Energy storage inverter types
Technical route: There are two major routes: DC coupling and AC coupling
The photovoltaic storage system includes solar panels, controllers, solar inverters, energy storage batteries, loads and other equipment. There are two main technical routes: DC coupling and AC coupling. AC or DC coupling refers to the way the solar panel is coupled or connected to the energy storage or battery system. The connection type between the solar panel and the battery can be AC or DC. Most electronic circuits use DC, solar panels generate DC, and batteries store DC, but most electrical appliances run on AC.
Hybrid photovoltaic + energy storage system, that is, the direct current generated by the photovoltaic module is stored in the battery pack through the controller, and the 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, and then charges the battery 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 discharges to supply the load, and the insufficient part is supplemented by the grid; when the grid is out of power, photovoltaic power generation and lithium batteries only supply power to the off-grid load, and the grid-connected load cannot be used. When the load power is greater than the photovoltaic power generation power, the grid and photovoltaic can supply power to the load at the same time. Because photovoltaic power generation and load power consumption are not stable, they rely on batteries to balance the system energy. In addition, the system also supports users to set the charging and discharging time to meet the user's power demand.
How a DC-Coupled System Works
Source: spiritenergy, Haitong Securities Research Institute
Hybrid photovoltaic + energy storage system
Source: GoodWe Photovoltaic Community, Haitong Securities Research Institute
The hybrid inverter integrates off-grid functionality to improve charging efficiency. Grid-tied inverters automatically shut down power to your solar panel system during a power outage for safety reasons. Hybrid inverters, on the other hand, allow users to have off-grid and on-grid capabilities at the same time, so power can be used even during power outages. Hybrid inverters simplify energy monitoring, allowing vital data such as performance and energy production to be checked through the inverter panel or connected smart devices. If the system has two inverters, they must be monitored separately. DC coupling reduces AC-DC conversion losses. Battery charging efficiency is about 95-99%, while AC coupling is 90%.
Hybrid inverters are economical, compact, and easy to install. Installing a new hybrid inverter with a DC-coupled battery may be cheaper than retrofitting an AC-coupled battery to an existing system because the controller is cheaper than a grid-tied inverter, the switch is cheaper than a distribution cabinet, and the DC-coupled solution can also be made into a controller-inverter all-in-one, saving both equipment and installation costs. Especially for small and medium-power off-grid systems, DC-coupled systems are very cost-effective. Hybrid inverters are highly modular, and it is easy to add new components and controllers. Additional components can be easily added using relatively low-cost DC solar controllers. And hybrid inverters are designed to integrate storage at any time, making it easier to add battery packs. Hybrid inverter systems are relatively compact, use high-voltage batteries, and have smaller cable sizes and lower losses.
DC coupling system configuration
Source: Zhongrui Lighting Network, Haitong Securities Research Institute
AC coupling system configuration
Source: Zhongrui Lighting Network, Haitong Securities Research Institute
However, hybrid inverters are not suitable for upgrading existing solar systems, and larger systems are more complex and expensive to install. If a user wants to upgrade an existing solar system to include battery storage, choosing a hybrid inverter may complicate the situation, and a battery inverter may be more cost-effective because choosing to install a hybrid inverter requires a comprehensive and expensive rework of the entire solar panel system. Larger systems are more complex to install and more expensive due to the need for more high-voltage controllers. If electricity is used more during the day, there will be a slight decrease in efficiency due to DC (PV) to DC (batt) to AC.
The coupled photovoltaic + energy storage system, also known as the AC transformation photovoltaic + energy storage system, can realize that the DC power generated by the photovoltaic module is converted into AC power through the grid-connected inverter, and then the excess power is converted into DC power and stored in the battery through the AC coupled energy storage inverter. The energy collection point is at the AC end. It includes a photovoltaic power supply system and a battery power supply system. The photovoltaic system consists of a photovoltaic array and a grid-connected inverter, and the battery system consists of a battery pack and a bidirectional inverter. The two systems can operate independently without interfering with each other, or they can be separated from the large power grid to form a microgrid system.
How AC-Coupled Systems Work
Source: spiritenergy, Haitong Securities Research Institute
Coupled household photovoltaic + energy storage system
Source: GoodWe Solar Community, Haitong Securities Research Institute
The AC coupling system is 100% compatible with the power grid, easy to install and easy to expand. Standard household installation components are available, and even relatively large systems (2KW to MW level) are easily expandable and can be combined with grid-connected and stand-alone generator sets (diesel units, wind turbines, etc.). Most string solar inverters above 3kW have dual MPPT inputs, so long strings of panels can be installed in different orientations and tilt angles. At higher DC voltages, AC coupling is easier, less complex and therefore less costly to install large systems than DC coupled systems requiring multiple MPPT charge controllers.
