Development Of Heat Pump System For Electrical Cars

Energy conservation and emission reduction as well as environment protection arouse general interest all over the world at present. Thus now is the golden age of development for electrical vehicles due to its unique advantages including low energy consumption and low emission. Air-conditioning and PTC heater are the most used ways for temperature control in electrical vehicles, but the enormous energy consumption causes shorted travel distance. On the other hand, heat pump system has huge advantage over PTC with high efficiency. It is crucial to develop heat pump system for electrical vehicles considering current situation. One of the main issues about the heat pump system is outside heat exchanger(HX) frosting. To address the problem and improve the heat capacity for the system, the following research was done:1) A heat pump system with three HXs was built. Experimental results showed that MCHX frosted fast in heating mode. Small diameter HX was presented to solve the problem. The frost and defrost process in tube-fin heat exchanger and micro-channel heat exchanger was compared in the automotive air-conditioning test bench. Tube-fin heat exchanger proved to have better performance on drainage, frost and defrost during repeatedly cycle of frost and defrost.2) The numerical model of capacity and flow resistance for heat exchanger had been established based on ε-NTU method. Four kinds of small diameter tube-fin heat exchangers were designed and produced using the model, so that the optimized small diameter heat exchanger could have similar capacity with micro-channel heat exchanger.3) Five kinds of heat exchanger were tested in the heat pump system in cooling and heating modes. According to the results, heat pump with small diameter tube-fin heat exchangers suited low speed condition in cities best, fitting the objective of electrical vehicles. Since heat pump with three heat exchangers was complicated and expensive, heat pump system with four-way-valve was designed to simplify the system.4) To simplify the system, a bi-direction thermostatic expansion valve was designed to replace the two thermostatic expansion valves and two by-pass valves in the original system. The valve could improve the cooling capacity by 13.5%-20.2% with the same heating capacity. Experiment on the effect of superheat on the capacity was carried out to explain the improvement afterwards. Experimental results of frost and defrost in the optimized heat pump proved the advantages of small diameter tube-fin heat exchangers again. With increased wind velocity, the period of frosting becomes longer. Further study showed the utilization of distributor created more even flow distribution and frosting was eliminated.5) The designed heat pump system was installed in an electrical vehicle and evaluated in cooling and heating mode. In cooling mode, temperature ins ide decreased quickly and the efficiency of the system could reach 2.64-4.88, which also meets the requirement from State 863 Project. In heating mode, more heat was needed with lower ambient temperature. When the ambient temperature reaches 6℃, the system could still provide wind at 28℃, covering the heating requirement from the car. COP of the system is more than 1. Compared with PTC heater, the heat pump system could save energy by 50%-80%.