Part-load, startup, and shutdown strategies of a solid oxide fuel cell-gas turbine hybrid system

Abstract

Current work on the performance of a solid oxide fuel cell (SOFC) and gas turbine hybrid system is presented. Each component model developed and applied is mathematically defined. The electrochemical performance of single SOFC with different fuels is tested. Experimental results are used to validate the SOFC mathematical model. Based on the simulation model, a safe operation regime of the hybrid system is accurately plotted first. Three different part-load strategies are introduced and used to analyze the part-load performance of the hybrid system using the safe regime. Another major objective of this paper is to introduce a suitable startup and shutdown strategy for the hybrid system. The sequences for the startup and shutdown are proposed in detail, and the system responses are acquired with the simulation model. Hydrogen is used instead of methane during the startup and shutdown process. Thus, the supply of externally generated steam is not needed for the reforming reaction. The gas turbine is driven by complementary fuel and supplies compressed air to heat up or cool down the SOFC stack operating temperature. The dynamic simulation results show that smooth cooling and heating of the cell stack can be accomplished without external electrical power.

Key words： solid oxide fuel cell (SOFC); hybrid system; part-load strategy; startup; shutdown

Acknowledgments

The present work is supported by the National Basic Research Program of China (973 Program) (No. 2010CB227301).

Notations
A_{\mathrm{e}\mathrm{l}}	Active surface of fuel cell/m2
C_{\mathrm{P}\mathrm{R}}	Specific internal energy/(kJ·kg-1)
c	Heat capacity/(J·kg-1·K-1) or mole concentrations of species
F	Faraday constant/(C·mol-1) or molar flow rate/(mol·s-1)
G	Gas flow rate/(kg·s-1)
h	Heat transfer coefficient (W·m-2·K-1) or channel height (m) or specific enthalpy (kJ·kg-1)
\mathrm{▵}h	Enthalpy change of the reaction/(J·mol-1)
K	Equilibrium constant of reaction
k	Thermal conductivity/(W·K-1·m-1) or channel heat transfer coefficient/(kJ·m-2·s-1·K-1)
I	current density/(A·m-2)
\dot{m}	Flow rate
P	Pressure/Pa or power density/(W·m-2)
p	Partial pressure/Pa
Q	Heat flow/W
R	Gas constant/(J·mol-1·K-1) or resistance/\mathrm{\Omega }
\dot{r}	Reaction rate/(mol·s-1)
T	Temperature/K
U	Voltage/V
u	Flow velocities/(m·s-1)
W	Power
x	Mole fraction or direction of air flow
Greek symbols
\eta	Efficiency
\rho	Density/(kg·m-3) or ohmic resistance/( \mathrm{\Omega }\cdot m )
ϵ	Compression pressure ratio or radiation coefficient
\delta	Expansion pressure ratio or thickness/m
\tau	Thickness/m
\sigma	Stefan-Boltzmann constant/(5.67 \times 10-8 W·m-2·K-4)
\lambda	Thermal conductivity/(kW·m-1·K)
Subscripts
C	Compressor
Fu	Fuel
F	Fuel channel
PEN	Positive-Electrolyte-Negative
Op	Over potential
Ocp	Open circuit potential
Ref	Reforming reaction
Shi	Water-gas shift reaction
Elc	Electrochemical reaction
T	Turbine

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