Thank you all once again for this long, rich thread; it has become my main inspiration for continuing to work on heat pump system design. I continue to struggle with knowing how to size the heat pump and how to design and size the source pipe array, but every day I read folk's posts brings me closer to a solution.
In addition to the heat pump system per se, I'm working on a heat sequestration idea which I suspect is not novel, but holds some promise for my situation. Our house has a large attic that collects a great deal of heat on even overcast days. I propose to collect that heat and store it in the same ground field that the heat pump will use for its source.
I've conducted some crude experiments, illustrated below in photographs, and I can obtain more than half the heat I need for our house from the attic space, but the difficulty has always been that, in general, we don't need the heat during the day when it's available. One 80 or 120 gallon tank of hot water can be collected pretty quickly, but it won't be nearly enough to keep the house warm through the evening and night. In another post I have referenced collecting data on soil temperature and heat movement in soil. I have discovered that heat moves quite slowly through the soil. I could circulate 60 to 110 degree water from the attic to the soil in the location of the source pipes, and heat absorbed by the soil from the source piping won't dissipate far from the pipe in the few hours until it is needed in the evening when the attic circulation system would shut down and heat pump system would start up. In this manner I could recharge the soil heat bank any time the temperature in our attic rose to above 55 degrees
The photos below show two attempts to salvage heat from the attic, and they proved equally effective. I circulated water during warm periods and collected the heat in a 55 gallon drum in order to measure potential BTU/h gain from the attic, and I felt the results were heartening enough to merit development.
System one utilizes slinky type coils held off the roof sheathing enough so that re-roofing wouldn’t endanger the coils. The coils were connected in series and covered with reflective mylar backed by 1” styrofoam. There is lots of space for such heat exchangers, but I installed only four panels for the experiment.
System two utilizes a fan coil and crude duct work that draws air from the attic peak and blows the cooled air down toward the floor of the attic. One small fan coil collected as much heat in a given time period as did the four coils of polybutylene, and it seems a much simpler solution. It also has the advantage of avoiding trapping heat next to the roof sheathing.
As long as the attic air temp remains above the soil temp near the source pips, heat can be sequestered in the soil until needed. It seems to be the case that nearly all the heat near the surface of the earth, say within the first few hundred feet, comes from solar radiation, and only a small amount comes from geothermal heat migrating to the surface. By sequestering heat from my attic in the ground, I’d only be boosting the normal activity of the sun and providing easy access for removal via the heat pump.
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