Geothermal energy is recovered energy from the heat generated deep inside the earth. It is one of the renewable energy sources as the heat is continuously generated in the earth core from the slow decay of the radioactive particles.
Image source: http://geo-energy.org/basics.aspx
The earth core is made of solid iron covered by very hot melted rock, called magma. This magma can come closer to earth surface as volcanoes or erupts as lava. Rock and water absorb the heat from this magma and get hotter and hotter as it gets deeper.
Geothermal energy currently has three main usages:
Direct Use of Hot Water from Hot Spring or Reservoir
The hot water that carries earth warmth and emerges near the surface as hot spring or reservoir can be used directly for beneficial usage. Other than bathing, the most common use of this hot water is piping it directly in the heating system of both residential buildings as well as commercial industries.
Geothermal Energy to Control Building Temperature
Almost everywhere ground temperature, generally 10 feet below from the top surface stays in the range of 10 to 16 degree centigrade. As a result, the surrounding soil temperatures are warmer than the air in winter and cooler than the air in summer. This stable earth temperature can be recovered and utilized for different usage like to heat and cool buildings. This can be done through a geothermal pump system that transfers recovered heat through hot water to buildings in winter, and reverse in the summer.
A geothermal heat pump system consists of a heat pump, an air delivery system (ductwork), and a loop of pipes buried in the ground near the building acting as a heat exchanger. In the winter, the heat pump collects heat through the buried pipes from the ground 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 and release it into the ground through the buried piping systems. This removed heat from the indoor air can also be utilized by delivering it to the hot water system in the building.
Geothermal Energy to Generate Electricity
Geothermal energy can be used to produce electricity. But high temperature is required to produce electricity that can only access from deep underground. Generally that heat is brought up by fluid circulation like water, oils etc. This heated fluid is then use to turn a turbine of a generator to produce electricity. When the fluid is cool it is returned to the heat source to repeat the process.
Where the earth’s crust is thin these circulations exist naturally like hot springs or magma conduits. If not, an artificial well also can be drilled in to a hot aquifer and most part of the world the well need to be at least several kilometer deep for electricity generation. Therefore location is very important to establish an efficient and economically viable geothermal power plant.
There are mainly three different types of geothermal power station:
Dry Steam Power Plants is the
oldest and simplest of all that directly uses the geothermal steam of 150°C or
greater to spin the rotor of a turbine. The turbine is connected to a generator
for producing electricity. The produced electricity can be distributed via
Flash Steam Power Plants collects
high pressure hot water from deep beneath and store that in a low pressure
tanks. Then they use the resulting flashed steam to drive a turbine for
producing electricity. The pulled up hot fluid temperature should be at least
180 0C or more.
Binary Cycle Power Plants uses moderately hot fluid (below
200 0C) to heat a secondary fluid with much lower boiling point via
heat exchanger. Then uses the secondary fluid to flash vaporizes to drive a
turbine. The cooled secondary fluid flows back to the heat exchanger and heats
up again. This type of plant is a recent development and most common type of
plant being constructed today.
Enhanced Geothermal Systems of Engineer Geothermal Systems (EGS) are referred as future of geothermal energy. In this type of system an artificial geothermal reservoir is created inside hot rocks by drilling deep down (at least 3 to 10 kilometers). Deep down earth core is made of hot dry rock. Then cold water is injected into that in a very high pressure. Cold water breaks down solid hot rock and creates an artificial reservoir.
The water gets hot from contact with the surrounding hot rocks. This water is then brought up using additional drill to run a binary cycle power plant. Theoretically, EGS can expand the reach of geothermal power enormously and provide access to an inexhaustible energy source (Hot Rocks and High Hopes, The Economist, Sept 2, 2010).
Source: Hot Rocks and High Hopes, The Economist, Sept 2, 2010; http://www.economist.com.
Geothermal energy is one of the cleanest energy sources. Geothermal power is considered a renewable and sustainable power source because the heat it uses is very small compared to the earth heat content. Although individual well can serve for many decades, sometime they may cool down or run out of water. But using some mitigation strategies like reducing production or re-injecting water, this can be prevented. With new technologies in development for reducing the cost and access of geothermal energy, the future of geothermal energy is very promising.
Existing technologies limit geothermal energy to existing geothermal reservoirs that exist mainly near the “Ring of Fire” a volcanic zone encircling the Pacific Ocean. Outside that geothermal energy is either not available or economical to produce.
Higher Cost of Production
The upfront cost associated with drilling wells and construction of the plants is relatively high and it is a drawback for expansion of geothermal energy.
Possibility of Earthquakes
There is potentiality that deeper drilling may cause an earthquake, although seismic activity has been reported small until today.
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