Geothermal Heat Pumps
From Thermal-FluidsPedia
Heat pumps, as the name implies, are devices that move heat from one place to another up the temperature gradient, i.e. removing heat from cold outside air in the winter and deliver heat to hot outside air in the summer. Geothermal heat pumps (GHPs) take advantage of the relatively constant temperature of water underground from one season to another. Because rocks and soil are good insulators, they are rather insensitive to variations in the ambient air temperature. They are warmer than the ambient air temperature above ground in the winter, and colder during the summer. Below a depth of approximately two meters, the temperature of the soil in most of the world’s regions remains stable between 7°C and 20°C.
Geothermal heat pumps exploit the earth or groundwater as heat sources by transferring heat from the soil to the house in winter and as a heat sink by transferring heat from the house to the soil in summer. In the simplest form, a mixture of water and ethylene glycol solution (antifreeze) circulating in underground pipes or loops is used as a medium for transferring heat into or out of a building. In the heating mode (during winters), the mixture is colder than the surrounding ground; thus it absorbs heat from the ground and warms up. The heated liquid is pumped into the building where it transfers its heat through a heat exchanger into the room. In the cooling mode (during summers), the process is reversed. Here, the water/glycol mixture is hotter than the surrounding soil, and thus it releases its energy to the ground and cools. The cold water exchanges heat with a refrigerant, circulating through a heat exchanger located inside the heat pump. A blower cools the air by forcing it across the refrigerant coil.
Just like conventional heat pumps, geothermal heat pumps can operate in a closed- or an open-loop cycle. In closed-loop systems, a small pump is used to circulate the fluid. In open-loop systems, water from an underground aquifer is piped directly from a well to a building, where it transfers its heat to a heat pump. The water is then returned to the aquifer through a second well some distance away. Open-loop systems are simpler and less costly, but can be used only when there is an abundant supply of ground water.
Depending on the location and the availability of ground space, loops can be installed either horizontally or vertically and in linear or loop configurations (See Figure 1). Horizontal installation is the most cost-effective for residential sites and in new construction where sufficient land is available. It requires trenches between one and two meters deep. Vertical installations are used in large commercial buildings, in places where land area is limited, where land is too rocky to dig trenches, or where digging causes major disturbances to the existing landscape. To save space, the pipe may be coiled or looped into a spiral. When there is an adequate body of water, submerging the loop into the water may be the most cost effective strategy. In such instances, the supply line is run underground from the building to the water and coiled into circles a few meters below the surface to assure that it is not susceptible to winter freezing.
Geothermal heat pumps are rated in two ways: the coefficient of performance (COP) (a) and the energy efficiency rating (EER) (b). Although COP is defined for either heating or cooling, EER is used to indicate the cooling efficiency. Geothermal heat pumps can be installed practically anywhere and, when compared to conventional cooling and heating systems, use 30-50% less electricity. If a geothermal system is planned before a building is constructed, installation is easy, and cost is relatively small. Furthermore, domestic hot water production is essentially free during summers. Unlike conventional rooftop models, geothermal heat pumps are small and can be installed indoors, typically in a basement or attic.
Worldwide, there are currently more than a half million geothermal heat pumps installed, totaling a thermal output of 7,000 megawatts; nearly 70% of them are in the United States.
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References
(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) COP is the ratio of heating or cooling achieved per unit of work supplied (See Chapter 5).
(b) EER = COP(cooling) x 3.413
Further Reading
Dipippo, R., Geothermal Power Plants: Principals, Applications and Case Histories, Elsevier, 2005.
Dickson, M. H., Fanelli, M., Geothermal Energy: Utilization and Technology, Stylus Pub., 2005.
Ochsner, K, Geothermal Heat Pumps: A Guide for Planning and Installing, Earthscan Ltd, 2007.
Gupta, H. , and Roy, S., Geothermal Energy: An Alternative Resource for the 21st Century, Elsevier, 2007.
Geothermics, Direct Science Elsevier Publish. Company, publishes articles on geothermal energy resources and technologies.
Geotimes, Journal of the American Geological Institute.
Geo-heat Center Quarterly Bulletin, covers how-to articles on various geothermal applications and equipment, progress in research and development activities of direct heat utilization
Journal of Volcanology and Geothermal Research, an international journal on the geophysical, geochemical, petrological, economic, and environmental aspects of volcanology and geothermal research.
External Links
National Renewable Energy Laboratory Geothermal Energy Program (http://www.nrel.gov/geothermal).
Idaho National Laboratory Geothermal Program (http://geothermal.id.doe.gov).
US Department of Energy Geothermal Technology Program (http://www1.eere.energy.gov/geothermal).
California Energy Commission ((http://www.energy.ca.gov/geothermal).
Geothermal Resources Council (http://www.geothermal.org).
Geothermal Energy Association (http://www.geo-energy.org).