pipe allows more pipes in a shorter trench, which cuts down on installation costs and makes horizontal installation possible in areas it would not be with conventional horizontal applications. 14.2.2 Vertical Nonresidential buildings often use vertical systems because the land area required for hori- zontal loops would be prohibitive. Vertical loops are also used where the soil is too shallow for trenching, and they minimize the disturbance to existing landscaping. For a vertical system, boreholes (also referred to as wells) are drilled about 20 ft apart and 100 ft to 400 ft deep. Into these boreholes go two pipes that are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with horizontal pipe (i.e., manifold), placed in trenches, and connected to the heat pump in the building. Of the four methods, vertical well fields extract the most amount of energy from the smallest area of land. A rough rule of thumb is that there should be a minimum of 225 ft2 of land area available per ton (12,000 Btu/hr) of design load capacity. See Fundamentals of Commercial Geothermal Wellfield Design for a discussion of the design considerations for various configurations.86 14.2.3 Pond/Lake If the site has an adequate body of water, this may be the lowest-cost option. A supply line pipe is run underground from the building to the water and coiled into circles at least 8 ft under the 86Jeppesen, K. C., Fundamentals of Commercial Geothermal Wellfield Design. http://www.ghpsystems.com/wp-content/ uploads/Fundamentals_of_Commercial_Geothermal_Wellfield_Design.pdf.

120 Renewable Energy Guide for Highway Maintenance Facilities surface to prevent freezing. The coils should only be placed in a water source that meets mini- mum volume, depth, and quality criteria. This will ensure adequate heat source availability and prevent the possibility of icing of the coils. 14.2.4 Open-Loop System This type of system uses well water/groundwater or surface body water as the heat exchange fluid that circulates directly through the GHP system. Once it has circulated through the system, the water returns to the ground through the well, a recharge well, or surface discharge. This option is practical only where there is an adequate supply of relatively clean water, and all local codes and regulations regarding groundwater discharge are met. 14.3 Applications 14.3.1 Site Evaluation for a Geothermal Heat Pump Because ground temperatures below about 5 ft are relatively constant throughout the United States, GHPs can be effectively used almost anywhere. However, the specific geological, hydro- logical, and spatial characteristics of the site will dictate the best type of ground loop for a par- ticular site. The depth of the soil and heat transfer properties can determine how much piping is needed or whether to use a vertical or horizontal installation method. Surface water can be used as a source of water for an open-loop system or as a repository for coils of piping in a closed-loop system, depending on factors such as depth, volume, and water quality. The amount of land and layout of the installation site, landscaping, and the location of underground utilities or sprinkler systems also contribute to the system design. Horizontal ground loops (generally the most eco- nomical) are typically used for newly constructed buildings with sufficient land. Vertical installa- tions or more compact horizontal Slinky installations are often used for existing buildings because they minimize the disturbance to the landscape. The screening of commercial-scale geothermal heat pump systems is best accomplished by the use of software tools using inputs specific to the site or local region. In general, the heat pump will be able to perform more efficiently in the heating mode when the source temperatures (heat from the ground or water) are higher. The reverse is true during the cooling season, when the ground or water serves as the sink for heat removed from the building. Even though ground temperatures are relatively constant as compared to air temperatures in a given location, there are seasonal variations and significant geographical differences. Figure 14-1 shows the variation in mean annual ground temperature in the continental United States. These are representative temperatures at depths of 30 ft or below and remain fairly constant at greater depths. In general, the colder regions can expect to have a seasonal COP of about 3 during the heating season, while moderate climates can expect to see a COP of 4 to 5 (excluding auxiliary pumps). For the cooling season, the reverse holds true, and geothermal heat pumps in colder regions will have better per- formance. Note that the comparison of geothermal heat pump energy use to alternative heating and cooling systems should also take into account the pump energy to move the fluid between the geothermal source (e.g., ground loop) and the heat pumps and any system differences if a different distribution system is used (e.g., radiant heating versus air). 14.3.2 Software Tools for Screening Geothermal Heat Pumps RETScreen (http://www.retscreen.net/) is a screening level tool that can be used for geother- mal heat pumps. It is free and can be downloaded as a stand-alone package. It also includes an economic analysis module. Note that assumptions about sizing the geothermal field (heat exchangers) are needed.

