Browse Publications Technical Papers 2003-01-0416
2003-03-03

A Comparison of Energy Use for a Direct-Hydrogen Hybrid Versus a Direct-Hydrogen Load-Following Fuel Cell Vehicle 2003-01-0416

Hybridizing a fuel cell vehicle has the potential to improve the vehicle efficiency largely due to the ability to recover braking energy. However, tradeoffs do exist, and the advantages (in terms of potential fuel savings) are largely dependent on the drive cycle. The tradeoffs include 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 direct-hydrogen (DH) 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 and hybrid vehicle platforms.
Several hybrid component configurations were studied. When the DC/DC converter is placed in the path of the fuel cell stack current, there does not appear to be much benefit, in terms of energy usage, in hybridizing the DH fuel cell vehicle. Specifically, on the US EPA cycles, the load following vehicle outperformed the hybrid on the HIWAY sequence, but the hybrid had slightly better results on the FUDS 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, the benefits in terms of improved fuel economy are larger than in the first configuration. This later configuration will be the design used for this study.
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 was reduced due to a ∼10% heavier vehicle, and a lower overall system efficiency (when including the battery and DC/DC converter losses). The important factor, therefore, is the regenerative braking capability and whether this gain outweighs the added losses.

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