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Geothermal Heat Pumps

Winter Heating and Summer Cooling Configurations

Republished from United States Geological Survey, Circular 1249, Geothermal Energy - Clean Power From the Earth's Heat

Geothermal Heat Pumps Can Be Used Anywhere

Geothermal heat pumps can be used for heating and cooling buildings virtually anywhere. Though initial installations costs exceed those for conventional heating and cooling systems, monthly energy bills are always lower. Thus, within a few years, cumulative energy savings equal the extra up-front cost of installation. Thereafter, heating and cooling costs are less than those associated with conventional systems.

How Do Geothermal Heat Pumps Work?

A heat pump is simply a machine that causes thermal energy to flow up temperature, that is, opposite the direction it would flow naturally without some intervention (see accompanying sketches). Thus, a heat pump is commonly used for space heating and cooling, when outside ambient air temperature is uncomfortably cold or hot, respectively. The cooling and heating functions require the input of "extra" work (usually electrical energy) in order to force heat to flow upstream, and the greater the "lift," or difference in temperature between the interior of a building and the outside, the more work is needed to accomplish the function. A geothermal heat pump increases the efficiency of the heating and cooling functions by substantially decreasing the thermal lift.

The Steady Temperature of Earth

Because rocks and soils are good insulators, they respond little to wide daily temperature fluctuations and instead maintain a nearly constant temperature that reflects the mean temperature averaged over many years. Thus, at latitudes and elevations where most people live, the temperature of rocks and soil only a few meters beneath the surface typically stays within the range of 5 to 10C.

Efficiency Compared to Air-Source Heat Pumps

For purposes of discussion, consider the functioning of a conventional air-source heat pump in a single-family residence, a system that exchanges thermal energy between air indoors and outdoors. Whereas such a heat pump must remove heat from cold outside air in the winter and deliver heat to hot outside air in the summer, a geothermal heat pump exchanges heat with a medium that remains at about 8C throughout the year. As a result, the geothermal-based unit is almost always pumping heat over a temperature lift much smaller than that for an air-source unit, leading to higher efficiency through less "extra" energy needed to accomplish the lift.

Higher Costs but Higher Savings

Some consumer resistance to geothermal heat pumps exists because initial purchase-and-installation cost is greater than that for an air-source system. The additional cost comes mostly from the need to bury piping through which fluid (water or antifreeze) is circulated to exchange heat with the ground or by drilling a shallow well to use ground water as the heat source/sink. Additional cost varies with the capacity and subsurface design of a given system. Experience to date indicates that the extra expense can be amortized in as little as 3 or 4 years for some systems. Other systems carry a longer pay-off period, but eventually all geothermal heat pumps provide savings that accrue as lower-than-normal utility bills.

Comparison With Electricity

Heat pumps provide significant energy savings, more than 75 percent as compared to electric baseboard heating and between 30 and 60 percent relative to other methods of heating and cooling. Many utilities, particularly in the Eastern United States, have subsidized the installation of geothermal heat pumps, also known as geoexchange systems, to help reduce peak demand for electric power. The lower electrical usage associated with the widespread use of geothermal heat pumps has allowed utilities to avoid or postpone construction of new power plants in areas where suitable land and transmission facilities are very difficult to acquire.

Tax Incentives

In the United States and in a few other countries governments are supporting geothermal energy use by offering tax credits or other incentives when individuals or businesses choose geothermal over fossil fuel heating/cooling systems. These incentives will reduce the cost of installation and offset some of the technology gains that might be attained by installing later rather than now.

Investigate before you build or replace an existing heating/cooling system. You might save a lot of money.

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geothermal heat pump configurations
Configura-tions of heat exchange piping either underground or underwater for geothermal heat pumps. USGS image.

geothermal heat pump in winter
Heat-flow directions are reversed between summer and winter. In winter heat is collected from underground and transferred to the building. USGS image.

geothermal heat pump in summer
Heat-flow directions are reversed between summer and winter. Heat is collected from the building and transferred to the ground in summer. USGS image.

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