Dieter Froning

Clean combined-cycle SOFC power plant — cell modelling and process analysis

Abstract The design principle of a specially adapted solid-oxide fuel cell power plant for the production of electricity from hydrocarbons without the emission of greenhouse gases is described. To achieve CO 2 separation in the exhaust stream, it is necessary to burn the unused fuel without directly mixing it with air, which would introduce nitrogen. Therefore, the spent fuel is passed over a bank of oxygen ion conducting tubes very similar in configuration to the electrochemical tubes in the main stack of the fuel cell. In such an SOFC system, pure CO 2 is produced without the need for a special CO 2 separation process. After liquefaction, CO 2 can be re-injected into an underground reservoir. A plant simulation model consists of four main parts, that is, turbo-expansion of natural gas, fuel cell stack, periphery of the stack, and CO 2 recompression. A tubular SOFC concept is preferred. The spent fuel leaving the cell tube bundle is burned with pure oxygen instead of air. The oxygen is separated from the air in an additional small tube bundle of oxygen separation tubes. In this process, mixing of CO 2 and N 2 is avoided, so that liquefaction of CO 2 becomes feasible. As a design tool, a computer model for tubular cells with an air feed tube has been developed based on an existing planar model. Plant simulation indicates the main contributors to power production (tubular SOFC, exhaust air expander) and power consumption (air compressor, oxygen separation).

Stochastic Aspects of Mass Transport in Gas Diffusion Layers

The relationship between the 3D morphology of gas-diffusion layers (GDL) of HT-PEFCs and their functionality is analyzed. A stochastic model describing the microstructure of paper-type GDL is combined with the Lattice-Boltzmann method (LBM) to simulate gas transport within the GDL microstructure. Virtual 3D microstructures representing paper-type GDL are generated by a stochastic model, where the binder morphology is systematically modified. On these structures, single phase single component gas flow is computed by the LBM. Quality criteria evaluating the spatial homogeneity of gas supply are introduced and related to the binder morphology. The spatial homogeneity of the gas supply is analyzed by a parametrized stochastic model describing the gas flow at the exit of the GDL. This approach gives insight into the spatial structure of the gas flow at the GDL exit. The quality of gas supply is quantified by characterizing size and arrangement of regions with high gas supply. This stochastic gas flow model predicts the quality of gas supply for further binder morphologies. Analyzing the quality criteria and the stochastic evaluation of the spatial structure of the gas flow field at the GDL exit, it is found that the binder morphology has an essential influence on the gas supply.

Experiences with a CFD Based Two Stage SOFC Stack Modeling Concept and Its Application

The parallel development of kW-range SOFC stacks and their systems at the Forschungszentrum Julich requires distinguished modeling capabilities by validated SOFC models targeting the three-dimensional stack structure itself as well as the entire system. For the full three-dimensional (3D) geometric resolution of a stack, Computational Fluid Dynamics (CFD) models are used whereas a one-dimensional (1D) behavior model based on ordinary differential equations (ODE) covers the system modeling requirements. The CFD model is using advanced data structures to reduce the computing time significantly. However, Computing times in the range of seconds or below required by system modeling are featured only by the 1D model. Both SOFC models are based on the same physical equations for electrochemical conversion and internal methane reforming. The application of these models is shown in practice, focusing on choosing the right model for an application, and how both models were validated against each other and against experiments.

Numerical and Experimental Analysis of a Solid Oxide Fuel Cell Stack

A numerical simulation of the Julich F-design solid oxide fuel cell stack is conducted by means of an original mathematical model. The model is implemented in the open source toolbox, OpenFOAM. This cell/stack level model is a key component of a programme to develop a fully integrated multi-scale fuel cell modelling capability, from nano-scale through to system level. It is intended to share the resulting software with other parties openly. Substantial experimental data is available for the Julich F-design; Stack configuration, flow conditions, interconnector geometries and porous electrode properties prescribed in the numerical model are based on field experiments. The numerical model generates distributions of fluid flow, species concentrations, current density and temperature. The results of the simulations are compared against experimental data obtained from short stacks.

Stability Issues for Fuel Cell Models in the Activation and Concentration Regimes

Code stability is a matter of concern for three-dimensional (3D) fuel cell models operating both at high current density and at high cell voltage. An idealized mathematical model of a fuel cell should converge for all potentiostatic or galvanostatic boundary conditions ranging from open circuit to closed circuit. Many fail to do so, due to (i) fuel or oxygen starvation causing divergence as local partial pressures and mass fractions of fuel or oxidant fall to near zero and (ii) nonlinearities in the Nernst and Butler-Volmer equations near open-circuit conditions. This paper describes in detail, specific numerical methods used to improve the stability of a previously existing fuel cell performance calculation procedure, at both low and high current densities. Four specific techniques are identified. A straight channel operating as a (i) solid oxide and (ii) polymer electrolyte membrane fuel cell is used to illustrate the efficacy of the modifications. (Less)

Stochastic Analysis of the Gas Flow at the Gas Diffusion Layer/Electrode Interface of a High-Temperature Polymer Electrolyte Fuel Cell

In polymer electrolyte fuel cells of the types PEFC, DMFC and HT-PEFC, the gas diffusion layer (GDL) connects the electrodes with the feeding channels of the bipolar plate. The GDL is typically composed of materials based on carbon fibers, e.g., paper, woven or non-woven textiles. Efficient fuel cell operation requires that the electrodes are sufficiently supplied by gaseous fluids from the channels. Also, reaction products must be transported away from the electrodes. The GDL also has to provide electronic contact to the bipolar plates, but its major task is the mass transport of fluids. The gas transport in through-plane direction is simulated in the porous structure of the GDL, represented by stochastic geometries equivalent to the real structure. In order to support multi-scale simulation, effective properties can be calculated from these mesoscale simulation results to provide model parameters for continuum approaches in cell-scale simulations. In this paper, the resulting gas flow is analyzed with statistical methods with the focus on the interface between GDL and electrode. This approach provides the opportunity to detect quantitative relationships between functionality and microstructure and to design virtual GDL materials with improved transport properties. The evaluation of the interface with stochastic methods provides substantiated properties suitable for connecting regions representing fuel cell components of different spatial scales.