J. Wilhelm

Development of direct methanol fuel cell systems for material handling applications

Direct methanol fuel cells (DMFCs) are attractive for various applications, above all, however, as replacements for batteries or accumulators. They may be used in different power classes. A market analysis indicated that the use of a DMFC energy system in the kW class had the best chance of commercial realization if applied in forklift trucks for material handling in large distribution centers or warehouses. An advantage of such energy systems is that there is no need for the relatively time-consuming recharging of the lead-acid batteries, nor is it necessary to have spare batteries available for multishift operation. This calls for DMFC energy systems that are capable of replacing the existing Pb accumulators in terms of space requirements and energy. However, this requires considerable improvements to be made in terms of power and stability over time of DMFC systems and, in comparison to their present status, an increase of overall efficiency. Recent cost analyses for the overall system; for example, show that for the DMFC stack, a durability of at least 5000 h must be achieved with an overall efficiency for the DMFC system of at least 30%, with the constraint that the system can be operated in a waterautonomous manner up to an ambient temperature of 35 °C. As part of a joint R&D project with industrial partners, two systems were constructed and each subjected to long-term testing for 3000 and more than 8000 h, respectively, with realistic load profiles from driving cycles. In this test, the stack from the first system, DMFC V 3.3―1, displayed an aging rate of approximately 52 μ V h ―1 at a current density of 100 mA cm ―2 . This corresponds to a performance degradation of 25% over a period of 3,000 h. The DMFC V 3.3―2 system, a modified and optimized version of the first system, also underwent long-term testing. In this case, the aging rate of the stack was only approximately 9 μ V h ―1 at a current density of 100 mA cm ―2 . The system has thus been operated to date for more than 8000 h under realistic load profiles.

Horizontal order picker driven by a direct methanol fuel cell

Direct methanol fuel cells (DMFCs) directly convert liquid methanol into electric energy. In addition to the very high energy density of methanol, the DMFC is characterized by easy handling and unproblematic refuelling. Direct methanol fuel cells are attractive for various applications, for example as replacements for batteries or accumulators, since DMFC systems permit longer operating times due to the high energy density of methanol. A market analysis was performed to find the best commercial application for the future. The results of this market analysis show that the best potential for systems lies in the material handling sector. Therefore, a DMFC system was designed for a horizontal order picker. In this paper, the system will be described in terms of process engineering and hybridization. For hybridization, the focus is on the used energy storage. The control strategy of the DMFC system was also described.

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.