Comprehensive investigation of membrane sorption and CFD modeling of a tube membrane humidifier with experimental validation

The humidification of PEM fuel cells is critical for their performance and efficiency and for ensuring a long durability. In most PEM fuel cell systems for mobile applications membrane humidifiers are used to humidify the fresh air. In this process, the water contained in the cathode exhaust gas is used to increase the humidity of the supply air. Despite the simple design of membrane humidifiers, the simulation of the water transfer is difficult and so far there exist hardly any precise models to calculate the absorption and desorption processes. Common approaches that use the Sherwood number to determine the sorption rates cannot account for the influence of the local water content of the membrane. This ultimately leads to an inaccurate simulation of humidifier behavior, as these models cannot consider the fact that desorption is nearly ten times faster than absorption.In this study, an empirical formula for an accurate determination of the sorption rate is derived based on experimental data. This function accounts for the different absorption and desorption rates by finding a sorption rate coefficient as a function of the local membrane water content, temperature, pressure and flow velocity.Furthermore, a CFD model is derived from the geometry of a commercially available membrane humidifier, which is also investigated on a test bench. Using the experimental data, the CFD model is validated and it is shown that the developed sorption rate formula leads to good agreements between simulations and experiments at steady-state operating points of the humidifier.     

Design methodology for membrane-based plate-and-frame fuel cell humidifiers

Currently, polymer electrolyte membrane fuel cells require some method of humidification to operate effectively. External gas-to-gas membrane-based humidifiers can provide an efficient method to recycle exhaust heat and product water from the fuel cell stack. This work describes a design methodology involving a series of design equations for plate-and-frame membrane humidifiers. Humidifiers of different flow channel geometries were created with a rapid prototyping technique. These humidifier units were tested at different operating conditions in an attempt to validate the design equations. The ratio between the residence time of gas in the humidifier over the diffusion time of water from the surface of the membrane into the channel can be used as a design parameter. This ratio was shown to offer a good starting point for humidifier design, and a target range between 2.0 and 4.0 was identified (with a nominal desired value of 3.0). A humidifier design procedure and suggestions are presented based on this parameter and the packaging requirements of the humidifier in a fuel cell system. This algorithm was validated by creating a further prototype humidifier.     

A parametric study of the performance of a planar membrane humidifier with a heat and mass exchanger model for design optimization

The performance of a proton exchange membrane fuel cell (PEMFC) is seriously changed by the humidification capability available when equipped with a PTFE® membrane. Typically, the humidification of a fuel cell is carried out by means of an internal or external humidifier. A membrane humidifier is applied to the external humidification of residential power generation fuel cell due to its convenience and high performance. In this study, a static model is constructed to understand the physical phenomena of the membrane humidifier in terms of geometric parameters and operating parameters. The model utilizes the concept of planar type heat exchanger with mass transport through the membrane. The model is constructed with FORTRAN in a Simulink® environment for consistency with other components of the model we previously developed. The results show that the humidity of the wet gas and the channel length, the membrane thickness and wet gas inlet humidity are critical parameters affecting the performance of the humidifier.     

燃料电池空气系统增湿器建模与仿真

建立气-气增湿器的数学理论模型,并基于Amesim软件建立燃料电池增湿器及空气系统仿真模型,从燃料电池系统层面分析干湿侧不同温度、压力、水含量等输入条件下的干侧出口空气的湿度变化情况,并采用水转移率（water vapor transfer rate,WVTR）对增湿器增湿性能进行评价,结果表明此模型可进行前期验证,能较好地预测汽车运行过程中增湿器的动态响应特性。     

Design of an air humidifier for a 5 kW proton exchange membrane fuel cell stack operated at elevated temperatures

Air humidification is a crucial issue for superior performance of proton exchange membrane fuel cell (PEM fuel cell) stacks. In this work, an air humidifier is proposed for a 5 kW PEM fuel cell stack working at elevated temperatures, e.g., 90–95 °C. The high temperature coolant exiting the stack is utilized to pre-heat the air in the heat exchanging tubes of the humidifier, and the heated air is humidified with deionized water supplied by a nozzle fixed in a top cavity. Both the tubes and the nozzle are properly designed to ensure sufficient heat transfer and superior atomization. Humidification performance is evaluated under different operation conditions. The nozzle is able to inject well-atomized water with uniform droplet diameter. With the variation of inlet air flow rate, the relative humidity (RH) of the outlet air increases at the beginning, then decreases gradually due to the attenuation of dew point (DP) temperature. However, the humidification performance can be improved when higher temperature deionized water is injected or high temperature coolant is supplied. At a coolant temperature of 95 °C, the outlet air DP temperature is maintained over 80 °C with 25 °C injection water. Moreover, better humidification performance is achieved when the injection water flow rate is controlled according to the working conditions of the stack.     

