Geothermal Energy

The Technology

Under Earth's crust heat is continually produced, mostly from the decay of naturally radioactive materials. The amount of heat within 10,000 meters of Earth’s surface contains 50,000 times more energy than all the oil and natural gas resources in the world. This heat energy is known as geothermal energy. Geothermal energy can be used in many ways, from large and complex power stations to small and relatively simple pumping systems.
In modern direct-use geothermal systems, a well is drilled into a geothermal reservoir to provide a steady stream of hot water. The water is brought up through the well, and a mechanical system—piping, a heat exchanger, and controls—delivers the heat directly for its intended use. A disposal system then either injects the cooled water underground or disposes of it on the surface.

Applications and Efficiency

Typical applications of geothermal technology include; electricity generation, supply of hot water and heat for use in buildings, agriculture, and industrial processes.

Electricity Generation

Thermal power plants require steam to generate electricity. The steam drives a turbine that activates a generator which produces electricity. Geothermal power plants use steam produced from reservoirs of hot water found deep below the Earth's surface. There are three types of geothermal power plants: dry steam, flash steam, and binary cycle.

• Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant, where it is directed into a turbine/generator unit.
• Flash steam power plants are the most common. They use geothermal reservoirs of water with temperatures greater than 182°C. This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.
• Binary cycle power plants operate on water at lower temperatures of about 107°-182°C. These plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.

Small-scale geothermal power plants (under 5 megawatts) have the potential for widespread application in rural areas as distributed energy resources. Distributed energy resources refer to a variety of small, modular power-generating technologies that can be combined to improve the operation of the electricity delivery system.
In the Eastern Africa, most geothermal reservoirs are located in Rift valley.

Heat pumps

The shallow ground, the upper 10 feet of the Earth, maintains a nearly constant temperature between 10° and 16°C. Like a cave, this ground temperature is warmer than the air above it in the winter and cooler than the air in the summer. Geothermal heat pumps take advantage of this resource to heat and cool buildings.
Geothermal heat pump systems consist of basically three parts: the ground heat exchanger, the heat pump unit, and the air delivery system (ductwork). The heat exchanger is basically a system of pipes called a loop, which is buried in the shallow ground near the building. A fluid (usually water or a mixture of water and antifreeze) circulates through the pipes to absorb or relinquish heat within the ground.
In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to heat water, providing a free source of hot water.
Geothermal heat pumps use much less energy than conventional heating systems, since they draw heat from the ground. They are also more efficient when cooling your home. Not only does this save energy and money, it reduces air pollution.

Capability and Limitations

While geothermal resource is ideal for distributed energy supplies, it is very site specific except for large scale generation that can be fed into the national grid.  For distributed generation, minimum economical size is typically 5 MW.  Turnkey development and installation is estimated at $2.25 million/MW compared to $2.5 million/MW for hydro power plants

Links to further reading

http://en.wikipedia.org/wiki/Geothermal_power
http://www1.eere.energy.gov/geothermal/