The applicability of earth coupled heat pumps and factors affecting their performance

Earth coupled heat pumps are closed-loop systems placed underground to take advantage of favorable thermal characteristics of the subsurface. Design concepts have been divided into three categories: shallow horizontal coils, multiple shallow wells, and single deep wells. In some areas earth coupled heat pumps offer the potential of more efficient heating and cooling of buildings than conventional heat pumps, and they may be applicable in places where groundwater heat pumps and conventional geothermal development are not. Heat transfer occurs primarily by conduction making the thermal conductivity of the subsurface a critical factor in determining the performance of earth coupled heat pumps. Thermal conductivity is dependent on porosity, moisture content, and the mineralogy of the solid phase. The cost of drilling or excavating into the subsurface and installing the system is an important element of the overall cost.