The work presented in this article combines experimental analysis and theoretical studies of temperature effects on the performance of an open-cathode, self-humidified PEM fuel cell system for the design of optimization strategies. The experimental analysis shows the great potential of improving the system performance by proper temperature management. The most significant temperature dependent parameters of the system under study are the activation polarization and the water content of the ionomer of the catalyst layer. An Extremum seeking control algorithm is proposed to regulate the temperature to a voltage maximum. However, the slow dynamics of the temperature related catalyst-drying effect on performance complicate the optimal thermal management via model-free control strategies.
Proton Exchange Membrane Fuel Cells (PEMFC) are energy efficient and environmentally friendly alternatives to conventional energy conversion systems in many yet emerging applications. In order to enable prediction of their performance and durability, it is crucial to gain a deeper understanding of the relevant operation phenomena, e.g., electrochemistry, transport phenomena, thermodynamics as well as the mechanisms leading to the degradation of cell components. Achieving the goal of providing predictive tools to model PEMFC performance, durability and degradation is a challenging task requiring the development of detailed and realistic models reaching from the atomic/molecular scale over the meso scale of structures and materials up to components, stack and system level. In addition an appropriate way of coupling the different scales is required. This review provides a comprehensive overview of the state of the art in modeling of PEMFC, covering all relevant scales from atomistic up to system level as well as the coupling between these scales. Furthermore, it focuses on the modeling of PEMFC degradation mechanisms and on the coupling between performance and degradation models.
In this paper we present a new dynamic multiscale model of an open-cathode Polymer Electrolyte Membrane Fuel Cell (PEMFC). The model describes two-phase water transport, electrochemistry and thermal management within a framework that combines a Computational Fluid Dynamics (CFD) approach with a microstructurally-resolved model predicting the water filling dynamics of the electrode pores and the impact of these dynamics on the evolution of the electrochemically active surface area (ECSA). The model allows relating for the first time the cathode electrode structure to the cell voltage transient behavior during experimental changes in fuel cell temperature. The effect of evaporation rates, desorption rates and temperature changes on the performance of four different electrode pore size distributions are explored using steady-state and transient numerical simulations. The results are discussed with respect to water management and temperature control.
Abstract.The durability of polymer electrolyte membrane fuel cells (PEMFC) is governed by a nonlinear coupling between system demand, component behavior, and physicochemical degradation mechanisms, occurring on timescales from the sub-second to the thousand-hour. We present a simulation methodology for assessing performance and durability of a PEMFC under automotive driving cycles. The simulation framework consists of (a) a fuel cell car model converting velocity to cell power demand, (b) a 2D multiphysics cell model, (c) a flexible degradation library template that can accommodate physically-based component-wise degradation mechanisms, and (d) a time-upscaling methodology for extrapolating degradation during a representative load cycle to multiple cycles. The computational framework describes three different time scales, (1) sub-second timescale of electrochemistry, (2) minute-timescale of driving cycles, and (3) thousand-hour-timescale of cell ageing. We demonstrate an exemplary PEMFC durability analysis due to membrane degradation under a highly transient loading of the New European Driving Cycle (NEDC).
In this work we present a dynamic, control-oriented, concentrated parameter model of an open-cathode proton exchange membrane fuel cell system for the study of stability and efficiency improvement with respect to thermal management. The system model consists of two dynamic states which are the fuel cell temperature and the liquid water saturation in the cathode catalyst layer. The control action of the system is the inlet air velocity of the cathode air flow manifold, set by the cooling fan, and the system output is the stack voltage. From the model we derive the equilibrium points and eigenvalues within a set of operating conditions and subsequently discuss stability and the possibility of efficiency improvement. The model confirms the existence of a temperature-dependent maximum power in the moderate temperature region. The stability analysis shows that the maximum power line decomposes the phase plane in two parts, namely stable and unstable equilibrium points. The model is capable of predicting the temperature of a stable steady-state voltage maximum and the simulation results serve for the design of optimal thermal management strategies.Postprint (author's final draft
Abstract:The objective of this study is to describe the methodology used to isolate the main voltage loss indicators through a simple and effective treatment of a current interrupt and current sweep. The voltage loss indicators are activation polarization, mass transport, and ohmic losses. The indicators for these losses are the Tafel slope, mass transport resistance and ohmic resistance respectively. The use of this methodology to isolate the individual voltage losses works quite well. Even though there may be some inaccuracies, the trends can be clearly seen. This methodology can be used in any PEMFC system to monitor the state of health of the fuel cell. If this method were to be automated and implemented on a periodic basis, then an on-line measurement of the individual voltage losses could be monitored. The data analysis indicates that a major objective of the controller will be to minimize the mass transport losses by implementing proper temperature control.
Water concentration in proton exchange membrane (PEM) fuel cells strongly influences performance and durability which demands for fundamental understanding of water transport mechanisms. The system efficiency can be significantly improved with greater understanding of water flux dynamics through the membrane and its dependence on the internal conditions of the fuel cell. Therefore, a two-dimensional, non-isothermal, dynamic model of a 100 W open cathode, self-humidified PEM fuel cell system has been developed, that is capable of representing system specific control mechanisms for water and thermal management. The model consists of three coupled sub models based on energy, momentum and water mass balance of the system. The work is based on experimental observations of the investigated fuel cell stack, for which the crucial coefficients for water transport, namely the diffusion and the electroosmotic drag (EOD) coefficient have been determined. The diffusivity of water vapor through the MEA at 30• C was determined to be 3.3 × 10 −8 m 2 s −1 and increases by 3 × 10 −10 m 2 s −1 per • C up to 50• C stack temperature. The EOD coefficient was measured as 0.47 to 0.48 water molecules per proton at stack currents from 1 to 3 A. Validation of the steady state and the dynamic model by using experimental data, directly obtained from laboratory tests, has shown that the model predictions match the experimental data well.
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