We have spoken of heat pumps and of an ancient form of geothermal energy that can heat and light homes. Now a fairly recent innovation has wedding the two and made possible a source of home heat and cooling that can be used virtually everywhere in the world. At a price.
Geothermal heat pumps (GHPs) work like a conventional heat pump that extracts warmth from even very cold air, uses it to warm interior space and then functions in reverse in hot weather. In the case of a geothermal pump, however, the heat is not taken from the air but from the earth by means of a ground loop pipe.
GHPs are not cheap but they can be used both for new construction and to retrofit existing homes. The Environmental Protection Agency estimates that such a system can save 30 to 70 percent on home heating and 20 to 30 percent on cooling over conventional system. Add to that that geothermal energy is a virtually endless resource and, because machinery such as air conditioner condensers are not required, GHPs operate quietly, reducing noise pollution.
A geothermal system uses a heat pump located indoors to extract heat from the earth and use it to warm a fluid in a ground loop or pipe from where it is moved indoors to a heat exchanger, used to heat the interior air, and then forced into ducts for distribution through the building. In an open ground loop system the fluid is fresh water which is drawn from a well and dissipated into a drainage field after use. In a closed loop system the fluid is water mixed in an antifreeze solution which is circulated over and over.
Geothermal heat pumps are more efficient than those that work above ground because of the consistent temperature of the earth. Traditional pumps have to work in air that is colder than the earth in winter and warmer in summer so extraction and return are not very efficient. While the range of temperatures between winter and summer and among climate zones in the U.S. can be extreme - from dozens of degrees below zero in northern tier states to over 120 in the Southwest deserts - there is little variation in subterranean temperature. Depending on latitude the temperature six feet below the earth's surface runs between 40° F to 75° F.
The ground loop pipe can be installed vertically in a well. It is the more expensive method and is usually used when space or rocky ground are considerations. A horizontal loop is a more common installation.
A vertical installation requires a well of 150 to 450 feet. A pipe with a U-bend at the bottom is inserted into the well and is connected to a horizontal pipe at the top which carries the heated fluid indoors to the exchange unit. A horizontal installation requires a trench three to six feet deep into which 400 to 600 feet of pipe are installed in parallel segments. So much pipe is needed in a horizontal installation because the fluid must travel a fair distance at such a shallow depth to extract or disperse sufficient heat while the vertical installation puts the loop into a higher temperature environment.
Various add-ons can contribute to the usefulness of a GHP system. In localities where the winter temperature and the summer temperature are very different, variable speed blowers and compressors can improve comfort and efficiency. An even better add-on is a "desuperheater" which takes heat from the house before it is expelled into the loop and uses it to heat household water. This provides essentially free hot water in the summer but a dedicated hot water heater will still be needed in spring, fall, and winter.
GHPs are now among the products rated by the EnergyStar® program operated by EPA. Because of the simplicity of the system and because much of it is protected by the earth, maintenance costs are often lower than for conventional systems.
A geothermal heat pump installation can be done by a qualified HVAC contractor and excavation for a vertical installation by a well digger. Excavation for a horizontal installation can be done by a backhoe or Bear Cat operator. During new construction there will be little disruption.
GHP equipment is widely available, but the cost is 50 to 100 percent higher than equipment for a conventional system. Interior duct work will be the same as for any forced hot air or air conditioning system but one source reports that the heat pump, indoor tubing, and circulating pump for a three-ton system will probably cost $1,000 to $2000 to install. The ground loop cost will run between $1,000 and $3,000 per installed ton. One should expect to pay between $4,000 and $11,000 more for a three-ton GPS system than for a comparable air extraction system.
Another source puts the cost of equipment at about $2,500 per ton or $7,000 for the average-sized house compared to $4,000 for conventional heating. Add to this the cost of drilling which can range from $10,000 to $30,000 depending on soil conditions and labor costs.
The range of projected costs and the estimated cost savings spread (in one study between $358 and $1,475 annual) makes it nearly impossible to project a cost recovery period. However, one suggestion I found was that financing with an energy efficiency mortgage which would cover the cost of installation and amortize it over 30 years would allow out of pocket savings to begin almost immediately.