Geothermal Energy 121 14.3.3 Economic Screening The economic screening requires knowledge of how much purchased fuel the GHP system has saved relative to a base technology (e.g., space heating system and cooling system), the cost of the fuel, and the capital and any nonfuel operating and maintenance costs of the system. Typical assumptions and information sources are: • System costs: $4,000/ton to $6,000/ton of rated cooling capacity. • Annual O&M costs: 2% of system costs. • Energy prices (per unit): depends on location (see EIA for statewide costs as a default: http:// 205.254.135.7/electricity/state/ and http://205.254.135.7/dnav/ng/ng_pri_sum_dcu_nus_a.htm). • Energy savings: energy savings = electricity used by the GHP – energy in fuel used by base system (in same energy units). • Energy cost savings: energy cost savings = electricity used × unit price of electricity – fuel used by base system × unit price of fuel. 14.4 Best Practices Rules of Thumb for Sizing • Commercial systems should be sized using appropriate software tools and not be sized using rules of thumb. The size of the ground loop will vary depending on loads, properties of the geothermal heat source, and the type of configuration: – Horizontal loop fields. 100 ft to 400 ft per refrigeration ton. – Vertical loop fields. 200 ft to 600 ft per refrigeration ton. – Lake or pond. 300 ft per refrigeration ton. Siting Best Practices • Check environmental permitting requirements before determining the type of geothermal system. This is especially important when considering groundwater systems. Many states and localities have specific regulations governing the installation of geothermal heat pumps. Figure 14-1. Mean earth temperature. Source: U.S. DOE FEMP, Ground Source Heat Pumps Applied to Federal Facilities. http://www1.eere.energy.gov/femp/pdfs/FTA_gshp.pdf.

122 Renewable Energy Guide for Highway Maintenance Facilities Selecting a Designer • Make sure designer has experience with GHP projects of similar scale. Retain a GHP consul- tant that will have authority in approving the design. Design Best Practices87 • Make sure block loads on the peak heating and cooling loads are used as inputs for system sizing. This is important to properly account for load diversity. • Make sure that the thermal properties of the ground/heat source are well known. This may require drilling test wells and taking soil samples. • Individual zones in the building are best served by separate heat pumps. • The ground loop should be configured based on the layout/zones within the space. For com- pact floor plans, a common circulating loop connecting a common set of wells can be effec- tive. For areas that are spaced out, separate fields for each heat pump unit could be more appropriate. • Consider radiant heating distribution, particularly in the maintenance shop areas. • Use thermally enhanced grout in boreholes to improve heat transfer with the soil and improve performance. • Pumping energy should be minimized through proper design and the use of high-efficiency pumps and variable-speed drives. Installation Best Practices • Make sure installation contractor has experience with GHP projects of similar type and scale. The lessons learned from the Elm Creek Park Administrative and Maintenance Center are instructive with regard to the problems that can arise without careful attention to ground- loop piping installation (see Case Studies, 22.10). • Installers should be certified by the International Ground Source Heat Pump Association (IGSHPA) and have the tools and knowledge necessary. • Consider certification by the National Ground Water Association. Commissioning • Make sure that vendor products are readily available. 87Phetteplace, G. A Guide for Best Practices for Ground-Source (Geothermal) Heat Pumps.

123 15.1 Overview Biomass refers to woody materials, grasses, agricultural crops, and other plant-based materials that can be used directly as fuels or processed into gaseous or liquid fuels for generating heat and/or power. Wood and wood waste are the most commonly used solid biomass fuels. They can be burned directly in wood combustors/boilers, or they can be converted to a gas through gasification (biogas). Other biomass fuels are derived from waste products and include landfill gas and gas from wastewater treatment plant operations. Biogas can be burned in gas boilers, engine generators, or gas turbine generators (e.g., microturbines, combustion turbines) to gen- erate power and heat [combined heat and power (CHP)]. Solid fuel combustors/boilers can also generate steam for steam turbine generator-based CHP. The types and uses of biomass will vary with what is available as a supply (local resources and waste products) and what the demands are (electric generation, heat, or both). With all biomass fuels, a major consideration is proximity to a reliable supplier for this renewable resource. A series of biomass resource maps are available from NREL that provide a general idea of the amount of biomass available in the United States by type (http://www.nrel.gov/gis/biomass.html). 15.2 Types of Systems and Strategies 15.2.1 Biomass Heat Biomass heating systems that use wood consist of wood combustors/boilers along with the ancillary equipment for fuel storage and handling, systems for dealing with the by-products of the combustion process (exhaust systems, ash handling systems), control systems, and systems to distribute the heat to the building. Space for wood storage is required to ensure an adequate supply and is sized based on facility requirements (e.g., number of days supply security and space constraints). Wood handling/conveyance systems move the wood to the combustion box. These can be fully automated or semi-automated systems. The heat from the wood combustion can be transferred to either air or water (e.g., via the boiler) for ultimate use in facility heating. Since these are direct combustion systems, they are subject to environmental rules regarding emissions, waste disposal, and so forth. Environmental rules are an important consideration, and regulations must be researched before proceeding. For buildings smaller than 10,000 ft2, wood pellet systems are appropriate. For larger buildings or central systems serving a number of buildings totaling more than 100,000 ft2, wood chip systems are likely to be the most economical88 (see Figure 15-1). Backup boilers are often used with these systems and enable the wood-fired boilers to be sized more economically (e.g., smaller than that required to meet peak heating C H A P T E R 1 5 Biomass 88Whole Building Design Guide. “Biomass for Heat.” http://www.wbdg.org/resources/biomassheat.php.

