Geo (Earth) thermal (heat) energy means harnessing heat from inside the Earth. Our planet's core is incredibly hot - 5,500° Celsius (9,932° F) by recent estimates - so it's no surprise that even the top three metres of Earth's surface stay a nearly constant 10-16° Celsius (50-60° F) year round. Plus, thanks to various geological processes, at some places much higher temperatures can be found in some places.
Putting that heat to work
Where geothermal hot water reservoirs are near the surface their hot water can be piped directly to where the heat is needed. This is one way geothermal is used for hot water, to heat homes, to warm greenhouses and even to melt snow on roads.
Geothermal energy plant near Reykjavik. Iceland has abundant amounts of free geothermal energy.
Even where there isn't an easily accessible geothermal reserve, a ground heat pump can bring warmth to the surface and into buildings. This works nearly anywhere,and because the temperature underground stays nearly constant year round the same system that helps heat a building in the winter can help cool it in the summer.
Geothermal power plants use wells up to 1.5 km (1 mile) or more deep sometimes to tap into boiling hot geothermal reserves. Some of these power plants use the steam from these reserves to directly turn turbines. Others pump high-pressure hot water into low-pressure tanks. This causes "flashed steam", which is used to turn a turbine generator. A newer kind of plant uses the hot water from the ground to heat another liquid, like isobutene, which boils at a lower temperature than water. When this liquid vaporizes and expands it turns the turbine generator.
Advantages of geothermal power
Geothermal power generation causes virtually no pollution or greenhouse gas emissions. It's also quiet, and extremely reliable. Geothermal power plants produce electricity about 90 percent of the time, compared to 65-75 percent for fossil fuel power plants.
Unfortunately, even in many countries with abundant geothermal reserves, this proven renewable energy source is being massively under utilised.