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Page 111 7 Renewable Energy: Generation, Storage, and Utilization John Turner National Renewable Energy Laboratory National security depends on energy security. —President-elect George W. Bush Energy is the major input for overall socio-economic development. —C. R. Kamalanathan, Secretary, Ministry of Non-Conventional Energy Sources, Government of India. The Americans in this area are very much the villains of the piece. They've not gone along with Kyoto and yet they are unquestionably the largest polluter with 4% of the world's population and 25% of greenhouse gas emissions. —Sir Crispin Tickell, the former British ambassador to the United Nations. For our own security we must reduce our dependence on foreign sources of energy. However, for our economy to grow we must obtain additional energy. Developing countries will also compete for this energy, because they must have additional energy sources before they can raise their standard of living and join the world's developed nations. Like a dark cloud hanging over all of this are the pollution and global climate change these fuels produce. As we use more coal, oil, and gas, we only exacerbate the problem. As we seek to address the issues of global climate change and carbon management, we must remain aware of all these other issues. To control and sequester carbon emissions will require additional energy. This will put additional pressure on our fossil reserves, our supply lines, and the energy infrastructure. Currently, this world is inhabited by more than 5 billion individuals and is powered ultimately by solar energy. The food we eat and the oxygen we breathe comes from photosynthesis. Can we use this energy from the sun that indirectly powers our bodies to provide the energy we need to run our society? This chapter discusses the possibilities and offers some suggestions as to the pathways to a sustainable energy infrastructure. Sustainable in this context means capable of supplying a growing population with energy without destroying the environment within which it is used. It must also include the ethics of using the earth's resources, particularly fossil fuels. It is completely unethical for us to squander the finite nonreplaceable
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Page 112resources of the planet in a one-time use without any accountability to future generations and our environment. We must try to look ahead, envision a future society, give it a voice, and use this to point out those issues that would clearly affect future lives. To consume a resource without developing a replacement is clearly an issue that will affect future generations. The author's perspective in writing this article include the following: To modify an existing energy infrastructure or build a new energy infrastructure requires money and energy—energy that must come from existing resources. Advanced renewable energy systems can provide long-term benefits to society—namely, sustainability. Manufacturing renewable energy systems for the developing world provides an economic benefit to the United States because a very large portion of the energy demand will occur in these regions. The points to consider here are the following: 1. What is the market for renewable technologies versus sequestration technologies? 2. Distributed generation that uses indigenous local resources reduces the need to build and maintain large electrical grids. SEQUESTRATION VERSUS RENEWABLES Our current energy and transportation infrastructure is plagued by many problems. It pollutes and damages our environment, it makes us dependent on foreign governments for more than 50% of our supply, and the trade deficit resulting from our oil purchases (at more than $1 billion per week) has a destabilizing effect on our economy. In addition, we must maintain an enhanced military presence in the Middle East to keep our access to this oil, which puts our military at risk. If we implement large-scale sequestration, it solves only the environmental issue, but the rest of the problems are actually exacerbated—we will have to burn more fossil fuel in order to generate the additional energy needed to power the sequestration systems to remove the CO2 that our fossil fuel systems generate. This means importing more oil (increasing our trade deficit) or burning our own reserves at a faster rate. Sequestration is only a temporary fix; eventually we will have to replace fossil fuels. Since we have technologies that will “do the job,” we should implement a sustainable energy infrastructure that doesn't emit carbon dioxide and can supply all our energy needs using our indigenous resources. Renewable technologies also have the ability to expand as our usage of and needs for energy grow. Furthermore, renewable energy systems can be configured to supply not only all the electrical needs, but also all our transportation requirements. The United States can spend its money and energy resources building sequestration systems or implementing renewable energy technologies; it is not likely that we can or will do both. The first part of this chapter therefore discusses the vision and possibilities of renewable energy such that the reader will be convinced of the viability of this approach. In the second part, a general list of research areas is presented. Because it would be impossible to cover all of the associated technologies fully, the author has chosen to present those technologies he feels could have an impact in the immediate to 10-year time frame. FEASIBILITY OF RENEWABLES We first address the question, Can we really supply all our energy needs from renewable energy? The power of renewable energy can easily be shown using the United States as an example. 1 The United
