Research On Voltage Uniformity And Control Strategy Of Proton Exchange Membrane Fuel Cell Stack

With the increasing of environmental pollution and energy scarcity, fuel cell technology has been the focus of worldwide attention as a very promising new energy technology. Among them, the proton exchange membrane fuel cell (PEMFC) has been widely used in distributed power station, portable power source and new energy vehicles because of its high power density, low operating temperature, quickly start-stop, environmental protection, etc.In this article, the design and build of water-cooling PEMFC stack test platform, single cell voltage uniformity test and control strategy during the steady operation and dynamic loading, start-stop strategy taking into account the cell voltage uniformity, and test and analysis of application working conditions are studied. The main findings are as follows.(1) Based on the analysis of water-cooling PEMFC stack’s operation frame and principle of the test platform, the water-cooling PEMFC stack test platform was designed and assembled previously. This paper gives the structure scheme of the test platform, thus the gas supply system, humidification system, thermal management system, pressure regulating system, data acquisition, and the main function control unit of control system were given for selection. On the basis of the hardware platform, the data acquisition card and LabVIEW software platform were utilized for compiling the PC software program of test program, and the various operational control processes were given. The test platform is the basis for the subsequent experimental studies;(2) On the basis of water-cooling PEMFC test platform, by using the 9ssl V4 stack packaged by Ballard company, the actual testing and data analysis of PEMFC stack steady-state output voltage and cell voltage uniformity were made under the influence of the four operation conditions of the stack temperature, excess air coefficient, gas temperature reported rarely and the pressure deviation between anode and cathode. It provides the basis for the operating conditions setting during the dynamic loading of PEMFC in this article;(3) On the basis of the research on dynamic loading step response of a PEMFC stack, the controlling strategies were deeply studied. Simulating the loading strategy of the locomotive PEMFC stack, analyzing the specific reasons of single cell voltage uniformity changing during the loading process, and based on above, focusing on analyzing the influence of the loading frequency and loading amplitude for the single cell voltage uniformity, a PEMFC stack CFCA loading control strategy was proposed. It gives a better loading control mode of the voltage uniformity, which provides a reference for actual working conditions of the stack current loading control.(4) For the problem of air supply during the PEMFC stack loading process, the measured data was used to fit the relationship between stack current and excess air coefficient under the optimum conditions of stack voltage uniformity, and the excess air coefficient function λ=f (I) was used as a calculation basis for the air flow and the current loading rate during the loading process. Under the premise of maintaining the current loading speed, the policy can significantly improve the stack cell voltage uniformity performance.(5) In view of the slower response speed and lower control precision of traditional temperature control strategy during the control process, combining with the temperature control requirements during the dynamic loading process, an improved temperature control strategy was proposed, which can control circulation cooling water velocity by controlling the cooling water inlet pressure. It realizes the decoupling control of the circulating pump and radiator fan, and allows the stack temperature control to respond quickly with substantial large amplitude current loading, which maintains the temperature of the internal stack and avoids transient high temperature, thus ensures the stack performance and life.(6) According to the single cell oxygen starvation or the inhomogeneous distribution of the remnant gas during the constant auxiliary load start-stop process of PEMFC stack, the start and stop strategies counted the voltage uniformity are proposed respectively. The strategies can improve the cell voltage uniformity performance significantly by dynamically adjusting auxiliary load during the start-stop process of the PEMFC stack, and at the same time, avoid the oxygen starvation effectively during the startup process and shorten the time of the cell in a high potential. In addition, it keeps the cells of stop-state in a more balanced low voltage state, dose good to avoid uneven consumption of the remnant gas, and effectively prevent the generation of hydrogen-air interface.(7) Using of PEMFC locomotive stack loading data and test standards of fuel cell bus power system, under these two actual working conditions, the water-cooling PEMFC stack test platform contributed and relevant control strategy were tested comprehensively and verified. There is a critical point that the control effect of CFCA loading control strategy on stack cell voltage uniformity during the process of loading, and the performance response of improved temperature control strategy with the variation of dynamic loading.