Design And Performance Investigation Of Bipolar Plate Channels For PEMFC Based On The Sieved Structure Of Corn Veins

With increasingly severe environmental problems,it has become an important global issue to reduce the consumption of fossil fuels and implement the efficient utilization of clean energy.Hydrogen energy is a clean secondary energy,which is of great significance for the utilization of renewable energy and solving energy and environmental problems.Proton exchange membrane fuel cell（PEMFC）is an electrochemical energy conversion device that can efficiently utilize hydrogen energy by directly converting chemical energy into electricity.The working process of PEMFC is different from the traditional heat engine and is not restricted by the Carnot cycle.So,it has the characteristics of high energy conversion efficiency,no pollution,low noise,etc.,and thus has been considered as an ideal way to utilize energy.Bipolar plate is an important part of PEMFC and its flow field directly affects the distribution of reactants in the cell and the mass transfer efficiency,which in turn affects the performance of the cell,especially at high current density.In order to obtain a better flow field,bionic approaches can be introduced into the design of the flow channel and flow field.In this thesis,a bionic flow channel based on the sieve plate structure of corn leaf veins has been designed for PEMFC and its performance has been investigated by numerical simulation to optimize the sieve plate structure and obtain the optimal bionic flow channel.The work in this thesis mainly includes followings.1.The structure of the sieve plates in the vascular bundles of the bionic prototype corn leaf has been studied.The structure and position of the sieve plates in the vascular bundles have been examined by confocal laser microscope,and the distance between the sieve plates and the diameter of the sieve holes have been measured,which provides a basis for the design of bionic sieve plate.2.The mathematical model and simulation assumptions of PEMFC have been outlined.A concise description of the PEMFC mathematical models is given,and the basic assumptions of the simulation are made.The mathematical models mainly include hydrodynamic equations,electrochemical equations and polarization control equations.3.The opening ratio,the number of sieve holes,and the arrangement of sieve holes have been studied and optimized by numerical simulation for the sieve plate of bionic flow channel.The introduction of the bionic sieve plate in the cathode flow channel can change the gas transport from diffusion to the mixed state of diffusion and convection,which can function to increase the concentration of reactants in the gas diffusion layer of the membrane electrode assembly（MEA）.It is found that the sieve plate with an opening ratio of 22.3%can achieve an optimal cell performance based on the comparison of polarization curves,power density curves and reactant distribution.Then the effect of the number of sieve holes on the cell performance has been investigated at the fixed opening ratio of 22.3%.It has been shown that the cell with 4 sieve holes delivers better performance than the cells with 1 sieve hole or 16 sieve holes,and its peak power density is 1.4%higher than that of the traditional straight channel without sieve plates.Next,the effect of sieve hole arrangement on the PEMFC performance has been studied to further optimize the sieve plate structure.The sieve hole arrangement rotated by 22.5°demonstrates better performance than the others,exhibiting a peak power density of 0.70 W/cm~2,which is 2.7%higher than that of the conventional straight channel without sieve plates.Therefore,the optimal parameters for the sieve plate structure is determined as the opening ratio of 22.3%,4 sieve holes and 22.5°rotation.4.The effect of the sieve plate number on the cell performance has been investigated.It is shown that the number of convection areas increases with the number of sieve plates,but too many sieve plates can cause a blocking effect,which weakens the effect of the middle and rear sieve plates.An optimal cell performance has been demonstrated for the cell with 5 sieve plates based on the comparison of polarization curves,power density curves,velocity field distribution,reactant distribution and net power output.The peak net power output for the cell with 5 sieve plates is as high as0.97 W,exhibiting an increase of 5.1%compared to the cell with the conventional straight channel without sieve plates.In summary,bionic flow channels have been designed for PEMFC bipolar plate in this work,via a bionic approach based on the structural characteristics of sieve plates in corn leaf veins.The effects of opening ratio,sieve hole number,sieve hole arrangement and sieve plate number on cell performance have been investigated by numerical simulation.This thesis can afford some insights for the design of PEMFC flow channel and flow field.