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Simulation and optimisation of a high temperature polymer electrolyte membrane fuel cell stack for combined heat and power
Nomnqa, Myalelo Vuyisa
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High temperature polymer electrolyte membrane fuel cells (PEMFC) operating between 120-180 oC are currently of much research attention. The acid doped polybenzimidazole (PBI) membranes electrolyte are known for their tolerance to relatively high levels of carbon monoxide impurity in the feed. Most fuel cell modelling are theoretical in nature and are solved in commercial CFD platforms such as Fluent. The models require a lot of time to solve and are not simple enough to be used in complex systems such as CHP systems. This study therefore, focussed on developing a simple but yet accurate model of a high temperature PEMFC for a CHP system. A zero dimensional model for a single cell was developed and implemented in Engineering Equations Solver (EES) environment to express the cell voltage as a function of current density among others. Experimental results obtained from literature were used to validate and improve on the model. The validated models were employed for the simulation of the stack performance to investigate the effects of temperature, pressure, anode stoichiometry and the level of CO impurity in the synthesis gas, on the cell potential and overall performance. Good agreement was obtained from the simulation results and experimental data. The results showed that increasing temperature (up to 180oC) and acid doping level have positive effects on the cell performance. The results also show that the cell can operate with a reformate gas containing up to 2% CO without significant loss of cell voltage at elevated temperatures. The single cell model was extended to a 1 kWe high temperature PEMFC stack and micro-CHP system. The stacks model was validated with experimental data obtained from a test station. The model was used to investigate the performance of PEMFC and CHP system by using uncertainty propagation. The highest combined cogeneration system efficiency of 87.3% is obtained with the corresponding electrical and thermal efficiencies are 41.3% and 46 % respectively. The proposed fuel processing subsystem provides an adequate rate of CH4 conversion and acceptable CO-level, making it appropriate for integration with an HT PEMFC stack. In the steam methane reformer 97% of CH4 conversion is achieved and the water gas shift reactors achieve about 98% removal of CO.