The word geothermal comes from the Greek words geo (earth) and therme (heat). So, is a renewable resource originating deep within the Earth where temperatures can reach up to 6000°C. At a depth of five kilometres from the Earth’s surface, the temperature decreases to around 200°C. We can use the steam and hot water produced inside the earth to heat buildings or generate electricity. Geothermal energy is a renewable energy source because the water is replenished by rainfall and the heat is continuously produced inside the earth.
It is geothermal energy that is responsible for volcanoes, earthquakes, geysers, hot springs and mud-pots and are derived from hot water and steam formed in porous or fractured rock at relatively moderate depths from 100 metres to five kilometres. High geothermal activity occurs where the Earth’s crust is thin and molten rock and steam at high pressure are able to force their way to the surface.
The hot water and steam are formed from the intrusion of molten magma into the Earth’s crust or the deep circulation and heating of groundwater through faults and fractures. High-grade hydrothermal resources are used to generate electricity and lower grade resources can be used in direct heating applications.
To generate electricity, hot water at temperatures ranging from 180°C to 350°C is brought from the underground reservoir to the surface through production wells, and is flashed to steam in special vessels by release of pressure. The steam is separated from the liquid and fed to a turbine engine, which turns a generator.
Spent geothermal fluid is injected back into peripheral parts of the reservoir to help maintain reservoir pressure.
In direct heating, the geothermal water is usually fed to a heat exchanger before being injected back into the Earth. Heated domestic water from the output side of the heat exchanger is used for a variety of purposes including home heating, greenhouse heating and vegetable drying