Geothermal power: how does it work?

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Geothermal power generation extracts energy from heat deep within the Earth. It is accessible in some places, but in others, it is impractical. The binary cycle is the most common method of operation, and hot dry rock geothermal power is another type. Flash steam plants and direct heat are also used. Geothermal energy accounts for only 0.1% of global power.

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 energy easily accessible. In other places, sufficient heat is only found more than a mile (1.6 km) below the surface, making the extraction of geothermal energy impractical. Holes dug up to this level anywhere can usually be used to produce geothermal energy.

The first plant for the production of geothermal energy was built on July 4, 1904, at the dry steam reservoir of Larderello in Italy and tested by Prince Piero Ginori Conti, exploiting geothermal production just over a century old. However, geothermal power generation is not a major source of energy worldwide, accounting for only 0.1% of global power or a capacity of about 100 gigawatts. Geothermal energy is widely used only 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, in which moderately hot water from shallow geothermal heat sources is passed over pipes, called heat exchangers, containing a secondary fluid – typically butane or pentane hydrocarbon – with a boiling point much lower than water. The heat flashes the secondary fluid into steam, which drives turbines that generate electricity. The fluid is then recondensed and used for multiple cycles. The secondary fluid pressure is usually quite high, around 500 PSI.

Hot dry rock geothermal power is another type of geothermal power generation; this method pumps water deep into the Earth, bringing it into contact with the hot, dry rock that gives the method its name. The rock heats the water, which is then pumped and used 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 20-30 years before the rock gets too cold for the plant to be economical. Full heat recovery in 50-100 years.

Another type of geothermal power generation is flash steam plants. These plants extract hot, high-pressure water from deep within the Earth in tanks, where the pressure drop blasts the water into steam, which is then used to drive turbines. One of the oldest types is direct heat, which draws hot water near the earth’s surface into pipes for use in heating buildings, heating water for fish farming, growing greenhouse plants, pasteurizing milk, etc. water is available near the surface.

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