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geological, hydrogelogical and heat transfer characteristics. Most geothermal heat is trapped or stored in rocks. A liquid or gas is usually required to transfer the heat from the rocks. Heat is transferred in three different ways, convection, conduction, and radiation. Conduction is the transfer of energy from one substance to another, through a body that may be solid. Convection is the transfer of energy from one substance to another through a working moving medium, such as water. The medium usually transfers the energy in an upward direction. Radiation is the transfer of energy out of a substance through the excitement of gas molecules surrounding a substance. Radiation is dependent upon two things the object emitting the heat and the surrounding’s ability to absorb heat. Convective geothermal systems are characterized by the natural circulation of a working fluid or water. The heated water tends to rise and the cool to sink continually circulating water throughout the ground. The majority of the heat transfer is done through convection and conduction, radiation hardly ever effects heat flow. When geothermal heated water collects into a reservoir one form of a geothermal resource is created. One can approximate the amount of thermal energy present in a geothermal resource by comparing the average heat content of the surface rocks with the enthalpy of saturated steam. Enthalpy is energy in the form of heat released during a specific reaction or the energy contained in a system with certain volume under certain pressure. It is generally accepted that below a depth of ten meters, the temperature of the ground increases one degree Celsius for every thirty or forty meters. At a depth of ten meters annual temperature changes no longer affect the temperature or the earth. The most common geothermal resources used for the production of human consumed energy are hydrothermal. Hydrothermal systems are characterized by high permeability by liquids. There are two basic types of hydrothermal systems, vapor and liquid dominated systems. In a liquid based system, pumps must be placed very deep in the well where only the liquid phase is present. By keeping the liquid under pressure it is possible to keep the liquid at a much higher temperature than the liquid s normal boiling point. If the liquid is not kept under pressure, it will flash. Flashing is the process of vaporization. It requires 540 calories per gram of heat to vaporize water. The super heated pressurized water is pumped up a long shaft into the plant. When it reaches the plant, controlled amounts of the pressurized water is allowed to flash or vaporize. The rapidly expanding gas pushes or turns the turbine. A power plant may have numerous flash cycles and turbines. The more flash cycles the higher the efficiency of the power plant. Once the heated liquid has been used to the point where it has cooled to an unusable temperature it is reinjected into the ground in hopes that it will replenish the geothermal well. Vapor systems work in much of the same way. The super heated gas flows through surface reboilers that remove all of the non-condensable gases from the mixture of gases. The gas is pumped into pressurization tanks where extreme pressure causes the gas to condense. The super heated liquid is then allowed to flash. The rapidly expanding gas turns the turbine. Specific examples and sites of electrical energy production will be discussed later. Conductive geothermal systems consist of heat being transferred through rocks and eventually being transmitted to the surface. The amount of heat transferred in a conductive geothermal is considerably less than the heat transferred in a convective system. Conductive geothermal systems lack the water to efficiently transfer the heat, so water must be artificially injected around the hot rocks. The heated water is then pumped from the underground reservoir to the surface. This system is not as effective as others because the temperature that the heated water reaches is not very great. Geopressured geothermal systems are similar to hydrothermal systems. The only difference is the pressure of the high temperature reservoir. Geopressured geothermal systems may be associated with geysers. Some geopressured geothermal systems reach pressures of fifty to one hundred megapascals (MPa) at depths of several thousand meters. These systems provide energy in the form of heat and water pressure making them more powerful and useful. Currently most electricity producing geopressured geothermal systems are only experimental. There are many factors in this type of system that are very hard to predict such as the reservoirs potential energy. It is very hard to predict the force at which the water will be projected from the well since the pressure of the high temperature is constantly changing. The salinity of the liquid projected is also very high. In some instances the liquid consists of twenty to two hundred grams of impurities per liter. Today with the depletion of many other natural resources using geothermal resources in more important than ever. Hot springs are natural devices that bring geothermal heated water to the surface of the earth. This processes is very efficient, little heat is lost during the transportation of the water to the surface. The heat is brought to the surface via water circulation in either the liquid or gaseous form. Geothermal hot springs are a good source of energy because it is probable that they will never be exhausted as long as water is not pumped from the spring faster than it naturally replenishes itself. A simplified version of a vapor run geothermal electric plant might operate under the following conditions. Holes are drilled deep into the ground and fitted with pipes that resist corrosion. When the hole is first opened, steam escapes into the atmosphere. Once the pipes are inserted into the holes the steam expansion becomes adiabatic. An adiabatic system is a system in