Test of an improved gas engine-driven heat pump

A new generation of natural gas engine-driven heat pump (GEHP) was introduced to the marketplace recently. While the units installed have performed exceptionally well and earned rave reviews for comfort and savings on utility bills, the higher initial cost and relatively long payback time have affected the wide commercialization of this advanced technology. According to a study done for the southeastern US in the Atlanta metropolitan area, the annual operating cost of the GEHP is less than that of a baseline system consisting of a 92% efficiency gas furnace and a SEER 12 air conditioner. The estimated payback time is around 10 years to cover the difference in initial equipment price between the new and the baseline system. It has been projected that a liquid overfeed (LOF) recuperative cycle concept can simplify the hardware design of a GEHP, resulting in reduced cost and improved performance. Laboratory tests have shown that LOF would improve the energy efficiency of a vapor compression unit by 10%. In addition, LOF will reduce the compressor pressure ratio and thereby improve equipment reliability. Based on the assumed performance improvements and cost reduction, a simple payback calculation indicates LOF can reduce the payback time for an improved GEHP considerably in the Atlanta metropolitan area. Laboratory testing of an improved GEHP has been carried out at Oak Ridge National Laboratory. This paper reports on the equipment design modifications required to implement LOF and the results of performance tests at steady-state conditions. The preliminary cooling test results have indicated that the LOF in conjunction with orifice-type expander can be applied to GEHP for cost and performance enhancements. The improvements in energy efficiency will be dependent upon several controlling parameters including the proper refrigeration charge, the selected ambient temperature, and the system operating condition