Two dimensional dynamic modeling of a shell-and-tube water-to-gas membrane humidifier for proton exchange membrane fuel cell

Water management is a crucial factor in determining the performance of proton exchange membrane fuel cell (PEMFC) for automotive application. The shell-and-tube water-to-gas membrane humidifier is useful for humidifying the PEMFC due to its good performance. Shell-and-tube water-to-gas membrane humidifiers have liquid water on one side of the tube wall and a dry gas on the other. In order to investigate humidifier performance, a two-dimensional dynamic model of a shell-and-tube water-to-gas membrane humidifier is developed. The model is discretized into three control volumes – shell, tube and membrane – in the cross-sectional direction to resolve the temperature and species concentration of the humidifier. For validation, the dew point temperature of the simulation result is compared with that of experimental data and shows good agreement with only a slight difference. The distribution of humidification characteristics can be captured using the discretization along the air-flow direction. The humidification performance of two different flow configurations, counter and parallel, are compared under various operating conditions and geometric parameters. Finally, the dynamic response of the humidifier at the step-change of various air flow rates is investigated. These results suggest that the model can be used to optimize the inlet flow humidity of a PEMFC.     

Maintaining desired level of relative humidity throughout a fuel cell with spatially variable heat removal rates

A concept of using the product water to internally humidify the air stream in a PEM fuel cell without external humidification is investigated by a simple, pseudo 2-D model along a single channel. This model takes into account the mass and energy balance, water and heat generation rates, heat removal, and water transport through the membrane. The model and thus the concept were confirmed experimentally using a 5-segment fuel cell. The temperature of each segment could be individually controlled, and the temperature and humidity of air could be measured between each segment. A temperature profile has been established, by applying spatially variable heat removal rates along the cathode channel, that results in relative humidity being close to 100% throughout the cell without any external humidification. The concept may be applied to a fuel cell stack resulting in simplification of the suporting system by avoiding external humidification.     

Effect of operating parameters on dynamic response of water-to-gas membrane humidifier for proton exchange membrane fuel cell vehicle

An analytic multi-dimensional dynamic model of a membrane type humidifier has been developed for the study of transient responses of the humidifier under proton exchange membrane fuel cell vehicle operating conditions. The dynamic responses of heat and mass transfer and fluid flow in a membrane humidifier are mathematically formulated and modeled with a newly developed pseudo-multi-dimensional concept. The model is used to analyze the performance of the humidifier under various operating conditions and the dynamic response of the humidifier under transient operating conditions. The simulation results show that, in the case of the water-to-gas type membrane humidifier modeled in this study, the time constant of water diffusion in the membrane is less than 1 s. Thus, the delay of the response of the humidifier induced by the vapor diffusion in the membrane is not significant in vehicle operation. However, it is also found that the dynamic behavior is mainly due to the thermal resistance and heat capacity of the membrane humidifier.     

Mass transfer characteristics of the hydrophilic membrane with the isolation of heat transfer effect by isothermal bath

The efficiency and lifetime of a proton exchange membrane fuel cell (PEMFC) system is critically affected by the humidity of incoming gas which should be maintained properly for normal operating conditions. But the experimental characteristics of the humidifier are rarely reported. Water transport through the hydrophilic membrane is a coupled phenomenon of heat and mass transport. In this study, a laboratory scale test bench is designed to investigate the characteristics of water transport through the hydrophilic membrane. The mass transfer capability of the hydrophilic membrane is evaluated over various flow rates, temperature, pressure, and flow arrangements. In the experiment, the test bench is submerged in a constant temperature bath in order to isolate the effect of temperature variation between dry air and humid air. The results show the water transport of the hydrophilic membrane is significantly affected by operating temperature and operating pressure. Additionally, the flow arrangement demonstrates a minor effect but it should be considered along with the heat transfer effect.     

