Experimental And Theoretical Study On A Novel Hybrid Vapor Compression Refrigeration System Based On Liquid Desiccant Dehumidificationa And Evaporative Cooling

In the traditional vapor compression refrigeration system,a great deal of condensation heat which accounts for 120%-130%of the refrigerating capacity is directly dissipated to the environment in summer.This dissipated heat not only causes severe pollution in the surrounding areas,but also wastes energy.From thermodynamic perspectives,the energy saving methods of the vapor compression refrigeration system include further cooling of refrigerant leaving the condenser and effective utilization of condensation heat.However,current refrigerant subcooling methods have different drawbacks and novel methods remain to be proposed.In addition,to make use of condensation heat,higher condensering temperature?above 60??is required in general,which reduces system performance.To solve the above problems,in this dissertation,a novel hybrid vapor compression refrigeration system is proposed which consists of a vapor compression refrigeration cycle,a liquid desiccant dehumidification cycle and an indirect evaporative cooling cycle.The condensation heat from the condenser?40-60??is used to drive the liquid desiccant dehumidification cycle to produce dry air,and the cooling water with low temperature is obtained through the heat and mass transfer between dry air and spray water in the evaporative cooler,which is used to subcool the refrigerant leaving the condenser.Therefore,the subcooling degree of the proposed system is increased and correspondingly the system performance will be improved.The system performance and optimization strategies have been studied thoroughly based on the thermodynamic analysis and experimental study.What's more,molecular dynamics simulations have been made on the air-solution interface to study the microscopic properties of heat and mass transfer,which provides guidelines for enhancing heat and mass transfer and reducing the condensing temperature of solution regeneration.The detailed content and results are shown in the following.First of all,a mathematical model is built based on the proposed system,and effects of solution concentration,condensing temperature,ambient air temperature and relative humidity on the system performance are analyzed,and the system performance is compared with the traditional air-cooled chiller.In addition,comparative studies have been made between the hot air and cold air driven regeneration.Theoretical results show that the proposed system achieves much higher performance than the traditional air-cooled chiller.COPch and COP_sys can be improved by 20.6%and 10.5%,respectively.The proposed system is appropriate to work with higher condensing temperature,lower ambient air temperature or lower relative humidity.In addition,solution concentration should be chosen reasonably to achieve better system performance.In general,the regeneration with hot air is better for the system performance which can be maximumly improved by 3.7%,compared with the cold air driven regeneration.Second,three commonly used liquid desiccants?LiCl,LiBr and CaCl₂?are chosen as the working solution in the liquid desiccant dehumidification cycle,respectively.The optimum solution concentration is obtained with different working conditions,and the fitting formulas between the optimum solution concentration and condensing temperature are derived.Under standard working conditions,the optimum solution concentration for LiCl,LiBr and CaCl₂ is 0.31,0.45 and 0.42,respectively.For LiCl and LiBr solution,the correlation between the optimum solution concentration and condensing temperature is linear,while it is nonlinear for the CaCl₂ solution.In addition,to suitably assess the performance of the regenerator,the sensible heat regeneration efficiency is proposed and then analyzed for the three liquid desiccants with different solution concentration and condensing temperature.The sensible heat regeneration efficiency can be larger than 1,lower than 1 or equal to 1.Under standard working conditions,the maximum sensible heat regeneration efficiencies for LiCl,LiBr and CaCl₂ are around 1.0.Thirdly,to efficiently utilize the condensation heat,regeneration modes driven by condensation heat are studied for the liquid desiccant dehumidification cycle.Three different utilization modes?Mode 1,Mode 2 and Mode 3?are realized through allocating the condensation heat between the solution and ambient air(R_lat).In Mode 1,all the condensation heat is used to heat solution;in Mode 2,condensation latent heat is to heat air and sensible heat is for solution;in Mode 3,one part of condensation latent heat is used to heat air,and the remaining condensation heat is to heat solution.Effects of the condensation latent heat distribution ratio on the system performance are studied with different condensing temperature and solution concentration,and the Mode 1 system is further analyzed.When the condensing temperature is relatively low or solution concentration is higher,Mode 1 is better for the system performance.However,when the condensing temperature is relatively high or solution concentration is lower,there is no much difference among the three modes.In addition,supply of the condensation heat exceeds the demand of the regeneration heat,and redundant condensation heat should be used for other purposes to make full use of the condensation heat.Therefore,a heat pump driven liquid desiccant dehumidification system with two-stage condensation heat utilization is proposed.Fourthly,two different air flow patterns?close-pattern and open-pattern?are studied to achieve the optimum configurations of the indirect evaporative cooling cycle.Comparisons are made between the close-pattern and open-pattern with three different climate conditions:typical condition,humid condition,and dry condition.Effects of ambient air to solution mass flow ratio,mass transfer unit and fresh air ratio(R_amb)on the system performance are discussed.The optimum air flow patterns are obtained for different climate conditions.Under the typical condition,the open-pattern with R_amb=0.3 is better for the proposed system,and the suggested range of ambient air to solution mass flow ratio is between 0.5 and 2 in the regeneration process.Under the humid condition,the close-pattern is suggested and the optimum ambient air to solution mass flow ratio is 0.4.Under the dry condition,the open-pattern with R_amb=1 is proposed.In addition,experimental setup of the proposed system is built,and effects of key parameters?air mass flow rate in the dehumidifier and regenerator,solution mass flow rate in the dehumidifier and regenerator,and spray water mass flow rate in the evaporative cooler?on the system performance are experimentally studied.Comparisons are made between the proposed system and the traditional water-cooled chiller.The results show that the proposed system can achieve low temperature cooling water,whose temperature can be lower than the wet bulb temperature of the ambient air,to subcool the refrigerant.Compared with the traditional water-cooled chiller,the proposed system can achieve better system performance under relatively humid climate conditions.COP,h can be improved by 18.6%and COP_sys 8.2%.The air mass flow rate in the dehumidifier has a significant effect on the system performance,and the optimum value is around 0.25 kg/s with given working conditions,With the increase of air mass flow rate in the regenerator,system performance increases significantly at first and then slowly.With the increase of solution mass flow rate in the regenerator,system performance increases gradually at first and then remains stable.In general,larger spray water mass flow rate is better for the system performance.Finally,studies on microscopic properties of heat and mass transfer on air-solution interfaces have been made,which provides guidelines for enhancing heat and mass transfer.Molecular dynamics simulation model is built with different system temperatures?300 K,325 K and 350 K?.LiCl and LiBr are chosen as the working solution.Density profiles of ions are plotted and distribution preferences of ions on the air-solution interface are discussed.Water molecules prefer to stay in the bulk,while the ions are more likely to occur in the interface.What's more,in the air-solution interface,the chloride and bromide ions prefer to stay above the lithium ions.The air-solution interface is pretty thin.When the system temperatures are 300 K and 325 K,the thickness of the interface is approximately 6 A,while the system temperature is 350 K,it is about 9 A.In addition,vapor pressure above the solution surface is predicted with different system temperature and compared with the empirical data.When the system temperature is 300 K,simulation data matches well with the empirical data.With the increase of the system temperature,the deviations become larger,but the variation trends of the vapor pressure are same.