Geothermal heating and cooling systems are becoming increasingly common as a smart way to heat and cool buildings. The big reason for this is a 30 percent federal tax credit that effectively reduces the cost by that much. They’re not for everyone, but according to the EPA, geothermal systems are “the most energy efficient, environmentally clean and cost-effective space conditioning system available today.”
The term geothermal is also used in reference to the heat from molten magma, which in some places is close enough to the Earth’s surface that it can be used to generate steam and create electricity. What we’re actually talking about in this article is ground-source heat pumps, much like the more common air-source heat pumps that the electric utilities like to promote.
The traditional heat pumps that were popularized 20-30 years ago use the outside air as a source of heat in the mildly cold winters, and a medium in which to dump heat in the summer. In fact, it’s just your air conditioner working in reverse. In the summertime, the AC unit extracts warm, moist air from inside the home, and dumps it outside using a liquid coolant/heat transfer fluid. In the winter, the same process extracts heat from the outside and pumps the warm air into the house.
You might be thinking “how can you get heat out of cold winter air?” Actually, your refrigerator is also a heat pump, extracting heat from the freezer and pumping that warm air out the bottom of the refrigerator. It’s not terribly efficient – that’s why you need a backup heat source for traditional heat pumps on really cold days.
Ground-source heat pumps use the same principle, except rather than pulling heat from the cold winter air, the unit extracts the heat from the earth, which is a fairly predictable source of heat that usually averages 55 degrees. In the summer, the warm, humid air extracted from the home is much more efficiently dumped into a 55° tunnel in the earth than in 90° air on a hot July afternoon.
The key, of course, is those tunnels, and this is largely what makes geothermal so expensive. For a typical home that needs 3 tons of heating and cooling capacity, you need 3 six-inch holes drilled 150 feet deep into the earth, with holes spaced ten feet apart. This vertical loop method, which is often the only alternative, is usually the most expensive option.
If you have sufficient land, you can create a horizontal loop through an open field by burying the lines six feet deep in the ground. Again, the lines need to be 150 feet long for each ton of heating and cooling capacity, and should be at least 10 feet apart.
If a water source – such as a lake or large pond – is available, a third option is to install a closed pond loop. The size and depth of the body of water is critical, and requires an engineering analysis to ensure that the water can handle the building loads. Open loop well water systems are also an option, but are relatively uncommon.
For new buildings, the case can often be made to support geothermal because the added cost of the wells can be lower during the construction. In existing buildings, a retrofit can be cost-effective if both the heating and cooling systems need to be replaced, and there is suitable ground source available to construct the loops. The equipment life is typically 20 years or more, but once the investment is made in the loop, replacement of the equipment is comparable to the replacement of traditional equipment.
With the 30 percent federal tax credit, the payback on these systems is usually about 4-6 years. The energy savings are dramatic, with heating and cooling costs about half of the cost of traditional HVAC units. Of course, it’s important to have a well-insulated and sealed home to minimize the heat transfer. But the combination of a tight building envelope and a geothermal HVAC is unbeatable.
This was published in the Going Green section of the April 2011 issue of Spirit Seeker magazine.