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Science corner

Sophie Duncan goes geothermal

It may look conventional, but this power station does not rely on fossil fuels. And this energy source is not just used to produce electricity, it is also used to directly heat homes.

Across the world power stations like this generate more than 8,000 Megawatts of electricity. Geothermal power has been used to cook food and to keep warm for thousands of years, but it is only in the last 50 years that it has been harnessed to generate electricity. Unfortunately there are few suitable sites for this type of power station, as it relies on the production of high pressure steam.

Different methods are used. The power station shown here, the Geysers geothermal plant in California, is a flash plant. The water is pumped into an area of hot rocks and is heated as it is squeezed through the cracks in the rocks. The hot, pressurised water converts to steam as it reaches the surface, a so called flash, and is carried through the pipes to a turbo generator. As the steam is impure it has to be purified before reaching the generator as the impurities cause the turbine to fur up like the kettle.

The pipes carrying the water have expansion areas, as the steam is at 350oC and causes the metal to expand. The steam can travel so quickly that it can take less than a minute to leave the ground and reach the cooling tower. The water can take less than two days from being injected to reemission as steam.

Sophie Duncan is a physicist and programme manager with Science Year www.scienceyear.com

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