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Geothermal energy

One of Iceland's geothermal power plants.

Credit: Ásgeir Eggertsson

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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.

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