Control-relevant modeling and simulation of a SOFC-GT hybrid system

Solid Oxide Fuel Cells (SOFC) integrated in Gas Turbine (GT) cycles (often denoted as 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, studies show that the net electrical efficiency can be increased up to 70% Pålsson et al. (2000). The hybrid system uses natural gas as fuel and the percentage of pollutant flue gases is low compared to conventional power production 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 satisfying disturbance rejection and part load operation. Such a design procedure is usually called an integrated process design, see eg. van Schijndel & Pistikopoulos (1999). 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 low complexity hybrid system model which includes the relevant dynamics for controllability analysis and control design. The paper is outlined as follows: The models of the components of the hybrid system are described. Dynamic simulations and the motivation for developing a control system is presented. The control structure and simulation results with the control structure are reported. A nomenclature can be found at the end of the paper.