Study On Performance Of Cascade Heat Pump System

When the ambient temperature of single-stage compression heat pump system is low in winter,the compression ratio of the system is too large,the volume efficiency is reduced,and the performance coefficient is reduced.The performance of the two-stage compression heat pump system is better than that of the single-stage compression heat pump system.However,when the ambient temperature is below – 25?,the performance coefficient of the two-stage compression heat pump system approaches to 1,and the heating effect is poor.The performance of cascade heat pump system is better than that of two-stage compression heat pump system.The cascade heat pump system still keeps high efficiency when the ambient temperature is below – 25?.Therefore,the cascade heat pump system with high efficiency under low ambient temperature has a good development prospect.In this paper,the thermal performance of cascade heat pump system at low ambient temperature is studied by theoretical analysis and experimental research.The main contents of the research are as follows: thermodynamic analysis of cascade heat pump system through thermodynamic cycle,the variation of performance parameters of cascade heat pump system under different ambient temperatures and intermediate temperatures is obtained.The results show that When the ambient temperature rises,the system evaporation temperature rises and the heating coefficient increases;when the evaporation temperature and the condensation temperature are fixed,the system heating coefficient increases first and then decreases with the increase of the intermediate temperature.Therefore,when the operating conditions change,it is necessary to change the flow rate ratio of high and low temperature refrigerants to adjust the intermediate temperature of the system so as to make the system under the optimal COP.Stable operation,the change of intermediate temperature is achieved by adjusting compressor frequency and refrigerant flow rate of the system.In the selection of equipment,the high and low temperature compressor uses frequency conversion compressor and R410 A refrigerant as working fluid.In the experiment,high temperature evaporation temperature is used as the intermediate temperature of cascade heat pump system.The experimental results are as follows:(1)Under certain condensation temperature and high and low temperature compressor frequency,the range of ambient temperature is-30?~-24?.When ambient temperature increases,the compression ratio of low temperature stage decreases by 5.3%,3.1% and2.8% respectively,and the compression ratio of high temperature stage decreases by 5.9%,3.8% and 2.6% respectively.The compression ratio of high temperature stage is larger than that of low temperature stage.The intermediate temperature of cascade heat pump system increases gradually,and the variation of the temperature difference of the intermediate heattransfer is 5.1? ? 4.8? ? 4.6? and 4.5?,respectively.The difference of cascade temperature decreases gradually with the increase of environmental temperature.(2)Under certain conditions of evaporation temperature and condensation temperature,the compressor frequency of high-temperature stage is fixed,the compressor rate of low-temperature stage frequency varies from 50 Hz to 80 Hz,the cascade temperature difference of the system is about 4.5?,the compression ratio of high-temperature stage varies from 3.92 to 3.71,and the compression ratio of low-temperature stage varies from4.41 to 4.67.The compressor frequency of low-temperature stage is fixed,the compressor rate of high-temperature stage frequency varies from 50 Hz to 80 Hz,the cascade temperature difference of the system is about 5.1?,high-temperature stage compression ratio range is 3.88~4.01,low-temperature stage compression ratio range is 4.54 ~ 4.45.The heating coefficient of low-temperature stage is larger than that of high-temperature stage,so the influence of frequency change of low-temperature stage compressor on system performance is better than that of high-temperature stage compressor.(3)When the condensation temperature is 50 ?,the high temperature compressor frequency is fixed,the COP of the system increases first and then decreases with the increase of the low temperature compressor frequency;when the system runs at the optimum frequency,the evaporation temperature increases from-38? to-32?,and the optimum COP increases by 7.7%,3.8% and 1.8%,with the evaporation temperature increasing.The higher the degree,the slower the COP increase of the system heating coefficient;when the evaporation temperature increases,the optimal frequency of the cryogenic compressor operating under the optimal COP decreases.In summary,cascade heat pump system solves the problems of high compression ratio,poor heating effect and low performance coefficient of single-stage and double-stage compression heat pump system at low ambient temperature.Under different working conditions,the system can operate efficiently and stably at low ambient temperature by adjusting the compressor frequency and changing the refrigerant flow rate at high and low temperatures.