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||The most common type of heat pump
extracts heat from the outdoor air and transfers it to the house at a higher temperature.
The performance of a heat pump depends on the temperature difference between the outside
and inside and, consequently the performance of the common air-to-air unit is lowest when
the outside temperature is the lowest and this is when heating of the home is most needed.
Therefore auxiliary heat, which is much lower in efficiency, must be utilized on the
colder days. Having a large number of these heat pumps accents the peak power that a
utility must provide on the coldest days which results in their operation of lower
efficiency generating equipment which increases overall consumer power costs and increases
pollution of the air.
The ground source or geothermal heat pump has been recognized as having superior efficiency to the standard air units because the ground is almost always at a more favorable temperature than the ambient air. For example, when outside air temperature is 10o F, the ground temperature six feet under the surface may be about 40 degrees F. Again, since the heat pump efficiency is a function of the temperature difference, the geothermal system will be superior. In this case, for example, the temperature difference for the air unit will be 65 F (assuming 75 F inside temperature) but for the ground unit the temperature difference will be 75 40 = 35 F.
Geothermal heat pump installations involve placing pipes in the ground and circulating a fluid within the pipes to extract the heat. Arrangements can be pipes laid horizontally in a pit or trench or placed vertically in holes drilled into the earth. A flowable fill material is frequently poured around the pipes to ensure good heat transfer contact between the pipes and the surrounding soil.
The direct exchange geothermal (DXG) heat pump utilizes buried copper pipe filled with circulating refrigerant to remove heat from the earth. Other types use a secondary water loop and requires an additional water pump and heat exchanger. The DXG system does not require an additional pump to move the refrigerant through the ground or any intermediate heat exchanger. In tests performed to date, the DXG system has reduced residential heating and cooling bills by as much as 60%.
Sponsors of this work include:
|Test Program||Director: Prof. W. Stan Johnson
The University of Tennessee
Field testing of DXG heat pumps, utilizing Flowable Fill
under high thermal stress.
|System No. 1
Installed September, 1996
7229 Evanel Way
|System No. 2
Installed September, 1997
7232 Evanel Way
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