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