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Applications Depending on the source and temperature, geothermal heat can be used either directly or by converting it to electricity. Geothermal resources can be classified as low-temperature (less than 100°C), moderate-temperature (100-200°C), and high-temperature (greater than 200°C). In addition to hot water and steam reservoirs, hot dry rocks and magma have enormous potential. In principal, the natural gradients present in these rocks can be used to extract an unlimited amount of energy - if the technology to exploit them is developed. Magma cannot be economically developed in the near future and will not be discussed further. Hot Water and Steam Geothermal Power Plants The highest temperature resources are generally best for electric power generation. When dry (superheated) steam is available, it can be directly expanded through a turbine to generate electricity. For hot water reservoirs under high pressure, the geothermal fluid is brought to the surface and sprayed into a tank. As the pressure drops, some of the water flashes into 210 steam. The steam is subsequently cleaned and piped directly into steam turbines, which drive electric generators. The remaining water is injected back into the reservoir to help maintain its high temperature and pressure. Dry steam reservoirs (fumaroles) are highly efficient, but rare. The largest plant of this type in the world is at the Geysers in northern California. Moderate steam temperatures are not practical for use in turbine facilities because the steam condenses before it has appreciably expanded through the turbines. Under such circumstances, binary-cycle power plants are more efficient. In a binary plant, the geothermal water is pumped at high pressure through a heat exchanger where it passes its internal heat to a secondary fluid with a lower vapor pressure (alcohol, isopropane, or other refrigerants), causing it to boil and turn into high-pressure steam. The steam is then used in a closed-loop cycle to drive a turbine/generator assembly. Because binary plants do not use the steam or hot water directly, the fluids can be reinjected back into the reservoir. This maintains the pressure and prevents any toxic or noxious gases from entering the atmosphere. To optimize efficiency, flash and binary cycles are often combined in a hybrid design. One such plant is the binary power plant near Mammoth Lakes in California; this system transfers heat from steam at 170oC to isobutene, which vaporizes and drives the turbines for a net generating capacity of 37 megawatts. Warm Water Systems: Direct Use At temperatures below 100°C, geothermal sources are available in the form of liquid water. Hot water can be pumped through pipes and used directly for residential, office, and greenhouse heating, domestic water heaters, swimming pools and spas, or can be passed underground to boost agricultural and aqua-cultural production in colder climates. Worldwide, around 12,000 megawatts of thermal energy is available from warm water systems for direct use. Iceland boasts the title of the largest consumer of hot water thermals; 87% of all homes are heated with geothermal water.5 Hot Dry Rocks Another method for using geothermal energy is to utilize the heat from aquifers and hot dry rocks (HDR). Hot dry rocks are impermeable solid slabs of hot rocks found a few kilometers below the surface. Granite usually contains trace amounts of radioactive uranium and thorium and therefore is substantially hotter than surrounding nonradioactive rocks. Cold water is injected through a bore hole to a bed of hot dry underground rock under pressures high enough to fracture the surrounding rock. Water is heated through conduction as it moves toward one or more nearby production wells where water can be extracted and, depending on the temperature, used either directly or for the generation of electricity. Since 5 Gawell, K., and Greenberg, G., “2007 Interim Report: Update on World Geothermal Development,” US Geothermal Energy Association, May 2007. 211 Chapter 9 - Geothermal Energy the pressure is high, water is still liquid even at temperatures of 200°C or more. When electricity generation is of interest, the hot water gives off its heat to a secondary fluid such as a refrigerant and turns it into vapor before returning to the well to become reheated -- closing the loop. The refrigerant steam drives a turbine-generator to produce electricity. A plant operated in such a closed-loop fashion is virtually pollution free and sustainable.6 An experimental plant of this type was constructed by the Los Alamos National Laboratory in New Mexico in early 1980s, which operated for 20 years before it was shut down.7 Many technological issues must be resolved before this type of plant is suitable for commercialization. Enhanced Geothermal Systems (EGS) Hydrothermal reservoirs are useful only when they are in the geographical areas where rocks are sufficiently permeable to allow easy flow of fluid. Enhanced Geothermal Systems essentially use the same technology developed for creating reservoirs in HDR. The fractures induced in surrounding rocks increase the permeability, which allows geothermal exploration in areas that otherwise were deemed inoperable. Many geothermal plants in Europe and Japan employ EGS techniques, thus increasing the productivity of existing hydrothermal reservoirs rather than creating new ones. Magma Magma is hot molten rock in the earth’s crust, much of it resides within the 5-km of the surface. Thermal energy from magma can be recovered by drilling holes and injecting cold water through magma. The magma heats the water into steam which will rise buoyantly through a second pipe. Magma is, however, highly corrosive and when solidified, creates a layer of insulation that limits the operability of such systems. Finding materials that can withstand hot corrosive magma for an extended time is a major obstacle to commercial development of this technology.


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