GeoExchange Systems

What is a GeoExchange System?
A geoexchange system is an electrically powered heating and cooling system for interior spaces. This system utilizes the earth (or a pond or lake) for both a heat source and a heat sink. Components of this system include a heat pump, a hydronic pump, a ground heat exchanger, and a distribution subsystem. Most geoexchange systems utilize air ducting for the distribution system, and polyethylene piping in the earth for the heat exchanger.

What is a Heat Pump?
A heat pump is an electrically powered mechanical device that takes heat from one location and moves it to another location. A typical air conditioner is a form of a heat pump in that it takes heat out of the interior space and then rejects that heat outdoors. However, a true heat pump can work in either direction, unlike a typical air conditioner. A heat pump can take heat out of an interior space, or it can put heat into an interior space.

What is a GeoExchange Heat Pump?
Unlike the air conditioner that rejects heat into the surrounding air through the condenser (which resembles a caged box, sits outside the house, and makes noise when it turns on), the geoexchange heat pump rejects heat into the earth during the cooling mode, and takes heat out of the earth while in the heating mode. GeoExchange Heat Pumps are more commonly referred to as Ground Source Heat Pumps. (See Notes)

Understanding the GeoExchange System
(Portions of the following are excerpted from "Outstanding Home Comfort Through Advanced Technology" by the Geothermal Heat Pump Consortium)

How GeoExchange Systems Work
GeoExchange systems, like common heat pumps and air conditioners, make use of a refrigerant to help transfer (or pump) heat into and out of your home. The refrigerant helps the GeoExchange system take advantage of two primary principles of heat transfer:

Heat energy always flows from areas of higher temperature to areas of lower temperature.
The greater the difference in temperature between two adjacent areas, the higher the rate of heat transfer between them.

Refrigerators, air conditioners, and heat pumps all operate by pumping refrigerant through a closed loop in a way that creates two distinct temperature zones--a cold zone and a hot zone.

The simplest example of such a system is the universally familiar home refrigerator. In a refrigerator, a fan blows the air inside the box over tubes containing refrigerant that is very cold (typically below 0° F). Heat flows from the interior air to the cooler refrigerant. The refrigerant is then pumped to the high-temperature section, which is exposed to room air outside the refrigerator box. Because the refrigerant is hot in this zone, it gives up heat to the relatively cooler air in the room, before flowing back to the cold zone to begin the loop again.

An air conditioner works in exactly the same way, except that it extracts heat from the air inside a room or building and transfers it to the air outside the building.

A conventional heat pump adds a reversing capability, so the hot zone and the cold zone can be switched. With the zones reversed, it can extract heat from the outside air in the winter and transfer it inside.

Granted, being able to extract heat from frigid winter air seems like it shouldn't work, but it will if we can expose the cold air to refrigerant that's even colder than it is. And modern heat pumps can do that.

When the outside air gets extremely cold, the conventional (air source) heat pump has to resort to electric resistance heating. This reduces efficiency dramatically.

The Super Efficiency of GeoExchange
Standard (air source) heat pumps, while relatively simple to operate, face one major challenge: their operating efficiency is lowest when demand is highest. That is, heat pumps (air source) have to work hardest when we demand the most performance from them.

As we've just seen, a regular heat pump (air source) extracts heat energy from outside air in the winter, and rejects heat to outside air in summer. Unfortunately, the colder the outside air, the more difficult it is to extract heat from it, and the hotter the outside air, the harder it is to transfer heat to it. The temperature difference between the air and the refrigerant is small in both cases, lowering heat transfer rates within the system.

Yet, the colder it gets outside, the higher the rate of heat loss through windows, around doors, and through walls and roofs, and the more heat we need to pump inside to keep indoor temperatures comfortable. In summer, we face a similar dilemma. The hotter it gets outside, the higher the rate of heat infiltration into the house, and the more heat removal we need to maintain comfort.

A GeoExchange system eliminates this dilemma by using the relatively constant temperature of the earth as a heat source in winter and a heat sink in summer, instead of outside air.