An experimental study on vapor transport of a hollow fiber membrane module for humidification in proton exchange membrane fuel cells

Water management is key in the optimization of proton exchange membrane fuel cell performance and durability. Humidifiers can be used to provide water vapor to cathode air, ensuring the proper operation of proton exchange membrane fuel cells. In this study, water vapor transport characteristics of hollow fiber membrane modules were investigated in shell-tube humidifiers under isothermal conditions, using two different test jig constructions: a convection jig and a diffusion jig. The mass transfer rate of water vapor was evaluated via the impact of various operating parameters, including temperature, flow rate, pressure, and relative humidity of inlet wet air, flow arrangements, and surface area of the tube side. The result was presented by the water vapor transport rate from wet air flow to dry air flow across the hollow fiber membrane. It was found that humidification performance could be improved with higher operating temperature, flow rate, and relative humidity of inlet wet air, lower pressure, larger membrane surface area, higher convection effect, and substituting co-current with counter-current flow configuration.     

Analysis of membranes used in external membrane humidification of PEM fuel cells

This paper presents experimental water vapour transfer results and model analysis, using theory proposed within, of state of the art flat sheet membranes used in external membrane humidification of polymer electrolyte membrane (PEM) fuel cells. The membranes have been tested in an experimental set-up which simulates the dry inlet air and the highly humidified exhaust air of a fuel cell stack. Five effects have been identified in order to fully characterise the membranes for all possible conditions that may occur in an automotive PEM fuel cell stack. These effects are the relative humidity (RH) gradient effect, pressure gradient effect, temperature gradient effect, pressure effect, and temperature effect. Each effect has been tested with a varying channel Reynolds Number between approximately 80 and 2100, therefore remaining in the laminar range. The models in this paper use a lumped parameter version of Fick's law of diffusion with an extra dimensionless function X, which relates the partial vapour pressure of the bulk flow to the partial vapour pressure at membrane surface, as well as the percentage of membrane area which is used due to the channel geometry of the membrane test module. Using this extra dimensionless function X allows a membrane specific parameter P′ to be calculated using experimental data to compare each membrane. This membrane specific parameter P′ can be applied to membranes without knowledge of all membrane properties.     

Reactants flow behavior and water management for different current densities in PEMFC

Computational fluid dynamics analysis was carried out to investigate the reactants flow behavior and water management for proton exchange membrane fuel cell (PEMFC). A complete three-dimensional model was chosen for single straight channel geometry considering both anode and cathode humidification. Phase transformation was included in the model to predict the water vapor and liquid water distributions and the overall performance of the cell for different current densities. The simulated results showed that for fully humidified conditions hydrogen mole fraction increases along the anode channel with increasing current density, however, at higher current densities it decreases monotonically. Different anode and cathode humidified conditions results showed that the cell performance was sufficiently influenced by anode humidification. The reactants and water distribution and membrane conductivity in the cell depended on anode humidification and the related water management. The cathode channel–GDL (Gas Diffusion Layer) interface experiences higher temperature and reduces the liquid water formation at the cathode channel. Indeed, at higher current densities the water accumulated in the shoulder area and exposed higher local current density than the channel area. Higher anode with lower cathode humidified combination showed that the cell had best performance based on water and thermal management and caused higher velocity in the cathode channel. The model was validated through the available literature.     

膜加湿器热质交换过程的理论分析

膜加湿器是保证质子交换膜燃料电池(PEMFC)正常高效运行的重要组成部分.以燃料电池的板式膜加湿器为研究对象,根据热质交换原理对膜加湿器的传热传质过程进行了理论计算,分析了空气质量流量、膜内加湿侧进口温度和膜内加湿侧进口湿度对传热传质过程的影响.在传热方面:当空气质量流量不同时,随着膜内加湿侧进口温度的变化,膜内的热流量变化趋势不一致;当膜内加湿侧进口相对湿度为95%时,随着空气质量流量的变化,膜内热流量变化不大.在传质方面:当加湿侧进口相对湿度不变时,膜中水传输速率随着空气质量流量的增大而减小;当空气质量流量不变时,膜中水传输速率随着加湿侧进口相对湿度的增大而增大.     

Water and pressure effects on a single PEM fuel cell

A fuel cell is a promising energy conversion system that will eventually become the first-choice for producing power because of its clean or zero-emission nature. A steady-state, two-dimensional mathematical model with pressure and phase change effects for a single PEM fuel cell was developed to illustrate the inlet humidification and pressure effects on proton exchange membrane (PEM) fuel cell performance. This model considers the transport of species along the channel as well as water transfer through the membrane. It can be used to predict trends of the following parameters along the fuel cell channels: mole number of liquid water and water vapor, pressure, temperature, density, viscosity, velocity, saturation pressure, pressure drop, vapor mole fraction, volume flow rate, required pumping power and current density.     

