As the costs of fuel and electricity rise, geothermal energy has a promising future. Underground heat can be found anywhere on Earth, not just where oil is pumped, coal is mined, where the sun shines or where the wind blows. And it produces around the clock, all the time, with relatively little management needed. Here's how geothermal energy works.
No matter where you are, if you drill down through the Earth's crust you will eventually hit red-hot rock. Miners first noticed in the Middle Ages that deep mines are warm at the bottom, and careful measurements since that time have found that once you get past surface fluctuations, solid rock grows steadily warmer with depth. On average, this geothermal gradient is about one degree Celsius for each 40 meters in depth, or 25°C per kilometer.
But averages are just averages. In detail, the geothermal gradient is much higher and lower in different places. High gradients require one of two things: hot magma rising close to the surface, or abundant cracks allowing groundwater to carry heat efficiently to the surface. Either one is sufficient for energy production, but having both is best.
Magma rises where the crust is being stretched apart to let it rise—in divergent zones. This happens in the volcanic arcs above most subduction zones, for instance, and in other areas of crustal extension. The world's largest zone of extension is the mid-ocean ridge system, where the famous, sizzling-hot black smokers are found. It would be great if we could tap heat from the spreading ridges, but that is possible in only two places, Iceland and the Salton Trough of California (and Jan Mayen Land in the Arctic Ocean, where no one lives).
Areas of continental spreading are the next-best possibility. Good examples are the Basin and Range region in the American west and East Africa's Great Rift Valley. Here there are many areas of hot rocks that overlie young magma intrusions. The heat is available if we can get to it by drilling, then start extracting the heat by pumping water through the hot rock.
Hot springs and geysers throughout the Basin and Range point to the importance of fractures. Without the fractures there is no hot spring, only hidden potential. Fractures support hot springs in many other places where the crust is not stretching. The famous Warm Springs in Georgia is an example, a place where no lava has flowed in 200 million years.
The very best places to tap geothermal heat have high temperatures and abundant fractures. Deep in the ground the fracture spaces are filled with pure superheated steam, while groundwater and minerals in the cooler zone above seal in the pressure. Tapping into one of these dry-steam zones is like having a giant steam boiler handy that you can plug into a turbine to generate electricity.
The best place in the world for this is off limits—Yellowstone National Park. There are only three dry-steam fields producing power today: Lardarello in Italy, Wairakei in New Zealand and The Geysers in California.
Other steam fields are wet—they produce boiling water as well as steam. Their efficiency is less than the dry-steam fields, but hundreds of them are still making a profit. A major example is the Coso geothermal field in eastern California.
Geothermal energy plants can be started in hot dry rock simply by drilling down to it and fracturing it. Then water is pumped down to it and the heat is harvested in steam or hot water.
Electricity is produced either by flashing the pressurized hot water into steam at surface pressures or by using a second working fluid (such as water or ammonia) in a separate plumbing system to extract and convert the heat. Novel compounds are under development as working fluids that could boost efficiency enough to change the game.
Ordinary hot water is useful for energy even if it isn't suitable for generating electricity. The heat itself is useful in factory processes or just for heating buildings. The entire nation of Iceland is almost completely self-sufficient in energy thanks to geothermal sources, both hot and warm, that do everything from driving turbines to heating greenhouses.
Geothermal possibilities of all these kinds are shown in a national map of geothermal potential issued on Google Earth in 2011. The study that created this map estimated that America has ten times as much geothermal potential as the energy in all of its coal beds.
Useful energy can be obtained even in shallow holes, where the ground isn't hot. Heat pumps can cool a building during summer and warm it during winter, just by moving heat from whichever place is warmer. Similar schemes work in lakes, where dense, cold water lies on the lake bottom. Cornell University's lake source cooling system is a notable example.
Earth's Heat Source
OK, so geothermal energy is heat from underground. But why is the Earth hot at all?
To a first approximation, Earth's heat comes from radioactive decay of three elements: uranium, thorium and potassium. We think that the iron core has almost none of these, while the overlying mantle has only small amounts. The crust, just 1 percent of the Earth's bulk, holds about half as much of these radiogenic elements as the whole mantle beneath it (which is 67 percent of the Earth). In effect, the crust acts like an electric blanket upon the rest of the planet.
Lesser amounts of heat are produced by various physico-chemical means: freezing of liquid iron in the inner core, mineral phase changes, impacts from outer space, friction from Earth tides and more. And a significant amount of heat flows out of the Earth simply because the planet is cooling, as it has since its birth 4.6 billion years ago.
The exact numbers for all these factors are highly uncertain because the Earth's heat budget relies on details of the planet's structure, which is still being discovered. Also, Earth has evolved, and we cannot assume what its structure was during the deep past. Finally, plate-tectonic motions of the crust have been rearranging that electric blanket for eons. The Earth's heat budget is a contentious topic among specialists. Thankfully, we can exploit geothermal energy without that knowledge.