Fuel Cell Powered Vehicles Using Supercapacitors: Device Characteristics, Control Strategies, and Simulation Results

The fuel cell powered vehicle is one of the most attractive candidates for the future due to its high efficiency and capability to use hydrogen as the fuel. However, its relatively poor dynamic response, high cost, and limited life time have impeded its widespread adoption. With the emergence of large supercapacitors (also know as ultracapacitors, UCs) with high power density and the shift to hybridization in the vehicle technology, fuel cell/supercapacitor hybrid fuel cell vehicles are gaining more attention. Fuel cells in conjunction with supercapacitors can create high power with fast dynamic response, which makes it well suitable for automotive applications. Hybrid fuel cell vehicles with different powertrain configurations have been evaluated based on simulations performed at the Institute of Transportation Studies, University of California-Davis. The following powertrain configurations have been considered: (a) Direct hydrogen fuel cell vehicles (FCVs) without energy storage (b) FCVs with supercapacitors directly connected in parallel with fuel cells (c) FCVs with supercapacitors coupled in parallel with fuel cells through a DC/DC converter (d) FCVs with fuel cells connected to supercapacitors via a DC/DC converter Simulation results show that hybridization of fuel cell vehicles with supercapacitors with load leveling control can significantly reduce the stress on fuel cells electrically and mechanically and benefit fuel economy of the vehicles. Compared to fuel cell vehicles without energy storages, fuel cell-supercapacitor hybridization achieved fuel economy increases of up to 28% on the FUDS cycle and up to 24% on the US06 cycle for mid-size passenger vehicles. In general, the maximum fuel economy improvements are greater using supercapacitors than batteries. The simulation results show that the power assist control strategy is better than load-level control for batteries because of the lower losses in the DC/DC converter and batteries, but load level control is better for supercapacitors. The best approach for hybridization of the fuel cell vehicles is to use supercapacitors with load leveled control as it greatly mitigates the stress on fuel cells and results in a near maximum improvement in fuel economy and fuel cell durability.