You can make electricity from heat in the Earth, just ask residents of Northern California. Their electricity comes from natural geothermal energy, or hydrothermal energy, and here's how it works. It starts with water 6,562 to 13,123 feet (2 to 4 kilometers) underground, trapped in holes or cracks in rocks. The water and rock get heated by Earth's hot mantle or by radioactive minerals in the rock. Energy companies drill wells into the rock and pump up hot water or steam. The steam drives turbines in generators, which send electricity to residents' houses.
Since nature provided the hot rock, the connected holes or cracks and the water, it's considered natural geothermal energy. By contrast, enhanced or engineered geothermal systems (EGSs) don't wait for the full setup. They start with hot rock and add the water or the cracks and connections or all of it. So, all geothermal electricity comes from hot water inside hot rock; in natural geothermal, nature makes the system. In engineered geothermal, engineers make some part of it.
Why bother to build the system if nature can give it to you for free? In a way, it's a chance to design the perfect geothermal system. You're no longer stuck with what nature provides, which could be colder water or more of a puddle than a vast reservoir. You don't have to hunt for the natural sources, and you aren't limited to regions of the world where natural sources exist. For a cost, you can engineer a geothermal system anywhere. And you can make it more efficient than anything nature provides.
In this article, we'll explore the benefits, limits and promise of a future powered by EGS. First, we'll tour an EGS power plant.
Inside an Engineered Geothermal System Power Plant
To understand how engineered geothermal systems (EGSs) work, it helps to start with how the systems are built. They're built into hot, deep rocks: basement, sedimentary or volcanic rocks. Developers drill wells 1.9 to 6.2 miles (3 to 10 kilometers) into the rocks, using conventional oil drills. The temperature down there measures some 160 degrees F to 600 degrees F (71 degrees C to 315 degrees C). The depth is deeper than that used in natural geothermal systems, but the temperature is about the same.
The rocks need to have a special history. These rocks, like all rocks, were stressed long ago -- thereby becoming cracked. Over time, the cracks resealed with crusts of minerals, but that's all part of the plan. The next step is to force water into the rock using high-pressure pumps.
Here's where another piece of the rock's history comes in. The rock is still under stress, so it's just aching to break along its old cracks. Forcing water into it does the trick, and it slips along its cracks. The rock's rough edges prop it open.
Now, we're ready to talk about electricity. The power plant on the surface has pairs of wells -- injection wells and production wells. Cold water gets pumped down the injection wells. As it percolates through cracks in the hot rock, it heats. Once it's hot enough, it rises by its own heat or by the pressure from incoming water up the production well. The rest is geothermal as usual: Hot water makes steam and drives turbines. Cooling towers or pipes cool the water and recycle it back into the injection wells.
Almost any site can be used to build an EGS because hot rock is everywhere. But the best sites occur where the hot rock is most stressed and closest to the surface. Developers can drill temperature wells and look for stress in the surface geology to assess sites. In several countries, including the United States, governmental surveyors are making systematic maps.
Earthquakes and Other Risks of Artificial Geothermal Energy
Harvesting engineered geothermal energy requires construction underground, so it carries risks, but they can be controlled.The first risks are vibrations at the surface. When engineers build an engineered geothermal system (EGS), they create something like an earthquake underground. It happens during fracturing, as the hot rock collapses on itself and slips. The slipping is on a much smaller scale than when a big fault slips to cause an earthquake we can easily feel. We rarely feel these man-made quakes at the surface, but if we do, it's as a light vibration.
The rock movements are monitored and controlled. By planting seismometers around the rock to be fractured, engineers can watch the cracks spread. Since their own water pumps control the cracking and slipping, if engineers want it to stop, they can turn off the water.
With good planning, no large earthquakes will occur. Developers wouldn't put an EGS site near a big fault, where high-pressure pumping could disturb the fault. Developers can check regional geological maps to know where big faults are. And just in case, developers measure seismicity at sites before they start working in an area.
Water use poses an even bigger issue than surface vibrations. EGS sites use water during building and operation. The first water gets invested to prop open the cracked rock and measures 2 million gallons or more (about three Olympic swimming pools or 7.6 million liters). Once the rock is unsealed, it will suck down nearby reservoirs, lowering the water table, unless you add millions to billions more gallons of water at the surface. In some systems, more water is used for cooling the power plant.
The good news is that all water added at the surface can be reused, so it's invested only once. It also doesn't have to be drinking-grade water. EGS is most economical in the arid West, because that's where the hot rocks are shallowest, so developers have to buy water rights.
Water pollution is another issue. As water circulates through the hot rock, it may pick up arsenic and other poisonous substances. The contaminants shouldn't leak at the surface or into underground freshwater. To try to ensure they don't, engineers keep the circulating water contained. On the surface, it flows through pipes that dive down into the wells, and when the water flows through the cracked rock, a jacket of uncracked rock serves as insulation.
Benefits of Artificial Geothermal Energy
All geothermal power, whether natural or engineered, has economic and environmental benefits, like reliability. It can supply electricity nonstop, owing to the Earth always being hot and radiating heat predictably. The same can't be said of wind or solar power, since the wind dies down, and the sun sets.
Engineered geothermal, like natural geothermal, is renewable, meaning it doesn't deplete Earth's heat. That's not to say that sites don't wear out.
They do. "You mine heat from this local region underground faster than it's being resupplied by radioactive minerals in the rock and conduction through the Earth," says Jefferson Tester, an EGS expert at MIT. Eventually, the rock between the injection and production well gets cold. But like crop rotation, new wells can be drilled nearby, where the rock is as hot as ever. By rotating among several pairs of wells, you can keep getting hot water. Overall, the process extracts a small fraction of the heat in a big block of rock and a laughably small amount of Earth's heat. No system could dream of cooling off the Earth.
