AC Coupled VS DC Coupled

In recent years, photovoltaic power generation technology has advanced rapidly, and the installed capacity has increased rapidly. However, photovoltaic power generation has shortcomings such as intermittent and uncontrollable. If electricity is directly connected to the power grid on a large scale before these problems are dealt with, it will have a great impact and affect the stable operation of the power grid. Therefore, the addition of the energy storage link can allow the smooth and stable output of photovoltaic power generation to the power grid, and large-scale access to the power grid will not affect the stability of the power grid. In addition, photovoltaic and energy storage have a wider range of system applications.

Content

What is Coupling?
What is DC?
What is AC?
What is DC Coupling?
What is AC-Coupling?
AC-Couple VS DC-Couple

There are four main operating modes of photovoltaic household energy storage system：

First, it is stored during the day when photovoltaic power generation is generated, and then released when the user needs it at night.
The second is that the electricity price can be charged in the valley section and discharged in the peak section, and the difference between the peak and valley can be used to achieve maximum income.
Third, if you can’t sell electricity online, you can install an anti-counter current system. When the photovoltaic power is greater than the load power, the excess electrical energy can be stored to avoid waste.
Fourth, when the power grid is out of power, the photovoltaic can continue to generate electricity, the inverter is switched to an off-grid operating mode, the system continues to work as a backup power supply, and the photovoltaic and battery can power the load through the inverter.

Optical 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 gathered at a certain point. At present, there are two topologies: DC Coupling and AC Coupling.

What is Coupling?

The transmission process (or transmission method) of energy from one medium to another is called coupling. For example, the transmission process of electrical energy from metal wires to optical cables is a kind of coupling, the kinetic energy of the motor becomes the pressure energy of the hydraulic pump, and the intermediate transmission process is also a kind of coupling. It can be understood that coupling is a transmission tool, or a way of transformation.

What is DC?

What is a DC system?
Direct Current is referred to as DC for short, which refers to a current that does not change periodically in direction and time, but the size of the current may not be fixed, resulting in a waveform, also known as a constant current.The circuit through which it passes is called a DC circuit, which is a closed conductive circuit composed of a DC power supply and a resistor.Such as: dry batteries, lead-acid batteries.

The DC system provides reliable DC power supply for control, signal, protection, automatic devices, and accident lighting in substations.It also provides a reliable operating power supply for operation.The reliability of the DC system plays a vital role in the safe operation of the substation and is the guarantee of the safe operation of the substation.

Advantages of DC direct current:
There is no stability problem in the DC grid. Direct current is used for DC loads, and it can be used without rectifier and filtering equipment, which saves energy and reduces consumption compared to alternating current.

Disadvantages of DC direct current:
DC voltage boost is difficult, and the use of transformers to boost voltage is not feasible. Only special electronic circuits can be used to achieve voltage boost.If the power generation capacity is the same, the DC power generation equipment is more complicated than the AC power generation equipment. So far, there is no large-scale special case of power grid, and the technical standards and technical systems are not complete.

What is AC?

The current whose intensity and direction change periodically over time is called alternating current, or alternating current for short.The direction and size of the current will change over time.The generator of a power plant uses power to make the coil in the generator run. The direction of the output current of the generator will change once every 180° revolution, so the size of the current will also change regularly over time. This kind of power supply is called “AC power supply”.

Advantages of AC alternating current
1.As long as the AC power supply uses a transformer, the voltage (boost, buck) can be easily converted.
2.When transmitting power, it can maintain high voltage/low current and reduce the phenomenon of voltage drop (I2R loss).
3.It can easily convert AC power supply into DC power supply, and it is easy to supply power to DC drive equipment.

However, there are stability problems in the AC power system. If it is an AC power supply, the load is DC, and it must be rectified and filtered several times before it can be used.

What is DC Coupling?

As shown in the figure below, the direct current emitted by the photovoltaic module is stored in the battery pack through the controller, and the power grid can also charge the battery through a two-way DC-AC converter. The gathering point of energy is at the DC battery terminal.

The working principle of DC coupling: When the photovoltaic system is running, the battery is charged by the MPPT controller; when the electrical load is in demand, the battery will release power, and the size of the current is determined by the load. The energy storage system is connected to the power grid. If the load is small and the battery is fully charged, the photovoltaic system can supply power to the power grid. When the load power is greater than the photovoltaic power generation power, the power grid and photovoltaic can supply power to the load at the same time. Because neither photovoltaic power generation nor load electricity consumption are stable, it is necessary to rely on the battery to balance the energy of the system.

What is AC-Coupling?

As shown in the figure below, the direct current emitted by the photovoltaic module is turned into alternating current through the inverter, which is directly fed to the load or into the power grid. The power grid can also charge the battery through a two-way DC-AC two-way converter. The gathering point of energy is at the AC terminal.

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

Cost: AC-Couple VS DC-Couple

DC coupling includes a controller, a two-way inverter and a switch, and AC coupling includes a grid-connected inverter, a two-way inverter and a power distribution cabinet. From the cost point of view, the controller is cheaper than the grid-connected inverter, and the switch is cheaper than the power distribution cabinet. The DC coupling scheme can also be made into a control inverter all-in-one machine, equipment costs and installation costs can be saved, so the cost of the DC coupling scheme is lower than that of the AC coupling scheme.

Applicability:AC-Couple VS DC-Couple

Efficiency: AC-Couple VS DC-Couple

From the point of view of the utilization efficiency of photovoltaic, 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. Photovoltaic modules directly power the load through grid-connected inverters, and the efficiency can reach more than 96%. If the user has relatively little load during the day and more at night, photovoltaic power generation during the day needs to be stored and reused at night. It is better to use DC coupling. Photovoltaic modules store electricity to the battery through the controller, and the efficiency can reach more than 95%. If it is AC coupling, photovoltaic must first be converted into alternating current through the inverter, and then into direct current through the two-way converter, the efficiency will drop to about 90%.

The above is the difference between AC coupling and DC coupling.The combination of photovoltaic and energy storage batteries can be more energy-efficient and save money.Photovoltaic and energy storage are constantly developing, and lithium iron phosphate batteries have gradually developed into the primary choice for energy storage batteries.We also have many batteries for you to choose from

Prev Article: How to Choose: UPS, Whole-home Backup or Solar Generators?

Next Article: Four Types of Photovoltaic Energy Storage Systems