AC coupling is suitable for system transformation, and it is more efficient to use AC loads during the day. Existing grid-connected PV systems can be transformed into energy storage systems with low investment costs. It can provide users with safe power protection when the grid is out of power. It is compatible with grid-connected PV systems from different manufacturers. Advanced AC coupling systems are often used for larger off-grid systems and use string solar inverters combined with advanced multi-mode inverters or inverter/chargers to manage batteries and grids/generators. Although relatively simple to set up and powerful, they are slightly less efficient (90-94%) when charging batteries compared to DC coupling systems (98%). However, these systems are more efficient when powering high AC loads during the day, reaching more than 97%, and some systems can be expanded with multiple solar inverters to form microgrids.
AC coupling is less efficient and more expensive for small systems. The energy going into the battery in AC coupling must be converted twice, and when the user starts using that energy, it must be converted again, adding more losses to the system. Therefore, when using a battery system, AC coupling efficiency drops to 85-90%. AC coupled inverters are more expensive for small systems.
The off-grid household photovoltaic + energy storage system is generally composed of photovoltaic modules, lithium batteries, off-grid energy storage inverters, loads and diesel generators. The system can realize direct charging of batteries by photovoltaics through DC-DC conversion, and can also realize bidirectional DC-AC conversion for battery charging and discharging. During the day, photovoltaic power generation first supplies the load, and then charges the battery; at night, the battery discharges to supply the load, and when the battery is insufficient, the load is supplied by diesel generators. It can meet the daily electricity demand in areas without power grids. It can be combined with diesel generators to enable diesel generators to supply loads or charge batteries. Most off-grid energy storage inverters do not have grid connection certification, and even if the system has a grid, it cannot be connected to the grid.
Off grid inverter
Source: Growatt official website, Haitong Securities Research Institute
Off-grid home photovoltaic + energy storage system
Source: GoodWe Photovoltaic Community, Haitong Securities Research Institute
Applicable scenarios for energy storage inverters
Energy storage inverters have three major functions, including peak shaving, backup power supply and independent power supply. From a regional perspective, peak shaving is a demand in Europe. Taking Germany as an example, the electricity price in Germany reached 2.3 yuan/kWh in 2019, ranking first in the world. In recent years, German electricity prices have continued to rise. In 2021, the German residential electricity price has reached 34 euro cents/kWh, while the photovoltaic/photovoltaic distribution and storage LCOE is only 9.3/14.1 euro cents/kWh, which is 73%/59% lower than the residential electricity price. The residential electricity price is the same as The difference between photovoltaic distribution and storage electricity costs will continue to widen. Household photovoltaic distribution and storage systems can reduce electricity costs, so users in areas with high electricity prices have strong incentives to install household storage.
Residential electricity prices in various countries in 2019
Source: EuPD Research, Haitong Securities Research Institute
Electricity price level in Germany (cents/kWh)
Source: EuPD Research, Haitong Securities Research Institute
In the peak load market, users choose hybrid inverters and AC-coupled battery systems, which are more cost-effective and easier to manufacture. Off-grid battery inverter chargers with heavy transformers are more expensive, and hybrid inverters and AC-coupled battery systems use transformerless inverters with switching transistors. These compact and lightweight inverters have lower surge and peak power output ratings, but are more cost-effective, cheaper and easier to manufacture.
Backup power supply is needed by the United States and Japan, and independent power supply is in urgent market demand, including South Africa and other regions. According to the EIA, the average power outage duration in the United States in 2020 exceeded 8 hours, which was mainly affected by the scattered residence of American residents, the aging of some power grids, and natural disasters. The application of household photovoltaic distribution and storage systems can reduce dependence on the power grid and increase the reliability of power supply on the user side. 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 an urgent market demand. In countries such as South Africa, Pakistan, Lebanon, Philippines, and Vietnam, where the global supply chain is tight, the national infrastructure is not enough to support people's electricity consumption, so users must be equipped with household photovoltaic energy storage systems.
U.S. power outage duration per capita (hours)
Source: EIA, Haitong Securities Research Institute
In June 2022, South Africa began level six power rationing, with many places experiencing power outages for 6 hours a day.
