6
Renewable Energy

This chapter reviews the status of renewable resources as a source of usable energy. It describes the resource base, current renewables technologies, the prospects for technological advances, and related economic, environmental, and deployment issues. While the chapter’s focus is on renewables for the generation of electricity, it also includes short discussions of nonelectrical applications. The use of biomass to produce alternative liquid transportation fuels is not covered in this chapter but rather in Chapter 5.

CURRENT STATUS OF RENEWABLE ELECTRICITY

Generation of Renewable Electricity in the United States

Renewables currently account for a small fraction of total electricity generation. According to the U.S. Energy Information Agency (EIA, 2007), conventional hydropower is the largest source of renewable electricity in the United States. Representing about 71 percent of the electric power derived from renewable sources, hydropower generated 6 percent of the electricity—almost 250,000 GWh out of a total of 4.2 million GWh—produced by the electric power sector in 2007.1

The nonhydropower sources of renewable electricity together contributed 2.5 percent of the 2007 total. Within this group, biomass electricity generation (called

1

The electric power sector includes electricity utilities, independent power producers, and large commercial and industrial generators of electricity.



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6 Renewable Energy T his chapter reviews the status of renewable resources as a source of usable energy. It describes the resource base, current renewables technologies, the prospects for technological advances, and related economic, environ- mental, and deployment issues. While the chapter’s focus is on renewables for the generation of electricity, it also includes short discussions of nonelectrical applica- tions. The use of biomass to produce alternative liquid transportation fuels is not covered in this chapter but rather in Chapter 5. CURRENT STATUS OF RENEWABLE ELECTRICITY Generation of Renewable Electricity in the United States Renewables currently account for a small fraction of total electricity generation. According to the U.S. Energy Information Agency (EIA, 2007), conventional hydropower is the largest source of renewable electricity in the United States. Rep- resenting about 71 percent of the electric power derived from renewable sources, hydropower generated 6 percent of the electricity—almost 250,000 GWh out of a total of 4.2 million GWh—produced by the electric power sector in 2007.1 The nonhydropower sources of renewable electricity together contributed 2.5 percent of the 2007 total. Within this group, biomass electricity generation (called 1The electric power sector includes electricity utilities, independent power producers, and large commercial and industrial generators of electricity. 271

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272 America’s Energy Future “biopower”)2 is the largest source, having produced 55,000 GWh in 2007. Wind power and geothermal supplied 32,000 GWh and 14,800 GWh, respectively, dur- ing that year. Except for wind power, none of these sources has grown much since 1990 in terms of either total electric power production or generation capacity. The largest growth in the use of renewable resources for electricity genera- tion is currently in wind power and, to a lesser extent, in solar power. Wind power technology, having matured over the last two decades, now accounts for an increasing fraction of total electricity generation in the United States. Though wind power in 2007 represented less than 1 percent, it grew at a 15.5 percent compounded annual rate over the 1990–2007 period and at a 25.6 percent com- pounded annual growth rate between 1997 and 2007. Wind power supplied almost 6,000 GWh more in 2007 than it had the year before. According to the American Wind Energy Association, an additional 8,300 MW of capacity was added in 2008 (AWEA, 2009a), representing an additional yearly generation of 25,000 GWh assuming a 35 percent capacity factor.3 By the end of 2008, the overall economic downturn had caused financing for new wind power projects and orders for turbine components to slow, and layoffs began in the wind turbine manufacturing industry (AWEA, 2009a). Thus new capacity in 2009 recently looked to be considerably smaller than in 2008. However, AWEA (2009b) recently reported that 2.8 GW of new wind power generation capacity was installed in the first quarter of 2009. Further, analysis of the American Recovery and Reinvestment Act (ARRA) of 2009 shows that by 2012 wind power genera- tion will more than double what it would have been without the ARRA (Chu, 2009). Central-utility electricity generation from concentrating solar power (CSP) and photovoltaics (PV) combined was 600 GWh in 2007, just 0.01 percent of the U.S. total—a fraction that has been approximately constant since 1990. However, this estimate does not include contributions from residential and other small PV installations, which now account for the strongest growth in solar-derived electric- ity. Installations of solar PV in the United States have grown at a compounded annual growth rate of more than 40 percent from 2000 to 2005, with a genera- 2Biopower includes electricity generated from wood and wood wastes, municipal solid wastes, landfill gases, sludge wastes, and other biomass solids, liquids, and gases. 3The capacity factor is defined as the ratio (expressed as a percent) of the energy output of a plant to the energy that could be produced if the plant operated at its nameplate capacity.

