Geothermal energy represents the inner heat of the earth, produced largely by the decay of radioactive elements in the mantle and center. The three ways that the heat is found is both wet and dry steam (wet steam has drops of water in it), hot water and dry volcanic rocks. We know that the temperature of the earth at depths of 25 to 50 km range from 200`C to 1000`C. There are areas of the earth where local concentrations of heat occur, just as mineral concentrations do. Most of these are located along oceanic ridges and continental rifts, such as the ‘Ring of Fire’.
Geothermal energy is not free from environmental problems. The steam contains large amounts of hydrogen sulphide with its smell of rotten eggs, and both steam and hot water contain substantial amounts of dissolved minerals, many of which are poisonous to the aquatic life in the streams and rivers into which they are eventually discharged. Also, the removal of steam or water causes the earth to subside in that region.
Geothermal energy was first used in the era of the ancient Roman bath house, in which they bathed in hot salt springs. Even though the Romans found an easy way to use geothermal energy, and we still enjoy natural hot springs, the process of removing geothermal energy from the earth to use it efficiently is rather difficult. The only three ways to use geothermal energy is for hot water, space heating and generating electricity.
In order to turn geothermal energy into electricity it must be brought up from the earth within a metal cased borehole that was driven deep into the naturally hot ground and put through a geothermal power station, (see diagram). The high pressure steam for these wells is used to drive turbines to generate 300 million kilowatts of electricity each year. Which if you compare all the power that the world produces from all the geothermal power plants, in 1985, to that of one average nuclear power plant the geothermal power will not yet compare.
Due to the acid gases in the steam delicate machinery was getting damaged. The hot water is corrosive and eating away at the pipes and expensive equipment making it even harder to removing it from the earth. The problem was solved by using the acid steam to heat acid-free water. This provided clean steam that would not damage the machinery. As a bonus, useful by-products were extracted from the acid steam, including boric acid, ammonia and carbon dioxide.
The possibility of tapping the heat energy stored in subsurface rocks is also being considered. The plan is to pump water into such regions by means of deep wells, and then pump the heated water back to the surface. One of the major problems is the poor thermal conductivity of rocks, and it is thus very desirable to have hot rocks that lie at relatively shallow depths.
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