Three-Dimensional Simulations Of Ducted Air-Breathing Direct Methanol Fuel Cells

In the cathode of an air-breathing direct methanol fuel cell (DMFC), the transport of oxygen through diffusion and natural convection is accomplished in a passive way. Therefore, there is no need of extra devices such as pumps, fans or blowers; this would not only increase the energy output of the fuel cell and improve the cell efficiency, but also make the fuel cell system simpler and more compact. Hence, the air-breathing DMFCs have a broad application prospect in portable electronic equipment. There are two cathode types of air-breathing fuel cells:ducted type and ribbed type. Compared with the ribbed type, the ducted type has more practical value with unique advantages of making the assembling and compact designing of cell stack much easier. However, in the recently reports, most studies about ducted cathode type were based on hydrogen-oxygen fuel cells, only few were based on DMFCs. In this thesis, a three-dimensional muti-phase thermal model was presented for a ducted DMFC, considering the differences between a single cell and a cell stack.Firstly, the effects of proton exchange membrane (PEM) properties and methanol concentration on cell temperature, methanol crossover, fuel efficiency and cell performance were investigated. The numerical results indicate that using the modified PEM with low methanol crossover, for a large cell stack with good thermal insulation, can improve fuel efficiency and power density. High fuel efficiency and power density of such cell stack with modified PEM can be maintained in a wide current density range while using 2M methanol.Secondly, according to the characteristic of assembling cell stack easily, the effect of ducted air-breathing fuel cell stack size and cell length was investigated. The numerical results indicate that as a key factor in determining cell temperature, stack size have great impact on designing and selecting other important parameters, such as PEM properties and methanol concentration. When the fuel cell works at a high current density, the effect of cell length on the mass transfer of oxygen was remarkable, the other parameters should also be adjusted according to the cell length.Finally, the heat management of the fuel cell was investigated and the heat dissipation condition was analyzed. The effect of heat dissipating method and ambient temperature on heat dissipation of the fuel cell was also discussed. The numerical results indicate that natural convection and radiation at the solid wall were the main methods of heat dissipating. Therefore, the heat dissipation condition can be influenced greatly by the natural convection coefficient and the emissivity of the solid wall. In order to keep the cell temperature in an appropriate range for better cell performance, the emissivity of the solid wall can be changed by some surface treatment method, such as plating metal on the wall, according to different sizes of the cell stack with different heat preservation quality. Ambient temperature had more influence on the cell stack with small size compared with big size. Cell performance increased with increasing ambient temperature due to the increasing cell temperature.