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BackPage Primer from Environmental Building News

September 1, 2007

Ground-Source Heat Pumps:
Tapping the Earth's Mass

Ground-source heat pumps (GSHPs), often called “geothermal heat pumps,” exploit the relatively stable temperatures found just 5 feet (1.5 m) or more below the surface, either depositing or extracting low-intensity heat. Heat pumps—whether ground-source or air-source—are basically air conditioners that can be run in reverse to provide heating as well as cooling.

Air conditioners (and refrigerators) make a relatively cool place cooler by extracting heat from it and releasing the heat in a warmer place. The key to this magic is the refrigerant, a fluid that changes from a liquid to a gas at ambient temperatures. After absorbing heat, the gas moves through a mechanical compressor that squeezes the heat back out, not unlike squeezing water out of a sponge. By releasing this heat to the temperate earth rather than into hot outdoor air, GSHPs cool more efficiently than air conditioners or air-source heat pumps. They provide heat by using the same principle in reverse—drawing heat from the relatively warm earth rather than from cold outdoor air.

GSHPs should not be confused with geothermal heat, which lurks roughly six miles below the surface, where the earth’s crust gives way to a layer of molten rock. This geothermal energy occasionally explodes to the surface as a volcano, creates natural geysers and hot springs, and, in places like Iceland, it is tapped to produce electricity.

There are many variations on the ground-source theme. In some installations, especially those for residential or small commercial buildings, the connection to the earth is through pipes laid in horizontal trenches. Larger systems typically use drilled wells that may descend over 1,000 feet (300 m), using groundwater as the primary source of stable temperatures.

Some GSHP configurations come with ecological burdens. Groundwater may be extracted and dumped, or it may be contaminated by pollutants leaking in through the wells. The refrigerants used in some systems contribute to ozone depletion (although not as much as the older generation, which has been phased out of use). Finally, a poorly engineered system can change the temperature underground, with potential ecological implications, not to mention a loss in energy efficiency. Even with these caveats, their advantages in energy efficiency make GSHPs worth considering for projects that have significant heating and cooling loads in roughly equal proportions—although for homes, it’s still better to make them small and efficient enough that it simply isn’t worth investing in such a complex heating and cooling system.

For more information:

Ground-Source Heat Pumps: Are They Green? EBN Vol. 9, No. 7

The Refrigerant Revolution: Cooling Buildings ... But Warming the Earth? EBN Vol. 6, No. 2

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