Researching On The Performance Of Air Source Heat Pump In Low Temperature

Using the quasi-two stage compression system not only can effectively improvethe heating efficiency, but also reduce the discharge temperature at low ambienttemperature condition. The refrigerant R32is widely used, because of its low GWPand the basic thermodynamic parameters similar with R410A. The dischargetemperature of conventional air source heat pump using R32is too high under the lowambient temperature condition. In this paper, the status of the quasi two stagecompression system is summarized. The performance of the quasi two stagecompression heat pump system with R32is investigated theoretically andexperimentally.Firstly, based on the characteristic of scroll compressor, the mathematic model ofthe compressor with the vapor injection was set up. The mathematic models of theother components in the system were established using lumped parameter method.According to the mass conservation and the energy conservation equations, themathematical model of the system was built up. The performance of the quasi twostage compression heat pump system and conventional heat pump system wassimulated under different evaporating temperatures. The influence of theinjection pressure and the injection ratio on the performance of the system wasanalyzed. Secondly, in order to investigate the heating performance of the quasi twostage compression heat pump system with R32, the experimental setup wasestablished. The heating performance was tested under the condensing temperature of35℃and the evaporating temperature ranged from-2℃to-15℃. The experimentalresults were analyzed and compared with the simulation results. The followingconclusion can be obtained from the simulation and experimental results:(1)The injection ratio, heat capacity and power consumption of quasi-two stagecompression heat pump system are proportionate to the injection pressure. The maxinjection ratio of the quasi two compression heat pump system is about33%, and itsheating capacity increases from8.4kW to10kW with the increase in the injectionpressure under the evaporating temperature of-15℃.(2)For the quasi two stage compression heat pump system, the COP of thesystem decreases with the increase in the injection pressure when the evaporatingtemperature is above-6℃. When the evaporating temperature is below-6℃, with therise of the injection pressure, the COP of the system increases firstly and thendecreases, which achieve maximum value as the injection pressure ranges from1.50MPa to1.54MPa.(3) The discharge temperature of the quasi two compression heat pump systemdrops by11℃with the rise of the injection pressure under the evaporatingtemperature of-15℃. The reliability of the system is improved significantly in alower temperature.(4) The heating capacity of both the quasi-two stage compression heat systemand the single stage compression heat system decreases as the evaporatingtemperature decreases. The heating COP of the quasi two stage heat pump system is lower than that of single stage heat pump system when the evaporating temperature isabove-8℃. It shows that the growth of the heating capacity which caused by the riseof the injection ratio is less than the growth of power consumption of the system.Thus, when the evaporating temperature is below-8℃,the COP of the quasi two stageheat pump system is higher than that of the single stage heat pump. It shows that thegrowth of heat capacity which caused by the rise of the injection ratio is more than thegrowth of the power consumption of the system. Compared with the single stage heatpump system, the discharge temperature of the quasi two stage heat pump systemdrops by10℃under the evaporating temperature of-10℃. From the economicefficiency and reliability considerations, the quasi two stage compression heat systemis suitable for lower temperature working conditions.Compared with the conventional heat pump, the quasi two stage compressionheat pump system can improve the performance of air source heat pump effectivelyand decrease the discharge temperature under the working conditions of lowerambient temperature, and enlarge the operating range of the air source heat pump.