Application of Supercapacitor in Elecrical Energy Storage

Abstract

Understanding the supercapacitor characteristics is mandatory to the application of supercapacitor. In order to do so, an electric circuit model to describe the performance of supercapacitor has to be constructed. The basis of supercapacitor modeling is the parameter acquisition itself, which poses a challenge due to difference in parameter values using different acquisition methods. A series of experiments were done and it was determined that only the AC Electrochemical Impedance Spectroscopy (EIS) and constant current pulse methods were reliable parameter acquisition methodology. It was discovered that the Equivalent Series Resistance (ESR) of the supercapacitor obtained through the constant current pulse method varies at different point of data acquisition. A novel method is presented which allows the conversion of DC ESR in time domain to the frequency domain. Doing so allows the comparison of AC and DC ESR, which were close in value. This ultimately enables the unification of ESR values: There shouldn't be terms such as AC or DC ESR, but an ESR at stated frequency. It was experimentally proven that the supercapacitor ESR and capacitance increases with its energy level, which is in line with general findings and knowledge. In order to model the transient characteristics of supercapacitor without taking into account redistribution effect, a modified single branch Resistor-Capacitor (RC) model was proposed, which reflects the change in capacitance and ESR with capacitor voltage. The simulation result of the model is in close proximity of the experimental results, which prove the effectiveness of the model. With decent understanding of supercapacitor behavior, a bidirectional hexa-mode buck-boost converter was investigated for implementation with the supercapacitor to achieve peak load shaving as well as Uninterruptible Power Supply (UPS) functionalities. Due to the need to operate in both buck-boost and boost modes, a tri-state hybrid mode was proposed to bridge the buck-boost and boost modes. It was proven experimentally that the implementation of hybrid mode can bridge both the modes well. The hexa-mode Switch Mode Power Supply (SMPS) was used to implement a supercapacitor based offline UPS for Hard Disk Drive (HDD). Simulation of real life applications was performed using the programmable electronic load and proves that the hexa-mode SMPS was very versatile in operation and could implement active peak load shaving as well. This SMPS has vast applications especially for low voltage load applications.