Journal of Power Sources 158 (2006) 303–315 Control strategy for a solid oxide fuel cell and gas turbine hybrid system Christoph Stiller a,∗ , Bjørn Thorud a , Olav Bolland a , Rambabu Kandepu b , Lars Imsland b a Department of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjorn Hejes vei 1B, NO-7491 Trondheim, Norway b Department of Engineering Cybernetics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway Received 8 April 2005; received in revised form 27 August 2005; accepted 6 September 2005 Available online 19 October 2005 Abstract This paper presents a multi-loop control strategy for a SOFC/GT hybrid system. A detailed dynamic model of the system is presented and its part-load performance is studied. The control objectives are discussed, with the main issue being a fairly constant fuel cell temperature under all conditions. Based on the system configuration and part-load performance, input and output variables of the control system are detected. Control cycles are introduced and their design is discussed. The responses of the resulting system on load changes, external disturbances as well as malfunction and degradation incidents are investigated. The system is stable under all incidents. An error in fuel flow measurement or assumed fuel quality provokes a steady-state fuel cell temperature offset. For a degraded system, it may be advisable to readjust the control system to the new characteristics. © 2005 Elsevier B.V. All rights reserved. Keywords: SOFC; Hybrid cycle; Modelling; Part-load; Control 1. Introduction A large amount of modelling work [1–4] and a demonstra- tion plant [5] have proven that a solid oxide fuel cell (SOFC) integrated with a gas turbine (GT) has a potential for high effi- ciency electricity production with low environmental emissions. The good scalability of such systems makes them especially advantageous for distributed generation. Provided that quick load-following is feasible, stand alone power generation is a possible application. However, SOFC/GT hybrid systems face many challenges when it comes to load change and part-load operation. A gas turbine alone has good dynamic properties, but part-load perfor- mance can be rather poor. At any operation point, compressor surge must be prevented. A SOFC is generally able to respond quickly to load changes [6], but it might be destroyed or seri- ously degraded either due to thermally induced stresses caused Abbreviations: SOFC, solid oxide fuel cell; GT, gas turbine; FU, fuel util- isation; AU, air utilisation; TIT, turbine inlet temperature; TOT, turbine outlet temperature; IIR, indirect internal reforming ∗ Corresponding author. Tel.: +47 735 93723; fax: +47 735 98390. E-mail addresses: christoph.stiller@ntnu.no (C. Stiller), bjorn.thorud@ntnu.no (B. Thorud), olav.bolland@ntnu.no (O. Bolland), rambabu.kandepu@ntnu.no (R. Kandepu), lars.imsland@ntnu.no (L. Imsland). by different thermal expansion coefficients in the cell materials or from carbon deposition at the anode. Another phenomenon that may occur during load change is backflow of gas from the burner to the anode cycle, exposing the anode to oxygen. These incidents must not occur in any operation instance. Furthermore, for high efficiency and low degradation of the fuel cell due to thermal cycling, the fuel cell temperature should remain fairly constant during operation. The fulfilment of the mentioned tasks requires a comprehensive control strategy. Results from part-load operation modelling have already been discussed by some authors. Costamagna et al. [7] investigated a hybrid system using a non-dimensional tubular SOFC model. In all simulations they assumed constant fuel utilisation (FU). If shaft speed was assumed constant, power output could only be controlled by varying the fuel flow. These simulations showed large variations in air utilisation (AU) and loss of efficiency for fixed shaft speed when operating at part-load. For variable shaft speed, however, AU and FU as well as SOFC inlet temperatures could remain fairly constant in part-load operation with only a small penalty on system efficiency. This effect was mainly due to increased recuperator efficiency owing to reduced air flow rate. Campanari [8] also used a non-dimensional tubular SOFC model to investigate the hybrid system. Assuming constant FU of 80%, for constant shaft speed he suggested reducing AU and 0378-7753/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2005.09.010