Room-temperature fuel cells and their integration into portable and embedded systems

Direct methanol fuel cells (DMFCs) are a promising next-generation energy source for portable applications, due to their high energy density and the ease of handling of the liquid fuel. However, the limited range of output power obtainable from a fuel cell requires hybridization the introduction of a battery to form a stand-alone portable power source. Furthermore, the stringent operating conditions to be met by active DMFC systems mandate complicated balance of plant (BOP) control. We present a complete hybrid active DMFC system design and implementation in which a DMFC stack and a li-ion battery are linked by a hybridization circuit to share the applied load to exploit high energy density of the fuel cell and high power density of the battery. We describe systems for fuel delivery, air supply, temperature management, current and voltage measurement, DC-DC conversion and power distribution, motor driving, battery charge management, DMFC and circuit protection, and control of the DMFC and battery as a hybrid. We have designed and implemented an embedded system controller that consists of a 32-bit microcontroller, running under a real-time operating system, that incorporating multiple cascaded feedback control loops which manage the dynamics of BOP control. We demonstrate reliable and efficient maintenance of a constant fuel cell output current in spite of severe fluctuation of the load current.