Maximum Fuel Economy-oriented Power Management Design for a Fuel Cell Vehicle Using Battery and Ultracapacitor

Single energy storage element (ESE) has been used in fuel cell (FC) vehicle to regulate power flow between the FC and electric drive for improved dynamic response and system efficiency. An energy storage system (ESS) consisting of hybrid ESEs extend the capabilities of single ESE by providing overall higher efficiency, lower total weight, and better performance. In this paper, a fuel cell vehicle power configuration structure that does not require a dc/dc converter to interface FC with inverter DC bus is provided. A three-port isolated triple-half-bridge (THB) dc/dc converter with high efficiency and high power density is applied to interface an ESS of two energy storage elements - battery unit (BU) and ultracapacitor (UC). A new routine is presented to size the BU and UC to achieve lightest mass and 95% efficiency. Furthermore, a new control strategy to achieve maximum fuel economy and reduced size of FC is proposed. The FC provides average electric drive power during driving cycles, the BU and UC compensate for the difference between electric drive requirement and power provided by FC. The state of charge (SOC) of BU and UC are also maintained after driving cycles. Two alternatives of this control strategy have been examined to identify the different impacts of fuel cell ohmic polarization loss and the combined BU/UC internal loss on system efficiency. Simulation and experimental results based on simplified urban driving cycles are presented to validate the proposed maximum fuel economy design and efficiency comparisons of two control alternatives.