Hongtan Liu

A two-phase flow and transport model for the cathode of PEM fuel cells

A unified two-phase flow mixture model has been developed to describe the flow and transport in the cathode for PEM fuel cells. The boundary condition at the gas diffuser/catalyst layer interface couples the flow, transport, electrical potential and current density in the anode, cathode catalyst layer and membrane. Fuel cell performance predicted by this model is compared with experimental results and reasonable agreements are achieved. Typical two-phase flow distributions in the cathode gas diffuser and gas channel are presented. The main parameters influencing water transport across the membrane are also discussed. By studying the influences of water and thermal management on two-phase flow, it is found that two-phase flow characteristics in the cathode depend on the current density, operating temperature, and cathode and anode humidification temperatures.

A parametric study of PEM fuel cell performances

The effects of different operating parameters on the performance of proton exchange membrane (PEM) fuel cell have been studied experimentally using pure hydrogen on the anode side and air on the cathode side. Experiments with different fuel cell operating temperatures, different cathode and anode humidification temperatures, different operating pressures, and various combinations of these parameters have been carried out. The experimental results are presented in the form of polarization curves, which show the effects of the various operating parameters on the performance of the PEM fuel cell. The possible mechanisms of the parameter effects and their interrelationships are discussed. In addition, a comprehensive three-dimensional fuel cell model is briefly presented and the modeling results are compared with our experimental data. The comparison shows good agreements between the modeling results and the experimental data.

Modelling of performance of PEM fuel cells with conventional and interdigitated flow fields

A simple mathematical model is developed to investigate the superiority of the interdigitated flow field design over the conventional one, especially in terms of maximum power density. Darcys equation for porous media and the standard diffusion equation with effective diffusivity are used in the gas diffuser, and a coupled boundary condition given by the Butler–Volmer equation is used at the catalyst layer interface. The performance of PEM fuel cells with a conventional flow field and an interdigitated flow field is studied with other appropriate boundary conditions. The theoretical results show that the limiting current density of a fuel cell with an interdigitated flow field is about three times the current density of a fuel cell with a conventional flow field. The results also demonstrate that the interdigitated flow field design can double the maximum power density of a PEM fuel cell. The modelling results compared well with experimental data in the literature.

An Analytical Solution of a Half‐Cell Model for PEM Fuel Cells

A mathematical model for polymer electrolyte membrane (PEM) fuel cells is developed and rigorous analytical solutions of the model are obtained. The modeling domain consists of the cathode gas channel, gas diffuser, catalyst layer, and the membrane. To account for the composite structure of the gas diffuser and for its gradient in liquid water content, the gas diffuser is modeled as a series of parallel layers with different porosity and tortuosity. Starting from the oxygen transport equations and Ohms law for proton migration, expressions for the oxygen mass fraction distribution in the gas channel, gas diffuser, and catalyst layer, and current density and membrane phase potential in the catalyst layer and membrane are derived. The solutions are presented in terms of the physical and thermodynamic parameters of the fuel cell. The polarization curve is expressed parametrically as a function of the surface overpotential. Expressions for cathode internal and overall effectiveness factors, active fraction of the catalyst layer, catalyst layer resistance, limiting current density, and the slope of the polarization curve are also presented. Due to the advantage of the closed-form solutions this model can he easily used as a diagnostic tool for a PEM fuel cell operating on H 2 and air.

Performance analysis of photovoltaic thermal air heaters

The performance of single-pass and double-pass combined photovoltaic thermal collectors are analyzed with steady-state models. The working fluid is air and the models are based on energy conservation at various nodes of the collector. Closed form solutions have been obtained for the differential equations of both the single-pass and double-pass collectors. Comparisons are made between the performances of the two types of combined photovoltaic thermal collectors. The results show that the new design, the double-pass photovoltaic thermal collector, has superior performance. Important parameters for both types of collector are identified, and their effects on the performances of the two types of collectors are presented in detail.

