Hybrid Fuel Cell Electric Vehicles

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Power sources hybridization for fuel cell vehicles can overcome short comings such as slow cold start-up, slow dynamic response to load change, and no regenerative energy capture. The hybridization can also reduce the vehicle weight and power source's cost as well as increase the vehicle fuel efficiency.

The system analysis in this report shows hybrid control is necessary to make sure the fuel cell and the energy storage device working properly and safely Batteries are favored as the energy storage device in hybridization with a fuel cell system from the consideration of energy density, technology maturity, and cost. The Panasonic D-size battery under study has high internal impedance, and wide variation with temperature. It would be difficult to control the hybrid system to achieve consistent performance. It is recommended to look at the alternative battery options that could offer high power and low impedance. The conclusions are highly dependent on the fuel system and energy storage system. The use of hybrid batteries with significant power density and the use of fuel cells with much quicker load response might result in a different hybridization strategy and different fuel efficiency.

Fuel cell powered vehicles are well known for their extremely high cost, less efficiency at lower power operation. They are not compatible with the regenerative braking system (regenerative fuel cell might fix this problem). A FC system with a reformer takes at least minutes to start from cold, and has sluggish dynamic response to load change.

A "hybrid" vehicle usually refers to one that incorporates a minimum of two separate power sources in its power train system. In the application of fuel cell vehicles, this means a fuel cell system coupled with one or more power sources or energy storage devices such as batteries or super capacitors. One of the primary advantages of this dual power supply system is the flexibility in power distribution between sources. This versatility yields greater potential to optimize the vehicle power train to meet vehicle performance, fuel economy and cost requirements. In order to take the advantage of this system flexibility, the sizes of the fuel cell system and supplemental power system have to be properly matched.

It is equally critical to integrate the system with an intelligent control strategy that uses each component to optimize the overall system performance.

This paper reviews the general concept of hybrid fuel cell EV with the emphasis on the fuel cell and battery combination. The issues related to the hybrid system design are discussed. The system trade-off study was performed to show how to meet the requirements of FCV performance targets, and the improvements achieved in vehicle acceleration and cold start time with the hybrid over the existing reformer fuel cell technology. This paper also explains why the appropriate control is necessary in such a hybrid system and how it affects the system efficiency.Finally, the cost and life issues are briefly discussed.

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