Source: GoodWe Photovoltaic Community, Haitong Securities Research Institute
Hybrid inverters have some limitations as backup power. Compared with dedicated off-grid battery inverters, hybrid inverters have some limitations, mainly limited surge or peak power output during power outages. In addition, some hybrid 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 power outages, and many systems will have a 3-5 second delay during power outages. Off-grid inverters provide very high surge and peak power output and can handle high inductive loads. If users plan to power high-surge equipment such as pumps, compressors, washing machines, and power tools, the inverter must be able to handle high inductive surge loads.
Hybrid inverter output power comparison
Source: clean energy reviews, Haitong Securities Research Institute
DC coupled hybrid inverter
At present, most photovoltaic energy storage systems in the industry use DC coupling to achieve integrated photovoltaic and energy storage design, especially in new systems, where hybrid inverters are easy to install and low cost. When adding a new system, using a photovoltaic and energy storage hybrid inverter can reduce equipment costs and installation costs, because one inverter can achieve integrated control and inverter. The controller and switching switch in the DC coupling system are cheaper than the grid-connected inverter and distribution cabinet in the AC coupling system, so the DC coupling solution is cheaper than the AC coupling solution. In the DC coupling system, the controller, battery and inverter are serial, the connection is relatively tight, and the flexibility is poor. For newly installed systems, photovoltaics, batteries, and inverters are designed according to the user's load power and power consumption, so they are more suitable for DC-coupled hybrid inverters.
DC-coupled hybrid inverter products are the mainstream trend, and major domestic manufacturers have deployed them. Except for AP Energy, major domestic inverter manufacturers have deployed hybrid inverters, among which Sineng Electric, GoodWe, and Jinlong have also deployed AC-coupled inverters, and the product form is complete. Deye's hybrid inverter supports AC coupling on the basis of DC coupling, which provides installation convenience for users' stock transformation needs. Sungrow, Huawei, Sineng Electric, and GoodWe have deployed energy storage batteries, and battery inverter integration may become a trend in the future.
Layout of major domestic inverter manufacturers
Source: Official websites of various companies, Haitong Securities Research Institute
Three-phase high-voltage products are the focus of all companies, and Deye focuses on the low-voltage product market. Currently, most hybrid inverter products are within 10KW, products below 6KW are mostly single-phase low-voltage products, and 5-10KW products are mostly three-phase high-voltage products. Deye has developed a variety of high-power low-voltage products, and the low-voltage 15KW product launched this year has begun to sell.
Domestic inverter manufacturers hybrid inverter products
The maximum conversion efficiency of new products from domestic inverter manufacturers has reached about 98%, and the on-grid and off-grid switching time is generally less than 20ms. The maximum conversion efficiency of Jinlong, Sungrow, and Huawei products has reached 98.4%, and GoodWe has also reached 98.2%. The maximum conversion efficiency of Homai and Deye is slightly lower than 98%, but Deye's on-grid and off-grid switching time is only 4ms, far lower than the 10-20ms of its peers.
Comparison of maximum conversion efficiency of hybrid inverters from various companies
Source: Official websites of each company, Haitong Securities Research Institute
Comparison of switching time of hybrid inverters of various companies (ms)
Source: Official websites of each company, Haitong Securities Research Institute
The main products of domestic inverter manufacturers are mostly targeted at the three major markets of Europe, the United States, and Australia. In the European market, traditional photovoltaic core markets such as Germany, Austria, Switzerland, Sweden, and the Netherlands are mainly three-phase markets, which prefer products with higher power. The traditional manufacturers with advantages are Sunshine and Goodwe. Ginlang is accelerating to catch up, relying on The price advantage and the launch of high-power products above 15KW are favored by users. Southern European countries such as Italy and Spain mainly need single-phase low-voltage products. Goodwe, Ginlang and Shouhang performed well in Italy last year, each accounting for about 30% of the market. Eastern European countries such as the Czech Republic, Poland, Romania, and Lithuania mainly demand three-phase products, but their price acceptance is low. Therefore, Shouhang performed well in this market with its low price advantage. In the second quarter of this year, Deye began shipping 15KW new products to the United States. The United States has larger energy storage systems and prefers higher power products.
Domestic inverter manufacturers’ hybrid inverter products target the market
Source: Official websites of each company, Haitong Securities Research Institute
The split type battery inverter is more popular among installers, but the all-in-one battery inverter is the future development trend. Solar-storage hybrid inverters are divided into hybrid inverters sold separately and battery energy storage systems (BESS) that sell inverters and batteries together. Currently, with dealers controlling the channels, direct customers are relatively concentrated, and products with separate batteries and inverters are more popular, especially outside Germany, because they are easy to install and expand, and can reduce procurement costs. , if one supplier cannot supply batteries or inverters, you can find a second supplier, and the delivery will be more guaranteed. The trend in Germany, the United States, and Japan is all-in-one machines. The all-in-one machine can save a lot of after-sales troubles, and there are certification factors. For example, the fire system certification in the United States needs to be linked to the inverter. The current technological trend is towards all-in-one machines, but in terms of market sales, the split type is more accepted by installers.
Most domestic manufacturers have begun to deploy battery-inverter integrated machines. Manufacturers such as Shohang Xinneng, Growatt, and Kehua have all chosen this model. Shougang Xinneng’s energy storage battery sales in 2021 reached 35,100 pcs, an increase of 25 times compared with 20 years; Growatt’s energy storage in 2021 Battery sales were 53,000 sets, a five-fold increase from 20 years ago. The excellent quality of Airo energy storage inverters has driven the continued growth of battery sales. In 2021, Airo battery shipments were 196.99MWh, with revenue of 383 million yuan, more than double the revenue of energy storage inverters. Customers have a high degree of recognition of inverter manufacturers that make batteries because they have a good cooperative relationship with the inverter manufacturers and have trust in the products.
Shouhang New Energy Storage Battery’s Revenue Proportion Increases Rapidly
rce: EIA, Haitong Securities Research Institute
Airo’s energy storage battery revenue will account for 46% in 2021
Source: GoodWe Photovoltaic Community, Haitong Securities Research Institute
In DC coupled systems, high voltage battery systems are more efficient, but more expensive in case of high voltage battery shortage. Compared with 48V battery systems, high voltage batteries have an operating voltage range of 200-500V DC, lower cable losses and higher efficiency, because solar panels usually operate at 300-600V, similar to the battery voltage, and very low loss and high efficiency DC-DC converters can be used. High voltage battery systems have higher battery prices and lower inverter prices than low voltage systems. Currently, high voltage batteries are in high demand and insufficient supply, so high voltage batteries are difficult to purchase. In case of high voltage battery shortage, it is cheaper to use low voltage battery systems.
DC coupling between solar array and inverter
Source: clean energy reviews, Haitong Securities Research Institute
Direct DC coupling to compatible hybrid inverters
rce: clean energy reviews, Haitong Securities Research Institute
Hybrid inverters from major domestic manufacturers are suitable for off-grid systems because their backup power output during power outages is not limited. The backup power supply power of some products is slightly lower than the normal power range, but the backup power supply power of new products of Goodwe, Jinlang, Sungrow, and Hemai is the same as the normal value , that is, the power is not more restricted when running off-grid, so domestic Inverter manufacturers' energy storage inverters are suitable for off-grid systems.
Comparison of backup power supply power of hybrid inverter products from domestic inverter manufacturers
Data sources: Official websites of each company, Haitong Securities Research Institute
AC coupled inverter
DC-coupled systems are not suitable for retrofitting existing grid-connected systems. The DC coupling method mainly has the following problems: First, the system using DC coupling has problems with complex wiring and redundant module design when modifying the existing grid-connected system; second, the delay in switching between grid-connected and off-grid is long, which is difficult for users to use. The electricity experience is poor; third, the intelligent control functions are not comprehensive enough and the control response is not timely enough, making it difficult to implement microgrid applications for whole-house power supply. Therefore, some companies have chosen the AC coupling technology route, such as Yuneng.
AC coupling system makes product installation easier. Yuneng realizes the two-way flow of energy by coupling the AC side and the photovoltaic system, eliminating the need for access to the photovoltaic DC bus, making product installation easier; it realizes off-grid integration through a combination of software real-time control and hardware design improvements Millisecond-level switching; through the output control of the energy storage inverter and the innovative combined design of the power supply and distribution system, the microgrid application of 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 inverters. Jinlong and GoodWe have also deployed AC-coupled products, mainly targeting the stock transformation market. The maximum conversion efficiency of AC-coupled products is 94-97%, which is slightly lower than that of hybrid inverters. This is mainly because the components have to undergo two conversions before they can be stored in the battery after generating electricity, which reduces the conversion efficiency.
Comparison of AC-coupled products from domestic inverter manufacturers
Source: Official websites of various companies, Haitong Securities Research Institute
Post time: May-20-2024
