3D transient multiphysics modelling of a complete high temperature fuel cell system using coupled CFD and FEM

Full commercialisation of the solid oxide fuel cell (SOFC) technology faces many technological challenges that prevent the incorporation of the technology into the global energy sector. The effort to increase the transient thermomechanical reliability of the interacting fuel cell components and the associated fuel cell system requires a comprehensive understanding of the complex multiphysics, occurring within the system. State of the art dynamic fuel cell system modelling comprises sub-models of the assembly, or is based on empirical nature. The present study introduces a transient, coupled 3D computational fluid dynamics/computational solid mechanics model of a complete solid oxide fuel cell system and its experimental validation. The model includes all system components; namely the fuel cell stack, afterburner, pre-reformer, air pre-heater and the auxiliary components. All components are presented in their real geometrical resolution. The capabilities of the 3D system level model are demonstrated by simulating the heating-up process and the critical system locations susceptible to thermomechanically induced stress, over time.     

1D and 3D numerical simulations in PEM fuel cells

The potential of fuel cells for clean and efficient energy conversion is generally recognized.The proton-exchange membrane (PEM) fuel cells are one of the most promising types of fuel cells. Models play an important role in fuel cell development since they enable the understanding of the influence of different parameters on the cell performance allowing a systematic simulation, design and optimization of fuel cells systems. In the present work, one-dimensional and three-dimensional numerical simulations were performed and compared with experimental data obtained in a PEM fuel cell. The 1D model, coupling heat and mass transfer effects, was previously developed and validated by the same authors [1] and [2]. The 3D numerical simulations were obtained using the commercial code FLUENT - PEMFC module.The results show that 1D and 3D model simulations considering just one phase for the water flow are similar, with a slightly better accordance for the 1D model exhibiting a substantially lower CPU time. However both numerical results over predict the fuel cell performance while the 3D simulations reproduce very well the experimental data. The effect of the relative humidity of gases and operation temperature on fuel cell performance was also studied both through the comparison of the polarization curves for the 1D and 3D simulations and experimental data and through the analysis of relevant physical parameters such as the water membrane content and the proton conductivity. A polarization curve with the 1D model is obtained with a CPU time around 5 min, while the 3D computing time is around 24 h. The results show that the 1D model can be used to predict optimal operating conditions in PEMFCs and the general trends of the impact on fuel cell performance of several important physical parameters (such as those related to the water management). The use of the 3D numerical simulations is indicated if more detailed predictions are needed namely the spatial distribution and visualization of various relevant parameters.An important conclusion of this work is the demonstration that a simpler model using low CPU has potential to be used in real-time PEMFC simulations.     

Wind turbine CFD modeling using a correlation-based transitional model

This paper describes the development of a Horizontal Axis Wind Turbine 3D CFD model using the Ansys Fluent solver. The model was developed to predict wind turbine performance and evaluate the capabilities of the 1D model (based on BEM Theory) developed by the authors. The two models were compared in terms of accuracy, predictability and calculation time.The strategy of generating a high quality mesh and optimizing the turbulence models (two equations SST k–ω fully turbulent and four equations Transitional SST models) is presented. In particular, a high quality unstructured 3D grid was generated to optimize spatial discretization and meet turbulence model requirements. The mesh was subsequently converted from a tetrahedral into a polyhedral geometry to considerably reduce the number of cells and better align the cell faces and flow. Polyhedral cells also reduce interpolation errors and false numerical diffusion. The empirical correlations of the Transitional SST turbulence model were modified to improve it for wind turbine applications. A significant number of numerical 2D airfoil tests were implemented to calibrate the turbulence model. The results of these tests were applied to the turbulence model by modifying the local correlation parameters. The same parameters were used in the 3D wind turbine model. A Moving Reference Frame model was used to simulate rotation and evaluate 3D flow along the rotor blades.The numerical results were compared to the fully turbulent SST k–ω simulation data to demonstrate the superior capabilities of the modified Transitional model.The 3D CFD model was validated using NREL PHASE VI experimental data available from scientific literature.An application of the 3D model to a new micro wind turbine is presented at the end of this paper. The micro rotor was designed and optimized using the 1D code and actually built.     

