Geothermal energy taps into the heat stored in the earth and uses it for things like heating and cooling (Wind power) or generating electricity. In a geothermal power plant, a deep well goes down a mile or more into the earth, where the temperature is much hotter than at the surface.
There are three kinds of geothermal plants:
Dry steam plants use geothermal steam to turn turbines and produce electricity.
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Some sites use water to produce steam. Engineers drill two deep holes and then use explosives or high-pressure water to break up the rock and create a reservoir at the bottom of the holes. To generate electricity, the plant pumps water down one hole. The earth heats the water, and steam rises through the other hole to turn the plant's turbines.
Flash steam plants create steam by tapping into very hot, highly pressurized water that's deep underground and pulling it into lower-pressure tanks. This creates the steam that drives the turbines.
Binary cycle plants take moderately hot water that's been heated by the earth and pass it by a second fluid that has a lower boiling point than water. The fluid vaporizes and drives turbines to generate power.
Geothermal power is clean—it gives off little or no greenhouse gases or other pollutants—and operates without using fossil fuels. And it's got something going for it that the other technologies mentioned so far don't have: reliability. The earth's underground temperature remains constant, so geothermal plants produce electricity, on average, 95% of the time.
Compared to other kinds of power plants, geothermal plants are relatively small, and once they're built, they require little maintenance and cost almost nothing to run.
Geothermal power has its problems:
Siting. It can be a challenge to find a good spot for a geothermal plant. The site needs to have the right kind of rocks and the right amount of heat at a suitable depth for drilling.
Setup costs. Building a new geothermal power plant is really expensive. Much of the cost is related to drilling deep into the earth, especially because it may take several attempts to find an appropriate site.
Tapped-out sites. The wells at a particular plant may go dry or cool down.
Escaping gases. Although most geothermal plants produce clean energy, in some cases they can contribute to pollution. This happens when a well releases greenhouse gases, particularly CO2 and hydrogen sulfide, trapped in the earth. Some plants have emission-control systems that mitigate the release of polluting gases, and even plants that don't have these systems release only miniscule amounts of such gases compared to traditional power plants that burn fossil fuels.
Sludge. Some geothermal plants produce sludge, rocks and sediment that need to be disposed of at an approved site. But on the plus side, some of the stuff in sludge, like zinc, silica, and sulfur, can be separated from the other materials and sold.