Performance Analysis And Optimization Of Liquid Desiccant Dehumidification System For Frost Prevention In Cold Storage

In 2017,the total capacity of China’s cold storage exceeded 123 million cubic meters,but the energy consumption level of China’s cold storage is much higher than that of developed countries.Because of the low temperature and high humidity environment in cold storage,frosting phenomenon exists widely in all kinds of cold storage and increased the energy consumption of cold storage.If the frost phenomenon in the cold storage is restrained,the energy consumption of the cold storage will undoubtedly be reduced and the contribution to energy saving and emission reduction will be made.The traditional defrosting methods mostly defrost after the formation of frost layer,time lag,the adverse effects of frost have been produced.On the other hand,the traditional defrosting method not only needs the cold storage refrigeration unit shutdown,but also may cause the temperature fluctuation.Some studies have shown that the most important external factor affecting frost growth is humidity ratio.Therefore,reducing the humidity ratio of air flowing through the cold surface becomes a new idea of defrosting.In recent years,heat pump driven liquid desiccant system has attracted much attention in humidity treatment area due to its efficient dehumidification ability,flexible structure and excellent operation stability.In this paper,a solution dehumidification and frost suppression system（LD-FP）for cold storage driven by heat pump is proposed,and the following work is done:Firstly,the dehumidification process（hereinafter referred to as"low temperature dehumidification"）in cold storage is experimentally investigated by using the existing solution dehumidification test bench.It is found that the heat and mass transfer between gas and liquid in the process of low temperature dehumidification is similar to that in the process of conventional air conditioning dehumidification（hereinafter referred to as"normal temperature dehumidification"）.The dimensionless heat and mass transfer correlations of low temperature dehumidification process are fitted by experimental data,and the applicability and accuracy of NTU-Le model and the fitted correlations under low temperature dehumidification conditions are verified.The different characteristics of the processing loads in the process of low temperature dehumidification and normal temperature dehumidification are compared and analyzed.It is pointed out that the LD-FP system may have the situation that condensation heat is insufficient for solution regeneration.The reason for this phenomenon is that the sensible heat ratio of the load treated by low-temperature dehumidification is quite different from that of normal-temperature dehumidification and the condensation waste heat from refrigeration in cold storage is assisted to regenerate solution.Secondly,the simulation program of LD-FP system is compiled using MATLAB/Simulink software platform for system performance analysis.Through simulation analysis,it is found that the sensible heat ratio of the dehumidified air load may be greater than 0,equal to 0 or less in the process of low temperature dehumidification,and the range of change of the sensible heat ratio of the dehumidified air load is much wider than that of normal temperature dehumidification（sensible heat ratio is greater than 0）;the condensation heat of LD-FP system itself is generally insufficient for solution regeneration.It is pointed out that the heat matching(I_m,C)of the condenser/regenerator in the system is not only affected by the total heat load on the dehumidification side,but also related to the heat release from the regeneration side.Next,it is demonstrated that there may be insufficient condensation heat in the heat pump driven solution dehumidification system,whether at normal temperature or at low temperature condition.This phenomenon is a defect caused by the structure of the heat pump solution dehumidification system itself,but it is easier to observe this phenomenon when the system is applied to the low temperature dehumidification with a wide range of load sensible heat ratio.Quantitative analysis is made of the effects of the internal and external environment parameters,the efficiency of the intermediate heat exchanger and the self-circulation ratio on the heat matching(I_m,C),the need for supplementary condensation heat and the system performance,and the reasons behind them are explained.The LD-FP system can reach the limit dew point of-10^oC,the corresponding operating concentration range of LiCl solution is 29%³4%under limit of the condensation waste heat grade（45⁵5^oC）.The system is suitable for dehumidification and frost suppression of fresh-keeping warehouse with storage temperature of-2^5oC.The maximum COP of the system can reach2.3,and the specific moisture extraction ratio（SMER）can reach 3.5 kg/（kg·h）.Compared with the traditional hot-gast defrosting system,the SMER can be increased by 45.95-94.12%when the LD-FP system runs in Nanjing monthly.Finally,the system is analyzed from the perspective of exergy.It is pointed out that when the system runs under typical operating conditions in summer,the exergy loss mainly comes from the heat pump subsystem.When the system operates under typical conditions in winter,the regenerator and auxiliary heater become two major sources of exergy loss due to the increase of the average heat transfer temperature difference in the regenerator and the supplement of condensation waste heat.The irreversibility of the regenerator was reduced by installing an air sensible heat recovery device on the regeneration side.The exergy loss and exergy efficiency of the system decreased by 25.5%and increased by 26.7%respectively in winter operation.