Geothermal energy

Harnessing the heat stored inside the earth is a particularly attractive option: it is an inexhaustible renewable energy source and its use helps reduce consumption of fossil fuels such as oil, thus reducing the emission of pollutant substances.

To harness this heat, the ground is drilled and hot water is extracted to be used directly for generating electricity from heat and for heating and even cooling systems.

Geothermal deposits

In some areas of the earth, rocks under the ground are at very high temperatures. This heat energy, known as geothermal energy, has many different applications, which vary depending on the temperature of the hot water contained in the rocks and the depth at which it is found.

In places where there are no natural underground aquifers, a technique is used which consists of drilling down and piping in high-pressure water to fracture the rock and artificially give it a certain porosity. Water can then be injected which, after heating, is extracted and can be used for generating electricity or for heating systems.

• The techniques used for drilling to extract the water are similar to those used for drilling for oil. The area to be explored must be at a depth that is accessible using modern drilling techniques.

• The underground heat cannot be extracted directly. Instead there needs to be a fluid-water or steam-near to the areas of high-temperature from which the heat can be extracted and which can be transported to the surface to harness the energy.

High temperature

High-temperature geothermal energy (150ºC-400ºC)

Some high-temperature geothermal deposits are located in areas of volcanic activity. The fluid-generally steam-has been heated by the magma to a high enough temperature to allow it to be used in a turbine for generating electricity. This technique was first used in Larderello, Italy, in 1904.

• Producing electricity

  The steam is introduced into turbines which rotate, turning the generator and producing electricity.

• Condenser

  The fluid used in this second circuit is condensed and cooled for re-use.

• Heat exchanger

  The underground water may contain large proportions of harmful substances and cannot therefore be used directly. Using a heat exchanger, part of the heat is transferred to a second "clean" fluid from which the energy is harnessed.

• Extraction pump

  Two holes are drilled down to the area of the aquifer. One of them is used to extract the water and the other is used to replace after the energy has been used.

• Reinjection well

  When the fluid is injected back into the aquifer, the water heats up again and can be re-used. This prevents alterations in the hydrological make-up of the area and ensures an inexhaustible supply from the deposit.

Low temperature

Low temperature geothermal energy (below 100ºC)

Most geothermal deposits are found at temperatures below 100ºC. Because of this low temperature, they are not generally used for generating electricity but instead for individual and district heating systems, industrial heat (such as for heating greenhouses, for example) and for therapeutic heat in health spas.

• The hot water can also be piped under roads to prevent them freezing over in winter.

• Geothermal heat pump

  The temperature of the subsoil remains very constant throughout the year: colder than surface temperature in summer, and hotter in winter.

  Using a system of underground pipes air or liquid anti-freeze is introduced, which is then piped to the building. The heat thus obtained keeps the entire building at a comfortable temperature. In summer, the system can be inverted and the heat coming from the process of cooling the buildings can be injected into the earth for cooling.

   
• The system used for extracting and re-injecting the water into the aquifer is the same as with high-temperature deposits.
   

• Cities like Rekjavik in Iceland have geothermal district heating systems.

  The hot water is pumped to an exchanger which transfers the heat to clean water. This is piped to the buildings where the heat is used in heating systems. This system can be used in individual buildings and also in collective distribution systems.