Modeling and control of air stream and hydrogen flow with recirculation in a PEM fuel cell system—II. Linear and adaptive nonlinear control

In this part of the paper, linear and nonlinear multivariable controllers are designed for the air stream and hydrogen flow with recirculation in a proton exchange membrane (PEM) fuel cell system. The focus of the model is to obtain the desired transient performance of air stoichiometric ratio, cathode inlet pressure, and pressure difference between the anode and the cathode. Based on linearization of the nonlinear dynamic model in the first part of this paper, the coupling between control inputs and performance is analyzed first. The phase relationship between the stack voltage and water transport in frequency domain is meaningful to the future humidity estimation and active purge operation. Then, linear quadratic Gaussian (LQG) algorithm based on observer feedback is used for set-point tracking, and a model-predictive controller (MPC) with an on-line neural network identifier is also designed to improve robustness. Compared with decentralized PI controllers, the multivariable controllers improve the transient response and shows better disturbance rejection capability.     

Transient air cooling thermal modeling of a PEM fuel cell

Fuel cell utilization for automobile and residential applications is a promising option in order to help reduce environmental concerns such as pollution. However, fuel cell development requires addressing their dynamic behavior to improve their performances and their life cycle. Since the temperature distribution in the cell is known to be an important factor to the fuel cell's efficiency, a cooling device is often added to homogenize the temperature in the cell and to ensure temperature control.     

Optimization of hydrogen feeding procedure in PEM fuel cell systems for transportation

The hydrogen feeding sub-system is one of balance of plant (BOP) components necessary for the correct operation of a fuel cell system (FCS). In this paper the performance of a 6 kW PEM (Proton Exchange Membrane) FCS, able to work with two fuel feeding procedures (dead-end or flow-through), was experimentally evaluated with the aim to highlight the effect of the anode operation mode on stack efficiency and durability. The FCS operated at low reactant pressure (<50 kPa) and temperature (<330 K), without external humidification. The experiments were performed in both steady state and dynamic conditions. The performance of some cells in dead-end mode worsened during transient phases, while a more stable working was observed with fuel recirculation. This behavior evidenced the positive role of the flow-through procedure in controlling flooding phenomena, with the additional advantage to simplify the management issues related to hydrogen purge and air stoichiometric ratio. The flow-through modality resulted a useful way to optimize the stack efficiency and to reduce the risks of fast degradation due to reactant starvation during transient operative phases.     

Air mass flow and pressure optimisation of a PEM fuel cell range extender system

In order to eliminate the local CO₂ emissions from vehicles and to combat the associated climate change, the classic internal combustion engine can be replaced by an electric motor. The two most advantageous variants for the necessary electrical energy storage in the vehicle are currently the purely electrochemical storage in batteries and the chemical storage in hydrogen with subsequent conversion into electrical energy by means of a fuel cell stack. The two variants can also be combined in a battery electric vehicle with a fuel cell range extender, so that the vehicle can be refuelled either purely electrically or using hydrogen. The air compressor, a key component of a PEM fuel cell system, can be operated at different air excess and pressure ratios, which influence the stack as well as the system efficiency. To asses the steady state behaviour of a PEM fuel cell range extender system, a system test bench utilising a commercially available 30 kW stack (96 cells, 409 cm² cell area) was developed. The influences of the operating parameters (air excess ratio 1.3 to 1.7, stack temperature 20 °C–60 °C, air compressor pressure ratio up to 1.67, load point 122 mA/cm² to 978 mA/cm²) on the fuel cell stack voltage level (constant ambient relative humidity of 45%) and the corresponding system efficiency were measured by utilising current, voltage, mass flow, temperature and pressure sensors. A fuel cell stack model was presented, which correlates closely with the experimental data (0.861% relative error). The air supply components were modelled utilising a surface fit. Subsequently, the system efficiency of the validated model was optimised by varying the air mass flow and air pressure. It is shown that higher air pressures and lower air excess ratios increase the system efficiency at high loads. The maximum achieved system efficiency is 55.21% at the lowest continuous load point and 43.74% at the highest continuous load point. Future work can utilise the test bench or the validated model for component design studies to further improve the system efficiency.     