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273 Renewable Energy tion capacity of almost 480 MW that, assuming a 15 percent capacity factor, pro- duces approximately 630 GWh. Current Policy Setting At present, electricity generation from non-hydropower renewable sources is gen- erally more expensive than from coal, natural gas, or nuclear power—the three leading U.S. options. Thus policies at the state and federal levels have provided the key incentives behind renewable sources’ recent penetration gains. One such policy is the renewables portfolio standard (RPS), which typically requires that a minimum percentage of the electricity produced or sold in a state be derived from some collection of eligible renewable technologies. Given that these RPSs have been developed at the state level, there are many different ver- sions of them. The policies differ by the sources of renewables included (some states specify conventional hydropower or biopower); by the form, timeline, and stringency of the numerical goals; and by whether the goals include separate tar- gets for particular renewable technologies. As of 2008, 27 states and the District of Columba had RPSs and another 6 states had related voluntary programs. Wiser and Barbose (2008) estimate that full compliance with these RPSs would result in an additional 60 GW of new renewables capacity by 2025. Assuming a 35 percent capacity factor, which means that the capacity produces electricity for approxi- mately 3070 hours per year, an additional 180,000 GWh from renewable sources would be generated. This is compared to the estimated total of 4.2 million GWh generated in 2007. Federal policies are also contributing to this era of strong growth in renewable-energy development. The major incentive, particularly for wind power, is the Federal Renewable Electricity Production Tax Credit (referred to simply as the PTC), which provides a $19 tax credit (adjusted for inflation) for every megawatt-hour (equivalent to 1.9¢/kWh) of electricity generated in the first 10 years of life of a private or investor-owned renewable electricity project brought on line through the end of 2008.4 Congress most recently extended the PTC and expanded incentives for 1 year in the Emergency Economic Stabilization Act of 2008 and the ARRA of 2009. These two bills together extend the PTC for wind through 2012 and the PTC for municipal solid waste, qualified hydropower, biomass, geothermal, and marine and hydrokinetic renewable-energy facilities 4After adjusting for inflation, the current PTC is 2.1¢/kWh.

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274 America’s Energy Future 160 140 Average Price of Wind Power without PTC 2005 Dollars per Megawatt-hour 120 100 Operating Cost of Natural Gas Combustion Turbine 80 Average Price of Wind Power with PTC 60 40 20 Operating Cost of Wholesale Price Range Natural Gas Combined Cycle for Flat Block of Power 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Year FIGURE 6.1 Impacts of the PTC on the price of wind power compared to costs for natural-gas-fired electricity. Source: Wiser, 2008. through 2013. Because of concerns that the current slowdown in business activ- ity will reduce the capabilities of projects to raise investment capital, the ARRA allows owners of nonsolar renewable-energy facilities to elect a 30 percent invest- ment tax credit rather than the PTC. Figure 6.1 shows the impact of the PTC on the price of wind power versus that of natural-gas-fired electricity, though it should be noted that other current electricity sources, such as coal, hydropower, and nuclear, have lower operating costs than do natural gas combined-cycle plants.

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275 Renewable Energy RESOURCE BASE Size of Resource Base The United States has significant renewable-energy resources. Indeed, taken col- lectively they are much larger than current or projected total domestic energy and electricity demands. But renewable resources are not evenly distributed spa- tially and temporally, and they tend to be diffuse compared to fossil and nuclear energy. Further, although the sheer size of the resource base is impressive, there are many technological, economic, and deployment-related constraints on using these sources on a large scale. The United States has significant wind energy resources in particular; Figure 6.2 shows their distribution across the country. The total estimated electri- Wind Resource Wind Power Wind Speed Wind Speed Power Potential Density at 50 m at 50 m at 50 m Class W/m2 m/s mph 2 Marginal 200–300 5.6–6.4 12.5–14.3 3 Fair 300–400 6.4–7.0 14.3–15.7 4 Good 400–500 7.0–7.5 15.7–16.8 5 Excellent 500–600 7.5–8.0 16.8–17.9 6 Outstanding 600–800 8.0–8.8 17.9–19.7 7 Superb 800–1600 8.8–11.1 19.7–24.8 FIGURE 6.2 U.S wind resource map showing various wind power classes. Areas shown in white have class 1 wind resources. Source: NREL, 2007a.

