Hybrid Energy Storage With Multimode Fuzzy Power Allocator for PV Systems

The ineluctable variations of the solar radiation, ambient temperature, and partial shading affect the performance of the photovoltaic (PV) arrays, and hence incur supply-demand mismatches in the PV-based system. This paper proposes a hybrid energy storage (HES) composed of lithium-ion batteries and ultracapacitors that can be incorporated in the PV-based system to complement the supply-demand mismatches by using a multimode fuzzy-logic power allocator. The battery used in the HES was modeled based on the experimental data and can accurately reflect the battery dynamics under varying conditions. The power allocator optimally adjusts the power contributions of the batteries and ultracapacitors and exchanges energy between them, thus compensating the supply-demand mismatches without accidentally depleting or saturating the two components. The proposed HES was evaluated in both short- and long-term scenarios. A good ac-side performance in the short-term scenario is observed due to the instant response of the ultracapacitors to the high-frequency requests. In the long-term scenario, the power allocator restrains the frequency fluctuations caused by the supply-demand mismatches within ±0.2 Hz, while maintaining the batteries and ultracapacitors in a safe working region. Additionally, the participation of ultracapacitors in supplying high-frequency power is beneficial for relaxing the stress on batteries, and the simulation results show that the lifetime of batteries in the HES can be extended.