How It Works: Geothermal Concepts
The science behind geothermal heat pump systems is fairly simple. All you need to know are a few basic concepts: |
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Energy efficiencyIn general, energy efficiency is calculated as the "useful work" or "energy delivered" divided by the amount of energy supplied to do that work. With heat pumps, energy efficiency is measured in two different ways. Heating efficiency is expressed as a Coefficient of Performance (COP). The higher the COP, the more efficient the system. For example, a residential-sized geothermal system might have a COP of 3.4 or higher, meaning for every one unit of energy used to power the system, more than three units are put back into the home as heat. This compares to efficiencies of 0.92 for a high-efficiency natural gas furnace. Cooling efficiency is measured as an Energy Efficiency Ratio (EER). The higher the EER, the more efficient the system. Keep in mind that both COP and EER are dependent on many factors, and that high-efficiency equipment comes with a higher price tag - but the energy savings can pay back in the difference in just a few years. |
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Ground temperaturesAt depths below six feet, ground temperature stays a constant 50 to 55 degrees Fahrenheit year-round. During the winter, a geothermal system absorbs this extra heat from the earth and transfers it into your home. During the summer, the system takes heat from indoors and moves it back underground. Annual air temperature, moisture content, soil type and vegetative cover (i.e., trees and plants) all have an effect on underground soil temperature. As you might expect, the earth's temperature changes in response to weather changes, but there is less change at greater depths. |
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Soil and rockNo single factor is more important to the successful operation of a ground-coupled heat pump system than the efficiency of the heat transfer between the ground loop and the surrounding soil. Soil properties and the thermal performance of rocks vary widely. These variations indicate the importance of an accurate estimate before any geothermal loop design can be finalized. Your contractor might consult a helpful source like the Natural Resources Conservation Service Soil Survey. This is available from local NRCS offices located in approximately 3,000 counties across the United States. It gives a detailed layer-by-layer description of the soil down to a five- or six-foot depth, along with any rock content, density, soil types and available water capacity. |
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Loop fluidsA good heat transfer fluid is vital to the operation of a geothermal heat pump. In most areas, geothermal contractors use a corrosion-inhibited antifreeze solution with a freeze point of 10 degrees or more below the minimum expected temperature. The antifreeze solutions used are biodegradable, non-toxic, non-corrosive and have properties that will minimize pumping power needed.
Ordinary water can be used in warmer climates where the ground temperature stays warm and the heat pump's heat exchanger refrigerant temperature does not drop below freezing. |
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Air distributionThe air distribution system can make a big difference in both the cost and the effectiveness of geothermal heating and cooling. It also has an important effect on personal comfort and health. The air handling component is either a separate cabinet or is part of the cabinet that houses the geothermal heat pump, and includes the blower assembly that forces air through the ductwork. The supply ductwork carries air from the air handles to the rooms. Typically, each room has at least one supply duct and larger rooms may have several. The return ductwork moves air from the room back to the air handler. Most buildings have one or more main return ducts located in a central area. |
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Learn more about:Working With It: Designing a Geothermal System Working With It: Selecting Geothermal Equipment |
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Brochures and fact sheets:GeoThermal: Bringing Comfort to Your World |
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