Geothermal power technology

In 1999, 8 GW/sub e/ of electrical generating plant worldwide was powered by geothermal heat. Geothermal sources generated 49 terawatt hours of virtually pollution-free power that year. Coal- or oil-fired plants generating this amount would discharge /spl sim/40 million tons per year of carbon dioxide into the atmosphere. Geothermal capacity has doubled in the past 20 years, and increased 17% in the past five years. The United States and the Philippines account for 50% of that installed capacity worldwide. The thermal energy stored in the upper 10 km of the Earth's crust is vast. For example, below the land surface of the US, it is estimated to be 10/sup 6/ times annual energy demand for the country, but most of it is not economically accessible. The only thermal energy currently economic to extract is from sites where temperatures above 200/spl deg/C are easily accessible by conventional drilling, and reservoirs of hot water or steam exist in the subsurface. To unlock a significant fraction of this vast energy source major technological advances are needed. These include improvements in drilling technology to lower the cost of wells, in fracture stimulation technology to create and control permeability in tight rocks, and in geophysical and chemical tracer technology to characterize thermal and hydraulic regimes in the subsurface. This will take a coordinated effort by government and industry over one or two decades.