Performance of a double pass photovoltaic thermal solar collector suitable for solar drying systems

The photovoltaic thermal solar collector, sometimes known as the hybrid solar collector generates both thermal and electrical energies simultaneously. A double pass photovoltaic thermal solar collector suitable for solar drying applications has been developed and tested. A steady state closed form solution to determine the outlet and mean photovoltaic panel temperature has been obtained for the differential equations of the upper and lower channels of the collector. Hence, the photovoltaic, thermal and combined photovoltaic thermal efficiencies can be obtained. An experimental setup has been developed. For given sets of operating and design conditions the theoretical and experimental outlet and mean photovoltaic panel temperatures can be obtained. Comparisons are made between the experimental and theoretical results and close agreement between these two values are obtained.

A parametric study of the cathode catalyst layer of PEM fuel cells using a pseudo-homogeneous model

Abstract A pseudo-homogeneous model for the cathode catalyst layer performance in PEM fuel cells is derived from a basic mass–current balance by the control volume approach. The model considers kinetics of oxygen reduction at the catalyst/electrolyte interface, proton transport through the polymer electrolyte and oxygen diffusion through porous media. The governing equations, a two-point boundary problem, are solved using a relaxation method. The numerical results compare well with our experimental data. Using the model, influences of various parameters such as overpotential, proton conductivity, catalyst layer porosity, and catalyst surface area on the performance of catalyst layer are quantitatively studied. Based on these results, cathode catalyst layer design parameters can be optimized for specified working conditions.

Characteristics and applications of the cold heat exergy of liquefied natural gas

Abstract A mathematical model for predicting the low temperature exergy, pressure exergy and total cold heat exergy of Liquefied Natural Gas (LNG) is developed in this paper. In the model, the liquid mixture densities are calculated by a shape factor Corresponding State method, Vapor–Liquid-Equilibrium data of LNG are predicted by an improved method and the influences of real fluid effects are considered. The model is used to determine the various exergies, and the influences of ambient temperature, system pressure and mixture component concentrations on the cold heat exergies are analyzed. Different schemes for applying the low temperature exergy and pressure exergy are proposed. Based on the modeling results, it is proposed that the schemes for applying the cold heat exergies of LNG be determined by thermodynamic cycle optimization, while considering the magnitudes of low temperature exergy and pressure exergy, as well as the required gas supply pressures.

Two-phase flow pressure-drop type and thermal oscillations

Abstract This work focuses on the theoretical investigation of thermal pressure-drop type instabilities in forced convection boiling in a vertical single channel system, with Freon-11 as the working fluid. Experiments with two nichrome tubes of 7.5 mm inner diameter and 9.5 mm outer diameter, one bare and one coated with Linde High-Flux coating, have been carried out. One series of experiments was conducted with constant fluid inlet temperature and various heat inputs, and another with constant heat input and varying inlet liquid temperature. Under the experimental conditions of the study, pressure-drop type and thermal oscillations, as well as pressure-drop type oscillations with superimposed density-wave oscillations, have been observed. A numerical model has been developed to predict the steady-state characteristics of the forced convective two-phase flow and the pressure-drop type and thermal oscillations in a boiling single channel. The drift-flux model is used for numerical predictions. Good agreement between the theory and experiments is obtained.

Transport Phenomena Analysis in Proton Exchange Membrane Fuel Cells

The objective of this review is to provide a summary of modeling and experimental research efforts on transport phenomena in proton exchange membrane fuel cells (PEMFCs). Several representative PEMFC models and experimental studies in macro and micro PEMFCs are selected for discussion. No attempt is made to examine all the models or experimental studies, but rather the focus is to elucidate the macro-homogeneous modeling methodologies and representative experimental results. Since the transport phenomena are different in different regions of a fuel cell, fundamental phenomena in each region are first reviewed. This is followed by the presentation of various theoretical models on these transport processes in PEMFCs. Finally, experimental investigation on the cell performance of macro and micro PEMFC and DMFC is briefly presented.