Frost And Defrost Mechanism In Heat Pump Air Conditioning For Electrical Vehicle

Electric vehicle will be challenging technology in near future as its advantage of both environmental friendly and energy safety. Compare with fuel vehicle, electric vehicle without residual heat from gas engine, electrical vehicle’s maximum range is affected seriously by air condition energy consumption and will be prolonged by use of high efficient heat pump air condition system, but vehicle heat pump air conditioning usually frost during winter operation, thus couldn’t meet its warming requirements. Frosting problem become main restrictive factor of heat pump air conditioning.Adopt a method-theoretical and experimental analysis to explore mechanism of frost and defrost process in heat pump air conditioning system of electrical vehicle, using CCD camera to capture frosting and defrosting process image to analyze the effects of contact angle and wall surface’s temperature to frost, and the impact of ambient temperature, ambient humidity, air volume to heat exchanger’s frosting also be researched. A frost thickness image recognition system was put forward to defrost in heat pump air conditioning.In the solid-liquid phase of water the drive force of phase change was affect by super cooled degree. And in the solid-gas phase change it was affect by partial vapor pressure and vapor temperature. The micro-frosting experiment found that water drop will frost when steam super-cooling temperature is-7℃, which is considered as critical temperature of frost. Phase transition force of liquid to solid is 153J/mol. When surface temperature of frost is higher than 1℃, frost will rapidly melt. both the hydrophilic surface and the hydrophobic surface could restrain the frost growth, the time of start to frost on the hydrophobic surface was 10 s latter than the hydrophobic surface while the frost on the hydrophobic surface was sparser than hydrophilic surface.The result of heat pump air conditioning’s frost experiment shows that, the shape of frost in the surface of heat exchanger is anomaly “W”. when water drop’s temperature is between-6.8 and-7.2℃, steam partial pressure is between 331 Pa and 343 Pa, it will rapid frosting, The observed phenomenon conform to microscopic experiment. The development of frost can considered as evolve of-7℃ critical isotherm in the heat exchanger’ surface。Heat pump system efficiency will decline when heat exchanger’s surface frost, the experimental data show that in the normal heating mode, when power of fan is early constant, in the standard heating conditions, when outdoor fan power is constant, the outside heat exchanger’s airflow rate is reduced from 1500m3/h to 900m3/h, refrigerant flow rate is reduced from 0.67kg/min to 0.4kg/min, heating capacity from 2300 W to 1500 W, the system COP from 4.8 to 3.3.Outdoor air temperature, humidity and airflow rate of heat exchanger influence on the frost performance of the heat exchanger. When operating mode temperature is lower than 0℃,there is no frost formation on the surface of heat exchanger. When operating mode temperature is reduced from 9℃ to 3℃, evaporator surface have frost phenomenon, the smaller outdoor air temperature, the more serious of frost formation. The increase in relative humidity resulted in a significant increase of frost’ quality. When airflow rate of heat exchanger is reduced from 1500m3/h to 900m3/h, the rate of frost formation is faster.The result of heat pump air conditioning’s frost experiment show that, frost will rapidly melt when surface temperature is exceed 1℃。The defrosting process problem is the residual water can’t evaporate, the Secondary freezing will be a greater impact on the system. When airflow rate of heat exchanger fall to 600m3/h, The outlet temperature of the refrigerant is promoted to 31℃;when using block cover heat exchanger’s surface, the center surface temperature of heat exchanger bottom is increased from 8.4℃ to 35.2℃. both way can effective evaporate residual water,but need to pay attention to the area of surface, lest cause the compressor exhaust temperature is too high.A heat pump frost layer thickness of image recognition and transmission defrosting program was invented, Using computer program to simulation of continuous tone image after the sample quantization is converted into digital image, using mathematical statistical method to get quantity of frost on the surface of heat exchanger area, it can judge about the frost on the surface of heat exchanger efficiently and accurately. And it also can work with other performance parameters of the system as the basis of a heat exchanger defrosting starting points.