Heat is formed in the Earth crust. It is produced as a result to the influents of the gravitation, atomic reactions and the radioactive disintegration of the Magma. The most of the heat is stored in the mountain and in the water that fills out the mountains pore- and crack system. Only a very little part reaches the surface. It is this stored heat that is called geothermal energy. The supplies are very large. In practice, there is only a fraction that can be extracted, but itís anyway enough to give a contribution to the energy supply.The temperature in the Earth crust is rising with the deep. Like an average value 30° C/km is used to mention. The temperature increase varies within widely limits depending on where on the Earth you are. In volcanic areas (for example Iceland) the increase can amount to more than 100° C/km while the old and stable Scandinavian primary rocks often only reach 15° C /km. In Iceland and on a number of other places in the world, with volcanic activities, the geothermal energy can be utilized for power production.

Geothermal power plants take advantage of a natural, clean energy source ñ heat from the Earthís interior ñ to produce electricity. Under the right geological conditions, the Earthís heat collects in large underground reservoirs of steam or hot water. This energy is tapped by drilling wells into the reservoirs and piping the steam or hot water to power plants, which convert the heat to electricity. The used geothermal water is then reinjected into the reservoir to maintain pressure and to sustain the reservoir.

3.1.1 Hydrothermal Geo-energy

3.1.1.1 Hot Water Hydrothermal Energy

3.1.1.2 Dry Steam Hydrothermal Energy

Around the world there are Geysers, which is an underground steam reservoir. Having access to a Geyser means that you can pipe the steam directly to the turbines.
Dry steam is produced when the hot mantle or magma superheats water trapped deep in the Earth. High-pressure steam forces its way upward and is released at the surface as an extremely hot gas having little or no liquid content. Dry steam sources are generally used to directly drive a steam turbine with a generator for the production of electricity. The largest dry-steam hydrothermal electricity generating plants are located northeast of San Francisco and have a total capacity of more than 1400 MW.

3.1. Hot-rock Geo-energy

Hot dry rock technology, currently under development, is one of the future geothermal technologies. It has the potential to provide geothermal power by generating electricity from the heat in deep rock formations that contain no water.
Hot-rock geo-energy is closely related to hydrothermal energy in that extremely hot rock deep in the Earthís crust is used to superheat water, turning it into highly pressurized steam. In this case, the rock is naturally dry and water is pumped down from the surface to be superheated. The water is pumped down to a hot rock bed, which can be as hot as 1000 degrees Celsius. The resulting steam is channeled to the surface to drive a steam turbine generating system. The steam is condensed in a massive cooling unit and then recycled down back to the hot rock bed. Ideal locations for hot rock installations are those that have hot rock beds at depths no greater than 3 or 4 km, and have crust that permit easy drilling. The most suitable places are found in volcanic regions.

3.1.3 Geopressurized water energy

Geopressurized water is water that is trapped in huge chambers or reservoirs deep down in the Earthís crust. The water down there is held under great pressure and temperature. Geologists can trace these reservoirs. Once the reservoir is found, it must be successfully tapped for energy. Thatís the hard part. If the reservoir is tappeble, hot water, steam and many natural gases are usually available and can be used for heating and generation of electricity. This form of geothermal energy tapping is very risky. It is not a recycling process, the chamber can only bee emptied once. Therefore it must be determined that the reservoir is sufficient in size to cover the cost of labor, equipment and facilities.

3.1.4 Direct usage of magma

Because of the high temperature of the magma (3000° C-5000° C), it would be great if we could use that heat directly to warm up water. In some regions, magma has been found in reachable deep, but unfortunately, we have not developed the equipment that is needed.

Because geothermal plants do not burn fuel, they have an inherent environmental advantage over power plants that do. The geothermal fluids are drawn from the Earth and returned to the Earth, so the environmental emissions are very low. Although there is of course some emissions and waste products. But overall, geothermal energy is one of the cleanest power sources available today.

