空气抽吸式甲醇燃料电池传热与传质特性

空气抽吸式直接甲醇燃料电池不仅具有被动式燃料电池的优点,同时又便于将其串联成电堆提高输出电压。建立以阴极为管道抽吸式结构的直接甲醇燃料电池的三维、两相、非等温稳态数值模型,研究了质子交换膜性能、供给甲醇浓度以及电堆规模对电池性能及燃料利用率的影响。对于保温较好的大电堆,采用低甲醇穿透的改性质子交换膜能同时提升燃料利用率和比功率;此类电堆若采用穿透率低的改性膜,则2 mol/L的甲醇浓度就能保证电池在较大的电流密度区间内维持较高的功率与效率。作为影响电池运行温度的重要因素,电堆规模的大小将直接影响质子交换膜种类与甲醇浓度等关键参数的设计与选择。     

Analytical modeling of PEM fuel cell i–V curve

The performance of a fuel cell is characterized by its i–V curve. In this study, the performance of a bench scale fuel cell stack, run on hydrogen/air, is measured experimentally for different air flow rates and temperatures. The experimental data, obtained from the 40-W proton exchange membrane fuel cell (PEMFC), are used in estimating the parameters of a completely analytical model that describes the i–V curve. The analytical model consists of the three fundamental losses experienced by a fuel cell, namely: activation, ohmic, and concentration losses. The current loss is also considered in the model. While the Tafel constants, ohmic resistance, and the concentration loss constant are estimated through regression, the limiting current density and the current loss are obtained through measurements. The effect of temperature on the fuel cell performance, exchange current density, and current loss is also investigated. Both the exchange current density and the current loss are plotted against temperature on an Arrhenius-like plot and the related parameters are estimated. The theoretical equations derived in the literature, which model fuel cell performance, are found to reasonably fit the obtained experimental data.     

Highly efficient and durable anode-supported SOFC stack with internal manifold structure

We have developed a solid oxide fuel cell (SOFC) stack with an internal manifold structure. The stack, which is composed of 25 anode-supported 100-mm-diameter SOFCs, provided an electrical conversion efficiency of 56% (based on the lower heating value of methane, which was used as a fuel) and an output of 350 W when the fuel utilization, current density, and operating temperature were 75%, 0.3 A cm⁻², and 1073 K, respectively. The electrical efficiency and the output were maintained for 1100 h. The cell voltage fluctuation was ±2% for 25 cells. The relationship between average cell voltage and current density in the 25-cell stack was as almost the same as that in the 1- and 10-cell stacks, which suggests that our stack provides almost the same cell performance regardless the number of the cells.     

Parametric analysis of a hybrid power system using organic Rankine cycle to recover waste heat from proton exchange membrane fuel cell

Using fuel cell systems for distributed generation (DG) applications represents a meaningful candidate to conventional plants due to their high power density and the heat recovery potential during the electrochemical reaction. A hybrid power system consisting of a proton exchange membrane (PEM) fuel cell stack and an organic Rankine cycle (ORC) is proposed to utilize the waste heat generated from PEM fuel cell. The system performance is evaluated by the steady-state mathematical models and thermodynamic laws. Meanwhile, a parametric analysis is also carried out to investigate the effects of some key parameters on the system performance, including the fuel flow rate, PEM fuel cell operating pressure, turbine inlet pressure and turbine backpressure. Results show that the electrical efficiency of the hybrid system combined by PEM fuel cell stack and ORC can be improved by about 5% compared to that of the single PEM fuel cell stack without ORC, and it is also indicated that the high fuel flow rate can reduce the PEM fuel cell electrical efficiency and overall electrical efficiency. Moreover, with an increased fuel cell operating pressure, both PEM fuel cell electrical efficiency and overall electrical efficiency firstly increase, and then decrease. Turbine inlet pressure and backpressure also have effects on the performance of the hybrid power system.     

