Performance Study Of Air Source Heat Pump With Segmented Evaporator Under Special Working Condition

Air source heat pumps are highly efficient,environmentally friendly,easy to install,have both heating and cooling functions,and can operate uninterruptedly throughout the year.However,under some special conditions,there are still some problems in its actual use: 1)Conditions for preparing high-temperature hot water: due to the limitations of compression refrigeration technology in terms of refrigerant,condensing pressure and temperature,singlestage heat pumps in heating conditions its production of hot water is generally around 60℃,which is difficult to meet the demand for hot water above 65℃ in spa hotels,electroplating,metal processing,food,chemical and other industries.In addition,when the unit runs for a long time in summer,problems such as high condensing temperature of the system,large compression ratio,increased electricity consumption,decreased performance coefficient and unstable operation will occur;2)under wet and cold weather conditions in winter: the evaporator fins of the unit are prone to frost,and if the frost layer accumulates too much,it will lead to serious attenuation of the heating performance of the system.At present,most heat pump systems still use reverse cycle defrost to solve the problem of heat exchanger frost,but the unit will stop heating during defrost and affect the comfort of the heating room.In order to improve the problems of the unit under the above two special working conditions,an air source heat pump system with segmented evaporator is proposed in this topic,which divides the evaporator on the outside of the room into three segments arranged in parallel with pipelines for heat exchange.When preparing high-temperature hot water: the unit control terminal automatically switches the valve when the compressor discharge pressure rises to3.0MPa,converting one section to "condenser" bypass heat dissipation and the other two sections to evaporate and absorb heat normally,which can reduce the pressure of the unit and does not affect the continued heating of hot water;when defrosting is needed in wet and cold weather in winter: the unit control terminal sequentially switches the valve to control the rotation defrost of three sections of evaporator,and the remaining two sections will not be affected when any section is defrosted,and the unit can produce heat without interruption.The stability and comprehensive performance of the experimental prototype operating under two special operating conditions were investigated by combining theoretical analysis and experimental research.The main work includes the design of the air source heat pump system with segmented evaporator,the selection of the prototype equipment and the construction of the experimental bench;Theoretical analysis and calculation of thermal cycle of air source heat pump system and the establishment of defrost model;the comprehensive performance test of the experimental prototype under two special operating conditions and the analysis of the results.The main conclusions drawn from the above studies are:(1)Calculation results of defrosting model: 1)defrosting time increases as ambient temperature decreases and condensing temperature rises.Under different ambient or condensing temperatures,increasing the fan frequency will increase the defrosting time;2)Under different ambient or condensing temperatures,increasing the fan frequency will increase the average defrosting heating capacity,but there is a boundary effect on the effect of fan frequency.(2)Preparation of high-temperature hot water conditions: 1)When preparing hightemperature hot water at different temperatures,the normal heating mode can only prepare hot water of about 62℃ due to the limitation of condensing pressure and temperature.But after adopting the bypass cooling technology,the maximum water temperature of the unit can rise smoothly to the target setting value of 67℃,which is about 5℃ higher than the normal heating mode;2)The suitable opening degree of electronic expansion valve(EEV)on the bypass side can enable the unit to obtain more heat production while taking into account the stability,and the experimental test results show that the optimal opening degree of EEV on the bypass side of the prototype is 30%;3)Under the optimal bypass opening,when the ambient temperature is 25°C,the average heating capacity and average COP of the unit are 3.58 k W and 0.69,respectively,which are 19.7% of the normal heating mode.When the ambient temperature rises to 35℃,the average heating capacity and average COP of the unit rise to 7.46 k W and 1.36,respectively,reaching 34.2% of the normal heating mode.At the same time,the time taken for the water temperature to rise from 62°C to 67°C is also shortened from 850 s at 25°C to 735 s at35°C;the discharge pressure of the compressor is relatively stable with the increase of water temperature,and is always lower than the threshold of 3.0MPa.The exhaust temperature increased after switching the valve,but it was always within the safe value range.(3)Segmented defrosting conditions: 1)The working condition with ambient temperature tc=1℃,relative humidityφ=80%,condensing temperature to=40℃,and fan frequency of50 Hz and 30 Hz respectively during defrosting is the typical working condition.Under the typical working condition,the frosting time of the unit is 3662 s,the average heating capacity of frost-free operation is 10.6k W,the segmental defrosting time is 386 s,and the average heating capacity and COP during defrosting are 2.25 k W and 0.91 respectively;2)Raising the condensing temperature and lowering the ambient temperature will prolong the frosting time of the unit and reduce the overall heating capacity of the unit.The size of air relative humidity only affects the frosting time;the lower the relative humidity,the longer the frosting time;3)After the unit runs continuously for many cycles under different operating conditions,the frosting is shorter and shorter,and the heating capacity of the frosting-defrosting period will be decayed;4)The increase of air volume will increase the heat dissipation of the refrigerant in the defrosting section,thus prolonging the defrosting time.The average defrost heat production and COP tend to rise and then slowly decrease with the increase of fan frequency,and the maximum value is achieved at the fan frequency of 20 Hz.The results show that an appropriate fan frequency can improve the heating performance during defrosting,but it should not be too large.