In the case of medium level geothermal energy; the water utilised is in the form of a liquid and is maintained under pressure to prevent its boiling.
If the water temperature is below 140°C, a heat exchanger is generally used: the water heats liquid in a secondary circuit (for example an alkane –a saturated hydrocarbon- or a light alkane and brings it to the boil. The gas so produced in the secondary circuit turns a turbine and produces electricity before being re-liquefied.
If the water is at a higher temperature, its pressure is lowered in a container designed for this purpose. In its vapour form, it is used to drive a turbine, as in the case of high-level geothermal energy.
In the case of high-level geothermal energy, the water is used in the form of steam at high temperature. The steam is dispatched into a turbine that produces the electricity. In general, steam is humid, that is to say it contains a proportion of water in liquid form. This liquid water must be separated before the steam is dispatched into the turbine. In the rare cases of dry steam, the water can be dispatched directly into the turbine without any special treatment other than purification as it comes out of the well. |