Dynamic behavior of a PEM fuel cell stack for stationary applications

We discuss the behavior and performance of a proton exchange membrane fuel cell stack under fast load commutations. We present experimental results for the polarization curves, energy balance sheet, and time response of the fuel cells. Although load transients are present both in the voltage and current generated, it is found that the fuel cell system response is faster than 0.15 s to load commutations. The experimental results were also compared to the Amphlett et al. and Kim et al. models, which were found to describe the data well.     

Thermal and electrical experimental characterisation of a 1 kW PEM fuel cell stack

The present work describes the experimental characterisation of a self-humidified 1 kW PEM fuel stack with 24 cells. A test bench was prepared and used to operate a PEMFC stack, and several parameters, such as the temperature, pressure, stoichiometry, current and voltage of each cell, were monitored with a LabView platform to obtain a complete thermal and electrical characterisation. The stack was operated in the constant resistance load regime, in dead-end mode (with periodic releases of hydrogen), with 30% relative humidity air and with temperature control from a cooling water circuit. The need to operate the stack for significant periods of time to achieve repeatable performance behaviour was observed, as was the advantage of using some recuperation techniques to improve electrical energy production. At low temperatures, the individual cell voltage measurements show lower values for the cells nearer to the cooling channels. The performance of the fuel cell stack decreases at operating temperatures above 40 °C. The stack showed the best performance and stability at 30 °C, with 300 mbar of hydrogen and 500 mbar of air pressure. The optimised hydrogen purge interval was 15 s, and the most favourable air stoichiometry was 2. Between 15 A and 32 A, the maximum electrical efficiency was 40%, and the thermal energy recovery in the cooling system was 40.8%; these values are based on the HHV. Electrical efficiencies above 40% were obtained between 10 and 55 A. The variation in the electrical efficiency is explained by the variation in the following independently measured factors: the fuel utilisation coefficient and the faradic and voltage efficiencies. The deviation between the product of the factors and the measured electrical efficiency is below 0.5%. Measurements were taken to identify all the losses from the fuel cell stack; namely, the energy balance to the cooling water, which is the main portion. The other quantified losses by order of importance are the purged hydrogen and the latent and sensible heat losses from the cathode exhaust. The heat losses to the environment were also estimated based on the measured stack surface temperature. The sum of all the losses and the electrical output has a closure error below 2% except at the highest and lowest loads.     

Anion exchange membrane fuel cell modelling

A parametric model predicting the performance of a solid polymer electrolyte, anion exchange membrane fuel cell (AEMFC), has been developed, in Matlab environment, based on interrelated electrical and thermal models. The electrical model proposed is developed by modelling an AEMFC open-circuit output voltage, irreversible voltage losses along with a mass balance, while the thermal model is based on the energy balance. The proposed model of the AEMFC stack estimates its dynamic behaviour, in particular the operating temperature variation for different discharge current values. The results of the theoretical fuel cell (FC) stack are reported and analysed in order to highlight the FC performance and how it varies by changing the values of some parameters such as temperature and pressure. Both the electrical and thermal FC models were validated by comparing the model results with experimental data and the results of other models found in the literature.     

Mitigating performance deterioration analysis of VC-PEMFC with dead-ended anode by pulsation fuel supplying mode

Hydrogen starvation and water flooding are two principal factors resulting in performance deterioration of the proton exchange membrane fuel cell stack at the dead-end anode. This paper proposes a novel hydrogen supply mode called the pulsation mode aimed at mitigating the problems of performance deterioration in a 10-cell open-cathode vapor chamber proton exchange membrane fuel cell stack to increase the performance. This method does not require complex equipment and structure, only four controllable solenoid valves are sufficient to generate periodic pulsation inside the anode channels. The experiments were used to validate the effectiveness of the new mode and to compare the effects of different pulsation frequencies on the performance of the stack. A series of parameters such as voltage, power growth rate, and voltage stability index are used to analyze the operating characteristics of the stack. The results show that the periodic pulsations generated by the new mode are potent of increasing the species mass transfer rate within the anode channels, and the species mass transfer rate increases with the increase of pulsation frequency. Meanwhile, selection of a suitable pulsation frequency can effectively improve stack water management and reduce the probability of hydrogen starvation. Finally, the new mode is able to enhance the voltage down valley of the stack under large external load variation. The ohmic resistance of the stack in the new mode has proved to be lower by the current interruption method. Furthermore, it is capable of increasing the net power of the stack by up to 7.71%.     

