Determination of an Empirical Transfer Function of a Solid Oxide Fuel Cell Gas Turbine Hybrid System Via Frequency Response Analysis

Abstract

This paper presents the study of the effect variations in the heat effluence from a solid oxide fuel cell (SOFC) has on a gas turbine hybrid configuration. The SOFC is simulated through hardware at the U.S. Department of Energy, National Energy Technology Laboratory (NETL). The gas turbine, compressor, recuperative heat exchanger, and other balance of plant components are represented by actual hardware in the Hybrid Performance Test Facility at NETL. Fuel cell heat exhaust is represented by a combustor that is activated by a fuel cell model that computes energy release for various sensed system states System structure is derived by means of frequency response data generated by the sinusoidal oscillation of the combustor fuel valve over a range of frequencies covering three orders of magnitude. System delay and order are obtained from Bode plots of the magnitude and phase relationships between input and output parameters. Transfer functions for mass flow, temperature, pressure, and other states of interest are derived as a function of fuel valve flow, representative of fuel cell thermal effluent. The Bode plots can validate existing analytical transfer functions, provide steady state error detection, give a stability margin criterion for the fuel valve input, estimate system bandwidth, identify any nonminimum phase system behavior, pinpoint unstable frequencies, and serve as an element of a piecewise transfer function in the development of an overall transfer function matrix covering all system inputs and outputs of interest. Further loop shaping techniques and state space representation can be applied to this matrix in a multivariate control algorithm.

Multiple-Model Adaptive Control of a Hybrid Solid Oxide Fuel Cell Gas Turbine Power Plant Simulator

“…The TF matrices used in this study are those related to the cold air (CA) bypass valve and the electrical load (EL) actuation. These BoP inputs have the most weight in the thermal management of the system, and are essential to the performance and efficiency of the FC hybrid [7,15]. The inputs u 1 and u 2 in Eqs.…”

“…A unique feature of having a cyber-physical plant is the ability to derive empirical models of all the operating points of the entire system. The previous work has identified multiple operating points with the use of transfer functions [15]. These transfer function (TF) matrices can be derived with ease for the entire operating envelope using classical identification techniques, by modulating the BoP actuators, one at a time at different frequencies, or with the use of open-loop tests [14].…”

Experimental characterization of fuel cell gas turbine power system and determination of optimal trajectories of operation using a model predictive controller

“…The system was composed of a GT cycle coupled to an atmospheric FC. Tsai et al [26], [27], [28], and [29] disturbances, and optimize operation.…”

“…The second case uses transfer functions (TFs) identified previously by Tsai ([26], [27], [28], and [29]). These TFs were identified to linearize the HyPer plant around an operating point, in order to design a multivariable control system.…”

“…27 shows the measured output and input data for a case in which the electric load was increased from 0kW to 25kW in approximately one second. This change in load decreases the turbine speed.…”