More Efficient than Air-source Heat Pumps
By George Harvey
Regular readers of Green Energy Times probably do not need any introduction to heat pumps. For anyone who does, a heat pump takes the heat out of a cold place, making it colder, and moves it into a warm place, making it warmer. A refrigerator has a heat pump, removing heat from inside it and moving it into the kitchen, so the refrigerator warms the kitchen while cooling the food.
Geothermal heat pumps take heat from the area just below the surface of the earth, typically five feet down or more, and move it into buildings or anything else that needs to be heated. They are more efficient than air-source heat pumps, partly because air does not transfer heat as easily as earth or water, and partly because the ground that far down is much warmer than the air on cold days. They are also more expensive because of the costs of burying a large amount of pipe, which could mean digging up a large area of a back yard or drilling a set of wells.
For anyone who has easy access to a suitable
body of water, a pond loop heating system
should be less expensive to install and
to run than a typical geothermal system.
There is a special kind of heat pump that uses a “pond loop” as a heat source. In this case, the heat is obtained from a pond, lake, or river that acts as a thermal mass. For anyone who has easy access to a suitable body of water, this should be less expensive to install than a typical geothermal system and less expensive to run. The heat exchanger in a pond loop is often just a large coil of polyethylene pipe spread out on the bottom of the pond. The two ends of the pipe are run through a ditch to the building being heated, and anti-freeze is circulated through it, delivering the pond’s heat to the heat pump.
The advantages of the pond loop depend on the unique nature of water, which many scientists regard as one of the most amazing things in nature. Water is one of the hardest things in nature to heat or cool, making it quite possibly nature’s best thermal mass. It transfers heat rather easily when it is liquid, but with much more difficulty when it is ice, which covers the water, helping to slow cooling. Given a large enough pond, it will store the heat of the summer sun for a cold winter day, below the ice on the pond’s surface.
Another important fact about water is that it is very unusual in the way it expands or contracts as it changes temperature. Where most physical objects contract as they cool, water only contracts until it is 39° F., at which point it starts to expand as it is cooled; so water is densest at that temperature. And as water is frozen it expands a lot, which is why ice floats so nicely. The result of this is that the temperature of a body of water of more than a few feet depth is pretty much expected to be 39° F. or higher, at least in our area. This means that we are very likely to know the minimum temperature of the heat exchanger in a proper pond loop installation will never fall below 39° F, as long as pond is big enough to give up the amount of heat needed.
A large lake is not inferior to a pond in any way for the pond loop system, provided permissions can be easily obtained. A river is a bit trickier, because the agitation of the water means we no longer know what the temperature at the bottom might be.
While a do-it-yourself pond loop heating system is probably feasible, we would not recommend it. The engineering may not be tricky, but it is vital it be done right. In all cases, we would suggest a professional installation.
One thought that would probably cross the minds of many people is that their land might be suitable to build a pond for the purpose of a pond loop geothermal system. This is a real possibility in many places. A pond is a wonderful thing to have, and there are many reasons to install one. Having a copious supply of water at hand can lower fire insurance costs in some places. A pond can be stocked with fish, and can be used to grow edible water plants. With the strong suggestion that professionals be included in the planning process, we highly recommend the idea where it is practical.
Pull Quote: For anyone who has easy access to a suitable body of water, a pond loop heating system should be less expensive to install and to run than a typical geothermal system.
Caption: There was a time when farms often had fish ponds on them, and many could be used for heat, in addition to other uses. This is a fish farm Sainte-Famille, île d’Orléans, Quebec. Photo by Selbymay, downloaded from Wikimedia Commons.