Source http://myecoproject.org/get-involved/energy-conservation/renewable-energy-sources/geothermal-energy/
An electrical grade hydrothermal system is by definition one that can generate electricity by means of driving a turbine with geothermal fluids. Only high- and moderate-temperature hydrothermal systems have sufficiently high temperatures to generate electricity. The temperature that separates electrical-grade from cooler hydrothermal systems changes with improvements in technology, but it falls within the 100EC to 150EC range.
There are three types of electrical grade hydrothermal systems (Figure 19):
Hot-Water Hydrothermal Systems of about 200 C or more are capable of producing steam at pressures sufficient to drive turbine generators. These systems are in porous and permeable rock saturated with water, which partly boils to steam when it rises up production wells. This steam is routed to a turbine generator to produce electricity. See example.
Vapor-Dominated Hydrothermal Systems are saturated with steam rather than liquid water. This is a less common system than the Hot-Water type. The origin of these systems requires a combination of potent heat source and a restricted source of water recharge. In these situations, only steam is produced through the wells, and this steam can be routed directly into turbine generators. These are the most desirable type of hydrothermal system for electrical production, but their occurrence is uncommon.
Moderate-Temperature Hydrothermal Systems are incapable of the production of steam at high enough pressure to directly drive a turbine generator. They are however, hot enough to produce a high-pressure vapour through heat transfer to a second “working” fluid, which in turn drives a turbine generator. These are termed binary generator systems. See example.
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