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Page 113States is the world's largest energy consumer. Total U.S. annual electrical demand for 1997 was about 3.2 × 1012 kWh (representing 25% of the world's consumption). If we assume flat fixed-plate collectors with a system efficiency of 10% (current commercial technology) covering only half of the employed land area, a photovoltaic (PV) array 104 miles (166 km) on one side (~10,900 square miles or 27,600 km2), placed in southwest Nevada, would, over one year supply all this energy. This area represents less than 0.4% of the available land area of the United States. A system efficiency of 15%, which should be available in the next three to five years, would decrease the area to 7,200 square miles. If we add wind to the energy mix, this area for PV decreases further. If we add geothermal and hydropower, the area gets smaller still. The point is clear: we can gather more than enough renewable energy to power our society and yet have an abundance of renewable resources available for future growth. Also, one should note that wind alone or solar thermal alone could provide all our electrical energy needs. Although we have used Nevada for this calculation, PV panels can be placed across the entire United States. The U.S. average solar irradiance is 1,800 kWh/m2 per year. Implementation would involve the rooftops of homes and businesses, parking lots, and even landfills, along with 1 to 10 square mile “energy farms.” Landfills are especially viable. There are more than 10,000 landfills in the United States, and only 3,000 are active. The land is typically unusable for 20 to 30 years, and this is exactly the lifetime of PV systems. Furthermore, landfills produce methane as the organic matter decomposes. If methane is collected and used in a fuel cell, for example, this provides an additional energy input from the land. The way in which electricity is generated is as important as the area required to supply this amount of energy. If we look at solar irradiance data (how much sunshine is available per day), we see that, in southwest Nevada, the sun shines only for an annual average of about 6 hours a day. That effectively means that this system generates the same amount of electricity in 6 hours that the United States uses in 24 hours. The remaining 18 hours of electricity must be taken from the energy stored during the hours of sunlight. Energy storage and its efficiency then become critical; any efficiency losses must be made up by an increase in the area of the PV array. This points out one of the major drawbacks to many forms of renewable energy—their intermittency and the need for energy storage. Dealing with the intermittent nature of certain forms of renewable energy and energy storage systems are topics in this discussion. ENERGY PAYBACK Implementing an energy infrastructure that uses more energy in its manufacture and deployment than it produces in its lifetime is not a viable pathway for the future. In fact, our current energy infrastructure has an energy payback ratio of about 0.3 meaning it converts only 30% of the input energy (in the form of fossil fuel) into electricity. A sustainable energy system will have an energy payback ratio greater than one. There is, however, the persistent myth that it takes more energy to manufacture renewables than they produce in their lifetime. Actual calculations show a very rapid payback. For example, the energy payback time for current PV systems has been calculated to range from three to four years depending on the type of PV panel (thin-film technology or multicrystalline silicon respectively). 2, 3 This includes the energy costs for processing the semiconductor material and assembling it into a module, the frame, and the support structure. This means that PV with a lifetime of at least 30 years has a payback ratio of 8 to 10. Wind energy has an even faster payback—two to three months, and this includes scraping the turbine at the end of its life. 4 For wind, with a 20-year lifetime, the payback ratio is an impressive 80! Renewable energy resources can therefore be used to manufacture additional renewable energy systems, like a breeder plant, producing more energy than they use in the manufacture—the ultimate in sustainable energy systems.
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Page 114 With the combination of PV, solar-thermal, wind, geothermal, biomass, and hydro, the United States has sufficient renewable resources for virtually unlimited energy growth. Moreover, PV, wind, geothermal, and biomass are commercial systems that are available now. We now discuss some of the renewable generation technologies and then highlight some of the research issues. PHOTOVOLTAICS (SOLAR CELLS) Photovoltaics (solar cells) convert light energy directly into direct current d.c. electricity with no moving parts. 5 Developed in the early 1950s, primarily for the space industry, PV is now a $1.5 billion industry for terrestrial applications. Worldwide production in 2000 was approximately 278 megawatts (MW), representing a yearly growth rate of about 37%. As of January 2001, the production of major manufacturers was sold out for next two years, and many new production facilities are being built. Levelized electricity costs are reported to range from 15 to 30/kWh ($5-$10 per watt installed). The major application in the past has been for remote applications far from the grid, but it is becoming more common to find PV on the rooftops of homes and integrated into buildings ( Figure 7.1) In fact, building integrated PV has become a major market. In these days of heightened awareness of energy shortages, and in particular their effect on Internet businesses, energy reliability is becoming a major factor in the decision of companies to integrate PV into their buildings. The siting issues are very important. PV does not impact the area with emissions as diesel generator sets do, and because there are no moving parts, maintenance is minimal. During the summer months, PV provides its maximum power just when air conditioning loads are the greatest. Integrating PV into a building design decreases the installation costs, provides for additional energy reliability and reduces the load on the local grid since the electricity is generated at point of use. While the major technology being installed today is based on single and poly crystalline silicon, thin film solar cell technologies offer the potential for very low cost and high volume manufacturing resulting in a levelized cost in the 6 cents/kWh range. Thin film technologies utilize 1-5-µm-thick films of semiconductors on glass or stainless steel. The advantages that thin films provide are high efficiency, reduced materials requirements, and an inexpensive and rapid manufacturing process. The disadvantages are that current systems use toxic or rare materials and that the long-term stability (>20 years) of current technology is unknown, although data to date show excellent stability. Before these materials can make a major impact in the PV market, improved understanding of the scientific and technological base for today's thin films