Geothermal literally means “of or relating to the heat in the interior of the earth”. Practically speaking, it is the use of the temperature of the earth in any given location to generate an energy transfer to provide heating and cooling. On a commercial level, certain regions around the world are viable sites for large energy-generating stations. Many businesses are now considering this option on a smaller scale. On a residential level, almost any home that you can get a drilling rig to can enjoy the benefits of a geothermal system.

The “geo” part of the system is the ground loop. The system uses the constant temperatures found under the earth's surface to provide basic heating and cooling. The loop consists of tubing (polyethylene or copper) which is usually filled with water or a mixture of water and anti-freeze.

A scientific fact that heat energy readily flows to cooler areas is the basis for all geothermal systems. The heat from the sun warms the earth in the winter and that stored energy is then used to heat the home. In summer, the earth becomes a “sink” where the the in-side air's heat is discharged.

The only energy required for this part of the system after the loop's installation is the power the circulating pump uses to move the fluid through the loop. There are four types of closed loop systems and, in some cases, variations within each type.

Closed Loop Systems

In the vertical loop configuration, the tubing is buried in the ground in a series of drilled holes anywhere from 50' to 250' deep. The tubing forms a “U” in each hole and is connected to other holes in the field by a manifold. The holes are filled with a thermally-conductive grout mixture [bentonite, cementitious and other new materials under development] to insure the best transfer of heat to and from the earth. Then the entire area is filled with earth and returned to its previous appearance. Graphic from Department of Energy [www.energysavers.com]

In a horizontal loop system, the tubing is laid into an open pit in “slinky” or in horizontal loops laid into trenches that are then buried. This configuration does not require drilling but does require more land to execute due to the size of the pits/trenches required to execute it. Also, as the loop is closer to the surface, theoretically, the full impact of the earth's constant temperature (55 to 65 degrees at a depth of 50' depending on geographic location) is not realized. Graphic from Department of Energy [www.energysavers.com]

A diagonal loop system is used by only one manufacturer. In this system, the holes are drilled at a 45 degree angle. This manufacturer also utilizes copper tubing for the loops and a different method of exchanging heat from the loop to the structure's air.

A closed loop pond system is installed just as it says – in a closed loop configuration along the bottom of a pond. As you may know, even if the surface of a pond freezes in winter, the water even 5' or 6' below the surface is warm enough to allow fish to live. Converesely, in summer, the water lower in the pond is cooler than water near the surface. As the system is closed, nothing is transferred into the water so pollution is not a consideration. Graphic from Department of Energy [www.energysavers.com]

Open Loop System

An additional system is the open loop which uses a well to exchange and discharge the water in the system. This last type is not as widely used as other loop systems because not that many people have wells these days. Among various concerns is the quality of the water in the well. Mineral deposits in the water can have a deleterious effect on the inner workings of the exchanger. Great care must be exercised in testing the well water to see if this option is available. The up-side of this method is that almost no drilling or excavation is required and less tubing is used than in the closed systems. Graphic from Department of Energy [www.energysavers.com]

The Details – How it Works

If you think of the loop as the veins of the system, then the heat exchanger/heat pump is its heart. This is where the “thermal” part takes place. The earth has an ambient temperature of about 55 degrees, depending on where the system is installed. The geothermal heat pump is where the fluid that transfers heat to and from the earth is processed. A compressor in the unit increases the temperature to a level suitable for all but the coldest of days. On those days a secondary source of energy boosts the temperature of the heated air or water before it circulates in the home.

During the winter, the loop fluid (which is cooler as it exits the exchanger) absorbs heat from the earth surrounding it. This heated fluid is pumped back to the heat exchanger where a “pipe-within-a-pipe” coil uses the absorbed energy to heat the fluid contained within the heat exchange unit. This heated fluid becomes a gas at much lower temperatures than water and is used to heat the air (or water if using a radiant system) that circulates from the home through the exchanger.

During the summer, the temperature below the surface of the earth is less than the air temperature. So, the warm air in the home is cycled through the exchanger where the heat is extracted by reversing the heating process. The fluid in the loop – now warmed – is pumped back through the ground loop where the absorbed heat is released into the cooler earth and the cool fluid is recycled back to the home to repeat the process.

A geothermal system can provide heating and cooling through the forced-air system ductwork you're likely familiar with. It can also be used to heat water for in-floor radiant systems and use surplus heat to provide supplemental residential hot water. Geothermal can be installed as a replacement system to a current fossil fuel system that uses forced air to heat and cool. It is an obvious choice for consideration in new construction where the system cost is amortized over the term of the mortgage.

Some of the factors that will influence a decision to invest in a geothermal system are energy savings from changing systems, rebates and subsidies from local, State and Federal government programs, projected system life cycles and your ability to negotiate the purchase. The recovery of investment can occur in as few as 6 or 7 years or as many as 12 years. A reputable dealer will be able to construct a computer model to help you analyze your personal recovery term.