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.     

Development,manufacture and validation of an open cathode LT-PEMFC stack at HySA systems

This work presents open cathode low temperature polymer electrolyte membrane fuel cell stack development and validation process project performed at HySA Systems as a part of a long-term programme funded by Department of Science and Innovation in South Africa. A detailed explanation of the stack design, manufacturing, assembly and validation is given as well as detailed analysis of results is presented. Prototype stack has an electrode active area of 50 cm², bipolar plates made of graphite composite material (Eisenhuth) and membrane electrode assemblies manufactured in South Africa - HyPlat (Pty) Ltd. A short 10-cell stack is validated using FuelCon Evaluator stack test station and custom designed stack control system integrated with complete balance of plant components. The stack maximum current and power densities are 1.2 Acm⁻² at 0.5 V and 0.6 Wcm⁻², respectively. Performed current hold (300 h) and open circuit voltage (60 h) durability tests resulted in degradation rates of 0.64 mVh⁻¹ and 3.83 mVh⁻¹, respectively.     

Three-dimensional multi-physics modelling and structural optimization of SOFC large-scale stack and stack tower

Multi-physics modelling of the Solid Oxide Fuel Cell (SOFC) stack requires significant computational resources. Design optimization of large-scale stacks and stack towers has always been a challenge in recent years. This study establishes a three-dimensional multi-physics model based on a two-step coupling using the BP neural network. The comparison between the novel model and the traditional fully coupled model in both accuracy and computing resource requirements are explored. The novel method has high effectiveness for modelling the large-scale stacks. Based on this, planar SOFC 50-cell stacks and 150-cell stack towers are simulated. The results show that, the flow uniformity of fuel distribution of the stack towers can decrease more than 30% comparing with the 50-cell stack, which leads to significant deterioration of the voltage and temperature distribution. The parameters of manifold and buffer area and channel height of the stack tower is optimized to achieve better uniformity of flow and voltage distribution and lower temperature gradient simultaneously.     

Numerical modeling of manifold design and flow uniformity analysis of an external manifold solid oxide fuel cell stack

Computational fluid dynamics (CFD) technique and experimental measurement are combined to investigate the effects of several geometric parameters on flow uniformity and pressure distribution in an external manifold solid oxide fuel cell (SOFC) stack. The model of numerical simulation is composed of channels, tubes and manifolds based on a realistic 20-cell stack. Analysis results show that gas resistance in the channel can improve the flow uniformity. However, channel resistance only has a limited effect under high mass flow rate. With the increase of inlet tube diameter, the flow uniformity improves gradually but this has little impact on pressure drop. On contrary, the larger diameter of outlet tube reduces the pressure drop effectively with minor improvement on flow uniformity. The dimensions of the flared inlet tube and the round perforated sheet in the manifold are designed to optimize both flow uniformity and pressure drop. Practical experimental stack is established and the velocity in the outlet of the channel is measured. The trends of the experimental measurements are corresponding well with the numerical results. The investigation emphasizes the importance of geometric parameters to gas flow and provides optimized strategies for external manifold SOFC stack.     

Numerical simulation of flow distribution for external manifold design in solid oxide fuel cell stack

In this study, a three dimensional model is constructed to investigate the flow distributions and the pressure variations for a 40-cell solid oxide fuel cell (SOFC) stack. Computational fluid dynamics (CFD) is used to optimize the design parameters of external manifold in the stack. The model consists of equations for the network with chamber structure of manifold. Simulation results indicate that the flow uniformity strongly depends on geometric shapes of manifold, including the joined position between tube and manifold, the dimension of manifold and the number of tubes. The ratio of flow velocity which describes the uniformity of flow distribution can be decreased by optimizing the geometrical structure of manifolds. In addition, it is found that the flow distribution can be intensively influenced by the gas resistance of the stack, which is closely related to the configuration of interconnect channels. The results summarize the importance of structure design of external manifold for stack performance. The numerical results are in good agreement with the experimental measurement in a 40-cell stack.     

