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.     

Modeling and experimental validation of H2 gas bubble humidifier for a 50 kW stationary PEMFC system

Ensuring uniform membrane hydration in a PEMFC (Proton Exchange Membrane Fuel Cell) is important for its performance and durability. In this study, a bubble humidifier for humidifying hydrogen in a 50 kW PEMFC pilot plant was designed, built, and modeled. Initial tests, carried out by humidifying air, show that a dew point temperature of higher than 59 °C is attained when operating the PEMFC plant at nominal power at 65 °C. The model simulation results show good agreement with experimental data and the model is used for studying humidifier performance at other conditions. Steady state simulation results suggest that by increasing the heating water flow rate, the humidifier outlet dew point temperature can be increased by several degrees because of improved heat transfer. Finally, dynamic simulation results suggest that the humidity of the hydrogen can be controlled by manipulating the heat supply to the humidifier.     

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.     

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。     

Analyze the effects of flow mode and humidity on PEMFC performance by equivalent membrane conductivity

A three‐dimensional and two‐phase numerical model is developed for a 25‐cm² proton exchange membrane fuel cell (PEMFC) to investigate the effects of flow mode (coflow and counterflow) and relative humidity (anode 0%/100%; cathode 60%/100%) on the cell performance. Experimental studies are performed to validate this developed model. An equivalent membrane conductivity is proposed to describe the match level between current flux and membrane conductivity. It is found that the cell performance is enhanced under low relative humidity conditions because of the optimized equivalent membrane conductivity. More specifically, the voltage is improved from 0.611 to 0.637 V, and the equivalent membrane conductivity is enhanced from 10.35 to 11.11 S m^?1 by replacing the coflow mode with counterflow mode at 1000 mA cm^?2 when anode gas is dry and cathode gas is 100% hydrated. Both the anode and cathode relative humidities show an obvious influence on the PEMFC performance, and a suitable inlet humidity could ensure adequate hydration of membrane and avoid water flooding in gas diffusion layers simultaneously.     

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.     

Numerical study of PEMFC heat and mass transfer characteristics based on roughness interface thermal resistance model

The thermal contact resistance (TCR) is the main component of proton exchange membrane fuel cell (PEMFC) thermal resistance due to the existence of surface roughness between the components of PEMFC, and the influence of TCR is often ignored in traditional three dimensional PEMFC simulations. In this paper, the heat and mass transfer characteristics including polarization curve, power density curve, temperature distribution, membrane water content distribution, membrane current density are studied under different component surface roughness conditions, and finally the effect of each TCR on the PEMFC performance is studied. It is found that under the same operating conditions, the TCR makes the radial heat transfer of the PEMFC decrease, and the temperature of the membrane electrode and the temperature difference of each component of the PEMFC is higher than that of the model without TCR. When the surface roughness of components in the PEMFC equals 1 μm, 2 μm, 3 μm, the cell current density decreases by 6.56%, 12.46% and 17.17% respectively when the output cell voltage equals 0.3 V, and the cell power density decreases by 3.64%, 7.54%, 13.14% respectively when the cell current density equals 1.2 A·cm^?2. When the TCR between the CL and PEM equals 0.003 K·m²·W^?1, 0.005 K·m²·W^?1, 0.01 K·m²·W^?1, the cell current density is increased by 2.30%, 3.65%, 6.74% respectively under the condition that the output cell voltage equals 0.3 V, and the cell power density is increased by 1.24%, 1.85%, 3.10% respectively when the cell current density equals 1.2 A·cm^?2. The results show that the numerical simulation of PEMFC cannot ignore the effect of TCR.     

