Cooling Strategies for PEM FC Stacks (2010)

Fuel cell technology, even though not as advanced as predicted, is still one of the promising candidates for future efficient and sustainable energy conversion. Out of several types of fuel cells the proton exchange membrane fuel cell (PEMFC) is regarded as the future power source for transportation and portable electronic devices, because of its low operating temperature between 80-180ºC. PEMFC, like other types of fuel cells, converts the chemical energy of the hydrogen rich fuel into electrical energy, and as a by-product it generates water and heat. Part of this heat is a result of the thermodynamic reaction entropy, the bigger part is due to irreversible transport mechanisms. A removal of this low grade heat requires an efficient cooling system.--- This thesis provides a variety of cooling strategies for the efficient removal of heat, so that the temperature of the PEMFC stack remains under a tolerable limit and as much as possible spatially homogeneous. In first place it is important to know the magnitude and location of heat sources in PEMFC, then one can hunt for the cooling strategies. A systematic study of the heat sources in PEMFC is made in this thesis, and it is concluded that the irreversible sources of heat generation are amenable by using the toolbox of thermodynamics of irreversible processes. The reversible part of heat generation due to the entropy change taking place on behalf of the electrochemical reaction at the electrodes of the cell is uncertain. This brings a major limitation on the vast number of non-isothermal PEM fuel models developed in last two decades.--- A complete computational modeling of the commercial size PEMFC stack is still far away from reality, this necessitates to decrease the complexity of the model which can allow us to simulate a large size stack. In this regard a sophisticated computational fluid dynamics (CFD) model of one single fuel cell is developed as a first step, which is capable of calculating the non-uniform heat flux distribution at the electrodes based upon the concentration changes occurring at the electrode due to the electrochemical reaction. Local current density distribution is predicted for two possible reactant flow arrangements in a stack. This single cell model gives rise to a simplified model for the stack simulation as a second step.--- Larger PEMFC stacks are always cooled by liquid cooling. In this work different liquid cooling channel designs with single and multiple channels are analyzed numerically and the temperature distribution of the cooling plate is shown. The criterion for optimization of the design is made by calculating the total entropy generation. All different designs show stratification of temperature. Better uniformity of temperature can be achieved through offsetting and intelligent zoning of the temperature in the stack as proposed in the thesis. Two known benchmark problems are solved in this study to validate the calculation methods.--- A four cell stack is modeled using CFD and the temperature distribution is experimentally validated by a four cell stack which is designed and manufactured in-house. An important result of this experimental work is the existence of temperature variation within a small active area of 25 cm2. In the end a commercial size stack having 30 cells is simulated with cooling plates at a space of every cell, every third cell and every fifth cell. The resulting temperature distribution shows that the difference in maximum and minimum temperature of the stack decreases as the number of cooling plates increases. In a PEM-FC stack, just as in other types, the spatial temperature distribution is pivotal for its stability and for its lifetime expectation. The results of this thesis give important design criteria for an optimum thermal management of PEMFC.