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Geothermal Plants

From the boiling vapors at the heart of the Earth to the electrical grid: here’s how a geothermal power plant produces renewable energy

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Geothermal plants use heat from the depths of the Earth to produce renewable electrical energy. Our planet’s internal temperature increases gradually the closer we get to its core. This increase in temperature, called the geothermal gradient, is on average about 3° every 100 meters of depth, but in some areas – under certain geological structural conditions – this is much higher, with temperatures of 250-350°C at depths between 2000 and 4000 meters.
 

How Geothermal Plants Work

Through the fracturing of rock layers, heated water and steam from heat sources (for example, magma ascent from low depths and/or the thinning of the earth’s crust) rise to the surface, where they are intercepted by geothermal extraction wells. The steam from the wells is then conveyed into pipes, called steam pipes, and sent to operate a turbine, where the energy is transformed into mechanical rotational energy.

The turbine axis is connected to an alternator rotor that, by turning, transforms the mechanical energy into alternating electrical energy, which is then transmitted to a transformer. This raises the voltage value to 132,000 volts and puts it into the distribution grid.

The steam coming out of the turbine is returned to a liquid state in a condenser, while the non-condensable gases in the steam of the subsoil are dispersed into the atmosphere only after specific treatments are done to break down the main pollutants, like hydrogen sulfide and mercury (Hydrogen Sulfide and Mercury Abatement Systems, AMIS).  A cooling tower allows the water produced by the condensation of the steam to be cooled: at this point the cold water is either used in the condenser, to lower the temperature of the steam, or is injected into deep rock with injection wells, to initiate a new production cycle of renewable energy. 

1. Cooling tower | 2. Pipeline | 3. Pipes

Types of Geothermal Plants

Geothermal energy production plants use three main technologies: dry steam, flash and a binary cycle

  • Dry Steam: the most common technology, this involves the use of steam at high temperatures (over 235°C) and pressure to move a turbine paired with an electrical energy generator. 
  • Flash: dominant-water tanks (temperatures over 150-170°C) are used to power systems in single or double flash. Water comes to the surface through wells and, because of the rapid change in pressure from the tank to the atmosphere, is separated into steam sent to the plant and liquid then injected into the tank (single flash). If the geothermal fluid gets to the surface at very high temperatures, it can be put through the process twice (double flash).
  • Binary cycle: in tanks that produce water at moderate temperatures (between 120 and 180°C), the geothermal fluid is used to vaporize, through a heat exchanger, a second liquid (usually isobutane or isopentane), with a lower boiling point than water. The secondary fluid expands in the turbine and is condensed and sent back to the exchanger in a closed circuit, without any exchanges with the outside.
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