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Geothermal can cut power bills in half

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[September 25, 2007]  URBANA -- There is an age-old but little used source of alternative energy available to just about everyone, just about everywhere. It can save you as much as 50 percent on your heating and cooling bills -- and it's right beneath your feet.

Just 6 to 8 feet below the earth's surface, the ground remains at a relatively constant temperature. The stable, even heat of the earth can be used as a heat source in the winter and a heat sink in the summer to provide residential and commercial buildings with heating and air conditioning, using a geothermal heat pump.

"The geothermal heat pump is one of the most efficient and environmentally friendly heating and cooling systems available today," said Xinlei Wang, a professor in the Department of Agricultural and Biological Engineering at the University of Illinois.

A normal heat pump system extracts heat from outdoor air and transfers it inside, where it is circulated through your home's ductwork by a fan, said Wang.

"If you want to keep it 70 degrees inside and it's below zero outside, a heat pump would have to work pretty hard to produce those temperatures," Wang explained. "But if we use underground heat as our source, even on the coldest day the temperature is still 45 to 50 degrees 6 to 8 feet below ground. Going from 50 to 70 degrees, your efficiency will be much higher."

A geothermal heat pump, known as a GHP, works by collecting the earth's natural heat through a series of pipes, called a loop, that are installed below the surface of the ground. These loops can also be submersed in a pond or lake. Fluid in the loop, either water or an environmentally safe antifreeze solution, circulates through the pipes in a closed system and carries the heat to the house. An electrically driven compressor and a heat exchanger concentrate the energy and, using typical ductwork, release it inside the home at a higher temperature.

In the summer, the loop draws excess heat from your home and transfers it back into the ground. The system works the same way your refrigerator keeps your food cool -- by drawing heat from the interior, not by blowing in cold air.

There are several loop configurations that can be used, said Wang, but one of the most common is the horizontal ground loop.

"Horizontal ground loops are used where there's sufficient land available and where trenches are easy to dig," said Wang.

Workers dig trenches 4 to 6 feet deep, 2 feet wide, and then lay a series of parallel plastic pipes. A typical horizontal loop will be 400 to 600 feet long for each ton of heating and cooling, and an average residence requires a 3- to 4-ton unit.

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If you have little yard space or you don't want to disrupt the landscape, a vertical ground loop can be used, said Wang.

Vertical holes, about 4 inches in diameter, are bored into the ground 150 to 450 feet deep. A single loop of pipe with a U-bend at the bottom is inserted before the hole is backfilled. The vertical pipe is then connected to a horizontal underground pipe that carries fluid to and from the indoor exchange unit.

A third alternative is the pond loop system. If your property has a pond or lake with an adequate supply of water, a supply line pipe can be run underground from your residence to the water. The pipe is coiled into circles at least 8 feet under the surface to prevent freezing. The water source must meet minimum volume, depth and quality criteria.

Because of the technical knowledge and equipment needed to properly install a GHP system, Wang advises finding a qualified installer to do the job. Your local utility company or the Geothermal Heat Pump Consortium should have a listing of qualified installers in your area

On average, a geothermal heat pump system costs about $2,500 per ton of capacity, although a horizontal ground loop system will generally cost less than a system with vertical loops. And though more conventional heat pump systems cost about half that amount to install, Wang said the savings over time more than make up for the GHP's installation costs.

"Heating efficiencies are 50 to 70 percent higher than other heating systems, and cooling efficiencies are 20 to 40 percent higher than available air conditioners," Wang said. "Most of the components are sheltered from the harsh weather, and the underground piping is guaranteed to last anywhere from 25 to 50 years, so there's very little maintenance. These systems generally pay for themselves in five to eight years."

Many people worry about the reliability or durability of some alternative energy technology, said Wang, because it's not mature yet.

"But this is one technology that is very mature," Wang concluded. "People can use it."

[Text from file received from the University of Illinois College of Agricultural, Consumer and Environmental Sciences]

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