Instant expert

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.

So what's the catch?

Geothermal energy is considered renewable because the heat source it uses is so vast (the entire Earth's mantle). However, the productivity of a plant can decline over time as the rocks in direct contact with the water cool down.

Just as a field can be overgrazed, the rocks can be overused, transferring heat to so much water that their own heat reserves are depleted.

But, if you give a field enough time to recover, it eventually will. Likewise, if left alone, the rocks will be reheated by heat sources deeper underground … you just have to wait a couple of hundred years!

Anything else?

A plant's long-term productivity can also be compromised by incorrect placement - for example, in an area where the rocks aren't stable or hot enough.

There are also the issues of pollution and contamination. The drilling of boreholes for geothermal plants produces carbon emissions, although these are significantly less than what's emitted from fossil fuel power plants! Also, the water pumped through the rocks may pick up traces of mercury, arsenic and other harmful elements, and could be leaked into surrounding rivers.

Can Australia even use geothermal power plants?

It already does! Well, sort of. Although no geothermal energy is being fed into the grid just yet, several companies are already exploring granite rock systems deep underground in Central Australia, and some large scale projects are under construction. For example, by 2012, Geodynamics Ltd are hoping to have established a plant that will generate enough electricity to power 50,000 homes.