will be necessary ( Figure 7.2). Figure 7.3 shows the historical trends in laboratory efficiencies for the various thin film systems. For solar cells, the efficiency of commercially produced panels usually lags the laboratory efficiencies by about 10 years. For commercial applications, panel efficiencies need to be in the range of 10%, although many of these thin-film systems can be successfully marketed with efficiencies of 5-8%. ADVANCED DEVELOPMENT OF SOLAR CELLS Increasing the efficiency of any solar converter will decrease the area that must be covered to collect a fixed amount of energy. Depending on the cost of the solar converter system, this can also lead to lower costs. For photovoltaics, one can greatly increase the conversion efficiency by designing cells to utilize specific areas (colors) of the solar spectrum and stacking them on top of one another in a series configuration. A commercial example is found in the GaAs-GaInP2 tandem cell currently used to power communication satellites. This solar cell consists of a gallium arsenide (GaAs) bottom cell connected to
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Page 115 ~ enlarge ~ FIGURE 7.1 4 Times Square, a 48-story skyscraper at the corner of Broadway and 42nd Street, was the first major office building constructed in New York city in the 1990s. The building's most advanced feature is the photovoltaic skin, a system that uses thin-film PV panels to replace traditional glass cladding material. The PV curtain wall extends from the thirty-fifth to the forty-ninth floors on the building's south and east walls. The developer, the Durst Organization, has implemented a wide variety of healthy building and energy-efficient strategies. Kiss and Cathcart architects designed the building's PV system in collaboration with Fox and Fowle, the base building architects. Energy Photovoltaics of Princeton, New Jersey, developed the custom PV modules.
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Page 116 ~ enlarge ~ FIGURE 7.2 Thin copper indium diselenide solar cell. Progress in manufacturing is mostly empirical, with little understanding of material properties, devices, and processes that lead to higher efficiency. ~ enlarge ~ FIGURE 7.3 Thin-film solar cell efficiencies in the laboratory from 1975 until 2000.
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Page 117 a gallium indium phosphide (GaInP2) top cell via a tunnel diode interconnect. The top p/n GaInP2 junction, with a bandgap of 1.83 eV, is designed to absorb the visible portion of the solar spectrum. The bottom p/n GaAs junction, with a 1.42-eV bandgap, absorbs the near-infrared portion of the spectrum, which is transmitted through the top junction. While single gap electrodes have a solar conversion efficiency limit of 32%, tandem junction devices have an efficiency limit of 42%. 6 The maximum theoretical solar-to-electrical efficiency for the present combination of bandgaps is about 34%, 7 and more than 29% efficiency has been realized experimentally. Research work is currently under way to develop systems with four junctions, which have a theoretical efficiency of more than 50% and a realizable efficiency greater than 40%. One approach to deal with the high cost of these materials is to use them in a solar concentrating system, where most of the area of the expensive semiconductor is replaced by an inexpensive optical concentrator. The GaInP2-GaAs system has been shown to operate at up to 1,000 times light concentration (with active cooling). To show the power of this approach, if we take a PV manufacturing plant that is producing 10 MW of PV material per year and the material is capable of 1,000 times concentration, with the use of an optical concentrator system, that plant now produces 10,000 MW (10 GW) of PV per year. While current research involves mainly single-crystal material, applying multijunction technology to thin-film devices would provide great efficiency and cost benefits. WIND ENERGY Wind is the world's fastest-growing energy resource. In 2000, worldwide wind-generating capacity was 18,000 MW. 8 (This is equivalent to the amount of nuclear power installed worldwide by 1970.) The growth rate is about 28% per year (1995-1998) and was 36% from 1998 to 1999. Wind systems produce energy at the lowest cost of any renewable energy system, thus wind is projected to produce 10% of the world's energy supply by the year 2020. Wind can be very cost-effective in the displacement of fossil-generated electrons. Electricity from the Lake Benton I 107.25-MW wind farm in Minnesota is sold to Northern States Power Company at an average cost of 3 cents/kWh. Wind energy turbines range from a few hundred watts to multimegawatt systems. Denmark is currently producing a 2-MW system designed to be used mainly in offshore locations. Because of the lower air turbulence over water, offshore wind turbines produce about 50% more energy than land-based turbines. They also last about 10 years longer. One of the great advantages of wind is that it is a dual-use technology—its footprint uses only 5% of the land, and the rest of the land can still be used for farming, ranching, and forestry. The land-leasing revenue for a landowner leasing to a wind farm ranges between $500 and $2,000 per turbine per year. That same land used for farming would generate about $100 per year. The farming areas in the Midwest from Texas to North Dakota could be used to provide all of the U.S. electrical needs. The average capacity factor for wind is about 30%, meaning that the wind farm generates electricity only about 30% of the time. Because of this, utilities often discount wind since they cannot predict or fully depend on the wind farm to generate electricity when it is needed. However, it has been determined that widely separated wind sites provide a more constant power supply; the more wind farms connected to the grid, the more will the short-term fluctuations from one farm cancel out the fluctuations from another. This has been particularly noted in Denmark, which in 2000 was already supplying 13% of its electricity by wind generation. This figure will increase to at least 16% by 2003 and is expected to be 50% of its consumption by the year 2030. One of the issues with wind is that wind sites are often located away from electrical loads and transmission lines. This also applies to large-scale PV arrays and solar-thermal electric plants situated in the desert areas of the American West.
