On January 16, 2006, the Energy Analysis Office (EAO) of the National Renewable Energy Laboratory (NREL) issued for the Office of Science a DRAFT analysis, for comment, of the technical potential for renewables. EAO's preliminary analysis included a summary table representing near-term and ultimate technical potential for renewable energy resources (economic and market considerations are not taken into account). The seven-page document is entitled "Near-Term Practical and Ultimate Technical Potential for Renewable Resources."

The representation for the near term potential is given in percentage of electric generation in the United States in 2020. Near-term potential is restricted by near-term challenges, such as infrastructure and reliability problems, electricity storage, and technological ability to use the resource. Nonetheless, the "near-term practical" potential of renewable resources as a percent of U.S. electricity generation in 2020 is estimated to be 99-124 percent, or - in terms of primary energy - as 47-55 quads/year (electricity only).

The ultimate technical potential is a compilation of previous estimates and calculations based on those estimates. While the analysis assumes some near-term challenges will be overcome, the ultimate potential does account for constraints on technologically insurmountable goals, such as generally accepted restrictions on offshore wind facility distance from shore (200 meters), and on drilling capability for enhanced geothermal systems (10 km of depth). The table suggests that the ultimate technical potential for renewable resources could be as much as 8,529 quads/year

The resulting estimates offer rough estimates of the potential contributions from renewable resources, not economically or market-feasible projections.

The text of the DRAFT paper reads as follows:

METHODOLOGY

Current Renewable Resource Use

Currently used renewable energy resources are drawn from a variety of sources. The current installed nameplate capacity total is a summation of verified, functioning electric-generation facilities (REPIS 2005). Delivered electricity is based on 2004 electricity production (EIA 2005a). For all the renewable electric technologies except biomass, primary energy required to produce electricity is calculated based on an average heat rate of 10,000 Btu/kWh for existing thermal power plants (EIA 2005b). For biomass, a measured heat rate for power plants, 9,000 btu/kwh, is used (EIA 2005b). For those renewable energy forms that also contribute to heat and fuels markets, total primary energy shown is larger than the thermal energy required to produce only electricity (EIA 2005a).

NEAR-TERM PRACTICAL POTENTIAL

The amount of electricity potentially produced by renewables is shown as a percentage of the total projected U.S. generation in 2020: 5,085 billion kWh (EIA 2005b).

BIOMASS

Biomass is the only renewable energy form cited that can be used as either electricity or fuel. Because we cannot predict the distribution of biomass use between electricity and fuel, we make two estimates. The first assumes 100 percent of biomass is used for electricity, and the second assumes that 100 percent is use for fuel. The baseline amount of energy for these is the same, because it is limited by physical availability of biomass. Perlack (2005) estimates 1.3 billion dry tons of biomass is possible with the use of non-food cropland and forestland in the long run. To determine the near-term potential the mid-range scenarios from Perlack (2005) to identify a near-term range of 593 million to 968 million dry tons. The biomass-to-energy conversion used is an average of energy from biomass types of just more than 12 million btus per ton (NREL 2005c). This range yielded a potential of between 8 and 13 quads of energy in the near term. To estimate the amount of electricity that can be generated from the range, we assume a power plant heat rate of 9,000 Btu/kWh (EIA 2005b). The result is 17-28 percent of total U.S. electric generation. Biomass as a fuel potential is expressed as a percentage of projected 2020 petroleum demand: 26 million barrels per day (EIA 2005b). Using 8-13 quads of available biomass energy, and a 49 percent fuel plant conversion efficiency, biomass could contribute 9-14 percent of the national petroleum demand in 2020.

GEOTHERMAL

Because of technology limitations, only hydrothermal energy is considered in the short term. In 1979, the United States Geological Survey (USGS) estimated that there were about 22 GW of discovered hydrothermal resources (USGS 1979). While this estimate is dated, there has been no authoritative study of the potential since that time. Using a 95 percent capacity factor (NREL 2005c), 22 GWs represents 2 quads of energy (or 4 percent of U.S. electric generation) in 2020.

HYDROELECTRIC

Full hydroelectric potential is 140 GW (Hall et al 2003), which would provide 9.4 percent of electric generation in 2020, assuming today's national average capacity factor of 0.39 (NREL 2005c). Assuming a 10,000 Btu/kWh power plant heat rate conversion, this is equal to about 5.0 quads of primary energy.

