Design of the Dynamic Power Compensation for PEMFC Distributed Power System

Transient load power may bring damage to the proton exchange membrane fuel cell (PEMFC) and shorten the lifetime of the stack. This paper introduces a dynamic power compensation unit consisting of a bidirectional dc/dc converter and a supercapacitor pack, used for compensating the slow dynamic response of the PEMFC and guaranteeing the operation safety of the fuel cell (FC) during load transitions. In this paper, the characteristics of PEMFC are first studied by an experiment, and then, the target of the dynamic power compensation is set based on the experimental results. Subsequently, an analysis on the dynamic power compensation and derivation of the design target is presented. The controller and the filter design based on these analyses are given. With regard to the hardware realization, the bidirectional dc/dc converter is then introduced. Finally, the experimental results on a 5-kW FC power system with dynamic power compensation are given to verify the theoretical analysis and the design. With the dynamic power compensation unit, the FC only needs to supply a slowly changing output power during the sharp load transition process. A more reliable operation condition can be achieved for the FC.

I. Introduction

In a proton Exchange Membrane Fuel Cell (PEMFC), the output characteristics are limited by the mechanical devices which are used for maintaining the air pressure, the temperature, and the humidity in the cell. Moreover, the time constant of the activation in the electrochemical reaction is hundreds of milliseconds [1]–[4]. For these reasons, the fuel cell (FC) is considered as a slow dynamic response equipment with respect to the transient load requirement. It will take a long time, about several seconds, for the FC to shift from one steady state to another once there is a sharp load variation. During this process, the FC may suffer partial temperature excess or overair pressure, which will shorten its lifetime and cause membrane degradation. In other words, the sharp dynamic load current of the power will affect the FC [5], [6]. Therefore, an auxiliary energy storage device is needed to compensate the transient power of the load. Thus, the FC only needs to supply the slow-changing part of the load power. In other words, a more reliable operation condition can be achieved for the FC [6]–[12].