Residential Environmental Design Featured Article - Geothermal Heating and Cooling Systems
The application of geothermal heating/cooling, also known as ground source heat pumps, has been named "the most energy-efficient and environmentally sensitive of all space conditioning systems", by the Environmental Protection Agency. The system's basic concept takes advantage of the earth's constant temperature, approximately 55 degrees, to heat and cool a building. By tapping this steady flow of heat from the earth in the winter, and displacing heat in the earth in the summer, a geothermal heat pump can save homeowners 40 to 70 percent in heating costs and 30 to 50 percent in cooling costs compared to conventional systems. Now you can also realize up to 30% rebates from both federal and state green stimulus programs.
Ground source heat pumps work in a similar manner as air source heat pumps, minus the high cost. Atypical household can save $1500 a year or more. This can give most systems a payback period of three to five years. GSHP's are more than three times as efficient as the most efficient fossil fuel furnace. By moving heat that already exists in the earth, instead of burning a combustible fuel, GSHP's deliver three units of energy for every one unit used to power the heat-pump system.
Ground source heat pumps work by circulating water or a water/antifreeze solution through a closed loop of polyethylene pipe that is buried in the ground or set beneath the water. GSHP systems can be lad out in different orientations, depending on the situation. A closed loop system, the most popular, can be laid out either vertically in 50 -250 foot deep holes drilled like a well, or horizontally in 3-6 foot deep trenches. The less common open loop system circulates a constant source of ground water and dispels the water back to its origin, such as a stream, well, or pond.
The principle action of a heat pump moves heat from lower temperature location to a higher temperature location. This principle can be witnessed in an air conditioning window unit, or air source heat pump, where cold air is blown into the house and warm air is released out of the back of the unit. A ground source heat pump works in a similar manner, except that its heat source is the warmth of the earth. The process of elevating low-temperature heat to over 100 degrees F and transferring it indoors involves a cycle of evaporation, compression, condensation and expansion. A refrigerant, like freon, is used as a heat-transfer medium which circulates within the heat pump.
The cycle starts as the cold, liquid refrigerant passes through a heat exchanger (evaporator) and absorbs heat from the low-temperature source (liquid from the ground loop). The refrigerant evaporates into a gas as heat is absorbed. The gaseous refrigerant then passes through a compressor where the refrigerant is pressurized, raising its temperature to over 180 degrees F. The hot gas then circulates through a refrigerant-to-air heat exchanger where heat is removed and pumped into the house at about 100 degrees F. When it loses the heat, the refrigerant changes back to liquid. The liquid is cooled as it passes through an expansion valve and begins the process again. To work as an air conditioner, the flow is reversed.
The ductwork is no different than that of a conventional forced-air system. The difference is found in the temperature of the air flowing from the registers in the winter. With a conventional air source heat pump, the air flow is seldom warmer than 80 degrees. But because water transfers a greater volume of heat than air, the Ground source heat pump is able to deliver warmer air, typically about 110 degrees F.
Another benefit of a ground source heat pump can be found when teamed with a desuperheater. This component skims residual warmth from the compressor to heat water. Which means that in the summer, when the system is working to get rid of heat, the desuperheater can provide practically free hot water. And since most systems are oversized, there is usually enough warmth left over for low cost hot water in the winter too.
While GSHP's require a small amount of electricity to concentrate the energy and circulate it through the system, most systems derive approximately 70 percent of their energy from a clean, renewable source- the earth.
Other advantages of GSHP's include the fact that all components of the unit are housed inside the building, thereby reducing the wear and tear on the unit by Mother Nature, and also eliminating the fear of vandalism or theft. GSHP's do not require a flue, and since there is no on-site combustion, there's less chance of fire, and no chance of carbon monoxide infiltrating the home. GSHP's also carry the Environmental Protection Agency's Energy Star Label, which is used to designate energy-efficient equipment. Often homeowners may find tax benefits, lower mortgages, or utility rebates.
The only problem with GSHP's is one of economics. GSHP's can cost as much as 30 percent more than conventional heating and cooling systems. Another drawback to GSHP installation is the lack of qualified contractors who know how to properly design, install, and service the systems. Interested homeowners must take extra precautions in screening both the contractor and the equipment manufacturer for a solid track record.
Geothermal technology is still relatively new, with only about 50,000 systems (approximately 1 percent) installed nationwide last year. There are only 150,000 units installed in homes in the United States at this time, but the Geothermal Heat Pump Consortium, which includes the EPA, the Department of Energy, 240 electric utilities and 20 heat pump manufacturers, is out to change that. With $100 million at its disposal, the consortium is offering rebates, reduced utility bills and other incentives to entice homeowners.