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Direct Use
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| Not all geothermal waters are warm enough to be used to make
electricity, but they are can be valuable nevertheless because they all
contain heat energy. Resources of low to moderate temperatures (20 -150
C) can be used for a wide variety of applications in the residential,
commercial and industrial sectors. From our earliest records we know that hot water has been used for spas, bathing, cooking and heating, especially in Japan and Europe. Such “direct uses” of geothermal have jumped dramatically in recent years, especially in the United States. And the range of use has expanded as well, to include food dehydration, laundries, textile processes, fish farming (aquaculture), resorts, greenhouses for growing everything from vegetables and fruits to flowers. U.S. geothermal greenhouses today cover more than 110 acres, and domestic aquaculture annually yields an impressive 17,545,000 kilograms (38,600,000 pounds) of fish. Both these applications can be found just east of the Arizona border, in southwestern New Mexico, south of Lordsburg. One of the most widespread and helpful direct uses of geothermal energy is in the form of district heating of homes, workplaces, and other facilities. In Iceland, for example, most space heating is provided by geothermal resources that concentrate there. In addition to the added security and economic benefits of using such local energy resources, such use has also removed most of the air pollution that used to plague the urban areas. In the U.S., there are many examples of direct use of geothermal energy in the western states. Boise, Idaho has been operating a district heating system continuously since 1892 and two of its production wells are still in service. District heating is also well advanced in Susanville, California; Reno, Nevada; San Bernardino, California; and at the Oregon Institute of Technology, where virtually all their heat requirements are met with geothermal energy. In addition, the Geo-Heat Utilization Center is headquartered at OIT. As the diagram to the left illustrates, geothermal waters are employed for various needs. They are even used to a small degree in a variety of industrial applications, including enhanced heap leaching of precious-metal ores and the drying of crops and building materials. The engineering considerations of direct-use geothermal systems are quite simple, usually consisting of a well to convey the heated water to the surface, a mechanical system including piping, a pump and a heat exchanger to convey the heat energy to where it is required, and a disposal system, such as an injection well or storage pond to receive the cooled fluid. The worldwide application of direct use geothermal in 2000 was 15,000 MW of thermal (heat) energy. Electrical generating capacity, by contrast, is about 8,000 MW. Even though there is much more direct-use capacity, each form yields about the same amount of energy for our use, approximately 50,000 GWh/yr. The rough symmetry of the energy use between the two forms results from the higher capacity factor of the electric systems. If, for example, a geothermal power plant operates every hour of the year at full capacity, its capacity factor would be 100%. As it turns out, the capacity factor for electrical systems is 71% and for non-electric use, 40%. The summary of the direct use of geothermal includes its use in Europe (35%), Asia (46%), and North America about 13%. In a recent survey conducted by OIT, 21 reported having electrical generating facilities, while 58 reported direct-use. |
Figure 10 - This Lindal diagram identifies several applications for hydrothermal resources, from fish farming to absorption chilling. Technical improvements since this diagram was first published in 1973 allow electrical energy today to be generated in certain systems down to 85C. Source: http://iga.igg.cnr.it/geo/geoenergy.php |
Figure 11 - Current direct-use applications of geothermal energy in the U.S |
From: Valgardur Stefansson and Ingvar B. Fridleifsson,Geothermal Energy
European and Worldwide Perspective, 1998
| Remembering that
geothermal resources must be utilized on site, one of the most attractive
possibilities for future geothermal energy application is to increase
its use in those locations where the resource is collocated with existing
communities. A collocated community is defined as being within 8 km of
a geothermal resource with a temperature of at least 50°C. Over 1900
thermal wells have been identified by State Teams as having temperatures
greater than or equal to 50°C, and 1469 are collocated with communities.
There are 271 such cities, with a population of 7.4 million in the 10 western
states that could potentially utilize geothermal energy for district heating
and other applications, according the James Witcher, of the Southwest Technology
Development Institute, New Mexico State University. One of the most popular uses of geothermal energy is largely passive, heat pumps. These devices operate on the same principal as refrigerators, but can move heat in either direction. The devices therefore take advantage of the relatively constant temperature of the earth several feet below the surface, using it as a source and sink of heat for both heating and cooling. In summer, heat is extracted from buildings being cooled and dissipated into the earth. In winter, heat is removed from the earth and pumped into the building. Even marginal geothermal resources with temperatures as low as 20C can be utilized. Such systems are used widely in Switzerland and the Scandinavian countries, and are now being installed in the many thousands in the U.S. One of the most efficient designs for the direct use of geothermal energy is in a “cascade system”. In such an arrangement, the waste water of one process becomes the feed water for the next, with the highest temperature user closest to the well, and the lowest temperature user farthest away from the well. The accompanying diagram illustrates this concept. To best use the resource in this manner users must be in the proper sequence. Sometimes this sequence is already in place, such as is the case in Susanville, California. Most of the time, however, land use planning should and can be coordinated around the use of the resource, especially in places where no development currently exists. |
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