The systems used for application of geothermal energy for residential purposes are called either a Geothermal Heat Pump (GHP) or a Ground-Source Heat Pump (GSHP). A geothermal heating and/or cooling system utilizes the earth's near constant underground temperature to maintain comfortable living conditions inside residences. The temperature of the earth and ground water a few feet below the surface is relatively stable throughout the year depending on the annual climate in the specific location. The majority of the United States is in a temperate climate zone and benefits from nearly constant ground and ground water temperatures regardless of the outside temperatures. During the winter the GHP system uses the ground as a heat source to bring energy into your home in order to provide space heating. In the summer time the system absorbs heat in the air and transfers this energy into the ground effectively cooling the house. The geothermal heat pump does not have to work as hard as an air source heat pump (AC) which has to extract heat from cold air in winter and transfer heat to already hot air in the summer. The energy efficiency of a geothermal heat pump is therefore much higher than a conventional air conditioner.

Geothermal Heat pumps can be classified into two categories, closed loop and open loop. In either case, heated or cooled air is distributed through the house's ductwork in the same way as conventional heating and cooling systems. Geothermal Heat pump systems utilize a heat transfer fluid (usually water or ethylene glycol) to heat or cool air from the house. In a closed loop system the heat transfer fluid is passed through a series of underground pipes where it give up or receives heat, depending on the time of year, before being brought back into the house at a temperature equal to that under the ground (typically between 68 and 72 degrees Fahrenheit). Closed loop systems can be further divided into vertical and horizontal systems. To install vertical systems 4 inch diameter borings are drilled into the ground. Then two 1 inch diameter plastic pipes are inserted into each boring, one for fluid delivery and one for fluid return. See picture 1 below. The average depth for each boring is 150 to 200 feet. Most residences will require about 450 feet of total boring in order to supply the typical requirement of ~3 tons of cooling or heating load.

Figure 1: Vertical boring used for geothermal heating and cooling

Horizontal closed loop systems are typically installed in areas where drilling is difficult or expensive. Horizontal systems only need to be buried 4 or 5 feet below the ground but since the temperature here will have slightly more variability than the deeper borings these systems require more surface area and thus longer piping to produce the same amount of heating and cooling. See picture 2 below for an example of a typical horizontal closed loop system.

Figure 2: Horizontal Closed loop system.

A nearby pond or lake can also be used a heat source and sink. This type of system can avoid the expense digging into the ground but does have the difficulty of an underwater installation. The pipes have to be placed at the bottom of the pond which must be at least 8 feet deep in order to maintain good temperature control. See picture 3 below for a typical pond geothermal heat pump system configuration.

 

Figure 3: Pond used for heating and cooling

An open loop system will actually pull ground water up to the surface level, use that water for exchanging heat, and then return it below surface at a different location. These systems are less common but can avoid a lot of earth moving when ground water is readily available. See picture 4 below for an example of an open loop geothermal system.

Figure 4:Â Open loop system

Evaluation of Performance

For cooling systems performance is evaluated by an index called EER (Energy Efficiency Ratio). The EER is defined as the cooling effect produced by the unit (BTU/hr) divided by the electrical input to the unit (Watts). Some representative values for EER's of typical units are:

Conventional Air Conditioning Unit: 12 (BTU/Wh)

Geothermal Heat Pump: 16 (BTU/Wh)

This demonstrates that in general the GHP is 33% more efficient than a conventional air conditioner.

For heating systems performance is measured by an index called COP (Coefficient of Performance). The COP is defined as the heating effect produced by the unit (BTU/hr) divided by the energy equivalent electrical input (BTU/hr). Some representative values of COP's for typical units are:

Electrical Heating:  1.0

Geothermal Heat Pump: 3.6

This means that when using a geothermal heat pump for heating you get out 3.6 times more heat energy than you put in as electricity.

In order to estimate the energy savings from a geothermal heating/cooling system you must first estimate your typical heating and cooling demand. Then calculate the energy requirement for a conventional HVAC system and that for a GHP system using the above performance indicators. The difference in these two values indicates the energy savings and can then be used to calculate savings in dollars based on local energy prices.

The cost of installing a GHP system is in the neighborhood of $6,000 (depending on the type you select). This installation cost is typically higher than conventional HVAC systems however when the value of the energy savings are taken into consideration then the added initial investment usually pays for itself very quickly. Of all the HVAC systems evaluated the Geothermal Heat pump system had the lowest overall life cycle costs. Please see our RENEWABLE ENERGY CALCULATOR in order to estimate the monetary and emissions savings you could enjoy by implementing a Geothermal Heat Pump system.