Starting about five feet beneath your feet, the earth maintains a constant temperature with nearly infinite energy. In Michigan, that temperature is about 52 degrees Fahrenheit. By using a geothermal heat pump system that taps into the constant temperature of the earth, you can very efficiently heat your house in winter and cool it in summer. The system can also supply a good portion of your hot water for essentially free.
Geothermal heat pumps work exactly like your refrigerator or air conditioning systems. Using the refrigeration cycle, those devices convert a fluid from gas to liquid and back using a compressor and a heat exchanger. This “latent heat of transformation” releases heat energy into a heat sink during the cooling cycle. This heat sink with a standard air conditioning system is the outside air, while a refrigerator’s heat sink is the air in your home.
With a geothermal heat pump, the heat sink is the ground, which is significantly cooler than ambient air. This makes a heat pump very efficient for cooling. Better yet, with a geothermal heat pump, we can run this cycle in reverse, drawing energy from the earth and using the air in a home as a heat sink. This heats a home effectively far more efficiently than standard heating equipment.
A geothermal system uses electricity to run the heat pump, but for every unit of electricity we use, we can capture nearly 4.5 units of heating power and 3.5 units of cooling power. Why the discrepancy between the two? The electrical energy we use to run the system in the home ultimately creates heat, and this works against the system when we are trying to remove heat from the home.
Geothermal earth fields can be run in a variety of methods. Most commonly, fields are either installed as horizontal trenches on the lot or as vertical fields which are drilled into the ground using well-drilling equipment. Horizontal fields are cheaper and more efficient, but require a lot that is big enough to make several trenches that are around 100 feet long. Vertical drilling works well on smaller lots where space is at a premium. Both vertical and horizontal loop systems are closed-loop systems, meaning the fluid that transfers energy re-circulates in the fields.
Geothermal systems can be used for heating with forced air or radiant floor systems. Because the maximum temperature we can achieve with a geothermal system is about 125° Fahrenheit, traditional hot water or steam boiler systems do not work efficiently with these systems
A geothermal system works best when the building envelope is already tight and well-insulated. It is important to view a geothermal system as part of a highly efficient home, not just the end goal. Insulation and air-sealing are much less expensive to install, and the money invested in these items can frequently be offset by a smaller geothermal system that costs less to install.
Because geothermal heating and cooling is so efficient, however, both DTE Energy and the federal government offer incentives to homeowners who use it. DTE Energy gives geothermal energy users a lower electrical rate, bring the cost of the electrical energy closer to the low price of natural gas, and beating the cost of propane handily.
The federal government also offers a 30 percent unlimited tax credit for geothermal systems. With this tax credit, 30 percent of the total cost for all components of a geothermal system and the labor to install them are given back to consumers as a credit directly off their total tax bill. With these incentives, the high upfront cost of geothermal makes sense, particularly if a new furnace is being installed anyway.
An efficient building envelope coupled with geothermal heating and cooling can bring average monthly heating and cooling bills to about $50 or lower. This savings combined with available incentives can make geothermal systems have a reasonable payoff period for the high cost of installation, particularly if you are installing a new heating and cooling system anyway.
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