Photovoltaic power generation systems can be divided into grid-connected photovoltaic power generation systems and stand-alone photovoltaic power generation systems according to whether they are 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 power 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 to be widely used, flywheel energy storage as a new type of mechanical energy storage, 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. Thermal energy storage is the storage or release of energy through material phase change, the relevant domestic research is in the laboratory stage, has not been practical applications. Battery energy storage is a mature technology, widely used and low cost energy storage technology. Among them, the lead-acid battery as a storage device for stand-alone photovoltaic power generation products have been commercially available, and in recent years, with the development of lithium battery technology and its cost reduction, the construction of lithium battery energy storage device is currently a research hot spot.
1, grid-connected photovoltaic energy storage system
Grid-connected photovoltaic power generation system directly connected to the distribution network, the power directly into the grid, currently generally not configured energy storage system, with photovoltaic, wind power generation "abandoned light limited electricity" phenomenon is serious, as well as photovoltaic, wind power generation system power output fluctuations and other factors on the use of renewable energy and the promotion of increasingly serious restrictions, after the grid-connected The configuration of energy storage in grid-connected photovoltaic systems has become one of the research directions of large-scale energy storage systems.
The configuration of energy storage in grid-connected photovoltaic power generation systems is determined by the energy storage objectives, which can be specifically divided into: smooth output, economic dispatch, and microgrid composition.
1) Smooth output
Photovoltaic power generation is the process of converting solar energy into electrical energy, and its output power is affected by the intensity of solar radiation, temperature and other environmental factors and changes dramatically. In addition, because the output of photovoltaic power is DC current, it needs to be converted into AC power by inverter and then connected to the grid, and harmonics will be generated in the process of inverter. The unstable power of PV power and the existence of harmonics make the access of PV power will cause impact on the grid. Therefore, an important purpose of grid-connected photovoltaic power generation system with energy storage is to smooth the output of photovoltaic power and improve the quality of photovoltaic power.
Energy storage system configuration with the goal of smoothing PV power output is generally configured on the PV power generation side with a centralized energy storage system, and the structure of the PV energy storage system with the goal of smoothing the output is generally shown in the following figure.
The capacity of the energy storage system is determined by the grid-connected smoothing strategy, while the energy storage power is generally determined by the smoothing objective. The grid-connected PV smoothing strategy based on the energy storage system currently has a fixed time constant low-pass filtering smoothing strategy, fuzzy control/SOC (storage battery state of charge) smoothing strategy, PV power prediction smoothing strategy, etc. Low-pass filtering smoothing strategy smoothing effect in general, but the control is simple, low cost, is currently a more promising application of the control strategy.
2) Power peaking
After the PV power is connected to the grid, it needs to accept the grid dispatch, but its power output peak phase and the grid load peak phase are not consistent, coupled with the power market peak and valley tariff factors, the use of energy storage system to achieve PV power generation in the time coordinate shift, so that PV power to participate in grid peaking is also one of the current PV energy storage system research hotspots. Through power peaking, the access capacity of PV power in the grid and the economy of PV power can be improved.
The PV energy storage system based on power peaking purpose is generally configured with centralized energy storage system on the grid side. The configuration of energy storage on the grid side is illustrated as follows.
The capacity of the energy storage system in this configuration is generally larger, and the cost of the energy storage system is higher and unreasonable charge and discharge control will seriously damage the life of the energy storage system, so the current capacity and power configuration of the centralized energy storage system configured on the grid side is determined by the grid peaking requirements, energy storage charge and discharge control strategy, energy storage cost and other factors. The algorithms for solving the peak-shaving and valley-filling strategies of battery energy storage systems mainly include gradient-type algorithms, intelligent algorithms, and dynamic planning algorithms. Different grid peaking requirements, energy storage control strategy for power, capacity requirements vary greatly, in the practical application of the need for a variety of practical conditions under the constraints of the configuration of the energy storage system. At present, domestic large-scale energy storage power plants are still in the initial stage, only experimental or demonstration energy storage power plants are operating, not yet put into large-scale commercial use.
3）Micro-grid application
Microgrid is a new type of grid structure proposed to promote the use of renewable energy, specifically by renewable energy, energy storage systems and load composed of regional grid form, as an independent whole, can either be grid-connected operation, but also in the off-grid state islanding operation. As a component unit of microgrid, energy storage system is the energy buffer link in microgrid, which plays an important role in improving control stability, enhancing power quality of microgrid, maintaining power balance of microgrid, and improving anti-interference capability of microgrid. In addition, the energy storage system in the microgrid can also be used as an emergency backup in case of grid power interruption.
Energy storage systems in microgrids are generally configured in parallel with renewable energy generation systems and have an independent energy storage management system (e.g., battery control system, BESS), whose operation mode varies with the microgrid operation mode (off-grid/on grid). The capacity and power configuration of the energy storage battery depends on different microgrid compositions and operation modes, and is also governed by the operation mode of the energy storage system. The configuration and control strategy of energy storage system in microgrid is the hot spot of microgrid related research at present.
2、Stand-alone photovoltaic energy storage system
Stand-alone PV system is relative to the grid-connected PV system, which refers to the PV system that does not connect to the grid and operates independently. 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 system total configuration storage type effectively enhance the use of PV power output, enhance the stability of the system of effective means, 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 parts: PV power generation, control/inverter, and energy storage.
The current commercially available stand-alone PV systems generally use batteries as energy storage devices. At present, the life of PV modules and control/inverter can reach more than ten years, but the life of the battery is only 6-7 years, while the cost of the battery can account for more than 25% of the whole system, so the current configuration and control of energy storage in the stand-alone PV system aims to extend the battery life as much as possible and reduce the system cost. Under this premise, the primary goal of the configuration of energy storage capacity and power in stand-alone PV 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 (20.20%, Leye PV) and price reduction ($2.86/W, JinkoSolar, 2017.3,) 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.
