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The term "geothermal energy" might bring to mind hot springs and billows of steam rising from the soil, but you can get energy from the ground without moving to Iceland or Yellowstone. You just need a geothermal heat pump.
"We call anything below the ground geothermal," said John Lund, director of the Geo-Heat Center at the Oregon Institute of Technology.
This includes geothermal heating, in which hot underground water is used to heat a building, and geothermal power, in which steam from very hot underground rock (more than 300 degrees Fahrenheit) is used to drive an electric generator.
However, these hydrothermal resources are only available in select areas. A geothermal heat pump (sometimes called a ground source heat pump) can work anywhere.
"They are the fastest growing geothermal use in the world," Lund told LiveScience, with about 20 percent annual growth.
Refrigerate the outdoors
If you've ever touched the tubes on the back of a working refrigerator, you know that it is pulling heat from the inside and radiating it to the rest of the kitchen.
A heat pump is like a refrigerator run backwards. It pulls heat from outdoors (as if it were trying to cool the outside) and releases it indoors.
In both a fridge and a heat pump, a system of tubes circulates a refrigerant fluid that becomes hot when compressed and cold when expanded.
To heat a home, the hot compressed fluid is typically passed through a heat exchanger that warms the air that feeds into a duct system. This "spent" fluid is then cooled through expansion and brought into contact with a ground source, so it can "recharge" with heat.
Although pumping the fluid requires electricity, a geothermal heat pump is more efficient than any alternative heating system. In fact, current models can produce as much as 4 kilowatts of heat for every 1 kilowatt of electricity. This is because they are not generating heat, but rather moving it from the outside.
And some heat pumps can cool as well as heat a home. A valve controls the direction of the fluid, so that heat can flow in both directions.
Down to earth
Some people are familiar with heat pumps that exchange heat with the air outside. These sometimes get lukewarm reviews because they do not work well when the temperature drops below freezing — just when you need them the most.
Geothermal heat pumps overcome this problem by exchanging heat with the ground, which maintains a constant temperature between 45 and 70 degrees Fahrenheit, depending on the location.
"You wouldn't notice the difference between a home with a geothermal heat pump and one with a gas furnace," Lund said.
There are a number of ways to pull heat from the ground.
The most popular is a vertical geothermal heat pump, in which holes are drilled 150 to 200 feet below the surface. Pipes installed in these holes circulate water (with a dash of anti-freeze) that brings up heat to warm the refrigerant fluid.
An alternative is the horizontal heat pump, where the water-filled pipes are laid about 6 feet deep over a wide area. Although less expensive, these systems require a lot of land to heat a moderate-size building.
For those who live near a body of water or who have their own water well, it is possible to use that water directly as the outside heat source.
The biggest drawback for geothermal heat pumps is that their initial cost can be several times that of traditional heating and cooling systems. The installation for a typical house can run from $6,000 to $13,000, according to ToolBase Services, a housing industry resource.
However, geothermal heat pumps can pay for themselves over time with reduced energy bills. A homeowner can save 30 to 70 percent on heating and 20 to 50 percent on cooling costs over conventional systems, according to the U.S. Environmental Protection Agency.
This may be why their popularity is growing. The United States leads the way with close to a million geothermal heat pumps, mostly in the Midwest and East Coast. Another million units can be found throughout Europe and Canada.
"Maybe in Antarctica it wouldn't work, but everywhere else it does," Lund said.