Throughout most of the U.S., the temperature of the ground below the frost line (about 3 to 5 feet below the surface) remains at a nearly constant temperature, generally in the 45 ° -50 ° F range in northern latitudes, and in the 50 ° -70 ° F range in the south. So, in the winter, a GeoExchange unit can extract heat from the earth that's relatively warm compared to the cold outside air, and in the summer, it can discharge heat to the earth that is relatively cool, compared to the hot outside air. Since the difference between the refrigerant temperature and the ground temperature remains relatively high in both seasons, so do heat transfer rates. Consequently, the GeoExchange system operates at much higher year-round efficiencies than a standard heat pump.

The Cleanliness of GeoExchange Systems
Installing a GeoExchange system is environmentally responsible. Since a GeoExchange system merely transfers heat from the ground into your home in winter, you don't need to burn any fossil fuels to create a warm interior environment. The approach drastically reduces carbon dioxide emissions (a greenhouse gas) compared with the operation of other heating systems, and completely eliminates the heating system as a potential source of carbon monoxide fumes within your home - making the GeoExchange system an environmentally friendly as well as safe and healthy alternative to traditional oil and gas furnaces.

Making The Ground (Earth) Connection
The unique aspect of the GeoExchange system, and the key to its lengthy list of benefits, is the "ground loop." The ground loop provides the means of transferring heat to the earth in summer, and extracting heat from the earth in winter. There are "closed loop" and "open loop" systems. First, let's look at typical closed loop systems that recycle the same water (the refridgerant) endlessly.

Physically, the ground loop consists of several lengths of plastic pipe typically installed either in horizontal trenches or vertical holes that are subsequently covered with earth and landscaping of your choice. Water inside the ground loop piping is pumped through a heat exchanger in the GeoExchange unit. In the summer, it absorbs heat from the refrigerant hot zone and carries it to the ground through the ground loop piping. In winter, it absorbs heat from the earth through the ground loop, and then transfers that heat to the refrigerant cold zone.

The length of the ground loop will be determined by the heating and cooling loads, which are determined in turn by the size of your home, its design and construction, its orientation, and the climate where you live. Whether the ground loop is most efficiently installed in horizontal trenches or in vertical boreholes depends on the type of soil near the surface (rocky, sandy, clay-laden, etc.), the geology of the deeper terrain in your area, and the amount of land available. Generally, horizontal loops are less expensive to install, but require more land area. Vertical holes require much less land area, but require the expense of drilling.

Another ground connection scheme - an "open loop" system - involves using wells instead of closed loop piping. Where water is plentiful, it can be pumped out of a well, through the heat exchanger at the GeoExchange unit, and then pumped back into another well to return to the groundwater. Since the water merely absorbs or gives up heat, but is not altered in any other way, it leaves the GeoExchange unit as pure as it was when it entered it.

Any one of these installation schemes results in the same high efficiency, when properly sized.

Moreover, once the ground loop is installed, you can typically forget about it. The polyethylene piping (the same type used for cross-country natural gas lines) does not degrade, corrode, or break down in ground or water contact, so sound installations are expected to last 50 years or more.

Free Hot Water
As a side benefit, most GeoExchange systems can be designed to produce free hot water during the summer, by using waste heat extracted from the interior air during the air conditioning season. Even in the winter, waste heat from the GeoExchange heat pump can be converted to hot water to reduce the energy costs of the hot water heater.

Conclusion

GeoExchange is the most energy-efficient, environmentally clean, and cost-effective space conditioning system available, according to the Environmental Protection Agency. The EPA confirmed the superior efficiency of GeoExchange, finding that even on a source fuel basis - accounting for all losses in the fuel cycle including electricity generation at power plants - GeoExchange systems average 40% greater efficiency than air source heat pumps, 48% greater efficiency than gas furnaces, and 75% higher efficiency than oil furnaces. Today's best GeoExchange systems outperform the best gas technology, gas heat pumps, by an average of 36% in the heating cycle and 43% in the cooling cycle.

GeoExchange systems use the Earth's energy storage capability to heat and cool buildings, and to provide hot water. The earth is a huge energy storage device that absorbs 47% of the sun's energy -- more than 500 times more energy than mankind needs every year -- in the form of clean, renewable energy. GeoExchange systems take this heat during the heating season at an efficiency approaching or exceeding 400%, and return it during the cooling season.

In addition to operating cost benefits, GeoExchange provides:

Heating without combustion of fossil fuels
No carbon monoxide or carbon dioxide
Increased safety
Simpler design, maintenance, and operation
Free hot water in the summer
No unsightly/noisy air conditioning or air source heat pumps in the yard