3
Renewable Electricity Generation Technologies

A renewable electricity generation technology harnesses a naturally existing energy flux, such as wind, sun, heat, or tides, and converts that flux to electricity. Natural phenomena have varying time constants, cycles, and energy densities. To tap these sources of energy, renewable electricity generation technologies must be located where the natural energy flux occurs, unlike conventional fossil-fuel and nuclear electricity-generating facilities, which can be located at some distance from their fuel sources. Renewable technologies also follow a paradigm somewhat different from conventional energy sources in that renewable energy can be thought of as manufactured energy, with the largest proportion of costs, external energy, and material inputs occurring during the manufacturing process. Although conventional sources such as nuclear- and coal-powered electricity generation have a high proportion of capital-to-fuel costs, all renewable technologies, except for biomass-generated electricity (biopower), have no fuel costs. The trade-off is the ongoing and future cost of fossil fuel against the present fixed capital costs of renewable energy technologies.

Scale economics likewise differs for renewables and conventional energy production. Larger coal-fired and nuclear-powered generating facilities exhibit lower average costs of generation than do smaller plants, realizing economies of scale based on the size of the facility. Renewable electricity achieves economies of scale prmarily at the equipment manufacturing stage rather than through construction of large facilities at the generating site. Large hydroelectric generating units are an exception and have on-site economies of scale, but not to the same extent as coal-and nuclear-powered electricity plants.



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3 Renewable Electricity Generation Technologies A renewable electricity generation technology harnesses a naturally existing energy flux, such as wind, sun, heat, or tides, and converts that flux to electricity. Natural phenomena have varying time constants, cycles, and energy densities. To tap these sources of energy, renewable electricity generation technologies must be located where the natural energy flux occurs, unlike conven- tional fossil-fuel and nuclear electricity-generating facilities, which can be located at some distance from their fuel sources. Renewable technologies also follow a paradigm somewhat different from conventional energy sources in that renewable energy can be thought of as manufactured energy, with the largest proportion of costs, external energy, and material inputs occurring during the manufacturing process. Although conventional sources such as nuclear- and coal-powered elec- tricity generation have a high proportion of capital-to-fuel costs, all renewable technologies, except for biomass-generated electricity (biopower), have no fuel costs. The trade-off is the ongoing and future cost of fossil fuel against the present fixed capital costs of renewable energy technologies. Scale economics likewise differs for renewables and conventional energy pro- duction. Larger coal-fired and nuclear-powered generating facilities exhibit lower average costs of generation than do smaller plants, realizing economies of scale based on the size of the facility. Renewable electricity achieves economies of scale prmarily at the equipment manufacturing stage rather than through construction of large facilities at the generating site. Large hydroelectric generating units are an exception and have on-site economies of scale, but not to the same extent as coal- and nuclear-powered electricity plants. 

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Electricity from Renewable Resources  With the exception of hydropower, renewable technologies are often disrup- tive and do not bring incremental changes to long-established electricity industry incremental sectors. As described by Bowen and Christensen (1995), disruptive technologies present a package of performance attributes that, at least at the outset, are not valued by a majority of existing customers. Christensen (1997) observes: Christensen Disruptive technologies can result in worse product performance, at least in the near term. Disruptive technologies bring to market very different value propositions than had been available previously. Generally, disruptive technologies underperform estab- lished products in mainstream markets. But they have other features that a few fringe customers value. Disruptive technologies that may underperform today, relative to what users in the market demand, may be fully performance-competitive in that same market tomorrow. Traditional sources of electricity generation at least initially outperform non- hydropower renewables. The environmental attributes of renewables are the initial value proposition that have brought them into the electricity sector. However, with improvements in renewables technologies and increasing costs of generation from conventional sources (particularly as costs of greenhouse gas production are incor- porated), renewables may offer the potential to match the performance of tradi- tional generating sources. This chapter examines several technologies for generation of renewable elec- tricity. It discusses the technology associated with each renewable resource, the state of that technology, and research and development needs until 2020, between 2020 and 2035, and those beyond 2035. WIND POWER Wind power uses a wind turbine and related components to convert the kinetic energy of moving air into electricity and other forms of energy. Wind power has been harnessed for centuries—from the time of the ancient Greeks to the present. The modern era of wind-driven electrical generation began with the oil shocks of the 1970s and accelerated with the passage of the Public Utilities Regulatory Poli- cies Act (PURPA). Both the development of wind technology and the installation of wind power plants have grown ever since.

