How geothermal heat is turned into electricity

The word geothermal means “related to the heat of the earth”. It comes from the Greek words geo (earth) and therme (heat).

In most countries, geothermal heat occurs in rock that is dry. To make use of this heat, "hot dry rock technology" is being developed to inject water from ground level to several kilometres below the surface so that it can be heated. This technology is still at the experimental stage; it has not yet advanced to a commercial scale. Geothermal Electric Limited does not anticipate using this technology in Fiji.

In some areas, water from rainfall or the ocean seeps into deep cracks in the earth, and geothermal heat can turn the water into steam. Unfortunately, the deeper the steam and hot water is, the more difficult it is to find. However, in Fiji there are a number of "hot springs" where hot water flows naturally to the surface. This is a good indication that hot water and steam may be in the vicinity under ground -- but exactly where it is, and whether volumes of hot water and steam are sufficient to generate electricity, can really only be discovered by drilling.

The technology to generate electricity from this hot water and steam is called "wet geothermal technology". In contrast to the experimental hot dry rock technology mentioned above, wet geothermal technology has existed for more than a century. It is commercially proven, highly reliable technology that provides electricity for 60 million people around the world -- a figure that is increasing rapidly.

Electricity can be generated from wet geothermal resources in several ways. Geothermal Electric Limitedplans to install a "binary" system, which involves two separate circuits for fluids. There are five stages, as shown in the diagram:

1.  Deep holes are drilled to gain access to the hot water underground.

2.  The water is pumped out of the ground, and passes through a heat exchanger that transfers the heat (but not the water) across to the second circuit.

3.  The water, which is now cooler, is pumped back deep under ground, not far from where it was pumped out.

4.   In the second circuit is a fluid which, when heated, boils into a vapour -- at a lower temperature than water would boil into steam. This vapour rotates a turbine at very high speed, in much the same way that steam powers turbines in coal, nuclear and thermal solar power stations.

5. An electrical generator, on the same shaft as the turbine, rotates to generate electricity that feeds into transmission lines.

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