A stand-alone photovoltaic system is a photovoltaic system that operates independently of a grid-connected photovoltaic system and that does not connect to the grid. At present, the widely used independent systems such as solar street lamps, solar mobile power supplies, etc., their photovoltaic power generation output and load power consumption are not in the same period of time, and the photovoltaic power generation output can not always meet the load requirements, so the independent photovoltaic power generation The system is generally equipped with energy storage type to effectively improve the utilization of photovoltaic power output and enhance the stability of the system. At the same time, the energy storage system also has the function of providing starting current and clamping voltage for the load. The widely used independent photovoltaic system is generally composed of photovoltaic power generation, control/inverter and energy storage.
A battery is generally used as an energy storage device in a stand-alone photovoltaic system that has been commercially produced and applied. At present, the life span of PV modules and control/inverters can reach more than ten years, but the service life of batteries is only 6~7 years, and the cost of batteries can account for more than 25% of the whole system. Therefore, it is currently in independent photovoltaic system. The configuration and control objectives of the medium energy storage are to maximize battery life and reduce system cost. Under this premise, the primary goal of the allocation of energy storage capacity and power in an independent photovoltaic system is to optimize the battery charge and discharge energy storage life. In an independent photovoltaic system, the capacity of the energy storage unit is usually small, and generally no independent control system and control strategy are configured. The energy storage and photovoltaic solar mounting structure components are controlled by the same control system. At present, the most widely studied and most widely used power point tracking (MPPT) control system is to optimize the output of photovoltaic modules as the control target, and lacks optimal control of the energy storage system. As the efficiency of photovoltaic modules increases and the price decreases, and the cost of energy storage remains high, the development of independent energy-saving control strategies and control systems in independent photovoltaic systems has broad prospects.