Application of supercapacitors in electric traction storage systems

Abstract

Energy storage systems (ESS) are based on electrochemical batteries and supercapacitors (SC). SCs have a lower energy density compared to batteries, but have an advantage over them in power density. The combined use of the two in a hybrid ESS (HESS) gives both high energy density and high power density of ESSs. Fractional-order impedance describes the SC dynamics better than commonly used integer order impedance. Such a description can be based on the dielectric relaxation equations e.g. Cole-Cole equation used in this work. The paper compares results of the impedance approximation of a number of SC’s with various fractional and integer models, on the basis of which the time responses are analysed. The methods of capacitance and ESR measurement recommended by the IEC standard are examined. They are compared with the methods used in practice by SC producers. The problem of energy losses in ESS with respect to charging/discharging current pulses is presented. A proper ESS designing requires an accurate approximation of the SC impedance over the frequency band in which the dominant part of the power spectrum of current pulses is located.