This paper first describes the salient features of the various types of geothermal energy resources potentially available for exploitation: (1) convective (“hydrothermal”) systems, (2) enhanced geothermal systems (“EGS”), (3) conductive sedimentary systems, (4) produced water from oil and gas fields, (5) geopressured systems, and (6) magma energy. Of these six types, only hydrothermal systems have been commercially exploited to date; there are still technical or economic barriers to exploiting the others. In the U.S., the resource base in EGS is two orders of magnitude higher than in the other types combined; the same is likely to be true for the world. The paper then considers how long geothermal energy can supply the U.S. and the world.

It can be argued that commercial geothermal energy exploitation is primarily a heat mining operation rather than tapping an instantly renewable energy source, such as, solar, wind or biomass energy. At the current annual energy consumption rate, geothermal heat mining can theoretically supply the world for several millennia. If a commercial geothermal exploitation project is operated for a typical life of 20 to 30 years and then shut down, the resource would be naturally replenished and available for exploitation again in about a century. With such a scheme, a geothermal project could be made entirely renewable, and therefore, practically inexhaustible. This situation is comparable to sustainable forestry.

Finally, the paper considers the potential rate of growth in installed geothermal power capacity. It is concluded that between years 2010 and 2050, geothermal power capacity in the world would increase from 10,000 MW to perhaps as high as 58,000 MW. The rate of growth in power capacity could be higher if adequate commercial incentives are offered by governments and international agencies.