OCEAN

In the short term, the full potential of mechanical (wave, tidal, and current) electrical generation is assumed. This resource is estimated to have a full potential of 30 GW installed nameplate capacity. Assuming constant power and a power plant conversion heat rate of 10,000 Btu/kWh, this translates to 2.3 quads of primary energy (or 4.5 percent of the electric generation) projected for 2020.

SOLAR

For the near-term technical photovoltaic potential, it is assumed that there will be no storage for solar energy, and no PV generation will be wasted. This implies that none of the nighttime loads can be met by solar, and much of the load at dawn cannot be met (if PV capacity were sufficient to meet such loads, PV output at midday would exceed loads, wasting energy). These assumptions severely limit the impact of PV on the electric system. The PV impact would be even more limited if one also took into account the many conventional fossil and nuclear plants that must run all the time. In this case, the PV capacity would have to be even smaller to keep from wasting PV generation.

The near-term potential for concentrated solar power (CSP) is assumed to be the minimum of the projected in-state electrical load and the actual CSP resources in that state. In all cases, the projected state electrical load is the minimum. Therefore, the near-term CSP potential is the electric load of the state in which the CSP resource resides. In 2020, the projected load for states for CSP potential is expected to be 12 percent of the total U.S. generation, creating an upper bound for CSP electrical generation. Assuming a 10,000 Btu/kWh heat rate for power plants, the estimated primary energy to create this electricity is 6 quads/year.

WIND

The short-term wind potential is limited by grid reliability/stability concerns to be 20 percent of total generation [based on Wan and Parsons (1993) estimate of between 4 percent and 50 percent]. Assuming a power plant heat rate of 10,000 Btu/kWh, the primary energy equivalent is 10 quads.

ULTIMATE TECHNICAL POTENTIAL

Ultimate technical potential differs from the short-term potential by a set of general assumptions for each resource type and one more general assumption. The general assumption is that the electricity grid can adjust to the diverse electricity fed into it by adding storage, transmission, ancillary services, etc. Moreover, the ultimate assumptions do not limit the amount of renewable electricity as a function of total projected electricity demand. As with the short-term assumptions, economic and market constraints are not accounted for in this long-term technical potential.

BIOMASS

Biomass is the only renewable energy form cited that can be used as either electricity or fuel. Because we cannot predict the distribution of biomass use between electricity and fuel, we make no assumption regarding the differences between the use of biomass for electricity and biomass for fuel. The baseline amount of energy for these is the same, because it is limited by physical availability of biomass. Perlack (2005) estimates 1.3 billion dry tons of biomass is possible with the use of non-food cropland and forestland. The biomass-to-energy conversion used is an average of energy from biomass types of just more than 13 million btus per ton (NREL 2005c). The total energy potential for biomass is 17 quads. To estimate the amount of electricity that can be generated from 17 quads, we assume a power plant heat rate of 9,000 BTU/kWh.

GEOTHERMAL

The hydrothermal estimate includes approximately 72-127 GW of as yet-undiscovered resource (USGS 1979). The enhanced geothermal systems estimate is based on an estimate of 42 TW, which includes the entire potential heat source (Tester 1994).

HYDROELECTRIC

The ultimate potential is assumed to be the same as the near-term potential.

OCEAN

The ultimate potential estimate or ocean-based power expands the near-term potential to include power from ocean thermal energy of 0.11 TW (Sands 1980). The primary energy required for electricity generation, assuming a heat rate of 10,000 Btu/kWh, is 9 quads.

SOLAR

Unlike the near-term potential, the ultimate potentials for both PV and CSP are not assumed to be constrained by grid limitations, e.g., storage is assumed, transmission is assumed available, etc. For PV, the total resource potential (NREL 2003b) was restricted by excluding federal and sensitive lands, assuming only 30 percent of land area can be covered with PV, allowing only slopes that are less than 5 degrees, and requiring a minimum resources of 6 kwh/m2/day. This results in an ultimate technical potential of about 219 TW or 4,200 quads/year for PV systems, assuming a 22 percent capacity factor.

The CSP resource is restricted to areas with resource potential -- the southwestern United States. The potential reduces that amount of land that can be used for CSP by federal and sensitive lands, land with a slope greater than a 5 percent gradient, major urban areas and features, and parcels less than 5 km2 in area. The remaining area determined the technical potential for CSP, assuming 50 MW/km2 (Price et al 2003).

