Theoretical Modeling Of Heat And Mass Transfer,and Performance Control Based On Electrolyte Membrane Dehumidification System

Precision manufacturing process instruments,instrument interior and the storage of valuables in the accuracy of air humidity requirements are relatively strict,and the dehumidification space is small.However,the traditional dehumidification technologies had the disadvantages of complex equipment,such as large thermal inertia,resulting in they were not suitable for high-precision dehumidification requirements in small space environment.The electrolyte membrane dehumidification technology is driven by low voltage DC field to realize the electrolysis and transfer of water molecules in the air.It has the advantages of compact structure,small volume,simple operation and can realize accurate,portable,energy-saving dehumidification,and it is a promising new dehumidification technology.The structure of the electrolyte membrane dehumidification dehumidification system mainly includes membrane electrode,titanium mesh conductive plate and air flow channel.The membrane electrode is composed of proton exchange membrane(PEM),catalyst layer(CL)and gas diffusion layer(DL).It is similar to other PEM electrochemical systems,but its internal heat and mass transfer and electrochemical mechanism are significantly different,and the performance control principle is also different,which need to further study.Furthermore,the current research on the new dehumidification technology was mainly focused on experiments,and the theoretical research was mostly black box or semi-empirical models,leading to large deviations in performance predictions and ambiguity about the factors that affect performance.The performance control and key material research and development could not be guided,which was not conducive to its commercialization and industrialization process.In view of this,the research work of this paper mainly includes the following aspects.(1)Considering the effect of electrochemical reaction,electro-osmosis and back diffusion,a two-dimensional steady-state heat and mass transfer theoretical model of multilayer electrode(GDL、CL、PEM)for the dehumidification module was established.The finite difference algorithm was used to solve the model and optimize the calculation of anode side over-potential and the heat and mass transfer coefficient by using the self-developed C++language program.The results showed that the calculation results of the model were consistent with the experimental trend,and the problem of excessive current prediction(4～5)times that was common in the previous model was solved.Especially when the Re_awas greater than 2 or the applied voltage was greater than 3 V,the prediction error of the new model was reduced by more than 50%.The multi-physical field distribution diagram inside the PEM element showed that the highest temperature occurred in the PEM and the potential dropped sharply at the CLA interface.(2)Based on the real structure of the electrolyte membrane dehumidification system(air flow channel,titanium mesh conductive plate,multilayer membrane electrode),a 3-D transient theoretical calculation model was established and numerically solved by COMSOL Multiphysics software.The dynamic start-up characteristics of the system and the uneven distributions of physical field inside the dehumidification component during start-up were revealed.The results showed that the parameters such as temperature,current and water vapor concentration changed rapidly in the first 200 s,and finally reached a stable state at about1200 s.When the air mass flow rate and voltage was doubled,the start-up time was reduced by about 50%and 67%,respectively.The temperature at the edge of the PEM was maximum,up to 357.5 K.The higher water vapor concentration and current density occurred at the corresponding position of the hole and the edge of the hole of the titanium mesh conductive plate,respectively.At the corner away from the anode air inlet,the air temperature was higher and the water vapor concentration was lower.Compared with the air flow channel,the titanium mesh conductive plate structure at the anode side had a greater influence on the dehumidification rate of the system and the multiple physical field distributions inside the dehumidification component.(3)A multi-site electrolyte membrane dehumidification test platform was developed to test the dehumidification performance of the system under multiple working conditions and the temperature and humidity distribution inside the components for the verification of the theoretical model.The results showed that the change of anode side working condition parameters had a greater impact on the dehumidification rate of the system.The relative humidity(RH)of the air at the inlet of the anode channel had the greatest influence on the current,followed by the air velocity and voltage,and the temperature was least.The temperature rise and concentration difference of the test site far away from the air inlet was larger,while the test site near the air inlet was opposite.In addition,due to oxygen reduction reaction(OER)/hydrogen evolution reaction(HER),a small amount of hydrogen was generated in the air flow channel on the cathode side,but the amount was small,it could be ignored in the development of theoretical calculations model.(4)Based on the theoretical model and genetic optimization algorithm,the key materials and structural design parameters of the membrane electrode for the dehumidification module were optimized.The results showed that the change of CLA material physical parameters had the greatest influence on the dehumidification rate,followed by PEM and DLA.When the CLA’s tortuosity coefficient was doubled,the system’s dehumidification rate increased by approximately 85%.The current decreased significantly with the increase of CLA and PEM thickness during the dehumidification.When the thickness of the PEM was doubled,the resistance increased by about 40%during the dehumidification.Results of genetic optimization algorithm showed that the optimal value of CLA thickness and tortuosity coefficient increased with the increase of air mass flow rate,while CLA porosity and pore size were opposite.The dehumidification rate of the system was greatly improved by the optimized mental conductive plate structure.Compared with the original structure,the“口”shape,X-shape metal conductive plate improved the dehumidification rate by about 35%and41.9%,respectively.The optimal metal conductive plate was“米”,which made the dehumidification rate increase by 57%.Furthermore,the temperature and moisture distributions inside the dehumidification component were more uniform,effectively preventing the occurrence of local dry points and hot spots.The uniformity of current density,energy efficiency and COP of the system were also greatly improved.In this paper,two dimensional steady state heat and mass transfer and three dimensional transient coupled heat,mass and electricity transfer theoretical calculation models for electrolyte membrane dehumidification system were established respectively,and the heat and mass transfer characteristics and electrochemical reaction mechanism inside the component were illustrated.The prediction accuracy was significantly better than previous black box or semi-empirical models.Through the study,the dynamic start-up characteristics of the dehumidification system were clarified,which provided a theoretical basis and direction for the design of the key materials and structure of the membrane electrode,and it is of great significance for promoting the commercialization and industrialization of the technology.