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Geothermal energy

Can we generate electricity using hot rocks underground?

One of Iceland's geothermal power plants.

Credit: Ásgeir Eggertsson

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What does 'geothermal' mean?

The word 'geothermal' refers to heat underground. The heat comes from a layer of the Earth called the mantle, which is made up of molten rock or 'magma'. It's the same stuff that occasionally erupts out of volcanoes.

Magma is very hot, and as a result it heats up everything near it; including rock in the outer layer of the Earth (called the crust). The further down you dig, the hotter the rocks will be. New technology is being developed to harness that heat in order to make electricity.

So how exactly do we use the heat to produce electricity?

The process first involves drilling holes several kilometers into the ground, where, at around 200°C, the rock is considerably hotter than at the surface. Water is then pumped in at high pressure through one of the newly created wells.

As the water passes between the rocks, it is heated up. It is then pumped back up to the surface through a different drilled hole, and into a geothermal power plant.

In the plant the water passes through lower pressure tanks. Some of the water turns to steam, which drives a turbine that is attached to a generator. This produces electricity.

The water is then recycled and pumped back down the borehole to be heated up again.

Is geothermal energy reliable?

Yes. Unlike solar and wind energy, the heat resources underground are not dependent on the weather or daylight. In theory at least, a geothermal plant can deliver a consistent level of electricity generation all the time it's operating.

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