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1. Types of energy storage inverter
Optical storage system, including solar modules, controllers, inverters, batteries, loads and other equipment. At present, there are mainly two technical routes for energy storage inverter, DC coupling and AC coupling. AC or DC coupling refers to the way solar panels are coupled or linked to an energy storage or battery system. The type of connection between the solar module and the battery can be AC or DC.
The hybrid photovoltaic + energy storage system, that is, the direct current generated by the photovoltaic modules, is stored in the battery pack through the controller, and the power grid can also charge the battery through the bidirectional DC-AC converter. The energy collection point is at the DC battery end. During the day, photovoltaic power generation first supplies the load. Secondly, the battery is charged through the MPPT controller, the energy storage system is connected to the grid, and the excess power can be connected to the grid. At night, the battery is discharged to supply the load, and the insufficient part is supplemented by the grid.
The hybrid energy storage inverter integrates the off-grid function to improve the charging efficiency. The grid-connected energy storage inverter automatically shuts down the power to the solar panel system during a power outage for safety reasons. Hybrid energy storage inverters allow users to have both off-grid and grid-connected capabilities, so electricity can be used even during power outages. Hybrid energy storage inverters simplify energy monitoring, allowing important data such as performance and energy production to be checked via the energy storage inverter panel or connected smart devices.
If the system has two energy storage inverters, they must be monitored separately. Hybrid energy storage inverters are economical, compact and easy to install. Installing a new hybrid energy storage inverter with DC-coupled batteries may be cheaper than retrofitting AC-coupled batteries to an existing system. Because the controller is cheaper than the grid-connected inverter, the switch is also cheaper than the power distribution cabinet.
The DC coupling scheme can also be made into a control and inverter integrated machine, which can save both equipment cost and installation cost. Especially for low to medium power off-grid systems, DC-coupled systems are extremely cost-effective. However, hybrid inversion is not suitable for upgrading existing solar systems, and systems with higher power are more complicated to install and more expensive. If a user wants to upgrade an existing solar system to include battery storage, choosing a hybrid energy storage inverter may complicate the situation, while a battery inverter may be more cost-effective.
Coupled photovoltaic + energy storage system, also known as AC retrofit photovoltaic + energy storage system. It can realize that the DC power emitted by the photovoltaic modules is converted into AC power by the grid-connected inverter, and then the excess power is converted into DC power by the AC-coupled energy storage inverter and stored in the battery. The energy collection point is at the AC side. The photovoltaic system consists of photovoltaic arrays and grid-connected inverters, and the battery system consists of battery banks and bidirectional inverters. AC coupling is suitable for system modification, and it is more efficient to use AC loads during the day.
The existing grid-connected photovoltaic system can be transformed into an energy storage system with low input cost. When the power grid is out of power, it can provide users with safe power protection. Compatible with grid-connected photovoltaic systems from different manufacturers. Although relatively simple to set up and powerful, they are slightly less efficient (90-94%) in charging the battery than DC-coupled systems (98%). AC-coupled charging is less efficient and more expensive for small systems. The energy that goes into the battery in AC coupling has to be converted twice, and when the user starts using that energy, it has to be converted again, adding more losses to the system.
Off-grid home photovoltaic + energy storage systems are generally composed of photovoltaic modules, lithium batteries, off-grid energy storage inverter, loads and diesel generators. The system can realize the direct charging of the battery by the photovoltaic through the DC-DC conversion, and can also realize the bidirectional DC-AC conversion for the charging and discharging of the battery. It can meet the daily electricity demand in areas without power grid. Most off-grid energy storage inverter do not have grid-connection certification, so even if the system has a grid, it cannot be connected to the grid.
2. Application scenarios of energy storage inverter
The energy storage inverter has three functions, including peak regulation, backup power supply and independent power supply. From a regional perspective, peak shaving is a European demand. Taking Germany as an example, the electricity price in Germany has continued to rise in recent years. In 2021, the electricity price for German residents has reached 34 euro cents/kWh. The LCOE of PV/PV distribution and storage is only 9.3/14.1 cents per kilowatt-hour, which is 73%/59% lower than the residential electricity price. The difference between the residential price and the cost of PV distribution and storage will continue to widen.
The household photovoltaic distribution and storage system can reduce the cost of electricity consumption, so users have strong incentives to install home energy storage in areas with high electricity prices. In the peak shaving market, users choose hybrid energy storage inverters and AC-coupled battery systems, which are more cost-effective and easier to manufacture. These compact, lightweight energy storage inverters have lower surge and peak power output ratings, but are more cost-effective, cheaper and easier to manufacture.
The backup power supply is needed by the United States and Japan, and the independent power supply is just demanded by the market, including South Africa and other regions. The photovoltaic energy storage system in the United States is larger and equipped with more batteries, because it needs to store electricity to deal with natural disasters.
Independent power supply is just a market demand. In South Africa, Pakistan, Lebanon, the Philippines, Vietnam and other countries under the situation of tight global supply chains, the national infrastructure is not enough to support people's electricity consumption, so users should be equipped with household photovoltaic energy storage systems. Inversion has some limitations compared to dedicated off-grid battery inverters, mainly limited surge or peak power output during outages.
In addition, some hybrid energy storage inverters have no backup power capability or limited backup power, so only small or necessary loads such as lighting and basic power circuits can be backed up during a power outage, and many systems will have 3-5 seconds during a power outage. Delay. If the user plans to power high-surge equipment such as compressors, washing machines and power tools, the energy storage inverter must be able to handle high inductive surge loads.
3. DC-coupled hybrid inverter
At present, most photovoltaic energy storage systems in the industry use DC coupling to realize the integrated design of solar energy storage. Especially in newly added systems, the hybrid energy storage inverter is easy to install and low cost. When adding a new system, the use of solar-storage hybrid energy storage inverter can reduce equipment costs and installation costs, because one energy storage inverter can realize the integration of control and inverter. In the DC-coupled system, the controller, battery and inverter are serial, which is less flexible.
For newly installed systems, photovoltaics, batteries, and inverters are all designed according to the user's load power and electricity consumption, so they are more suitable for DC-coupled hybrids. DC-coupled hybrid energy storage inverter products are the mainstream trend, and major Chinese manufacturers have layouts. SINENG, GOODWE, and GINLONG have also deployed AC-coupled inverters, and the product form is perfect. DEYE's hybrid inverter supports AC coupling on the basis of DC coupling, which provides the convenience of installation for the needs of users' stock transformation.
SUNGROW, HUAWEI, SINENG and some of top 10 home energy storage battery companies deploy energy storage batteries, and the integration of battery inverters may become a trend in the future. The split battery inverter is more popular with installers, but the integrated battery inverter is the future development trend. Most Chinese manufacturers have begun to deploy battery-inverter all-in-one machines. SOFARSOLAR, GROWATT and other manufacturers have all chosen this model. SOFARSOLAR's sales of energy storage batteries in 2021 will reach 35,100 pcs, an increase of 25 times compared with 20 years.
In DC-coupled systems, high-voltage battery systems are more efficient, but cost more in high-voltage battery shortage situations. Compared to 48V battery systems, high voltage batteries operate in the 200-500V DC range with lower cable losses and higher efficiency. Because solar panels typically operate at 300-600V, similar to battery voltages, high-efficiency DC-DC converters with very low losses can be used. High-voltage battery systems are more expensive than low-voltage systems, and inverters are less expensive. At present, the demand for high-voltage batteries is strong and the supply is insufficient, so it is difficult to purchase high-voltage batteries.
In the case of shortage of high-voltage batteries, the cost of using low-voltage battery systems is lower. Hybrid inverters from major Chinese manufacturers have unrestricted standby power output in the event of a power outage, making them suitable for off-grid systems. The backup power of some products is slightly lower than the normal power range, but the backup power of the new products of GOODWE, GINLONG, SUNGROW and Hoymiles is the same as the normal value. That is, the power is not more restricted during off-grid operation, so the energy storage inverter of Chinese inverter manufacturers is suitable for off-grid systems.
4. AC coupled inverter
DC-coupled systems are not suitable for retrofitting existing grid-connected systems. The AC coupled system makes product installation easier. APsystems realizes the bidirectional flow of energy by coupling the AC side and the photovoltaic system, eliminating the need for the access of the photovoltaic DC bus and making the product installation easier. Through the combination of software real-time control and hardware design improvement, the millisecond-level switching between on-grid and off-grid is realized.
Through the innovative design of the output control of the energy storage inverter and the power supply and distribution system, the microgrid application of the whole house power supply under the control of the automatic control box is realized. The maximum conversion efficiency of AC-coupled products is slightly lower than that of hybrid energy storage inverter.
GINLONG and GOODWE have also deployed AC coupling products, which are mainly aimed at the stock transformation market. The maximum conversion efficiency of AC-coupled products is 94-97%, which is slightly lower than that of hybrid energy storage inverter, mainly because the components need to undergo two conversions after power generation before they can be stored in the battery, which reduces the conversion efficiency.
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