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Renewable energy: what is geothermal energy?

Increasingly countries are turning to renewable energy such as solar, wind and hydropower to generate electricity. But geothermal energy is also used in many countries as a source of renewable energy – we look at how it’s used around the world.

The term geothermal comes from the ancient Greek words ‘geo’ meaning earth and ‘therme’ meaning heat. Geothermal energy uses the heat from the earth making it clean and sustainable.

What is geothermal energy?

Geothermal energy can be found both deep down in the depths of the earth where temperatures are so high that rock is molten (magma), to shallow ground where hot water and rocks are found. Temperatures at the Earth’s core can reach over 4,000°C.

Geothermal energy has been used for thousands of years. Hot springs are used for bathing now just as they were in the past. Geothermal energy was also used by the Romans to heat their homes. But more recently, geothermal energy has been used to generate electricity.

Geothermal heat pumps

The shallow ground (usually the top 10 feet of the Earth’s surface) stays at a constant temperature – around 10–16°C. Geothermal heat pumps use this energy to heat buildings in the winter and cool them in the summer. The heat removed from buildings in the summer can be used to provide a source of hot water.

Geothermal power

Currently there is around 12.8GW of geothermal power globally. But according to the Geothermal Energy Association (GEA) only around 6% of the global potential has so far been tapped. There are a number of countries generating over 15% of their electricity from geothermal sources including Iceland, Kenya and Costa Rica. Around 87% of Iceland’s heating and hot water comes from geothermal energy – with 73% of the country’s electricity coming from hydropower.

The first geothermal power generator was tested in July 1904 in Italy by Prince Piero Ginori Conti – it lit four light bulbs. The first commercial geothermal power plant was built there in 1911.

Geothermal electricity

Conventional power stations use fossil fuels to generate steam which then turns a turbine. Geothermal power stations use steam produced from reservoirs of hot water below the ground. There are several types of geothermal power plants – dry steam, flash steam and binary cycle.

Dry steam, flash steam and binary cycle

  • In dry steam power plants steam is drawn from underground and piped straight into the power plant.
  • Flash steam power plants are the most common using geothermal reservoirs where the temperature is more than 182°C. Wells are drilled into the ground and the hot water flows up them under its own pressure. As it makes its way up the well the pressure drops and some of the water boils and creates steam. It’s this steam that is then used to power the turbine. The remaining water and steam is pushed back into the reservoir.
  • Binary cycle power plants use water that is at a lower temperature – between 107°C and 182°C. The heat from the hot water is used to boil a fluid – something with a low boiling point. Steam is created in a heat exchanger and again used to rotate a turbine. The water is then pushed back into the ground to be reheated.

Business energy costs

Geothermal energy is increasingly used in homes and businesses. The potential of this sustainable energy source is huge. It can be used to heat and cool buildings and generate electricity. But until it’s available in all offices and manufacturing units along with solar power, the best way to reduce business energy costs is compare prices. can help by comparing 17 energy suppliers and advising on the best option for your business. Call 0800 411 8830 for a quick quote or complete the form at the top right of this page.

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