Fuel Cell System Model: Analysis and Simulation

Abstract

The system model development in Chapters 2 and 3 focuses on the reactant supply systems that include the air flow control, hydrogen feed from a highpressure tank, and the humidification of the reactant feeds. In a direct H₂ system, the PEM fuel cell becomes the main heat source. Due to the low operating temperature in PEMFCs, the dynamics of the stack temperature are considered to be relatively slow and, thus, can be viewed as a separate subsystem. As a result, the stack temperature is considered as a setpoint to the reactant systems. The control inputs are the compressor motor voltage, the hydrogen valve, and the humidifier water injection commands. In this chapter, we integrate into the model a static controller for the humidifier and a proportional controller for the hydrogen tank valve. Note here that when a fuel cell system runs based on compressed H₂ that is stored in cylinders, the air flow dynamics and the humidity management dominate the fuel cell system response. By assuming a perfect controller for the humidification, we decouple the phenomena of the air flow from the humidity. This enables us to focus on the air supply dynamics behavior and its control design. A steady-state analysis of the model presented in Section 4.2 is performed to determine the optimal air flow setpoints in terms of maximum net system power. The result corresponds with the value given in the literature as fuel cell specifications. In addition to the steady-state simulation, the dynamic model developed is also able to simulate the transient behavior of the system. The results from transient simulation are shown in Section 4.3. The transient behaviors agree with experimental data published in the literature.