An assessment of a geothermal heat pump in an application with high internal gains

With the increasing interest and need for more energy efficient buildings, high efficiency mechanical systems are being specified more frequently. Geothermal or earth contact heat pumps are being increasingly selected because of their potential for high efficiency. When adequately sized and used with a properly sized field, these systems offer high efficiency with little sensitivity to ambient conditions. Field sizing guidelines and relatively simple computer programs are available which make sizing fields for residences and simple buildings a straightforward and reasonably accurate process. For larger buildings more sophisticated building load and field sizing computer programs have been developed. Field sizing is critical for the proper operation of geothermal heat pumps. For instance, an undersized air source heat pump will work well, although it may not meet the load it was chosen for, and its efficiency may be less than one would expect from manufacturers data. If one undersizes the field for a geothermal heat pump, the pump not only doesn't meet the load; it causes the field to overheat. An overheating field decreases the efficiency of the heat pump. This, in turn, causes the field to further overheat. This cascading of effects can result in efficiencies that may be much lower than what the system is expected to have. This paper reports on the performance of a closed loop geothermal heat pump installed to condition a space with high internal gains. The test has shown that the current sizing criteria for conventional small buildings are inadequate for spaces with high internal gains. The field overheated and the efficiency decreased by 40% from the manufacturers rating. The geothermal heat pump continued to operate satisfactorily with the field seriously overheated. Had the system not been heavily instrumented, one would likely not have realized there was a problem.