Simulation on a Two-Stage Compression Heat Pump with Focus on Optimum Control

An appropriate control logic is very important to optimize the performance of a two-stage compression heat pump system. In this paper, a numerical simulation model of two-stage compression heat pump system with intercooler was first presented and then validated by comparing simulation results with measured data. In addition, the model was used to analyze the performance characteristics of the heat pump, according to the superheat at suction line of the high-stage compressor and injection point, with frequency of low-stage compressor varying from 30 to 80 Hz. The simulation results showed that the heating capacity increased linearly with an increase of frequency of low-stage compressor, and the optimum heating capacity and COP were determined when the quality of the injection point was very close to unity. It was suggested that the superheat at the injection point was kept at 2 degrees C by controlling the 2nd EEV's opening, and the frequency of low-stage compressor was adjusted to meet the heating command of the user during space heating.