Restoration of solid oxide fuel cell stacks after failure of partial cells

We investigated a solid oxide fuel cell stack that employs anode-supported planar cells in which two intermediate plates are installed every 10 cells to determine the influence of the separation and reconnection of the intermediate plates after high temperature operation. We showed that this separation and reconnection caused no significant degradation in stack performance. A 30-cell stack, which was constructed by removing two 10-cell sub-stacks from a 50-cell stack that had operated stably 1200 h, functioned well. The difference between the average voltages of the cells in the 50- and 30-cell stacks was less than 3% when the current density, fuel utilization, and oxygen utilization were 0.30 A cm⁻², 60%, and 15%, respectively. The 30-cell stack operated stably for about 1200 h with almost no degradation. These findings indicate that our stack can be restored after cells in the stack have broken down simply by removing the 10-cell sub-stacks that contain the broken cells and replacing them with undamaged 10-cell sub-stacks.     

Dynamic behavior of PEM FCPPs under various load conditions and voltage stability analysis for stand-alone residential applications

In this paper, dynamic behavior and performance of a fuel cell power plant (FCPP) which operates in parallel with a battery bank is tested under classified load conditions, such as mostly resistive, mostly inductive, resistive-inductive and non-linear loads. Thereafter, voltage stability analysis is performed using the dynamic response of the FCPP for stand-alone residential applications. Simulation results are obtained using the MATLAB^® and Simulink^® software packages, based on the mathematical and dynamic electrical models of the system. Using the experimental results, a validated model has been realized and voltage stability analysis is performed through this model.     

PEMFC system simulation in MATLAB-Simulink environment

Modelling and simulation of PEM fuel cell stack operation is developed in Simulink^® environment and validated through experimental data. The present work is the starting point for the development of a user friendly and versatile tool aimed at controlling and optimizing the operation of a PEMFC stack; in addition, it could be of help in stack and BOP components design, for instance feeding and humidification systems, cooling circuit, temperature control logic and electrical interface. The constitutive equations used to model the FC stack operation are the fundamental equations of electrochemistry. First, the model is used to describe the behaviour of a single cell under steady-state conditions upon varying variables such as temperature, pressure and relative humidity of reactants; then, it is applied to simulate the operation of a stack configuration, including also fluid-dynamics aspects, thermal and kinetic behaviour of feed systems. In particular, thermal control modelling is based on a simplified approach where different heat removal mechanisms are accounted for in a separate way. In its present state, the simulation tool so developed allows a feasible investigation of some process variables influence on the FC stack performances. The stack modelling is tested against benchmark results obtained from a 300W 20-cell air-cooled stack under variable operative conditions. MEAs based on Nafion 112 and Carbon cloth GDLs developed ad hoc are assembled into each cell of the stack. Although the model is quite simple, these preliminary results point out that it may be an adequate tool to set design targets and support further steps of optimization.     

Computational study of effects of contact resistance on a large‐scale vanadium redox flow battery stack

Computational models are developed to allow for a deeper understanding of design factors that affect the lifetime of a vanadium redox flow battery (VRFB) stack, particularly related with the contact‐resistance issue of end cells in a large‐scale stack. A simplified microcontact‐resistance model and a physics‐based macrocontact‐resistance model are constructed to investigate the effect of contact resistance on the performance and longevity of VRFB stacks. A microcontact‐resistance model predicts significant heat accumulation in the current‐collector plate that can result in irreversible damage of plastic materials and an electrical‐voltage loss if the contact resistance is not properly engineered in the stack design. Furthermore, the physics‐based macrocontact‐resistance model investigates abrupt voltage and current distortion in the bipolar plate that is in imperfect contact with the current collector; this results in the local corrosion of the bipolar plate. To ensure a long lifetime of VRFBs, a stack design with minimal contact resistance (less than 0.1 Ω cm²) is required. The structural design of the endplate as well as the selection of a high‐stiffness material is critical to mitigate the bending issue and reduce contact resistance.     

Dynamic behavior of PEM fuel cell and microturbine power plants

This paper presents a comparison between the dynamic behavior of a 250 kW stand-alone proton exchange membrane fuel cell power plant (PEM FCPP) and a 250 kW stand-alone microturbine (MT). Dynamic models for the two are introduced. To control the voltage and the power output of the PEM FCPP, voltage and power control loops are added to the model. For the MT, voltage, speed, and power control are used. Dynamic models are used to determine the response of the PEM FCPP and MT to a load step change. Simulation results indicate that the response of the MT to reach a steady state is about twice as fast as the PEM FCPP. For stand-alone operation of a PEM FCPP, a set of batteries or ultracapacitors is needed in order to satisfy the power mismatch during transient periods. Software simulation results are obtained by using MATLAB^®, Simulink^®, and SimPowerSystems^®.     

Monolithic flat tubular types of solid oxide fuel cells with integrated electrode and gas channels

A new monolithic solid oxide fuel cell (SOFC) design stacked with flatten tubes of unit cells without using metallic interconnector plate is introduced and evaluated in this study. The anode support is manufactured in a flat tubular shape with fuel channel inside and air gas channel on the cathode surface. This design allows all-ceramic stack to provide flow channels and electrical connection between unit cells without needing metal plates. This structure not only greatly reduces the production cost of SOFC stack, but also fundamentally avoids chromium poisoning originated from a metal plate, thereby improving stack stability. The fuel channel was created in the extrusion process by using the outlet shape of mold. The air channel was created by grinding the surface of pre-sintered support. The anode functional layer and electrolyte were dip-coated on the support. The cathode layer and ceramic interconnector were then spray coated. The maximum power density and total resistance of unit cell with an active area of 30 cm² at 800 °C were 498 mW/cm² and 0.67 Ωcm², respectively. A 5-cell stack was assembled with ceramic components only without metal plates. Its maximum power output at 750 °C was 46 W with degradation rate of 0.69%/kh during severe operation condition for more than 1000 h, proving that such all-ceramic stack is a strong candidate as novel SOFC stack design.     

Hydrogen PEMFC stack performance analysis through experimental study of operating parameters by using response surface methodology (RSM)

Although many studies have been done on finding operating conditions of hydrogen-fed fuel cells before, it remains one of the most critical points in determining its parameters in the process. So this paper aims to investigate experimentally the reactant gases flow rate and cell voltage which have a significant impact on the current density of a 3-cell Proton Exchange Membrane fuel cell stack having a 150 cm² active layer. In this case, to determine the optimum values, Design of Experiment and Response Surface Methodology was applied to the experimental system at low 1.5 V, medium 1.8 V, and high 2.1 V. Then, they were compared with each other. In this context, keeping the hydrogen flow rate low and obtaining high current density is one of the main targets; at low voltage values, it was concluded that the flow rate should be increased due to the reaction rate increases with temperature. In general, the effect of humidification and cell temperature on performance was seen more prominently at 1.8 V. The highest current density values that were 313.66 mA/cm², 336.75 mA/cm², and 323.48 mA/cm², respectively, were reached at flow rates of 1 L/min,1.3 L/min,1.6 L/min.     

Effect of thermal cycling on durability of a solid oxide fuel cell stack with external manifold structure

A 5-cell stack with external manifold is thermal cycled between room temperature and 750 °C fifteen times. The electric performances after each cycle are measured and compared. The stack has an initial peak output of 328.44 W and shows excellent stability in thermal cycling. The average operating voltage degradation rate is only 0.8% corresponding each thermal cycle. A cell from the stack is randomly chosen for electrochemical evaluation. Its performance is found to be comparable to a cell which is not thermal cycled. Post-test examination shows deterioration of cathode contact materials at points of contact and cracks throughout the oxide layer between corrugated and bipolar plates to be the main causes of the degradation.     

