Geothermal heat pumps are much like the heat pump in the refrigerator – its purpose is to absorb the heat from the earth loop and concentrate it to a high enough temperature so that it can be effectively transmitted to the condition spaces within the building.
Just like the refrigerator, it has a series of closed-loop tubing through which a refrigerant fluid is recirculated. The ground loop piping comes into close contact with cold – evaporator – side of the heat pump, transmitting some of its heat to refrigerant causing it to boil and vaporize and its temperature to rise slightly. The vaporized fluid is then compressed which concentrates its heat, raising its temperature dramatically as it passes to the condenser side of heat pump. Now the fluid is hot enough to transmit its heat to conditioned spaces within the building. Before the fluid returns to the evaporator, it passes through an expansion valve which drops its temperature below 30oF so that it can absorb more heat from the ground loop.
A series of by-pass valves (not shown in the schematic) enables the evaporator and the condenser to switch positions, converting the heat pump to air-conditioning mode.
Ground loops are typically closed systems, a series of pipes through which a water-alcohol solution is recirculated. Most of the piping is outdoors, buried beneath several feet of earth (or water, in the case of pond loops). A short section of the loop runs into the building where it comes into close contact with the tubing within the heat pump – this is like the continuous supply of warm water to the container in the refrigerator – before returning to the outdoor section.
As the solution is pumped through the buried piping, it absorbs heat from the earth which raises its temperature before it reaches the cold side of the piping in the heat pump. After transferring heat to the heat pump, the now-cooler solution is returned to outdoor section to be re-heated.
If there is sufficient outdoor space and adequate soil conditions, a horizontal loop (buried 5 to 6 below the surface, where the temperature is fairly constant between 40 and 50oF) is preferable as it is the least expensive configuration. If outdoor space is at a premium and/or soil conditions dictate, one or more holes can be bored (a few hundred feet down) to create a vertical loop. If a pond or lake of sufficient size and depth is nearby, a horizontal loop can be laid in it to minimize the amount of excavation (and associated cost and inconvenience) required.
In certain circumstances, an open loop system may be preferable. Here, water is pumped from a well, pond, lake, or river. As in the case of the closed loop, the water is pumped through piping to come in close contact with the piping in the heat pump before it is redirected for other purposes or returned to the earth. Unique issues with an open loop include filtration of the incoming water and disposal of the effluent. Special permits may be required by some jurisdictions.
Heat from the hot side of the geothermal heat pump coil is transmitted to spaces within the building by one or both of two means:
- Blow air across the coil and then through a series of ducts in the case of a forced-air system
- Pump liquid (usually water-based) across the coil and then through a series of closed-loop tubes which pass through a radiator, a hot-water tank, and/or a series of tubing embedded in a floor or wall.
Distribution systems can be quite complex depending on the needs of the client. For example, auxiliary heat can be provide to the domestic hot water system while one or more forced-air systems heats the building and a hydronic (water) system heats a bathroom floor and an outdoor hot tub.