A steady state thermal duct model derived by fin-theory approach and applied on an unglazed solar collector

This paper presents the thermal modelling of an unglazed solar collector (USC) flat panel, with the aim of producing a detailed yet swift thermal steady-state model. The model is analytical, one-dimensional (1D) and derived by a fin-theory approach. It represents the thermal performance of an arbitrary duct with applied boundary conditions equal to those of a flat panel collector. The derived model is meant to be used for efficient optimisation and design of USC flat panels (or similar applications), as well as detailed thermal analysis of temperature fields and heat transfer distributions/variations at steady-state conditions; without requiring a large amount of computational power and time. Detailed surface temperatures are necessary features for durability studies of the surface coating, hence the effect of coating degradation on USC and system performance. The model accuracy and proficiency has been benchmarked against a detailed three-dimensional Finite Difference Model (3D FDM) and two simpler 1D analytical models. Results from the benchmarking test show that the fin-theory model has excellent capabilities of calculating energy performances and fluid temperature profiles, as well as detailed material temperature fields and heat transfer distributions/variations (at steady-state conditions), while still being suitable for component analysis in junction to system simulations as the model is analytical. The accuracy of the model is high in comparison to the 3D FDM (the prime benchmark), as long as the fin-theory assumption prevails (no ‘or negligible’ temperature gradient in the fin perpendicularly to the fin length). Comparison with the other models also shows that when the USC duct material has a high thermal conductivity, the cross-sectional material temperature adopts an isothermal state (for the assessed USC duct geometry), which makes the 1D isothermal model valid. When the USC duct material has a low thermal conductivity, the heat transfer course of events adopts a 1D heat flow that reassembles the conditions of the 1D simple model (for the assessed USC duct geometry); 1D heat flow through the top and bottom fins/sheets as the duct wall reassembles a state of adiabatic condition.     

Dynamic modelling of PEM fuel cell system for simulation and sizing of marine power systems

This article presents a model of a proton exchange membrane fuel cell (PEMFC) system for marine power systems. PEMFC in marine hybrid power sources can have various power ranges and capacities in contrast with vehicle applications. Investigating PEMFCs behaviour and performance for various conditions and configurations is demanded for proper sizing and feasibility studies. Hence, modelling and simulation facilitate understanding the performance of the PEMFC behaviour with various sizes and configurations in power systems. The developed model in this work has a system level fidelity with real time capabilities, which can be utilized for simulator approaches besides quasi-static studies with a power-efficiency curve. Moreover, the model can be used for scaling the PEMFC power range by considering transient responses and corresponding efficiencies. The Bond graph approach as a multi-disciplinary energy based modelling strategy is employed for the PEMFC as a multi domains system. In the end, various PEMFC cell numbers and compressor sizes have been compared with power-efficiency curves and transient responses in a benchmark.     

Evaporation Cooling Using a Bionic Micro Porous Evaporation System

The temperature increase due to incident solar radiation has an adverse impact on the electrical output of photovoltaic (PV) modules. A theoretical model of the fabricated and tested bionic evaporation backside cooling was established and verified by experimental investigation. A microfluidic structure featuring micropores consists of two polymer layers attached on the backside of a PV cell model. The thermal performance of roof-mounted PV modules with rear panel air ventilation was mathematically described and extended by the cooling capabilities of the developed bionic evaporation foil. The results of experimental investigations performed in a roof equivalent test environment consisting of a wind tunnel within a climate chamber are in good accordance to the established model. Experimentally, temperature reductions at low incident solar power of less than 575 W causing an efficiency gain for up to 4.8% have been demonstrated while the model implicates an efficiency increase of 10% for real roof systems at an incident solar radiation of 1000 W.     

Extended description of tunnel junctions for distributed modeling of concentrator multi-junction solar cells

One of the key components of highly efficient multi-junction concentrator solar cells is the tunnel junction interconnection. In this paper, an improved 3D distributed model is presented that considers real operation regimes in a tunnel junction. This advanced model is able to accurately simulate the operation of the solar cell at high concentrations at which the photogenerated current surpasses the peak current of the tunnel junction. Simulations of dual-junction solar cells were carried out with the improved model to illustrate its capabilities and the results have been correlated with experimental data reported in the literature. These simulations show that, under certain circumstances, the solar cell's short circuit current may be slightly higher than the tunnel junction peak current without showing the characteristic dip in the J-V curve. This behavior is caused by the lateral current spreading toward dark regions, which occurs through the anode/p-barrier of the tunnel junction.     

