A variation of a water-source heat pump is a WSHP that rejects heat to the ground, instead of to the air at a cooling tower. The water from the refrigerant-to-water heat exchanger is circulated by a small in-line pump through a series of wells. The fluid is in turbulent flow throughout the loop, to maximize heat transfer with the ground.
The key to understanding the GSHP is that the heat is rejected to the ground, which is always colder than the air in the summer and warmer in the winter. As a result, the GSHP is more efficient than a WSHP.
A significant limitation of the GSHP is that the system can be used only about 8 hours a day, maximum, 5 days a week. This is because the ground must have time to “recover” overnight, slowly dissipating the heat built up over the day. Operation of the unit for prolonged periods reduces the unit efficiency, and consequently the working capacity, of the GSHP. (These limitations apply to a hot and humid climate such as along the Gulf Coast in the United States.)
This limitation can be mitigated by drilling the wells deeper, increasing the well spacing, or drilling additional wells—e.g., to oversize the well field. Another possible resolution is a hybrid system with a cooling tower to reject part of the heat or to reject all of the heat after the day’s operating hours are over, using the ground loop as a thermal storage capacitor. Several computer simulation programs are available to make an accurate determination of the ground’s capacity to handle the annual design building load.
Hybrid systems are common in northern climates. The well field is sized for the heating load, and that part of the cooling load that can’t be handled by the ground loops is rejected by a cooling tower.
The GSHP can be installed in two configurations. Each unit can be piped to a dedicated set of wells (typically one well per ton of cooling capacity), or many units can be on a single primary loop circulating fluid through many wells. The latter must be done for a hybrid system. One advantage of a common header loop is that one large high-efficiency pump is used to move the water rather than a multitude of small, fractional- horsepower, relatively inefficient pumps—one at each unit. The latter uses several times more energy, reducing the overall system efficiency. One applicable variation is to install horizontal piping, often in the form of a coiled, slinky-type of piping. These coils are much easier to install, do not require a skilled driller and expensive equipment, and can often be incorporated into the overall site plan for new construction projects.
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Excerpt from Energy Conservation Projects.