Research On System Model And Key Technologies Of Micro Reformed Methanol Fuel Cell

Energy is so crucial to sustainable economic and enviromental well-being that protecting the environment was,is and will be important to us.It is our responsibility that we should continue putting effort in discovering and researching for better type and higher efficiency of energy resources.Nowadays,proton exchange membrane fuel cell?PEMFC?fed with pure hydrogen is regarded as one of the most promising technologies which have the potential to replace the conventional less efficient and more polluting batteries or internal combustion engines.However,it has the drawbacks of cumbersome storage and transport for compressed high-pressure hydrogen.Therefore,alternative systems using liquid fuels are of great interest.One is the reformed methanol fuel cell?RMFC?which refers to a system mainly contains a PEMFC stack and an on-board hydrogen supplying device where a mixture of methanol and water is reformed via the chemical reaction of methanol steam reforming.When it comes to portable application,the miniaturization of RMFC is faced with the the following problems.First,the design methods of the micro RMFC system need to be improved because micro RMFC is a complex system and system design has an important impact on its working mode and energy conversion efficiency.Second,due to limitation of the structural design and processing methods,hydrogen production is pulled down by the poor performance of traditional methanol reformer.Third,system shows poor stability when driving different electronic loads.Therefore,in order to solve the above problems,a system model of micro RMFC,a high-performance micro methanol reformer,the design of high temperature PEMFC and the key technologies of micro RMFC prototype are studied in this PhD research.Firstly,a dynamic closed-loop system model for micro RMFC was enstabished in this work based on the foundamental theory of electrochemistry,chemical reaction engineering and thermodynamics.Simulating the working process of RMFC system,the mass transfer and energy conservation law were revealed and fuel comsumption rates with different output powers were obtained.The numerical results reveal that the energy conversion efficiency of the system increases first and then decreases when the the output power increases,which means maximum output power and maximum energy conversion efficiency can not be obtained simultaneously.The results of the simulation provide a strong theoretical support for the realization of the miniature RMFC system,and can guide the design of a RMFC prototype.Secondly,this research is focused on the performance of methanol steam reformer and a U-shaped micro reformer is presented in this study.By using a single copper matrix with both the reforming and combustion chamber,the performance of the reformer was improved effectively.Three-dimensional finite element modeling shows that the temperature in the U-shaped reformer is more uniformly distributed and the average temperature is higher than that in a tube reformer,resulting in higher reactivity and homogeneity for methanol steam reforming.Then,precision machining and vacuum brazing technology were used to fabricate the micro reformer.The experimental results show that the hydrogen production of the micro reformer can reach more than 1100 sccm with high stability in long time run,and meets the application requirements in RMFC system.Finally,a micro RMFC prototype was designed and implemented based on above works.The system consists of a 10-cell high temperature PEMFC stack,which was able to output 33.4 W at 180°C and exhibited good stability in long time discharging test with an output voltage degradation rate of 2.45×10^-3 V/h under the current density of 200mA/cm².A real-time operating system and a feed-forward PID algorithm were implemented in the control unit of the micro RMFC system for the workflow and fuel feeding rate control.Experimental results show that at least 21 min was needed for the start-up of the prototype and the time would gradually increase with the degradation of the combustion catalyst.Fuel feeding rate was self-adjusted in portable application,showing the miniature RMFC prototype has a good dynamic response.A maximum dynamic energy conversion efficiency of 36%was obtained when the output power was22.5W,an overall energy conversion efficiency of 30%can be guaranteed when single operation runtime is more than 2.5h,and the energy conversion efficiency of the RMFC system would decrease with the performance degradation of the fuel cells.The system model developed in this research was validated through the comparison between the theoretical and the practical studies.