Energy consumption,energy R&D and real GDP in OECD countries with and without oil reserves

The objective of the study is to shed light on the contributions of energy consumption and energy R&D on economic growth. We examine two sets of causal relationship between (1) capital stock, energy consumption and real GDP and (2) capital stock, energy R&D and real GDP using panel-based fully-modified ordinary least squares (FMOLS) and dynamic ordinary least squares (DOLS) for 20 OECD countries over the period of 1980–2010. Since different countries may respond differently to energy consumption and energy R&D, the sample is further divided into two groups: OECD countries with oil reserves and without oil reserves. Similarly, energy consumption and energy R&D are also further divided into fossil fuel energy and renewable energy. The results show that the role of energy R&D should not be overlooked and fossil fuel R&D is found to drive economic growth more than fossil fuel consumption. The findings also show that while capital stock and fossil fuels are the key factors driving economic growth, renewable energy promotes real output, specifically in the countries without oil reserves.     

A fast modelling tool for plate heat exchangers based on depth-averaged equations

In this study, the depth-averaged flow and energy equations for plate heat exchangers are presented. The equations are derived by integrating the original 3D flow and energy equations over the height of the gap between the bottom and top plates. This approach reduces the equations from 3D to 2D but still takes into account the frictions on the surfaces and heat transfer through the plates. The depth-averaging reduces the elapsed time of CFD simulations from hours to minutes. Thus, it is very practicable modelling method in real time design work. 2D CFD simulations with depth-averaged equations are compared with full 3D models for five different corrugation angles and corrugation lengths. The simulation results show that the 2D model predicts with relatively good accuracy the profile of the pressure drop and the temperature change as a function of the corrugation angle and the function of the corrugation length. In order to get more extensive information about the significance of the different geometry parameters on the efficiency of the heat exchanger, we simulated 30 different geometries with the fast 2D model. The results suggest that the temperature change is not as sensitive for the geometrical modifications as the pressure drop.     

A reduced 1D dynamic model of a planar direct internal reforming solid oxide fuel cell for system research

A reduced 1D dynamic model of a planar direct internal reforming SOFC (DIR-SOFC) is presented in this paper for system research by introducing two simplifications. The two simplification strategies employed are called Integration and Average, respectively. The present model is evaluated with a detailed 1D SOFC model, which does not introduce the two simplifications, and a lumped parameter (i.e., 0D) SOFC model. Results show that under the operating conditions investigated the accuracy of the reduced model is not significantly compromised by the two simplifications in prediction of the outlet gas flow rates and molar fractions, the outlet temperatures, and the cell voltage, while its computational time is significantly decreased by them. Moreover, it is quite simple in form. Therefore, the reduced SOFC model is attractive for system research. Compared with the lumped model, the reduced SOFC model is an improvement with regard to accuracy because it takes into account the spatially distributed nature of SOFCs to a certain extent. The discretized node number for solving the reduced model can be taken as an adjustable parameter in modeling, and is determined according to specific modeling requirements.     

电子钻井实时模拟系统

介绍了新型实时钻井模拟系统——电子钻井系统的工作原理。介绍了电子钻井系统核心部分——实时钻井分析模型原理和结构。实时钻井分析模型包括数据质量模型、实时模型、数据流和计算机基础设施、三维可视化等。同时,分析了与温度有关的流动模块、扭矩、机械钻速、井壁稳定性以及孔隙压力等实时模型子模块的工作原理、计算方式、优点以及应用案例等。     

Modeling and optimization of multi-tubular metal hydride beds for efficient hydrogen storage

This work presents a novel systematic approach for the optimal design of a multi-tubular metal hydride tank, containing up to nine tubular metal hydride reactors, used for hydrogen storage. The tank is designed to store enough amount of hydrogen for 25 km range¹, for a fuel cell vehicle. A detailed 3D Cartesian, mathematical model is developed and validated against a 2D cylindrical developed by Kikkinides et al. [1]. The objective is to find the optimal process design so as to increase the overall thermal efficiency, and thus minimize the storage time. Optimization results indicate that almost 90% improvement of the storage time can be achieved, over the case where the tank is not optimized and for a minimum storage capacity of 99% of the maximum value.     

