December 1, 2009

The Future of Geothermal

Sonal Patel

Steam and water heated by Earth’s crust have long been used for cooking and bathing, but it was not until the early 20th century that geothermal energy was harnessed for industrial and commercial purposes. In 1904, electricity was first produced using geothermal steam at the vapor-dominated field in Larderello, Italy. Since that time, other hydrothermal developments—at The Geysers, Calif.; Wairakei, New Zealand; Cerro Prieto, Mexico; Reykjavik, Iceland; and in Indonesia and the Philippines—have led to an installed world electrical generating capacity of nearly 10,000 MWe and a direct-use, nonelectric capacity of more than 100,000 MWt at the beginning of the 21st century.

But, according to Dr. Subir K. Sanyal, president of the California-based geothermal consulting and services firm GeothermEx Inc., the conventional means of harnessing geothermal energy—by relying on finite hydrothermal (hot water) aquifers—does not technically qualify it as a renewable energy source. It could be—if the energy extraction rate did not exceed the natural heat loss rate from the earth’s surface, which is of the same order of magnitude (about 1026 J) as the worldwide energy consumption rate today. But the natural heat loss rate per unit area of the earth’s surface (on the order of 50 kW per square kilometer) is so low that commercial geothermal energy extraction is primarily “heat mining,” he explains. One way of ensuring that hydrothermally exploited resources would naturally be replenished—and therefore practically inexhaustible—is to operate a project for a typical life of 30 years, stop it, and resume operations in about a century.

The future of geothermal energy will be driven by six primary technologies, Sanyal believes, but each will pose its own challenges.