Energy efficiency of supercapacitors in hybrid energy storage systems

The supercapacitor supports the main energy source of the hybrid electrical energy storage system. In short periods, it supplies additional energy or absorbs braking energy. The paper addresses the issue of energy losses in supercapacitors used in such a pulsed mode of operation. Commonly used first-order RC capacitor impedance models are not a good basis for accurate calculation results of energy losses in supercapacitors. The article presents fractional-order supercapacitor impedance models based on the Cole-Cole relaxation equation. The following part presents the results of measurements of energy losses in real supercapacitors in comparison with simulation calculations using both above-mentioned supercapacitor impedance models. The article also discusses the problem of using the equivalent series resistance (ESR) values in RC impedance models, called AC ESR and DC ESR, specified by different supercapacitor manufacturers. It shows that the simulation results using the presented fractional impedance model are the closest to reality. The use of the other discussed models leads to even a several-fold underestimation of energy losses in supercapacitors used in electric energy storage systems.