燃料重整固体氧化物燃料电池发电系统性能分析及多变量参数优化

以燃料重整的固体氧化物燃料电池发电系统为研究对象,通过数值模拟方法对固体氧化物燃料电池发电系统的性能、(火用)损、(火用)效率以及多变量运行参数优化进行了分析。研究结果表明：重整反应中燃料利用系数、电池工作温度、水碳比、电堆电流密度等参数对系统性能影响显著;电堆工作在不同电流密度下都有其对应的最佳工作温度、最佳燃料利用系数工况点;水碳比会改变重整反应产氢量,从而影响电化学反应速率,空气加热器的(火用)损所占份额最大;优化后的系统效率及(火用)效率为0.480 9和0.462 6,效率提升约4%。     

Experimental study of a fuel cell power train for road transport application

The development of fuel cell electric vehicles requires the on-board integration of fuel cell systems and electric energy storage devices, with an appropriate energy management system. The optimization of performance and efficiency needs an experimental analysis of the power train, which has to be effected in both stationary and transient conditions (including standard driving cycles).In this paper experimental results concerning the performance of a fuel cell power train are reported and discussed. In particular characterization results for a small sized fuel cell system (FCS), based on a 2.5 kW PEM stack, alone and coupled to an electric propulsion chain of 3.7 kW are presented and discussed. The control unit of the FCS allowed the main stack operative parameters (stoichiometric ratio, hydrogen and air pressure, temperature) to be varied and regulated in order to obtain optimized polarization and efficiency curves. Experimental runs effected on the power train during standard driving cycles have allowed the performance and efficiency of the individual components (fuel cell stack and auxiliaries, dc–dc converter, traction batteries, electric engine) to be evaluated, evidencing the role of output current and voltage of the dc–dc converter in directing the energy flows within the propulsion system.     

Experimental analysis of internal gas flow configurations for a polymer electrolyte membrane fuel cell stack

The internal gas distribution system utilised for supplying fresh reactants and removing reaction products from the individual cells of a fuel cell stack can be designed in a parallel, a serial or a mixture of parallel and serial gas flow configuration. In order to investigate the interdependence between the internal stack gas distribution configuration and single cell as well as overall stack performance, a small laboratory-scale fuel cell stack consisting of identical unit cells was subject to operation with different gas distribution configurations and different operating parameters. The current/voltage characteristics measured with the different gas distribution configurations are analysed and compared on unit cell- as well as on stack-level. The results show the significant impact of the internal stack gas distribution system on operation and performance of the individual unit cells and the overall stack.     

PEMFC purging at low operating temperatures: An experimental approach

In this paper, the effect of operating temperature on optimal purge interval for maximum energy efficiency in a proton exchange membrane fuel cell (PEMFC) with dead‐ended anode (DEA) was experimentally investigated. The study was conducted with a focus on challenges associated with operation at temperatures lower than the recommended designed temperature. With DEA, gradual voltage drop happens due to the accumulation of water and impurities such as nitrogen. Hence, periodic purging of the anode side is required to remove excess water and impurities that are accumulated at the anode side over time. Short purge intervals increase hydrogen loss that translates into low fuel utilisation, whereas long purge intervals result in voltage drop due to high water and impurity accumulations. Therefore, an optimal purge strategy should be implemented to maximise the stack energy efficiency. Depending on the operating conditions and loads, there are instances that a fuel cell stack operates at temperatures lower than its recommended designed temperature. Considering the temperature effect on the cell water management, operating temperature is an important factor to consider for optimising the purge strategy in PEMFCs. At lower operating temperatures (ie, below 50°C), more water is formed in liquid form, which makes the optimisation of purge strategy more challenging. For a stack temperature of 40°C, it was observed that with an increase in stack current from 0.25 to 0.45 A cm^?2, the optimal purge interval decreases from 90 seconds to around 45 seconds, respectively. Increasing the stack temperature from 40°C to 50°C resulted in an increase in the optimal purge interval to 120 seconds and 90 seconds for stack currents of 0.25 (ie, low current density) and 0.45 A cm^?2, respectively. At lower operating temperatures, more frequent purging schedules are needed accordingly to remove the liquid water from the cell. These results indicated that at lower operating temperatures, water accumulation at the anode side becomes more dominant compared with higher operating temperatures.     