The prime cause of acid rain is sulfur oxides, and the geothermal power produces minimal amounts of this gas. The coal emission for sulfur oxide is 5.44 kg/MWh, for oil 4.99 kg /MWh and for geothermal power only 0.16 kg/MWh.
Geothermal power plants manage the demands for clean air. Geothermal plants emit no nitrogen oxides and very low amounts of sulfur dioxide, and this involve that they easily meet the most strict clean air standards.
The low levels of air emissions produced by geothermal plants are caused almost entirely by the release of gases from geothermal fluids, except for water vapor from the cooling towers. These gases are for the most part carbon dioxide, which is not a pollutant, but which does act as a greenhouse gas to trap heat within Earthís atmosphere. Emissions of carbon dioxide from geothermal plants are much less than from fossil-fuel power plants, which produce about 1000 to 2000 times as much.
The gases released from the geothermal fluid may include hydrogen sulfide, which causes the characteristic odor often obvious near natural hot springs. At most general hot-water power plants, hydrogen sulfide is present in such low concentrations that it becomes insignificant. Also carbon dioxide, methane, and ammonia can be released in the process. But methods have been and are being developed to capture these secondary products and put them to useful applications.
Hydrogen sulfide is much more abundant in geothermal steam reservoirs. At the Geysers, the steam contains up to 0.15 percent hydrogen sulfide by weight, but the treatment processes remove more than 99.9 percent. These processes are much simpler, less expensive, and more effective than emissions-control equipment at fossil-fuel power plants. Typical emissions of hydrogen sulfide from geothermal plants are less than 1 parts per billion-well below what people can smell.

A second concern is in this area of water pollution. Underground water contains many minerals, including large quantities of salt. If for example a pipeline breaks or if this water is re-injected in the superficial mountain layers and mixed with the surface water, plants and animals will be destroyed and the local ecological system will be disrupted. The solutions to this problem seem to be purification or re-injection of subterranean waters back into the underground reservoirs.

A third concern is that of ground subsidence. As water is extracted from the crust, and exhausting steam relieves underground pressure, there may be (and in some cases have been), a tendency for the ground to crack or sink. Careful geological studies must be performed to insure a solid crust structure before energy extraction can begin.

Noise disturbances are limited to the building time for a geothermal foundation. An area with a diameter of approximately 1 km is then going to be exposed for noise from among other things drilling.

Many geothermal plants generate no appreciable solid waste, but some of them do, and they require special handling.
Geothermal water contains a high amount of dissolved corrosive salts that can be dangerous for the environment. One way of solving this problem is to separate the salt from the water. This can be done through a process where the salts are crystallized.
The solid by-products by these power plants often contain just enough heavy metals to require special disposal. The plants produce as much as 45 kg of solids per megawatt-hour (MWh) of electricity generated, but recent technical advances are greatly reducing the amount requiring disposal. Some plants are now able to dewater the by-products and rinse them to remove the heavy metals. The rinse water can then be injected back into the reservoir, and the remaining solids, mostly silica, are used as filler in concretes for building roads and buildings.
In another process the heavy metals are removed from the solid by-products by using microbes. Many of the metals are valuable and their recovery and sale may improve plant economics.

Another way of using the heat of the Earth is with heat pumps. The deep of the drilled holes are just a few meters. This is a more common way to use the Earth heat in households. In addition to making electric power, geothermal fluids are used directly to provide heat to buildings, greenhouses and in resorts and spas. This way of using the heat in the ground is called direct-use.
Geothermal direct-use technology is also available in the form of geothermal heat pumps. These heat pumps use the thermal properties of soil and rock just beneath the surface of the ground to provide a reliable, efficient source of home heating and cooling. In addition, they help the environment by allowing homeowners to use less energy.

Geothermal plants are compatible with scenic areas because the plants require very little land compared to coal and nuclear plants. This advantage is especially notable when the total effects of these plants are compared, including the mining operation needed for coal and uranium to support for 30 years. Depending on the technology, an entire geothermal field uses 7300-32000 m²/MW, versus 76500 m²/MW for coal and 20200 m²/MW for nuclear options.
A typical geothermal plant requires several wells. Although drilling these wells has an impact on the land, using advanced directional or slant drilling can minimize the impact. This allows several wells to be drilled from one drilling pad, minimizing the amount of land needed for drilling pads, access roads, and geothermal fluid pumping.
In arid regions, geothermal power plants can use air cooling to minimize their water consumption. Air-cooled plants have low profiles that blend in well with scenic areas.

For electricity and direct use: Geothermal reservoirs that are close enough to the surface to be reached by drilling can occur in places where geologic processes have allowed magma to rise up through the crust, near to the surface, or where it flows out as lava. The crust of the Earth is made up of huge plates, which are in constant but very slow motion relative to one another. Magma can reach near the surface in three main geologic areas:

Since the first geothermally generated electricity in the world was produced at Lardello, Italy, in 1904 the use of geothermal energy for electricity has grown worldwide to about 8200 megawatts in twenty-one countries around the world. About 60 million people are supported with electricity produced by geothermal power plants.