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Page 118 SOLAR-THERMAL TECHNOLOGY—CONCENTRATING SOLAR POWER Solar-thermal systems, also known as concentrating solar power plants, produce electrical power by concentrating the sun's energy with various mirror configurations and generating high-temperature heat. 9 They thus follow the path from solar to heat to electricity. The systems are classified by the way in which they concentrate the sun's energy and collect the heat. They include central receiver, parabolictrough, and dish systems. Although each of these has been demonstrated, the parabolic-trough systems have the longest operating experience. Parabolic-Trough Technology Parabolic-trough technology offers the lowest-cost, near-term option for large-scale solar power. 10 It is also the technology that has the longest commercial experience of any solar plant. Electricity costs with current technology are around 10 cent/kWh now, with 5 cent/kWh predicted for the future (2010) using thermal energy storage. The trough systems collect energy in a receiver pipe located along the focal length of a parabolically curved trough-shaped reflector. A heat-collecting fluid, usually oil, is pumped through the receiver pipes and is heated to about 400°C. The hot oil is then used to produce steam that generates electricity in a conventional steam generator. Systems have been designed that incorporate thermal storage; they can be used to generate electricity when the sun is not shining. However, all parabolic-trough plants currently operate as hybrid solar-fossil plants. These plants use standard fossil fuels to generate electricity when the sun is not shining or during periods of low solar intensity. This points out the great advantage with this technology—it couples nicely with existing fossil fuel technology, thus integrating easily with conventional central station power plants. These hybrid systems provide continuous power, with significantly lower carbon emissions than a stand-alone fossil plant. There are nine parabolic-trough plants with a combined 354 MW(e) (megawatts of electrical power) of generating capacity operating in the Mojave Desert ( Figure 7.4). The first one was installed in 1984. These plants have a combined 100 plant-years of commercial operating experience. ENERGY STORAGE Whereas biomass and geothermal can produce energy at will, wind, solar-thermal, and PV cannot. Therefore, any renewable-based energy scheme must have integrated energy storage before it can be considered as a viable, sustainable energy system. Energy storage systems include hydrogen, biofuels, batteries, pumped hydro, compressed air, thermal storage, flywheels, and superconducting magnetic storage. Energy storage is site specific and can be time dependent. Pumped hydro can be very inexpensive but is obviously site dependent. Flywheels and batteries have a high turnaround efficiency (>90%) but degrade at 0.1-5% per hour. Hydrogen has a turnaround efficiency of about 50% but is not time dependent. Energy must be dispatchable, so only those renewable energy systems that contain a viable energy storage technology can be considered for large-scale implementation. A utility cannot call on solar and wind generation systems to produce at will; it can call on its energy storage system—but the energy must be there! Managing energy storage will be the key for a successful utility. Resource assessment for storage scenarios will be very important for the implementation of renewable energy technologies. The energy storage system will likely be dependent on the local environment, so there has to be the capability to match the energy storage system with the energy generation system. Algorithms and control strategies must be developed to match the generation, storage, and distribution systems. There is a need
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Page 119 ~ enlarge ~ FIGURE 7.4 Nine trough power plants in California's Mojave Desert provide the world's largest generating capacity of solar electricity, with a combined output of 354 MW. These systems provide large-scale power generation from the sun and, because of their proven performance, are gaining acceptance in the energy marketplace. to couple the forecast of energy usage (time and magnitude) with the forecasting of energy generation. Weather forecasts that include wind and solar insolation forecasts become very important. They are lacking at the moment. In fact, the author sees the lack of a large program focused on energy storage and its integration into the energy infrastructure as a major gap in current U.S. energy policy. Flywheel systems 11 are possibly the lowest-cost energy storage technology, predicted to be in the range of about $500 per kilowatt. Flywheels have a very high turnaround efficiency (>90%), meaning energy in versus energy out, but current technology degrades at about 2% per hour, which limits their energy storage capability to a few hours. Superconducting bearings would give a 0.1% per hour decay 12 and would extend their energy storage capabilities from days to weeks. Only kilowatt-hour energy storage systems have been demonstrated. For utility applications, multimegawatt-hour systems will have to be developed and demonstrated. Hydrogen can replace fossil fuels as the energy carrier for transportation and electrical generation when renewable energy is not available. Since it can be transported via gas pipelines or generated onsite, any system that requires an energy carrier can use hydrogen. Conversion of the chemical energy of H2 to electrical energy via a fuel cell produces only water as waste. This pollution-free attribute of a hydrogen economy is one of the major driving forces for research in the generation, storage, distribution, and utilization of hydrogen. 