The idea behind the geothermal heat pump is similar to that of a central
HVAC system; both systems use an outside unit, called a condenser, and
an inside unit, called an evaporator. However, while the central HVAC
system uses compressed refrigerant to remove heat from the system, a
geothermal heat pump uses the stable, consistent temperature of the
earth to provide winter heating and summer cooling. Studies have shown
that as much as 70 percent of the energy used in a geothermal heat pump
system is renewable energy obtained from the earth's surface, and that
geothermal heat pump systems are capable of reducing monthly heating and
cooling bills by as much as 40 percent.
Rather than using refrigerant, a geothermal heat pump uses a series of
piping, installed below the earth's surface or submersed in a pond or
lake, to form a loop through which water or another environmentally
safe fluid is pumped. During the winter months, the fluid carries
heat into the house, where a compressor and heat exchanger concentrate
the earth's energy before releasing it into the home at a higher
temperature. The heat is then distributed throughout the house through
a series of ductwork, just like a forced-air furnace. During the
summer months, the fluid carries heat from the home, releasing it back
into the earth or the water body in which it is submerged.
The length of the piping loop depends on several factors: the heating and
cooling requirements of the home, climate and soil conditions, landscaping,
and type of loop used. Horizontal ground closed loop systems are typically
the most cost effective, assuming the lot size is sufficient. In a
horizontal ground closed loop system, a series of parallel plastic pipe
is buried three to six feet below the earth's surface, with a typical
length of 400 to 600 feet per ton of heating and cooling. Piping can
also be coiled in a slinky shape to increase the amount of piping that
can be located in a given space. Fluid is then
used to carry heat to and from the indoor exchange unit. A vertical
ground closed loop system is a more practical option for small yards or
when you want to disrupt the landscape as little as possible. In this
type of system, vertical holes 150 to 450 foot deep are drilled into the
earth, and a single loop with a U-bend at the bottom is installed.
Because the temperature of the earth is more stable as the depth increases,
a vertical ground loop system typically requires less piping. Pond closed
loop systems may be the most cost-effective choice for those near shallow
bodies of water, such as ponds or lakes. The pond closed loop system
functions just like the two ground loop systems, but transfers heat to
and from the water body, rather than the earth's crust.
FIGURE 1: Typical geothermal heat pump configurations.
According to the Consumer Energy Center,
geothermal heat pumps cost
approximately double the amount of a typical forced-air system with
air conditioning. The energy savings, however, will more than make
up for the additional cost of a geothermal heat pump system, particularly
as fossil fuel prices, such as natural gas, continue to rise. Geothermal
heat pump systems can also be added to existing homes to reduce monthly
energy bills, with the investment recouped in two to ten years.
While a geothermal heat pump system was considered for the Green House
on Watson, the small lot size and extremely rocky soil would have made
this an expensive option. However, the donation of a forced-air system
made the decision an easy one. Since the ICF wall system gives the
home such an impressive thermal
envelope, the realitively small high efficiency forced air furnace
requires only a minimal amount of natural
gas in order to keep the interior of the home warm on even the
coldest days.
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