Abstract
Controls of a PEM fuel cell stack are one of the crucial issues for securing efficient and durable operations of the stack. When the air is excessively supplied, the efficiency of the system drops. Conversely, insufficient supply of the air causes oxygen starvation at a dynamically varying load. In particular, proper cooling strategy ensures rejecting heat produced and prevents any thermal stress on thin layers of cells. Excessive cooling decreases the working temperature and consequently drops efficiency of the stack. In contrast, the thermal stress imposed by insufficient cooling may reduce the lifespan of the layers.Design of controls needs a model for the plant that sufficiently represents its dynamics. Current models available are either empirical or computationally intensive, which do not allow for analysis of a stack behavior and associated controls. The paper addresses development of a high dynamic model for a stack that is based on transport of charges, flow of fuels and byproducts taken into temperature effects. The stack is constructed with single cells composed of sandwiched multiple layers that are thermally and electrically coupled. Air is supplied by a blower, which voltage is regulated. Two representing control strategies for the air supply system are designed and compared. Heat is rejected by a thermal circuit that consists of a pump, a three-way valve, and a radiator with a fan and a reservoir. In order to control the coolant flow rate, a linear cascade and a state feedback control are designed and compared, which includes a feed-forward function that is derived from load profile. In addition, the temperature effect on air flow rate is compensated, so that a deviation of the oxygen excess ratio can be suppressed.The dynamics and performance of the designed controllers are evaluated and analyzed by simulations using dynamic fuel cell system models at a multi-step current and a current profile measured at the Federal Urban Driving Schedule. The results show that the control strategy proposed reduces not only temperature rise in the catalyst layer but also the parasitic power needed for operation of the air and coolant pumps maintaining the oxygen excess ratio set.
Highlights
J bbll t bbllThe open circuit voltage (OCV) decreases when the temperature rises
Mass m2 kg m-3 J kg-1 K-1 A s mol-1 m2 A m-2 W m-2 K-1 kg m2 kg
The model used in this paper is based on empirical equations and takes into account three additional major effects – water balance in the membrane, gas dynamics in the gas diffusion layer and temperature distribution in a cell described below
Summary
The OCV decreases when the temperature rises. The 2nd cell has lower temperature than the one in the 5th, which leads to less water vapor. The flow rate of the air blower is given as a function of the angular velocity and pressure and the efficiency as a function of the flow rate and the angular velocity [14]: 2.2 Air Supply System. The air supply system should continuously replenish the air to the fuel cell stack as the load varies. It consists ω ( ω (( ) ) ) Wbbll wbbll ⋅ −200. Dynamic characteristic of the inlet and outlet manifold pressures are described by using the mass conservation equation
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