Thermodynamic performance experiment and cooling number calculation of a counter-flow spray humidifier in the HAT cycle

An experimental investigation of the thermodynamic performance of a counter-flow spray humidifier was conducted on the basis of theoretical analysis of the heat and mass transfer mechanism inside the humidifier. Critical parameters such as the temperature and relative humidity of air and the temperature of water at the inlet and outlet were measured. The influence of every measured parameter on the thermal performance of the humidifier was obtained under different experimental conditions. The cooling number, whose variation was also obtained, was calculated according to the measured data. The experimental results show that both the temperature and the temperature increment of outlet humid air and the temperature of outlet water increase with an increase of the water-gas ratio, whereas the cooling number decreases. Under all experimental conditions, the outlet humid air reaches or is close to the saturation level. The lower cooling number is favorable for the system, but it has an optimal value for a certain humidifier. __________ Translated from Journal of Power Engineering, 2006, 40(9): 1263–1267 [译自: 动力工程]     

Predictions of moisture removal rate and dehumidification effectiveness for structured liquid desiccant air dehumidifier

In hot and humid climates such as in the Sultanate of Oman, the humidity puts extra load on the electric vapor-compression air conditioning (VAC) systems. Liquid and solid desiccants can reduce the moisture content of humid air and thus reduce the latent load imposed on the VAC systems. In the present work, the performance of air dehumidifiers using triethylene glygol (TEG) as desiccant was investigated. Three differently structured packing densities were used (77, 100 and 200 m²/m³). The performance of the dehumidifier was evaluated and expressed in terms of the moisture removal rate (m_cond) and the dehumidifier effectiveness (ε_y). The experimental work was undertaken to study the effects of several influencing design factors on this performance. The design factors covered included the air and TEG flow rates, air and TEG inlet temperatures, inlet air humidity and the inlet TEG concentration. The desiccant flow rate investigated was much less than that covered in previous studies and the range of the inlet temperatures of air and desiccant was significantly wider. The objective this study was to use the multiple regression method and the principal component analysis to obtain statistical prediction models for the water condensation rate and the dehumidification effectiveness in terms of these design factors. The results of both techniques agree with each other affirmed that the desiccant flow rate, desiccant inlet concentration and air inlet temperature are the most significant variables in predicting m_cond, whereas desiccant flow rate, air inlet temperature and packing density are the most significant variables in predicting ε_y.     

Phase change characteristics and their effect on the performance of hydrogen recirculation ejectors for PEMFC systems

The working fluid of the hydrogen recirculation ejector in proton exchange membrane fuel cell (PEMFC) systems is humid hydrogen containing water vapour. However, previous studies on the hydrogen recirculation ejector using computational fluid dynamics (CFD) were based on the single-phase flow model without considering the phase change of water vapour. In this study, the characteristics of the phase change and its effect on the ejector performance are analysed according to a two-phase CFD model. The model is established based on a non-equilibrium condensation phase change. The results show that the average deviation of the entrainment ratio predicted by a single-phase flow model is 25.8% compared with experiments involving a hydrogen recirculation ejector, which is higher than the 15.1% predicted by the two-phase flow model. It can be determined that droplet nucleation occurs at the junction of the primary and secondary flow, with the maximum nucleation rate reaching 4.0 × 10²⁰ m^?3s^?1 at a primary flow pressure of 5.0 bar. The higher temperature, lower velocity, and higher pressure of the gas phase can be found in the mixing region due to condensation, resulting in a lower entrainment performance. The nucleation rate, droplet number, and liquid mass fraction increase remarkably with an increasing primary flow pressure. This study provides a meaningful reference for understanding phase change characteristics and two-phase flow behaviour in hydrogen recirculation ejectors for PEMFC systems.     

Thermodynamic performance experiment and cooling number calculation of a counter-flow spray humidifier in the HAT cycle

An experimental investigation of the thermodynamic performance of a counter-flow spray humidifier was conducted on the basis of theoretical analysis of the heat and mass transfer mechanism inside the humidifier. Critical parameters such as the temperature and relative humidity of air and the temperature of water at the inlet and outlet were measured. The influence of every measured parameter on the thermal performance of the humidifier was obtained under different experimental conditions. The cooling number, whose variation was also obtained, was calculated according to the measured data. The experimental results show that both the temperature and the temperature increment of outlet humid air and the temperature of outlet water increase with an increase of the water-gas ratio, whereas the cooling number decreases. Under all experimental conditions, the outlet humid air reaches or is close to the saturation level. The lower cooling number is favorable for the system, but it has an optimal value for a certain humidifier.     

