Proceedings of ECOS 2005 Trondheim, Norway June 20–22, 2005 CONTROL-RELEVANT SOFC MODELING AND MODEL EVALUATION Rambabu Kandepu ∗ , Lars Imsland ∗ , Bjarne A. Foss ∗ Department of Engineering Cybernetics Christoph Stiller † , Bjørn Thorud † and Olav Bolland † Department of Process Engineering Norwegian University of Science and Technology 7491 Trondheim, Norway ABSTRACT In this paper, a dynamic, lumped model of a Solide Oxide Fuel Cell (SOFC) is described, as a step towards developing control relevant models for a SOFC integrated in a gas turbine process. Several such lumped models can be aggregated to approximate the distributed nature of important variables of the SOFC. The model is evaluated against a distributed dynamic tubular SOFC model. The simulation results confirm that the simple model is able to capture the important dynamics of the SOFC. It is concluded that the simple model can be used for control and operability studies of the hybrid system. Keywords: SOFC, control relevant, fuel cells, modeling INTRODUCTION Solid Oxide Fuel Cells (SOFC) integrated in Gas Turbine (GT) cycles (often denoted hybrid systems) is a promising concept for production of efficient and low-polluting electrical power. The SOFC can produce electric power at an electrical efficiency of about 55%, and when it is combined with a GT, stud- ies show that the net electrical efficiency can be in- creased up to 70% [8]. The hybrid system uses nat- ural gas as fuel and the percentage of pollutant flue gases is low compared to conventional power pro- duction from fossil fuels. Due to the tight integration between the SOFC and the GT in a hybrid system, dynamic operability (and hence control) of the process is a challenge. It is important not only to design a good control system, but also to choose a process design that together with the appropriate control structure allows satis- fying disturbance rejection and part load operation. Such a design procedure is usually called a inte- grated process design, see eg. van Schijndel [15]. To be able to design control structures and analyze dynamic behavior, it is very beneficial to have low complexity models of the components of the hybrid system. Such models are also valuable for online optimization. The aim of this article is to develop a ∗ {Rambabu.Kandepu,Lars.Imsland,Bjarne.A.Foss}@itk.ntnu.no † {Christoph.Stiller,Bjorn.Thorud,Olav.Bolland}@ntnu.no low complexity mechanistic SOFC dynamic model which includes the relevant dynamics for operation in a hybrid system. There are several dynamic, dis- tributed models reported in the literature. For exam- ple, Achenbach [1] developed a three dimensional, dynamic, distributed model for a planar SOFC stack. Chan et al. [4, 3], Thorud et al. [14], Stiller et al. [12] and Magistri et al. [10] all developed dis- tributed, dynamic tubular SOFC models for designs similar to that of Siemens Westinghouse, for use in hybrid systems. In this paper, a simple, lumped, dy- namic model of the SOFC based on mass and energy balances, with methane as fuel, is developed. The modeling approach proposed by Padulles et al. [11] for use in power systems simulation has some sim- ilarities with the approach described in this paper, however, therein hydrogen is used as fuel and only mass balances are considered. The paper is outlined as follows: First a simple con- trol relevant SOFC model is described with no re- gard to the geometric layout (tubular or planar). Ex- tension of the simple lumped model towards captur- ing the distributed nature of the process by aggregat- ing single volumes is explained next. Both SOFC models are evaluated against a distributed tubular SOFC model [14] with a considerably higher com- plexity. The two models are simulated for different realistic scenarios and the corresponding simulation results are presented. Applicability and shortcom- 1139