A pond loop is another closed ground loop for a geothermal system, except instead of trenching or drilling holes, large coils of tubing or a purpose built heat exchanger is submerged in a local body of water, like a pond or a lake. This makes the water of the pond the heat exchanger of the system.
This method is a great option if there is a sufficiently sized pond close to the structure, because there is very little trenching and excavation needed. Because of that it can be very cost effective to install this system over another type.
The size of the loops on this ground loop will need to take into account more things than the standard earth ground loop.
Things like pond size, depth and flow will also come into play along with the standard heat load of the house and flow through the pipes.
Other considerations also need to be taken with a pond system. It will significantly affect the temperature of the pond all year around, making it colder in the winter when heat is being sucked out, and warmer in the summer when heat is being put back in. This can affect any fish or wildlife that inhabit the pond, so precautions need to be taken. Also local water use permits and zoning requirements may be required, depending on your location.