Geothermal heat pumps, or ground-source heat pumps, use the relatively
constant temperature of soil or surface water as a heat source and sink
for a heat pump, which provides heating and cooling for buildings.
Geothermal heat pumps are similar to ordinary heat pumps, but use the
ground instead of outside air to provide heating, air conditioning and,
in most cases, hot water. Because they use the earth's natural heat,
they are among the most efficient and comfortable heating and cooling
technologies currently available.
Geothermal heat pumps are durable and require little maintenance.
They have fewer mechanical components than other systems, and most of
those components are underground, sheltered from the weather. The
underground piping used in the system is often guaranteed to last 25 to
50 years and is virtually worry-free. The components inside the house
are small and easily accessible for maintenance. Warm and cool air is
distributed through ductwork, just as in a regular forced-air system.
Since geothermal systems have no outside condensing units like air
conditioners, they are quieter to operate.
How do they work?
Remember, a geothermal heat pump doesn't create heat by burning fuel,
like a furnace does. Instead, in winter it collects the Earth's natural
heat through a series of pipes, called a loop, installed below the
surface of the ground or submersed in a pond or lake. Fluid circulates
through the loop and carries the heat to the house. There, an
electrically driven compressor and a heat exchanger concentrate the
Earth's energy and release it inside the home at a higher temperature.
Ductwork distributes the heat to different rooms.
In summer, the process is reversed. The underground loop draws excess
heat from the house and allows it to be absorbed by the Earth. The
system cools your home in the same way that a refrigerator keeps your
food cool - by drawing heat from the interior, not by blowing in cold
air.
The geothermal loop that is buried underground is typically made of
high-density polyethylene, a tough plastic that is extraordinarily
durable but which allows heat to pass through efficiently. When
installers connect sections of pipe, they heat fuse the joints, making
the connections stronger than the pipe itself. The fluid in the loop is
water or an environmentally safe antifreeze solution that circulates
through the pipes in a closed system.
Another type of geothermal system uses a loop of copper piping placed
underground. When refrigerant is pumped through the loop, heat is
transferred directly through the copper to the earth.
Types of Loops
Geothermal heat pump systems are usually not do-it-yourself projects.
To ensure good results, the piping should be installed by professionals
who follow procedures established by the International Ground Source
Heat Pump Association (IGSHPA). Designing the system also calls for
professional expertise: the length of the loop depends upon a number of
factors, including the type of loop configuration used; your home's
heating and air conditioning load; local soil conditions and
landscaping; and the severity of your climate. Larger homes requiring
more heating or air conditioning generally need larger loops than
smaller homes. Homes in climates where temperatures are extreme also
generally require larger loops.
Here are the typical loop configurations:
Horizontal Ground Closed Loops
This type is usually the most cost effective when trenches are easy
to dig and the size of the yard is adequate. Workers use trenchers or
backhoes to dig the trenches three to six feet below the ground in which
they lay a series of parallel plastic pipes. They backfill the trench,
taking care not to allow sharp rocks or debris to damage the pipes.
Fluid runs through the pipe in a closed system. A typical horizontal
loop will be 400 to 600 feet long for each ton of heating and cooling.
Vertical Ground Closed Loops
This type of loop is used where there is little yard space, when
surface rocks make digging impractical, or when you want to disrupt the
landscape as little as possible. Vertical holes 150 to 450 feet deep -
much like wells - are bored in the ground, and a single loop of pipe
with a U-bend at the bottom is inserted before the hole is backfilled.
Each vertical pipe is then connected to a horizontal underground pipe
that carries fluid in a closed system to and from the indoor exchange
unit. Vertical loops are generally more expensive to install, but
require less piping than horizontal loops because the Earth's
temperature is more stable farther below the surface.
Pond Closed Loops
This type of loop design may be the most economical when a home is
near a body of water such as a shallow pond or lake. Fluid circulates
underwater through polyethylene piping in a closed system, just as it
does through ground loops. The pipes may be coiled in a slinky shape to
fit more of it into a given amount of space. Since it is a closed
system, it results in no adverse impacts on the aquatic system.
Although they are less applicable to California, there are other loop
systems described at the
Geothermal Heat Pump Consortium's Web Site. These include an Open
Loop System in which ground water is pumped into and out of a building,
transferring its heat in the process; and Standing Column Well Systems,
which can be up to 1,500 feet deep and can also furnish potable water.
In a few places, developers have installed large community loops,
which are shared by all of the homes in a housing project.
To date, geothermal heat pumps are an under-used technology, merely
because few people are aware of it's potential. The Department of
Energy's Office of Geothermal Technologies, however, wants to increase
installations of geothermal systems to about 400,000 a year by 2005. If
the goal is reached, that would mean that 2 million systems would be in
service, saving consumers over $400 million per year in energy bills and
reducing U.S. greenhouse gas emissions by over 1 million metric tons of
carbon each year.
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