At the bottom of the trench are 6, 600 ft long loops of pvc tubing that will cycle water near ground temperature (here that means about 35 F at its lowest) to a heat exchanger in the house and then back to the underground system of loops.
The system works according to Boyle's Law...the short version of that is when a fixed volume of gas expands it cools, and when a gas is compressed it heats. If you have a compressor that runs air-powered tools you may have noticed that it gets HOT when it's compressing air but when you release the air the air that expands out the exit valve is cold. That's the phenomenon of gas physics called Boyle's law.
In a geothermal assisted heat pump, the water from the underground system of tubing exchanges its thermal content with a heat exchanger in the basement of the building being heated or cooled. In the winter water entering the heat exchanger from the ground warms the refrigerant (relative to outside air) and the refrigeratn expands. When that refrigerant is then compressed the heat given off can be used to warm the building, and the cooled water returns to the ground to be rewarmed.
In the summer, the heat produced by compression can be absorbed by the heat exchanger and 'dumped' into the ground via cooler water moving through the heat exchanger. The compressed refrigerant when allowed to expand cools and is used to air condition the house in summer.
In the summer it's works much like a refrigerator...you may have noticed the radiator coils on the back or bottom of a refrigerator you've moved. Although a refrigerator is used only to cool, it dumps heat into the room where it sits--you can feel that at the back of your refrigerator if it's easily accessible.
Unlike a refrigerator or an air-conditioner, a geothermal assisted heat pump can run in both directions heating in winter, cooling in summer. It also generates hot water year round.
The 'field' is used to exchange relative warmth (or coolness) with the ground. In the bottom of the field in the picture, aka the trench in the picture, now lay 3600 linear feet of water containing tubing that exchanges its heat content with the ground. This 3600 feet of tubing is the 'heat source/sink' for the system.
The economics of this type of heating depends a temperature difference between the air in the house and the temperature of the ground. It works out favorably because currently available energy used for heating (fuel oil, propane, electricity--but in Wisconsin not natural gas--not available at this rural building location) is more expensive than the electricity required to compress the refrigerant that "pumps" the heat out at the heat exchanger.
If global warming continues (and it should across my remaining life) this system will become more and more cost effective for this building in southern WI. Although the effectiveness of the system is challenged by prolonged cold periods below zero which can result in cold (heat stripped) returning water actually freezing the ground, climate change will make such now infrequent periods, uncommon or absent.
Estimates of cost of heating this building with propane on 2012 prices is $1500 and another $200 for cooling annually and another ~$200 for hot water (~$1900 total), with the geothermal assisted heat pump that yearly cost, depending on 'avg' weather, these costs could be $700-$1000 per year.
Hard to say if the economics actually works out until we get there.
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