Technology Description

With STES, low-cost or waste energy (either heat [positive] or cold [negative]) is harvested when it is produced or when it is available, stored in the subsurface using borehole heat exchangers or water wells, and then utilized when the heat or cold is expensive or difficult to obtain. The anticipated application would consist of two sets of borehole heat exchangers that are tied into a ground source (geothermal) heat pump inside a building. The heat pump operates on a standard refrigeration cycle. During the winter heating season, heat is extracted from water that is circulated through the hot borehole heat exchangers, and it is used to evaporate the working fluid in a heat pump heat exchanger. The heat pump compressor compresses the vapors back into a liquid in a different heat exchanger, releasing the heat into either air or water that is used to warm the building. During the summer cooling season the cycle is reversed, and the heat pump working fluid now evaporates in the heat exchanger that is tied to the building return air (or water), thus absorbing heat and cooling the building. After compression by the heat pump compressor, the refrigerant vapor condenses in a heat exchanger, giving off heat. Water that is circulated through the cold borehole heat exchangers is used to remove this heat from the building by cooling the condensing refrigerant in the heat pump.

The STES geothermal heat pump system to be demonstrated differs from the conventional, single ground-loop, geothermal heat pump operation, because it takes advantage of free or inexpensive sources of heat and cold to create two ground loops (borehole arrays containing heat exchangers), one artificially hot and the other artificially cold. In contrast, a conventional geothermal heat pump system uses heat (cold) from a single ground loop at a constant ambient temperature in both the winter and summer cycles. By using a hot water source in the summer and a cold water source in the winter to maintain the hot and cold ground loops, respectively, the STES geothermal system can achieve a higher heating and cooling efficiency than the conventional geothermal system. Then, net heating of the hot loop and net cooling of the cold loop become benefits.