High-fidelity stack and system modeling for tubular solid oxide fuel cell system design and thermal management

Effective thermal integration of system components is critical to the performance of small-scale (<10 kW) solid oxide fuel cell systems. This paper presents a steady-state design and simulation tool for a highly-integrated tubular SOFC system. The SOFC is modeled using a high fidelity, one-dimensional tube model coupled to a three-dimensional computational fluid dynamics (CFD) model. Recuperative heat exchange between SOFC tail-gas and inlet cathode air and reformer air/fuel preheat processes are captured within the CFD model. Quasi one-dimensional thermal resistance models of the tail-gas combustor (TGC) and catalytic partial oxidation (CPOx) complete the balance of plant (BoP) and SOFC coupling. The simulation tool is demonstrated on a prototype 66-tube SOFC system with 650 W of nominal gross power. Stack cooling predominately occurs at the external surface of the tubes where radiation accounts for 66-92% of heat transfer. A strong relationship develops between the power output of a tube and its view factor to the relatively cold cylinder wall surrounding the bundle. The bundle geometry yields seven view factor groupings which correspond to seven power groupings with tube powers ranging from 7.6-10.8 W. Furthermore, the low effectiveness of the co-flow recuperator contributes to lower tube powers at the bundle outer periphery.     

Numerical analysis of start-up operation of a tubular solid oxide fuel cell

The purpose of the current study is to numerically predict the start up behavior of a tubular solid oxide fuel cell (SOFC) using a 2-D transient model. The developed model provides the transient response of the start-up mode as well as the steady state operation of the SOFC. A code based on finite volume method is utilized to solve the transient nonlinear transport equations of the cell (momentum, species and energy equations). To account for the Ohmic losses and Joule heating of the current that passes through the cell body, a discretized network circuit is adopted. The local electrochemical parameters are calculated based on the local pressure, temperature, and concentrations of the species. At each time step an iterative procedure is used to solve the electrochemical, electrical and transport equations simultaneously. The model predicts the cell output voltage, the local EMF and the state variables (pressure, temperature and species concentration) during the start up. It also predicts the cell heat-up rate for hot input gases as well as the start up time of the SOFC. The results show that the gases mass flow rate and temperature affect the heat-up rate. Also during the start-up, the cell electrical response is about 2.5 times quicker than the cell temperature response. The start-up time for the cell output voltage is about 50 min.     

An error in solid oxide fuel cell stack modeling

This paper points out an error in the literature and analyzes its effect on electrochemical models of solid oxide fuel cell stacks. A correction is presented.     

Numerical investigation of a direct ammonia tubular solid oxide fuel cell in comparison with hydrogen

Nowadays, carbon-rich fuels are the principal energy supply utilized for powering human society, and it will be continued for the next few decades. Connecting with this, modern energy technologies are very essential to convert the available limited carbon-rich fuels and other green alternative energies into useful energy efficiently with an insignificant environmental impression. Amongst all kinds of power generation systems, SOFCs running with high temperatures are emerging as a frontrunner in chemical to electrical transformation efficiency, allows the engagement of all-embracing fuel varieties with negligible environmental impact. This study investigates the effect of ammonia usage in tubular SOFC performance. Firstly, the use of ammonia and hydrogen in the electrolyte-supported SOFC (ES-SOFC) has investigated. Then, the effect of using ammonia in anode-supported SOFC (AS-SOFC), ES-SOFC and cathode-supported SOFC (CS–SOFC) on performance has been examined by using COMSOL software. As a result of the study performed, it is found that the ammonia can be used in tubular SOFC's as a carbon-free fuel and CS-SOFC shows better performance compared with ES-SOFC and AS-SOFC. Besides, the findings of this study indicate that the use of ammonia as a fuel for SOFCs is comparable to the use of hydrogen.     

A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack

A novel design of cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack is presented in this paper. The cone-shaped tubular anode substrates are fabricated by slip casting technique and the yttria-stabilized zirconia (YSZ) electrolyte films are deposited onto the anode tubes by dip coating method. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 7 μm is successfully obtained. The single cell, NiO-YSZ/YSZ (7 μm)/LSM-YSZ, provides a maximum power density of 1.78 W cm⁻² at 800 °C, using moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC is fabricated. Its typical operating characteristics are investigated, particularly with respect to the thermal cycling test. The results show that the two-cell-stack has good thermo-mechanical properties and that the developed segmented-in-series SOFC stack is highly promising for portable applications.     

