Electrochemical Capacitors as Energy Storage in Hybrid-Electric Vehicles: Present Status and Future Prospects

The development of electrochemical capacitors (ultracapacitors) has continued since the early 1990s. Activated microporous carbon and hybrid carbon devices from a number of developers world-wide have been tested and evaluated for use in hybrid vehicles of various types. The test data indicate that the useable energy density of the activated carbon devices is about 5 Wh/kg and that of the hybrid carbon devices is 10-12 Wh/kg. The power capability of the carbon/carbon devices can be very high (> 2000 W/kg for a 95% high power pulse); the power capability of the hybrid carbon devices are significantly lower being 500-1000 W/kg for a 95% pulse. This means that the P/E ratio of the hybrid carbon devices is much lower than the carbon/carbon devices and as a consequence, it may be difficult to take full advantage of the higher energy density of the hybrid carbon devices in some applications. Simulation results for various types of hybrid vehicles are presented. The results for micro-hybrids are particularly interesting and surprising, because of the large fuel economy improvements predicted. The improvements were about 40% on the FUDS and ECE-EUD cycles and 20% on the Federal Highway and US06 cycles using the carbon/carbon ultracapacitor units. The improvements were significantly less using the hybrid carbon units because of their lower round-trip efficiencies.