Geothermal power generation works by extracting energy from heat deep within the Earth. In some places, such as the islands of Iceland, the Philippines, and Japan, geothermal hot springs are common, making the energy easily accessible. In other places, sufficient heat is only found more than a mile (1.6 km) beneath the surface, making geothermal energy extraction impractical. Holes dug down to this level anywhere can usually be used to produce geothermal energy.
The first station for geothermal power generation was built on 4 July 1904, at the Larderello dry steam field in Italy and tested by Prince Piero Ginori Conti, making the use of geothermal power generation just over a century old. Still, geothermal power generation is not a major energy source worldwide, only making up about 0.1% of global power, or a capacity of around 100 gigawatts. Geothermal energy is only used extensively by the countries of the Philippines and Iceland, where they make up 15-20% of electricity generation.
The most common method of operation of a geothermal plant is the binary cycle, whereby moderately hot water from shallow geothermal heat sources is passed over pipes, called a heat exchanger, containing a secondary fluid — typically butane or pentane hydrocarbon — with a boiling point much lower than water. The heat causes the secondary fluid to flash to vapor, which drives turbines that generate electricity. The fluid is then recondensed and used for more cycles. The pressure of the secondary fluid is usually quite high, around 500 PSI.
Hot dry rock geothermal energy is another type of geothermal power generation; this method pumps water deep into the Earth, putting it in contact with the hot, dry rock that gives the method its name. The rock heats the water, which is then pumped out and used either directly with steam turbines, or pumped through a binary cycle system. Also known as Enhanced Geothermal Systems, these plants have a useful life of about 20 to 30 years before the rock cools down too far for the plant to be economical. Full heat recovers in 50-100 years.
Another type of geothermal power generation are flash steam plants. These plants pull hot, high-pressure water from deep in the Earth up into tanks, where the pressure drop causes the water to burst into steam, which is then used to turn turbines. One of the oldest types is direct heat, which pulls hot water from near the Earth's surface into pipes for use to heat buildings, heat water for fish farming, grow plants in greenhouses, pasteurize milk, etc. This only works in areas where sufficient hot water is available near the surface.