Analysis of design parameters in anodic recirculation system based on ejector technology for PEM fuel cells: A new approach in designing

In this study, the anodic recirculation system (ARS) based on ejector technology in polymer electrolyte membrane PEM fuel cell is studied with employing a theoretical model. A practical method is presented for selecting or designing the ejector in an ARS, that offers the best selection or design. A comprehensive parametric study is performed on the design parameters of a PEM fuel cell stack and an ARS ejector. Four geometrical parameters consist of cell active area, cell number, nozzle throat diameter, and mixing chamber diameter in the design of ARS are intended. The effect of each contributes to the overall system performance parameters is studied. In this parametric study, the correlation between stack design parameters and ejector design parameters are studied. Eventually, based on the results, two dimensionless parameters are useful in the design process are proposed.     

PEM fuel cell performance with solar air preheating

Proton Exchange Membrane Fuel Cells (PEMFC) have proven to be a promising energy conversion technology in various power applications and since it was developed, it has been a potential alternative over fossil fuel-based engines and power plants, all of which produce harmful by-products. The inlet air coolant and reactants have an important effect on the performance degradation of the PEMFC and certain power outputs. In this work, a theoretical model of a PEM fuel cell with solar air heating system for the preheating hydrogen of PEM fuel cell to mitigate the performance degradation when the fuel cell operates in cold environment, is proposed and evaluated by using energy analysis. Considering these heating and energy losses of heat generation by hydrogen fuel cells, the idea of using transpired solar collectors (TSC) for air preheating to increase the inlet air temperature of the low-temperature fuel cell could be a potential development. The aim of the current article is applying solar air preheating for the hydrogen fuel cells system by applying TSC and analyzing system performance. Results aim to attention fellow scholars as well as industrial engineers in the deployment of solar air heating together with hydrogen fuel cell systems that could be useful for coping with fossil fuel-based power supply systems.     

Speed controlling of the PEM fuel cell powered BLDC motor with FOPI optimized by MSA

In this study, Brushless DC (BLDC) motor, which is commonly used as a drive element in the unmanned aerial vehicle (UAV), electric vehicles, and mobile robots today, is powered by hydrogen technologies as environmentally friendly and controlled by a fractional-order PI (FOPI) controller structure. Proton Exchange Membrane (PEM) electrolyzer, PEM Fuel Cell (PEMFC), storage tank, BLDC motor, and motor driver system are modeled and integrated into the Simulink environment in MATLAB. PEM electrolyzer that is energized from the AC grid via the AC/DC converter generates the hydrogen. This generated hydrogen is stored in the storage tank and used by PEMFC to energize to the BLDC motor. The model of the BLDC motor is controlled by using a closed-loop FOPI controller for the variable speed and torque reference values. Parameters of the FOPI are determined by Moth Swarm Algorithm (MSA) optimization method. It is observed from the results that the PEMFC powered FOPI controlled BLDC motor operates stably at high performance for different speed and torque values as expected from the modern drive systems. Furthermore, it is seen that the required energy for the BLDC motor is provided by the PEMFC-PEM electrolyzer system without interruption and the FOPI controlled BLDC motor successfully follows the reference speed values for the different torque values.     

A gas management strategy for anode recirculation in a proton exchange membrane fuel cell

The anode configuration and gas management strategy are two of factors that affect the energy efficiency of a proton exchange membrane fuel cell. In order to improve the hydrogen utilization, unused hydrogen can be recirculated to the inlet using a pump. However, impurities diffusing from the cathode to the anode may cause the dilution of hydrogen in the anode. As a result, a gas management strategy is required for the anode recirculation configuration. In this preliminary study, a novel configuration for anode recirculation and a gas management strategy are proposed and verified by experiments. Two valves are installed in the recirculation line. The anode is operated in four modes (dead-end, recirculation, compression, and purge), and the real-time local current density (LCD) is monitored for gas management purposes. The results show that the LCD distribution is uniform during the recirculation mode and nonuniform during the dead-end and compression modes. With this configuration and gas management strategy, the cycle duration is increased by a factor of 6.5.     

