Report to Congress on Renewable Energy Resource Assessment Information for the United States

Selected Extracts

EXECUTIVE SUMMARY

Section 201 of the Energy Policy Act of 2005 (EPACT), also known as Public Law 109-58, directs the U.S. Department of Energy (DOE) to prepare a report to Congress presenting a detailed inventory of U.S. renewable resource assessment information for solar, wind, biomass, ocean (including tidal, wave, current, and thermal), geothermal, and hydropower energy resource technologies. Resource assessment products include reports, maps, in-situ measurement data, remotely sensed data, and modeled data that characterize the physical potential for renewable energy generation. This report reviews the status of available resource assessment products, along with related data and tools that integrate resource information with other factors influencing renewable energy development and considers how well these products meet user needs.

All regions of the United States have access to one or more renewable energy resources. Solar, wind, biomass, and hydropower resources are found in most states, while ocean and geothermal resources have a more limited distribution. Resource potential varies considerably by location due to climate, land-use, terrain, and other factors.

Although the type, amount, and regional distribution of resource information vary widely by resource category, resource assessment products are available for each renewable energy category. DOE and various public and private institutions have conducted these resource assessments. The solar, wind, and geothermal resource categories have resource assessment products available at national, regional, and site-specific scales, although gaps exist in certain areas. The biomass and hydropower categories have national resource products available, with more limited information at regional and site-specific scales. The ocean energy resource category has the least resource assessment information available. The differences in breadth and type of information are influenced by historical support from the federal government for category-specific resource assessment activities, their characteristics spatially and temporally, and the maturity of the program.

The results of these various resource assessments are difficult to compare because of differences in assumptions and methodologies. To date, a comprehensive, integrated resource-based study of national renewable energy potential exploring a variety of technical, economic, and market assumptions is not yet publicly available because of the difficulty of unifying assumptions for all geographic areas and technologies.

As required by EPACT, this report also provides a brief summary of information on other factors that influence renewable energy development. These factors include land use, availability of energy infrastructure, and proximity to energy infrastructure and load centers. Access to transmission lines is considered one of the largest barriers to renewable energy project
development.

3.2 Wind

3.2.1 Resource Characteristics

Large-scale winds result from the uneven heating of the Earth; the poles receive less solar energy than the equatorial regions, and land masses warm and cool more quickly than water bodies. The uneven heating creates temperature gradients in the atmosphere as air moves from cold (high pressure) to warm (low pressure) areas. In general, the stronger the pressure gradient, the harder the wind blows between the regions in order to equalize the pressure gradients.

The continental United States is characterized by a large-scale westerly flow (winds from the west) at higher levels of the atmosphere. A key component of the westerly flow is the jet stream. This fast moving westerly air is located about 6 to 10 km above the surface and steers weather systems across the country. Its influence is felt most strongly across the northern tier of states, but the entire continental United States is affected by the jet stream during the course of the year.

Regional wind climates in the United States are caused by the interplay of jet stream-induced storm tracks and meteorological factors that occur closer to the surface. An example of one of these meteorological factors is the low-level jet, a fast-moving southerly (winds from the south) flow situated only a few hundred meters above the surface, that occurs frequently across the Central Plains during the warm season. The presence of the low-level jet is one factor that produces the high wind resource in the central part of the United States. Terrain and local temperature gradients can significantly change the wind flow characteristics near the surface.

Terrain can amplify wind by constricting the area through which it flows, or block the wind flow to create a low resource shadow. Differential local heating patterns can also modify the wind resource, such as sea-breeze and mountain-valley flows. The wind resource at a specific site is also impacted by local ground cover. Large obstacles provide a friction surface that can significantly reduce the available resource.

The available wind resource is quite variable both horizontally along the ground and vertically. Small-scale effects from terrain and from local heating and cooling can significantly impact the resource intensity over very short distances. For example, increases in wind resource due to sea-breeze effects may degrade quickly (within a few hundred meters of shore) from land-cover friction. In general, wind resource increases with height above ground in the atmospheric layer used for energy production, but the rate of increase varies due to local terrain and meteorology.

