The present and projected performance and cost of double-layer pseudo-capacitive ultracapacitors for hybrid vehicle applications

The performance of carbon/carbon double-layer and pseudo-capacitive ultracapacitors are assessed based primarily on testing done in the laboratory at UC Davis. The useable energy density of commercially available carbon/carbon devices is between 3.5 and 4.5 Wh/kg. The corresponding power density for high efficiency (95%) discharges is between 800-1200 W/kg. The pseudo-capacitive devices have higher energy densities in the range of 10-13 Wh/kg for constant power discharges less than 500 W/kg, but the energy density decreases significantly at higher power densities. Projections of future development indicate that the energy density of the carbon/carbon devices will increase to 5-6 Wh/kg in the relatively near future along with higher power densities of 1500-3000 W/kg. In the case of the pseudo-capacitive approaches, the carbon/PbO₂ device looks particularly promising from both the performance and cost points-of-view primarily because of its relationship to the low cost lead-acid battery. Costs of the carbon/carbon ultracapacitors are expected to continue to decrease from the present costs of 2-3 cents/Farad to about 0.5 cents/Farad. Further reductions in cost require a low cost carbon of $5-$10/kg. Comparisons of the costs of ultracapacitor and battery energy storage units for mild hybrid vehicles depend critically on the energy storage requirement (Wh) assumed for the ultracapacitor unit. If the storage requirement is less than 100 Wh, there is a strong possibility that ultracapacitors can compete with nickel metal hydride and lithium-ion batteries in the future hybrid vehicles.