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Geothermal energy (geo meaning "earth" and thermal meaning "heat") is energy recovered from the heat of the earth's interior. Geothermal heat can appear in the form of volcanoes, hot springs and geysers. The high temperatures in the earth's interior are a result of heat trapped during the formation of the planet, as well as the decay of naturally occurring radioactive elements. Temperatures below the surface can reach 4,200° C, decreasing to 650 - 1200° at depths of 80km-100km. Through the deep circulation of groundwater and the intrusion of molten magma into the earth's crust at depths of only 1km-5km, the heat is brought closer to the earth's surface. The molten rock then heats the surrounding groundwater, which is forced to the surface in certain areas in the form of steam or hot water (i.e. hot springs, geysers). The heat energy close to or at the earth's surface can be utilized as a source of energy.
Using modern scientific and engineering techniques, geothermal systems may be sustained commercially for decades. The Geysers (California) and Wairakei (New Zealand) have produced electric power continuously for 40 years. The pioneering Larderello field in Italy has been productive since 1904. In fact, not a single geothermal field has been exhausted to date, although reservoir pressures and temperatures have declined slowly in response to production.
The heat source for geothermal energy comes primarily from large, magmatic systems deep in the earths crust. These are still partially molten or crystallized, but are hot igneous intrusions that yield their heat gradually over hundreds of thousands of years. As the earth cools over time, there is a constant movement of thermal energy that travels outward through highly permeable fracture zones to the surface.
Geothermal energy is classified as a renewable source of electricity. It is considered to be a clean, environmentally friendly, sustainable method of electrical power generation. The use of geothermal energy is an established method of electrical power generation with 2,000 megawatts (MW) of installed capacity in the Western United States.
The benefits of geothermal energy are far reaching. A geothermal power plant does not burn fuel; energy production is clean, and renewable. It is a base load source; therefore, geothermal plants are designed to run 24 hours a day, every day with no emissions.
The US market demand for electricity is rising, creating a growing dependency on non-renewable non-sustainable resources. Consumption of fossil fuels has produced significant environmental problems including air pollution, smog and global warming. As a result of the environmental impact, rises in the world oil price and thinning reserves the world has looked to other sources of energy that will help alleviate these issues. Geothermal energy is rapidly growing into a $1.5 billion per year industry. There are approximately 12,000 MW of direct use and over 10,715 MW of generating capacity globally.
Binary Cycle Geothermal Power Plants
In a binary cycle geothermal power plant hot water is produced to a steam piping and gathering system from wells drilled into the geothermal reservoir. The hot water flows to a heat exchanger called a vaporizer where it vaporizes a secondary working fluid causing the original hot water to become cool. All of the cooled water is then pumped to injection wells where it is injected to help recharge the geothermal reservoir. The vaporized working fluid passes through a turbine which drives an electrical generator that is tied into the electrical transmission grid. Upon discharging the turbine the secondary working fluid is condensed before piping it back to the vaporizer where the process is repeated.
Dry Steam Geothermal Power Plants
An example of a vapor dominated geothermal system is as The Geysers in central California. Dry steam is produced from wells through a piping system and run directly through a turbine. The turbine drives an electrical generator that delivers power to the electrical transmission grid. Steam discharges from the turbine into a condenser where it is condensed forming hot water. The hot water is pumped to a cooling tower where additional heat is removed. The cooled water from the cooling tower is recycled back to the condenser to repeat the process. Any excess water from the cooling tower is pumped through a piping system to injection wells where it is injected back into the reservoir which helps to recharge the geothermal reservoir.
Flash Geothermal Power Plants
In hot water geothermal systems (temperatures greater than approximately 400 degrees Fahrenheit), flash production systems are often used. The hot water is produced from wells drilled into the geothermal reservoir. The hot water from the various production wells is piped to a flash tank where the pressure is reduced. The reduction in pressure in the flash tank causes part of the hot water to flash to form steam and part to remain as water. The flash tank also acts a separator, separating the steam from the water. The hot water separated from the steam is pumped through a pipeline system to injection wells and injected into the reservoir for reservoir recharge. The steam coming off the flash tank/separator is piped directly to a turbine where the process is identical to that used for dry steam geothermal power plants.