In Sweden, the temperatures are not so high that the energy can be used for this aim. On more places, the temperature however is high enough so that the geothermal energy can be utilized for house heating, possibly in combination with a heat pump. The sun heat up the Earth crust and this warmth remains in a part of the winter. This thermal energy can be extract by a use of a fluid, often water with an anti frees medium. This circulates in a tube system in the ground, and heat is then pumped from the water in the house, that you want to warm up, with a heat pump.
Economical conditions for geothermal energy extract in Sweden can today be found only in porous sandstone and in soft sand. The drilling costs become otherwise too high. The possibilities for hot water extract are largest in the south of Sweden but conditions can also be found in the lake Vättern, Siljans district and on Gotland. Solely in southwest of Skåne it is technical possible with a heat socket considerable bigger than the need.
At Klintehamn in Gotland there is a smaller geothermal foundation that is warming up a service house, a school and block of flats. Water with a temperature of 18° C is pumped up from the bedrock and is released, after cooling in a heat pump, out in the ocean.
In Sweden it can, depending on the bedrockís low temperature, be actual with heat production, when in other countries it can be actual with electricity production because of higher available bedrock temperature.

In Lund, a geothermal foundation is used since December 1984 to produce heat to the cityís district heating net. Approximately the half of Lunds district heating is produced with geothermal energy. Water with a temperature of 21° C is pumped up from a deep of 700 meters. After cooling in a heat pump the 4° C cold water is brought back to the bedrock.

In a few years the city of Malmö is planning to get some of its heat supply from a new power plant. Geothermal heat water will be the main source, but also biological fuels and industrial waste heat will be used. There will be two holes drilled, both with deeps of two kilometers. The water at this deep has got a temperature of around 70° C, but with a heat pump the temperature will be increased to 100° C. This power plant is planned to produce 50 MW, and 20 MW of this will be from the geothermal heat water. This 20 MW fulfil about 5% of the heat supply of the city. When the heat has been taken from the water, the water will be led down to the ground again, but then through an other hole 1,5 kilometer away from the first hole. The temperature of the water is then about 5° C.
The first testing drills will be this year and the heat plant is planned to be in use the autumn 2003. The cost of the project is calculated to approximately 160 MKr. Hopefully there will be a continuing of this project and then the drilling deeps will be around six kilometers.

The cost for geothermal energy varies around the world. Today the cost for electricity is about 40 to 65 öre/kWh. The cost is predicted to decrease when technology develops. The U.S. Department of Energy is working on a geothermal power plant that will produce electricity for 24 öre/kWh. Such a cost is expected to result in further 15 000 MW in new installations within a following ten-year-period.

Thousands more megawatts of power than are currently being produced could be developed from already-identified hydrothermal resources. With improvements in technology, much more power will become available. Usable geothermal resources will not be limited to the "shallow" hydrothermal reservoirs at the crustal plate boundaries. Much of the world is underlain (3-6 miles down), by hot dry rock - no water, but lots of heat. Scientists in the U.S.A., Japan, England, France, Germany and Belgium have experimented with piping water into this deep hot rock to create more hydrothermal resources for use in geothermal power plants. As drilling technology improves, allowing us to drill much deeper, geothermal energy from hot dry rock could be available anywhere. At such time, we will be able to tap the true potential of the enormous heat resources of the Earth's crust.

There are many reasons why geothermal energy isnít expanding. Here are a few:

We think that geothermal energy could be an interesting market for ALSTOM in Finspong. All geothermal electricity power plants require steam turbines. Thatís why it is so relative to ALSTOMís business. The only problem is that the steam that goes through the turbines contains dirt and corrosive salts, which in a long term damages the turbine. If ALSTOM Finspong decides to enter the market they need to develop their turbines to bee more insensitive to dirt in the steam.

Mark E. Hazen, Alternative Energy, Prompt Publications, 1991, ISBN 0-7906-1079-5

Delegationen för energiforskning, förnybara energikällor, 1979, ISBN 91-38-05214-8.

Statens energiverk, Hans Rode, El- och värmeproduktion, 1989:5, ISBN 91-38-12325-8.

Statens energiverk, Ett miljöanpassat energisystem, Rapport 3724, 1989:3, ISBN 91-38-12323-1