A voltage and state of charge control technique for battery-supercapacitor hybrid energy storage system for standalone photovoltaic application

In this paper, hybrid energy storage system (ESS) consisting of battery and supercapacitor (SC) is employed in a standalone photovoltaic system. The battery ESS is characterized by high energy density, low power density, degradation due to frequent and partial charge/discharge cycles. By incorporating supercapacitor ESS which possesses high charge/discharge rates, a system having high energy and power capabilities can be designed. In the proposed control scheme, high frequency current reference in addition with battery error current is used as the reference for the SC ESS resulting in a better voltage profile as compared to that of the traditional methods. Also, a state of charge (SOC) controller is employed to maintain the SOC within the desired limits. When the SOC limit is violated, battery SOC is controlled by diverting charge/discharge currents to SC. The system results in overall life span improvement, reduction in size and cost of the battery. The control scheme is verified using Matlab/Simulink models under varying conditions of irradiation and load.