124 Renewable Energy Guide for Highway Maintenance Facilities demand). Systems that use cordwood are also available but are best suited for smaller building applications. A key drawback is the need to manually fuel the systems. The main benefit is the relatively wide availability of cordwood. 15.2.2 Biomass Power Wood-fired biomass boilers can be used to generate power by coupling the steam output to steam turbine generators. Units are available in sizes from 50 kW to utility-scale units in the multi-megawatt range. These are most commonly used in larger industrial or utility applications. A back-pressure turbine can be used for driving the generator as well as providing steam for space heating or water heating. The other biomass power technologies—gas turbines, microturbines, or internal combustion engine generators—use biogas as the fuel. Waste heat from the engines/ turbines can be used as a heat source for space heating or water heating, increasing the overall efficiency of biomass fuel utilization. Gasifiers can be used to convert the solid biomass to gaseous form. Alternatively, if landfill gas is available, this can be a cost-effective fuel source. Gasified fuels generally require some degree of cleanup to enable their use in the power systems. This rids the fuel of constituents that can be damaging to the power systems.89 89U.S. Environmental Protection Agency, 2007. Biomass Combined Heat and Power Catalog of Technologies, Chapter 6, Power Generation Technologies. http://www.epa.gov/chp/documents/biomass_chp_catalog_part6.pdf. Figure 15-1. Woodchip boiler in Mount Wachusett Community College (MA) biomass plant. Source: http:///www.fpl.fs.fed.us/documnts/fplgtr/ fpl_gtr168.pdf.

Biomass 125 15.3 Applications 15.3.1 Biomass Heat Biomass furnaces/boilers that use either pellets or wood chips are the most common biomass heat equipment. Equipment efficiencies range from 70% to 85%. Pellets cost about $200/ton and have a heat content of about 16 million Btu/ton. Wood chips cost about $50/ton and have a heat content of 10 to 12 million Btu/ton. Equipment costs are about $165,000 for 0.5 million Btu/h, $195,000 for 1 million Btu/h, and $265,000 for 1.7 million Btu/h output. The economic sizing of the system will require looking at several scenarios, from meeting all the loads with the boiler(s), or only a portion with the boiler and the balance with conventionally fueled equipment. This price can be compared to the price of conventionally fueled equipment to determine whether it makes sense to evaluate biomass heat further. Biofuels such as landfill methane can be used with conventional heating equipment, without the need for solid fuel storage and conveyance systems. However, the gas would require pretreatment or cleanup prior to use and would need to be piped to the site. Furthermore, the energy content per unit volume would need to be accounted for in evaluating its performance and overall economics. 15.3.2 Biomass Power Biomass power systems have considerations similar to biomass heating systems insofar as the price of the biomass fuel is a main driver for competitiveness. In general, CHP systems require spark spreads of 4 to 5 [ratio of purchased electricity price to CHP fuel source price on an equivalent fuel basis (e.g. $/million Btu)] to be competitive with electric power. For example, if the biomass fuel price is $11/million Btu, then the competing electricity rate should be $44/million Btu, or $0.15/kWh or higher. 15.4 Best Practices Rules of Thumb for Sizing • Sizing of biomass systems for heating should take into consideration options that include backup conventional fuel sources to meet peak demand. This could result in a system that is less costly than one where the biomass system is sized to meet peak heating demands. • Consider multiple, smaller boilers to meet demand, which improves overall performance due to boilers operating closer to their design rating. • Consider thermal storage to reduce boiler cycling. Siting Best Practices • Check environmental permitting requirements and impacts on equipment costs. There may be a need for additional pollution control equipment, especially for particulate control. In addition, the amount of time for permitting should not be underestimated. Selecting a Designer • Make sure designer has experience with biomass projects of similar scale.