Effects of operating conditions on the performance of a micro-tubular solid oxide fuel cell (SOFC)

A parametric analysis is carried out to study the effects of the operating conditions on the performance and operation of a micro-tubular solid oxide fuel cell. The computational fluid dynamics model incorporates mass, momentum, species and energy balances along with ionic and electronic charge transfers. Effects of temperature, fuel flow rate, fuel composition, anode pressure and cathode pressure on fuel cell performance are investigated. Polarization curves are compared to allow an understanding of the effects of different operating conditions on the performance of the fuel cell. Effects of anode flow rate on fuel cell efficiency and fuel utilization are also investigated. Moreover, influence of operating temperature on the internal electronic current leaks is outlined. Temperature distributions, current density profiles and hydrogen mole fraction profiles are also utilized to have a better understanding of the spatial effects of operating parameters. It is predicted that at 550 °C, for an output current demand of 0.53 A cm⁻², fuel cell needs to generate 0.65 A cm⁻² ionic current density where the difference in these values is attributed to internal current leaks. On the other hand for temperatures lower than 500 °C, the effect of electronic leakage currents are not significant.     

Two-dimensional temperature distribution estimation for a cross-flow planar solid oxide fuel cell stack

The uniform temperature distribution of a cross-flow planar solid oxide fuel cell (SOFC) stack plays an essential role in stack thermal safety and electrical property. However, because of the strict requirements in stack sealing struture, it is hard to acquire the temperature inside the stack using thermal detection devices within an acceptable cost. Therefore, accurately estimating the two-dimensional (2-D) temperature distribution of the cross-flow stack is crucial for its thermal management. In this paper, Firstly, a 2-D mechanism model of a cross-flow planar SOFC stack is established. The stack is divided into 5*5 nodes along the gas flow directions, which can reflect the stack states with moderate computational burden. Then, experimental test data is utilized to modify and validate the stack model, guaranteeing the model accuracy as well as the reliability of model-based state estimator design. Finally, easily-measured stack inputs and outputs are selected, and a temperature distribution estimator combined with unscented kalman filter (UFK) approach is developed to achieve accurate and fast temperature distribution estimation of a cross-flow SOFC stack. Simulation results demonstrate that the UKF-based temperature distribution estimator can precisely and quickly achieve the temperature distribution estimation of the cross-flow stack under both static state and dynamic state changes and is applicable to cross-flow stacks with different size or cell number as well, the maximum estimated absolute error is less than 0.15 K with an absolute error rate of 0.015%, which indicates the developed estimator has good estimation performances.     

Experimental study of dry reforming of biogas in a tubular anode-supported solid oxide fuel cell

The performance of a tubular Ni/YSZ anode supported SOFC directly fed by an anaerobic digestion simulated biogas, with an extra addition of carbon dioxide to operate in conservative operating conditions to avoid coking on the anode support, was investigated. The fuel cell has been tested at a fixed oven temperature of 800 °C and under biogas/CO₂ mixtures with different volumetric ratios, fuel utilization (FU) and current densities. Polarization curves and performance maps were obtained to better understand the influence of the investigated operational parameters on the cell behavior. Furthermore, since the tubular geometry enables an easy separation of the anode and cathode exhaust gases, the anode off-gas has been collected and monitored through a gas-chromatograph under open circuit voltage to investigate on the catalytic behavior of a Ni-based state-of-the-art anode. For corresponding operative conditions, performances of the cell for biogas/CO₂ 1/1.5 (i.e. CH₄/CO₂ 30/70) and 1/2 (i.e. CH₄/CO₂ 24/76) were at least 2% and 4% lower than the case 1/1 (i.e. CH₄/CO₂ 20/80), respectively. The highest efficiency of 43.4% was reached at 17.5 A and FU = 70%.     

Performance comparison of planar,tubular and Delta8 solid oxide fuel cells using a generalized finite volume model

A generalized, finite volume-based SOFC model is presented which includes charge, mass and heat transport as well as a Langmuir–Hinshelwood type applied kinetic model for steam reforming reactions. The model development aimed at fast applicability to various cell geometries, short calculation times to allow for system analysis calculations and high fuel flexibility. In the first part of the paper, the model approach and assumptions are presented in detail. In the second part, the generalized model is applied to a planar, the standard tubular and the triangular tube cell geometry. The validation against experimental and benchmark test data is stressed to assure comparability of the model results for the three investigated cell designs. In the last part, the three cell designs are compared, highlighting the differences with respect to internal heat- and mass transfer and the impact on the electrochemical performance. It is shown that the performance of the triangular tube cell is almost double that of the standard tubular cell designs. The planar cell is outperformed by the triangular tube cell as well.     

