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Geothermal energy

Geothermal Plant: What It Is, How It Works and Types of Power Plants

Geothermal plant: what it is, how it works and types of power plants

Geothermal power plants are the plants in charge of converting geothermal energy into electricity. A geothermal power plant is a facility where electricity is generated by heat from the Earth.

Geothermal energy is the thermal energy contained within the Earth. These facilities carry out a thermodynamic exchange at a certain depth of the earth's layers to heat a fluid and generate electricity.

The temperature inside the Earth increases approximately between 25 and 30 degrees Celsius per kilometer of depth. This variation in temperature is called a geothermal gradient.

Because the heat energy inside the Earth is inexhaustible, it can be said that this form of electricity generation is a renewable energy source. In addition, it is a clean energy that does not favor global warming.

How Does a Geothermal Plant Work?

Geothermal power plants are similar to other steam turbine power plants.

In all thermoelectric plants, heat is generated in one form or another to heat a fluid and generate steam. With the steam generated, a steam turbine connected to an electric generator is driven. In this way electricity is generated. Subsequently, the fluid is cooled and is returned to the heat source.

The difference between different thermoelectric plants is based on the way in which the initial heat is obtained. In conventional thermal power plants, heat is obtained by burning fossil fuels, usually coal or natural gas. In nuclear power plants, heat is obtained through nuclear fission reactions of atoms.

In the case of geothermal power plants, heat is obtained from geothermal reservoirs in the inner layers of the Earth.

Types of Geothermal Power Plants

Three types of geothermal power generation 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.

  • Geothermal flash steam plants or flash steam plants.

Binary Cycle Geothermal Plants

This type of thermoelectric power plants can operate with temperatures that are not very high, of only 57 degrees Celsius.

The main characteristic of this type of power station 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 steam turbines.

Binary cycle geothermal power plants are the most recently developed. 

This is the most common type of geothermal power plant in projects currently under construction. Both the Rankine cycle and the Kalina cycle are used.

The thermal efficiency of binary cycle geothermal plants is about 10-13%.

Dry Steam Geothermal Power Plants

Dry steam geothermal power plants are the simplest and oldest designed geothermal plants. They directly use geothermal steam at 150 degrees Celsius or higher to drive steam turbines.

Dry steam geothermal power plants directly take advantage of steam that comes out of fractures in the ground and to drive a turbine that generates electricity.

Flash or Flash Steam Geothermal Power Plants

Flash or flash steam geothermal plants use water from deep within 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.

Flash steam plants raise high pressure hot water through wells and into low pressure reservoirs. Due to this pressure change, the thermodynamic process occurs by which liquid water turns into steam.

Because the center of the Earth is under high pressure, water at this temperature is kept in a liquid state thanks to pressure and thermodynamic laws. The moment liquid water is pumped out, where the pressure is atmospheric pressure, it turns into steam.

The steam generated at the outlet is used to drive the steam turbine connected to an electric generator. In this way, electrical energy is generated.

The rest of the water that has not vaporized is returned to the reserve. The excess liquid water and condensed steam can be injected into the geothermal reservoirs again. This action contributes to making the process a more sustainable process.

What Is the Energy Efficiency 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 heat engines.

In the extraction of useful energy during electricity production, waste heat is lost unless it can be used directly and locally, for example in greenhouses, sawmills or in district heating.

The poor energy efficiency of the system does not affect operational costs as much as for a coal or other fossil fuel plant, but it does weigh on the viability of the plant.

To produce more energy than heat pumps consume, generating electricity requires high-temperature geothermal fields and specialized thermal cycles.

As geothermal energy does not rely on intermittent energy sources, like other renewable energy sources such as wind energy or solar energy, its load factor can be very high.

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Published: July 2, 2019
Last review: October 15, 2021