Control of hybrid battery/ultra-capacitor energy storage for stand-alone photovoltaic system

Battery life is an important criterion in a stand-alone photovoltaic system operation due to intermittent characteristic of solar irradiation and demand. This paper presents a stand-alone photovoltaic system with Ni-MH battery and ultra-capacitor serving as its energy storage elements. A control strategy is proposed in this paper to reduce charging and discharging cycles and avoid deep discharges of battery. The battery converter is controlled in current mode to track a charging/discharging reference current which is given by energy management system, whereas the ultra-capacitor converter is controlled to corporate solar irradiation fluctuations, load spikes and variations to maintain a stable dc-link voltage. Isolated PV system with the proposed control schemes is created using MATLAB SIMULINK. An optimum performance is achieved to serve as both high power and high energy sources due to complementary characteristic of battery and ultra-capacitor.