Modeling and optimization of two-stage compression heat pump system for cold climate applications

When an air source heat pump runs at very low ambient temperatures, several issues could arise and thus limit its applications. These issues include lower coefficient of performance (COP), deteriorated heat output and higher compressor discharge temperature resulting from an increased pressure ratio. Therefore, there is a need to develop more efficient heat pump systems for low ambient temperature (--30 degrees C) applications. Under this circumstance, it becomes advantageous to use devices in which the compression of refrigerant vapor occurs in two or more stages. Two-stage vapor compression heat pumps can operate more efficiently at a greater dif-ference in temperatures between evaporator and condenser and improve their performance under cold climate conditions. In this study, a model for a two-stage compression heat pump system is developed with the system configuration containing a closed economizer cycle and inter-stage refrigerant injection. In the two-stage heat pump, the intermediate temperature and injection pressure (or displacement ratio) between low-pressure compressor and high-pressure compressor are important parameters that influence the operating performance. In this paper, the optimal intermediate parameters corresponding to the maximum possible COP have been determined. The performance of the two-stage heat pump using R410A and R290 are modeled and compared. It is shown that the two-stage heat pump achieves a COP of 2.85 and a COP of 2.7 at an ambient temperature of-30 degrees C using R290 and R410A, respectively. In addition, the optimized total fin surface areas of the evaporator and the condenser heat exchangers are calculated from the developed model.