What are my ground loop options for a ground source heat pump?
With ground source heat pumps (GSHP) gaining traction in Alaska, the options for designing them are expanding too. Back in 2010, almost all heat pumps in Fairbanks used a horizontal slinky ground loop configuration. Today other configurations are taking over that are often easier and cheaper to install.
GSHPs use energy from the ground to heat a home, operating much like a refrigerator in reverse. Whereas a refrigerator moves heat from inside the appliance and rejects it outside through coils of pipe, a GSHP gathers energy by sending fluid through pipes buried in the ground. Using electricity, the heat pump then “steps up” the temperature to use for heating the interior of a home. The ground loop fluid then returns to the ground to gather more energy and repeat the cycle. Heat pumps can also be used for cooling in the summer months, in which case the fluid is reversed to gather heat from inside a home and dump it into the ground.
Ground loop configurations
Traditionally, GSHPs employed a horizontal slinky ground loop configuration, where the ground loop was buried in trenches dug next to the building.
In Fairbanks, trenches are typically between 6 and 12 feet, though the depth depends on several factors, including soil type, water table depth, and solar radiation on the ground. As you may imagine, this method involves considerable excavation work, as a medium-sized house might require six, 100-foot trenches for a ground loop. Thus, your yard needs to be big enough for both the trenches and the excavation equipment.
Another option is to drill vertical wells. In this scenario, holes are drilled to around 150 -250 feet deep (depending on the geology) and the fluid travels up and down each one to gather energy. Vertical wells require much less surface area than horizontal slinky loops, and can also be drilled beneath a parking lot rather than an open yard.
The most recent type of ground loop available in Fairbanks is horizontal directional drilling. This technique still installs horizontal pipes underneath the surface, but using a directional drilling machine rather than excavation equipment, minimizing disturbance to the surface landscape. In the Lower 48, for instance, ground loops have even been installed under soccer fields using this drilling method. To install a directionally drilled ground loop, header and footer trenches are dug to connect the pipes and then a drill is used to install underground pipes in a shallow arc between the two trenches.
There are other “ground” loop configurations that actually involve placing the ground loop coils in a body of water, such as a lake or the ocean. Water provides a good heat conductor for the piping, but you need a permit from the state to install these loops (through the Alaska Department of Natural Resources, which also consults with Department of Environmental Conservation, Fish and Game, and the Army Corps of Engineers before issuing the permit to the installer). Pipes should be fully covered in water bodies in order to stay out of the way of boats and wildlife.
The Cold Climate Housing Research Center installed a GSHP at its Fairbanks facility in 2013 to heat part of the building. The heat pump uses a horizontal slinky ground loop because horizontal directional drilling was not available at the time, and vertical drilling was not an option because of site conditions–our building sits on cold bedrock (20°F) starting at approximately 60 feet. CCHRC is monitoring the system for 10 years to record its efficiency and effect on the ground temperature.