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ZEH Info!

Geothermal Heat Pumps

Installing a geothermal heat pump system can be a smart way to save energy and money. Unlike other types of heating systems, which convert fuel to heat, a heat pump is designed to move heat from one place to another. Even at relatively cold outdoor temperatures, a heat pump is able to extract heat from outside to heat the home. In the summer the system reverses the direction of heat movement to cool the home.

Geothermal heat pumps extract heat from the ground or from water, either below or on the surface. Because ground and ground water temperatures are a constant 7 - 13 C (45-55 F) year-round, this type of system is much more efficient.

There are two basic types of geothermal systems, open loop and closed loop. An open loop system uses a conventional well as its heat source. Water is pumped from the well through the heat pump's heat exchanger, where heat is extracted and transferred to a refrigerant system. The heat is then transferred to the air in the home. The water is then returned to a pond, stream, or second well. Local conditions such as quantity and quality of available water can affect the use of this type of system. Local water use and disposal regulations may also limit the use of open loop systems.

 

Closed loop systems circulate a heat transfer fluid (usually a water/antifreeze solution) through a system of buried or submerged plastic piping, arranged either horizontally or vertically. Ground-based horizontal loop systems draw their heat from loops of piping buried 1.8 to 2.4 meters (six to eight feet) deep in trenches. The piping for water loop systems is installed below the winter ice level in pond or lake, or below low tide level in the ocean. Vertical loop systems use holes bored 45 to 60 meters (150-200 feet) deep with U-shaped loops of piping. They work the same as horizontal loop systems, but can be installed in locations where space is limited due to size, landscaping or other factors.

Another type of geothermal heat pump is called a "Direct Exchange" or "DX" system. This type of system uses a much shorter loop of piping buried below ground, through which the refrigerant itself is circulated, replacing the heat transfer fluid used in other geothermal systems because the heat is transferred directly between the refrigerant and the ground, the amount of piping can be drastically reduced. This type of system is ideal for situations where the amount of space for the piping loop is very limited.

How the Heat Pump Works

The heat pump operates on the principle that heat can be transferred by a cycle of alternating vaporization and condensation, the same cycle used by refrigerators, freezers and air conditioners. When a liquid vaporizes, heat is absorbed, and when a gas condenses, heat is released. By alternately pressurizing and depressurizing a liquid with a very low boiling point (called a refrigerant), the heat pump can absorb heat from a relatively cool medium and transfer it to a warmer one.

Even though the ground water temperature may be a relatively cool 10 C (50 F), the circulating fluid can absorb some heat, and the vapor compression cycle of the heat pump can transfer it to the indoor air.

During the summer, the same fluid is circulated through the loop of piping and the heat pump's heat exchanger, but the heat pump's cycle is reversed. Instead of absorbing heat from the fluid and transferring it to the indoor air, it now absorbs heat from the indoor air and transfers it to the fluid, where it is given off to the ground or ground water. Because of the constant relatively cool temperature of the ground water, the geothermal system is actually more efficient for cooling than the typical air conditioner, which must reject heat to hot outdoor air.

The geothermal system is a highly efficient and economical year-round space conditioning system. It can save over 50% on heating costs compared with electric resistance heating (e.g. electric furnace), and up to 30% on air conditioning costs, while providing clean, safe comfort year-round.