Hybrid Supercapacitor and Direct Borohydride Fuel Cell DC-DC Converter for Portable Applications

Access to electrical energy is of ever increasing importance in modern society. Devices that require portable power to operate while not connected to a typical grid continue to proliferate and there is a large scale search underway across academia and industry to find the best solutions for providing portable power. Fuel cells offer a great promise in both stand-alone and portable applications. The research presented herein regards a high energy-density fuel cell system paired with a power converter for portable applications. A class of fuel cells known as direct-borohydride fuel cells offer higher energy density than typical proton exchange membrane fuel cells fuel using hydrogen or hydrocarbon fuels. Both the steady state and dynamic electrical characteristics of this fuel cell are studied using V-I curve characterization and electrochemical impedance spectroscopy. To create a regulated, usable power supply, a non-isolated, synchronously switched DC/DC boost converter topology was selected. Using state-space averaging techniques, transfer functions for this converter are derived. With these transfer functions and the fuel cell characterization a custom control loop is designed. Additionally, the inclusion of an input supercapacitor is proven to improve converter transient response and is added to the model. The supercapacitor is also shown to have the added benefit of energy storage, eliminating the need for batteries or additional interface converters for remote startup. A prototype power converter was constructed and the final fuel cell system was tested under various load conditions and dynamics. The control algorithm was implemented digitally via a Texas Instruments TMS320F28035 processor.