AC-Coupled BESS Vs DC-Coupled BESS

What is Coupling in BESS?

In the context of Integrated Battery Energy Storage Systems, coupling refers to how battery energy storage is electrically interfaced or interconnected with the electrical power grid or other sources of energy or distributed energy resources.

?Solar PV array generates DC power that is converted into AC power through an Inverter, which is suitable for domestic or industrial loads. Solar PV batteries store energy in DC form. So, the difference between AC-coupled and DC-coupled batteries lies in whether the electricity generated by your solar panels is inverted?before?or?after?being stored in your battery.?

AC-Coupled Integrated BESS

In an AC-coupled Integrated BESS, the Battery system is interconnected to the grid or externally to the solar PV system on the AC side of the PV inverter. The BESS has its own dedicated inverter connected to the battery known as Battery Inverter that converts the DC output of the battery into AC power. AC coupled BESS has two different types of inverters which are PV or interactive Inverter and Battery or multimode Inverter.

In an AC-coupled system, DC power flows from a solar PV array to a, which converts the DC power into AC power. The output AC power from the PV inverter then flows to either the connected AC loads or to a Battery Inverter that converts the electricity back to DC for charging the battery storage and also converts the DC power from the battery storage to AC power to feed the load or export to the grid when there is excess energy from the Solar PV. In other words, the output from the PV array is fed through a PV or interactive inverter?before?it reaches the battery storage system. This means that the output AC power from the PV inverter must be converted to DC power before charging the battery energy storage system, and the DC power from the battery energy storage system must be converted once again to AC power. ?This means that in an AC-coupled Integrated BESS, any energy stored in the battery storage system must be inverted three times (DC – AC, AC - DC and DC-AC) before being fed to the connected AC loads. There are energy losses in the system during this inversion process due to conversion inefficiencies.

Since AC-coupled BESS have separate inverters for the solar PV and the battery storage system, the grid, solar PV and the battery storage system can supply power to the connected loads at full power simultaneously or independently, creating flexibility in how the system operates. The solar PV array and battery can both share a common AC bus to the grid as shown in Figure 1.0 or run on separate interconnections. In an AC-coupled system, power from the PV modules is converted to AC before connecting to the ESS. To achieve this, an additional battery storage inverter is required.

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AC-couple BESS Schematic

?Charging and Discharging of Energy in an AC coupled integrated BESS?

One of the key advantages of AC-coupled BESS is its ability to interact or work in conjunction with the grid. It has the ability for the grid to supply power to the load simultaneously with the Solar PV system and/or Battery inverter.

Below is how typically this works between the grid, Solar PV system and the Battery Energy Storage system:

?Grid power Supply: When no power is available from other sources ( for example, from PV systems and battery energy storage), the grid serves as the primary source of power to charge the battery and at the same time continuously supplies power to the load.

Solar PV System: AC coupled system can receive power both from the grid and solar PV system simultaneously. When there is excess energy produced by the solar PV array, the DC energy is converted to AC power by the PV inverter. This excess power from the solar PV system can exported to the grid or charged the battery energy storage, this effectively reduces the energy consumption from the grid and offsets energy tariffs.

Battery Power Supply: When there is insufficient energy from the solar PV array, and the energy storage has sufficient energy stored in it, and there is a need for additional energy beyond what the Solar PV array is generating, the battery energy storage can discharge to supply power to the load via the battery inverter.

Note: The coordination of power supply to the load between these three power sources is managed by the energy management systems (EMS) and the control strategies, as well as monitoring the charging and discharging of the battery storage system by the Battery Management System (BMS), to ensure that the power is supplied efficiently to meet the load demand and prioritize power supply to the load between the solar PV system and battery energy storage to reduce the reliance on the grid. Also, the dua-inverter set-up in an AC-coupled system allows for the storage of energy and self-consumption optimization.

?The AC-DC conversion has major system design implications. As previously mentioned, solar PV array produces DC power. The DC power must be converted to AC power to be used in AC loads in most residential, commercial, and industrial applications. In contrast, battery energy storage must be charged with DC power and outputs AC power.