13 Currently, hydrogen is manufactured in large quantities from steam reforming of natural gas; however, H2 from solar energy can be generated through a number of paths. 14 The conversion of biomass to
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Page 120H2, although fairly straightforward, has a low sunlight-to-hydrogen conversion efficiency, and any system designed to generate significant amounts of H2 must cover a rather large land area. Nonetheless, if the biomass is a waste by-product (such as wood chips or nutshells), then this is perhaps the least expensive of the renewable H2 generation technologies. Wind energy and photovoltaic systems coupled to electrolyzers are perhaps the most versatile of the approaches and are likely to be the major hydrogen producers of the future. These systems are commercially available, but only at high prices. Advanced photolysis systems combine the two separate steps of electrical generation and electrolysis into a single system. These direct conversion systems include photoelectrolysis and photobiological systems. These systems are based on the fact that visible light has sufficient energy to split water. 15 The coupling of hydrogen to the energy and transportation infrastructures requires the development and deployment of fuel cell technology. Fuel cells are devices that take chemical energy and, without combustion, convert the fuel directly to electricity. 16 Since there is no burning of the fuel, there is no generation of airborne pollutants; the only effluent is water. High-efficiency fuel cell technologies are well known; perhaps the best-known current application is the use of fuel cells to power the space shuttle. Fuel cells represent the most energy-efficient link between renewable-based fuels (hydrogen, methanol, ethanol, and other biofuels) and electricity. In a typical fuel cell, hydrogen and oxygen are supplied to individual electrodes separated by an ion-conducting layer. The electrochemical reactions produce electricity, heat, and water. Fuel cells are classified by the electrolyte used. These include the proton exchange membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), alkali fuel cell (AFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC). Solid-electrolyte fuel cells such as the SOFC and PEMFC have a number of advantages in comparison to other types. The solid electrolyte removes liquid electrolyte management problems, reducing or eliminating corrosion, leakage, pore flooding, and so forth. The high temperature used in the SOFC system (>650°C) promotes rapid kinetics and eliminates the need for precious metal catalyst, but places severe requirements on materials, as well as requiring thermal management. It also dictates an extended startup and shutdown period, virtually eliminating its use for intermittent power generation. PEM systems operate at lower temperatures (<90°C), but they require strict water management for membrane humidification and platinum or other precious metal catalysts to increase the kinetics. The strategic potential of fuel cell technology is enormous. Fuel cells integrate into renewable energy power packages and facilitate distributed generation. Biomass-fired fuel cells, where gasified biomass is fed into molten carbonate or solid-oxide fuel cells, produce a concentrated CO2 stream that if sequestered, actually reduces atmospheric CO2. Fuel cells can be integrated into building energy systems to provide both heat and electricity. Fuel cells are the key enabling technology for future transportation vehicles, providing high efficiency, high fuel economy (80-100 mpg [miles per gallon] equivalent), and zero emissions. 17 Fuel Cell Conundrum—The Current Fuel Infrastructure Won't Work For fuel cell vehicles to be acceptable to consumers there must be a readily accessible and reasonably priced fuel for refueling. However, a production and distribution system for a new fuel will not be built unless there is a known demand. The most versatile way to solve this problem is with electrolysis. Electrolysis uses electricity in an electrolyzer to break apart water into hydrogen and oxygen. Although current commercial electrolyzers are very expensive and used primarily in industry to produce high-purity H2, companies are working to bring the costs down and develop small inexpensive systems suitable for individual use.
