Modeling and Control of a Sofc-gt Hybrid System with Single Shaft Configuration

This article focuses on issues related to control and operability of a Solid Oxide Fuel Cell (SOFC) Gas Turbine (GT) hybrid system with single-shaft GT configuration. The models of all the components of the hybrid system are developed and integrated to constitute the hybrid system. An autonomous power grid is modeled as load. The main control objectives considered are control of Fuel Utilization (FU) in the SOFC and SOFC solid temperature during dynamic operation of the hybrid system. INTRODUCTION In the foreseeable future, fossil fuels including natural gas will be a major source of energy. With today’s increasing concern about global warming and climate change, there is an incentive to investigate natural gas power processes that operate efficiently, thus emitting less per kWh produced, and also power production processes with CO2 capture capabilities. It is widely accepted that fuel cells are power sources that will become increasingly important, due to high efficiency, low levels of pollution and noise, and high reliability. One of the most promising fuel cell technologies is the Solid Oxide Fuel Cell (SOFC), due to its solid state design and internal reforming of gaseous fuels, in addition to its high efficiency [1]. The SOFC converts the chemical energy of a fuel directly to electrical energy. Since SOFCs operate at high temperatures (about 1000 C), natural gas can be used directly as fuel. The electrical efficiency of a SOFC can reach 55%. Another significant advantage of the SOFC is that since it operates at high temperature and its efficiency increases when pressurized, and it naturally lends itself as a heat source for a gas turbine (GT) cycle. The combined (hybrid) cycle can theoretically have an overall electrical efficiency of up to 70% with a power range from a few hundred kWs to a few MWs. The main applications of the hybrid system include remote area power supply and distributed power generation. There are several models available in literature for the SOFC-GT hybrid system [2], [3], [4], [5]. In [6], a dynamic model of grid connected SOFC model is developed. However, the SOFC-GT hybrid system with single shaft configuration is integrated in an autonomous power system. The reason for procuring an integrated model is to obtain a comprehensive understanding of the operability of the system which has close dynamic interactions between the power generation system and the local grid. Further, the hybrid system consists of tightly integrated dynamic subsystems with strict operating criteria making the control design more challenging in terms of disturbance rejection, part load operation and in particular startup, shut down and load shedding. Suitable system actuation must be chosen, good control structures must be devised, and good controllers must be designed. As a basis for all these tasks, control relevant models must be developed for the subsystems, and for the total system. Such models should have limited complexity to allow for the necessary analysis, and at the same time should include the important dynamic interactions. In this paper we present an integrated model of a SOFC-GT hybrid system with a power grid connecting to an electrical load. The process is described on a system level and modeling of each component is discussed briefly. The model is subsequently used to perform analysis of system dynamics and