Photovoltaic power generation system can be divided into grid-connected photovoltaic power generation system and stand-alone photovoltaic power generation system according to whether it is grid-connected or not. Grid-connected photovoltaic power generation system mainly refers to the photovoltaic system that is connected to the grid and receives grid dispatch, such as various centralized or distributed photovoltaic power plants. Stand-alone photovoltaic power generation system mainly refers to various photovoltaic power generation systems that operate independently of the grid, such as solar street lights, rural household photovoltaic power supply, etc.
Energy storage system, as a proven way to store electrical energy, can be divided into four categories: mechanical energy storage, battery energy storage, electromagnetic energy storage and phase change energy storage.
Among them, mechanical energy storage such as pumping energy storage has been widely used, flywheel energy storage as a new type of mechanical energy storage, is currently in the research and demonstration stage. Electromagnetic energy storage includes superconducting energy storage, super capacitor energy storage, high energy density capacitor energy storage, etc.
Battery energy storage is currently a mature technology development, widely used and low cost energy storage technology. In recent years, with the development of lithium battery technology and its cost reduction, the energy storage device constructed by lithium battery is the current research hot spot.
Stand-alone photovoltaic system is relative to the grid-connected photovoltaic system, refers to not connected to the grid and independent operation of the photovoltaic system.
At present, the more widely used stand-alone system such as solar street lights, solar mobile power, its PV power output and load power consumption is not in the same time period, while the PV power output does not always meet the load requirements, so in the stand-alone PV power generation system configuration storage is an effective means to enhance the use of PV power output, enhance system stability, while the energy storage system also has for the load Provide the role of starting current, clamp voltage, etc.
Currently, widely used stand-alone PV systems generally consist of three components: PV power generation, control/inverter, and energy storage.
The batteries are generally used as energy storage devices in commercially produced and used stand-alone PV systems. PV modules and control/inverter life can reach more than ten years, but the battery life is only 6-7 years, while the cost of the battery can account for more than 25% of the entire system, so the current configuration and control of energy storage in stand-alone PV systems target to extend the battery life and reduce system costs as much as possible.
Under this premise, the primary goal of the configuration of energy storage capacity and power in stand-alone PV energy storage systems is to optimize the battery charge and discharge storage life. The capacity of energy storage units in stand-alone PV systems is usually small, and generally not configured with independent control systems and control strategies, with energy storage and PV modules controlled by the same control system.
The most widely researched and applied maximum power point tracking (MPPT) control system is to optimize the PV module output as the control goal, lacking optimal control of the energy storage system. With the increasing trend of PV module efficiency improvement and price reduction, and the high cost of energy storage, the development of control strategies and control systems that prioritize energy storage in stand-alone PV systems holds great promise.