126 C H A P T E R 1 6 16.1 Overview Water moving downhill represents significant energy potential, and even small quantities can make a good renewable energy source. Small hydroelectric power projects rarely require dams or water diversion techniques. This type of installation is called a run-of-the river system. • Small, run-of-the-river hydroelectric power systems consist of these basic components: – Water conveyance. Channel, pipeline, or pressurized pipeline (penstock) that delivers the water. – Turbine or waterwheel. Transforms the energy of flowing water into rotational energy. – Alternator or generator. Transforms the rotational energy into electricity. – Regulator. Controls the generator. – Wiring. Delivers the electricity. Two variables, volume and pressure, determine the amount of energy and thus the power extractable from the water source. Pressure is produced by the vertical distance the water flows between the collection point and the turbine generator (also called elevation head or head). The greater the vertical distance, the higher the pressure and the greater the power generated. • Power = density of water × volumetric flow rate × acceleration due to gravity × net head × system efficiency. • Energy = power × time. Net head is the difference between the elevation head and any hydraulic losses (e.g., fric- tion losses in piping). The system efficiency accounts for inefficiencies in the hydro turbine, the generator, the transformer, and so forth. Hydroelectric systems are classified by the amount of electrical power they generate (large: >30 MW, small: 100 kW to 30 MW, micro: <100 kW) and the head: • High head: >50 m (164 ft). • Medium head: 10 m to 50 m (33 ft to 164 ft). • Low head: <10 m (33 ft). 16.2 Types of Systems and Strategies Different turbine designs have been developed based on the head and flow rate, which repre- sent a trade-off in cost and performance (e.g., efficiency). In general, the turbines are classified as reaction- or impulse-type turbines. Impulse-type turbines such as Pelton and Turgo require a medium or high head and use nozzles to spray a jet of high-velocity water onto the turbine blades, which can spin the wheel at an efficiency of 70% to 90%. Low-head systems require a Hydroelectric (Small Scale)

Hydroelectric (Small Scale) 127 reaction-type turbine in which all blades are in contact with the water at all times. This type of turbine is commonly used in large-scale hydro projects but is generally too complicated for small installations. A variation of the reaction turbine, the propeller-type turbine, is ideally suited for micro-hydro sites with sufficient volume but very low head. The most effective of the propeller types for this situation is the Kaplan turbine, which allows adaptable propeller angles. Kaplan turbine efficiencies are typically over 90% but may be lower in very low head applications. 16.3 Applications Hydropower screening is site-specific and requires flow data and elevation data. Ideally, the flow data would be on an hourly basis and enable the development of a flow duration curve. The curve is a plot of flow as a function of percent time (or annual hours) that that flow is met or exceeded. Given the flow range and head, the general type of turbine can be selected. This enables estimation of the efficiency as a function of flow. The potential power available from the water can be determined from the following relationship: • Power = density of water × volumetric flow rate × acceleration due to gravity × net head × system efficiency. A rule of thumb for this calculation is that power = 7 × Q × H, where power is watts, Q is in cubic meters per second, and H is the gross head or elevation head in meters.90 The hydro- turbine generator output power duration curve can then be plotted with this information. The facility’s power electric load duration curve can be superimposed on the hydro-turbine gen- erator output power duration curve to help size the system. The size should be between the minimum firm flow rate and the maximum flow rate, and should take into account whether net metering is allowed and net metering capacity rules (e.g., capacity limits, whether the basis for net generation is monthly or annual). The useful energy can then be calculated as the area under the curve bounded by the output of the hydro-turbine generator and the load requirement curve. 16.3.1 Software Tools for Screening Hydropower Systems Hydropower system performance software includes: • RETScreen. This software is sponsored by NRCAN. It is free and can be downloaded as a stand-alone package. It also includes an economic analysis module. http://www.retscreen.net/. 16.3.2 Economic Screening There is a wide variation in costs for hydro turbines, ranging from about $2,000/kW to $4,000/kW for systems between 1 MW and 10 MW, to $3,000/kW to $7,000/kW for systems between 100 kW and 1 MW. In general, the smaller the system, the higher the normalized cost. 90RETScreen, Small Hydro Project Analysis, www.retscreen.net/download.php/ang/109/0/textbook_hydro.pdf.