An improved dynamic voltage model of PEM fuel cell stack

The voltage dynamic properties of PEM fuel cell stack have been analyzed through experimental investigation. Different behaviours between voltage overshoot and undershoot are found under load commutations. A semi-empirical dynamic model for stack voltage is introduced on the basis of experimental investigation. The proposed model can predict the transient response of stack voltage under step change in current. Comparing with previous model, the suggested model indicates a better agreement between tests and simulations.     

Operational characteristics of the direct methanol fuel cell stack on fuel and energy efficiency with performance and stability

This paper is presented to investigate operational characteristics of a direct methanol fuel cell (DMFC) stack with regard to fuel and energy efficiency, including its performance and stability under various operating conditions. Fuel efficiency of the DMFC stack is strongly dependent on fuel concentration, working temperature, current density, and anode channel configuration in the bipolar plates and noticeably increases due to the reduced methanol crossover through the membrane, as the current density increases and the methanol concentration, anode channel depth, and temperature decreases. It is, however, revealed that the energy efficiency of the DMFC stack is not always improved with increased fuel efficiency, since the reduced methanol crossover does not always indicate an increase in the power of the DMFC stack. Further, a lower methanol concentration and temperature sacrifice the power and operational stability of the stack with the large difference of cell voltages, even though the stack shows more than 90% of fuel efficiency in this operating condition. The energy efficiency is therefore a more important characteristic to find optimal operating conditions in the DMFC stack than fuel efficiency based on the methanol utilization and crossover, since it considers both fuel efficiency and cell electrical power. These efforts may contribute to commercialization of the highly efficient DMFC system, through reduction of the loss of energy and fuel.     

A semi-empirical model for efficiency evaluation of a direct methanol fuel cell

Energy density and power density are two of the most significant performance indices of a fuel cell system. Both the indices are closely related to the operating conditions. Energy density, which can be derived from fuel cell efficiency, is especially important to small and portable applications. Generally speaking, power density can be easily obtained by acquiring the voltage and current density of an operating fuel cell. However, for a direct methanol fuel cell (DMFC), it is much more difficult to evaluate its efficiency due to fuel crossover and the complex architecture of fuel circulation. The present paper proposes a semi-empirical model for the efficiency evaluation of a DMFC under various operating conditions. The power density and the efficiency of a DMFC are depicted by explicit functions of operating temperature, fuel concentration and current density. It provides a good prediction and a clear insight into the relationship between the aforementioned performance indices and operating variables. Therefore, information including power density, efficiency, as well as remaining run-time about the status of an operating DMFC can be in situ evaluated and predicted. The resulting model can also serve as an important basis for developing real-time control strategies of a DMFC system.     

Improving open-cathode polymer electrolyte membrane fuel cell performance using multi-hole separators

We developed a new separator with a multi-hole structure (MHS) in the rib region for open-cathode polymer electrolyte membrane fuel cell (OC-PEMFC) stack to improve performance. The electrochemical current–voltage performance results clearly demonstrate that the performance of the OC-PEMFC stack using the MHS design was higher than that using the conventional parallel design at high current regions (i.e., over 7 A). The current increased by 11.24% at 12 V (i.e., 0.6 V/cell). The effects of supplying additional oxygen and removing generated water were identified as factors improving the performance. The individual cell voltages demonstrate that the initial value of standard deviation for the OC-PEMFC stack using MHS was somewhat high, but it exhibited better uniformity at higher current regions.     

