To develop electricity from geothermal resources, wells are drilled into deep subsurface permeable regions containing natural hot water or steam, far below the shallow groundwater depth. The wells bring the hot fluids to the surface for use in generating electricity.
Because hot geothermal fluids will cool if transported more than a few miles from the wellheads, geothermal power plants must be built at the site of the reservoir. If the deep resource contains hot compressed liquid water, it may be expanded to create steam or used to heat a secondary fluid that is converted to steam. The expanding steam is separated in a surface vessel (steam separator) and delivered to the spinning turbine , and the turbine powers a generator. Flash, the most popular type, starts with hot water to produce electricity, while dry steam starts with hot steam to produce electricity. In the binary process, the geothermal water heats another liquid that boils at low temperature than water. The two liquids are kept completely separate through the use of a heat exchanger, so geothermal's already low emissions are reduced to zero. A combination of flash and binary technology, known as the flash/binary combined cycle, has been used effectively to take advantage of the benefits of both technologies.
Two cooling options are available: water or air cooling. The most popular type, water cooling, requires a continuous supply of cooling water and creates vapor plumes. Air cooled systems, in contrast to the relative stability and continuous efficiency of water cooled systems, can be extremely efficient in the winter months, but are less efficient in hotter seasons when the contrast between air and water temperature is reduced, so that air does not effectively cool the organic fluid. Air cooled systems are preferred in areas where the viewshed is particularly sensitive to the effects of vapor plumes, as vapor plumes are only emitted into the air by water cooling towers and not air cooling towers. Most geothermal air cooling is used in binary facilities.
While geothermal resources capable of power generation are only economically viable in the geologically active western states at this time, geothermal fluids and the heat they contain are currently used in 26 states for - direct uses in spas, for space and district heating, and in industry, agriculture and aquaculture. Besides, electricity production and direct use, the third type of geothermal use, often called - geothermal heat pump or - ground source heat pump technology, does not require a geothermal reservoir. Rather, heat pumps make use of stable temperatures of approximately 65 °F just a few feet underground, and can be used anywhere in the world.