Experimental Research Of Variable Condition Performance With Trans-critical CO₂ Heat Pump System

The main direction of this study is the application of the natural refrigerant CO₂ in heatpump field, the first of its theoretical analysis and numerical simulation, and then set up thetest bed to study the performance in variable conditions, to find out the condition that keep theheat pump sysytem in optimizing state, which can provide data support and test reference.From the thermodynamics of trans-critical CO₂ heat pump system cycle, we know thatall the basic parameters of the system have a certain influence on the performance of the heatpump, then study the connection between basic parameters and system performance COP_H bysimulation. In the simulation process, only change one parameter, other parameters remainunchanged. The results showed that: when the evaporating temperature increased, the COP_Hof system is also increased; and the outlet temperature of gas cooler refrigerant side increased,the COP_H of system is decreased; the compressor efficiency and system COP_H are alsoproportional; the absorption gas superheat has little effect on the performance of system, butshowed decreasing tendency; when the high side pressure increased, the system system COP_Hfirst increased and then decreased, there is a maximum.For measuring the performance of trans-critical CO₂ heat pump system in variousconditions, and finding out the measures to improve the performance of heat pump system.We improved the structure of heat pump system, which has the following characteristics:(1) Heat Exchanger: Select coaxial tube heat exchanger, spiral tube of nickel whitecopper, that can strengthen the disturbance of the water side and refrigerant side, increase theheat transfer coefficient; meanwhile it also can weaken the scaling phenomenon of water side.(2) Throttle: The system uses two types of throttle, the first uses one throttle; the seconduses two throttles, the one adjust the high pressure side, the other throttle regulate the lowpressure side. Between the two throttles, the liquid reservior balances the flow fluctuations inthe throttle process.(3) Internal Heat Exchanger: The system can be run in two states which are with andwithout internal heat exchanger, so we can research on the performance of trans-critical CO₂heat pump system.Results showed that: (1) The diameter of the system′s pipeline is 10mm, the thickness is 1mm,which can fully competent the system′s pressure requirements; The heat exchanger iscoaxial, and it′s inner tube is the spiral of nickel white copper. The heat exchangers have agood effect, the maximum heat transfer coefficient of gas cooler and evaporator were636w/(m2?k) and 450w/(m2?k), the system is stable. (2) The evaporation temperature is -5℃,when the termination of water temperature is 45℃and 50℃, the COP_H of trans-critical CO₂heat pump system is increasing with the high side pressure; when the termination of watertemperature is 55℃, 60℃and 65℃, the COP_H of trans-critical CO₂ heat pump systemincreases first and then reduces with the high side pressure increasing, which exists amaximum value, it corresponds to the high side pressure is the optimal. (3) When the highside pressure and evaporation temperature are invariant, the COP_H of heat pump systemdecreases with the termination of water temperature increasing. (4) When the evaporationtemperature is -5℃, the water source temperature is 15℃, the initial temperature of water is25℃, the termination temperature of water is 50℃, when the high side pressure is lower than9.5Mpa, the COP_H of system that has intermediate heat exchanger is below than none. But itcan increase the compressor suction temperature, prevent the compressor from liquid blowing,and have a benefit for the oil return;In addition, the intermediate heat exchanger can reducedthe differential pressure of evaporator and gas cooler.(5) The lower evaporation temperature,the higher the compressor discharge temperature. When the evaporation temperature is -10℃,the high side pressure is 7.5MPa, the termination temperature of water is 45℃, thecompressor discharge temperature can reach 110℃; If the high side pressure is 9.5MPa, thecompressor discharge temperature can reach 150℃, so the evaporation temperature of systemcan not be lower than -10℃.