A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells

A direct carbon fuel cell based on a conventional anode-supported tubular solid oxide fuel cell, which consisted of a NiO-YSZ anode support tube, a NiO-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode, has been successfully achieved. It used the carbon black as fuel and oxygen as the oxidant, and a preliminary examination of the DCFC has been carried out. The cell generated an acceptable performance with the maximum power densities of 104, 75, and 47 mW cm⁻² at 850, 800, and 750 °C, respectively. These results demonstrate the feasibility for carbon directly converting to electricity in tubular solid oxide fuel cells.     

Novel solid oxide fuel cell system controller for rapid load following

A novel SOFC system control strategy has been developed for rapid load following. The strategy was motivated from the performance of a baseline control strategy developed from control concepts in the literature. The basis for the fuel cell system control concepts are explained by a simplified order of magnitude time scale analysis. The control concepts are then investigated in a detailed quasi-two-dimensional integrated dynamic system model that resolves the physics of heat transfer, chemical kinetics, mass convection and electrochemistry within the system.     

Application of a detailed dimensional solid oxide fuel cell model in integrated gasification fuel cell system design and analysis

Integrated gasification fuel cell (IGFC) systems that combine coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally sensitive utilization of coal for power production. Most IGFC system analysis efforts performed to-date have employed non-dimensional SOFC models, which predict SOFC performance based upon global mass and energy balances that do not resolve important intrinsic constraints of SOFC operation, such as the limits of internal temperatures and species concentrations. In this work, a detailed dimensional planar SOFC model is applied in IGFC system analysis to investigate these constraints and their implications and effects on the system performance. The analysis results further confirm the need for employing a dimensional SOFC model in IGFC system design. To maintain the SOFC internal temperature within a safe operating range, the required cooling air flow rate is much larger than that predicted by the non-dimensional SOFC model, which results in a larger air compressor design and operating power that significantly reduces the system efficiency. Options to mitigate the challenges introduced by considering the intrinsic constraints of SOFC operation in the analyses and improve IGFC design and operation have also been investigated. Novel design concepts that include staged SOFC stacks and cascading air flow can achieve a system efficiency that is close to that of the baseline analyses, which did not consider the intrinsic SOFC limitations.     

Performance of a biohydrogen solid oxide fuel cell

For solid oxide fuel cells (SOFCs), biohydrogen is an ideal fuel, which introduces a clean renewable energy source to a highly efficient energy conversion technology with minimum complications. The performance of a SOFC working with biohydrogen, and the effects of fuel composition, working temperature, load, and air utilization are less well understood. In this study a comprehensive numerical model was employed to investigate the biohydrogen fueled SOFC in different working conditions. Direct electrochemical oxidation of H₂ and CO and water gas shift reaction (WGSR) were considered in the model. An experimental set up was built to verify the simulation results. Results from the numerical model were validated against experimental polarization curves and cell temperature measurements. The results showed how different parameters affect the performance of a biohydrogen SOFC and how different working conditions can be selected to meet certain criteria.     

A 2D transient numerical model combining heat/mass transport effects in a tubular solid oxide fuel cell

The purpose of this study is to present a 2D transient numerical model to predict the dynamic behavior of a tubular SOFC. In this model, the transient conservation equations (momentum, species and energy equations) are solved numerically and electrical and electrochemical outputs are calculated with an equivalent electrical circuit for the cell. The developed model determines the cell electrical and thermal responses to the variation of load current. Also it predicts the local EMF, state variables (pressure, temperature and species concentration) and cell performance for different cell load currents. Using this comprehensive model the dynamic behavior of Tubular SOFC is studied. First an initial steady state operating condition is set for the SOFC model and then the time response of the fuel cell to changes of some interested input parameters (like electrical load) is analyzed. The simulation starts when the cell is at the steady state in a specific output load. When the load step change takes place, the solution continues to reach to the new steady state condition. Then the cell transient behavior is analyzed. The results show that when the load current is stepped up, the output voltage decreases to a new steady state voltage in about 67 min.     

Experimental activity on two tubular solid oxide fuel cell cogeneration plants in a real industrial environment

The EOS project carries on an industrial research on stationary fuel cells based on the SOFC technology developed by Siemens, through a co-operation between TurboCare S.p.A. and Politecnico di Torino. Two SOFC units have been installed and operated in the TurboCare workshop. The running hours of the CHP100 generator are more than 16,400 h; the 5-kW Alpha6 generator is in operation since more than 10′000 h.     