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Renewable Electricity Generation Technologies  Status of Technology System Components A typical wind turbine consists of a number of components: rotor, controls, drive- train (gearbox, generator, and power converter), tower, and balance of system.1 Each of these components has undergone significant development in the last 10 years, with improvements integrated into the latest turbine designs. In addition, improved understanding and better modeling capabilities have contributed to the rapid introduction of technical improvements. What were initially small clusters of 100 kW turbines in the early 1980s have grown to clusters of hundreds of machines, including machines of 1.5 MW or more. In general, wind speed increases with height, and the energy capture capabil- ity depends on the rotor diameter. Figure 3.1 shows the change in rotor diameter and rated capacity over time. In 2006 the most common installed machine had hub heights of 275 ft (84 m) and a rotor diameter of 220 ft (67 m). Turbines as big as 5 MW have been installed in offshore locations; these have 505 ft (154 m) hub height and 420 ft (128 m) rotor diameter (IEEE, 2007a).2 As noted in Chap- ter 1, the U.S. wind energy industry installed almost 14,000 MW of capacity dur- ing 2007 and 2008. The U.S. wind power capacity is now more than 25 GW and spans 34 states; the world’s largest wind power plant, Horse Hollow Wind Energy center with a capacity of 750 MW, was recently commissioned in Texas (SECO, 2008). U.S. wind farms will generate an estimated 52,000 GWh of electricity in 2008, about 1.2 percent of the U.S. electricity supply. As discussed in Chapter 1, the installed wind power generating capacity worldwide at the end of 2006 was 75,000 MW. 1In general, the balance of system (BOS) is the system between the technologies that convert the renewable flux (wind or solar) into electricity and the electricity grid (for power production) or load (for direct use). The BOS might include the power-conditioning equipment that adjusts and converts the DC electricity to the proper form and magnitude required by an alternating- current (AC) load. For solar PV, the BOS consists of the structure for mounting the PV arrays and storage batteries. For wind turbines, it typically includes all the related electronics required to provide the connection to the grid. 2Background description and information on activities of the wind industry can be found on the American Wind Energy Association website at http://awea.org.

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Electricity from Renewable Resources 0 140 The 1980s The 1990s 2000 and Beyond 5MW Offshore 120 3.6MW Rotor Diameter in Meters 100 Arklow, Ireland GE 3.6MW Land Based 2.5MW 104m Rotor 80 1.5MW Buffalo Ridge, MN Zond Z-750kW 60 46m Rotor Medicine Bow, WY Altamont Pass, CA Clipper 2.5MW Kenetech 33-300kW 750kW 93m Rotor 33m Rotor 40 500kW Altamont Pass, CA Kenetech 56-100kW 300kW 17m Rotor 20 Hagerman, ID 100kW GE 1.5MW 50kW 77m Rotor 0 1980 1985 1990 1995 2000 2005 2010 2015 Year FIGURE 3.1  Increase in rotor dimensions over recent past.  R 3.1 Source: IEEE, 2005. Copyright 2005 IEEE. Reprinted by permission.  Electrical Output Controls Besides the mechanical characteristics, the development of the turbine mechani- cal to electrical conversion characteristics have evolved from machines based pri- marily on fixed-speed induction generators (Type 1), to variable-speed machines with electronic control (Type 2), and then machines incorporating vastly different outputs and controls (Type 3). These Type 3 machines are able to control for low- voltage ride-through (LVRT),3 voltage,4 output5 and ramp rate,6 and volt-ampere- 3Under FERC order 661A, low-voltage ride-through is the capability to continue to operate down to 15 percent of rated line voltage for 0.626 s and continuously at 90 percent of rated line voltage. This capability keeps the plant from shutting down as a result of short-term voltage fluctuation. 4Voltage control ability provides control of wind turbine voltage output. 5Output control ability allows the power produced to be reduced by feathering the blades. 6Ramp rate management allows the power output to stay within the increase or decrease lim- its required by the system.

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Renewable Electricity Generation Technologies  Rotor Squirrel Cage Induction Generator Grid Gearbox Past Compensating Double Feed Capacitors Rotor (Wound Rotor) Induction Generator Gearbox Grid Present Rotor Converter Grid Future Converter Direct Drive Synchronous Generator FIGURE 3.2  Evolution of wind turbine technology.  R 3.2 Source: IEEE, 2005. Copyright 2005 IEEE. Reprinted by permission.  reactive (VAR) support.7 While wind generators have increased in height and rotor diameter, the major changes in internal operating characteristics are not as apparent. Figure 3.2 depicts the evolution of the internal operating characteristics. Many perceptions of wind technology’s negative impact on the electrical system, such as the inability to remain connected to the electricity grid during voltage dis- turbances and the draw on the grid’s reactive power resources, stem from Type 1 machines. The evolution of control technologies has made wind generators and their electricity output easier to integrate into the utility system. With these new con- trol technologies, wind power plants are better at mimicking traditional generat- ing plants. This capability led to Federal Energy Regulatory Commission (FERC) Order 661-A, issued December 2005, which deals with machine design and system integration. It calls for wind facilities of 20 MW or larger to provide the ability 7VAR support provides reactive power compensation to aid in electricity grid stability.

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Electricity from Renewable Resources  to maintain operations, including LVRT, during disturbances on the electric grid; provide reactive power; and maintain continuous real-time communications and data exchange with the control area operator. These power integration capabilities have been incorporated into Type 3 machines. However, wind power generation takes place where and when the wind blows, and electricity must be used when it is generated. This intermittency has raised concerns about integrating wind power into the existing power system and requires wind turbines to provide LVRT, volt- age control, output and ramp rate controls, and VAR support. Integrating Type 3 machines into existing grids is not without its challenges. Circumstances such as wind fluctuations and overall grid stability are unique to each particular control area. Thus, even as technologies improve, it will be critical to carry out site-specific analyses of each control area, which will better aid grid operators in balancing the system within their control area. Integration into Utility System Operation A number of studies on the integration of wind power into a utility capacity and dispatch structure indicate that wind can be integrated at up to approximately 20 percent of the total electricity mix without requiring storage, although the exact level depends on the power system (Parsons et al., 2006; ETSO, 2007; DOE, 2008).8 The specifics of these studies are discussed in this report in the chapters on economics (Chapter 4), deployment (Chapter 6), and scenarios (Chapter 7). As the studies point out, achieving such levels of renewables penetration will depend on upgrades to the grid (necessary regardless of the energy mix) and new transmis- sion lines for more remote sources. Modern electricity grid systems are designed to handle loss of the largest power plant without disruption; to have ramp up and ramp down capabilities: and to increase or decrease generation as demand increases or decreases. However, each system has its own generating capacity structure, transmission capabilities, and ability to purchase power outside its own boundaries, making wind power integration somewhat unique for each utility. 8A number of studies can be found on the Utility Wind Integration Group (UWIG) website at http://www.uwig.org.

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Renewable Electricity Generation Technologies  Small Wind Systems The vast majority of wind power is generated by large wind turbines feeding into the electricity grid, while small wind turbines generally provide electricity directly to cus- tomers. The United States is the leading world producer of small wind turbines. These residential turbines are erected and connected directly to the customer’s facility or to the electricity distribution system at the customer’s site. The manufacture and mar- keting of wind-powered electric systems sized for residential homes, farms, and small businesses have experienced major growth in the past decade. These small wind tur- bines (Figure 3.3), defined as 100 kW or less in capacity, have seen significant market growth, and the industry has set ambitious targets: growth at 18–20 percent through 2010. FIGURE 3.3  Small wind turbine, shown near home with rooftop photovoltaic panels  installed.  Source: Courtesy of National Renewable Energy Laboratory. 

