1. What is a photovoltaic energy storage system?

1. 1 Definition of photovoltaic energy storage

Photovoltaic energy storage system, also known as solar photovoltaic energy storage system, is an important energy utilization method that combines solar photovoltaic power generation with energy storage technology. The system converts solar energy into electrical energy through photovoltaic panels and uses energy storage devices (such as battery packs) to store this electrical energy so that it can supply electricity when needed.          

   1.2 Relationship between photovoltaic energy storage and photovoltaic power generation

The relationship between photovoltaic energy storage and photovoltaic power generation is complementary. By combining energy storage technology with photovoltaic power generation, an integrated photovoltaic energy storage system can be formed, which brings many advantages and solves practical problems.

Photovoltaic power generation depends on sunlight and has obvious intermittency and volatility, that is, the power generation is large when the sunshine is sufficient, but the power generation is reduced or even zero at night or on cloudy days. This characteristic leads to a mismatch between the output power of photovoltaic power generation and the demand of the power grid, which may cause instability of the power grid. In order to solve this problem and improve the stability and reliability of the power system , the introduction of energy storage systems becomes particularly important. The energy storage system can effectively smooth the output fluctuations of photovoltaic power generation and improve the stability of the power grid through charging and discharging operations. Releasing the stored electric energy during peak power load can reduce the pressure on the power grid; charging and storing excess electric energy during low load can achieve the effect of peak shaving and valley filling. By rationally configuring the capacity and power of the energy storage system, the operating cost of the power grid can also be reduced and the overall economic benefits can be improved.

2. How to store energy from photovoltaic power generation

2.1 Working principle of photovoltaic energy storage system

The working principle of the photovoltaic energy storage system mainly includes two processes: photovoltaic power generation and energy storage.

The photovoltaic power generation process uses the photoelectric effect to convert light energy into electrical energy. In a photovoltaic cell, when sunlight shines on a semiconductor material, the energy of the photons causes the electrons in the material to be excited, thereby generating an electric current. Photovoltaic cells are generally composed of multiple modules, each of which contains many solar cells, which are assembled together in series or parallel to provide higher voltage and current. The entire photovoltaic power generation process is the process of converting solar energy into direct current through photovoltaic cells.

2.2 Composition of photovoltaic energy storage system

As an important component of modern renewable energy utilization, photovoltaic energy storage system integrates multiple key technologies such as photoelectric conversion, electric energy storage, system control, inverter conversion, energy management, grid interaction and emergency backup , and realizes efficient collection, storage and flexible utilization of solar energy. The following is a detailed description of the main components of photovoltaic energy storage system:

（1）conversion is the starting point of photovoltaic energy storage systems, which is mainly achieved through photovoltaic cells (also known as solar panels) . Photovoltaic cells use the photoelectric effect of semiconductor materials to directly convert sunlight into DC power . When sunlight shines on the surface of photovoltaic cells, photons excite electrons in semiconductor materials to form current, completing the conversion from light energy to electrical energy. This process does not require fuel, is clean and pollution-free, and has almost unlimited resources.

（2）Energy Storage Due to the intermittent nature of sunlight, the electricity generated by photovoltaic cells is unstable , so it needs to be regulated through energy storage. Energy storage is mainly achieved through battery packs (such as lithium-ion batteries, lead-acid batteries, etc.) , which can store excess energy when there is sufficient sunlight and release electricity for load use when there is insufficient sunlight or at night. The capacity and efficiency of the energy storage system directly affect the operating stability and economy of the entire photovoltaic energy storage system.

（3）System control system control is the core of the photovoltaic energy storage system , responsible for monitoring the operating status of the entire system and adjusting the system's operating mode according to real-time data and preset strategies. The control system automatically adjusts the working point of the photovoltaic cell, the charging and discharging status of the battery pack, the grid voltage and frequency , and other information to ensure safe and efficient operation of the system. At the same time, the control system also has fault detection and protection functions, which can detect and handle system abnormalities in a timely manner.

（4）Inverter conversion Since photovoltaic cells generate direct current , and most electrical equipment requires alternating current, inverter conversion is an indispensable part of photovoltaic energy storage systems. The inverter converts direct current into alternating current and adjusts the output voltage, frequency and phase to meet the requirements of the grid or load. High-quality inverters should have the characteristics of high efficiency, low harmonic distortion, wide input voltage range and fast response capability to ensure the reliability and stability of power conversion.

3. Classification of Photovoltaic Energy Storage Systems

3.1 Classification by investment entity

According to different investment entities, photovoltaic energy storage systems can be divided into two categories: commercial and household.

（1）Industrial and commercial photovoltaic energy storage system Industrial and commercial photovoltaic energy storage systems are widely used in factories and shopping malls, photovoltaic storage charging stations, microgrids, and new application scenarios, such as data centers, 5G base stations, battery replacement heavy trucks, port shore power, etc. The scale of industrial and commercial photovoltaic energy storage systems is large, generally covering an area of 120-160 acres or more, and the single-point grid-connected installed capacity is required to be less than 6MW. The application scenarios of industrial and commercial photovoltaic energy storage systems include peak-valley price arbitrage, participating in related transactions of virtual power plant products, or participating in demand response events. The main purpose is to reduce corporate electricity costs. Participants in the industrial and commercial photovoltaic energy storage market include traditional power generation and power grid companies, end users, distributed investors, energy-saving service providers, etc. It is a blue ocean market with unlimited room for innovation.

（2）Household photovoltaic energy storage systems are usually installed on the roof or courtyard of a family home . This type of system is relatively small, covering an area of about 20 acres, and the installed capacity generally does not exceed 50kw . The economic model of household photovoltaic energy storage systems includes self-generation and self-use and surplus power access to the grid, aiming to improve the level of self-generation and self-use of electricity, and use the peak-valley electricity price difference to increase the energy storage kilowatt-hour income, thereby saving electricity expenses. The application of household photovoltaic energy storage systems is not only conducive to environmental protection and energy saving, but also has the advantage of high flexibility. With the adjustment of household photovoltaic subsidy policies such as photovoltaic grid-connected electricity prices (FIT) and net metering electricity prices, the economic efficiency of household photovoltaic energy storage systems has been improved, further promoting the growth of the household energy storage market.

3.2 Classification by connection mode to the power grid, energy management and storage method

(1) Grid-connected system

This system directly connects the photovoltaic power generation system to the public power grid, using the grid to regulate and manage power supply and demand. Grid-connected systems are often used in large commercial and industrial applications because they can take full advantage of the stability and reliability of the public power grid.

(2) Off-grid systems Off-grid systems are independent of the public power grid and are suitable for remote areas or places without access to the public power grid. These systems usually include energy storage devices to provide electricity during periods without sunshine.

(3) Off-grid energy storage system This system combines the characteristics of on-grid and off-grid. It has an energy storage device that can rely on the energy storage device to supply power when there is no sunlight, and can feed back excess electricity to the grid when there is sunlight.

(4) Microgrid system A microgrid system is a small power generation and distribution network that can operate independently or be connected to the public power grid. It usually includes multiple components such as distributed power sources, energy storage equipment, and load management. It can achieve self-control and autonomous management and is suitable for occasions that require independent power supply but want to maintain a certain connection with the external power grid.