Dry-Steam Power Plant Generator Turbine Condenser
Cooling tower
Water Water Water Production well Steam Flash-Steam Power Plant Generator Turbine Steam Steam Brine Waste brine Production well Binary Power Plant Generator Turbine Isobutane Heat exchanger Hot brine Pump Production well Geothermal zone Injection well
> Geothermal power plants. Dry-steam power plants are the most basic style of geothermal power plants (top). Steam piped from a hydrothermal reservoir directly enters turbines to generate electricity. As the steam cools and condenses, the water is gathered and injected back into the reservoir where it is reheated as it travels through the formation to the production well. Flash-steam plants (middle) use hot water that is below the boiling point while at reservoir pressure but that flashes to steam at lower surface pressures. Binary power plants (bottom) use a closed system to exploit even cooler reservoirs whose water temperatures are less than 150°C [302°F]. Water flows or is pumped to the surface and enters a heat exchanger where it brings a second fluid, in this case isobutane, to its boiling point, which must be below that of water. The second fluid expands into a gaseous vapor that then powers electricity-generating turbines. This fluid may be circulated through the heat exchanger for reuse rather than being disposed of and, because the water does not come into contact with the power generator, maintenance costs are usually lower than with dry-steam or flash-steam hydrothermal plants.
Condenser Air Water
Cooling tower
Cool brine Air and water vapor Air Water Geothermal zone Condenser
Cooling tower
Water Water Air Air and water vapor Air
Direct heat uses
Injection well Geothermal zone Injection well Air Air and water vapor Air
Hydrothermal energy is a specific form of
geothermal resource. Characterized by high temperature, high permeability and rock that contains large volumes of water, it is often found at relatively shallow depths. Without stimulus, or aided only by high-temperature electrical sub- mersible pumps, these formations can deliver superheated water or steam to the surface through large-diameter production wells. The steam, or hot water flashed into steam at the sur- face, is funneled to drive turbines that generate electricity. Such formations exist in relatively few places around the world. Hydrothermal reser- voirs are found predominantly in areas of high tectonic activity where hot-water reservoirs are abundant and pressured, such as in the area of the Pacific Ocean known as the “Ring of Fire.” Most formations around the world that have
the requisite water and permeability do not have sufficient heat to be considered geothermal energy sources. But there are others with deep, high- temperature zones that lack only sufficient water or permeability, and it is these that may hold the most promise as future sources of geothermal energy. The solution to tapping such widely avail- able heat resources is through enhanced, or engi- neered, geothermal systems (EGS). Put simply, EGS projects create or sustain
geothermal reservoirs. In cases of low permeabil- ity, the formation may be hydraulically fractured. Formations with little or no liquid or without a sufficient recharge source may be supplied with water through injection wells. Today, engineers and geophysicists are bringing techniques for EGS to high-temperature dry reservoirs at depths of 3 to 10 km [10,000 to 33,000 ft] below the sur- face. At these depths, the rock is hot enough to convert water to superheated steam. These hot dry rock (HDR) systems are a unique
type of EGS, characterized by very hot basement formations with extremely low permeability. They require hydraulic fracturing to connect water- injection wells to water-production wells. Other prospective formations contain permea-
bility and water but are not hot enough for geo- thermal applications. To exploit these resources, less ambitious concepts are being advanced through binary power plants. These plants use water that is below the boiling point to heat a sec- ond fluid with a boiling point that is below that of water. The vaporized second fluid is funneled to turbines to generate electricity (left).2 This article focuses on hydrothermal and HDR
AUT09–RVF–02
technology. The state of EGS technology is dis- cussed through preparations for an EGS-expansion project in Nevada, USA, a case history from
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Oilfield Review
Isobutane vapor
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