Deep inside the Earth lies the top layer of the mantle consisting of hot liquid rock called magma. Underground, water can be heated up to boiling temperatures and turn into steam through the heat emitted by these rocks. Harnessing the thermal energy fomr hot water and steam, we can produce electricity through environmentally-friendly means.
What is Geothermal Energy?
The word "geothermal" means "Earth" plus "heat". Most of the geothermal energy inside the Earth that we use is in the form of subterranean reservoirs of water. A number of technologies have developed that has allowed us to take advantage of this heat.
How Geothermal Plants Work
Dry Steam Power Plants: Dry steam power plants were the first type of geothermal plants. They rely on steam pumped directly from underground wells to turn a turbine which drives a generator to produce electricity. These plants only emit excess steam and minor amounts of gases.
Flash Steam Power Plants: The most common type of geothermal plants. Flash steam power plants use water with temperatures greater than 200°C pumped at high pressures to the surface, where the pressure is suddenly dropped, causing the hot water to "flash" into steam. The steam is then used to power a turbine and generator. Any leftover water is pumped back into the reservoir, or into a second tank where it can be flashed again to generate more steam. The only by-products of this process are excess steam and trace gases.
Binary-Cycle Power Plants: Binary-cycle power plants operate at lower temperatures than flash steam power plants. Binary-cycle power plants use the heat of the hot water to boil a secondary fluid with a low boiling point. The heat from the water thus causes the secondary fluid to flash to steam, which will drive the turbines. After a cooling process, the water is then injected back into the reservoir to be reused again later. Because the two fluids are separated during the whole process, almost nothing is emitted to the atmosphere. Since water in underground reservoirs usually have moderate temperatures, binary-cycle power plants will likely be the main geothermal technology in the future. The disadvantage of this system is that it tends to be less efficient.
Geothermal has many environmental and economic advantages. In countries where hydrocarbon resources are limited, geothermal power can be cost competitive. As a plentiful and low emission energy source, research continues into geothermal energy to make it more widely available.