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.
Convective (“Hydrothermal”) Systems
Hydrothermal (or hot water) resources arise when hot water and/or steam is formed in fractured or porous rock at shallow-to-moderate depths (100 m to 4.5 km) as a result of either the intrusion in the earth’s crust of molten magma from the planet’s interior or the deep circulation of water through a fault or fracture. High-temperature hydrothermal resources (with temperatures from 180C to over 350C) are usually heated by hot molten rock. Low-temperature resources (with temperatures from 100C to 180C) can be produced by either process.
More than 9,000 MW of the world’s power is generated from conventional geothermal reservoirs—a third of that from the U.S. alone. Sanyal believes that the U.S. harbors the potential to generate another 10,000 MW to 30,000 MW, but convective systems have been commercially exploited for decades already, he says, and their growth could be hampered by their limited distribution worldwide.
Enhanced Geothermal Systems
This technology is still experimental, and several challenges—such as creating a pervasively fractured large rock volume, securing commercial well productivity, minimizing cooling, and minimizing water loss—will need to be overcome before it could become commercially viable. But, because it offers the promise of worldwide distribution, it offers the most potential, Sanyal says.
Conductive Sedimentary Systems
Many sedimentary formations, including some that contain oil or gas, may be hot enough to serve as commercial geothermal reservoirs. Though no fracturing will be needed for this commercially unproven technology, it may require deeply drilled wells. Sanyal believes that this system could be commercially feasible if reservoir flow and capacity and temperature are high enough.
Oil and Gas Field Waters
Hot water produced with deep drilling for oil or gas or from depleted oil/gas wells is being used more and more. But though it poses few technical challenges, the power cost using this process may not always be attractive.
Geopressured geothermal resources consist of hot brine, saturated with methane, found in large, deep aquifers under high pressure. The water and methane are trapped in sedimentary formations at a depth of about 3 km to 6 km, and the temperature of the water is in the range of 90C to 200C. Three forms of energy can be obtained from geopressured resources: thermal energy, hydraulic energy from the high pressure, and chemical energy from burning the dissolved methane gas. The major region of geopressured reservoirs discovered to date is in the northern Gulf of Mexico.
The method consists of drilling a bore into a geopressured-geothermal reservoir, allowing the fluid within the reservoir to escape through the bore, and using the fluid to turn an electricity-generating turbine. The concept has not been commercially proven yet, though a demonstration has shown technical feasibility. Even so, it poses a variety of technical challenges to making power at a cost that is commercially viable, not to mention that its distribution is very restricted, Sanyal says.
Magma, the largest geothermal resource, is molten rock found at depths of 3 km to10 km and deeper. It has a temperature that ranges from 700C to 1,200C. The concept of using this heat source theorizes that thermal energy contained in magmatic systems could represent a huge potential resource of energy. In the U.S., for example, useful energy contained in molten and partially molten magma within the upper 10 km of Earth’s crust has been estimated at 5 to 50 x 10 to the 22 power J (50,000 to 500,000 quads). This technology is far from becoming commercially viably, however, says Sanyal. Not only is it extremely localized, but it also poses a host of technical challenges, including developing drilling and completion techniques as well as developing a technology for extracting heat from magma.