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Page 121 The approach would be to provide a “home fueling appliance” (i.e., an electrolyzer) with each fuel cell vehicle. 18 The cost goal is about $1,500 per appliance and would be included in the vehicle cost. The consumer plugs the appliance into an outlet in the garage, and water is delivered via a garden hose. There would be no storage of hydrogen in the appliance; the consumer would connect the appliance up to the vehicle, which would receive its charge of hydrogen overnight. It should be recognized that with this approach, the consumer is purchasing most of her or his fuel up-front, with reoccurring charges from the electricity used to power the electrolyzer. Electrolyzers in the Infrastructure Using electrolyzers to provide hydrogen for fuel cell vehicles with our current fossil-based energy infrastructure actually doubles the CO2 produced per mile compared to commercial internal combustion engine technology 19 because of the low efficiency of the current electricity generation system and the efficiency of electrolysis. Therefore, this approach is viable only if there is a concurrent program for major introduction of renewable generation of electricity. Alternatively, small-scale reformers (100-1,000 cars per day) generating hydrogen from natural gas would cut emissions CO2 per mile by half. This approach may be viable for the short term, but it is not sustainable in the long term. Water Issues Water is already an issue for current fossil-fueled plant construction, especially in the arid western United States. Water is also a global issue. 20 It is apparent that generating hydrogen from water electrolysis will only exacerbate the issue. The conclusion is that water desalination plants will be necessary. If these desalination plants are placed in or near cities that currently have oil ports, then the existing liquid fuels distribution infrastructure could be used for “electrolysis-grade” water delivery. PATHWAY TO A RENEWABLE ENERGY INFRASTRUCTURE What would be the most reasonable approach to replacing our current energy infrastructure with a sustainable one? The author respectfully offers the following recommendations. Any renewable energy system that produces electrons should be connected to the grid in such a way as to directly reduce the CO2 emissions from current fossil energy generation and avoid construction of additional fossil fuel power plants. 19 PV comes very close to matching the early afternoon peak in energy use. Integrating PV into current buildings and future building designs should be strongly encouraged. Wind farms, being the least-cost renewable energy systems, should be encouraged with incentives and legislative mandates. There is some debate as to how much intermittent power the grid can accept, but even 10% would constitute a large amount of renewable energy and greatly reduce CO2 generation. Biomass-based power plants and parabolic-trough hybrid systems could integrate seamlessly into the current infrastructure, providing continuous power and lowered CO2 emissions. Fuel cell vehicles should be deployed with hydrogen as the on-board fuel. Hydrogen is the preferred fuel because it provides the greatest benefit in terms of fuel economy and emissions. 19 During the buildup of the renewable generation systems, research and development should be focused on energy storage technologies in the multimegawatt-hour to gigawatt-hour range. These would be deployed as the technology is proven.
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Page 122 Only minimal sequestration should be considered because it costs money and energy, and the money and energy are better spent deploying a sustainable energy infrastructure. RESEARCH ANDDEVELOPMENT ISSUES Because renewable energy technologies must cover an area for the collection of energy, the efficiency of every process is important. Efficiency also relates directly to costs. These efficiencies include the following: Efficiency of electrical generation (PV, wind, solar thermal, etc.) Efficiency of energy storage (hydrogen, flywheel, etc.) 1. For hydrogen: electrolysis efficiency and fuel cell efficiency 2. For flywheel: bearing losses Efficiency of utilization Efficiency of system coupling Silicon SolarCell Technology The PV industry has been very good at reducing costs; however, it is going to run up against a barrier in the cost of the silicon feedstock used to make solar cells. Low-cost, low-energy technologies must be developed that can take the raw material (quartz) and refine it into solar-grade silicon. Techniques are being developed to grow thin layers of silicon on various substrates to minimize the amount of silicon used in the manufacture of solar cells. Research is needed to determine ways to make thin-film silicon perform at high efficiencies and, in particular, how to mitigate the effect of grain boundaries. Thin-Film Solar Cell Technology For production in the range of about 30 GW per year the material availability with current technologies—particularly for the elements indium, gallium, tellurium, and germanium—would cause supply issues. Research is needed for the discovery and development of new thin-film semiconductors that will reduce or eliminate the necessary amount of these materials. Research is needed for the discovery and development of new thin-film semiconductors that will replace current toxic and heavy metals (cadmium, tellurium, lead) with nontoxic materials. In the meantime, in case they cannot be eliminated, research is needed on the recovery and recycling of these toxic materials. Work is needed on the replacement of toxic or explosive feedstock gases that are used in the manufacture of thin-film systems. Multijunction thin-film systems have to be developed for increased solar-to-electrical conversion efficiency. Continued work is necessary on the fundamental mechanism for the degradation of amorphous silicon devices. There may also be some stability issues with the other thin-film technologies. Wind Technical issues for improved turbine performance and lower costs include aerodynamics, structures and fatigue, advanced components, and wind characteristics.
