Ground Loops for Geothermal heating Systems
A key component of a residential geothermal system is the heating
loop. 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.
Open vs Closed-loop Systems
Ground source heat pumps are sometimes classified as open-system or
closed-loop. This refers to the design of the piping system
located outside of the home. In an open system a well or a pond
is used as the latent heat source. In using this type of system
the water is pumped directly from the water source to the primary heat
exchanger in the heat pump where heat is either subtracted for heating
or added for cooling.
In a closed loop system the heat is collected by means of a
continuous loop of piping that is buried underground. The fluid
in the pipes is usually an antifreeze type of solution which extracts
the heat from the soil. The antifreeze is then pumped to the
primary heat exchange in the heat pump where the latent heat is
extracted. One variation on this type of approach is what is called a
direct-expansion or DX system. In a DX system the refrigerant
runs directly from the heat pump to the underground piping without
passing through a heat exchanger. The advantage of this approach
is that it requires less electricity and so is about 10-15% more
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.
Because a lot of surface area is dug up when using a horizontal loop
it is usually easiest to put them in new homes or businesses prior to doing the
initial landscaping. 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.
Though they are usually less costly than vertical loops, there are some disadvantages
to horizontal ground loops. First, horizontal loops are really
not appropriate in colder climates where the frost goes down more than
about two inches. This is why in the Northeast U.S. nearly all
of the implementations use vertical rather than horizontal loops.
Another consideration is that horizontal loops do much more damage to the environment since more surface area
has to be dug up and replaced. If your property is close to
environmentally sensitive or protected areas this might not be your
best option. Also, in order to use this approach your property
has to have enough surface area to lay the ground loop. If your
property is a quarter acre or less or contains obstructions where you
cannot dig you may not have sufficient space for this type approach.
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.
One thing to consider when looking at using a vertical loop is to
determine if there might already be any existing wells on your
property. Most older wells do not have sufficient depth for a
vertical loop but if your groundwater level is deep then this might be
Pond or Lake 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.