There's a news item today about the decision of the airport in Portland, Maine to use "geothermal" heating and cooling, saving 50,000 gallons of heating oil a year. Because I frequently consult for the National Park Service, which is a leader in installing this technology, I have some experience with these systems. Engineers call these systems "ground source heat pumps," and they have nothing to do with getting energy from volcanoes. They work like this:
Ten feet below the surface, the temperature of the earth is about 55 degrees (12 degrees Celsius). This reservoir of effectively infinite mass and constant temperature can be used to cool buildings in the summer and heat them in the winter. The most common kind of system adds an extra loop to an ordinary heat pump. This loop sends water down into the ground to be warmed (in winter) or cooled (in summer). That water is then used to warm or cool the refrigerant in the heat pump. This increases the energy efficiency of the heat pump by 30 to 45 percent. The installation cost is high, and these systems take 7 to 10 years to pay for themselves in the US. (Especially when you are installing the system in a National Park where archaeology and other environmental studies are necessary.) Because the technology is fairly new, nobody knows how long these systems will last, but the NPS estimates that the ground loops of their systems (the expensive part) will last at least 50 years.
Improving the efficiency of an electric heat pump by 45 percent is a fairly small savings compared to what can be done by the most advanced environmental technologies, which can reduce the energy needed to heat and cool buildings to near zero. But those technologies can't be installed in old buildings, like the historic houses where the NPS is installing geothermal systems. So these systems are a good idea, and more property owners should install them.