Hydrogen purge and reactant feeding strategies in self-humidified PEM fuel cell systems

A 6 kW proton exchange membrane fuel cell system, operating in self-humidified conditions, was characterized in two anode operative modes: dead-end and flow through with exhaust recirculation. The anode sub-system was specifically designed in order to adjust the level of recycled anodic stream. The role of anode purge frequency, anode recirculation level and air stoichiometric ratio was analysed in the power range 1–5 kW. The aim of this study was to define management strategies to assure efficient and reliable cell performance during steady-state, warm-up and load variation phases. The results evidenced the combined effect of hydrogen purge, air flow rate impulse, and recycled anodic stream on individual cell performance recovery when unstable working conditions were detected during system start-up and load variations.     

Methanol fuel processor and PEM fuel cell modeling for mobile application

The use of hydrocarbon fed fuel cell systems including a fuel processor can be an entry market for this emerging technology avoiding the problem of hydrogen infrastructure. This article presents a 1 kW low temperature PEM fuel cell system with fuel processor, the system is fueled by a mixture of methanol and water that is converted into hydrogen rich gas using a steam reformer. A complete system model including a fluidic fuel processor model containing evaporation, steam reformer, hydrogen filter, combustion, as well as a multi-domain fuel cell model is introduced. Experiments are performed with an IDATECH FCS1200™ fuel cell system. The results of modeling and experimentation show good results, namely with regard to fuel cell current and voltage as well as hydrogen production and pressure. The system is auto sufficient and shows an efficiency of 25.12%. The presented work is a step towards a complete system model, needed to develop a well adapted system control assuring optimized system efficiency.     

Computational model of a PEM fuel cell with serpentine gas flow channels

A three-dimensional computational fluid dynamics model of a PEM fuel cell with serpentine flow field channels is presented in this paper. This comprehensive model accounts for the major transport phenomena in a PEM fuel cell: convective and diffusive heat and mass transfer, electrode kinetics, and potential fields. A unique feature of the model is the implementation of a voltage-to-current (VTC) algorithm that solves for the potential fields and allows for the computation of the local activation overpotential. The coupling of the local activation overpotential distribution and reactant concentration makes it possible to predict the local current density distribution more accurately. The simulation results reveal current distribution patterns that are significantly different from those obtained in studies assuming constant surface overpotential. Whereas the predicted distributions at high load show current density maxima under the gas channel area, low load simulations exhibit local current maxima under the collector plate land areas.     

Dynamic modeling of a PEM fuel cell with temperature effects

With the aim of dynamic simulation and control, a new non-linear state-space dynamic non-isothermal polymer electrolyte membrane fuel cell (PEMFC) model is developed in this paper. This mathematical model is developed based on mass and energy equation. The present model takes in account subsequent factors as the effects of charge double layer capacitance, the geometrical capacity and the effect of temperature gradient. In this paper, the authors propose a combination of several dynamic equations to study the effect of suddenly variation of some operating parameters like load resistance, gas pressure and gas temperature input. The results are compared to those of an isothermal model. This model will be extremely functional for the best possible design and real-time control of PEMFC systems. The present model is executed in MATHCAD software and the fuel cell is symbolized by an equivalent circuit which incorporates gas diffusion layer, membrane and electrodes. The analysis results show that the main elements that influence the performance of the cell are load resistance and functioning temperature.     

