UC Davis Institute of Transportation Studies

This paper is concerned with the use of ultracapacitors in hybrid vehicles in place of batteries. In the case of the mild, charge sustaining hybrid, the ultracapacitors would replace a lithium or nickel metal hydride battery: for a stop-start micro-hybrid, the capacitors would be used in combination with a lead-acid battery with the capacitors starting the engine, accepting energy during regenerative braking, and providing accessory loads during relatively short stop periods. Test data are shown for the performance of advanced carbon/carbon and hybrid lithium ultracapacitors indicating higher energy density (more than 2X) than that of commercially available carbon/carbon cells from Maxwell and NessCap. The advanced devices showed no sacrifice in high power capability in order to achieve the higher energy density. Simulations of mid-size passenger cars using the advanced ultracapacitors in micro-hybrid and charge sustaining hybrid powertrains were performed using the Advisor vehicle simulation program modified with special routines at UC Davis. The influence of the ultracap technology and the size (Wh) of the energy storage unit on the fuel economy improvement was of particular interest. Significant improvements in fuel usage were predicted for all the hybrid powertrains using ultracapacitors for energy storage. The results for the micro-hybrids indicated that a 7-25% improvement in fuel economy can be achieved using a small electric motor (4 kW) and small ultracapacitor units (5-10 kg of cells). The fuel economy improvements for the mild-HEV ranged from over 70% on the FUDS to 20% on the US06 driving cycle. In both microand mild-HEVs, the differences in the fuel economies projected using the advanced ultracapacitor technologies were very small. It is possible to store more energy using the advanced ultracapacitors, but the fuel savings appear be unaffected. The primary advantage of the advanced ultracapacitors is that the energy storage unit is smaller, lighter, and lower cost and there is more reserve energy storage to accommodate a wider range of vehicle operating conditions. In the mild hybrids, the fuel economy improvement was greater using ultracapacitors than with a lithium battery primarily because of the higher round-trip efficiency of the ultracapacitors.