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This paper compares the merits of operating a direct-hydrogen fuel cell (DHFC) system using a high-pressure air supply (compressor) versus one using a low-pressure air supply (blower). Overall, for the system modeled, it is shown that there is no inherent performance advantage for either mode of operation at the DHFC stack level. However, in practical applications, as will be shown in this paper, a systems analysis (stack and air supply) of power and efficiency needs to be performed. Equivalent PEM DHFC stack peak power values can be obtained using both high-pressure and low-pressure air supply systems. For each air supply configuration, air mass flow and pressure operating conditions can be found that result in an equal value of the oxygen partial pressure at the cathode catalyst layer surface. However, at the system level, the required air supply power needed to achieve the same DHFC stack performance values can be drastically different for high and low pressure operation.

This work presents a method to maximize the net power output of an indirect methanol PEM fuel cell system. This method establishes an operating strategy for the air supply based on the stack, air supply and water and thermal management (WTM) sub-system characteristics - holding anode conditions constant. It is shown that operating strategies based on individual components result in the inefficient operation of the overall system. Inclusion of the WTM modifies the optimal operating conditions for both low and high pressure systems. However the results for high pressure show an efficiency gain through reducing air pressure and increasing airflow, the opposite of what is expected. This work also outlines the components and issues not included and their importance in system operation.

This paper primarily compares costs and fuel economies of load following direct-hydrogen fuel cell vehicles with battery hybrid variations of the same vehicle. Additional information is included regarding load-following indirect methanol fuel cell vehicles. The key points addressed are as follows: the tradeoff between fuel cell system efficiency and regenerative braking ability; transient effects; and component cost differences. The difference in energy use and costs can vary significantly depending on the assumptions and the hybrid configurations. The mass of the battery pack creates the largest impact in energy use, while system efficiency losses roughly balance out with regenerative braking. For the direct-hydrogen fuel cell system, transient effects are small. These effects are expected to be significant for steam reformer/indirect-methanol systems (analyzed only graphically herein).

This paper addresses the critical need to incorporate realistic models of the air supply sub-system in fuel cell system performance analysis. The paper first presents the dominant performance issues involved with the air supply operation in the fuel cell system. The report then goes on to propose a methodology for an air supply model that addresses many of the performance issues. Most importantly, a model is needed with a defined set of performance criteria and data input format, one that can accommodate multiple air supply configurations, and one that realistically and accurately simulates the air supply operation and its effect on the system power and efficiency. The paper concludes that it is possible to compare alternative air supply components under the constraint of maximizing the instantaneous net fuel cell system efficiency for a dynamic vehicle driving cycle under various ambient conditions.

The question of whether or not direct-hydrogen fuel cell systems in automotive applications should be used in load following or load leveled (battery hybrid) configurations is addressed. Both qualitative and quantitative analyses are performed to determine the potential strengths and weaknesses of each option. It is determined that the amount of energy that can be recovered through regenerative braking has a strong impact on the relative fuel economy of load following versus load leveled operation. Further, it is demonstrated that driving cycles with lower power requirements will show an improvement in vehicle fuel economy from hybridization while those with higher power requirements will not. Finally it is acknowledged that the practical considerations of cost and volume must also weigh heavily into the decision between the two configurations.