Energy costs to heat and cool fire stations are increasing. Add to that concerns over global warming, and fire departments are looking to prove they can make good choices about energy and the environment. Geothermal heat pumps are one way to address these concerns.

Heat pumps compress a gas to make it hot and then expand it to make it cold. During the heating mode, room air is blown over a coil containing the compressed, heated gas. The gas is then allowed to expand into a coil on the outside of the building. The expanded gas is very cold, well below freezing. Outside air is then blown over the coil heating the gas up to close to the outdoor temperature. The gas is then drawn back through the compressor, which again makes it hot, and the cycle is repeated. So long as the expanded gas is much colder than the outside air, you get the benefit of “free” heat when you blow outside air over it “warming it up” before bringing it back to the compressor. The cooling mode is just the opposite. The very hot compressed gas is cooled on the outside of the building, and the expanded cooled gas extracts heat from the interior. This is called an air-to-air heat pump.

The problem with an air-to-air heat pump is that often the winter air is too cold to provide much “free” energy. For instance, if it is 50° outside, you can get from two to three units of heat energy out for one unit of electrical energy in. When the outdoor temperature drops to zero, you can get less than one units of heat energy out for one unit of electrical energy in. Similar problems exist during the cooling mode. The warmer it is outside, the less efficient the cooling.

Geothermal heat pumps solve this problem by using either ground water or the ground itself as a source of heating or cooling energy at subterranean temperatures that range between 40° and 72°. In the winter, heat energy can be extracted from the earth (or ground water) and added to the building. The process is reversed in the summer. Unwanted heat is extracted from the building and added to the earth or ground water. This system replaces the boilers and coolers.

Because of the relatively constant, mild ground temperature, the GHP typically will provide between three and five units of heating or cooling energy for one unit of purchased electricity, twice as much energy as you typically get from an air-to-air heat pump when averaged over the entire year.

Starting in the late 1950s, GHPs used pumped well water that was re-injected into the ground after the energy transfer with the equipment. This was referred to as a pump-and-dump system. Refinements in GHP efficiency have resulted in the majority now installed with a closed-loop system using continuous high-density polyethylene pipe buried in the earth. Because the closed-loop technology does not need access to high-output wells, GHPs can be installed almost anywhere.

In some instances, GHPs can be installed for the same cost as conventional systems, but installing the ground heat exchanger will make initial cost higher than that of a conventional system. Energy costs can be cut by more than half, offsetting the added initial investment, and simple payback for the additional cost of a geothermal heat pump system can fall between two and eight years. In addition, some states offer cash incentives.

Homes and commercial, and institutional buildings account for about 40% of primary U.S. energy consumption and greenhouse gas emissions, 72% of U.S. electricity consumption, 55% of U.S. natural-gas consumption, and significant heating oil and propane consumption. According to the Oak Ridge National Laboratory, switching over to GHPs could avoid the need to build 91 to 105 gigawatts of electricity generation capacity, or 42% to 48% of the net new capacity projected to be needed nationwide by 2030. In addition, reduced utility bills could save $33 billion to $38 billion annually (at 2006 rates).

For additional information, visit the Federal Emergency Management Program's energy-efficient products page, the New York State Energy Research and Development Authority and GeoExchange.org.

<p> Bob Mitchell is the principal architect at Mitchell Associates Architects.