FUEL CELL DESIGN AND MODELING: The micro-PEM fuel cell under consideration here is a miniature fuel cell stack fabricated from micro-machined silicon wafers. To achieve the power density required a hydrogen powered fuel cell stack was chosen. The cells use hydrogen and air as reactants and a thin polymer electrolyte as a separator. Polarization data from Chang et al., 2002 was utilized to model the PEM fuel cell. The fuel cell operates at two different points on the polarization curve depending on whether the MAV is climbing (72 W of net power) or cruising (20 W of net power). This represents a significant design challenge as a fuel cell's power output is a nonlinear function of the current draw on the stack. For any given fuel cell there is an optimum operating point for power output as a function of current density. An iterative calculation was performed to determine the best operating points on the polarization curve. The fuel cell stack would consist of 20 cells in series. This allows for a 12V power output during climbing which is appropriate for the electrical motors. An overall volume of 17 ml was obtained.
REFORMER DESIGN AND MODELING: It is not feasible under the present design constraints to carry hydrogen as the fuel for the power system. For this reason, our design utilizes a methanol-water mixture as the fuel. The methanol is reacted to form hydrogen in an onboard reformer. The reformer under consideration is a radial microfluidic packed bed reactor with an integrated micro-vaporizer that utilizes methanol to produce hydrogen of the necessary purity to be used by the micro-PEM fuel cell. Kinetic expressions from Peppley et al., 1999 and Peppley, 2006 were utilized to model the reforming reactions. It was assumed that the reactor was operated isothermally. The pressure drop was modeled via the Ergun equation. The gas viscosity of at any point in the reactor was estimated by using the kinetic theory of gases from Bird et al., 2004. Simulations in MATLAB indicated that a volume of 20 ml was required to produce the necessary hydrogen for both climbing as well as cruising situations. It was shown that the pressure drop in the radial flow reactor was negligible.
HEAT INTEGRATION AND START-UP ISSUES: To achieve appropriate conversion of methanol in the reformer it is necessary to operate the reformer at a temperature of 280oC. To heat the reformer from room temperature to the operating temperature an electrical resistance heater will be used, this same heater will supply power during operation to maintain the reformer temperature at the appropriate level. The PEM fuel cell was initially operated as a direct methanol fuel cell. In this mode, the power generated is not sufficient to climb; however it was shown that sufficient power could be generated to heat the reformer to the desired temperature to produce hydrogen. Once the reformer is at the desired temperature, the PEM fuel cell is operated as a hydrogen fuel cell, which results in sufficient power for climbing as well as cruising. An energy balance on the reformer indicated that to keep the temperature of the reformer at 280oC, it is necessary to provide additional heat. This additional heat requirement requires additional power generation from the fuel cell. It was estimated that a power output of 35 W is necessary during cruising to provide a net power of 20 W for the aircraft motor (the rest goes for providing heat to the reformer). Similarly, a power output of 115 W is necessary during climbing to provide a net power output of 72 W for the aircraft motor. This calculation assumes that heat integration is accomplished, using some of the energy in the reformed gases to preheat the methanol feed to the reformer.
We showed that we could design (37 ml weighing just 54g) micro fuel cell system with very high power density (2,507 W/l and 1,333 W/kg) capable of providing 9 – 14 VDC at up to 6A. This system can be constructed from standard materials with simple processing techniques. The 1.14:1 molar water:methanol mix fuel has very high energy density (1,099 Wh/l and 1,220 Wh/kg) which is much higher than current battery technology.
Bird, R.Byron, Stewart Warren E, Lightfoot Edwin N, Transport Phenomena, 2nd ed. Wiley, New York, NY, 2004
Chang, H., Kim, J. R., Cho, J. H., Kim, H. K., and Choi, K. H., “Materials and processes for small fuel cells,” Solid State Ionics, Volume 148, Issues 3-4, 2 June 2002, 601-606.
Peppley, BA, Amphlett JC, Kearns LM, and Mann RF. “Methanol-Steam Reforming on Cu/ZnO/Al2O3 Catalysts. Part 2. A Comprehensice Kinetic Model.” Applied Catalysis: General A 179 (1999): 31-49
Peppley, B.A. – Personal Communication (2006)