| Electric vehicles are commonly heated with PTC thermal resistance as there is no waste heat utilization from the engine.However,its inefficient heating has a serious impact on the distance range of electric vehicles.The heat pump type automotive air conditioning can achieve the integration of cooling and heating,and its advantages of high efficiency,energy saving and environmental protection can be an effective solution to the traditional PTC auxiliary air conditioning system.However,the adaptability of current heat pump type automotive air conditioners in high/low temperatures has not been well addressed.When the external ambient temperature is high,the heat transfer capacity of the outdoor heat exchanger decreases,resulting in insufficient cooling capacity.When the external ambient temperature is low,the compressor pressure ratio increases,causing its discharge temperature to be too high and the system heating performance to decrease sharply.Based on the above problems,how to make the heat pump air conditioning system of electric vehicles run stably and efficiently in a wide temperature range has become a key issue for further popularization of heat pump air conditioning system.In order to further widen the operating temperature range of electric vehicle heat pump system,a wide temperature range heat pump system using quasi-two stage compression technology and its operation strategy is investigated in this paper.Firstly,the optimal intermediate temperature,intermediate pressure and relative injection ratio for the quasi-two stage compression cycle are investigated using a theoretical derivation method,and the influence of condensing temperature,evaporating temperature,suction superheat,injection superheat,subcooling and the heat transfer efficiency of the economiser on the intermediate parameters is analysed.The optimal intermediate temperature of the quasi-two stage compression cycle is more sensitive to the subcooling of the system than to the superheat.Increasing the heat transfer efficiency of the economiser contributes to the system’s heating COP.The optimal injection pressure is higher than the empirical value(empirical equation)for the same operating conditions.The optimal injection pressure calculated by the theoretical derivation method is in good agreement with the experimental value,and the maximum error in the optimal injection pressure is within ±3%.Secondly,a calculation model for a scroll compressor with vapor injection was developed and fitted using experimental data to obtain a semi-empirical formula between the relative injection ratio and pressure ratio.On this basis,the main components of a wide temperature range heat pump air conditioning system are analysed for suitability.Based on the above theoretical research,a wide temperature range electric vehicle heat pump air conditioning system is built and experimental research on the system performance under high/low temperature operating conditions is carried out.The charge amount of the quasi-two stage compression system in different modes is investigated and a method for determining the appropriate charge amount of the system is obtained.Based on the determination of the charge amount,the cooling/heating performance of the vapor injection system is compared with that of the non-injection system under high/low temperature conditions.The vapor injection system can effectively reduce the compressor discharge temperature,especially at low temperatures,which offers the possibility to extend the temperature range of the system operation.The effect of injection pressure(intermediate pressure)on system performance under high/low temperature conditions is investigated.In low temperature heating conditions,the system heating COP rises and then falls as the injection pressure rises.In practice,the optimal injection pressure can be calculated according to the experimental conditions(condensing temperature,evaporating temperature,subcooling and superheating,etc.)and then adjusted towards the optimal injection pressure by regulating the expansion valve of the injection branch.In high temperature cooling conditions,as the injection pressure increases,the system cooling capacity increases and then decreases,and the cooling performance coefficient(EER)decreases.For the same operating conditions,there is an optimal injection pressure to maximize the cooling capacity of the system.According to the steady-state load calculation model,the curves of steady-state demand of cabin load at different ambient temperatures are obtained.At ambient temperatures of-5 ℃,-10 ℃,-15 ℃ and-20 ℃,the load demand in the cabin is 2.75 k W,3.40 k W,3.85 k W and4.30 k W respectively.On this basis,the operation strategy of a wide temperature range heat pump air conditioning system is investigated.At low temperature conditions,the outlet state of the main evaporator is in slightly wet,which helps to improve the heating performance of the system and reduce the compressor discharge temperature.In the tested working conditions,compared with the strategy 1 system,the system heating capacity and COP can be increased by up to 5.7% and 4.6% using Strategy 2 for vapor injection cycle.For two-phase injection cycle,compared with the strategy 3 system,the system heating capacity and COP can be increased by up to 4.4% and 4.7% using Strategy 4.With the above control strategy,the discharge temperature of the compressor can be reduced by 23 ℃ at most.A suitable injection strategy can further widen the operating temperature range of refrigerant injection system.When the inlet temperature of evaporation plate exchange is above-15 ℃,the vapor injection strategy can meet the demand.Below-15 ℃,a two-phase injection strategy is required,with a minimum operating temperature of-22 ℃.At high temperature conditions,the cooling capacity of the system is more important than the system performance coefficient(EER).For the vapor injection cycle,strategy 1 shows better performance in terms of the cooling capacity and EER compared to strategy 2.At a compressor speed of 5000 rpm,the cooling capacity and EER of the system with strategy 1 are increased by 20.1% and 20.3% respectively.According to the climatic conditions of typical cities in China,the operation of heat pump air conditioning system for electric vehicles in wide temperature range is evaluated.From the load demand of the passenger compartment,the operating temperature range of the quasi-two stage compression system is-20 ～ 48 ℃,while that of the non-injection single stage compression system is only-10～ 37 ℃.The temperature range of the conventional three heat exchanger heat pump system is basically the same as that of the non-injection single stage compression system(the injection branch is closed).In the Mohe region,the quasi-two stage compression system runs 57 days longer than three heat exchanger system for heating.In Turpan,the quasi-two stage compression system runs 45 and 20 days longer than three heat exchanger system for heating and cooling respectively.In this paper,the quasi-two stage compression technology is used to study the wide temperature range heat pump air conditioning system and its operation strategy mechanism from both theoretical and experimental aspects.Finally,according to the climate characteristics of China,the wide temperature range operation of the heat pump air conditioning system for electric vehicles is evaluated.The above research work has theoretical significance and application value,and provides technical support for promoting the wide temperature range operation of electric vehicles. |
