Ashley Fly

Model Based Investigation of Liquid Water Injection Strategies for Evaporatively Cooled PEM Fuel Cells

Evaporative cooling through liquid water injection directly into the fuel cell flow channels removes the requirement for external humidification and liquid cooling channels within the stack. However, the amount of liquid water injected must be accurately controlled, to prevent on one hand membrane drying due to lack of water vapor and on the other hand flooding due to excessive liquid water.In this paper a one-dimensional, non-isothermal model of an evaporatively cooled proton exchange membrane fuel cell (PEMFC) is produced. The model accounts for changes in relative humidity and temperature along the anode and cathode flow channels, water transfer through the membrane and liquid accumulation within the gas diffusion layers. The model was used to study liquid water injection strategies at both cell and localized level. The influence of current density, operating pressure and inlet humidity were investigated. Results show that provided high humidity is maintained throughout the cell, exhaust gas temperature increase from low to high current densities (0.4–1.4A/cm2) is less than 4.0°C, without the need for active temperature control. Furthermore both temperature regulation and good membrane hydration can be managed by uniform injection of liquid water throughout the cell to maintain a target cathode exhaust humidity.Copyright

Equivalent Stiffness Model of a PEM Fuel Cell Stack including hygrothermal effects and dimensional tolerances

Proton exchange membrane fuel cells (PEMFCs) require mechanical compression to ensure structural integrity, prevent leakage, and to minimize the electrical contact resistance. The mechanical properties and dimensions of the fuel cell vary during assembly due to manufacturing tolerances and during operation due to both temperature and humidity. Variation in stack compression affects the interfacial contact pressures between components and hence fuel cell performance. This paper presents a one-dimensional equivalent stiffness model of a PEMFC stack capable of predicting independent membrane and gasket contact pressures for an applied external load. The model accounts for nonlinear component compression behavior, thickness variation due to manufacturing tolerances, thermal expansion, membrane expansion due to water uptake, and stack dimensional change due to clamping mechanism stiffness. The equivalent stiffness model is compared to a three-dimensional (3D) finite element model, showing good agreement for multicell stacks. Results demonstrate that the correct specification of gasket thickness and stiffness is essential in ensuring a predictable membrane contact pressure, adequate sealing, and avoiding excessive stresses in the bi-polar plate (BPP). Increase in membrane contact pressure due to membrane water uptake is shown to be significantly greater than the increase due to component thermal expansion in the PEMFC operating range. The predicted increase in membrane contact pressure due to thermal and hydration effects is 18% for a stack containing fully hydrated Nafion® 117 membranes at 80 °C, 90% relative humidity (RH) using an eight bolt clamping design and a nominal 1.2 MPa assembly pressure.

Reliability modelling using Petri-Net simulation of polymer electrolyte membrane fuel cell degradation in automotive applications

Climate change concerns have increased in recent years, and technologies to reduce emissions from the transport industry have been put forward. Hydrogen fuel cells have the potential to mitigate emissions concerns as they only produce water vapor as an emission. However, their commercialization has been hindered due to reliability and lifetime concerns. They need to meet strict targets of 5000 hours of operation (equivalent to 150,000 miles), and currently struggle to do so. Reliability analysis of fuel cells can ascertain key information as to the reduction of current lifetimes due to degradation. Failure mode and effect analysis, and fault tree analysis was performed for a Polymer Electrolyte Membrane Fuel Cell (PEMFC) revealing interactions and relationships between failure modes which makes the use of FTA in this case unfeasible. Petri-Net simulation techniques have been pursued to alleviate these concerns and to develop an accurate degradation model of a PEMFC in an automotive context. The Petri-Net model is integrated into a PEMFC performance model for the purpose of incorporating key variables into each model. For example; membrane thickness is an input into the performance model, and this can be modified by the Petri-Net degradation model based upon failure modes such as radical attack of the membrane (which thins the membrane). Thus, outputs from the degradation relationships in the Petri-Net model directly feed into the running of the performance model. This work furthers the research in PEMFC reliability by providing an accurate degradation model of a PEMFC based upon Petri-Net simulation of interactions and relationships between failure modes. Integration of relationship concerns with a verified performance model of a PEMFC is invaluable to increase the accuracy of PEMFC degradation research. Future work will include PEMFC stack experimentation to fill holes in the literature regarding failure mode failure rates that are fed into the model.