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.
Read on to learn about the benefits of EGS.