Dynamic modeling of solid oxide fuel cell

In order to operate solid oxide fuel cell (SOFC) systems, it is necessary to investigate dynamic characteristics of SOFC through modeling and simulations. In this thesis, SOFC dynamics is presented in the form of non-linear state-space model (SSM). Performance and responses of SOFC are investigated through simulations. This thesis consists of four stages in solving the problems of interest.; First we investigate how fuel enters the cell surface and produces electricity. Dynamics led by diffusion process and inherent impedance is investigated and modeled. Dynamic correlations between parameters in the primary flow and in the immediate vicinity of the triple phase boundary (tpb) are considered in the form of transfer function as well as ordinary differential equation (ODE). A new equivalent circuit that can emulate both internal and external dynamic characteristics of SOFC is proposed to represent the effect of inherent resistance and double layer capacitance. Through simulations, a phenomenon of slow response of voltage in current interrupt experiment is explained.; In the second stage, we consider transport processes from the cell surroundings to a finite volume of tubular SOFC composite, such as internal reforming/shifting reaction, fluid transport, and heat transfer. Combined with dynamics developed from the first stage, a detailed SSM with 28 states is developed. Mole fractions, temperatures, flow velocities etc. are investigated and dynamically modeled through mass/energy/momentum balance. Dynamic responses of each physical variable to step changes of inlet variables as well as load changes are investigated through simulations.; In the third stage, the dynamic model for the finite volume of tubular SOFC is expanded to a one-dimensional (1-D) dynamic model, in the form of non-linear SSM. With known total current demand, the dynamic current density distribution is developed by solving the equivalent circuit. Non-flowing solid phase variables are dynamically modeled. Dynamics of the flowing phase variables and their distributions are developed in the form of partial differential equations (PDEs).; Aiming to solve the distributed parameter problem approximately, an innovative analytical solution for a 1-D reacting gas flow problem is developed. The solution is applied to the 1-D dynamic model of SOFC. The developed model can reduce computations while maintain reasonable precision. The explicit solution makes the 1-D dynamic model more applicable for further control studies. Dynamic performance and parameter distributions of SOFC are investigated through simulations.; Finally, with the aim for simpler control application, an 2nd order nonlinear lumped parameter SSM is built. Input-output parameters of the SOFC stack are analyzed. Faster processes are approximated by their steady state solutions. Solid phase temperatures are modeled by dynamic equations owing to their slow response and dominant role played in SOFC dynamic responses. Simulations show that the lumped model can reasonably approximate dynamics of the SOFC shown by the detailed model.