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Electricity from Renewable Resources  Key Technology Opportunities Short Term: Present to 2020 The key technological issues for wind power focus on continuing to develop better turbine components and to improve the integration of wind power into the elec- tricity system, including operations and maintenance, evaluation, and forecasting. Goals appear relatively straightforward: taller towers; larger rotors; power elec- tronics; reducing the weight of equipment at the top and cables coming from top to bottom; and ongoing progress through the design and manufacturing learning curve (Thresher et al., 2007; DOE, 2008). Table 3.1 summarizes the incremental improvements under consideration. Although no big breakthroughs are anticipated, continuous improvement of existing components is anticipated, and many are already being actively devel- oped. For example, there are advanced rotors that use new airfoil shapes specifi- cally designed for wind turbines instead of those based on the design of helicop- ter blades. These rotors are thicker at points of highest stress and reduce loads during turbulent winds by flying the blades using turbine control systems. Other improvements include the use of composite materials and advanced drivetrains. In particular, gearboxes are a major area of concern for reliability. Approaches for improving this component include direct-drive generators; greater use of rare-earth permanent magnets in generator design; possibility of single-stage drives using low-speed generators; and distributed drivetrains using the rotor to drive several parallel generators. Advanced towers are a major focus for innovation, given the current need for large cranes and transport of large tower and blade sections. Concepts under investigation include self-erecting towers, blade manufacturing on site, vibration damping, and tower–drivetrain interactions. There is certain to be some development of offshore wind in the United States in the near term, but it is not expected that this will have a significant impact before 2020. Nonetheless, there is a near-term opportunity to learn from offshore projects in Europe and the United States, if offshore wind is going to have an impact in the medium term. Other near-term opportunities will lie in improving the integration of existing wind power plants into the transmission and distribution system, which includes using improved computational models for simulating and optimizing system inte- gration (Ernst et al., 2007). Chapters 6 and 7 discuss the deployment and integra- tion of wind-generated electricity.

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Renewable Electricity Generation Technologies  TABLE 3.1 Areas of Potential Wind Power Technology Improvements Performance and Cost Increments Best/Expected/Least (%) Annual Energy Turbine Capital Technical Area Potential Advances Production Cost Advanced tower • Taller towers in difficult locations +11/+11/+11 +8/+12/+20 concepts • New materials and/or processes • Advanced structures/foundations • Self-erecting, initial, or for service Advanced (enlarged) • Advanced materials +35/+25/+10 −6/−3/+3 rotors • Improved structural-aero design • Active controls • Passive controls • Higher tip speed/lower acoustics Reduced energy • Reduced blade soiling losses +7/+5/0 0/0/0 losses and improved • Damage-tolerant sensors availability • Robust control systems • Prognostic maintenance Drivetrains (gearboxes • Fewer gear stages or direct-drive +8/+4/0 −11/−6/+1 and generators and • Medium- to low-speed generators power electronics) • Distributed gearbox topologies • Permanent-magnet generators • Medium-voltage equipment • Advanced gear tooth profiles • New circuit topologies • New semiconductor devices • New materials (gallium arsenide [GaAs], SiC) Manufacturing and • Sustained, incremental design and 0/0/0 −27/−13/−3 learning curvea process improvements • Large-scale manufacturing • Reduced design loads Totals +61/+45/+21 −36/−10/+21 aThe learning curve results from NREL (2008) (Cohen and Schweizer et al., 2008) are adjusted from 3.0 doubling in the reference to the 4.6 doubling in the 20 percent wind scenario. Source: DOE, 2008. Medium Term: 2020 to 2035 Mid-term wind technology development will have two thrusts: the movement toward offshore, and its implications for turbine design; and the development of efficient low-wind speed turbines. Development of offshore wind power plants has already begun in Europe (approximately 1200 MW of installed capacity), but