128 17.1 Overview Energy storage technologies can be used in combination with renewable energy systems to improve the availability of the systems to deliver heat or electricity. Batteries are the most com- monly used electric energy storage technologies with renewable energy electric systems, although other technologies are entering the market. While grid-connected systems frequently do not incorporate storage, higher penetration of wind and solar electric technologies will increase the value of electric energy storage. This is because the storage devices can help smooth out power fluctuations due to the variability of the power supplied by renewable energy sources. 17.2 Types of Systems and Strategies 17.2.1 Batteries Batteries are the most widely used electric energy storage devices with renewable energy systems. These are generally used in off-grid applications, where access to utility power is not available. The key factors in selecting batteries are the application duty cycle (charge-discharge cycle duration and depth of discharge) and operating environment (e.g., ambient temperatures). The depth of discharge—the percent of the battery’s capacity that is accessible—is a particularly important consideration. Batteries that are able to discharge 50% or more of their capacity without impairing their service lives are categorized as deep cycle. Batteries that are designed to regularly discharge less than this fraction are categorized as shallow cycle. The depth of discharge can be provided as an overall limit or as a daily limit. Applications that require high output over short periods of time—automobile or engine start-up or uninterruptible power supplies (UPSs)—and where the batteries otherwise remain fully charged can be well served by shallow-cycle batteries. In contrast, batteries for renewable electric systems are required to provide steady power for extended periods of time and are not fully charged for long periods. These applications are best served by deep-cycle batteries. If shallow-cycle batteries are used for this latter application, then more batteries will be required. Terminology that is useful to know for batteries is: • Battery state of charge. The percent of the battery’s rated capacity at a particular point in time. • Battery capacity. The amount of electrical charge (current over a specified period of time) stored in a battery measured in amp-hours. The energy capacity is derived by multiplying the capacity by the voltage under the specified condition. • Cut-off voltage. The specified minimum voltage below which further discharge will cause damage to the battery. • Charging and discharging regimes. The specified charging and discharging strategies to ensure reliable charging or discharging of the battery. This is accomplished by the use of C H A P T E R 1 7 Energy Storage

Energy Storage 129 charge controllers. Typical charge rates for PV batteries are 0.02 to 0.10 times the rated capacity of the battery, depending on the type of battery. • Battery life. Number of years or number of charge/discharge cycles before battery is no longer able to be charged. While there is a wide variety of batteries on the market—lead acid, lithium ion, nickel cad- mium, and newer/developmental batteries such as sodium sulfur batteries and flow batteries (iron chromium, zinc bromine, and vanadium redox)—the most commonly used batteries for renewable energy systems are deep-cycle lead acid batteries. While they are not the most efficient batteries, they represent a good compromise in price and performance. • Flooded lead acid battery. These are called flooded because the electrodes are totally covered by the sulfuric acid electrolyte. These require regular maintenance—adding water that has been lost due to the chemical reactions in the battery is the most frequently required maintenance item. Other maintenance includes keeping the terminals clean and, depending on the battery, may involve adding electrolyte. • Sealed absorptive glass matt (AGM) battery. AGM batteries incorporate an AGM to absorb the sulfuric acid, thereby reducing the formation of gases and reducing water loss. These are labeled as maintenance-free batteries. A trade-off to the lower maintenance is the requirement for more controlled, lower voltage charging to prevent overcharging. This type of battery costs more than flooded lead acid batteries. • Sealed gel-cell battery. In this battery, silica gel is mixed with the sulfuric acid, which reduces gas formation and water loss. This also results in a maintenance-free battery. The trade-off to the lower maintenance is the requirement for more controlled, lower voltage charging to prevent overcharging. This type of battery costs more than flooded lead acid batteries. Sealed gel-cell batteries do not usually have as long a life, nor are they able to be discharged as deeply as flooded lead acid or sealed AGM batteries. Note that valve-regulated lead acid and sealed lead acid batteries limit or eliminate the release of hydrogen gas to the atmosphere through the use of pressure vents or valves. Lead acid batteries cost from $140/kWh to $200/kWh, with the higher end of the range representing the maintenance-free types. The other commercially available and advanced batteries cost three to seven times as much as lead acid batteries. 17.2.2 Other Electric Energy Storage Technologies 17.2.2.1 Flywheels Flywheels use the kinetic energy of a rotating disk to store energy. For electric energy storage the flywheel is turned by an electric motor, which in turn is powered by electricity provided by the renewable energy system or conventional electricity generator. When electric power is needed, the flywheel turns the generator, which converts the kinetic energy (mechanical energy) back into electricity. Flywheels spin at very high speeds, and friction losses must be minimized to achieve the highest efficiencies (85% or more). Flywheels are best suited to applications that require electric power quickly (within seconds) and over short time periods. This makes them well suited for use in uninterruptible power supplies. Since flywheels can respond quickly to power requirements, they can be effective in frequency regulation of electric power systems. There are limited commercial products at this time, and current costs are higher than battery systems. 17.2.2.2 Pumped Hydro Pumped hydro systems use electricity to pump water from a storage unit or reservoir to a higher elevation. When needed, the water is allowed to flow through turbines that turn a generator that produces electricity. These are essentially hydro power systems. Pumped hydro systems are

