Energy management for a fuel cell/battery hybrid system

Direct methanol fuel cells (DMFCs) convert liquid methanol directly into electric energy. Due to the high energy density of methanol, they permit a longer operating time. Therefore, DMFC systems are attractive in various applications as replacements for batteries. A market analysis showed that the best application potential is to be found in the material handling sector. A DMFC system was designed for a horizontal order picker, which is in this application class. To fulfill the requirements of a highly fluctuating load profile, the DMFC has to be hybridized. The hybridization concept for the DMFC energy system is described, as well as the dimensioning of the DMFC stack and the battery. The main focus will be placed on the energy management for this fuel cell/battery hybrid system. The energy management strategy is used to control the power flow between the DMFC stack and the battery according to their actual states. Additionally, this strategy has three different aims. The first is to maintain the state of charge of the battery at a constant level. The other two concern avoiding and identifying aging of the DMFC stack. Several control strategies will be analyzed with simulations and experiments.