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Electricity from Renewable Resources  progress has been slower in the United States. Nine projects are in various stages of development in state and federal waters. In addition to technical risks and higher costs, these projects have been slowed by social and regulatory challenges (DOE, 2008). In the mid-term, offshore turbines will have a larger size and generating capacity than onshore turbines, but, owing primarily to technical and cost con- cerns, development will likely lag behind onshore machines. Transmission siting issues with offshore wind power plants will be simplified because of fewer siting impediments. However, underwater cables must be carefully constructed, and there will likely be a move to develop microgrids with high-voltage direct current to integrate the offshore resources. Offshore wind technologies face several transi- tion problems as they move from near-shore, land-based sites to offshore sites of various depths and, finally, floating designs. Assessment tools for sensitive marine areas, wind loads, and system design are not now ready for offshore development. Offshore projects must be built to handle both wind and wave loads, and com- ponents must be able to endure marine moisture and extreme weather. Offshore wind projects have a higher balance of station cost (approximately two-thirds of total costs) than do onshore projects, and thus will rely on cost reductions across the system in order to become more competitive. All of these developments pose both technological and organizational problems and will require continuous research and development in order to be feasible. It should be noted that chal- lenges posed by the greater technical difficulties of offshore wind power develop- ment are being addressed by other countries. However, political, organizational, social, and economic obstacles may continue to inhibit investment in offshore wind power development, given the higher risk compared to onshore wind energy development (Williams and Zhang, 2008). In terms of onshore development, as the higher wind speed sites are used, wind power development will move to lower wind speed sites, which will require turbines that are relatively efficient at lower wind speeds, necessitating larger rotors with lighter, stronger materials, as well as increased tower height. Long Term: After 2035 At present, no revolutionary technology to extract energy from wind has been proposed, but several designs, e.g., vertical wind turbines or eggbeaters, are again under consideration. There have been conceptual proposals to access high-altitude winds using balloons or kites. Component improvements will continue, with

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Renewable Electricity Generation Technologies  additional emphasis on offshore turbine installation. Floating offshore platforms may gain interest, but first must come experience from anchored offshore wind facilities. Summary of Wind Power Potential Wind-power technologies are actively deployed today, and there are no techno- logical barriers to continued deployment. Cost reductions will be possible as a result of wider deployment and incremental improvements in components. No other enhancing technologies are required for wind power to meet 20 percent and higher of U.S. electricity demand. SOLAR PHOTOVOLTAIC POWER Solar power involves the conversion of the radiant energy from the sun into elec- tricity by using photovoltaics (PV) or concentrating devices. When sunlight strikes the surface of the PV cell, some of the photons are absorbed and release electrons from the solar cell that are used to produce an electric current flow, i.e., electric- ity. A solar cell consists of two layers of materials, one that absorbs the light and the other that controls the direction of current flow through an external circuit (Figure 3.4). The absorbing materials can be silicon (Si), which is also used in integrated circuits and computer hardware; thin films of light-absorbing inorganic materials, such as cadmium telluride (CdTe) or gallium arsenide (GaAs), that have absorption properties well matched to capture the solar spectrum; or a variety of organic (plastic) materials, nanostructures, or combinations. Status of Technology The PV industry has grown at a rate greater than 40 percent per year from 2000 through 2008. Much of this growth is the result of national and local programs targeted toward growing the PV industry and improving the competitiveness of PV in the marketplace. In 2007, PV modules supplying 3.4 GW were produced worldwide, and approximately 220 MW were installed in the United States.9 Table 3.2 provides a breakdown of PV module shipments by technology type. 9See http://www.solarbuzz.com/Marketbuzz2008-intro.htm.

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Electricity from Renewable Resources  plant could provide a closed-loop system in which methane would not have to be transported. Summary of Storage Potential Analysis of the future for the various storage technologies is beyond the scope of this panel, but some summary statements are in order. In the near term, diabatic CAES and various battery technologies, especially sodium sulfur batteries, have found initial applications in the electricity sector. In the longer term, when pen- etrations of renewables in the electricity sector might reach levels requiring energy storage, there may be a variety of approaches, including adiabatic CAES or the use of renewable energy in the production of chemical fuels. Advances in ultracapaci- tors and other short-term storage solutions may provide additional mechanisms to effectively integrate and stabilize intermittent resources. Energy storage is a system resource that should be operated for the overall benefit of the system. The greatest value of energy storage is realized when it is operated for the benefit of the entire system, and not dedicated to balancing any particular resource on the system. Storage tied to smart transmission and distribu- tion grids would become a valuable component of any power system, and could provide numerous benefits to the system. Storage benefits the system without renewables, and renewables benefit the system without storage. The task is to manage variability with flexibility. Improved Grid Intelligence—the Smart Grid The architecture needed to improve integration of renewables into the electricity grid would incorporate a variety of technologies, such as advanced sensors; smart meters (net metering, turn-on/turn-off capability, and the capability to enable time- of-day pricing); power converters, conditioners, and other power-quality tech- nologies; source and load controls; improved software, including forecasting and operations models; and storage technologies (Kroposki, 2007). Most of these tech- nologies are part of the broad initiative to improve the intelligence of the mod- ern grid.33 The objectives to meet in modernizing the electricity grid go beyond 33The term “Smart Grid” has often been used to describe this initiative. The Smart Grid may be described as the overlaying of a unified electronic control system and two-way communica- tion over the entire power delivery infrastructure. Smart Grid capabilities optimize power supply and delivery, minimize loss, and enable maximum use of electricity generation resources, energy efficiency, and demand responses. However, this term suffers from overuse and multiple interpre-

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Renewable Electricity Generation Technologies  increasing intermittent renewables, and include improving security and power quality and creating a more efficient, adaptive electricity system. Demonstrations are under way in several U.S. cities (e.g., Boulder, Colorado), but widespread deployment is expected to take decades.34 More details on the objectives and tech- nologies involved in creating a future electricity grid with increased capacity and intelligence are presented in the upcoming report of the Committee on America’s Energy Future (NAS-NAE-NRC, 2009a). A truly intelligent modern grid would anticipate the fluctuations in the power output from intermittent renewable energy sources and maintain absolute supply/demand equivalency on a given transmission or distribution circuit, while requiring less compensating backup power and storage capacity. Instantaneous electronic control of the grid would allow each transmission line to operate at a higher load factor without risking thermal overload than is now feasible on the electromechanically controlled transmission system. This level of coordinated control would require improved communications and seamless connectivity, or interoperability,35 which would make the grid a dynamic, interactive infrastructure for the real-time exchange of power and information. Open connectivity archi- tecture would create a plug-and-play environment that would securely network grid components and operators. The current lack of uniform interconnection and operations codes and standards, as well as the acceptance of standardized open communications architecture, is restricting the timely implementation of the mod- ern grid. A system-wide integrated cyber security capability is also an important dimension of this communications architecture. The Smart Grid’s emphasis today is primarily on creating interstate high- voltage transmission capabilities to facilitate bulk wind power access. While important, transmission is only one element of the nationwide grid modernization effort needed to realize the potential benefits of renewable energy. The electronic modernization of the local electricity distribution network is equally essential to incorporating distributed renewable energy technologies such as photovoltaics and wind power. One critical objective of smart distribution grids is to enable the tations. The panel instead uses the improved term “grid intelligence” to refer to the collection of technologies needed to improve the integration of renewables into. 34EISA 2007 authorized the Smart Grid Advisory Committee and Task Force through 2020. An earlier (2003) DOE plan was called Grid 2030; the intention was to have 100 percent of elec- tricity running through a smart grid by 2030. 35Seamless, end-to-end connectivity of the hardware and software throughout the transmis- sion and distribution system to the electrical energy source.

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Electricity from Renewable Resources  seamless, uninterruptible balancing of electricity supply and demand, which could allow distributed renewable power generation to be broadly dispatchable. Dis- patchability would improve intermittent renewables’ compatibility with the reli- ability and operational requirements of the bulk power system. The result could help transform buildings into power plants and provide a more reliable, efficient, and clean electricity supply system. Advanced Metering Advanced metering—the use of electricity meters that provide detailed consump- tion profiles—is one technology for improving the intelligence of the grid that would be particularly important to increasing the use of distributed renewables. Unlike conventional metering, advanced metering would couple the cost of elec- tricity generation with the price to the consumer. In the context of renewables integration, the ability to do time-of-day pricing and net metering would better enable the deployment of renewables, especially solar PV. Such meters also could communicate real-time information to the consumer for billing and pricing pur- poses. Because solar PV generation peaks close to the late-afternoon price peak, meters allowing time-of-day pricing could improve the cost-competitiveness of solar PV at the consumer end. Advanced metering also helps to create incentives to use energy at off-peak times when possible, thereby reducing demands on the transmission and distribution systems. Chapter 4 discusses the use of real-time pricing to encourage the development of renewables. Furthermore, advanced metering technologies would enable net metering for those with on-site renewable generation. Net metering improves the integration into the grid of distributed renewable resources such as solar PV installed at resi- dential and commercial facilities. It measures both the consumption of electricity and the excess energy produced on-site, and at least partly credits the consumer for excess generation produced by consumer-owned solar PV or other renewable electricity technologies. Software/Modeling Support New grid operating tools are also needed to incorporate renewable energy resources, including operating models and system impact algorithms that address the transient behavior of renewable energy; improved operators’ visualization techniques and new training methodologies; and advanced simulation tools that can provide an accurate understanding of grid behavior. These grid operating tools