130 Renewable Energy Guide for Highway Maintenance Facilities typically used by utilities to provide additional electric generating capacity during peak demand periods. They are charged during off-peak/low-demand periods. 17.2.2.3 Compressed Air Compressed air can be used as a means of storing energy for later use. An electrically or mechanically powered air compressor is used for the compression process, and the air is stored in caverns or pressure vessels (storage tanks). The compressed air can be used to generate electricity by allowing it to expand through a turbine generator. There are several technology approaches that differ based on how they treat the heat generated by the compression process. If this heat is not used (e.g., for the expansion process), it represents wasted energy. Much of current activity has been on large-scale systems (multi-megawatt) that can be used by utilities, in the same manner as pumped hydro systems. They are charged during off-peak/low-demand periods (air is compressed and stored) and discharged (air is expanded through the turbine generator) to generate electricity during on-peak/high-demand periods. 17.2.2.4 Thermal Storage Electric energy can be used to heat water or ceramic material for discharge at a later time. For example, an oversized water storage heater can be used to provide additional capacity. However, the energy must be used to offset thermal loads and is not converted back to electricity. Assuming that the thermal loads are met by electric systems to begin with, the system shifts the electric loads. 17.3 Applications Energy storage screening can be done based on the value of the electricity stored versus the value of avoided purchased electricity. In general for grid-connected applications, battery storage is not economic on the basis of avoided purchased electricity alone. The batteries would also need to provide energy reliability and security—either to the user or the utility. For off-grid applica- tions, the sizing of the battery bank is typically based on the number of days storage desired. The economics would need to be weighed against electricity provided by distributed power sources (e.g., propane or diesel engine generator sets) also used at the site. 17.3.1 Software Tools for Screening Energy Storage Electric energy storage systems can be evaluated as part of a renewable energy system using the following software: • RETScreen. This software is sponsored by NRCAN. It is free and can be downloaded as a stand-alone package. It also includes an economic analysis module. http://www.retscreen.net/. • HOMER. This software can evaluate hybrid power systems that are grid-connected or off-grid. http://www.homerenergy.com/. 17.3.2 Economic Screening Energy storage systems are sized based on the duration of storage needed. Lead acid batteries cost from $140/kWh to $200/kWh, with the higher end of the range representing the maintenance- free types.

131 18.1 Combined Heat and Power Combined heat and power, also known as cogeneration, is an efficient approach to generating power and thermal energy from a single fuel source. While it is not a new technology, various smaller-scale CHP systems are under development. These include microturbines and fuel cells. By installing a CHP system designed to meet the thermal and electrical base loads of a facility, CHP can greatly increase the facility’s operational efficiency and potentially decrease energy costs.91 Cogeneration is a very useful energy efficiency tool but is only renewable when the fuel source itself is renewable. This largely limits truly renewable CHP systems to those that use biomass-derived fuels (although photovoltaic/thermal hybrid systems would technically qual- ify). CHP systems achieve total system efficiencies of 60% to 80% for producing electricity and thermal energy.92 The success of any biomass-fueled CHP project is heavily dependent on the availability of a suitable biomass feedstock. Biomass feedstocks include: • Forest residues and wood wastes, • Crop residues, • Energy crops, • Manure biogas, • Urban wood waste, • Food processing residue, • Wastewater treatment biogas, and • Municipal solid waste (MSW) and landfill gas. 18.2 DC Distribution Systems The use of DC rather than AC power distribution is being explored as a means to improve the overall efficiency of the distribution system within buildings or microgrids. Many devices use DC power, such as office equipment, fluorescent and LED lighting, security and fire alarm systems, occupancy and daylighting sensors, and a wide variety of building systems. Many renewable energy options produce DC power, and that power usually is connected directly to the grid through an AC inverter and a direct metering or net metering arrangement. By C H A P T E R 1 8 Emerging and Alternative Energy Technologies 91See the U.S. EPA website on combined heat and power for a good overview of the subject: http://www.epa.gov/chp/. 92The catalog of CHP technologies provides technical information on the various CHP options, including biomass-fueled systems: http://www.epa.gov/chp/documents/biomass_chp_catalog.pdf.

