What is... Geothermal Energy?

The earth is a renewable energy power source! Within the earth's molten core, temperatures reach around 6000°C. Although the planet's core is obviously impossible to tap into (the deepest well ever drilled is 'only' 12km!), heat naturally dissipates towards the earth's surface and is available for us to use. This is called geothermal energy. The first use of geothermal energy for generating electricity was in Larderello, Italy, in the early 1900s.

To exploit geothermal energy we usually use water that has been heated naturally by the earth. This water, in the form of steam, can be used to power turbines to generate electricity. Geothermal energy can also be used directly as heating, without first generating electricity.

The most obvious geothermal locations are those where there is active volcanism present. This includes locations near ocean spreading centres (e.g. Iceland), rift valleys (e.g. East African Rift) and Island Arcs (e.g. Japan).

The largest area with the potential for using geothermal energy is commonly known as the Ring of Fire. This is a term for the area surrounding the Pacific Ocean and is known for its high density of volcanoes caused by the subduction of the Pacific Plate under the surrounding plates of North and South America, Eurasia and Australia.

In these volcanically active geothermal settings, reservoir temperatures are around 160 – 250°C. Once a well is drilled into these hot reservoirs, water flows up the well due to the pressure gradient. Some of this water turns into steam as it depressurises (known as flashing) and drives a turbine creating electricity. This is the most common form of geothermal power. In some cases only steam is produced by the wells (with no liquid water), power plants utilising these sources are known as dry steam power plants.

Less obvious geothermal locations can be found miles away from any active volcanism. Sedimentary basins, ancient igneous intrusions and abandoned mines are some of the most common geothermal sources outside of volcanic regions. These utilise lower temperatures within the earth with reservoirs of water temperature ranging from 50 – 170°C. This can be exploited using binary systems which use an alternative fluid with a lower boiling point to drive the turbine. This fluid is heated by the water from the well, which turns it into a gas and drives the turbine. Binary plants are the most common type of geothermal power plant outside of volcanic regions. Increasingly, these binary systems are being added to flash power plants. This can utilise the excess warm water output, generating additional power before the water is re-injected underground.

Where temperatures are found below 50°C geothermal energy can no longer be used for producing electricity, but it is still widely exploited as a heat source. This is called direct utilisation and has been commonly used for centuries heating bathing areas and now more extensively homes through district heating systems. When geothermal energy is used in this way it is possible to do it on a much smaller scale, heating establishments such as universities and office blocks.

Iceland, with lots of active volcanism and hot springs is a perfect place to utilise geothermal energy. Geothermal sources account for 66% of Iceland's primary energy use, enabling roughly 85% of primary energy use in Iceland to come from non-imported renewable resources!

It is hoped other countries with the potential for geothermal power may be able to follow Iceland's example and use geothermal power as the basis for a cleaner energy mix.

Next week we will explain Energy Storage.
This article is part of a series explaining some of the key technologies that are enabling the energy transition. Click Here to see the full series.
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