What is a Ground Source Heat Pump?
A ground source heat pump (GSHP) uses electricity to convert free energy from the ground into heat for your home. While they are common in Europe and parts of the Lower 48, these heating appliances are still relatively new to Alaska. Well-known systems include the Juneau Airport, Weller Elementary School in Fairbanks, and the Alaska SeaLife Center in Seward. There are more than 50 residential systems located in Fairbanks as well.
How do heat pumps take advantage of “free energy?” The technology works like a refrigerator in reverse, but instead of taking heat from inside the fridge and rejecting it outside, a GSHP gathers heat from the ground, steps it up to a higher temperature using a compressor, and then “rejects” it inside a house. In the summer, some GSHPs can run in reverse mode to provide air conditioning–taking heat from a house and rejecting it to the ground. The main “fuel” for a GSHP is geothermal energy that naturally exists in the ground, but it also uses electricity to run the compressor. Since the electricity is only acting to boost the geothermal heat (which is free), heat pumps are more efficient than electric heating appliances.
A GSHP has three components: the ground loop, the heat pump, and the distribution system. The ground loop consists of loops of pipes buried in horizontal or vertical holes in the ground. As fluid is pumped through the pipes, it absorbs heat from the surrounding soil and returns to the heat pump at a higher temperature than when it left. Back at the heat pump, the fluid from the ground loop passes its heat to a refrigerant, which has a very low boiling point, and causes it to evaporate into a gas. The refrigerant then passes through a compressor and its heat is “stepped up” before being distributed throughout the home via air or water. Heat pumps can work with a forced air or a radiant hydronic distribution system, however they are not capable of producing water hot enough for baseboard hydronic systems.
Ground source heat pumps are considered partially renewable because the heat they take from the ground comes from solar and geothermal sources. They are entirely renewable if the electricity used to power the heat pump comes from a renewable source, such as solar or wind. Heat pumps are also very safe to operate because there is no combustion. They can, however, be expensive to install because you have to excavate land or drill to put in the ground loop.
What are my ground loop options for a ground source heat pump?
With ground source heat pumps gaining traction in Alaska, the options for designing them are expanding too. While most of the early heat pumps used horizontal slinky ground loops, today other designs are gaining popularity that are easier and cheaper to install.
In a horizontal slinky configuration, the ground loop is buried in trenches dug next to the building. The depth of the trenches depends on several factors, including soil type, water table depth, and solar radiation on the ground. In Fairbanks, trenches are typically between 6 and 12 feet. 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 local geology) and the fluid travels up and down vertical pipes 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 newest type of ground loop available in Fairbanks is horizontal directional drilling, which installs horizontal pipes underneath the surface using a directional drilling machine rather than excavation equipment, which minimizes disturbance to the surface. In the Lower 48, for instance, ground loops have been installed under soccer fields using this drilling method. This technique requires digging header and footer trenches to connect the pipes and then using a drill to install underground pipes in a shallow arc between the two trenches.
There are other 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). Pipes should be fully covered in water bodies in order to avoid interfering with boats and wildlife.