Performance of a gas engine heat pump (GEHP) using R410A for heating and cooling applications

A gas engine heat pump (GEHP) represents one of the most practicable systems which improve the overall energy utilization efficiency and reduce the operating cost for heating and cooling applications. The present work aimed at evaluating the performance of a GEHP for air-conditioning and hot water supply. In order to achieve this objective, a test facility was developed and experiments were performed over a wide range of engine speed (1200 rpm–1750 rpm), ambient air temperature (24.1 °C–34.8 °C), evaporator water flow rate (1.99 m³/h–3.6 m³/h) and evaporator water inlet temperature (12.2 °C–23 °C). Performance characteristics of the GEHP were characterized by water outlet temperatures, cooling capacity, heating capacity and primary energy ratio (PER). The results showed that the effect of evaporator water inlet temperature on the system performance is more significant than the effects of ambient air temperature and evaporator water flow rate. PER of the considered system at evaporator water inlet temperature of 23 °C is higher than that one at evaporator water inlet temperature of 12.2 °C by about 22%. PER of the system decreases by 16% when engine speed changes from 1200 rpm to 1750 rpm.     

泡沫陶瓷填料湿化器加湿性能实验研究

温化器是湿空气透平(HAT)循环的关键部件,其性能优劣对于循环的性能有着重要的影响.对采用新型SiC泡沫陶瓷填料的湿化器在加压条件下的湿化性能进行了实验研究,分析了水气比、进口水温、操作压力以及进口空气温度对湿化过程的影响,研究表明,提高水气比或进口水温会使进出口空气温差、含湿量差相应增加,湿化器节点温差增大.操作压力...     

Experimental investigation on the heat and water transfer enhancement in a membrane-based air-to-air humidifier at insulation condition

In the present study, a membrane-based air-to-air planar humidifier (MAPH) with baffle-blocked flow channels and a common MAPH are fabricated, tested and compared. These MAPHs are well thermal insulated from their surroundings. Polyoxymethylene (POM) plates with some unique properties such as large tensile and flexural strength, high chemical resistance and high stiffness are used to create channels at dry and humid sides of MAPHs. The obtained findings revealed that the higher heat and water transfer rates and smaller dew point approach temperature (DPAT) in entire tested flow rates occurs in baffle-blocked MAPH. To evaluate the MAPH performance with considering the pressure drop, a dimensionless parameter, performance evaluation criteria (PEC), is introduced. At flow rates less than 1 m³/h, PEC is less than 1, indicating a decline in MAPH performance with considering the pressure drop. In baffle-blocked MAPH using water trap in the inlet of dry side leads to the performance deterioration. Additionally, the increased relative humidity (RH) of humid side inlet causes an increase in DPAT, consequently, the performance deterioration.     

Study on voltage clamping and self-humidification effects of pem fuel cell system with dual recirculation based on orthogonal test method

Durability and reliability are still major challenges of vehicular polymer electrolyte membrane fuel cell (PEMFC) systems. With exhaust gas recirculation on both the anode and cathode sides, two important functions can be achieved: the voltage clamping in low current density, and the self-humidification without any external humidifiers. The former restrains catalyst decay in small load working conditions, and the latter is beneficial for improving the cold-start ability. In this study, dynamic performances and stable characteristics of a fuel cell system with dual exhaust gas recirculation are firstly experimentally studied using an orthogonal test method. System parameters, including humidification temperature of cathode external humidifier, fresh air stoichiometric ratio (SR), current density, cathode and anode recirculation pump speeds, are regarded as key factors in the experiments based on the testing conditions of the test-bench. Two four-factor and three-level orthogonal tables are designed, and the effects of key factors on system performance indices (average cell voltage, relative humidity (RH) at cathode inlet, high frequency resistance (HFR), oxygen concentrations at cathode inlet and outlet, and the concentration difference between these two positions) are investigated. Results show that: (1) with the cathode recirculation, the cell voltage can be reduced in low current densities by coordinately adjusting the recycled gas flow and reducing fresh air SR; (2) with the dual recirculation, the fuel cell membrane can be well hydrated, and system performance only shows 3% reduction compared with a system with an external humidifier; (3) the difference between the oxygen molar concentration at the inlet and outlet of cathode gas channels becomes small using dual recirculation.     