132 Renewable Energy Guide for Highway Maintenance Facilities connecting DC loads directly to the DC power generated by the renewable systems, two energy conversions are avoided. For example, for photovoltaic systems, the elimination of the inverter could result in savings of 5% to 10%. In addition to these efficiency gains, having a separate distribution system within the building would allow the DC system to operate either in conjunction with the AC grid distribution, or alone using renewable energy generation and either battery storage or an alternate energy source (generator). This can allow critical systems to operate through power outages without interruption, which may be useful if the mainte- nance facility is a critical function for incident or disaster response efforts. One standard, the EMerge Alliance standard,93 has set an early start for a 24-volt system and includes a list of companies that make compatible DC equipment. Another competing standard, the REbus DC Microgrid specification, has established a slightly different method of technical specifications.94 Buildings on DC systems could be interconnected to form a microgrid as a means of optimizing the use of distributed generators, such as renewable-power generators to enhance reliability, and optimizing their operation. 18.3 Microgrids Microgrid systems are small-scale electric power distribution networks that can be operated independently of the larger electric utility power network or be connected to it, depending on requirements. The independent operation is made possible by the use of dis- tributed generators—conventionally fueled engine generators, gas turbines, renewable energy technologies—together with electric storage (e.g., batteries), power conditioning equipment, switchgear, and controls. The ability of microgrids to optimally use the various technologies as a stand-alone or in conjunction with utility power enables increased reliability/energy security and reduced energy operating costs. While standby generators, UPSs, and distributed generators/ combined heat and power equipment have been serving facilities for many years, the microgrid provides a means of optimizing their operation. Microgrids are well suited for college cam- puses, military installations, or other multi-building situations [e.g., industrial parks, corporate campuses, and (potentially) transportation maintenance facility stations]. The service reliabil- ity of buildings on the microgrid is enhanced by providing access to multiple power sources. Microgrids can also support the larger electric power grid by going off-line (islanding) in the event of a power disturbance. Microgrids can improve the economics of intermittent renewable resources such as wind or photovoltaics by enabling each to be operated in an optimal fashion. Microgrids are largely in the development stage, with most activity underway in public institu- tions. The three leading applications that are under current deployment are listed in the follow- ing; each tends to have a different topology, especially remote systems:95 • Institutional/campus microgrids. The typical focus of these microgrids is to aggregate exist- ing on-site generation with multiple loads that are co-located in a campus setting. These microgrids tend to be among the largest and therefore may require master controller systems. • Military-base microgrids. The focus of these microgrids is on security, both cyber and physi- cal. Since the U.S. Department of Defense has a mandate to shift over to renewable energy supplies as a matter of national security, distributed renewables will have to play a vital role. • Remote off-grid microgrids. Since these microgrids never connect to a larger grid, and there- fore operate in an island mode on a 24/7 basis, there is a greater need for storage than with other topologies. 93EMerge Alliance home page: www.emergealliance.org, accessed 8/14/2012. 94REbus home page: http://rebuspower.com, accessed 8/14/2012. 95Lorand et al., 2012. NCHRP Project 20-85, Task 1: Literature Review, p. 40 (internal project document).

Emerging and Alternative Energy Technologies 133 The unique aspects of a microgrid that are the focus of current research and development efforts include:96 • Switch technologies for connecting to/disconnecting from the main power grid; • Inverters, converters, and power conditioning equipment that are lower cost and capable of working with a variety of distributed generators, and methods to optimize the control of multiple inverters; • Uniform standards and communications protocols; and • Common integration framework for various systems. According to Pike Research, it is anticipated that by 2018 nearly 4,000 MW of microgrid power will be in operation globally.97 96U.S. Department of Energy, 2011. Microgrid Workshop Report, pp. iii and iv. http://energy.gov/sites/prod/files/Microgrid%20 Workshop%20Report%20August%202011.pdf. 97Microgrid Enabling Technologies, http://www.navigantresearch.com/research/microgrid-enabling-technologies.