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Page 123 Basic aerodynamics research for three-dimensional computer simulations of airflows is rarely used in the aircraft industry, so wind turbine researchers have to develop new methods and computer simulation models to deal with these issues. Research in computational fluid dynamics (CFD), which is a group of methods that deal with simulating airflows around, for example, rotor blades for wind turbines, is also needed. Materials engineering is needed for advanced components to improve performance and reduce hardware costs. Research into innovative generators and advanced controls, including power electronics, is needed. Other activities that must be conducted include developing an updated, comprehensive national database for utility and industry access and improving resource assessment and mapping techniques and wind forecasting. Solar-Thermal Research issues related to increasing the efficiency and decreasing the costs of solar-thermal technology include the following: Optical materials—durability, flexibility (easily applied to compound curvature surfaces), high reflectivity, easy cleanability, low cost Concentrators (heliostats and dishes)—low-cost drives, lightweight structures, high optical accuracy, flexible control systems, low-cost, innovative system concepts. Receivers—high-efficiency volumetric reactors, secondary concentrators Storage—high-temperature, low-cost storage concepts Electrolyzers and Fuel Cells—PEM Systems One cannot emphasize enough the necessity for increased research in electrocatalysts. Hydrogen will be one of the main components of a renewable energy infrastructure, and the major conversion technologies for hydrogen, namely electrolyzers and fuel cells, both involve electrocatalysts. Catalysts will be needed for the oxygen reaction for both water oxidation and oxygen reduction. Other research that is needed for electrolyzers and PEM fuel cells includes the following: More reactive catalysts for direct methanol (and perhaps ethanol) fuel cells must be developed. Ways are needed to reduce (or better substitute for) platinum (180 g per system now for a 50-kW(e) fuel cell). Better manufacturability of fuel cells is necessary; including. 1. better manufacture of bipolar plates (injection molded or stamped from metal stock) and, 2. better membranea (Nafion—fluorine chemistry). REFERENCES 1. To estimate the land area needed for photovoltaic panels, the following information was used (from R.L. Hulstrom, National Renewable Energy Laboratory internal report): Flatplate photovoltaic (PV) collector modules are typically placed such that they cover one-half of the available land; 1 m2 of PV requires 2 m2 of available land. Total U.S. 1997 annual electricity demand was about 3.2 × 1012 kWh. Average solar resource per year for southwest Nevada is 2,300
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Page 124kWh/m2 per year. If we assume 10% net plant efficiency (current technology), that solar resource would provide 230 kWh/m2 per year. Therefore the total area needed is ~ enlarge ~ which equals 2.78 × 1010 m2 of land area, or about 10,900 square miles (a square 104 miles on one side). A system efficiency of 15% would drop the area to 7,200 square miles. 2. Alsema, E.A., and E. Nieuwlaar. Energy Policy 28(14): 999-1010 ( 2000 ). 3. Oliver, M., and T. Jackson. Energy Policy 28(14): 1011-1021 ( 2000 ). 4. See http://www.windpower.dk/tour/env/enpaybk.htm. 5. See http://www.nrel.gov/ncpv/. 6. Bolton, J.R., S.J. Strickler, and J. S. Connolly. Nature 316: 495 ( 1985 ). 7. Kurtz, S.R., P. Faine, and J. M. Olson. J. Appl. Phys. 68: 1890 ( 1990 ). 8. See http://www.awea.org/faq/ and http://www.windpower.dk/core.htm. 9. See http://www.eren.doe.gov/csp/. 10. See http://www.eren.doe.gov/power/pdfs/solar_trough.pdf and H. Price and D. Kearney. A Pathway for Sustained Commercial Development and Deployment of Parabolic-Trough Technology, NICH Report No. TP-550-24748 ( 1999 ), found at http://www.nrel.gov/docs/fy99osti/24748.pdf 11. Hull, J.R., IEEE Spectrum 34(7): 20-25, ( 1997 ). 12. Hull, J.R., Superconductor Science & Technology 13(2): R1-15 ( 2000 ). 13. See http://www.eren.doe.gov/hydrogen. 14. Turner, J., Science 285: 629-792 ( 1999 ). 15. Khaselev, O., and J. Turner. Science 280: 425 ( 1998 ). 16. See http://education.lanl.gov/resources/fuelcells. 17. Thomas, C.E., B.D. James, and F.D. Lomax, Jr. Analysis of Residential Fuel Cell Systems and PNGV Fuel Cell Vehicles, in Proceedings of the 2000 Hydrogen Program Annual Review, found at http://www.eren.doe.gov/hydrogen/pdfs/28890mm.pdf. 18. Fairlie, M.J., and P.B. Scott. Filling up with Hydrogen 2000, in Proceedings of the 2000 Hydrogen Program Annual Review, found at http://www.eren.doe.gov/hydrogen/pdfs/28890z.pdf. 19. Thomas, C., B. James, F. Lomax, and I. Kuhn. Integrated Analysis of Hydrogen Passenger Vehicle Transportation Pathways, in Proceedings of the 1998 Hydrogen Program Annual Review, found at http://www.eren.doe.gov/hydrogen/pdfs/25315o.pdf. 20. Vorosmarty, C. J., P. Green, J. Salisbury, and R. Lammers. Science 289: 284 ( 2000 ). APPENDIX A The following is an example of a residential distributed energy system