Parallel serpentine-baffle flow field design for water management in a proton exchange membrane fuel cell

In a proton exchange membrane fuel cell (PEMFC) water management is one of the critical issues to be addressed. Although the membrane requires humidification for high proton conductivity, water in excess decreases the cell performance by flooding. In this paper an improved strategy for water management in a fuel cell operating with low water content is proposed using a parallel serpentine-baffle flow field plate (PSBFFP) design compared to the parallel serpentine flow field plate (PSFFP). The water management in a fuel cell is closely connected to the temperature control in the fuel cell and gases humidifier. The PSBFFP and the PSFFP were evaluated comparatively under three different humidity conditions and their influence on the PEMFC prototype performance was monitored by determining the current density–voltage and current density–power curves. Under low humidification conditions the PEMFC prototype presented better performance when fitted with the PSBFFP since it retains water in the flow field channels.     

外界供给参数对PEMFC输出性能及水热平衡的影响

为了改善质子交换膜燃料电池(PEMFC)内部的水热平衡,从而进一步改善PEMFC的输出性能,文章建立了PEMFC的三维模型,通过改变PEMFC的外界供给参数(进气速度、加湿率以及冷却水流速),应用COMSOL模拟仿真得到了PEMFC的极化曲线和功率曲线、流道和气体扩散层(GDL)的水浓度分布情况,以及冷却水流速对PEMFC温度的影响。研究结果表明:随着进气速度和加湿率的逐渐增加,PEMFC的输出性能均逐渐提升,但是,过高的加湿率可能导致电极水淹;随着冷却水流速的增加,PEMFC温度加速下降,膜内温度分布变得更均匀。     

Experimental investigation of an adsorptive thermal energy storage

A zeolite‐water adsorption module, which has been originally constructed for an adsorption heat pump, has been experimentally investigated as an adsorptive thermal energy storage unit. The adsorber/desorber heat exchanger contains 13.2 kg of zeolite 13X and is connected to an evaporator/condenser heat exchanger via a butterfly valve. The flow rate of the heat transfer fluid in the adsorber/desorber unit has been changed between 0.5 and 2.0 l min^?1, the inlet temperature to the evaporator between 10 and 40°C. It turned out that the higher the flow rate inside the adsorber/desorber unit the faster and more effective is the discharge of heat. However, at lower flow rates higher discharge temperatures are obtained. Storage capacities of 2.7 and 3.1 kWh have been measured at the evaporator inlet temperatures of 10 and 40°C, respectively, corresponding to thermal energy storage densities of 80 and 92 kWh m^?3 based on the volume of the adsorber unit. The measured maximum power density increases from 144 to 165 kWh m^?3 as the flow rate in the adsorber increases from 0.5 to 2 l min^?1. An internal insulation in form of a radiation shield around the adsorber heat exchanger is recommended to reduce the thermal losses of the adsorptive storage. Copyright © 2006 John Wiley & Sons, Ltd.     

Energy consumption during Refractance Window® evaporation of selected berry juices

The Refractance Window^® evaporator represents a novel concept in the design of evaporation systems for small food processing plants. In this system thermal energy from circulating hot water is transmitted through a plastic sheet to evaporate water from a liquid product flowing concurrently on the top surface of the plastic. The objectives of this study were to investigate the heat transfer characteristics of this evaporator, determine its energy consumption, and capacity at different tilt angles and product flow rates. The system performance was evaluated with tap water, raspberry juice, and blueberry juice and puree as feed. With a direct steam injection heating method, the steam economy ranged from 0.64 to 0.84, while the overall heat transfer coefficient (U) was 666 W m^?2 °C^?1. Under this condition, the highest evaporation capacity was 27.1 kg h^?1 m^?2 for blueberry juice and 31.8 kg h^?1 m^?2 for blueberry puree. The energy consumption was 2492–2719 kJ kg^?1 of water evaporated. Installation of a shell and tube heat exchanger with better temperature control minimized incidences of boiling and frequent discharge of condensate. The steam economy, highest evaporation rate and overall heat transfer coefficient increased to 0.99, 36.0 kg h^?1 m^?2 and 733 W m^?2 °C^?1, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

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.