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Renewable Electricity Generation Technologies  would also assist system planners in designing reliable power systems for this new environment. Better forecasting algorithms would allow better use of temporally varying resources such as wind energy. The objective of this work is to improve the forecasting of wind and its use in electricity markets (Ahlstrom et al., 2005; Hawlins and Rothleder, 2006; Smith, 2007). Reactive Dynamic Power The demand that some renewables place on ancillary services, such as reactive power and dynamic voltage control, also must be considered. Reactive power is the portion of electricity that establishes and maintains the electric and mag- netic fields of alternating current (AC) equipment. Because wind and solar power produce direct current (DC), reactive power must be provided in the DC-to-AC conversion process, a requirement that is complicated by the variable/intermit- tent nature of these renewable energy sources: the reactive power must be equally dynamic to keep pace. Many early wind machines were induction generator wind turbines with a constant frequency and so required reactive power to be sup- plied from the grid. Although newer machines have solved this problem, voltage stability remains an issue. The European Transmission System Operators (ETSO) recently completed a study on the ancillary services required by wind power as the amount of installed wind capacity in Europe increased from 41 GW in 2005 to an expected 67 GW in 2008 (ETSO, 2007). In particular, the ETSO study looked at the effects of variable power output on the electricity grid and the ability of vari- ous wind turbine types to provide system service needed for the stable operation of an electricity grid. Another study describes technologies used to provide reac- tive power for a large wind farm and the interactions of the wind farm, reactive power compensation, and the power system network (Muljadi et al., 2004). FINDINGS The most critical elements of the panel’s findings on renewable electricity genera- tion technologies are highlighted below. Over the first timeframe through 2020, wind, solar photovoltaics and con- centrating solar power, conventional geothermal, and biopower technologies are technically ready for accelerated deployment. During this period, these technolo- gies could potentially contribute a much greater share (up to about an additional

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Electricity from Renewable Resources  10 percent of electricity generation) of the U.S. electricity supply than they do today. Other technologies, including enhanced geothermal systems that mine the heat stored in deep low-permeability rock and hydrokinetic technologies that tap ocean tidal currents and wave energy, require further development before they can be considered viable entrants into the marketplace. The costs of already-developed renewable electricity technologies will likely be driven down through incremen- tal improvements in technology, “learning curve” technology maturation, and manufacturing economies of scale. Despite short-term increases in cost over the past couple of years, in particular for wind turbines and solar photovoltaics, there have been substantial long-term decreases in the costs of these technologies, and recent cost increases due to manufacturing and materials shortages will be reduced if sustained growth in renewable sources spurs increased investment in them. In addition, support for basic and applied research is needed to drive continued tech- nological advances and cost reductions for all renewable electricity technologies. In contrast to fossil-based or nuclear energy, renewable energy resources are more widely distributed, and the technologies that convert these resources to use- ful energy must be located at the source of the energy. Further, extensive use of intermittent renewable resources such as wind and solar power to generate elec- tricity must accommodate temporal variation in the availability of these resources. This variability requires special attention to system integration and transmission issues as the use of renewable electricity expands. Such considerations will become especially important at greater penetrations of renewable electricity in the domes- tic electricity generation mix. A contemporaneous, unified intelligent electronic control and communications system overlaid on the entire electricity delivery infrastructure would enhance the viability and continued expansion of renewable electricity in the period from 2020 to 2035. Such improvements in the intelli- gence of the transmission and distribution grid could enhance the whole electricity system’s reliability and help facilitate integration of renewable electricity into that system, while reducing the need for backup power to support the enhanced utiliza- tion of renewable electricity. In the third time period, 2035 and beyond, further expansion of renewable electricity is possible as advanced technologies are developed, and as existing technologies achieve lower costs and higher performance with the maturing of the technology and an increasing scale of deployment. Achieving a predominant (i.e., >50 percent) penetration of intermittent renewable resources such as wind and solar into the electricity marketplace, however, will require technologies that are largely unavailable or not yet developed today, such as large-scale and distributed

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