Research On Optimization And Operational Characteristics Of Solar Absorption Refrigeration System Using Two Working Pairs

The use of solar thermal energy to drive the absorption cooling system has the advantages of cleanliness and friendly to environment.The solar-driven absorption refrigeration system can be used to ease the power shortage in the summer and shows high application value when it is associated with buildings.Considering the operation efficiency of LiBr absorption refrigeration system driven by solar energy is low and the work time is short during the daytime.This paper has proposed the solar absorption refrigeration system based on the absorption refrigeration cycle using two working pairs,which has been established in our previous research work.The optimization targets and the optimization results have been given based on the exergy analysis of the combined system.Combining with experimental research,the principle of high absorption rates exhibited by LiCl solution in falling film absorption has been analyzed by the coupled heat and mass transfer evaluation method.The operation characteristics of the system have been analyzed based on the simulation of the combined system.At the same time,the corresponding operation adjustment methods have been proposed.Firstly,the exergy destruction of each part of the cycle and its proportion to the total exergy destruction have been calculated and analyzed based on the second law of thermodynamics.The parts with large exergy destruction are the low-pressure absorber,the high-pressure generator,and the low-pressure solution heat exchanger.The three accounts for 71.93%of total exergy destruction.At the same time,the source of exergy loss has been pointed out based on the exergy destruction decomposition theory.The most potential part for further optimization is low-pressure solution heat exchanger since more than 80%of its exergy destruction can be avoided.Taking the values of COP and exergy efficiency of cycle reach the maximum at the same time as the optimization criterion.The optimal inlet heat source temperatures at different evaporation temperatures have been found out.Finally,the correlation of optimal inlet heat source temperature and evaporation temperature for the cycle have been provided.Furthermore,Taking the required collector area and efficiency of the combined system as the optimization indexes,the optimal collector area of the combined system has been correlated with the evaporation temperature.It can be used for providing the guidance for the initial investment and the operation of the combined system.The falling film absorber is functional implementation of absorption refrigeration cycle.Coupled heat and mass transfer evaluation method has been established to investigate the falling film absorption performance,considering that the traditional method based on log-mean-temperature and concentration difference is criticized for its lack of theoretical basis when the simultaneous heat and mass transfer processes occurs.The influence of solution properties on the falling film absorption rates have been analyzed based on the coupled heat and mass transfer evaluation method,and it is pointed out that the absorption solution with higher density value and lower the temperature partial derivative of solution enthalpy value is favorable to increase the absorption efficiency for the actual falling film absorber.The reasons why the absorption rates of LiCl solution is higher than that of LiBr solution are that the lower operating concentration of LiCl solution and a higher falling film mass transfer coefficient.The mixed solution has certain application advantages due to its flexible formulation and adjustable cost.In this paper,a mixed solution of LiCl and CaCl₂ with a mass ratio of 1:1 has been used as a replacement solution for LiCl solution.The performance of the mixed solution in the falling film absorption test platform is experimentally studied.Furthermore,a simulation model for the absorber and generator has been established applying the coupled heat and mass transfer evaluation method.Through the construction logic of small level to large level,a simulation model of the absorption refrigeration system using two working pairs has been formed.Finally,the simulation models of the flat plate solar collector and non-pressured water tank have been introduced to form a simulation model of the solar-driven absorption refrigeration system using two working pairs.To understand fully the operation characteristics of the solar absorption refrigeration system during the whole day,the control strategy and operational regulation of the combined system throughout the daytime have been reasonably set up.All the structural parameters required for the combined system have been determined.Taking the typical weather conditions in summer and transition season in Nanjing respectively as the input climate parameters of the combined system,the operational characteristics of the combined system have been analyzed.Considering that the high risk of solution crystallization encountered by the combined system when solar radiation is strong.Two types of operational adjustment methods have been constructed on the basis of the analyses on the combined system operational characteristics.The operation simulation correspondingly at the two adjustment methods have been carried out to provide the necessary basis and optimization reference to the combined system operation regulation.