Online voltage consistency prediction of proton exchange membrane fuel cells using a machine learning method

Widely acknowledged by experts, the inconsistency between the cells of the proton exchange membrane fuel cell stack during operation is an important cause of the fuel cell life decay. Existing studies mainly focus on qualitative analysis of the effects of operating parameters on fuel cell stack consistency. However, there is currently almost no quantitative research on predicting the voltage consistency through operating parameters with machine learning methods. To solve this problem, a three-dimensional model of proton exchange membrane fuel cell stack with five single cells is established in this paper. The Computational Fluid Dynamic (CFD) method is used to provide the source data for prediction model. After predicting the voltage consistency with several machine learning methods and comparing the accuracy through simulation data, the integrated regression method based on Gradient Boosting Decision Tree (GBDT) gets the highest score (0.896) and is proposed for quickly predicting the consistency of cell voltage through operating parameters. After verifying the GBDT method with the experimental data from the fuel cell stack of SUNRISE POWER, in which the accuracy score is 0.910, the universality and accuracy of the method is confirmed. The influencing sensitivity of each operating parameter is evaluated and the current density has the greatest influence on the predicted value, which accounts for 0.40. The prediction of voltage consistency under different combination of operating parameters can guide the optimization of structural parameters in the process of the fuel cell design and operating parameters in the process of fuel cell control.     

Computational Fluid Dynamics Study of Serpentine Flow Field Proton Exchange Membrane Fuel Cell Performance

Abstract

Fuel cell is an electro-chemical device that directly converts the chemical energy of fuel into usable electrical form without any toxic byproducts. Flow field plates are backbone of the fuel cell. It is the essential component of fuel cell with multifunctional character. In the present work, a complete three-dimensional steady-state isothermal single phase computational fluid dynamics model is proposed for a proton exchange membrane fuel cell with serpentine flow channel to analyze the species transport phenomenon. The numerical model was developed and analyzed using ANSYS FLUENT 15.0 and simulations were carried out to get the key parameters like variation of oxygen, hydrogen, liquid water activity, and water mass fraction in the flow channels, membrane water content, and membrane protonic conductivity. The numerical results show that, when the cell is operated at higher current densities, i.e. lower cell voltage, hydrogen and oxygen consumption is more as well as water generation is also more. The effect of cell operating temperature and reactants inlet humidity on cell performance was also studied. Finally, the polarization curve obtained was compared with the experimental results and it was found that the numerical results are having good agreement with the experimental results.     

Performance optimization of polymer electrolyte membrane fuel cells using the Nelder-Mead algorithm

The direct-search simplex method for function optimization has been adapted to performance optimization of polymer electrolyte membrane fuel cells (PEMFCs). The established method is strongly application oriented and uses only experimentally determined data for optimization. It is not restricted to discrete parameters optimums and does not require the use of third-party software or computational resources. Hence, it is easy to implement in fuel cell testing stations. The optimization consists of finding, for a given fuel cell load, an optimum set of values of the 7 fuel cell operating parameters: the fuel cell temperature, the reactants' stoichiometric ratios, the reactants' inlet relative humidity, and the reactants' outlet pressures, resulting in the highest fuel cell performance. The performance is measured using a scalar function of the operating parameters and the load and can be defined according to needs.Two PEMFC performance functions: the fuel cell voltage and the system-related fuel cell efficiency were optimized using the procedure for practically sized PEMFC stacks of two designs. With respect to the nominal operating conditions defined as optimal for each stack design by its manufacturer, the gains from the optimization procedure were up to over 12% and up to over 7% for the stack voltage and efficiency, respectively. The validation of the procedure involved 5 stack specimens and four laboratories and consistent results were obtained.