One‐way mesoscale–microscale coupling for the simulation of atmospheric flows over complex terrain

The microscale model WINDIE, initially developed for the simulation of neutral atmospheric flows over complex topography, is here extended to the study of stratified atmospheric flows with Coriolis effects, with particular focus on its application to wind farm development projects. The code now uses E–l and E– ?–l turbulence models, which have shown to be more adequate than the standard E– ? model for the simulation of atmospheric flows. The validation tasks include 1D atmospheric boundary layers from the first two cases produced by the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study: a stably stratified boundary layer and a diurnal‐cycle over land, respectively. To test the applicability of the new code to real situations, a series of simulations were performed of the time‐varying atmospheric flow (a 3‐month period between February and May 2012) over a moderately complex topography in the Portuguese mainland, using the Weather Research and Forecasting with Advanced Research WRF (WRF‐ARW) mesoscale code on a 3 km mesh to produce time‐varying boundary conditions for the microscale code, in a dynamic coupling fashion. Comparisons with sonic anemometer measurements at the hill top and with WRF‐ARW results from a finer horizontal resolution mesh ( Δx,y = 1 ∕ 3 km) showed that the code can adequately simulate real atmospheric flows over complex topography. Copyright © 2014 John Wiley & Sons, Ltd.     

A quasi-two-dimensional electrochemistry modeling tool for planar solid oxide fuel cell stacks

A quasi-two-dimensional numerical model is presented for the efficient computation of the steady-state current density, species concentration, and temperature distributions in planar solid oxide fuel cell stacks. The model reduction techniques, engineering approximations, and numerical procedures used to simulate the stack physics while maintaining adequate computational speed are discussed. The results of the model for benchmark cases with and without on-cell methane reformation are presented with comparisons to results from other research described in the literature. Simulations results for a multi-cell stack have also been demonstrated to show capability of the model on simulating cell to cell variation. The capabilities, performance, and scalability of the model for the study of large multi-cell stacks are then demonstrated.     

Transient analysis of alkaline anion exchange membrane fuel cell anode

Alkaline anion exchange membrane (AAEM) fuel cell has attracted increasing attention in recent years due to its several outstanding advantages over proton exchange membrane (PEM) fuel cell such as fast electrochemical kinetics and friendly alkaline environment for catalysts. In this study, a three-dimensional (3D) half-cell transient model is developed to study the dynamic characteristics of AAEM fuel cell under different step changes of operating conditions. It is found that the current density has significant effects on the distribution of liquid water, while the anode stoichiometric ratio effect is insignificant. More time is needed to reach a steady state when the current density decreases rather than increases, and the similar phenomenon also occurs when the operating temperature decreases rather than increases, however, this effect within a low temperature range becomes insignificant. Moreover, the overshoot and undershoot of water diffusion through membrane can also be observed with the step change of the anode stoichiometric ratio and anode inlet relative humidity. The model prediction also has reasonable agreement with published experimental data. The dynamic behaviors observed in this study are of significant importance to the development of AAEM fuel cells for portable and automotive applications.     

Parameterization and prediction of temporal fuel cell voltage behavior during flooding and drying conditions

This paper describes a simple isothermal two-phase flow dynamic model that predicts the experimentally observed temporal behavior of a proton exchange membrane fuel cell stack. This model is intended for use in embedded real time control where computational simplicity is of critical importance. A reproducible methodology is presented to experimentally identify six (6) tunable physical parameters based on the estimation of the cell voltage, the water vapor transport through the membrane and the accumulation of liquid water in the gas channels. The model equations allow temporal calculation of the species concentrations across the gas diffusion layers, the vapor transport across the membrane, and the degree of flooding within the cell structure. The notion of apparent current density then relates this flooding phenomena to cell performance through a reduction in the cell active area as liquid water accumulates. Despite the oversimplification of many complex phenomena, this model provides a useful tool for predicting the resulting decay in cell voltage over time only after it has been tuned with experimental data. The calibrated model and tuning procedure is demonstrated with a 1.4 kW (24 cell, 300 cm²) stack, using pressure regulated pure hydrogen supplied to a dead-ended anode, under a range of operating conditions typical for multi-cell stacks.     

