Geothermal Energy
Geothermal energy is the heat stored below ground level. Essentially this heat is present in the Earth due to the natural radioactivity of the rocks composing the earth’s crust and so lower the contribution of the thermal exchanges with the deepest parts of the Earth.
Almost everywhere, the shallow ground or upper 3 meters of the Earth’s surface maintains a nearly constant temperature between 10° and 16°C. Geothermal heat pumps can tap into this resource to heat and cool buildings. A geothermal heat pump system consists of a heat pump, an air delivery system, and a heat exchanger-a system of pipes buried in the shallow ground near the building. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to provide a free source of hot water.
Depending on the characteristics of the geothermal resource there are three types of geothermal power plants;
- Dry Steam Power Plant
- Flash Steam Power Plant
- Binary Cycle Power Plant
Dry Steam Power Plant
This is the least common type of geothermal power plant, utilising the dry steam straight from the production well, from the geothermal reservoir. The high pressure dry steam passes up the production well and through a rock catcher; a series of mesh filters which catch any rocks, stones or other debris, which would damage the turbine blades. The steam then passes through a steam turbine that drives an electrical generator, which produces electricity for the grid.
The steam exits the LP stage of the turbine and into the turbine condenser, that is under a vacuum and from the condensate is pumped through a series of scrubbing towers that remove the gases which are non-condensable. From here it is pumped on to the water cooling towers, where the condensate is cooled and any remaining incondensable gasses are re-circulated to the scrubbers before being re-injected with the cooled condensate down the injection well back into the geothermal reservoir.
Flash Steam Power Plant
This type of plant injects water and condensate into the geothermal reservoir through the injection well and forces water at a high temperature (71.1°C) up through the production well. From the production well it is pumped through a series of pressure vessels which are at a lower internal pressure than the hot geothermal fluid, causing it to flash off into low, medium and high pressure steam. The steam then passes through the steam turbine condensing and being cooled as in a dry steam plant, returning to the geothermal reservoir along with the non-condensable gasses through the injection well.
Binary Cycle Power Plant
This type of plant uses high temperature geothermal water to heat another fluid which has a lower boiling point than water. The secondary fluid is heated by the geothermal fluid through a heat exchanger and flashes of to a vapour.
This vapour is used to drive a turbine, similar to a steam turbine and condensed back to a fluid before returning to the heat exchanger to start the cycle again. Because the geothermal fluid passes from the productio well, through the heat exchanger and back down the well in a continuous circut this is a closed loop system. Therefore in this type of plant there is no escape of noxious gasses nor is there any gas scrubbing required.
