Heat pumps that use water as their source of heat and heat sink are called water-source heat pumps. In contrast to geothermal heat pumps, or ground-source heat pumps, in these systems water is used as a medium of direct transfer of heat. The water source can be groundwater filled extraction wells, in which case it is a open loop coupling system, or a pond, stream or lake, in which case it is a closed loop coupling system.
The water’s stable cool temperature offsets seasonal temperature variations in that it acts as a reservoir of stored heat in winter months and as a drain of heat in the summer months. Heat pumps reverse the direction of heat transfer, depending on whether cooling or heating is needed, by means of special reversing valves. Therefore, the outdoor coils of the pump act as a condenser during a cooling cycle, while in the heating cycle, it becomes the evaporator, and vive versa for the indoor coils.
Closed vs. Open Loop
In the closed loop systems, pipes circulate water between the lake, pond or stream and a refrigerant to water heat exchanger then back to the source. The closed loop nature of the system means that no water from the system is discharged back into the pond, stream or lake, avoiding environmental contaminant concerns. There remains the issue of having the natural water body’s temperature altered by the system, and it’s impact on wildlife ecosystems, however.
In open loop well systems, two columns of water are used. One well serves as the water source, extracting groundwater, the other as a discharge re-injection well. In some systems, a single standing column is used for both, allowing the heat pump’s discharge water to be re-injected into the source well, eliminating the requirement for a second column of water.
Pros and Cons
Because uniform regulations do not exist in many localities for water source heat pump systems, installation can be problematic. Of concern are any environmental regulations governing the discharge of the water from the heat pump. Local codes, as well as federal and state or provincial codes may apply, and need to be researched and adhered to.
Possible regulations may include requirements return discharge water wells to be installed by licensed drillers, who must submit well locations and logs, in the U.S., to the Dept. of Conservation and Natural Resources’ Bureau of Topographic and Geologic Survey.
Local codes could require that the water is discharged through a sewer, which means paying a hookup fee and a monthly sewer bill. Any water discharged into a lake, stream, pond or river may require obtaining a National Pollutant Discharge Elimination System permit.
The major advantage of water source heat pump systems is they are more efficient than air-source heat pump systems, since water temperatures are more stable and warmer than air in colder winter months.
The cooling and heating efficiency of heat pumps is measured by two methods; Seasonal Energy Efficiency Ratio (SEER), and Heating Season Performance Factor (HSPF). The higher the SEER number, the more efficient a heat pump system is at converting electricity into cooling power. The higher the HSPR number, the more efficient a heat pump will heat a house.