Investigation of liquid water heterogeneities in large area proton exchange membrane fuel cells using a Darcy two-phase flow model in a multiphysics code

Proper management of the liquid water and heat produced in proton exchange membrane (PEM) fuel cells remains crucial to increase both its performance and durability. In this study, a two-phase flow and multicomponent model, called two-fluid model, is developed in the commercial COMSOL Multiphysics® software to investigate the liquid water heterogeneities in large area PEM fuel cells, considering the real flow fields in the bipolar plate. A macroscopic pseudo-3D multi-layers approach has been chosen and generalized Darcy's relation is used both in the membrane-electrode assembly (MEA) and in the channel. The model considers two-phase flow and gas convection and diffusion coupled with electrochemistry and water transport through the membrane. The numerical results are compared to one-fluid model results and liquid water measurements obtained by neutron imaging for several operating conditions. Finally, according to the good agreement between the two-fluid and experimentation results, the numerical water distribution is examined in each component of the cell, exhibiting very heterogeneous water thickness over the cell surface.     

Neural-net based coordinated stabilizing control for the exciterand governor loops of low head hydropower plants

This paper presents a design technique of a new adaptive optimal controller of the low head hydropower plant using artificial neural networks (ANN). The adaptive controller is to operate in real time to improve the generating unit transients through the exciter input, the guide vane position and the runner blade position. The new design procedure is based on self-organization and the predictive estimation capabilities of neural-nets implemented through the cluster-wise segmented associative memory scheme. The developed neural-net based controller (NNC) whose control signals are adjusted using the on-line measurements, can offer better damping effects for generator oscillations over a wide range of operating conditions than conventional controllers. Digital simulations of hydropower plant equipped with low head Kaplan turbines are performed and the comparisons of conventional excitation-governor state-space optimal control and neural-net based control are presented. Results obtained on the nonlinear mathematical model demonstrate that the effects of the NNC closely agree with those obtained using the state-space multivariable discrete-time optimal controllers     

Performance prediction of proton exchange membrane fuel cell engine thermal management system using 1D and 3D integrating numerical simulation

The cooling system of proton exchange membrane fuel cell (PEMFC) engine was simulated by 1D and 3D collaborative simulation method. Firstly, the resistance characteristics of the flow channel are obtained by simulating the airside flow model. A three-dimensional simulation model including dual fans and radiator is also established to simulate the airflow distribution. The one-dimensional simulation model of 30 kW PEMFC engine cooling system that are mainly composed of a thermostat, water pump, and fan and radiator model is established. Secondly, the heat dissipation performance of the cooling system is calculated by using the coupled simulation model. It is found that the simulation results of the amount of heat transferred are in good agreement with the experimental data by compromising, which proves that the model is reasonable. Finally, the thermal performance of the extreme operating conditions of the PEMFC system is simulated by means of a simulation model. By monitoring the flow of the pump and the fan speed, we can maintain the stack internal heat balances, so that the stack efficient and stable operation. The results demonstrate that the 3D simulation can get the distribution of fluid flow more accurately, while the simulation time of 1D thermal system is short and can guide the matching of heat transfer parts quickly.     

Fouling in a Steam Cracker Convection Section Part 1: A Hybrid CFD-1D Model to Obtain Accurate Tube Wall Temperature Profiles

Abstract

To study fouling in steam cracker convection section tubes, accurate tube wall temperature profiles are needed. In this work, tube wall temperature profiles are calculated using a hybrid model, combining a one-dimensional (1D) process gas side model and a computational fluid dynamics (CFD) flue gas side model. The CFD flue gas side model assures the flue gas side accuracy, accounting for local temperatures, while the 1D process gas side model limits the computational cost. Flow separation in the flue gas side at the upper circumference of each tube suggests the need for a compartmentalized 1D approach. A considerable effect is observed. The hybrid CFD-1D model provides accurate tube wall temperature profiles in a reasonable simulation time, a first step towards simulation-based design of more efficient steam cracker convection sections.