134 19.1 Overview The following provides best practices that apply in general when considering renewable energy technologies. These should be used in conjunction with the technology-specific best practices included in the previous chapters in Part III. 19.2 Pre-Design • For new buildings or major renovations, consider an integrated delivery process and whole building design strategies. These should result in the best opportunity to achieve substantial energy reductions and enhance the value of renewable energy strategies. • Establish energy objectives for the project, including for renewable energy. These could be quantitative, such as a target percentage contribution of renewable energy technologies relative to a specific baseline, achieving a certain number of points from LEED through renewables, or a certification level. Make sure that criteria based on the objectives and priorities are also established as part of the process. • Include renewable energy consultants and commissioning agents on the team early in the process. • Make sure provisions are identified for performance monitoring of the renewable energy systems. Use visual displays of performance information as a means of verifying operation and raising energy awareness. • Use building energy models early in the process to establish a baseline. However, if the renewable energy project is not physically connected to the building, this may not be necessary. Instead, a technology-specific model such as those described in the technology screening sections of the guide can be used. • Incorporate solar-ready principles during the planning phase even if the current project will not incorporate solar. Guidelines for solar-ready buildings have been developed by NREL (Solar Ready Buildings Planning Guide: http://www.nrel.gov/docs/fy10osti/46078.pdf) and states (e.g., Minnesota’s Solar Ready Buildings Design Guidelines: http://mn.gov/commerce/energy/ images/Solar-Ready-Building.pdf). • Research environmental, permitting, and other regulatory issues related to the site and meet with the appropriate regulatory agency officials. Since these issues can be significant factors in project cost and schedule, they should be researched early. The Caltrans CREBs case study provides a good example of how a proactive process expedited the review of many photovoltaic systems across California (see Case Studies, 22.7). This would not have been possible with a piecemeal approach. • Understand the energy rate structures and utility requirements for on-site renewable power generation. Meet with the utility account representatives and review the implications of the various system options. C H A P T E R 1 9 General Best Practices for Implementing Renewable Energy Technologies and Strategies

General Best Practices for Implementing Renewable Energy Technologies and Strategies 135 • Make sure the plan takes into consideration potential changes to loads (future load increases/ decreases) so that these can be accommodated. This includes provisions for system expansion. • Develop initial estimates of energy requirements of the building and various end uses. 19.3 Design • Incorporate solar-ready design principles so that the building can add solar energy equipment (solar thermal or photovoltaic), even if it’s at a later date.98 This means: – Roof area should be free from shading by current or future structures or vegetation. – Roof area should be largely unobstructed to enable possible future installation of solar arrays. – Roof should be designed to be capable of supporting the weight of solar arrays, including support structures and other equipment. – Access areas for piping or conduits should be established. – Space should be allocated for solar storage tanks and heat exchange equipment (solar thermal). – Space should be allocated for inverters, breakers/disconnects, and electric meters (photo- voltaic). • Develop good estimates of energy requirements for the building and various end uses. These can be based on modeling or a combination of modeling and energy data for existing buildings. Ideally, load profiles showing energy use for typical, peak, and minimum days during the principal seasons should be generated. These can help with matching system capacities and operating strategies to the building. • Make sure designers have requisite experience with the technologies. Request references for projects of similar type and scope. If in-house staff are to be used, they should be given training first. • Make sure measurement and verification and system performance monitoring equipment is incorporated in the plans. This includes visual displays and remote access capability, as appropriate. • Bid documents: – Ensure that equipment specifications incorporate applicable safety and performance certifications. – Ensure that the specifications clearly assign warranty responsibilities for equipment and installation, including repairs. For example, if solar equipment is mounted on the roof and the roof needs to be repaired, it should be clear who is responsible for removal and re-installation, if required. – Incorporate requirements to ensure that the supplier can provide repair and replacement in a timely fashion, and request evidence supporting the claims. – Ensure that the commissioning of related items is incorporated, including O&M manuals and training. 19.4 Construction • Make sure contractor has experience with the renewable technologies in similar applications and on a comparable scale. Require installer training certifications, if available for the technol- ogy in question. Consider asking for a minimum number of references. Contact references or visit the project. • Follow commissioning plan and quality control processes. • Make sure there is close coordination among all the subcontractors. This was an important lesson learned drawn from several of the case studies included in Part IV. (For example, see Case Studies, 22.5 for Central Platte, CO, experience). 98Lissell and Watson, 2009. http://www.nrel.gov/docs/fy10osti/46078.pdf.

136 Renewable Energy Guide for Highway Maintenance Facilities 19.5 Operation and Maintenance • Educate staff and building occupants on any renewable energy system-specific considerations. This is particularly important for certain technologies where occupant interaction can have performance impacts (e.g., overriding controls). Daylight sensor placement and calibration, for example, can have a significant impact on the effectiveness of the systems. The systems should first meet the safety, health, and comfort (thermal, visual, acoustic) requirements. • Perform periodic recommissioning.