The United States has a significant amount of renewable energy resources. This chapter details the resource base from wind, solar, geothermal, hydroelectric, and biomass sources of energy that could make a material contribution to the nation’s electricity supply. Discussion of this resource base sets the stage for the scenarios of renewable energy deployment in Chapter 7.
Most renewable electricity generation must be located near the source of the renewable energy flux1 being captured and converted into electricity. Hence, renewable energy sources are by nature local or regional, and those that may be unable to contribute significantly to total U.S. electricity generation could still contribute to a substantial share of the renewable-based electricity generated in regions where that specific type of renewable energy flux is abundant and well suited for development.
2007 BASELINE VALUES
In 2007 total U.S. electricity generation was 4.2 million GWh and peak generation capacity nationally was 998 GW (EIA, 2008); the average annual U.S. electric generation load in 2007 was thus approximately 480 GW. For reference, total U.S. primary energy consumption in 2007 was approximately 100 EJ. At approximately 35 percent generation efficiency, 42 EJ (corresponding to 11.7 million GWh at 100 percent generation efficiency) was used to provide the 4.2 million GWh of electricity generated in the United States in 2007.
According to a study done by Pacific Northwest National Laboratory, the total estimated electric energy potential of wind for the continental United States is 11 million GWh per year from regions rated as Class 3 and higher2 (Elliott et al., 1991)—a value greater than the 4.2 million GWh of electric energy generated in the United States in 2007. In energy units, 11 million GWh represents 40 EJ of energy, as compared to the 2007 domestic primary energy consumption of 100 EJ.
The domestic large-scale wind electric energy resource estimate of 11 million GWh is uncertain, however, and the actual wind resource could be higher or lower. One source of uncertainty is that the yearly wind electricity potential from the PNNL study was estimated from point-source measurements of the wind speed at a height of 50 m (Elliott et al., 1986). Modern wind turbines can have hub heights of 80 m or higher, where more wind energy resource is likely to be available. However, computer simulations of very-large-scale wind farm deployment show that an agglomeration of point-source wind speed data over large areas can significantly overestimate the actual wind energy resource base (Roy et al., 2004). Just as a large wind turbine will overshadow a wind turbine farther downwind, so a very extensive wind farm will also overshadow other wind farms downwind. Specifically, when the downwind length of the wind farm is comparable to, or larger than, the scale length of the atmosphere (approximately 50 km), then the point-source measurement extrapolation is no longer valid, and significantly overestimates the actual available wind energy resource (Keith et al., 2004).
Another consideration is that wind field deployment at levels needed to produce 5 million to 10 million GWh of electricity would entail extraction of a significant portion of the energy from the wind field of the continental United States for conversion into electric energy. Continental-scale simulations indicate that high levels of wind power extraction could, to various degrees, affect regional weather as well as climate. In addition to limiting the efficiency of large-scale wind farms, model calculations suggest that the extraction of wind energy from very-large-scale wind farms could have some measurable effect on weather and climate at the local or even continental and global scales (Roy et al., 2004; Keith et al., 2004).
More detailed meso-scale modeling and measurements are needed to clearly delineate the total U.S. extractable wind energy potential and the portion that can
be extracted without significant environmental impacts. Modeling activities are under way to determine the optimal distance between wind farms to minimize power loss (Frandsen et al., 2007). Assuming an estimated upper limit of 20 percent of the energy in the wind field for extraction, both regionally and on a continental scale, and a total U.S. onshore wind electricity value of 11 million GWh/yr, an upper value for the extractable wind electric potential would be about 2.2 million GWh/yr, equal to more than half of the electricity generated in 2007. This estimate assumes that large-scale wind farms are installed over all suitable Class 3 and higher wind speed areas in the continental United States, as mapped in Figure 2.1 (AWEA, 2007; DOE, 2008). The preceding analysis is limited to onshore wind energy resources.
