Geothermal energy works because at a relatively shallow depth beneath the earth, the ground is a constant temperature. Geothermal systems work by exchanging heat between the earth and air pumped through it. Soil temperatures beneath the earth remain at a nearly constant 50 to 65 degrees F year-round, depending on your latitude. The earth serves as a low-temperature storage reserve. In the summer, the subsurface temperature is cooler than the air; in the winter, it's warmer.
Types of Systems
Geothermal systems use either water or nontoxic refrigerants like glycol. The principles are the same for both types of systems. The system can be a closed loop, which pumps refrigerant or water in a circuit through the system, into the ground and back. This system is more common. An open-loop system pumps water out of the ground from a water well and back into the ground after the heat exchange takes place. This is less common, and can be problematic if the water table in your area drops, forcing you to re-drill the well.
Underground Heat Exchanger
An underground-heat-exchange system is created by drilling a series of wells (usually about five) into the ground down to 200 feet, or more. If you are drilling the holes yourself, make sure the holes are large enough to accommodate two 3-inch pipes. Two pipes, connected at the bottom by a U-bend are lowered to the bottom of the well, then the hole is filled. The pipes are connected in sequence from each well, following very specific engineering specifications. Use thermal pipe connections for the strongest bond possible. Remember these will be in the ground for a long time. The "in" and "out" pipes are run back to the house in a ditch. Pressure test the pipes before you fill in the wells and ditches. Fill the ditch around the pipes with sand to protect the pipes from damage by rocks or sharp debris.
The pipes from the wells must be brought into the house and connected to a geothermal heat pump. This pump exchanges heat from the air in the house with heat from the coolant fluid as it is pumped back up from the ground. Unless you have extensive HVAC skills, hire a professional to install the heat pump and connect your home air conditioning and heating system.
Cost and Maintenance
In 2009, most geothermal systems cost about $2500 per rated ton. You can save money if you have the equipment and expertise to drill and install the wells yourself. Though setup can be expensive, geothermal energy systems have a long working life of 20 to 30 years with very low maintenance. In addition, they can reduce your home's energy bills by 30 to 50 percent.