A: Water-to-air geothermal heat pumps, as the term suggests, can be adapted to water supplies other than wells, such as a freshwater lake or a stream. Another alternative to a well system would be a closed-loop, earth-coupled system.
The closed-loop geothermal heat pump system requires only that the heat exchanger be buried in solid contact with the earth. In using closed-loop exchangers, there is no depletion of the aquifer, since no water is withdrawn from the ground. The earth-coupled exchanger is filled one time with potable water or other heat exchanger fluids; therefore, it does not contaminate the geothermal heat pump by precipitating out minerals from the soil.
Two general types of earth-coupled systems have been used with geothermal heat pumps. The first type is a vertical heat exchanger. The second type is the earth-coil or horizontal heat exchanger. A serpentine length of plastic pipe buried four to six feet below the earth’s surface is called an earth-coil or ground loop.
Both horizontal and vertical earth-coupled exchangers utilize low-wattage circulator pumps to send water from the geothermal heat pump out through the continuous closed-loop to exchange heat with the earth, and then back to the geothermal heat pump.
The vertical heat exchanger is typically constructed using water well drilling equipment. The water from the geothermal heat pump is circulated through the vertical loops of piping, exchanging heat with the earth before returning to the geothermal heat pump. This flow is achieved by means of a low-wattage, inexpensively operated circulation pump. Vertical heat exchanger piping is filled with water or a water/nontoxic antifreeze mixture (depending on location), pressure-tested, and sealed. A vertical closed-loop system does not draw water from the earth.
The horizontal serpentine earth-coil has two plastic exchanger pipes in a single trench typically dug 2 to 3 feet wide and 6-feet deep. In the case of a double layer design, the trench is partially backfilled after laying the first pipe, then completely backfilled after the second. In urban areas, earth coils can be laid out in a curved pattern around the boundary of a lot.
Heat exchange will drop to only 10 to 20 percent of normal performance if the soil around the pipe becomes dry. When multiple pipes are to be buried in the same trench, the horizontal or vertical separation required between pipes is two feet. The length of the earth-coil required will vary with the climate and soil conditions.
For example, in Oklahoma where many of these systems have been installed, approximately 300 linear feet of wetted pipe or 450 linear feet of dry pipe per nominal ton of geothermal heat pump cooling capacity is required.
In the northern U.S. where heating is clearly dominant and cooling is of minor importance, 10 to 25 percent of propylene glycol USP should be used in the closed loop vertical heat exchanger to prevent freezing of the water in the geothermal heat pump exchanger.
Using current technology, a properly sized geothermal heat pump, duct system, water piping and water pump can provide a midwinter COP (coefficient of performance), according to advocates of the earth-coupled geothermal heat pump, of up to 4.2 with proper loop field design and good soil conditions. Averages are somewhat lower in northern climates, but rise in the middle U.S. Their performance is reported to be higher in autumn and spring.