Significant offshore wind energy resources also exist, and Europe has begun to develop its offshore resources. The available offshore wind capacity has been estimated at 907 GW for distances 5–50 nautical miles offshore (NREL, 2004a), which corresponds to 1.6 GWh/yr, assuming extraction of 20 percent of the energy in the wind field, i.e., almost 40 percent of 2007 U.S. electricity generation. The water at these locations varies from less than 30 meters to greater than 900 meters deep. Since a large percentage of the population lives along the coasts of the continental United States, offshore wind could be a renewable resource located close to population centers. These resources are also mapped in Figure 2.1 for the continental United States. Several states are now focusing wind development efforts on offshore wind resources, especially where onshore wind resources are well developed. However, offshore projects have been fraught with siting controversies, including the proposed development off Cape Cod, Massachusetts.
The solar energy resource is extremely large. Taking 230 W/m2 as a representative midlatitude, day/night average value for solar insolation3 and 8 × 1012 m2 as the area of the continental United States yields a yearly averaged, area-averaged, power generation potential of 1.84 million GW (Clean Edge, 2008). The solar resource thus provides annually to the continental United States the equivalent of
about 16 billion GWh of electric energy and, at a 10 percent average conversion efficiency, would therefore provide 1.6 billion GWh/yr of electricity. At a 10 percent conversion efficiency, coverage of 0.25 percent of the land area of the continental United States would be required to generate the 4.2 million GWh of electric energy generated domestically in 2007.
Solar Photovoltaic Power
Flat-plate photovoltaic (PV) arrays effectively use both direct and diffuse sunlight, thus enabling deployment over a larger geographic region than is possible with concentrated solar power. Although the yearly averaged total insolation varies significantly over the continental United States, the regional variation is approxi-
mately a factor of two, as shown in Figure 2.2. Estimates of the rooftop area suitable for installation of PV systems have been performed state-by-state for the whole United States. An analysis by the Energy Foundation and Navigant Consulting eliminated roofs on residences that were not generally facing southward and roofs that had too high a slope for routine installation of solar PV panels; considered the impacts of shading by trees, the presence of heating and air-conditioning units, and other obstacles on the remaining viable portion of the rooftops, but did not account for snow; and added suitable flat commercial building rooftop space to the total (Chaudhari et al., 2004). The analysis concluded that 22 percent of the available residential rooftop space, and 65 percent of commercial building rooftop space, was technically suitable for PV system installation. This total
rooftop area, along with state-by-state values for the average insolation, yielded a technical solar PV-based peak capacity of 1500–2000 GW at commercially available PV system conversion efficiencies of 10–15 percent. At an average 20 percent capacity factor, this peak-capacity value would thus result in the production of 13 million to 17.5 million GWh/yr of electric energy, still much larger than the 4.2 million GWh/yr of electricity generated in the United States in 2007. More conservative estimates indicate that existing suitable rooftop space could provide 0.9 million to 1.5 million GWh/yr of PV-generated electricity (ASES, 2007). Clearly, with some (or perhaps no) amount of land set-aside for flat-plate PV-based solar electricity generation beyond that already available in existing rooftop areas, flat-plate solar PV has the potential to supply significantly more electricity than was generated in 2008 in the United States.
Concentrating Solar Power
Concentrating solar power (CSP) systems can only use the focusable, direct beam portion of incident sunlight and are thus limited to favored sites, primarily in the Southwest, that have abundant direct normal solar radiation. Figure 2.3 shows that despite variations in radiation intensity in the Southwest, all six states there have attractively high levels of insolation. A recent analysis by the Western Governors’ Association identified suitable land area that has a high average insolation of more than 6.75 kWm–2day–1; it excluded land areas having a slope greater than 1 percent or a continuous area of smaller than 10 km2, and national parks, nature reserves, and urban areas (WGA, 2006a). The analysis concluded that the Southwest has a concentrated solar power electricity peak generation capacity of 7000 GW. With an average annual capacity factor of 25–50 percent for CSP, depending on the thermal storage used for a plant, this land area could theoretically produce 15–30 million GWh of electric energy per year, again significantly more than the 4.2 million GWh total U.S. electricity supply in 2007.4 Only a fraction of this land area at present could be developed economically for CSP-based electricity generation due to factors such as generation and transmission costs discussed in later chapters.
Geothermal energy exists as underground reservoirs of steam, hot water, and hot dry rocks in Earth’s crust. Hydrothermal (sometimes referred to as conventional geothermal) electric generating facilities use hot water or steam extracted from these reservoirs and supply this energy to turbines to generate electricity. For reference, according to the U.S. Geological Survey (USGS, 1979), thermal energy stored as hydrothermal resources ranges between 2,500 EJ (0.67 billion GWh) and 9,700 EJ (2.7 billion GWh).
A regional study of known geothermal resources in the western United States found that 13 GW of electric power capacity exists in 140 hydrothermal sites identified in the region (Figure 2.4; WGA, 2006b). Of these 13 GW, the Western Governors’ Association reported that 5.6 GW of capacity was considered viable for commercial development by 2015, which reflects the consensus of geothermal technology, development, and power-generating operations experts. Since hydrothermal facilities typically operate at 90 percent capacity during much of their operational life, the 13 GW from identified hydrothermal resources could provide up to 0.1 million GWh/yr of baseload electric energy. These same western states consumed slightly more than 1 million GWh/yr of electricity from 2000 through 2003 (WGA, 2006a). A nationwide assessment of the shallow hydrothermal resource base estimates an availability of 30 GW, with an additional 120 GW potential from unidentified hydrothermal resources that show no surface mani-
festations (NREL, 2006). The NREL study estimated that 10 GW could be developed by 2015.
Enhanced Geothermal Systems
Enhanced geothermal systems (EGSs) are engineered reservoirs created to extract heat from low-permeability and/or low-porosity geothermal resources, as defined by the Department of Energy. EGSs tap the vast heat resources available due to temperature gradients between the surface and depths of up to 10 km, as shown in the maps in Figure 2.5. The geothermal energy resource base located beneath the continental United States, defined as the total amount of heat trapped to 10 km depth, is estimated to be in excess of 13 million EJ (3.6 trillion GWh) (MIT, 2006). Figure 2.6 separates this heat content into a function of temperature and depth. The total heat stored is more than 130,000 times the total 2005 U.S. energy consumption of 106 EJ of energy. The extractable portion of this resource has been estimated at 200,000 EJ, i.e., about 2,000 times more than the primary energy consumed in the United States in 2005. At a conversion efficiency of 15 percent, a reasonable value in view of the typical ~200ºC temperature difference between the temperature of the resource and the ambient temperature at the surface, the extractable geothermal resource could then, in principle, provide 30,000 EJ of electric energy.
In addition to the total amount of available energy, the rate at which it is extracted is also important. The mean geothermal heat flux over land at Earth’s surface is approximately 60 mW/m2 and in many areas is significantly less. An efficiency of 15 percent is estimated for electricity generation from this relatively low temperature heat in a turbine. Thus, on average, the extractable electric power density from the geothermal resource on a renewable basis is about 10 mW/m2. At an extracted, producible electric power density of 10 mW/m2, 100 GW of electric power (22 percent of the 2005 average U.S. electric load and 10 percent of the 2005 U.S. electric generation capacity) would thus require a minimum land area footprint of 1 × 1013 m2.5 For comparison, the land area of the continental United States is 8 × 1012 m2, so the footprint needed to provide 20 percent of the 2005 average electric load from sustainably produced geothermal energy would exceed the total land area of the continental United States.
In practice, the in-place geothermal heat would be extracted at rates in excess
of the natural geothermal heat flux; such extraction rates are not sustainable in the long term, because they would deplete the heat more rapidly than it would be restored by the natural geothermal flux. Such heat mining would reduce the land area needed to be tapped by allowing heat extraction to exceed the 10 mW/m2 replacement rate. Indeed, a recent MIT report (2006) notes that some temperature drawdown should occur if such reservoirs are used most efficiently. In its analysis of the resource potential for EGS, the MIT report limited this heat mining by assuming that geothermal reservoirs would be abandoned when the temperature of the rocks fell by 10–15ºC. Because heat extraction may not be uniform, the MIT report assumes that reservoirs would have a lifetime of 30 years, with periodic re-drilling, fracturing, and hydraulic simulation. The report estimates that reservoirs should be able to recover to their original temperature conditions within
100 years after abandonment. It contends that if only 10 percent or less of the stored heat is mined at any time, enhanced geothermal energy could be considered a renewable resource, because the huge resource base would support abandoning reservoirs for the 100-year period needed to restore the original temperature.
Conventional hydroelectricity generation in 2007 provided 0.25 million GWh. Hydroelectric generation capacity was 98 GW, representing about 9 percent of the total U.S. electric generation capacity (EIA, 2009).
Because use of the conventional hydroelectric resource is generally accepted to be near the resource base’s maximum capacity in the United States, further growth will largely depend on non-conventional hydropower resources such as low-head power6 and on microhydroelectric generation.7 A 2004 DOE study of total U.S. water-flow-based energy resources, with emphasis on low-head/low-power resources, indicated that the total U.S. domestic hydropower resource capacity was 170 GW of electric power, of which 21 GW was from low-head/low-power, 26 GW was from high-head/low-power, and 123 GW was from high-head/high-power (DOE, 2004). These numbers represent only the identified resource base that was undeveloped and was not excluded from development. A subsequent study assessed this identified resource base for feasibility of development (DOE, 2006). After taking into consideration local land-use policies, local environmental concerns, site accessibility, and power transmission, the total potential domestic hydroelectric resource capacity was estimated to be 100 GW of electric power. This value was reduced to 30 GW of potential hydroelectric capacity after applying development criteria (DOE, 2006). A report from the Electric Power Research Institute (EPRI) determined that 10 GW of additional hydroelectric resource capacity could be developed by 2025 (EPRI, 2007). Of the 10 GW of potential capacity, 2.3 GW would result from capacity gains at existing hydroelectric facilities, 2.7 GW would come from small and low-power conventional hydropower
facilities, and 5 GW would come from new hydropower generation at existing non-powered dams.
Hydrokinetic Power—Wave, Tide, and River Energy
Hydrokinetic energy is the energy associated with the flow of water, such as wave energy and the energy in water currents, including tides and rivers. As shown in Table 2.1, there is significant interest in developing such energy resources, based on permits filed with the Federal Energy Regulatory Commission. Permit activity is not a reliable predictor of future development of hydrokinetic resources, however, because often developers will apply before planning the facility or obtaining financing.
According to an EPRI report that assessed total U.S. wave energy potential (EPRI, 2005), all the wave energy in the coastal states of Washington and California combined could produce 0.44 million GWh/yr, and the wave energy from the Maine, New Hampshire, Massachusetts, Rhode Island, New York, and New Jersey coasts combined could produce 0.12 million GWh/yr (Figure 2.7). These values should be reduced by 10–15 percent to account for generation losses, resulting in a total electric generation potential of about 0.07 million GWh from the entire continental U.S. wave energy resource. Exhaustive use of the entire wave energy resource would therefore be required to produce less than 2 percent of the 4.2 million GWh of the electricity generated in the United States in 2007.
The largest U.S. wave resource lies off southern Alaska, which has an estimated resource base of 1.25 million GWh/yr, as shown in Figure 2.7 (EPRI, 2005). Extraction of this total amount of energy would involve tapping wave energy flows over relatively large areas of ocean, and the EPRI report also does
TABLE 2.1 Permit Activity for Hydrokinetic Resources (in megawatts of proposed capacity)
Source: Federal Energy Regulatory Commission; presented in Miles, 2008.
not indicate how the electric energy over such a large area of the ocean would be collected or transmitted to consumers in the lower 48 states.
The 2005 EPRI study also looked at tidal energy from a series of sites identified in Alaska, Washington, California, Massachusetts, Maine, New Brunswick, and Nova Scotia (EPRI, 2005). The total combined resource was estimated to have an annual average electric capacity potential of 152 MW, which corresponds to an annualized electric energy production of 1300 GWh/yr (EPRI, 2005)—enough to provide, if the stated resource were used in whole, 0.03 percent of the 2005 domestic generated electric energy.
EPRI’s study of the electric energy potential in river currents yielded a value of 0.11 million GWh/year (EPRI, 2005). Thus, development of the entire U.S. river current electricity potential would be required to produce 0.1 million GWh/yr, which would represent less than 3 percent of the 2005 domestic electric energy production.
Biomass Resource Base
The USDA/DOE billion-ton study (2005) identified the potential for use of 1.3 billion dry tons (1 dry ton = 1,000 kg) per year of biomass without adversely affecting food production. The area involved in producing this resource base comprises 448 million acres (1.8 × 1012 m2) of agricultural land (consisting of both cropland and pasture), which is 23 percent of the land area of the continental United States, and 672 million acres of forestland (2.7 × 1012 m2), representing 34 percent of the land area of the continental United States (USDA/DOE, 2005). Agricultural land totaled 455 million acres in 1997, the year of the most recent complete inventory of land use. Hence, the total land area assumed to be used for such biomass farms is just over 57 percent of the total land area of the lower 48 states.
The amount of biomass sustainably removed from domestic agricultural lands and forestlands is 190 million dry tons annually, with about 142 million dry tons coming from forestland and the remainder coming from croplands. Only about 20 percent of this biomass is now in use. The USDA/DOE report projected that approximately 370 billion tons (double the present biomass production) could be made available sustainably for biomass uses from 672 million acres of forestland. To accomplish this would require a variety of methods, including using wood for electric power generation instead of burning that wood for forest management (as is done at the present time), using pulp residues, and logging residues.
The USDA/DOE report also projected that agricultural lands (cropland, idle cropland, and cropland pasture), which produce approximately 50 million tons per year for biomass uses, have the potential, within 35 to 40 years, to yield nearly 1 billion dry tons of biomass. This represents a 20-fold increase in the sustainable biomass yield relative to the present value. Of this projected 1 billion dry tons that might be available in 35–40 years, 300–400 million tons would come from crop residues and 350 million tons would result from the substitution of high-yield perennial biomass crops for other land uses on at least 40 million acres of land.
The geographical distribution of the biomass resource base shown in Figure 2.8 comes from Milbrandt (2005), which estimated a lower overall biomass resource base than does the USDA/DOE report. This is because the billion-ton study estimated future potential biomass resources in the country, while Milbrandt evaluated currently available biomass resources (though it considers a case study
of switchgrass on Conservation Reserve Program lands). The resource assessment is performed at a county level and it includes (1) residues from agriculture and forestry, (2) urban wood (secondary mill residues, MSW wood, utility tree trimming, and construction/demolition wood), and (3) methane emissions from manure management, landfills, and domestic wastewater treatment.
According to the USDA/DOE study, providing 1.3 billion dry tons per year of biomass would require increasing the yields of corn, wheat, and other small grains by 50 percent; doubling residue-to-grain ratios for soybeans; developing more efficient residue-harvesting equipment; managing cropland with no-till cultivation; growing perennial crops whose output is primarily dedicated to energy purposes on 55 million acres of cropland, idle cropland, and cropland pasture; using animal manure in excess of what can be applied on-farm for soil improve-
ment; and using a larger fraction of other secondary and tertiary residues for biomass production. Attaining these levels of crop yield increases and collection would require research and new technologies such as genetic engineering to increase production. The ~50 million acres devoted to high-yield perennials were projected to have an average annual crop yield of approximately 8 dry tons/acre, in order to provide ~400 million dry tons of biomass annually from that portion of land. Supporting the billion-ton estimate was the assumption that agricultural lands in the United States could potentially provide in excess of 1 billion dry tons of sustainably collectable biomass, while continuing to meet food feed and export demands. This estimate included 446 million dry tons of crop residues (for example, more than 250 million tons from corn stover, as compared to the present value of 75 million tons annually), 377 million dry tons of perennial crops,8 87 million dry tons of grains used for biofuels, and 87 million dry tons of animal manure, process residues, and other residues generated in the consumption of food products.
The forthcoming report of the America’s Energy Future (AEF) Panel on Alternative Liquid Transportation Fuels (see Appendix A) provides another estimate of the biomass resource base (NAS-NAE-NRC, 2009). It estimates that an annual supply of 400 million dry tons of cellulosic biomass could be produced sustainably, using technologies and management practices available in 2008, an amount that could likely be increased to about 550 million dry tons by 2020. The AEF alternative liquid fuels panel judges that those estimated quantities of biomass can be produced from dedicated energy crops, agricultural and forestry residues, and municipal solid waste with minimal impacts on U.S. food, feed, and fiber production, and with minimal adverse environmental impacts. The AEF alternative liquid fuels panel did not extend its estimate to 2035, as did the 2005 USDA/DOE report.
Electricity Generation from Biomass
Based on 2005 biomass production levels, full use of the 190 million dry tons of sustainable biomass produced in the United States, at 17 GJ (1 GJ = 1 × 109 J)/dry ton, and at 35 percent efficiency for conversion of the heat produced from bio-
mass combustion into electric energy, would provide energy of 1.1 EJ.9 In other words, 100 percent of the sustainable biomass produced domestically in 2005, if used entirely for electricity generation, would produce 0.306 million GWh/yr of electricity, or 7.3 percent of the 2007 domestic electricity generation of 4.2 million GWh/yr. Using the AEF alternative liquid fuels panel’s more recent resource average value of ~500 million tons of biomass (NAS-NAE-NRC, 2009), a total of 0.8 million GWh/yr of electricity could be produced, which is 19 percent of 2007 U.S. electricity generation.
Increasing the available biomass production to 1 billion tons and using it solely for electricity generation would produce 6 EJ, which is equal to 1.6 million GWh/yr of electricity, representing approximately 40 percent of the domestic electric generation in 2007. If, however, 75 percent of this biomass was used to produce cellulosic ethanol or other biofuels, then only 25 percent of the biomass would be available for electricity generation. Thus, 250 million tons of biomass, projected as potentially available in 35–40 years through the use of more than 60 percent of the land area of the continental United States, would be capable of producing 0.416 million GWh of electricity, or 10 percent of the 2007 U.S. electricity generation. This potential represents more than 7 times the actual electric generation from biomass in 2005 (0.054 million GWh, which accounted for just above 1 percent of the 2007 U.S. electricity generation).
Shown below in bold text are the most critical elements of the findings of the AEF Panel on Electricity from Renewable Resources, based on its consideration of the U.S. resource base for generation of renewable electricity.
In summary, the United States has significant renewable energy resources, which, combined, have the potential, in principle, to provide more electric power than the total existing installed peak capacity and more electric energy annually than the total electricity consumed domestically in 2005. This resource base is spread widely across the United States. However, as described in the remainder of this report, many other factors will determine what portion of these resources will actually be incorporated into the electricity system; some of these factors include
the costs of technologies needed to transform these resources into electricity; the expanded capacity and associated costs for transmission to bring this electricity into load centers; and the need to compensate for intermittency.
Solar and wind renewable resources offer significantly larger total energy and power potential than do other domestic renewable resources. Although solar intensity varies across the nation, the land-based solar resource provides a yearly average of more than 5 × 1022 J (13.9 million TWh) and thus exceeds, by several thousand-fold, present annual U.S. electrical energy demand, which totals 1.4 × 1019 J (~4,000 TWh). Hence, at even modest conversion efficiency, solar energy is capable, in principle, of providing enormous amounts of electricity without stress to the resource base. The land-based wind resource is capable of providing at least 10–20 percent, and in some regions potentially higher percentages, of current electrical energy demand. Other (non-hydroelectric) renewable resources can contribute significantly to the electrical energy mix in some regions of the country.
Renewable resources are not distributed uniformly in the United States. 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 levels to promote the use of renewable resources for generation of electricity.
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