Burke, Andrew and Marshall Miller (2001) Update of Ultracapacitor Technology and Hybrid Vehicle Applications: Passenger Cars and Transit Buses. Institute of Transportation Studies, University of California, Davis, Presentation Series UCD-ITS-RP-01-12
Ultracapacitor technologies are reviewed with the emphasis on carbon-based devices using organic electrolytes. Recently available cells with capacitances up to 5000F have been tested at cell voltages of 2.5–2.7 V. These devices exhibited useable energy densities between 3.4–3.7 Wh/kg for constant power discharges at 500 W/kg. The calculated pulse power capability of the devices ranged between 1400–3900 W/kg for 95% efficient discharges. An 18-cell, 45V pack of 2600F ultracapacitors was assembled and tested to determine the effect of cell-to-cell variability and thermal effects on the operation of the pack for high power pulses over long periods. The pack was operated on the PSFUDS cycle for 36 hours with 10 minute rest periods between cycles without difficulty. The temperature and maximum differences between the cell voltages stabilized at acceptable levels indicating the pack could operate with minimal cooling and no voltage balancing circuitry.
The uses of ultracapacitors in engine-starting and hybrid-electric vehicle applications were studied to determine whether devices with presently available performance were suitable for those applications. Available devices with capacitances of 2500–5000F were determined to be well suited for vehicle applications with unit weights acceptable to vehicle designers of the various vehicle types. Ultracapacitors units for power-assisted hybrid passenger cars were heavier than the PNGV design targets, but such that they could be accommodated in the vehicles. Recent advances in ultracapacitor performance indicate that both the energy density and power capability of devices will continue to improve with the use of higher specific capacitance carbons and higher cell voltage resulting in energy densities approaching 6 Wh/kg in the next several years. The critical issue that will determine the evolution of markets for ultracapacitors will be device cost which has been decreasing rapidly in recent years and is projected to continue to decrease for the next several years. Unit costs will need to decrease to about $5/Wh for a rapid development of a mass market for ultracapacitors in vehicle applications. For this to occur, the price of the microporous carbons used in the devices needs to be $10&ndash$20/kg and the devices must be produced on automated equipment.