Dynamic modeling and analysis of a shell-and-tube type gas-to-gas membrane humidifier for PEM fuel cell applications

A gas-to-gas humidifier using membranes is the preferred technology for external humidification of fuel cell reactant gases in mobile applications because no extra power supply is required and there are no moving parts. In particular, a shell and tube structure is compact, which allows its easier integration in a fuel cell vehicle.

This paper proposes a mathematical model for the humidifier using the principles of thermodynamics, including analysis of heat and mass transfer and of static and dynamic behaviors. Firstly, the heat and mass transfer behavior was simulated and the results compared with the experimental data. Secondly, the model was used to investigate the sensitivity of the geometric parameters and the effects of various operating conditions on performance. Finally, step responses of the humidifier at various flow rates were analyzed.     

An experimental study on the bubble humidification method of polymer electrolyte membrane fuel cells

A necessary requirement for polymer electrolyte membrane fuel cell (PEMFC) performance is providing sufficient water content in the membrane. The bubble humidifier is the simplest and inexpensive method for PEMFC humidification. In this study, a prototype of bubble humidifier is designed, fabricated, and tested. The effects of water temperature in the reservoir, water level inside the reservoir and inlet air flow on the humidifier performance are investigated. The results show that the outlet air relative humidity decreases (about 6% - 11%) with an increase in the inlet air flow rate from 1 m³ h^?1 to 3 m³ h^?1 at four different water temperatures. The increase in the water temperature and water level inside the reservoir lead to the better humidifier performance. At the water temperature of 20^°C, increasing water level from 5 cm to 7.5 cm has a significant effect on humidifier performance but increasing water level from 7.5 cm to 15 cm does not offer any advantage.     

Analysis of the water and thermal management in proton exchange membrane fuel cell systems

Water and thermal management is essential to the performance of proton exchange membrane (PEM) fuel cell system. The key components in water and thermal management system, namely the fuel cell stack, radiator, condenser and membrane humidifier are all modeled analytically in this paper. Combined with a steady-state, one-dimensional, isothermal fuel cell model, a simple channel-groove pressure drop model is included in the stack analysis. Two compact heat exchangers, radiator and condenser are sized and rated to maintain the heat and material balance. The influence of non-condensable gas is also considered in the calculation of the condenser. Based on the proposed methodology, the effects of two important operating parameters, namely the air stoichiometric ratio and the cathode outlet pressure, and three kinds of anode humidification, namely recycling humidification, membrane humidification and recycling combining membrane humidification are analyzed. The methodology in this article is helpful to the design of water and thermal management system in fuel cell systems.     

Study of a porous membrane humidification method in polymer electrolyte fuel cells

A gas humidification sub-system that does not add to the parasitic power loss is advantageous for water management in PEMFC. A membrane humidifier was fabricated with porous membrane and the performance of the single cell using this humidifier has been evaluated. The study shows that the performance of the humidifier is comparable to that of the bubble humidifier. It was further found that the humidifier is suitable for both water and exhaust cathode air as the humidifying medium.     

Effect of humidification on distribution and uniformity of reactants and water content in PEMFC

Restricted by experimental conditions, it is difficult to analyze reactants distribution and uniformity through experiment. A simulation model is established by experiment fuel cell. Hydrogen humidification has a great impact on hydrogen distribution and concentrates in inlet and serpentine sections. At anode side membrane water content increases significantly when hydrogen relative humidity is over 50%. Air humidification has little effect on air distribution and water content. The oxygen mass fraction only decreases with relative humidity increase. Hydrogen humidification has greater influence on the distribution of reactants and membrane water content than air humidification, but hydrogen humidification needs to control the relative humidity of hydrogen within a suitable range. According to the simulation results in this article, the relative humidity of hydrogen should be controlled at 25%–50%. This paper proposes mass fraction difference coefficient, as uniformity evaluation index. When hydrogen relative humidity is 50%, uniformity of reactants distribution is the best.     

Relative humidity control in polymer electrolyte membrane fuel cells without extra humidification

The performance of polymer electrolyte membrane fuel cells is highly influenced by the water content in the membrane. To prevent the membrane from drying, several researchers have proposed extra humidification on the input reactants. But in some applications, the extra size and weight of the humidifier should be avoided. In this research a control technique, which maintains the relative humidity on saturated conditions, is implemented by adjusting the air stoichiometry; the effects of drying of membrane and flooding of electrodes are considered, as well. For initial analysis, a mathematical model reveals the relationship among variables that can be difficult to monitor in a real machine. Also prediction can be tested optimizing time and resources. For instance, the effects of temperature and humidity can be analyzed separately. For experimental validation, tests in a fault tolerant fuel cell are conducted.