The wind resource can have significant diurnal and seasonal variations. Figure 3-10 shows the wind speed measured at NREL's National Wind Technology Center (NWTC), and shows that the resource intensity can change rapidly over relatively short periods. The degree of variability depends on local site characteristics and season. Figure 3-11 shows the seasonal wind resource at six sites in northern Colorado. Offshore wind resources, moderated by the thermal cushioning of the surrounding water, generally display less diurnal variability, though inter-seasonal
differences can still be significant depending on the specific location.

Prevailing wind direction, frequency distributions of speed and direction, and wind shear (change of wind speed with height) are other parameters relevant to wind energy production. Wind speeds are expressed as meters per second (m/s) and wind power density values are expressed as Watts per square meter (W/m2).

3.2.2 Current Status of Wind Resource Assessments in the United States

Wind resource assessment maps and atlases are available nationally at varying horizontal resolution and heights above ground. These products are based on measurement and model-erived data. The majority of the resource data is oriented to land-based wind resource assessment, but some offshore wind resource maps are available. At the time of publication, the wind resource assessment products of the United States include:

• A national wind resource atlas completed by Pacific Northwest National Laboratory (PNNL) in 1987 (Elliott et al.). The wind assessment used empirical data analysis techniques to assess wind energy potential at a scale of 1/3º of longitude by 1/4º of latitude (approximately 25 km). This assessment identified the relative quality of wind resource for the various U.S. regions, and includes annual and seasonal wind resource estimates of power class at 50 m height above ground (Figure 3-12).

• In the last decade, updated state and regional wind resource assessment data and maps have been completed by federal, state, and commercial organizations for all or part of 41 states. An example for Colorado is shown in Figure 3-13. The methodologies and assumptions used to create these resource assessment products vary, but they have some general features in common. All have defined the wind resource at a finer spatial scale than the previous national assessment, varying from an estimate every 30 m to 1,000 m (most commonly 200 m). Most of these assessment products provide wind speed estimates at different heights above ground, including 30, 50, 70, 80, and 100 m (most commonly 50 m) and wind power density or class at 50 m. These assessments are primarily of land-based wind resources, but may include estimates of near-shore wind resource (5-10 nautical miles from shore) as well. Some of the assessments also include modeled estimates of diurnal, monthly, or seasonal wind speed at 50 m.

• A national map of 80 m wind speeds extrapolated from 10 m and weather balloon wind data for 1,587 surface stations and 97 offshore sounding locations was produced by Stanford University in 2003 and updated in 2004. The extrapolation was performed using a least-squares fit methodology for vertical wind profiles and was based on wind measurements taken in 2000. Although the updated land-based wind resource products included some offshore estimates, more comprehensive regional offshore wind resource products are being generated or planned for offshore areas extending 50 nautical miles from shore. Offshore assessments for Georgia, Texas, and Louisiana are currently being generated, and assessments are planned for additional regions, particularly the Atlantic and the Great Lakes. These products include estimates of wind speed at 10, 30, 50, 90, 150, and 300 m heights above ground and wind power density at 50 m above ground.

• Measurement data are available nationally from wind energy measurement programs (Kenetech data set, DOE Candidate Program, Cooperative Network for Renewable Resource Measurements [CONFRRM], Utility Wind Resource Assessment Program [UWRAP], and others - see Appendix A) and meteorological observations available through the National Climatic Data Center's (NCDC) Integrated Surface Hourly Observations data set. Several of the measurement programs are no longer collecting resource data, but the existing collections provide information across the United States at heights varying from 10 m to 50 m.

3.2.3 Regional Distribution

Many areas of good land-based and offshore wind resource potential exist throughout the United States. However, land-based resources are strongly influenced by terrain factors and regional wind climate. Broad areas of high quality wind resource are present in the Great Plains region of the United States. Strong wind resources are also present in the hilly and mountainous terrain of the Western, Mid-Atlantic, and New England States. Land-based coastal areas along the Pacific, North Atlantic, and Great Lakes regions also have good wind resources. The Southeast and Midwest have the lowest land-based wind resource potential, but may contain areas suitable for development, particularly at higher hub heights above ground.

Strong offshore wind resources are available along most of the U.S. shoreline. The strongest resource occurs offshore from the Pacific Northwest, Alaska, New England, Hawaii, and Mid-Atlantic States. The Southern Atlantic and Gulf of Mexico regions have good offshore wind resource potential, but less intense than the other coastal regions. Resource levels generally improve further offshore and at higher hub heights above sea level.