utilizing renewable generation and an electrolyzer-hydrogen storage-fuel cell combination for energy storage. The energy system for this home utilizes renewable energy generation (Solar cells on the roof of the homse) and hydrogen storage to provide continuous power. (See Figure 7.5.) The key to this house is the controller. The controller is connected to the Internet and monitors and controls all of the components. The components of this system include an electrolyzer that utilizes electricity to split water into hydrogen and oxygen, a hydrogen energy storage system, and a fuel cell to convert the stored hydrogen back into electricity. Using parameters set by the homeowner (similar to a thermostat), the controller maintains the state of charge for the hydrogen system and either purchases or sells power to the grid. If the sun is shining on the PV panels, electricity can be either used in the home, directed toward the electrolysis unit, or sold to the grid. At night, hydrogen is supplied to the fuel cell to generate electricity. Alternatively, if available, the controller could purchase off-peak power from the grid to operate the home and electrolyze water to generate additional hydrogen. The hydrogen would be stored in underground tanks, either as a pressurized gas, absorbed into advanced carbon nanotubes, or as a metal
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Page 125 ~ enlarge ~ FIGURE 7.5 Examples of a residential distributed energy system utilizing renewable generation and an electrolyzed/hydrogen storage/fuel cell combination for energy storage. hydride. In addition to providing energy storage, the hydrogen could also be used to fuel the homeowner's fuel cell vehicles. As an option, the homeowner could make power available to the electric utility. If the utility has a need for additional power, it would send out a query to the controller to negotiate a price and to determine the amount of power that would be available from the home. Power could come from either stored hydrogen via the fuel cell or PV electricity. While each home would not necessarily have a lot of power to sell, a large number of homes (a hundred thousand or more) would represent a very large energy storage and generation system. Communities of homes could act as energy generation and storage systems for local industry, the ultimate in distributed generation. DISCUSSION Dave Cole, Oak Ridge National Laboratory: I have two questions that pertain to silicon. First, on the issue of stability, could you elaborate on what you really mean by stability of the silicon-based compounds? Second, with regard to the thin film, at what degree of degradation of the thin film does it become inefficient or does the efficiency drop off so much that it can't function anymore? John Turner: In terms of silicon, there are two issues and I zipped through it so quickly that it has caused confusion. There is no issue of physical degradation of crystalline silicon; as long as you keep it dry, it lasts 30 years. The degradation is in the performance, and that has a stable, lasting effect. It is not
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Page 126a really well understood effect, but under light soaking, the system loses some of its efficiency in the first six months of operation. So, the films start out at 15% and then in six months they drop down to 10 or 12%. This particular degradation mechanism is somewhat understood, but how to control it is not. Dave Cole: Is moisture an issue in temperate zones where it is not always dry? Is water or humidity a factor? John Turner: Water is not an issue with current sealing technologies for single-crystal silicon cells. Dave Cole: Is that true for the amorphous silica also? John Turner: It is the same for amorphous silica. Amorphous silicon uses the same sealing technique. Dave Cole: What do you mean by “sealing technique”? John Turner: Typically, the silicon is deposited either on glass or on stainless steel and sealed with a plastic. The plastic is called ethoxybenzoic acid (EBA), but I've forgotten what that means now. It is a cover that seals everything up. It lasts about 20 years. The degradation of EBA is really what limits the lifetime of silicon solar cells. David Keith, Carnegie Mellon: I think I heard you say that the use of water in electrolysis to make hydrogen would lead to some water shortages. Is that correct? John Turner: It is going to be an issue if you only have water— David Keith: I don't get it. I mean about a meter of water a year falls in the typical temperate zones. That means on each hectare you get 104 square cubic meters of water per year. John Turner: Yes. David Keith: Yet the amount of water that you need to make hydrogen, assuming you— John Turner: Pretty small. David Keith: It is of order 10 cubic meters. So, there is a difference of 103. John Turner: Right, but it is the culmination that makes the difference. Water is used for food, people and industry. If you add hydrogen production to that, there may be a problem. If you take it by itself, you are absolutely right. David Keith: It is down by three orders of magnitude. John Turner: It is going to be an issue.
Representative terms from entire chapter: