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| Fig. 1: Illustration of a home and the subterranean elements of its GHP system (Courtesy of the DOE.) |
For decades, geothermal energy has been harnessed via ground source heat pumps to meet the heating and cooling needs of consumer homes. Geothermal energy systems that heat and cool consumer homes via a ground source heat pump have enjoyed many names, such as geoexhange, ground-water assisted, ground-water-source, and water furnace systems; for the purpose of this paper, we will simply refer to a ground source heat pump system as a Geothermal Heat Pump (GHP). While the GHP has been around for some time, it is still not nearly as prevalent as the ubiquitous air-source system.
A GHP system generally takes the form of a buried network of pipes, approximately four to six feet underground in the case of a system installed for a consumer home. [Fig.1] At this depth and depending on latitude, ground temperature is typically stable year-round at approximately 50 degrees Fahrenheit. Depending on whether the system is being used to cool or heat the home, the moderately tempered soil surrounding the pipes will act as either a heat sink or heat source. After the coolant has cycled through the underground pipes, it is pumped back into the house and interior components not very different from a standard refrigeration and heating system further condition the air to reach its final desired temperature.
Thanks to the stable and moderate temperature of the sub-surface Earth, a GHP system will not have to work as hard as an air-source system, which must manipulate air from outdoors, to meet desirable interior temperatures. The efficiency of the GHP system becomes more salient at greater temperature ranges, where traditional air-source systems must consume greater amounts of energy to condition external air to meet desired indoor temperatures.
GHP systems, depending on the quality of components, installation, and terrain, generally have a coefficient of performance (COP) between 3.0 and 4.5, meaning the system ranges from approximately 300% efficiency on the lower end to approximately 450% on the upper end. [1] The typical fossil fuel furnace burns at about 78%-90% efficiency. The monthly operational cost of a GHP system is significantly less than that of a traditional fossil fuel system for the typical consumer household.
Additionally, GHP systems are extremely low maintenance, with interior components requiring maintenance approximately every 20 years, and ground loop components requiring maintenance after approximately 100 years..
Perhaps the single greatest dilemma GHP systems face is the high capital cost of installation. Cost can vary as a result of numerous factors, such as the availability labor, composition of soil, demands imposed by climate, and the size of the project. The mean total cost of the installation of GHP system has been estimated to be $7,694 per ton. [2] For a 2500 square foot home in a temperate region of the country, approximately 4 tons of heating and cooling capacity is required.
However, the extremely high efficiency and low maintenance costs of GHP systems can make the return on this investment as short at 3 to 10 years, depending on climate, with climates with more extreme temperature ranges yielding faster returns, as this is when the inefficiencies of traditional air-source systems become very salient.
Additionally, you simply need the space to install a GHP system. Urban homes and suburban homes on small lots are unlikely to have the property space to install this technology. The installation is also a fairly destructive and invasive process, meaning that unless you have very little regard for your neighbors and the state of your backyard, GHP systems are best left to being installed during the construction of a new home or heavy renovation of an existing one, and in a place where there is sufficient land as to not make the system or its installation a burden. Additionally, new homes that include the installation of a GHP system may be eligible for a federal tax credit. [3]
© Erik Holmvik. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. L'Ecuyer, C. Zoi, and J. S. Hoffman, "Space Conditioning: The Next Frontier," U.S. Environmental Protection Agency, EPA 430-R-93-004, April 1993.
[2] S. Kavanaugh, M. Green, and K. Mescher, "Long-Term Commercial GSHP Performance. Part 4: Installation Costs," ASHRAE J. 54, No. 10, 26 (October, 2012).
[3] "Guide to Geothermal Heat Pumps," U.S. Office of Energy Efficiency and Renewable Energy, DOE/EE-0385, February 2011.