Characterization of electrochemical reaction and thermo-fluid flow in metal-supported solid oxide fuel cell stacks with various manifold designs

The metal-supported solid oxide fuel cell (SOFC), in which a metal plate is bonded to a ceramic cell, was recently introduced as a new fuel cell design. Metal-supported SOFCs do not suffer from gas leakage, because the metal plates are welded to the metallic interconnects, which also provides high mechanical strength. However, the bonding layer existing between the interconnect and the ceramic cell increases mass transfer resistance, resulting in decreased performance. To better understand and control the mass transfer rate, the manifold structure of the fuel cell stack as well as the channel design in each single cell should be studied. Using a numerical approach, physical property models, governing equations and electrochemical reaction models were calculated simultaneously. The experimentally measured current density–voltage curves were compared with the simulation data to validate the code. Current densities, temperatures and pressure distributions resulting from various manifold designs were presented as numerical results. The parallel manifold design displayed an average current density of 2820.1 A/m² and a relatively uniform current density distribution. The serpentine design yielded the highest average current density among the studied manifold designs, but the maximum pressure was 32 times higher than with the parallel design. Moreover, the large temperature difference observed with the serpentine design may result in a thermal expansion problem. The expanding manifold design yielded an average current density of 2885.9 A/m² and a maximum pressure of 6350 Pa. The pressure distribution with this manifold design was clearly related to the manifold structure. The tapering manifold design is the opposite of the expanding manifold; with this design, the average current density and maximum pressure were slightly lower than the expanding manifold. The dual-flow hybrid manifold design combines two different manifold structures: a serpentine hydrogen manifold and a parallel air manifold. The dual-flow hybrid design yielded an average current density of 2905.4 A/m² and a maximum pressure of 750 Pa.     

Shunt current loss of the vanadium redox flow battery

The shunt current loss is one of main factors to affect the performance of the vanadium redox flow battery, which will shorten the cycle life and decrease the energy transfer efficiency. In this paper, a stack-level model based on the circuit analog method is proposed to research the shunt current loss of the vanadium redox flow battery, in which the SOC (state of charge) of electrolyte is introduced. The distribution of shunt current is described in detail. The sensitive analysis of shunt current is reported. The shunt current loss in charge/discharge cycle is predicted with the given experimental data. The effect of charge/discharge pattern on the shunt current loss is studied. The result shows that the reduction of the number of single cells in series, the decrease of the resistances of manifold and channel and the increase of the power of single cell will be the further development for the VRFB stack.     

Modelling of polymer electrolyte membrane fuel cell stacks based on a hydraulic network approach

Polymer electrolyte membrane (PEM) fuel cells convert the chemical energy of hydrogen and oxygen directly into electrical energy. Waste heat and water are the reaction by‐products, making PEM fuel cells a promising zero‐emission power source for transportation and stationary co‐generation applications. In this study, a mathematical model of a PEM fuel cell stack is formulated. The distributions of the pressure and mass flow rate for the fuel and oxidant streams in the stack are determined with a hydraulic network analysis. Using these distributions as operating conditions, the performance of each cell in the stack is determined with a mathematical, single cell model that has been developed previously. The stack model has been applied to PEM fuel cell stacks with two common stack configurations: the U and Z stack design. The former is designed such that the reactant streams enter and exit the stack on the same end, while the latter has reactant streams entering and exiting on opposite ends. The stack analysed consists of 50 individual active cells with fully humidified H₂ or reformate as fuel and humidified O₂ or air as the oxidant. It is found that the average voltage of the cells in the stack is lower than the voltage of the cell operating individually, and this difference in the cell performance is significantly larger for reformate/air reactants when compared to the H₂/O₂ reactants. It is observed that the performance degradation for cells operating within a stack results from the unequal distribution of reactant mass flow among the cells in the stack. It is shown that strategies for performance improvement rely on obtaining a uniform reactant distribution within the stack, and include increasing stack manifold size, decreasing the number of gas flow channels per bipolar plate, and judicially varying the resistance to mass flow in the gas flow channels from cell to cell. Copyright © 2004 John Wiley & Sons, Ltd.     

