Browse Publications Technical Papers 2004-01-1476
2004-03-08

A Comparison of Energy Use for a Indirect-Hydrocarbon Hybrid versus an Indirect-Hydrocarbon Load-Following Fuel Cell Vehicle 2004-01-1476

Hybrid vehicles have been in the news quite a bit of late given the commercial introduction of a number of hybrid vehicles that sport significant improvements in fuel economy. The improved fuel efficiency of these vehicles can be directly attributable to the hybridized power train on board these internal combustion engine vehicles.
Similarly, hybridization of fuel cell vehicles not only helps improve fuel economy but can also help overcome other technical barriers (start up delays, transients). For fuel cell vehicles, hybridization of on-board fuel cell systems is expected to have the potential to improve the vehicle efficiency largely due to the ability to recover braking energy and via flexibility in designing the system controls. However, the advantages can be offset by the tradeoffs due to added energy losses associated with the DC/DC converter and the battery pack itself. Additional tradeoffs not explicitly addressed in this study include added overall complexity, additional packaging constraints, and potentially higher overall cost.
This report will focus on a quantitative analysis of the performance of the indirect-hydrocarbon (IH, onboard fuel processor using gasoline type fuel), hybrid and load-following fuel cell vehicles (FCVs) from the viewpoint of the energy use throughout the system. Specifically, the vehicle energy use and efficiency will be compared between the load following (non-hybrid) and hybrid vehicle platforms.
Several hybrid component configurations were studied and two representative configurations were investigated in depth. The first (Configuration 1), in which the DC/DC converter is placed in the path of the fuel cell stack current, there does appear to be some benefit, in terms of energy usage, in hybridizing the IH fuel cell vehicle.
Specifically, on the US EPA cycles, the hybrid vehicle outperformed the load following vehicle on the FUDS
sequence but the load following vehicle had slightly better results on the HIWAY cycle. However, if the DC/DC converter is placed in the battery current path only, with the fuel cell stack directly connected to the electric drive train (Configuration 2), the benefits in terms of improved fuel economy are larger than in the first configuration. The results corresponding to both these configurations will be analyzed and discussed in this paper.
Overall, three main factors affect these vehicle results, all of which will be explicitly examined in this study.
These factors are: vehicle weight, fuel cell system efficiency (including the battery), and regenerative braking capabilities. Specifically, the hybrid vehicle fuel economy can be reduced due to a ∼10% heavier vehicle, and a lower overall fuel cell system efficiency (when including the battery and DC/DC converter losses). One important factor is clearly the regenerative braking capability; but the other factor is associated with the ability to improve the efficiency of the fuel cell system itself by taking advantage of the flexibility offered energy storage sub-system and adopting better control strategies.. The real question however is whether these gains outweigh the losses introduced by the additional components needed to hybridize the vehicle.

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