Engineered and natural geothermal use the same power plants, which are very clean. Binary plants, the cleanest designs, emit no gas into the environment, not even steam clouds. The circulating water stays in a pipe and boils another fluid to turn the plant's turbines. Steam and flash plants, which puff out billowing steam clouds, naturally emit little sulfur dioxide, nitrogen oxide and carbon dioxide and have scrubbers that let virtually no hydrogen sulfide escape.
In addition, geothermal power plants don't occupy much land: 7,460 square meters (80,299 square feet) per megawatt. Let's see how other energy sources compare:
- Solar panels are the worst on space, sprawling over 710,418 square feet (66,000 square meters) per megawatt of electricity under ideal conditions.
- Coal plants and their strip mines occupy 430,556 square feet (40,000 square meters) per megawatt.
- A nuclear plant takes up 107,639 square feet (10,000 square meters) per megawatt.
Geothermal power also offers a country energy security. Because the resource is on home soil, and for all purposes, unlimited, there's no need to worry about the cost of imports. And unlike nuclear power, byproducts of geothermal can't be used for weapons.
Engineered geothermal's big advantage over natural geothermal is that it works almost anywhere. Engineered geothermal needs only hot rock. "You drill deep enough anywhere, and you'll hit hot rock," says Peter Rose, an EGS expert at the University of Utah.
The Cost of Artificial Geothermal Energy: Dollars, Cents and Watts
We can't avoid the economics. Investors want to know how fast engineered geothermal systems (EGSs) will pay for themselves, and consumers are concerned about the cost of the power.
The most expensive part of engineered geothermal energy is drilling the wells. To drill one 2.5-mile (4-kilometer) well, which is middle-range, it costs about $5 million. If the heat happens to be deeper, at 6.2 miles (10 kilometers), the drilling cost skyrockets to $20 million per well [source: Tester]. These costs could drop by the millions per well as drilling technology progresses.
Once the wells and power plant are built, the system is inexpensive to operate. The heat from the Earth is free. Operators pay to keep the water pumps pumping and to maintain the wells. They also pay to redrill wells every five to 10 years, says Tester.
A mature engineered geothermal power plant can churn out between 1 and 50 megawatts of electricity, enough to supply 800 to 41,000 average U.S. homes [sources: Tester, EIA]. The output is less than some natural geothermal plants and pales in comparison to the more than 2,000 megawatts that a coal-fired plant can supply [source: Tester].
Investors can get a good deal in the end, each year recovering 17 to 18 percent of the money they spend on building underground parts of the system, the same as what they'd get from an oil or natural gas field, says Tester. For consumers, the cost of the electricity depends on how well the system milks heat from the rock. The cost drops if more water circulates through the rock and if the recovered water is hotter.
Tester and his colleagues ran models on six U.S. locations where engineered geothermal systems would be practical. They estimated that the first engineered geothermal systems would be inefficient, getting 20 kilograms of hot water per production well per second, setting the cost of the electricity between 18 and 75 cents per kilowatt-hour. But with mature technology, able to harvest 80 kilograms of hot water from each production well per second, the cost could drop to 4 to 9 cents per kilowatt-hour, in-range or below the cost of electricity from coal [source: Tester].
Raising the power output and lowering the cost is a manageable engineering problem, says Tester. "We don't have to make significant new discoveries or find new materials. We have to re-engineer the subsurface [rock] system by better knowledge of what's down there. It's a much more tractable route."
Artificial Geothermal Energy Around the World
Engineered geothermal is still experimental worldwide, but a few small commercial power plants do exist.
Japan burst onto the engineered geothermal scene early by demonstrating it on the side of a volcano, at a site called Hijiori. Its longest test ran for a year and harvested enough heat to run a small, 130-kilowatt power plant. The test stopped because one well cooled a dramatic 63 degrees F (17 degrees C) in one year [source: Tester].
The prospects look good in Australia because throughout the continent, radioactive sources heat basement rock that's shallow, cracked and now under the right kind of stress. In the Cooper Basin, currently used for oil and gas, surveyors found a 386-square-mile (1,000-square-kilometer) slab of granite sizzling at 482 degrees F (250 degrees C). Geodynamics Ltd. scooped up the site, sunk in a pair of wells, aptly called "Habanero-1" and "Habanero-2," cracked the rock and started circulating water.
A power station is being built, which could generate hundreds to thousands of megawatts of electricity, the latter making it competitive with a coal plant, if many wells go into the big field [source: Tester].
France and Germany are now producing electricity by engineered geothermal. One plant, in Soultz-sous-Forêts, France, produces about 1 megawatt of electricity. The other, in Landau, Germany, produces 2 to 3 megawatts, says Rose. These small outputs could grow if the projects raise money to drill more wells.
In the United States, engineered geothermal is now starting. The first demonstrations will be at natural geothermal power plants at the Geysers in California and at Desert Peak and Brady in Nevada. In the demonstrations, engineered geothermal techniques will rescue some dry wells and boost power production at the sites.
The U.S. Geological Survey plans to demonstrate more engineered geothermal in the Midwest and in hot rock basins east of the Mississippi. "That would capture the imagination of a lot more states and congressmen and would help tremendously if it convinced them this wasn't only a Western resource," says Rose. If all goes well, stand-alone power plants might appear in the United States in five years, says Rose.
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