Study On Frosting/defrosting Improvement Technology Of Micro-channel Heat Exchanger Based ASHP

Compared with traditional tube-fin heat exchangers,micro-channel heat exchangers have many advantages such as high heat transfer coefficient,low refrigerant charge,small volume,good corrosion resistance,etc,It has been widely used as a condenser in the field of air conditioning system.However,when it is used as an evaporator of an air source heat pump,it faces problems such as uneven refrigerant distribution,fast frosting speed and long defrosting time.Based on the above,this paper conducted an experimental study on the shortcomings of the low-temperature adaptability of the air source heat pump based on the micro-channel heat exchanger.The specific work and research results are as follows:An experimental platform based on an air source heat pump system equipped with a microchannel heat exchanger is designed and constructed.The parameters that reflect the characteristics and performance of the micro-channel heat exchanger air source heat pump system under various operating conditions are measured and analyzed.The research mainly includes the growth rule of the frost layer,the temperature distribution of the heat exchanger surface,the system power consumption and heat(cold)changes,as well as the pressure and temperature test of the refrigerant at the key points of the system.It is proposed to add a low-pressure reservoir in the inlet section of the microchannel heat exchanger(between the thermal expansion valve and the evaporator)to change the gas-liquid ratio of the refrigerant entering the microchannel heat exchanger,and carry out experiments on the performance and characteristics of the air source heat pump under frosting conditions.The results show that adding a low-pressure reservoir is beneficial to the uniform distribution of refrigerant in the micro-channel heat exchanger,the frost on the evaporator surface is more uniform,and the heat exchange performance of the heat exchanger is improved.27.2%,heating performance coefficient increased by about 34.8%.However,during the operation of the system,the suction and discharge pressure of the compressor fluctuated,and the stability of the system was affected to some extent.An air source heat pump system with double micro-channel heat exchangers that provides uninterrupted heat supply during defrosting is proposed.By increasing the area of the microchannel evaporator(dual microchannel evaporator)to increase the evaporation temperature of the microchannel heat exchanger,the suction and discharge pressure,suction and discharge temperature,and evaporation temperature of the compressor under different environmental conditions were studied.The research results show that: under the conditions of relative humidity of 85% and the dry bulb temperature of 0℃,3℃,and 5℃,the average evaporation temperature of the heat pump unit equipped with dual microchannel heat exchangers increases by about 3℃.In addition,the average heating capacity increased by about 24.5%,26.4%,and 30.1%,and the COP increased by about 19.6%,19.5%,and 21.9%.Increasing the area of the evaporator(increasing the evaporation temperature)can suppress and slow the frost on the surface of the microchannel evaporator,and can also reduce the frost temperature range.Moreover,the problem of reduced heating performance caused by the growth of the frost layer of the micro-channel heat exchanger is effectively improved.The defrosting and cooling conditions of the air source heat pump system with uninterrupted heating during defrosting were studied in detail.The results of the study show that: compared with traditional single microchannel heat exchanger air source heat pump system,the energy efficiency ratio of the system is improved in winter heating and summer cooling conditions.Among them,the COP of the uninterrupted heating during defrosting is 1.1,and the COP of the unit under cooling is 3.9.More importantly,the average heating capacity in the defrosting process is about 35.8% of the average heating capacity of normal heating,which shows that the heat pump unit can achieve the purpose of still providing heat to the room during the defrosting process.The research analyzes the feasibility and economy of the air source heat pump system with uninterrupted heat supply during defrosting in typical cities in China.The research results show that by increasing the evaporator area by two times in Shanghai and Chengdu,the frosting time is reduced by about 54.8% and 71.7%,and the additional initial investment can recover the cost in the third year.In summary,the research work in this paper provides technical reserves for the application of microchannel heat exchangers in air-source heat pump systems,and provides an important theoretical basis for the application and promotion of air-source heat pumps in China’s hot summer and cold winter regions,severe cold regions,cold regions.It also provides important technical support for the energy conservation of construction equipment in China.