In geothermal power plants, hot water and steam are used to turn turbine generators that make electricity, but unlike fossil fuel power plants, no fuel is ignited. Geothermal plants release water vapor, not smoky pollution. There are four main types of geothermal power plants: (1) flashed steam, (2) dry steam, (3) binary, and (4) hybrid.
The most common type of geothermal power plant is the flashed steam power plant. Hot well water is sent through separators where, released from the pressure of the deep reservoir, some water instantly boils to steam. The steam's extreme force spins a turbine generator. To save water and keep up reservoir pressure, the residual water and condensed steam are sent back down an injection well into the reservoir to be recycled and reheated.
Dry steam plants are less common. They produce mostly steam and very little water. In dry steam plants, the steam shoots directly through a rock-catcher and into the turbine. It is thought that the first dry steam geothermal power plant was built in Larderello, Italy, in 1904. These Larderello power plants were destroyed during World War II, but have been rebuilt and expanded, and produce electricity today.
The Geysers (northern California) dry steam reservoir has been providing geothermal power since 1960. Known as the largest dry steam field in the world, it furnishes enough electricity to supply a city the size of San Francisco.
In a binary power plant the geothermal water is sent through one side of a heat exchanger (series of pipes), where heat is transmitted to a second (binary) liquid, known as a working fluid, in a separate, nearby set of pipes.
A working fluid (isobutane or isopentane) boils and flashes to a gas at a lower temperature than water.
The working fluid boils to a gas, like steam, and powers a turbine generator. A working fluid can be concentrated repeatedly back to a liquid and reused. The geothermal water passes only through the heat exchanger and is immediately sent back to the reservoir.
Although binary power plants are more expensive to build than steam-driven plants, they have several advantages: (1) a working fluid (isobutane or isopen-tane) boils and becomes gaseous at a lower temperature than water (electricity can be obtained from lower temperature reservoirs), (2) a binary system uses reservoir water more efficiently (hot water in a closed system results in less heat/water loss), and (3) binary power plants have almost no emissions.
Hybrid power plants combine processes. In some power plants, flash and binary processes are combined to produce power. A hybrid system has been developed in Hawaii that provides nearly 25% of the electricity used in the state.
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