Geothermal energy is the thermal energy contained inside the Earth. These facilities perform a thermodynamic exchange at a certain depth of the terrestrial layers to heat a fluid. This heat is used to generate electricity. Because the heat inside the Earth is inexhaustible, it can be said that this form of electricity generation is a source of renewable energy.
Geothermal power plants are similar to other steam turbine power plants. In all thermoelectric plants heat is generated in one way or another to heat a fluid and generate steam. With the steam generated, a steam turbine connected to an electric generator is operated. In this way electricity is generated. Subsequently, the fluid is cooled and returned to the heat source. The difference between the different thermoelectric plants is based on the way in which the initial heat is obtained. In thermal power plants, they are obtained through the combustion of fossil fuels, usually coal or natural gas. In nuclear power plants, heat is obtained by nuclear fission reactions of the atoms. In the case of geothermal power plants, heat is obtained from the inner layers of the Earth.
Three types of geothermal power plants can be distinguished depending on the temperature, depth and quality of the water and steam extracted.
- Binary cycle geothermal power plants
- Dry steam geothermal plants.
- Flash steam geothermal power plants or flash steam power plants.
Geothermal Plants of Binary Cycle
The geothermal binary cycle power plants are the most recent development. This type of thermoelectric power plants can work with temperatures not very high, of only 57 degrees Celsius. The main characteristic of this type of plant is that moderately hot water is passed along with another fluid with a boiling point much lower than that of water. Because the secondary fluid has such a low boiling point, it quickly vaporizes and can be used to power the steam turbines.
This is the most common type of geothermal power plant within projects currently under construction. Both the Rankine cycle and the Kalina cycle are used. The thermal efficiency of binary cycle geothermal plants is approximately 10-13%.
Dry Steam Geothermal Power Plants
The dry steam geothermal plants are the geothermal plants of simpler and older design. They directly use geothermal steam at 150 degrees Celsius or more to move the steam turbines.
The dry steam geothermal plants directly take advantage of the steam that comes out of the ground fractures and to move a turbine that generates electricity.
Flash or Flash Geothermal Steam Power Plants
Geothermal flash or flash plants use water from the depths of the Earth. This water is at a temperature around 200 degrees Celsius or 473 degrees Kelvin due to the thermal energy transmitted by the magma.
The flash steam power plants make hot water rise at high pressure through wells and introduce it into low pressure tanks. Due to this pressure change the thermodynamic process takes place by which the liquid water turns into vapor.
Because the center of the Earth is at high pressure, water at this temperature is maintained in a liquid state thanks to pressure and thermodynamic laws. At the moment when liquid water is pumped outwards, where the pressure is atmospheric pressure, it becomes vapor.
The steam generated at the outlet is used to move the steam turbine connected to an electric generator. In this way, electric power is generated.
The rest of the water that has not been vaporized is returned to the reserve. The remaining liquid water and condensed steam can be injected into the geothermal tanks again. This action helps to turn the process into a more sustainable process.
Energy Efficiency and Load Factor of Geothermal Power Plants
The energy efficiency of geothermal plants is low, approximately 10 to 23%, because geothermal fluids are at low temperatures compared to steam boilers. By the laws of thermodynamics, this low temperature limits the efficiency of thermal machines.
In the extraction of useful energy during the production of electricity, waste heat is lost unless it can be used directly and locally, for example, in greenhouses, sawmills or district heating. The poor energy efficiency of the system does not affect the operational costs as much as for a coal or other fossil fuel plant, but it weighs on the viability of the plant. To produce more energy than that consumed by heat pumps, the generation of electricity requires high temperature geothermal fields and specialized thermal cycles.
Since geothermal energy is not based on intermittent energy sources, such as other sources of renewable energy such as wind energy or solar energy, its load factor can be very high.