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276 America’s Energy Future cal energy potential for the continental U.S. wind resource in class 3 and higher wind-speed areas is 11 million GWh/yr (Elliott et al., 1991), greater than the 2007 electricity generation of about 4 million GWh. The 11 million GWh estimate was obtained from point-source measurements of the wind speed at a height of 50 meters (m); the actual value could differ substantially (Elliott et al., 1986). On the one hand, modern wind turbines can have hub heights of 80 m or greater, where larger wind energy resources are likely available. On the other hand, computer- model simulations of very-large-scale wind farm deployments have shown that an agglomeration of point-source wind speed data over large areas can significantly overestimate the actual wind energy resource (Baidya et al., 2004). Estimating the upper-bound limit for extraction of the resource at 20–25 percent of the energy in the wind field, and using the total domestic onshore wind electricity potential value of 11 million GWh, an upper bound for the annual extractable wind electric potential is perhaps 2–3 million GWh. This potential resource base is about half of the current electrical power use in the United States, and significant offshore wind energy resources also exist and increase the wind resource base considerably. The solar energy resource also is very large indeed. Taking solar insolation to be a representative midlatitude, day/night average value of 230 W/m2, in conjunc- tion with the area of the continental United States of 8 × 1012 m2, yields a yearly averaged and area averaged power-generation potential of 18.4 million GW. At 10 percent average conversion efficiency, this resource would therefore provide 1.6 billion GWh of electricity annually. For 10 percent conversion efficiency, coverage of 0.25 percent of the land of the continental United States would be required to generate the total 2007 domestic electrical generation value of 4 mil- lion GWh. However, the solar resource is very diffuse and, as shown in Figure 6.3, distributed unevenly across the country. Additionally, the various technologies for tapping solar energy utilize differ- ent aspects of sunlight. Because CSP, for example, can exploit only the focusable direct-beam portion of sunlight, highly favored sites are located almost exclu- sively in the Southwest. Further, because CSP can use only the direct-beam por- tion of sunlight, energy input to the CSP plants falls to zero in the presence of clouds. However, most designs today decouple energy collection from the power cycle through the use of thermal storage, and thus the power output of the CSP plant will not immediately fall to zero in the presence of clouds. A recent analy- sis, which identified lands having high average insolation (>6.75 kW/m2 per day) and excluded regions of such lands having a slope >1 percent or a small (<10 km2) continuous area, estimated that CSP could deliver an average of

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277 Renewable Energy kWh/m2/day >9.0 8.5–9.0 8.0–8.5 7.5–8.0 7.0–7.5 6.5–7.0 6.0–6.5 5.5–6.0 5.0–5.5 4.5–5.0 4.0–4.5 3.5–4.0 3.0–3.5 2.5–3.0 2.0–2.5 <2.0 FIGURE 6.3 Solar energy resources in the United States. Source: NREL, 2007b. 15–30 million GWh/yr of electrical energy, which is 4–7 times larger than the total U.S. supply (ASES, 2007). Flat-plate PV arrays can be distributed more widely than concentrated solar power systems because flat-plate systems effectively utilize both the diffuse and the direct-beam components of sunlight. Analyses of the total rooftop area that would be suitable for installation of PV systems have produced resource estimates ranging from 0.9–1.5 million GWh/yr (ASES, 2007) to 13–17.5 million GWh/yr (Chaudhari et al., 2004). Only a fraction of rooftops and other lands can be devel- oped economically at present for solar-based electricity generation, however; it is the economics of solar technologies, not the size of the potential resource, that

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278 America’s Energy Future significantly limit the ability of solar electricity alone to contribute substantially to electricity production. There are two components to the geothermal resource base: hydrothermal (water heated by Earth) that exists down to a depth of about 3 km, and enhanced geothermal systems (EGS) associated with low-permeability or low-porosity heated rocks at depths down to 10 km. There is some potential for expanding electricity production from the hydrothermal resources and thus affecting regional electricity generation—for example, a regional study of known hydrothermal resources in the western states found that 13 GW of electric power capacity exists in identified resources within this region (WGA, 2006)—but in general, the resources are too small to have a major overall impact on total electricity genera- tion in the United States. It is the heat stored in the low-permeability and/or low-porosity rocks at great depths that represents the much larger resource base. As noted in a recent Massachusetts Institute of Technology study, a much larger potential for energy exists with EGS resources (MIT, 2006). The estimated geothermal resource below the continental United States, defined as the total amount of heat trapped to 10 km depth, has been estimated to be in excess of 1.3 × 1025 J (MIT, 2006). This is more than 130,000 times the total 2005 U.S. energy consumption of 1.00 × 1020 J. However, beyond the total amount of potentially available energy, the rate of extraction of this energy is especially critical in assessing the actual practical potential of this energy source. The mean geothermal heat flux at Earth’s surface is on the order of 50 mW/m2, and in many areas, the geothermal heat flux is sig- nificantly less than this value. Given that the electrical generation efficiency from use of this relatively low-temperature heat in a steam turbine is about 15 percent, the extractable and sustainable electrical power density from the geothermal resource is on the order of 10 mW/m2. To provide substantial power, heat must be extracted at rates in excess of the natural geothermal heat flux (heat mining) in order to usefully tap sufficient geothermal resources. Indeed, the MIT report (2006) notes that some temperature drawdown should occur if EGS resources are to be used in their most efficient manner. The substantial technical challenges associated with tapping this resource are discussed later in this chapter. Other renewable resources, including conventional hydropower, hydro- kinetics (wave/tidal/current), and biomass, have significant resource bases, too. Because the conventional hydroelectric resource is generally accepted to be near its maximum utilization in the United States, further growth opportunities are relatively small. Regarding hydrokinetics, one study puts the size of the wave

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279 Renewable Energy energy resource for the East and West Coasts at more than 0.5 million GWh/yr (EPRI, 2005). This study also estimates the wave energy base in Alaska to be 1.3 million GWh/yr, though it is unclear whether such a resource could be fully exploited. EPRI (2005) put the capacity of the tidal energy resource at a 152 MW annual average, which corresponds to an annualized electrical energy production of 1300 GWh/yr. The biomass resource base is discussed in Chapter 5. Findings: Resource Base Solar and wind renewable resources offer significantly larger total energy and power potential than do other domestic renewable resources. Solar energy is capable, in principle, of providing many times the total U.S. electricity consump- tion, even assuming low conversion efficiency. The land-based wind resource also is capable of making a substantial contribution to meeting current U.S. electricity demand without stressing the resource base. For these reasons, solar and wind resources are emphasized, but other non-hydroelectric renewables can make signif- icant contributions to the electrical energy mix as well, at least in certain regions of the country. However, renewable resources are not distributed uniformly. Resources such as solar, wind, geothermal, tidal, wave, and biomass vary widely in space and time. Thus, the potential to derive a given percentage of electricity from renewable resources will vary from location to location. Awareness of such factors is important in developing effective policies at the state and federal level to promote the use of renewable resources for generation of electricity. RENEWABLE TECHNOLOGIES A renewable electricity-generation technology harnesses a naturally existing energy flux, such as wind, sun, or tides, and converts that flux into electricity. Such tech- nologies range from well-established wind turbines to pilot-plant hydrokinetic sys- tems to methods, such as those that exploit salinity and thermal ocean gradients, that are in the conceptualization or demonstration stages. Some of these tech- nologies produce power intermittently (technologies that rely on wind and solar resources), whereas others are capable of producing baseload power (technologies that rely on hydropower, biomass, and geothermal resources). Though renewable- electricity technologies show much variability, they do have several shared charac- teristics: (1) the largest proportions of costs, external energy needs, and material

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280 America’s Energy Future inputs occur during the manufacturing and installation stages; (2) there are no associated fuel costs (except for biomass-fueled electricity generation); (3) oppor- tunities for achieving economies of scale are greater at the manufacturing stage than at the generating site—larger-generation units do not necessarily reduce the average cost of electricity generation as much as they do for coal-fired or nuclear plants; and (4) renewable electricity technologies can be deployed in smaller incre- ments and come on line more rapidly. Technology Descriptions Wind Wind power uses a turbine and related components to convert the kinetic energy of moving air into electricity. A typical wind turbine assembly includes the rotor, controls, drive train (gearbox, generator, and power converter), other electronics (wiring, inverters, and controllers), and a tower. Each of these components has undergone significant development in the last 10 years, and the resulting modi- fications have been integrated into the latest turbine designs. Critical objectives for these and future improvements are to make it easier to integrate the wind power plants into the electrical system and to increase their capacity factors. Especially important has been the development of electronic controls that allow modern turbines to remain connected to the electricity grid during voltage dis- turbances and reduce the draw on the grid’s reactive power resources. Advances in computerized controls will allow more aspects of the turbine to be monitored, resulting in more efficient use and the potential to better target and deploy tech- nical upgrades. Along with advances in electronics have come improvements in wind turbine structures, allowing turbine size and generating capacity to grow. Based on the fact that wind speed increases with height and that energy-capture ability depends on the turbine’s rotor diameter, the most common turbines at present are three- bladed rotors with diameters of 70–80 m, mounted atop 60–80 m towers, that have a capacity of 1.5 MW. The rotor blade has gone through many generations of designs, using various types of materials and structures, to maximize its aero- dynamic performance without compromising stability. Wind power technologies are actively being deployed today, and no major technological breakthroughs are expected in the near future. However, evolution- ary modifications in various turbine components are expected to bring 30–40 percent improvement in cost-effectiveness (cost per kilowatt-hour) over the next

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281 Renewable Energy decade (Thresher et al., 2007). And while the turbine tower is not expected to get much taller, advances will likely occur in installing and maintaining these machines in difficult-to-reach locations. One possibility, for example, is self-erect- ing towers. In the future, turbine rotors will be made of advanced materials such as fiberglass, and they will have improved structural-aerodynamic designs, sophis- ticated controls, and higher speeds. By reducing the blade-soiling losses (e.g., through dust or insect buildup) and installing damage-tolerant sensors and robust control systems, reductions in energy loss and improvements in turbine availability can occur. In addition, drive trains will be modified to include fewer gear stages, medium- and low-speed generators, distributed gearbox topologies, permanent- magnet generators, and new circuit configurations. As shown in Table 6.1, these improvements will have significant impacts on annual wind energy production and capital costs over the next decade. It should be noted that future capital costs also will be greatly influenced by global supply and demand for wind turbines. Some of these issues are discussed in the section titled “Deployment Potential” later in this chapter, as well as in the report by the Panel on Electricity from Renewable Resources (NAS-NAE-NRC, 2009). Along with improvements in onshore wind-turbine designs, offshore wind- turbine technologies will soon be actively enhanced to take advantage of the abundant U.S. offshore wind-energy resources. The technologies associated with offshore wind turbines will face fundamentally different challenges, however, attributable to the difficulties of building and operating turbines in the ocean and installing and maintaining transmission lines underwater. Solar Photovoltaic Power When sunlight strikes the surface of a PV cell, some of the light’s photons are absorbed. This causes electrons to be released from the cell, which results in a current flow, namely, electricity. The two main PV technologies entail flat plates, which consist of crystalline silicon deposited on substrates, and concentrators, which typically involve lenses or reflectors that, together with tracking systems, focus the sunlight onto smaller and more efficient cells. Silicon is used to form semiconductors in PV cells by taking advantage of the conductivity imparted when impurities (“doping” elements) are introduced. Because the efficiency of these crystalline PV modules is only 12–18 percent, fur- ther development is required—not only to increase efficiency but also to lower production costs (DOE, 2007a).

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320 America’s Energy Future pact storage technologies, based on phase-change and thermochemical mecha- nisms (with higher storage density than water), and materials that replace the copper and low-iron glass used in today’s collectors. Reductions in the cost of manufacture, materials, assembly, and shipping weight may be possible through a shift from metal/glass components to integrated systems, such as those associ- ated with polymeric materials, that are manufactured using mass-production techniques. Geothermal Geothermal energy can be applied to a variety of end-uses, including agriculture (mainly greenhouse heating), aquaculture, industrial processes, and space heating and cooling of buildings. Direct-use geothermal taps heated groundwater, without a heat pump or power plant, for the heating of facilities, with the technology generally involv- ing resource temperatures between 38° and 150°C (Lindal, 1973). Current U.S. installed capacity of direct-use systems is 620 MWthermal(MWt).13 Municipalities and smaller communities provide district heating by circulating hot water from aquifers through a distribution pipeline to the points of use, though this applica- tion of geothermal energy remains modest, with systems in only seven states.14 The barriers to increased penetration of direct geothermal heating and cooling systems are the high initial investment costs and the challenges associated with locating and developing appropriate sites. The resource for direct heating is richest in the western states. Geothermal heat pumps have extended the use of geothermal energy into traditionally nongeothermal areas of the United States, mainly the midwestern and eastern states. A geothermal heat pump draws heat from the ground, groundwater, or surface water and discharges heat back to those media instead of into the air. The available land area and the soil and rock types at the installation site deter- mine the best solution. Ground-coupled heat pumps are the most common type used. The efficiency of the heat pump is inversely proportional to the temperature difference between the conditioned space and the heat source or heat sink. As a consequence, heating and cooling efficiencies are improved because ground tem- peratures remain relatively constant throughout the year. The coefficient of perfor- 13Geo-Heat Center, Oregon Institute of Technology; see geoheat.oit.edu. 14See geoheat.oit.edu/directuse/district.htm.

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321 Renewable Energy mance is 3.4–4.4,15 and the annual operating costs are 25–33 percent of compa- rable fossil-fuel heating costs. The electric heat pump is standard off-the-shelf equipment, with minor modi- fications to handle the heat transfer from geothermal fluids or soil. The heat-pump equipment is located indoors, which reduces maintenance costs, and the more sta- ble operating temperature and pressure of the compressor give it a longer life than in air heat pumps. The unique component is the heat-exchange interface with the soil or with the groundwater. Ground-coupled heat pumps use high-density poly- ethylene pipe buried either vertically or in horizontal trenches to exchange heat between a working fluid and the soil. Vertical loops cost more, but they provide access to more stable deep-soil temperatures and are the only option if land area is limited. Regulatory requirements for the bore holes vary across the country, with the primary regulatory issue being the potential for groundwater contamina- tion. This problem is addressed by grouting the bore hole; the most commonly used material (bentonite-based grout) reduces heat transfer to the soil, but more conductive grouts such as cement mixtures and bentonite/sand mixtures provide superior performance. Today, the United States has 700,000 installed units—with 8,400 MWt of capacity delivering about 7,200 GWh/yr16—and there are 1.5 million units world- wide. The rate of installation is estimated to be 10,000–50,000 units per year. One barrier to growth is the lack of sufficient infrastructure (i.e., trained designers and installers) and another is the high initial investment cost compared to conventional space-conditioning equipment. There are no major technical barriers to greater deployment. Biomass Burning wood to heat U.S. homes currently represents about 1 percent of fuel used for direct heating of buildings.17 One-half to two-thirds of residential wood com- bustion in the United States occurs in wood stoves, as opposed to fireplaces (Fine et al., 2004). Solid fuels include conventional wood logs, which may or may not 15See www.eia.doe.gov/cneaf/solar.renewables/page/heatpumps/heatpumps.html, Tables 3.3 and 3.4. The coefficient of performance is the ratio of heat output per unit of energy input. 16Geo-Heat Center, Oregon Institute of Technology; see geoheat.oit.edu. 17U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. 2008 Build- ings Energy Data Book, downloadable at buildingsdatabook.eere.energy.gov/. This figure does not include biomass that is used in electricity generation.

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322 America’s Energy Future have been harvested sustainably, and pellets. Advanced biomass-fuel appliances use pellets, which are produced by compressing woody material that may include waste wood and sawdust, agricultural wastes, wastepaper, and other organic materials. Some pellet-fuel appliances can also burn corn kernels, nutshells, and wood chips. Pellet stoves use electricity to run fans, controls, and pellet feeders. One of the concerns about solid-fuel combustion for home heating is air pol- lution. In areas where wood stoves are prevalent, wood smoke is a major source of fine particulates and gaseous pollutants, including nitrogen oxides, carbon monoxide, and organics. The mandatory smoke-emission limit set by the U.S. Environmental Protection Agency (EPA) for wood stoves is 7.5 grams of smoke per hour for noncatalytic stoves and 4.1 g/h for catalytic stoves.18 Modern noncat- alytic stoves have improved fireboxes to achieve high combustion efficiency. The most efficient wood-burning appliances also use catalytic converters to achieve nearly complete combustion of the feedstock and to reduce harmful emissions. Stoves are available with EPA-certified emissions as low as 1 g/h. Stoves require homeowner maintenance and catalyst replacement, however, to retain their high efficiencies and low emissions. In summary, modern solid-fuel stoves are efficient and clean compared to the fireplaces of the past. The economics of using a stove to combust biomass prod- ucts depends on the fuel being displaced and the distance from home to supplier. CONCLUSION A future characterized by a large penetration of renewable electricity represents a paradigm shift from the current electricity generation, transmission, and dis- tribution system. There are many reasons why renewable electricity represents such a shift, including the spatial distribution and intermittency of some renew- able resources, as well as issues related to greatly increasing the scale of deploy- ment. Wind and solar—two renewable-energy resources with the potential for large near-term growth in deployment—are intermittent resources that have some of their resource bases located far from demand centers. The transformations required to incorporate a significant penetration of additional renewables include transformation in ancillary capabilities, especially the expansion of transmission 18See www.epa.gov/woodstoves/basic.html.

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323 Renewable Energy and backup power resources, and deployment of technologies that improve grid intelligence and provide greater system flexibility. Further, supplying renewable resources on a scale that would make a major contribution to U.S. electricity generation would require vast investment in and deployment of manufacturing and human resources, as well as additional capital costs relative to those associ- ated with current generating technologies that have no controls on greenhouse gas emissions. The realization of such a future would require a predictable policy envi- ronment and sufficient financial resources. Nevertheless, the promise of renewable resources is that they offer significant potential for low-carbon generation of elec- tricity from domestic sources of energy that are much less vulnerable to fuel cost increases than are other electricity sources. Overall success thus depends on having technology, capital, and policy working together to enable renewable-electricity technologies to become a major contributor to America’s energy future. REFERENCES AEP (American Electric Power). 2007. Interstate transmission vision for wind integration. AEP white paper. Columbus, Ohio. ASES (American Solar Energy Society). 2007. Tracking Climate Change in the U.S.: Potential Carbon Emissions Reductions from Energy Efficiency and Renewable Energy by 2030. Washington, D.C. AWEA (American Wind Energy Association). 2008. 20 Percent Wind Energy Penetration in the United States: A Technical Analysis of the Energy Resource. Washington, D.C. AWEA. 2009a. Wind energy grows by record 8,300 MW in 2008 (January 27, 2009, press release). Washington, D.C. AWEA. 2009b. AWEA First Quarter 2009 Market Report. Washington, D.C. Baidya, Roy S., S.W. Pacala, and R.L. Walko. 2004. Can large wind farms affect local meteorology? Journal of Geophysical Research 109:D19101.1-D19101.6. Beaudry-Losique, J. 2007. Biomass R&D program and biomass-to-electricity. Presentation at the first meeting of the Panel on Electricity from Renewable Resources, Washington, D.C., September 18, 2007. Berry, J.E., M.R. Holland, P.R. Watkiss, R. Boyd, and W. Stephenson. 1998. Power Generation and the Environment: A UK Perspective. European Commission. June. Black & Veatch. 2007. 20 Percent Wind Energy Penetration in the United States: A Technical Analysis of the Energy Resource. Black & Veatch Project 144864. Prepared for the America Wind Energy Association, Walnut Creek, Calif.

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