WIND

The ultimate wind potential is not limited to 20 percent for intermittency and grid stability reasons, as battery storage is assumed. Instead, wind potential is limited by appropriate land selection (exclusions for federal land, etc.) and technical feasibility. For onshore wind potential, using estimated future capacity factors (NREL 2005b), and assuming complete use of Class 3 winds and better, the result is 324 quads of primary energy from wind. For offshore wind, Class 5 and better with a distance between 5 and 200 nautical miles (nm) were assumed. Between 5-20 nautical miles, only one-third of wind energy in Class 5 and better is captured, between 20 and 50 nautical miles, two-thirds; and between 50 and 200 nautical miles, the entirety. Assuming future capacity factors, the potential for offshore wind primary energy is found to be 272 quads.

REFERENCES, DATA SOURCES, BACKGROUND MATERIAL

EIA 2005a - U.S. Department of Energy, Energy Information Administration. Annual Energy Review 2004. DOE/EIA 0384-2004, Washington, DC: U.S. Department of Energy

EIA 2005b - Assumptions to the Annual Energy Outlook 2005 with projections for 2025. Washington DC: U.S. Department of Energy

EPRI/DOE. 1997. Renewable Energy Technology Characterizations, TR-109496. Washington, D.C.: DOE

Hagerman, G., R. Bedard. 26-June-2005. "Ocean Kinetic Energy Resources in the United States and Canada." EnergyOceans 2005, Washington, D.C.

Hall, D., R. Hunt, K. Reeves, G. Carroll. 2003. Estimation of Economic Parameters of U.S. Hydropower Resources. Idaho National Engineering and Environmental Laboratory

Land and Water Fund of the Rockies. 2002. Renewable Energy Atlas of the West. The Hewlett Foundation and The Energy Foundation. Page 10. http://energyatlas.org.

Morse, F. 2004. Presentations: The Concentrating Solar Power Global Market Initiative (GMI) as a Result of Research and Development. Presented at the Renewables 2004 Conference.

NREL 2003a - National Renewable Energy Laboratory. Assessing the Potential for Renewable Energy on Public Lands. 95 pp.; NREL Report No. TP-550-33530; DOE/GO-102003-1704. Golden, CO: NREL

NREL 2003b - National Solar Photovoltaics (PV) Data. U.S. Data http://www.nrel.gov/gis/index_of_gis.html

NREL 2005a - Assessing the Potential for Renewable Energy on National Forest Systems Lands. 123 pp; NREL Report No. BK-71036759. Golden, CO: NREL

NREL 2005b - Potential Benefits of Federal energy Efficiency and Renewable Energy Programs: FY 2006 Budget Request NREL-TP 620-37931. Golden, CO: NREL

NREL 2005c - Power Technologies Energy Data Book. Golden, CO: NREL. URL: http://www.nrel.gov/analysis/power_databook

Perlack, R., Wright, L., Tuhollow, A., Graham, R., Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply, April 2005

Price, H.; Stafford, B.; Heimiller, D; Dahle, D. 2003. California Solar Power Detailed Technical Report for Southern California Edison. 95 pp.; NREL Report No. MP-710-35284

REPIS 2005 - Renewable Electric Plant Information System: http://www.nrel.gov/analysis/repis/

Sands, D. 1980. "Ocean thermal energy conversion programmatic environmental assessment." Proceedings of the 7th Ocean Energy Conference, Volume 1, Paper 4.1., Washington, D.C.: U.S. Department of Energy, Publication No. Conf-800633-Vol 1.

Tester, J.W., H.J. Herzog, Z. Chen, R.M. Potter, and M.G. Frank. 1994. Prospects for Universal Geothermal Energy from Heat Mining. Science & Global Security. Volume 5, pp.99-121

Thresher, R. (NREL). 2005. E-mail communication to Elizabeth Brown. October 14, 2005

TroughNet. 2005. TroughtNet CSP Projects Deployed Web page. http://www.eere.energy.gov/troughnet/deployed.html

USGS (United States Geological Survey) 1979, "Assessment of Geothermal Resources of the United States - 1978". Geological Survey Circular 790, Edited by L.J.P. Muffler, United States Department of the Interior.

Wan Y. and Parsons, B. 1993. "Factors Relevant to Utility Integration of Intermittent Renewable Technologies." NREL/TP-463-4953. National Renewable Energy Laboratory: Golden, CO. Page 49

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The DRAFT document had earlier been available for inspection at: <http://www.nrel.gov/analysis/tech_potential/pdfs/tech_potential_table.p df> but now appears to have been withdrawn. Comments on the draft had been requested to be sent to Elizabeth Brown in NREL's Energy Analysis Office at elizabeth_brown@nrel.gov; 303-384-7489.