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Geothermal energy is just as inexhaustible and renewable as solar energy, and has the added advantage of being a continuously available energy source. Its production cost is less than that of fossil energy, but its front-end expenses can be high, if the production wells are deep and the ground is rocky. The temperature in the core of the earth, thanks to the decay of radioactive isotopes, is on the order of 4,000 °C, lava from volcanoes is about 1,200 °C, and thermal spring temperatures can reach 350 °C.
Geothermal energy can be used for both heat and electricity generation.
If the groundwater temperature exceeds 150 °C, “flash steam” power plants can be built. If the water temperature is between 100 °C and 150 °C, “binary cycle” power plants can be operated. At regular groundwater temperatures (as shown on the left of Figure 3), “geothermal heat pumps” (GHP) can be used to heat buildings in the winter and cool them in the summer. For example, the buildings at Bard College, Annandale-on-Hudson, N.Y., are heated by heat pumps that take heat from ground water, which is at year-around temperature of 50-60° F, and pump it into the buildings to supply hot air at around 120 °F. In all three cases, after the heat content of the geothermal water is used, the spent fluid is returned into the underground water reservoir to maintain its pressure.
In heating mode, the GHP operates the same way as a domestic refrigerator, except that instead of removing the heat from the inside and rejecting it to the outside, it takes the heat from groundwater, and moves it into the hot water or hot air that is used to heat the house.
In case of “flash steam” power plants, steam is either generated directly by the production wells, or the wells produce hot water from which steam can be separated to drive conventional steam turbine generators. The size of these plants ranges from 100 kW to 150 mW.
In “binary cycle” power plants (on the right of Figure 3), where the water in the ground is not that hot, a heat pump and a secondary working fluid are used to operate the turbine generator. In this case, the 100° C and 150° C water exchanges its heat content in a heat exchanger (evaporator) by vaporizing the working fluid. This vapor in turn drives the turbine generator, while the turbine exhaust is re-condensed by conventional cooling. These plants range in size from 100 kW to 40 mW.
In the next part of this series, I will discuss the design and control of the “zero energy home” of the future.
Béla Lipták is editor of the Instrument Engineers’ Handbook, former chief engineer of C&R (later John Brown) and former adjunct professor of Yale University. He can be reached at firstname.lastname@example.org.
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