Wei Jiang Ruixian Fang Jamil Khan Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208 Roger Dougal Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208 Control Strategies for Start-Up and Part-Load Operation of Solid Oxide Fuel Cell/Gas Turbine Hybrid System Control strategy plays a significant role in ensuring system stability and performance as well as equipment protection for maximum service life. This work is aimed at investigat- ing the control strategies for start-up and part-load operating conditions of the solid oxide fuel cell/gas turbine (SOFC/GT) hybrid system. First, a dynamic SOFC/GT hybrid cycle, based on the thermodynamic modeling of system components, has been success- fully developed and simulated in the virtual test bed simulation environment. The one- dimensional tubular SOFC model is based on the electrochemical and thermal modeling, accounting for voltage losses and temperature dynamics. The single cell is discretized using a finite volume method where all the governing equations are solved for each finite volume. Two operating conditions, start-up and part load, are employed to investigate the control strategies of the SOFC/GT hybrid cycle. In particular, start-up control is adopted to ensure the initial rotation speed of a compressor and a turbine for a system-level operation. The control objective for the part-load operation regardless of load changes, as proposed, is to maintain constant fuel utilization and a fairly constant SOFC tempera- ture within a small range by manipulating the fuel mass flow and air mass flow. To this end, the dynamic electrical characteristics such as cell voltage, current density, and temperature under the part load are simulated and analyzed. Several feedback control cycles are designed from the dynamic responses of electrical characteristics. Control cycles combined with control related variables are introduced and discussed. DOI: 10.1115/1.3006197 Keywords: power system, SOFC, turbine, hybrid, dynamic simulation 1 Introduction The solid oxide fuel cell SOFC has the distinct advantages of high energy conversion efficiency, low environmental impact, and flexibility of usable fuel types. The high-temperature exhaust gases of the SOFC stack makes it suitable to integrate with a heat engine, such as a gas turbine. The power system based on such a cycle is expected to achieve a higher efficiency and increased power output due to the higher thermal utilization 1,2. One important challenge for SOFC/GT hybrid systems is to design control strategies for the system operating under different working conditions, especially for the part-load operation. It is important to ensure system stability and performance as well as equipment protection for maximum service life. Several authors, with different detail levels of description, have developed and simulated a SOFC model in the past couple of years to study the part-load operation and control strategies. Most models are non- dimensional models 3,4 and cannot predict the temperature and current density profiles inside the cell. Pålsson and Selimovic 5 studied the part-load behavior of a simple nondimensional SOFC cycle by varying the shaft speed and keeping fuel utilization FU constant. Kimijima and Kasagi 6 developed a two-dimensional planar SOFC model for part-load analysis. Besides keeping the turbine inlet temperature TIT constant and varying the shaft speed, they used an air heater/cooler prior to the SOFC to adjust air inlet temperatures at the part-load operation. Stiller et al. 7 and Thorud et al. 8 presented a dynamic model for control of the integrated SOFC and turbine systems, showing that the power supplied by the SOFC system can be controlled by manipulating the fuel. In another work by Aguiar et al. 9, the temperature control of a stack level SOFC model was proposed. Those above mentioned models emphasize the use of process control to en- hance the reliability of SOFC. In this paper, a dynamic SOFC/GT hybrid cycle, based on the thermodynamic modeling of system components, has been devel- oped in the virtual test bed VTB simulation environment 10,11. The one-dimensional tubular SOFC model is based on the electrochemical and thermal modeling, accounting for voltage losses and temperature dynamics. This work is aimed at investigating the control strategies for start-up and part-load operating conditions of the SOFC/GT hy- brid system. The main control objective for the part-load opera- tion is to maintain constant fuel utilization and a fairly constant SOFC temperature within a small range. Important control param- eters, such as cell mean temperature, fuel flow rate, air flow rate, current density, rotational speed of gas turbine, steam carbon S/C ratio, and TIT, are introduced and discussed, followed by detailed control strategies and methods. Several feedback control layouts are introduced. Then the response of the controlled system to the load changes is studied. The remainder of this paper is organized as follows. The mod- els’ description is detailed in Sec. 2. In Sec. 3, the system con- figurations, operating conditions, and important assumptions of the SOFC stack are provided. The control variables and strategies are designed and discussed in detail in Sec. 4. The steady state simulation results analysis and part-load behavior investigation are presented in Sec. 5. Conclusions are finally made in Sec. 6. Manuscript received June 17, 2007; final manuscript received March 11, 2008; published online November 11, 2009. Review conducted by Ben Wilhite. Paper presented at the 5th International Fuel Cell Science Engineering and Technology Conference FUELCELL2007, Brooklyn, NY, June 18–20. Journal of Fuel Cell Science and Technology FEBRUARY 2010, Vol. 7 / 011016-1 Copyright © 2010 by ASME Downloaded 19 Jul 2010 to 129.252.23.139. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm