Study On Performance Of Low Temperature Heat Pump Air-conditioning System For Pure Electric Vehicle

Given lack of engine cooling heat source applied for indoor in winter from pure electric vehicle, the heat pump air-conditioning system that can both cooling and heating, has been the key technology to be resolved eagerly. Based on two-stage compression refrigeration cycle, Considering the low temperature property of specific refrigerant R134 a in our country and the structure characteristics of the system equipments, the gas-mixed and synergistic system of low temperature heat pump air-conditioning is put forward, the system is mainly to solve the key techniques heat pump air conditioning using R134 a as refrigerant ensure heating operation efficiently and reliably under low temperature operating conditions. Meanwhile, the system also proposes a feasible technology method to develop high efficiency heat pump air conditioning system which is more suitable for pure electric vehicles. The research results include the following aspects:(1) Mathematical model about low temperature heat pump air-conditioning system for pure electric vehicles is built. Mathematical model of the system mainly includes: mathematical models of injection scroll compressor, outside heat exchanger(condenser), heat exchanger inside(evaporator), electronic expansion valves, gas-mixing heat exchanger(intermediate heat exchanger), and thermal physical parameters of circulating working substance R134 a. According to the coupling performance among all components of the system, all components are connected to form a mathematical model to predict the performance of the system. Through simulation calculation, factors which can impact operation characteristics of low temperature heat pump pure air conditioning system for pure electric vehicles such as environment temperature outside, gas-mixed ratio and others are analyzed. Compared the simulating results with the experimental ones, they both have the same variation tendency and inosculate with each other preferably.(2) Designing low temperature heat pump air-conditioning system for pure electric vehicle. According to operation conditions and actual demands, the system can achieve two kinds of heat pump cycle technology, namely, medium pressure-supplied and low pressure gas-mixed. In addition, the system can realize multiple modes of basic work, such as refrigeration, heating and defrosting of external heat exchanger for pure electric vehicle. Besides, through the compressor cooling, gas-mixed and synergistic system and auxiliary electric heater, the system can operate the heating circulation efficiently and steadily at-20 ℃ outside temperature.(3) The experiment platform of low temperature heat pump air conditioning system for pure electric vehicles is designed and established. Performance testing has been completed respectively under basic work modes, i.e. electric vehicles cooling, common heating, low temperature heating and defrosting of outside heat exchanger. Experimental results show that exhaust temperature of compressor decrease significantly under the low temperature condition. When the environment temperature is-20℃, the system still running normally and exhaust temperature of compressor can be effectively controlled below 80℃. The method solves the problem that exhaust temperature is too high to work normally under immiscible gas recycling. The results also reveal that system heating capacity improve significantly when the environment temperature is10 ℃. Heating capacity of common heat pump air conditioning system is about 4200 W, while heating capacity of gas-mixing low temperature heat pump air conditioning system is around 5300 W. Heating capacity increases more than 20%. System efficiency is high in case of low temperature. when environment temperature is-20℃, the COP is around 1.5, which is higher than other heating options.(4) The method of defrosting rapidly by improving the temperature and increasing enthalpy under the reverse-cycle is put forward. The study of experiment shows : under the ambient temperature range from 0 ℃to 20℃, the outside heat exchanger can defrost completely in 50 to 100 seconds by using this method, realizing the fast defrosting of outside heat exchanger.(5) Optimizing the above system based on system simulation and experimental results, moreover, developing the first low temperature heat pump air-conditioning system for pure electric vehicles in China. Related vehicle performance experiments have been carried out in a large artificial environment laboratory. The experimental results indicate that the vehicle-mounted system possesses good performance under standard refrigeration condition, standard heating condition and low temperature heating condition. In addition, the system can ensure indoor temperature always maintain in a range of comfortable temperature either at high environment temperature or low temperature via controlling frequency conversion speed of the compressor and the PTC(Positive Temperature Coefficient) heating source. It achieves three great goals: high efficiency, energy saving and reliable operation.