Load sharing using fuzzy logic control in a fuel cell/ultracapacitor hybrid vehicle

Fuel cell (FC) and ultracapacitor (UC) based hybrid power systems appear to be very promising for satisfying high energy and high power requirements of vehicular applications. The improvement in control strategies enhances dynamic response of the FC/UC hybrid vehicular power system under various load conditions. In this study, FC system and UC bank supply power demand using a current-fed full bridge dc–dc converter and a bidirectional dc–dc converter, respectively. We focus on a novel fuzzy logic control algorithm integrated into the power conditioning unit (PCU) for the hybrid system. The control strategy is capable of determining the desired FC power and keeps the dc voltage around its nominal value by supplying propulsion power and recuperating braking energy. Simulation results obtained using MATLAB^® & Simulink^® and ADVISOR^® are presented to verify the effectiveness of the proposed control algorithm.     

Development and preliminary testing of a unitized regenerative fuel cell based on PEM technology

Results related to the development and testing of a unitized regenerative fuel cell (URFC) based on proton-exchange membrane (PEM) technology are reported. A URFC is an electrochemical device which can operate either as an electrolyser for the production of hydrogen and oxygen (water electrolysis mode) or as a H₂/O₂ fuel cell for the production of electricity and heat (fuel cell mode). The URFC stack described in this paper is made of seven electrochemical cells (256 cм² active area each). The nominal electric power consumption in electrolysis mode is of 1.5 kW and the nominal electric power production in fuel cell mode is 0.5 kW. A mean cell voltage of 1.74 V has been measured during water electrolysis at 0.5 A cm⁻² (85% efficiency based on the thermoneutral voltage of the water splitting reaction) and a mean cell voltage of 0.55 V has been measured during fuel cell operation at the same current density (37% electric efficiency based on the thermoneutral voltage). Preliminary stability tests are satisfactory. Further tests are scheduled to assess the potentialities of the stack on the long term.     

Parallel operation characteristics of PEM fuel cell and microturbine power plants

This paper reports on the dynamic behavior of a 250 kW proton exchange membrane fuel cell power plant (PEM FCPP) and a 250 kW microturbine (MT) when operating in parallel. A load sharing control scheme is used to distribute the load equally between the PEM FCPP and the MT. For stand alone operation of a PEM FCPP, a set of batteries or ultracapacitors are needed in order to satisfy the power mismatch during transient periods. Using MT in parallel with the PEM FCPP helps in eliminating the need for storage devices. Models for the PEM FCPP and the MT with power, voltage and speed controls are used to determine the dynamic response of the system to a step change in the load. Simulation results indicate viability of parallel operation of the PEM FCPP and the MT. These results are obtained using MATLAB^®, Simulink^®, and SimPowerSystems^®.     

Performance tests of a double-passive μDMFC stack with parallel/dendrite flow field

We report an experimental study on the effect of different flow fields on the cell performance of a double-passive (both anode/cathode) μDMFC stack. Cell performance measurements were made and analyzed for seven different flow field combinations at the anode/cathode of a passive micro direct methanol fuel cell (μDMFC) stack. An optimum flow field combination, after taking a series of tests under different operating conditions, was obtained. The results show that the conventional parallel type flow field used at the anode with an innovative/new dendrite perforated type of 80° flow field can provide the best power density for both single cell and 8-cell stack which have a power density of 16.9 mA/cm² at 50 °C and 1 M methanol solution. Moreover, for an 8-cell stack, both the gravimetric and volumetric power densities can be up to 7.4 W/kg and 37.2 W/L, respectively.