Air System Management for Fuel Cell Vehicle Applications

Master's Thesis Research and development of fuel cell systems for multiple applications has dramatically increased in the past few years. The vehicular application of the fuel cell system as the powertrain leads to a number of unique challenges, namely physical packaging within the vehicle, durability and operation under extreme environmental conditions, and demanding duty cycles that include high peak power requirements and a rapid response time. The focus of this research is on the air management system of the fuel cell powertrain in the vehicular application. Specifically, the work has solely focused on numeric simulation (modeling) using fundamental calculations and characterization of existing laboratory data. The motivation for the modeling project has been to create a tool for supplementing physical system research. As may be expected, the full system can be quite complex and the optimum configuration choice is not always clear. Using a modeling tool, a system designer can experiment with various configurations and analyze their relative tradeoffs prior to physically building the system of choice. Specific to the air system, various types of compressors and energy recovery devices exist, and with each component comes a unique optimum control scheme for the fuel cell system. This research, therefore, is designed to address the following motivating questions. First, what model design will realistically characterize the performance of the laboratory-tested air system? And second, what are the relative differences in system performance when the air system configuration is altered? Both of these questions are addressed in this thesis. Much of the work from this research was published in three independent papers, which are included in this thesis. A few of the research findings are included here. Section 2.1 highlights an analysis comparing an air system with and without the use of an expander (turbine). It is shown that the use of the expander (turbine) results in an improvement to the system efficiency at peak power levels. However, under normal driving conditions, peak power levels are demanded only a small fraction of the time. Therefore, it becomes less clear as to whether the added complexity and cost of an expander (turbine) would be beneficial. For example, for a fixed fuel cell stack size, the net efficiency is improved by approximately 4 % in the higher power region above 24kW net compared to the system without the expander. However, net efficiency is almost unchanged in the lower power region used most of the time. Alternatively, for a fixed peak power, the stack size can be reduced by about 13% using an expander compared to the fuel cell stack size required in a system without an expander. Section 2.2 presents findings from a study comparing a low pressure air system to that of a high pressure system. The results of the study demonstrate that equivalent direct hydrogen fuel cell peak net system power values (86kW) can be obtained with both types of air supply configurations but require different stack sizes. For the blower application, the stack size had to be increased by 16.3% (500 vs. 430 cells in this example) for the same peak net power of 86kW. Finally, Section 2.3 highlights research focused solely on the modeling structure of an air system in the context of the fuel cell engine. It was found that to maximize the performance of a particular fuel cell system configuration, it is useful to have a model that can compare various air supply technologies in the context of the system operation.