hybrid solar storage system-difference between AC coupling system and DC coupling system

Solar storage system, including solar modules, controllers, inverters, batteries, loads and other equipment. At present, there are many technical routes, but the energy needs to be collected at a certain point. At present, there are mainly two topologies of DC coupling "DC Coupling" and AC coupling "AC Coupling".

1, DC coupling
As shown in the figure below, the DC power 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.

The working principle of DC coupling: When the photovoltaic system is running, the MPPT controller is used to charge the battery; when the electrical load has demand, the battery will release electricity, and the magnitude 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, the grid and photovoltaic can supply power to the load at the same time. Because the photovoltaic power generation and load power consumption are not stable, it is necessary to rely on the battery to balance the system energy.

2,AC coupling
As shown in the figure below, the direct current generated by the photovoltaic modules is converted into alternating current through the inverter, which is directly fed to the load or sent to the grid. The grid can also charge the battery through the bidirectional DC-AC bidirectional converter. The gathering point of energy is at the AC side.

Working principle of AC coupling: including photovoltaic power supply system and battery power supply system. The photovoltaic system consists of photovoltaic arrays and grid-connected inverters; the battery system consists of battery banks and bidirectional inverters. The two systems can operate independently without interfering with each other, or they can be separated from the large power grid to form a micro-grid system.

Both DC coupling and AC coupling are currently mature solutions, and each has its own advantages and disadvantages. According to different applications, choose the most suitable solution. The following is a comparison of the two solutions.

1 Cost comparison
DC coupling includes controller, bidirectional inverter and transfer switch, and AC coupling includes grid-connected inverter, bidirectional inverter and power distribution cabinet. From the cost point of view, controller is cheaper than grid-connected inverter, The switch is also cheaper than the power distribution cabinet. The DC coupling scheme can also be made into an integrated control and inverter machine, which can save both equipment cost and installation cost. Therefore, the cost of the DC coupling scheme is lower than that of the AC coupling scheme.
2 Applicability comparison
In the DC coupling system, the controller, battery and inverter are connected in series, and the connection is relatively tight, but the flexibility is poor. In the AC coupling system, the grid-connected inverter, battery and bidirectional converter are parallel, the connection is not tight, and the flexibility is good. For example, in an installed photovoltaic system, an energy storage system needs to be installed, and AC coupling is better, as long as the battery and bidirectional converter are installed, it will not affect the original photovoltaic system, and the energy storage system In principle, the design is not directly related to the photovoltaic system and can be determined according to the needs. If it is a newly installed off-grid system, photovoltaics, batteries, and inverters must be designed according to the user's load power and electricity consumption, and the DC coupling system is more suitable. However, the power of the DC coupling system is relatively small, generally below 500kW, and the larger system is better controlled with AC coupling.
3 Efficiency comparison
From the perspective of photovoltaic utilization efficiency, the two schemes have their own characteristics. If the user has more loads during the day and less at night, it is better to use AC coupling. The photovoltaic modules directly supply power to the load through the grid-connected inverter, and the efficiency can reach More than 96%. If the user has less load during the day and more at night, the photovoltaic power generation needs to be stored during the day and used at night. It is better to use DC coupling. The photovoltaic module stores electricity to the battery through the controller, and the efficiency can reach more than 95%. If it is AC coupled , the photovoltaic must first be converted into alternating current through the inverter, and then into direct current through the bidirectional converter, and the efficiency will drop to about 90%.