Systems considerations in capacitive energy storage

Capacitive energy storage systems are being developed as an alternative to SMES for electric utility power quality applications and to supplement batteries in electric vehicles. Systems considerations show that the low voltages of "ultra-capacitors" impose a high penalty in terms of increased component count, greater complexity in the power electronics used for charging and discharging and large IR voltage drops. The consequent dissipation and reductions in efficiency exacerbate thermal management and stresses on the power electronics. Alternative strategies using ceramic hyper-capacitors in the 800-1500 V range are discussed in terms of system cost and performance. A system analysis indicates that hypercapacitors at 12 Wh/kg have higher effective energy density than ultra-capacitors at 20 Wh/kg when useable/recoverable energy, packing density and system issues are taken into account. The power quality delivered to the load, the cost, weight and volume of the power converters used in discharging and recharging are shown to be essential considerations. Cryogenic operation appears to offer numerous attractive performance benefits.