Study On Solar Absorption Refrigeration System And Its Heat And Mass Transfer Enhancement

With the successive warming of the Global climate,solar absorption refrigeration system gets more extensive research,because of its good seasonal matching,environmental friendly and energy saving.However,the solar energy absorption refrigeration system's COP value is relatively low,compared to the compression refrigeration system.Besides,because of the uneven distribution of solar radiation in time and space and the character of low energy density,the performance of solar absorption refrigeration system is easily influenced by weather factors.These factors largely restrict the application and popularization of solar energy absorption refrigeration system.So this paper investigates how to realize the solar absorption refrigeration system,driven by low grade solar water,to run efficiently.While driven by low grade solar hot water,the insufficient of refrigerant water evaporation capacity in generator is the main reason why the capacity solar absorption refrigeration system is low.In order to improve evaporation capacity of refrigerant water,i.e.,strengthen the phase change mass transfer process,the mass transfer driving force need to be increased and the mass transfer resistance need to be reduced.Refrigeration water's mass transfer driving force comes from the heat which the solar hot water transmits to lithium bromide solution in generator,and the resistance of mass transfer is determined by lithium bromide solution itself and generator's internal pressure.And the traditional passive strengthening method of heat and mass transfer isn't very effective to the lithium bromide solution driven by low grade heat.So this paper presents an active strengthening method of heat and mass transfer basing on ultrasonic vibration.This paper presented a model for the thermodynamic analysis of solar absorption refrigeration system,studied the heat and mass transfer characteristics and the operating characteristics of the solar absorption refrigeration system,analyzed the performance evaluation method with exergy and thermodynamics perfection,the above contents provides the theoretical basis and evaluation index to improve and the performance of the system.This paper studied the mechanism of mass transfer enhancement of the refrigerant water in lithium bromide solution,and verified the feasibility of ultrasonic enhancement.The ultrasonic transducer was placed at different heights from the surface,and the lithium bromide solution was heated to different temperature,then the influence of ultrasound on mass transfer was researched.The study shows when the heat source temperature varies from 65? to 80?,the enhancement ratio of mass transfer can reach 0.2 to 0.6.For the determinate power and frequency ultrasonic,transducer has an optimal distance so that the mass transfer enhancement rate of refrigerant water is the highest with ultrasound.Ultrasound can greatly reduce the mass transfer resistance at the gas-liquid interface of the lithium bromide solution,which effectively improves the mass transfer rate of refrigerant water in generator.This paper experimental studied the ultrasonic effect on heat transfer of copper tubes with different surface structures in lithium bromide solution pool boiling region(including sub-cooled boiling and saturated boiling),and investigated the mechanism of heat transfer enhancement by ultrasonic wave.The research shows that whether the effect of ultrasound on boiling heat transfer process is positive or negative is determined by the initial radius of bubble and the power and frequency of ultrasonic wave.The study shows the negative effect of the ultrasound on the heat transfer is limited for the proper ultrasonic frequency,and even can promote the heat transfer in generator.According to the thermodynamic analysis model of the absorption refrigeration system,this paper makes the simulation study of the solar absorption refrigeration system using MATLAB/SIMULINK numerical calculation and simulation software.And basing on the experiment data,this paper studied how ultrasound influenced the cooling capacity and cooling performance of absorption chiller.Finally,this paper designed a solar absorption refrigeration prototype basing on the experimental research and system simulation,and the experiment results showed that the prototype has a more significant improvement in cooling performance than the traditional solar absorption refrigeration system in low grade heat source.