High-Efficiency Active DMFC System for Portable Applications

We propose a highly efficient active direct-methanol fuel cell system for portable application in this paper. The system is composed of a fuel cell stack, mechanical parts, a smart battery, and circuit parts. The mechanical parts consist of several pumps, a mixer, a liquid separator, and heat exchange modules. The battery enables the operation of the mechanical parts and powers the output load during the start-up state. It can also improve the system dynamic characteristics by providing or absorbing the power when the load varies. Furthermore, the battery can also provide peak power at overload. The circuit parts consist of two buck-boost converters and a control circuit. The first-stage buck-boost converter maintains the constant stack current and charges the battery. The second-stage buck-boost converter regulates the output voltage. Buck-boost converters use synchronous rectifiers to increase the power conversion efficiency. In order to ensure a highly efficient and a highly reliable operation, four operating modes (battery discharging mode, start-up mode, hybrid mode, and shut-down mode) are defined and operating algorithms are proposed. These control modes facilitate the cold start at start-up and increase fuel efficiency. All control functions are implemented in software with a single-chip microcontroller MSP430F5438. A 25-W prototype is implemented to support the theoretical analysis.