What are the solar energy storage system?

 

Now the country has vigorously promoted photovoltaic poverty alleviation. I believe you have some understanding of photovoltaic. So, what is a photovoltaic energy storage system? Have you understood? Now let's understand what photovoltaic energy storage system is, hoping to help you.

The photovoltaic power generation system can be divided into grid-connected photovoltaic power generation system and independent photovoltaic power generation system according to whether it is connected to the grid. Grid-connected photovoltaic power generation system mainly refers to the photovoltaic system connected to the grid for operation and receiving grid dispatching, such as various centralized or distributed photovoltaic power stations. Independent photovoltaic power generation system mainly refers to various photovoltaic power generation systems operating independently of the grid, such as solar street lamps, rural household photovoltaic power supply, etc.

 

As an effective way to store electric energy, energy storage system 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 pumped energy storage, is 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, supercapacitor 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 and has not yet been applied in practice. Battery energy storage is a mature, widely used and low-cost energy storage technology. Among them, the independent photovoltaic power generation products with lead-acid batteries as energy storage devices have been commercialized. In recent years, with the development of lithium battery technology and the reduction of its cost, the energy storage devices built with lithium batteries are the current research focus.

1. Grid-connected photovoltaic energy storage system

The grid-connected photovoltaic power generation system is directly connected to the distribution network, and the electric energy is directly input into the power grid. At present, the energy storage system is generally not configured. With the serious phenomenon of "light abandonment and power limitation" of photovoltaic and wind power generation, and the increasingly serious restrictions on the use and promotion of renewable energy due to factors such as large fluctuations in the output of photovoltaic and wind power generation systems, Configuring energy storage in grid-connected photovoltaic systems has become one of the research directions of large-scale solar energy battery storage system.

The configuration mode of energy storage in grid-connected photovoltaic power generation system is determined by the energy storage target, which can be divided into smooth output, economic dispatching and microgrid.

 

1) Smooth output

Photovoltaic power generation is the process of converting solar energy into electric energy, and its output power changes dramatically due to the influence of environmental factors such as solar radiation intensity and temperature. In addition, since the photovoltaic power output is DC current, it needs to be converted into AC power through the inverter and then connected to the grid, and harmonics will be generated during the inverter process. The instability of photovoltaic power and the existence of harmonic wave will make the access of photovoltaic power impact the power grid. Therefore, an important purpose of configuring energy storage in grid-connected photovoltaic power generation system is to smooth the output of photovoltaic power and improve the quality of photovoltaic power. The configuration mode of energy storage system aiming at smooth output of photovoltaic power generation is generally to configure centralized energy storage system at the photovoltaic power generation side, and the structure of photovoltaic energy storage system aiming at smooth output.

THe capacity of the energy storage system is determined by the grid-connected smoothing strategy, and the energy storage power is generally determined by the smoothing target. The photovoltaic grid-connected smoothing strategy based on the energy storage system currently mainly includes the low-pass filtering smoothing strategy with fixed time constant, the fuzzy control/SOC (state of charge of the energy storage battery) smoothing strategy, and the photovoltaic power generation power prediction smoothing strategy. Low-pass filter smoothing strategy has general smoothing effect, but simple control and low cost. It is currently a control strategy with broad application prospects.

 

2) Power peaking

After the photovoltaic power is connected to the grid, it needs to accept the grid dispatching, but its peak power output stage is not consistent with the peak load stage of the grid. In addition, due to the influence of the peak-valley electricity price factors in the power market, the use of energy storage system to realize the translation of photovoltaic power generation in time coordinates, so that photovoltaic power can participate in the grid peak regulation is also one of the current research hotspots of photovoltaic energy storage system. Through power peak regulation, the access capacity of photovoltaic power in the grid and the economy of photovoltaic power can be improved.

The capacity of the energy storage system in this configuration mode is generally large, and the cost of the energy storage system is high and unreasonable charge and discharge control will seriously damage the life of the energy storage system. Therefore, at present, the capacity and power configuration of the centralized energy storage system configured on the grid side are determined by the factors such as the peak shaving requirements of the grid, the charge and discharge control strategy of the energy storage, and the energy storage cost. The algorithms for solving the peak-shaving and valley filling strategy of battery energy storage system mainly include gradient algorithms, intelligent algorithms and dynamic programming algorithms. Different power grid peak shaving requirements and energy storage control strategies have different requirements for power and capacity. In practical applications, the configuration of energy storage system needs to be carried out under the constraints of various practical conditions. At present, domestic large energy storage power stations are still in the initial stage, and only experimental or exemplary energy storage power stations have been operated, and have not yet been put into commercial use on a large scale.

3) Microgrid application

 

Microgrid is a new grid structure proposed to promote the utilization of renewable energy. It is a regional grid form composed of renewable energy, energy storage system and load. As an independent whole, it can operate in parallel or isolated island under off-grid conditions. The energy storage system, as a constituent unit of the microgrid, is the energy buffer link in the microgrid. It plays an important role in improving the control stability, improving the power quality of the microgrid, maintaining the power balance of the microgrid, and improving the anti-interference ability of the microgrid. In addition, the energy storage system in the microgrid can also be used as emergency standby in case of power supply interruption.

The energy storage system configured in the microgrid is generally configured in parallel with the renewable energy generation system, and has an independent energy storage management system (such as battery control system, BESS), whose operation mode changes with the microgrid operation mode (off-grid/grid-connected). The capacity and power configuration of energy storage batteries depend on different microgrid configurations and operation modes, and are also restricted by the operation modes of solar power energy storage systems. At present, the configuration and control strategy of energy storage system in microgrid is the focus of microgrid related research.

2. Independent photovoltaic energy storage system

 

The independent photovoltaic system refers to the photovoltaic system that operates independently without connecting to the grid, compared with the grid-connected photovoltaic system. At present, independent systems widely used, such as solar street lamps, solar mobile power supplies, etc., whose photovoltaic power generation output and load power consumption are not in the same time period, and the photovoltaic power generation output cannot always meet the load requirements, so the independent photovoltaic power generation system is generally equipped with energy storage as an effective means to effectively improve the utilization of photovoltaic power output and enhance the stability of the system, At the same time, the energy storage system can also provide starting current and clamping voltage for the load. At present, the widely used independent photovoltaic system is generally composed of photovoltaic power generation, control/inverter and energy storage.

 

At present, batteries are generally used as energy storage devices in independent photovoltaic systems that have been commercially produced and applied. At present, the service life of PV modules and control/inverter can reach more than 10 years, but the service life of battery is only 6~7 years, and the cost of battery can account for more than 25% of the whole system. Therefore, the current configuration and control goal of energy storage in independent PV system is to extend the service life of battery as much as possible and reduce the system cost. On this premise, the primary goal of energy storage capacity and power allocation in an independent photovoltaic system is to optimize the battery charging and discharging energy storage life. The capacity of the energy storage unit in an independent photovoltaic system is usually small. Generally, independent control systems and control strategies are not configured. The energy storage and photovoltaic modules are controlled by the same control system. At present, the most widely studied and applied maximum power point tracking (MPPT) control system is aimed at optimizing the output of photovoltaic modules, and lacks the optimal control of the energy storage system. With the increasing efficiency of PV modules (20.20%, Leye PV) and the decreasing price (2.86 yuan/W, Jingke Energy, 2017.3,), and the high cost of energy storage, the development of control strategies and control systems with priority to energy storage in independent PV systems has broad prospects.