Geothermal energy: definition, types and functioning

In its broadest and literal sense, the definition of geothermal energy could be specified as the energy that takes advantage of the internal heat of the Earth. The term geothermal comes from the Greek geo (Earth) and thermos (heat), literally the heat of the Earth. Geothermal energy is therefore a renewable energy that is obtained by utilizing the natural heat from within the Earth's crust. This thermal energy can be transmitted through hot rock bodies or reservoirs either by conduction or convection. In areas within the Earth's crust, interaction processes can occur between rocks and some fluids giving rise to geothermal systems. Through these systems, significant benefits for our daily lives are obtained, such as climate control and hot water for sanitary uses in both public buildings and homes.

Applications of geothermal energy: definition and application

The applications of geothermal energy depend on the source from which it is obtained. High-temperature geothermal resources (greater than 100°C) are mainly used for electricity production. When the temperature of the reservoir is not high enough to generate this resource, its main applications are thermal, especially in the industrial, residential, and service sectors. Thus, in the case of temperatures below 100°C, a direct utilization or through a geothermal heat pump can be made to produce heating and cooling. When the utilization is direct, hot water is channeled to the building. This geothermal water is pumped through a heat exchanger, which transfers its temperature to the water and in turn to the interior of the heating system of the home. The used water is returned to the reservoir to be reheated for reuse.

Finally, when it comes to very low temperature resources, below 25°C, the possibilities for use lie in air conditioning and obtaining hot water for sanitary use. These temperature levels can be reached just a few meters below the surface. In Spain, at 10 meters deep, the temperature remains constant at 17°C all year round, thanks to the thermal inertia of the soil.

Functioning of the geothermal heat pump

Therefore, just a few meters below the ground, some fluids like water and oil remain at a constant temperature close to 15°C throughout the year. A small amount of this heat is used to heat homes and offices. The fluids circulate through a series of pipes located in the underground or beneath the water of a lake, from where they are guided to the building. An electric compressor and a heat exchanger push the thermal energy through the pipes into the heating or cooling system of the building. During the summer, the process is reversed: the pipes carry the heat from the home to the ground or the exterior water, where it is absorbed.