Energy storage technology refers to the process of storing energy through media or equipment and releasing it when needed. And the hybrid technology of photovoltaic + energy storage combines solar photovoltaic power generation with energy storage technology to store the electric energy generated by the photovoltaic system so that it can be used when needed, ensuring stable power supply.
Currently, the system solutions for photovoltaic + energy storage on the market mainly include AC coupling and DC coupling. So what is the difference between AC coupling vs DC coupling? Let’s find out together.
Main content:
1. What is coupling
Before starting the introduction to AC coupling vs DC coupling, let’s first understand the concept of “coupling”. In the circuits, coupling refers to the close cooperation and mutual influence between the input and output of two or more circuit elements or circuit networks, and through the interaction, transfer the energy from one side to the other.
2. What is a DC coupled system
The schematic diagram of a DC coupled solar storage system is as the figure below:
In a DC-coupled system, photovoltaic modules and energy storage batteries are directly connected to the hybrid inverter (the charge control and multimode inverter modules are usually integrated into one product, called a hybrid inverter).
The power generated by the modules enters the battery storage through the MPPT controller (DC-DC), the current from the grid can also be stored into the battery through the hybrid inverter (AC-DC). The energy is gathered on the battery (DC) side, so it is called a DC coupled system.
3. What is an AC coupled system
The schematic diagram of a DC coupled solar storage system is as the figure below:
In an AC coupled system, there are two inverters:
- The photovoltaic inverter is connected to the photovoltaic modules and converts the power generated by the modules into alternating current output (DC-AC).
- The energy storage inverter (also commonly known as energy storage PCS, AC coupled inverter, etc.) are connected to the battery, and the electric energy stored in the battery is released to the load (DC-AC). The energy of the entire system is gathered on the AC side, so it is called an AC coupled system.
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4. AC Coupling vs DC Coupling - the differences
Through what we mentioned above, we understand what DC coupling is and what AC coupling is. Since the circuit structure and electrical equipment of the two are different, there are also big differences between AC coupling vs DC coupling in working principles, flexibility, efficiency, cost, etc.
① Working principle
- DC coupling:
As shown in the DC coupling scheme in the figure below, the solar inverter like 2000w pure sine wave inverter or 3000 watt solar inverter, and bidirectional converter are integrated into a hybrid photovoltaic and storage inverter, and are directly connected to photovoltaic modules, power grids, batteries, etc.
When the photovoltaic system is running, the battery is charged through the MPPT controller. When there is demand from the load, the battery will release power, and the size of the current is determined by the load.
The energy storage system is connected to the grid, and if the load is small and the battery is fully charged, the photovoltaic system can supply power to the grid. When the load power is greater than the photovoltaic power generation, the grid and photovoltaics can supply power to the load at the same time.
Or when the load power is greater than the battery output power at night, the grid and the battery can charge the load at the same time (battery mode is used first). For the best batteries, check battery stores near me.
- AC coupling:
As shown in the figure below, the AC coupling scheme includes two parts: the photovoltaic system and the energy storage system. The photovoltaic system consists of photovoltaic arrays and photovoltaic inverters; the energy storage system consists of energy storage inverters and batteries.
In AC coupling, photovoltaics and energy storage can operate independently of each other without interfering with each other. The electric energy generated by photovoltaic modules can be sent to the power grid through the photovoltaic inverter, or power supply to the load. When the power is insufficient, the power grid assists in power supply (this mode is a traditional photovoltaic system).
The LFP battery can supply power to the load through the energy storage inverter, and the grid can assist when the power is insufficient; the battery's power can also be sent to the grid, and the grid can also provide power to the battery. Photovoltaics and energy storage can also work together.
In self-consumption mode, the energy storage inverter will detect the energy generated by the photovoltaic inverter, which will be used by local loads first, and the rest will be stored in the battery through the AC coupling inverter, and the excess energy will be fed back to the grid.
During a power outage, the energy storage inverter will cut off the connection with the grid (anti-islanding protection) and simulate the waveform of the grid to allow the photovoltaic inverter to continue working, forming a microgrid system with each other.
② Flexibility
In terms of system flexibility, comparing AC coupling vs DC coupling, AC coupled systems are more advantageous.
- DC coupling:
Photovoltaic modules, integrated photovoltaic storage device and batteries are in a serial state and are closely connected to each other. The addition and removal of equipment is complex, which makes the flexibility relatively not so good.
It is mainly used in new photovoltaic markets, such as newly installed photovoltaic storage systems. Photovoltaic modules, photovoltaic storage integrated machines and batteries need to be designed according to the user's load power and power consumption.
- AC coupling:
When compare AC coupling vs DC coupling, the photovoltaic inverter, energy storage inverter and battery are connected in parallel in AC coupling system, with flexible connection, making it easier to add or remove the equipment. It can be applied to photovoltaic stock and new markets.
If a new energy storage system is added to the original photovoltaic system, it can be directly incorporated into the equipment for use without additional system design adjustments. In addition, the design of the energy storage system is not directly related to the photovoltaic system in principle and can be determined according to specific needs.
Generally speaking, in the field of home solar power system, comparing AC coupling vs DC coupling, AC coupling is mainly used in the stock market, that is, households that have already installed photovoltaic systems and want to add new energy storage systems; while DC coupling systems are mainly used in the incremental market, that is, installing a complete set of photovoltaic systems from the very beginning.
③ Solar utilization
From the perspective of solar utilization efficiency, both solutions have their own characteristics. Comparing AC coupling vs DC coupling in terms of energy utilization efficiency, DC coupling is better than AC coupling.
In an AC-coupled system, photovoltaic energy stored in the battery must undergo two conversions of DC-AC and AC-DC. When the battery energy is released, another DC-AC conversion is required. The three conversion processes cause a large energy loss in the system and reduce the overall efficiency.
The total efficiency is about 90%, and this solution is more suitable for situations where the electricity consumption during the day is greater than the electricity consumption at night.
DC coupling charges the battery through MPPT, and the energy loss during the DC-DC conversion process is extremely low. When the energy is released, it only needs to undergo one DC-AC conversion through the hybrid inverter.
The overall efficiency can reach more than 97%, and electric energy can be stored for future use. Prepared for night use, this solution is more suitable for situations where the electricity consumption during the day is less than the electricity consumption at night.
④ Cost
In terms of the cost, DC coupling is better comparing AC coupling vs DC coupling. A very intuitive point is that AC coupling requires two inverters, while DC coupling only requires one, and DC coupling also has an optional choice of hybrid inverter, which has certain advantages in terms of equipment cost and installation cost.
Summary
Solar storage systems mainly have two connection schemes: DC coupling and AC coupling. Currently, both methods are technically mature, each has its own advantages and disadvantages, and is suitable for different application scenarios. The characteristics and selection of AC coupling vs DC coupling are not absolute, and more considerations need to be made based on actual application requirements.
For example, for a family that mainly uses the load during the day, which can absorb the photovoltaic power generation, the AC coupling system can be directly loaded through the photovoltaic inverter, the power is only one DC-AC conversion, the comprehensive efficiency can also reach more than 97%.
Therefore, when choosing the system coupling method, more considerations need to be made according to the actual situation.
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