The technology behind geothermal involves capturing the thermal energy stored in the Earth’s interior for use either directly for heating or indirectly in electricity production.
The constant up-flow of heat from the Earth’s red-hot core makes for a rich source of geothermal energy. In certain geological areas, especially at margins of continental plates, the heat is shallow enough for us to access and use for our energy needs.
Mode of use 1 - Direct use or heating
The shallow depths of the earth’s crust maintain a constant temperature of 10-20°C. This ground source heat is generated largely from solar energy stored in the ground, which can be extracted through absorption.
Geothermal heat pumps
As the output temperature of ground source heat is often quite low, geothermal heat pumps can be used to raise the temperature. The result is a more useful output temperature of around 40-50°C, which can be used directly in heating systems or saunas. Heat pumps can also be operated in reverse to provide a cooling system.
A ground-source heat pump system comprises three basic elements:
- Ground loop (a pipe buried underground through which a refrigerant fluid is pumped to absorb heat from the ground)
- Heat pump (which receives the absorbed heat from a heat exchanger)
- Heat distribution system (such as under-floor heating, radiant panels)
Heat pumps do not produce electricity, but they do require electricity to extract and make use of low-grade heat. Because of this, they are not strictly regarded as a renewable source of energy, although it is possible to power the pumps with electricity from another renewable source.
Mode of use 2 - Electricity production
When rainwater and groundwater enters fissures in the earth’s crust, it is heated by the hot rock and begins to rise to the surface. This mixture of hot water and steam then either:
- Emerges on the surface, creating visible features such as hot springs, steam vents or geysers
- Is trapped in permeable and porous rocks below a layer of impermeable rock where it creates a geothermal reservoir
It is this trapping of steam and water underground which forms the start of the geothermal process of producing electricity.
Geothermal reservoirs can reach temperatures of 400°C. In certain geological areas across the globe, these reservoirs can be accessed by drilling wells to depths of over two miles into the earth’s crust.
If temperatures in the reservoir are hot enough, steam bubbles will force water to the surface naturally. If not, pumps may be needed to extract the hot water and steam from the well.
Above the surface, this steamy mixture is directed to a turbine and generator in a geothermal power plant where it is used to generate electricity in the traditional way. This is a cyclical process as once the geothermal energy has been extracted from the water, it is returned down the well into the reservoir and reheated.
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