Energy Storage System Requirements for

This paper summarizes a methodology for determining desirable characteristics and requirements of the energy storage system for fuel cell hybrid vehicles. The purpose of this work was to provide supporting analysis for the FreedomCAR Technical Teams in defining energy storage requirements for fuel cell vehicles. The power and energy requirements related to specific roles of the energy storage system have been determined for two vehicle class scenarios, a mid-size car and a mid-size SUV. The specific roles of the energy storage system considered included driveline traction power during fuel cell start-up, powerassist capability during the drive cycle, regenerative braking energy recapture capability, gradeability and acceleration performance improvement, electrical accessory loads, and fuel cell startup and shutdown loads. Considering the requirements for each role individually provides the ability to mix and match the characteristics desired based on the specific design scenario. In the short term, specific cost and specific volume of fuel cell systems will likely lead to hybridized fuel cell vehicles to satisfy cost and volume constraints. The process was applied to define the energy storage system characteristics for several scenarios. Through hybridization, it has been demonstrated that the powertrain system cost and volume can be reduced in the near-term scenario over the fuel cell only vehicle scenario