Geothermal Energy. Heat Flows and Geothermal Potential

Earth's heat is the only major nonsolar energy flux on our planet. Three distinct sources heat the Earth's surface from below: heat conducted through the lithosphere from the underlying hot mantle (its temperatures, at the core-mantle boundary, are as high as 4000° K); radiogenic decay of long-lived ⁴⁰K, ²³²Th, and ²³⁵U and ²³⁸U isotopes in the crust, and heat transported convectively by magmas and fluids.

We are still uncertain about the shares of the total geothermal flux originating in these processes, but numerous measurements of heat flows have made it possible to map their continental and oceanic patterns. As expected, average flows peak along mid-ocean ridges where new oceanic lithosphere is continually created by hot magma that rises from the mantle.

Heat Flows and Geothermal Potential. Heat flux associated with this process includes the latent heat of crystallization of newly formed and cooling basaltic ocean crust, and the heat of cooling from magmatic temperatures (around 1200° C) to hydrothermal temperatures (around 350° C). Total hydrothermal flux of 7-11 terawatts is about a third of the global oceanic heat flow of 32 terawatts; this, in turn, equals roughly 70 percent of the global heat flux of 44 terawatts.

Almost a third of the total oceanic heat loss takes place in the South Pacific and its rates decline with the age of ocean floor, from as much as 250 megawatts per square meter in the youngest crust to less than 50 through the sea floor older than 100 million years. Planetary heat flux of 44 terawatts prorates to about 85 megawatts per square meter (equal to a mere 0.05 percent of solar radiation reaching the Earth's surface), with the means of almost 100 megawatts per square meter for the oceans and only about half as much for the continents.

Continental heat flows range from 41 megawatts per square meter in Archean rocks to 49-55 in Phanerozoic formations. But unlike with solar radiation, this flux is available everywhere all the time. As a result of plate tectonics and hot spot phenomena, many places around the Earth have a much more intensive heat flow through the crust, often delivered by hot water or steam and suitable for commercial extraction of geothermal energy.

Total geothermal energy stored within the crust is at least five orders of magnitude larger than the annual heat flux, but estimates of its extractable potential depend on the category of the considered resources and on temperature and depth limits.

Hot magma that intrudes into the top 10 kilometers of the crust in many regions around the world contains an order of magnitude more of heat than do hydrothermal flows, and hot dry rocks within the same depth have an order of magnitude more heat than does the near-surface magma. However, these resources could be tapped only through drilling and injections of liquids to recover the heat.

But while drilling to depths of more than 7 kilometers to reach rock temperatures in excess of 200° C (average thermal gradient is 25° C per kilometer) is now possible it would be economically prohibitive to do so in order to inject water for steam generation.

Consequently, only those geothermal resources that are dominated by vapors and liquids can be used directly for electricity generation, but these flows reach the Earth's surface, or are accessible through relatively shallow drilling, at only a limited number of locations. Hydrothermal resources with temperatures below 100° C can supply hot water for various industrial processes or for household heating.

Reykjavik, Iceland is the best example of such application of hydrothermal resources; nearly a 11 of its houses are heated by hot water, which is also used in many outdoor swimming pools, greenhouses, aquaculture, electricity generation, and for melting snow and de-icing sidewalks. Accessible flows of pressurized water and water vapor with temperatures above 100° C add up to only tiny fractions of the enormous global geothermal potential.

A recent estimate puts their total at about 72 gigawatts of electricity-generating capacity, and enhanced recovery and drilling improvements could raise this to 138 gigawatts. The largest shares of this accessible geothermal potential are along the tectonically active Pacific margins of the North, Central and South America and Asia but significant geothermal sites are found also in interiors of most continents (United States (Wyoming), Czech Republic, Hungary, Tibet).

Geothermal Electricity Generation. The world's first geothermal electricity generation began at Italy's Larderello (in Toscana) field in 1902. New Zealand's Wairakei was added in 1958, Geysers in the northern California came on line in 1960, and Mexico's Cerro Prieto in 1970. All of these fields tapped high-temperature vapor that could be used directly for electricity generation.

Post-1970 diffusion of geothermal generation resulted in construction of new capacities in about a dozen countries. At the beginning of the twenty-first century the United States had the highest installed capacity (nearly 2.9 gigawatts), followed by the Philippines (1.8), Italy (768 megawatts), Mexico, and Indonesia.

Global geothermal total of 8.2 gigawatts is no more than 0.25 percent of the world's installed capacity (dominated by fossil-fueled power plants) and it is equal to only about 11 percent of the geothermal energy that could be harnessed with existing techniques. Even if we were to develop the prospective potential of 138 gigawatts, geothermal electricity would represent less than 5 percent of the world's total generating capacity.

This means that while the geothermal energy has a considerable scope for expansion, it cannot supply a significant share of the world's energy generation during the coming decades. But geothermal electricity can supply nationally and locally important shares and geothermal heat can make an even greater contribution in distributed supply to industries and households. Iceland's reliance on geothermal heat is the best- known example of these uses but even the U.S. geothermal heating capacity is already more than twice as large as is the installed capacity in geothermal electricity generation.

Geothennally assisted household heat pumps, preferably closed-loop systems storing summer heat and releasing it in winter, are a particularly efficient option. If they were used in all U.S. households with no access to natural gas they would have saved nearly 100 gigawatts of peak winter electric capacity during the late 1990s.

Geothermal heat should be also used more widely by industries, greenhouses, and in aquaculture. Combination of electricity-generating and heating applications could make the greatest difference in about forty low-income countries (many of them islands) with large geothermal potential and shortages, or outright absence, of other energy resources.