The Earth's crust is a bountiful source of energy called geo energy and fossil fuels are only part of the story. Heat or thermal energy of the Earth is by far the more abundant resource. To put it in perspective, the thermal energy in the uppermost six miles of the Earth's crust amounts to 50,000 times the energy of all oil and gas resources in the world.

Types of Geothermal Resources

The temperature at centre of the Earth at 6400 kilometres depth is estimated to be 4000°C or higher. Partially molten rock, at temperatures between 650° to 1200°C, is believed to exist at depths of 50 to 60 miles (80 to 100 km). Heat is constantly flowing from the Earth's interior to the surface. Most types of geothermal resources, hydrothermal, geo-pressured, hot dry rock, and magma-result from concentration of Earth's thermal energy within certain discrete regions of the subsurface.

Hydrothermal resources are reservoirs of steam or hot water, which are formed by water seeping into the earth and collecting in, and being heated by fractured or porous hot rock. These reservoirs are tapped by drilling wells to deliver hot water to the surface for generation of electricity or direct use. Hot water resources exist in abundance around the world. Technologies to tap hydrothermal resources are proven commercial processes.

Geo-pressured resources are deeply buried waters at moderate temperature that contain dissolved methane. While technologies are available to tap geo-pressured resources, they are not currently economically competitive.

Hot dry rock resources occur at depths of 5 to 10 miles (8 to 16 km) everywhere beneath the Earth's surface, and at shallower depths in certain areas. Access to these resources involves injecting cold water down one well, circulating it through hot fractured rock, and drawing off the now hot water from another well. This promising technology has been proven feasible, but no commercial applications are in use at this time.

Magma (or molten rock) resources offer extremely high-temperature geothermal opportunities, but existing technology does not allow recovery of heat from these resources.

Earth energy is the heat contained in soil and rocks at shallow depths. This resource is tapped by geothermal heat pumps.

The geothermal resource is the world's largest energy resource and has been used by people for centuries. It is environmentally friendly and is a renewable resource of energy.

Geothermal energy power plants

Electric power plants driven by geothermal energy already provide over 44 billion kilowatt hours of electricity worldwide per year, and world capacity is growing at approximately 9% per year. To produce electric power from geothermal resources, underground reservoirs of steam or hot water are tapped by wells and the steam rotates turbines that generate electricity. Typically, water is then returned to the ground to recharge the reservoir and complete the renewable energy cycle. There are several types of geothermal power plant available, depending on the temperature and pressure of the geothermal source.

Dry Steam Power Plants were the first type of geothermal power plant (in Italy in 1904). The Geysers in northern California, which is the world's largest single source of geothermal power, is also home to this type of plant. These plants use the steam as it comes from wells in the ground, and direct it into the turbine/generator unit to produce power.

Flash Steam Power Plants, which are the most common, use water with temperatures greater than 182°C. This very hot water is pumped under high pressure to equipment on the surface, where the pressure is suddenly dropped, allowing some of the hot water to "flash" into steam. The steam is then used to power the turbine/generator. The remaining hot water and condensed steam are injected back into the reservoir.

Binary Cycle Power Plants operate on the lower-temperature waters, 107° to 182°C. These plants use the heat of the hot water to boil a "working fluid," usually an organic compound with a low boiling point. This working fluid is then vaporized in a heat exchanger and used to turn a turbine. The geothermal water and the working fluid are confined to separate closed loops, so there are no emissions into the air. Because these lower-temperature waters are much more plentiful than high-temperature waters, binary cycle systems will be the dominant geothermal power plants of the future.

Developing and commercializing geothermal power technologies contributes not only to a cleaner environment, but to a healthy industrial base, as well. Around the developing countries of the world, demand for electric power is burgeoning�and nearly half of these countries have geothermal resources. These markets have proven particularly receptive to clean energy produced with indigenous resources, creating attractive export options for geothermal technologies and expertise.

Geothermal plants emit no nitrogen oxides and very low amounts of sulphur dioxide�allowing them to easily meet the most stringent clean air standards. The steam at some steam plants contains hydrogen sulphide, but treatment processes remove more than 99.9% of those emissions.

Direct Use of Geothermal Energy

Underground reservoirs are also tapped for "direct-use" applications in which hot water is pumped directly to heating system of greenhouses, spas, fish farms, and homes or buildings space heating and other hot water needs.

In a typical application, a well brings heated water to the surface; a mechanical system piping, heat exchanger, controls delivers the heat to the space or process; and a disposal system either injects the cooled geothermal fluid underground or disposes of it on the surface.

Direct-use systems typically require a larger initial investment, but have lower operating costs and no need for ongoing fuel purchases, therefore reducing life-cycle costs.

Ground-source heat pumps

Ground-source heat pumps (GSHP) extract heat from the earth which is a different form of geothermal energy. It is the low-temperature heat (10-20°C) that is found at relatively shallow depths within the earth's crust. This source of heat remains at a relatively constant temperature all year and can be taken from the ground itself or from groundwater. Heat pumps can increase the temperature to provide a more useful output temperature of around 40�50°C, ideal for low-temperature heating systems like under floor systems and radiant panels.

Ground-source heat pumps are not strictly a renewable source of energy, because they require electricity to extract and make use of low-grade heat. However, there is no reason why this electricity could not be generated by another form of renewable energy. Heat pumps can be very energy efficient, producing four or five times the amount of heat energy for every unit of electrical energy needed. A heat pump takes the heat from a refrigerant fluid (or water) that is in contact with the ground, extracts the heat from this source and transfers it to a heat sink where it can then be circulated through a heating system. Although the refrigerant fluid is cooled by this process, it can be re-circulated back through the ground where it will absorb more heat before being passed through the heat pump again.

In winter, heat pump systems draw thermal energy from the ambient temperature of the shallow ground, which ranges between 10° to 21°C depending on latitude. In summer, the process is reversed to a cooling mode, using the ground as a sink for the heat contained within the building. The system does not convert electricity to heat; rather, it uses electricity to move thermal energy between the building and the ground and condition it to a higher or lower temperature according to the heating or cooling requirements. Consumption of electricity is reduced 30% to 60% compared to traditional heating and cooling systems, allowing a payback of system installation in 2 to 10 years. And these low-maintenance systems have long lives of 30 years or more. Some systems are also capable of producing domestic hot water at no cost in summer and at small cost in winter. They can be used in most kinds of building and have both domestic and commercial applications.

An overview of the status of the geo-energy and GSHP technologies developments in the European Union is given on the European commission's website and that for India is available on the Indian Ministry of New and Renewable Energy (MNRE) website.

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