When conveniently located, lakes and ponds can provide a low cost source of water for a geothermal open-loop heat pump. While it offers the potential of a source of heat, as well as a heat rejection sink, direct use of the water is not recommended without filtration, since algae and turbid water can foul the heat pump's heat exchanger. The indirect use of this water is considered closed-loop and is addressed under Lakes/Ponds/Others (Closed-Loop).
In some cases, a dry well dug next to the water line can produce good results. The well can be constructed by using a backhoe to dig a 15 to 20 foot hole adjacent to the body of water. If this is done, the backhoe should be set as close to the water's edge as possible.
Once excavated, a perforated plastic casing is installed with a gravel backfill around it. This provides enough filtering of the water to allow reasonable heat pump performance. The discharge point back into the lake or pond should be as far away from the intake as is reasonably possible. This prevents heated water from the discharge from reentering the system before it has cooled. The discharge pipe is typically placed 100 feet or more from the intake.
Avoid aeration of the water since the presence of oxygen can form oxides on the return screen and cause gas binding.
WARNING: Any use of surface water and discharge back into a body of water should be done only with the permission of local authorities, and in accordance with any local or other applicable codes.
Some generalized "Rules Of Thumb" are:
- The lake or pond should be at least 1 acre (40,000 square feet) in surface area for each 50,000 Btu per hour of heat pump capacity. Another way to measure is to have a volume of water available that is the same volume as the structure being cooled or heated.
- The average water depth should be at least five feet, and there should be some areas where the depth is at least 6 feet, preferably 10 to 12 feet.>
- If possible, use a submersible pump suspended in the dry well casing. Jet and other types of suction pumps normally consume much more electric energy.
- Size the pump to deliver the necessary flow, in gallons per minute, needed to provide the design day cooling and heating capacity.