Geothermal Power

How geothermal power works

Geothermal power uses heat generated by the Earth's core to provide energy. This heat can be captured and used on both a residential and utility scale. Residential geothermal uses water running through underground pipes (called geothermal heat pumps) to regulate a building's internal temperature. In winter, the water in these pipes carries heat from the Earth into the building. In summer, the geothermal system carries excess heat out of the building. Although geothermal heat pumps are growing in popularity, this article focuses on utility-scale geothermal projects, which use geothermal plants to generate electricity for the power grid.
Utility-scale geothermal projects get energy from groundwater found deep in underground cracks and reservoirs. In places where the Earth's natural heat has brought these underground reservoirs to temperatures of 225 to 600 degrees Fahrenheit (107 to 315 degrees Celsius), the groundwater can be used to power a geothermal plant.
There are three common types of geothermal power plants: dry steam, flash steam, and binary cycle.
Dry steam plants are the simplest and most common. They rely on steam released from underground sources to turn turbines and generate electricity. Although dry steam plants are simple to operate, they are restricted by the relatively few locations that produce sufficient steam for a utility-scale plant.
Flash steam geothermal plants mine hot water through long pipes that extend into deep underground reservoirs, where extreme pressure allows water to remain in a liquid state above its surface boiling point. The water, usually at temperatures of more than 360 degrees Fahrenheit, is piped up to holding tanks. When the high-pressure hot water enters these low-pressure tanks, it becomes steam. Then, in dry steam plants, this steam powers turbines to generate electricity.
In a binary cycle plant, hot water is also piped from underground reservoirs, but then a different fluid, with a lower boiling temperature than water, captures the water's heat through a heat exchanger. The vapor from this other fluid is used to turn the plant's turbines and generate electricity. A binary cycle plant can operate in an area with geothermal fluid temperatures of 225 to 360 degrees Fahrenheit (107 to 182 degrees Celsius), lower than the temperature required of the other plants.
Geothermal plants release the cooled water back into the ground, where it seeps back into the underground reservoir, is reheated by the earth, and can be reused by the plant.
Through this full cycle, geothermal power provides a renewable and inexhaustible source of energy.

 http://www.bls.gov/green/geothermal_energy/geothermal_energy.htm