Hydrogen utilization enhancement of proton exchange membrane fuel cell with anode recirculation system through a purge strategy

Proton exchange membrane (PEM) fuel cells are widely considered as potential alternative energy candidates for internal combustion engines because of their low-temperature start, high energy density, and ease of scale up. However, their low hydrogen utilization rate is one of the main reasons for the limited commercial development. This study focuses on improving the hydrogen utilization rate of PEM fuel cells and system efficiency using optimal active recirculation system (ARS). An anode ARS and purging strategy are introduced to enhance the hydrogen utilization rate of PEM fuel cells. An ARS simulation model with purge strategy model is developed in a MATLAB/Simulink environment. A control-oriented dynamic model is developed to study the hydrogen recirculation system characteristics. The dynamic model is used as basis to propose a proportional integration differentiation controller to maintain the anode hydrogen concentration and increase the hydrogen utilization rate. Several experiments are performed using different purging strategies in conjunction with ARS. The hydrogen utilization rate is the highest when the purge time is 0.3 s and the purge period is 10 s. Simulation results show that the PEM fuel cells with an anode recirculation configuration exhibit a better performance than other configurations in terms of hydrogen utilization. Experimental results also demonstrate the feasibility of the proposed system, the performance of which is also superior to that of other hydrogen supply system.     

Economics of hydrogen production and utilization strategies for the optimal operation of a grid-parallel PEM fuel cell power plant

This paper integrates the hydrogen production and utilization strategies with an economic model of a PEM fuel cell power plant (FCPP). The model includes the operational cost, thermal recovery, power trade with the local grid, and hydrogen management strategies. The model is used to determine the optimal operational strategy, which yields the minimum operating cost. The optimal operational strategy is achieved through estimation of the following: hourly generated power, thermal power recovered from the FCPP, power trade with the local grid, and hydrogen production. An evolutionary programming-based technique is used to solve for the optimal operational strategy. The model is tested using different seasonal load demands. The results illustrate the impact of hydrogen management strategies on the operational cost of the FCPP when subjected to seasonal load variation. Results are encouraging and indicate viability of the proposed model.     

A parametric comparison of three fuel recirculation system in the closed loop fuel supply system of PEM fuel cell

Anodic fuel recirculation system has a significant role on the parasitic power of proton exchange membrane fuel cell (PEMFC). In this paper, different fuel supply systems for a PEMFC including a mechanical compressor, an ejector and an electrochemical pump are evaluated. Furthermore, the performances of ejector and electrochemical pump are studied at different operating conditions including operating temperature of 333 K–353 K, operating pressure of 2 bar–4 bar, relative humidity of 20%–100%, stack cells number from 150 to 400 and PEMFC active area of 0.03 m²–0.1 m². The results reveal that higher temperature of PEMFC leads to lower power consumption of the electrochemical pump, because activation over-potential of electrochemical pump decreases at higher temperatures. Moreover, higher operating temperature and pressure of PEMFC leads to higher stoichiometric ratio and hydrogen recirculation ratio because the motive flow energy in ejector enhances. In addition, the recirculation ratio and hydrogen stoichiometric ratio increase, almost linearly, with increase of anodic relative humidity. Utilization of mechanical compressor leads to lower system efficiency than other fuel recirculating devices due to more power consumption. Utilization of electrochemical pump in anodic recirculation system is a promising alternative to ejector due to lower noise level, better controllability and wide range of operating conditions.     

Numerical study of water management in the air flow channel of a PEM fuel cell cathode

The water management in the air flow channel of a proton exchange membrane (PEM) fuel cell cathode is numerically investigated using the FLUENT software package. By enabling the volume of fraction (VOF) model, the air–water two-phase flow can be simulated under different operating conditions. The effects of channel surface hydrophilicity, channel geometry, and air inlet velocity on water behavior, water content inside the channel, and two-phase pressure drop are discussed in detail. The results of the quasi-steady-state simulations show that: (1) the hydrophilicity of reactant flow channel surface is critical for water management in order to facilitate water transport along channel surfaces or edges; (2) hydrophilic surfaces also increase pressure drop due to liquid water spreading; (3) a sharp corner channel design could benefit water management because it facilitates water accumulation and provides paths for water transport along channel surface opposite to gas diffusion layer; (4) the two-phase pressure drop inside the air flow channel increases almost linearly with increasing air inlet velocity.     