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.     

Fabrication and characterization of a cathode-supported tubular solid oxide fuel cell

A cathode-supported tubular solid oxide fuel cell (CTSOFC) with the length of 6.0 cm and outside diameter of 1.0 cm has been successfully fabricated via dip-coating and co-sintering techniques. A crack-free electrolyte film with a thickness of ∼14 μm was obtained by co-firing of cathode/cathode active layer/electrolyte/anode at 1250 °C. The relative low densifying temperature for electrolyte was attributed to the large shrinkage of the green tubular which assisted the densification of electrolyte. The assembled cell was electrochemically characterized with humidified H₂ as fuel and O₂ as oxidant. The open circuit voltages (OCV) were 1.1, 1.08 and 1.06 V at 750, 800 and 850 °C, respectively, with the maximum power densities of 157, 272 and 358 mW cm⁻² at corresponding temperatures.     

Feedforward-feedback control of a solid oxide fuel cell power system

One of the main challenges for wide-spread utilization of the solid oxide fuel cell (SOFC) power systems is how to achieve high electrical efficiency without increasing the degradation rate of the fuel cells. To run the SOFC power system at high efficiency over a long period of time, properly designed controllers are indispensable.Although a number of various approaches to control SOFC have been proposed so far, it seems that the design of control system, along with simple tuning procedure, has not been treated in a consistent manner. This issue is addressed in the present paper resulting in a feedforward-feedback control structure. The feedforward part is based on the stoichiometry of electro-oxidation, reforming and combustion reactions, which allow immediate response to variable current demand. The feedback part performs additional fine adjustment of fuel and air supply in order to minimize the undesired system temperatures variations. The selection of pairings of manipulated and controlled variables for control is based on physical knowledge of the system. Input/output pairing for single-loop feedback control is assessed by the relative gain analysis. An efficient procedure for tuning the parameters of the feedback controllers is suggested, relying on simple open-loop step responses of the system.The proposed low-level control is assessed on a detailed physical model of a 2.5 kW SOFC power system by simulating two nonstationary load regimes. Simulations show that the control provides a robust operation under large load variations while meeting the operating constraints. Due to its simplicity, the control is feasible for implementation on a real SOFC system.     

Operation of micro tubular solid oxide fuel cells integrated with propane reformers

This paper presents the operating results of micro tubular solid oxide fuel cells (MT-SOFCs) integrated with propane catalytic partial oxidation (CPOX) reformers. The cells combined with the propane CPOX reformers successfully survived 1000 continuous thermal cycles totaling 1922 h of operation with only a 0.47 mW power loss per cycle, as well as surviving 100 extreme thermal cycles with a maximum ramp rate of 1000 °C.min⁻¹ without any power loss. This excellent thermal shock resistance is due to both the well matched coefficient of thermal expansion (CTE) among all of the cells’ layers as well as the homogenous anode structure present in all of the cells. Additionally, the cells operated on propane for more than 1500 continuous hours with an average degradation rate of 0.067 mW h⁻¹ (0.58%/1000 h). This degradation was attributed to the sintering of the nickel in the anode, degradation of the current collection and the reformer. The fact that the cells showed no sign of delamination, cracking or coking after these tests also proves the successful integration of cell and CPOX reformer. Overall, the 3D printed MT-SOFCs with integrated CPOX reformers exhibited a breakthrough in terms of cell thermal cycling and long-term stability which will significantly advance the development of portable SOFCs systems.     

Cycling of three solid oxide fuel cell types

One of the key problems of SOFCs is their slow start-up and cycling performance which is due to the thermal shock problems of zirconia electrolyte and its associated electrode and interconnect materials. Typical start-up times range from 2 to 6 h. Faster cycles can cause degradation in performance and in material integrity. The purpose of this paper is to study the transient performance of SOFCs under various (e.g. thermal and/or current load) cycling conditions, typifying start-up and shut-down as well as variable working conditions of the systems, in order to understand the degradation mechanisms. Three types of SOFC have been compared; the planar stack with metal interconnects represented by the Forschungszentrum Jülich (FZJ) configuration; the Rolls Royce Fuel Cell Systems Ltd. (RRFCS) integrated planar tube; and the Adelan pure tube. The objective was to cycle in temperature from ambient to the operating condition several times to check if degradation was occurring. To obtain thermally shock resistant systems, cell dimensions had to be reduced to the millimeter scale.