A: A geothermal heat pump system simply transfers thermal energy (heat) from the ground or ground water into the space being conditioned during the winter months and transfers excess heat from the structure back into the ground or ground water in the summer months. Because the temperature of the ground or ground water remains fairly constant throughout the year—ranging from about 45-50 degrees F in northern latitudes to 70-80 degrees F in the deep south—operating efficiencies are high year-round.
The typical geothermal heat pump system consists of three main parts:
- The air handling system (fan and ductwork) that distributes air within the spaces being heated or cooled
- The ground or ground water heat exchanger that absorbs heat from the earth or discharges heat to the earth
- A reversible refrigerant loop that transfers heat between the air handling system and the ground or ground water heat exchanger.
The air handling system is typical of any forced-air heating or cooling system. A fan moves heated or cooled air through ducts to the individual rooms in a building, and returns air to the geothermal heat pump system.
The ground-coupled heat exchanger can take a number of forms. In an open-loop system, two wells are typically used. These are similar to conventional water wells, with one acting as a source well, and the other acting as a sink. Water is pumped from the source well, through a water-to-refrigerant heat exchanger on the GeoExchange system, and returned to the second well. Alternately, the water from the source well can be returned to a lake, pond, or stream, if there is one in proximity to the site, and local regulations permit. The water remains unaffected by the system, except that its temperature is raised in summer and lowered in winter.
In a closed loop system, the ground-coupled heat exchanger takes the form of sealed high density polyethylene piping buried vertically or horizontally in the ground. In the case of vertical systems, a series of 4-in. to 6-in. diameter bore holes are made (typically using water well drilling equipment to attain depths of 150 to 300 feet), a loop of piping is inserted into each hole, the various loops are tied together by a manifold and then the holes are grouted and backfilled. For horizontal systems, similar piping loops are buried in horizontal trenches dug 4 to 6 feet deep, the piping is connected by headers, and the trenches backfilled. In both vertical and horizontal closed-loop systems, the water or water/nontoxic antifreeze mixture in these pipes remains within the pipes for the life of the system.
For purposes of describing system operation, let’s assume an open-loop system using two wells. The heating cycle begins when the ground water is pumped from the source well to a water-to-refrigerant heat exchanger (acting as an evaporator) on the geothermal heat pump. The tubes on the refrigerant side of the heat exchanger are filled with a liquid refrigerant at low temperature. As the liquid refrigerant flows through the heat exchanger, it absorbs heat from the ground water, and evaporates to form a cool gas (10° to 30°F cooler than the ground water). The water from the source well gives up heat as it flows through the heat exchanger, returning to the discharge well at a lower temperature.
The gaseous refrigerant from the evaporator passes through tubing to a compressor, which compresses it, raising its temperature and pressure (to an average of 180°F and 245 pounds per square inch (psi) pressure for most models). The hot, compressed gas then flows to a refrigerant-to-air heat exchanger, which acts as a condenser in the heating mode. Here, air flowing across the condenser tubing absorbs heat from the refrigerant and carries it throughout the house. As it releases heat, the refrigerant condenses to form a liquid, which then flows through an expansion device that reduces its pressure and consequently lowers its temperature again. Finally, the refrigerant re-enters the evaporator and the cycle is repeated.
For home cooling, the above process is reversed. The compressor sends the hot, dense gas directly to the water-to-refrigerant heat exchanger (now acting as a condenser). The water from the source well absorbs heat from the refrigerant and flows back to the discharge well at a higher temperature. As it gives up heat to the water, the refrigerant cools and condenses into a liquid. The cool liquid refrigerant flows through an expansion device (usually an orifice or valve), which further lowers its temperature and pressure. The cold liquid refrigerant then flows to the air-to-refrigerant heat exchanger, which now acts as an evaporator. Air from the home’s interior flows across the evaporator tubing, giving up heat to the refrigerant inside the tubes. The cooler air is moved through the house via the duct work. The warmed refrigerant evaporates as it absorbs heat from the air, and then returns to the compressor to repeat the cycle.
Most of the geothermal heat pumps installed today may also be equipped to provide hot water for domestic needs. In fact, hot water can be provided free during summer months, by using waste heat extracted from the home interior. Even in winter, the geothermal heat pump system can supplement the hot water provided by the gas or electric water heater, reducing overall hot water costs by about 30% annually. All of this is accomplished with a small supplemental heat exchanger called a desuperheater.