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which there is little or no heat loss. Next the pipe is connected to the central power station. No condensation takes place because the steam is superheated. Many drill holes are connected to the central power station which results in mass quantities of superheated water vapor pushing the turbine. The more drill holes that are connected to the power station the greater the pressure of the gas flowing through the turbine. The greater the pressure of the gas the faster the turbine turns and the more electricity produced. In some power plants the water vapor itself is not used to turn the turbines but only to heat another purer substance. This method is less efficient but does not corrode the machinery. Most superheated gas from geothermal resources is not pure water but a mixture of gases. Some of these gases can be extremely corrosive so using purer non-corrosive materials has its advantages. Some common gases used are ethyl chloride, butane, propane, freon, ammonia. The efficiency of these generators is limited by the second law of thermodynamics. The second law of thermodynamics states that a thermal engine will do work when heat entering the engine from a high temperature reservoir is at a different temperature than the exhaust reservoir. The thermal engine must take heat from the high temperature reservoir convert some of that heat to work and exhaust the remaining heat into a low temperature reservoir. The difference between the heat put into the engine and the heat deposited as waste energy is transformed by the engine into mechanical work. The maximum possible efficiency of a heat engine is called its Carnot efficiency. Carnot efficiency is never reached and the actual efficiency is always lower than the Carnot efficiency. The greater the difference in temperature between the superheated gas and the low temperature exhaust reservoir the higher the efficiency of the power plant. The average actual efficiency for a geothermal power plant ranges from the single digits to about twenty percent. The average actual efficiency for a fossil fuel burning electrical power plant is approximately thirty percent. While other methods of electricity production may have slightly better efficiency than a geothermal power plant, the less destructive environmental impacts of geothermal power plants offset the importance of the a higher efficiency. Direct use of geothermal heat for heating purposes can result in actual efficiencies of up to ninety percent. Fossil fuel powered heat systems can generally only reach actual efficiencies of seventy to eighty percent. As well as being used for electricity, geothermal energy is currently being used for space heating. Geothermal heated fluid used for space heating is widespread in Iceland, Japan, New Zealand, Hungary and the United States. In a geothermal space heating system, electrically powered pumps push heated fluid through pipes that circulate the fluid through out the structure. Geothermal heated fluid is also being used to heat greenhouses, livestock barns, fish farm ponds. Some industries use geothermal energy for distillation and dehydration. Although there are many pluses to using geothermal energy there are also some problems. It was generally assumed that geothermal resources were infinite or they could never be completely depleted. In reality the exact opposite is true. As water or steam is pumped out of the well the pressure may decrease or the well may go dry. Although the pressure and fluid will eventually return it may not do so fast enough to be useful. Drilling geothermal wells is very expensive. It is generally figured that a geothermal well should last 30 years in order to pay for itself. Another factor to take into consideration is the disposal of the waste water. Some geothermal fluid consists of several toxic materials such as arsenic, salt, dissolved silica particles. These materials can pollute drinking water and lakes. When the waste water is reinjected back into the earth the previously dissolved silica particles precipitate out of the liquid and can block up the pores in the reinjection well. The cool water can also create new passages through the rocks and create unstable ground above. There are three main problems that can plague a power plant when it is operated using geothermal energy, silting, scaling and corrosion. Scaling is caused by silting or when suspended particles build up on the insides of the pipes. Scaling is directly related to the pH of the liquid. In some cases chemicals or other additives such as HCl have been added to the liquid to try to neutralize the liquid. Silting is when the particles that were dissolved in the hot fluid precipitate out when the fluid cools. This generally occurs in the pipes and can cause considerable damage to the pipes if significant pressure builds. This problem can be solved by using simple filters that are periodically changed in the pipes. Corrosion occurs because of acidic substances incorporated in the geothermal fluid. Usually geothermal fluid contains some boric acid. Using pipes that are not affected by these liquid generally takes care of corrosion. Unfortunately most metals that are non-corrosive are very expensive. Most types of wildlife can not live in or consume saline water. If the cooled fluid containing dissolved toxins and salt contaminates lakes or streams the environmental effects can be disastrous. Air pollution from geothermal resources is also significant. The most common type of air pollution is the release of hydrogen sulfate gas into the air. At the geysers in California an estimated 50 tons per day of hydrogen sulfite is released into the atmosphere. Iron catalysts have been added to try to offset the effects of pollution but have failed because moisture and carbon dioxide reduce the efficiently of the catalysts so much that it is not effective. Noise pollution is another consideration that must be taken into account. When the steam and water escape from the