
271Renewable Electricity
Estimates of the U.S. hydrothermal resource are difcult to estimate pre-
cisely due to geology, hydrology, and other uncertainties. The DOE has esti-
mated known resources at 9 GW and undiscovered resources at 30 GW (U.S.
DOE 2011). Although these are relatively small quantities, a much larger
resource is available in what is termed “enhanced geothermal systems”
(EGSs). EGSs require that hot underground rock structures be fractured and
water circulated through and recovered from those fractures to produce
steam-driven power. The DOE (2011) estimates these resources could be as
large as 518 GW of electric capacity.
As shown ...