Investigating the stability and degradation of hydrogen PEM fuel cell

The hydrogen proton exchange membrane (PEM) fuel cells are promising to utilize fuel cells in electric vehicle (EV) applications. However, hydrogen PEM fuel cells are still encountering challenges regarding their functionality and degradation mechanism. Therefore, this paper aims to study the performance of a 3.2 kW hydrogen PEM fuel cell under accelerated operation conditions, including varying fuel pressure at a level of 0.1–0.5 bar, variable loading, and short-circuit contingencies. We will also present the results on the degradation estimation mechanism of four fuel cells working at different operational conditions, including high-to-low voltage range and high-to-low temperature variations. These experiments examine over 180 days of continuous fuel cell working cycle. We have observed that the drop in the fuel cells' efficiency is at around 7.2% when varying the stack voltage and up to 14.7% when the fuel cell's temperature is not controlled and remained at 95 °C.     

Effects of humidification temperatures on local current characteristics in a PEM fuel cell

It is well known that water plays a very important role in the performance of proton exchange membrane (PEM) fuel cells. Non-uniform water content in the membrane leads to non-uniform ionic resistance, and non-uniform liquid water fraction in the porous electrode causes varied mass transfer resistances. The objective of this work is to study the effects of different anode and cathode humidification temperatures on local current densities of a PEM fuel cell with a co-flow serpentine flow field. The method used is the current distribution measurement gasket technique [H. Sun, G.S. Zhang, L.J. Guo, H. Liu, J. Power Sources 158 (2006) 326–332]. The experimental results show that both air and the hydrogen need to be humidified to ensure optimal cell performance, and too high or too low humidification temperature can cause severe non-uniform distribution of local current density. From the experimental results of local current density distributions, the local membrane hydration, the optimal humidification temperature, and if flooding occurs can be obtained. Such detailed local measurement results could be very valuable in fuel cell design and operation optimizations.     

Computationally efficient modeling of the dynamic behavior of a portable PEM fuel cell stack

A numerically efficient mathematical model of a proton exchange membrane fuel cell (PEMFC) stack is presented. The aim of this model is to study the dynamic response of a PEMFC stack subjected to load changes under the restriction of short computing time. This restriction was imposed in order for the model to be applicable for nonlinear model predictive control (NMPC). The dynamic, non-isothermal model is based on mass and energy balance equations, which are reduced to ordinary differential equations in time. The reduced equations are solved for a single cell and the results are upscaled to describe the fuel cell stack. This approach makes our calculations computationally efficient. We study the feasibility of capturing water balance effects with such a reduced model. Mass balance equations for water vapor and liquid water including the phase change as well as a steady-state membrane model accounting for the electro-osmotic drag and diffusion of water through the membrane are included. Based on this approach the model is successfully used to predict critical operating conditions by monitoring the amount of liquid water in the stack and the stack impedance. The model and the overall calculation method are validated using two different load profiles on realistic time scales of up to 30 min. The simulation results are used to clarify the measured characteristics of the stack temperature and the stack voltage, which has rarely been done on such long time scales. In addition, a discussion of the influence of flooding and dry-out on the stack voltage is included. The modeling approach proves to be computationally efficient: an operating time of 0.5 h is simulated in less than 1 s, while still showing sufficient accuracy.     

In situ detection of anode flooding of a PEM fuel cell

This paper proposes an early detection scheme of anode flooding in a PEM fuel cell. Through experimental testing of an eight-cell hydrogen-fueled polymer electrolyte stack it is shown that anode flooding can be detected prior to a rapid voltage decline. The proposed detection scheme requires no additional costly instrumentation and uses the existing voltage scan cards.