Design of a Fuel Cell Hybrid Electric Vehicle Drive System

Research output: Book/ReportPh.D. thesis

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Fuel cells achieve more and more attention due to their potential of replacing the traditional internal combustion engine (ICE) used in the area of transportation. In this PhD thesis a fuel cell shaft power pack (FCSPP) is designed and implemented in a small truck. The FCSPP replaces the original supply system of the truck which was powered by a lead-acid battery package. The FCSPP includes fuel storage, a fuel cell system, an energy storage device, power electronics, an electric machine, and the necessary control. The FCSPP therefore converts the energy of the fuel to a shaft torque and speed of the electric machine.

In the thesis the High Temperature Proton Exchange Membrane Fuel Cell (HTPEMFC) is used as it has promising properties for being supplied by reformed methanol, instead of pure hydrogen, which is more practical feasible. It takes approximately 6 minutes before the fuel cell is ready to produce power. In this period an energy storage device is necessary in order to provide power for the electric machines, and to heat-up the fuel cell stack. The energy storage device also takes care of the peak loads, the high load dynamics, and it utilizes the braking energy in order to increase the efficiency. In this work a lead-acid battery, an ultracapacitor, or a combination of both are considered as energy storage devices.

A FCSPP is designed for 10 different configurations of connecting the energy storage device(s) and fuel cell to a common bus, which comply with the 42V PowerNet standard. Each of the ten configurations is designed for different fuel cell power ratings. The FCSPP is designed in an iterative process where the power flow through the system is under the influence of a certain energy management strategy and charging strategy, which sufficiently divides the power between the units.
The FCSPP is designed from a driving cycle which is constructed from field measurements of the original battery-powered truck.

Due to the long heating-time of the fuel cell, it is not appropriate to use ultracapacitors as the only energy storage device, because the system then becomes too big and heavy, even though they provides the highest system efficiency among the three options of energy storage devices. The system volume, mass, and efficiency are improved by increasing the rated fuel cell power. However, when a battery is included it has a negative effect on the battery lifetime to increase the fuel cell power rating, as the partial load cycles then becomes dominating. Simulation result indicates that the system efficiency and battery lifetime can be improved by adding ultracapacitors, because they can handle the shallow cycles, so they not are directed to the battery.
However, this indication is based on insufficient data of the battery lifetime at small cycles, and a better model for the battery lifetime is therefore necessary.

The used 42V PowerNet standard is within the range of the voltage characteristic of the used fuel cell stack. Therefore a non-inverting buck-boost converter is inserted in the between, which is able to both buck and boost the voltage depending on the actual fuel cell power level. A method where the converter is operated in a combination of buck-mode and boost-mode provides the smoothest transition between the two modes.
Original languageEnglish
PublisherDepartment of Energy Technology, Aalborg University
Number of pages199
ISBN (Print)978-87-89179-81-0
Publication statusPublished - Aug 2010


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