The operation of a direct methanol fuel cell with an internal combustion engine in a hybrid system is investigated in terms of fuel efficiency. The following work shows a potential for fuel saving because the engine's waste heat is utilized in preconditioning of methanol for the fuel cell and in postconditioning of the cell's anode exhaust for the engine. The low activity of methanol oxidation catalysts and methanol crossover are the main drawbacks of direct methanol fuel cells. H3PO4-doped polybenzimidazole membranes have lower methanol crossover, and allow a higher operational temperature and methanol concentration compared to Nafion membranes. The operation of the cell at higher temperature with polybenzimidazole membranes improves catalyst activity and mass transfer increasing cell efficiency. But the fuel feed to this type of membrane must be in vapor phase. Methanol solution can be evaporated by the engine coolant. Unutilized methanol in the anode exhaust is converted to H2 rich product gas in a reactor before feeding into the engine. The endothermic reaction enthalpy for this conversion is recovered from engine's exhaust gas. The system efficiency increases with the cell's fuel utilization, as long as the cell's efficiency is higher than the engine's efficiency. In order to increase the system efficiency with load, the current density of the fuel cell should not be increased beyond the point where the cell and engine efficiency meet. Beyond that, the product gas should be substituted with liquid methanol to meet the rest of the load because the engine charge's energy density can be increased with liquid methanol injection into the engine. If the engine charge is comprised of fuel cell exhaust only and the engine's indicated efficiency is 20%, the efficiency of the hybrid system will be 25.5% at a cell voltage of 0.4 V and a cell fuel utilization of 40%. This corresponds to a fuel saving of 28% compared to the internal combustion engine. The hybrid system efficiency will increase to 28.5% at this operating point, if the fuel cell's anode exhaust is further decomposed in a reactor prior to combustion in the engine. The addition of the reactor to the hybrid system corresponds to a fuel saving of 43% compared to the engine and a fuel saving of 12% compared to the hybrid system without the reactor.