?DC-Coupled Integrated BESS

In a DC-coupled system, the battery system and the Solar PV system are connected to the same DC bus with the DC side of an inverter as shown in the figures below. The solar PV array is connected to a DC-DC converter (MPPT solar Charge Controller) at the DC side of a hybrid inverter with an inbuilt MPPT Solar Charge Controller. DC power output from the solar PV array is fed to an MPPT?charge controller (DC-DC converter)?which converts the DC output from the Solar PV array into a suitable DC power form to charge the battery energy storage. ?Since there is no PV inverter in DC coupled system, it means there is no inversion of solar PV array output from DC to AC and back again before the battery stores the energy. The energy produced by the solar PV array will be inverted only once (from DC to AC) as it flows from battery energy storage to the load or the grid. The DC-coupled system typically uses a solar PV charge controller, or solar PV regulator, to charge the battery from the solar PV array along with a battery inverter to convert the DC power to AC Power. DC-coupled systems rely only on a single multimode inverter or battery inverter that is fed by both the PV array and battery energy storage system. With this system architecture, DC output power from the solar PV array can directly charge the battery energy system. Since there is no dc-to-ac conversion required between the solar PV array and battery energy storage system, the energy conversion loss is less as compared to that of an AC-coupled system, which makes a DC-coupled system more efficient than an AC-coupled system.?

In a DC-coupled system, the battery energy storage system can be charged by both grid power and solar PV systems. This is one of the key advantages of a DC-coupled system as it’s specifically designed to work seamlessly with solar PV systems. One of the key disadvantages of DC coupled system is that excess energy produced by the solar PV array is not fed or exported to the grid instead it is used to efficiently charge the battery as a backup power.

Note: While an AC-coupled BESS is more efficient when the PV array is feeding loads directly via a solar PV inverter, a dc-coupled system is more efficient when power is routed through the battery energy storage (i.e. when the battery energy storage is charged directly and discharged at a later time) since there is only one conversion from dc to ac via a single inverter, rather than two inverters, to pass through.

?Charging and Discharging of Energy in DC-coupled BESS

Grid Power Supply: Like AC-coupled BESS, the Solar PV system is typically the primary source of energy when other sources are unavailable to charge the battery energy storage system. DC-coupled BESS can also be connected to the grid, allowing power to be supplied to the load from the grid when necessary.

Solar PV Power: The DC-coupled BESS system is designed for solar PV system optimization. When there is excess energy generated by the solar PV array, the excess energy doesn’t feed the grid directly, instead, it is used to efficiently charge the battery energy storage.

Battery Power: Since there is no double conversion in DC-coupled BESS, the stored energy in the battery system is primarily supplied to meet the load demand.?

What are the advantages of AC-coupled BESS?

Below are the major benefits of an AC-coupled BESS:

Retrofitting:?AC-coupled BESS can be integrated easily with an existing grid or solar PV installation setups, making them a more versatile choice for retrofits, and more can be added to expand capacity.

Flexibility:?Installers are not restricted in where the inverters and batteries can be located. AC coupling works with any type of inverter.

Resiliency: AC-couple BESS has the flexibility to install multiple inverters and batteries in different locations. In the event that an inverter fails, or the battery system is faulty, this does not have any impact on the power generation, and this helps to avoid the risk of a power outage.

Versatility: AC-coupled systems enable batteries to charge from the grid as well as the solar PV array and the grid, so if there is insufficient energy production from the solar PV array, the battery can still charge from the grid.

Grid Support: AC-coupled BESS can provide various grid support services such as voltage regulation, frequency regulation, and power factor control for grid stability and black start.

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What are the disadvantages of AC-coupled BESS?

Lower Efficiency:?Due to the double conversion involved in an AC-coupled BESS (e.g. stored energy is converted three times, from the DC to AC current to supply the load and then back to DC to the battery and again back into AC). Each conversion in the system results in a small amount of energy loss due to conversion inefficiencies.

Higher Cost: The AC-coupled system?is more expensive than the DC-coupled system as it uses two inverters.

Supply limitations:?AC BESSs are not designed to be used off-grid and as they are transformerless, they cannot manage the surge loads from multiple appliances.

?What are the advantages of a DC-coupled BESS?

Where AC-coupled system suffers in terms of efficiency and cost, DC-coupled systems have the advantage:

Lower Cost: DC-coupled systems tend to be cheaper than AC-coupled systems as the solar panels and battery use a single inverter and less extra equipment, reducing the overall system costs, and making them cost-effective for certain applications.

Higher efficiency:?Unlike AC-coupled BESS which has multiple conversions of energy, DC-coupled BESS only convert the energy once. They directly store and discharge energy, reducing energy losses and making them more efficient.

?Solar PV Optimization: DC-coupled BESS allows solar PV modules to generate more energy than the inverter rating. They are well-suited for solar PV installations as they store DC energy generated by the Solar PV array efficiently thereby reducing energy loss, whereas in an AC-coupled system, the energy is lost.

?What are the disadvantages of a DC-coupled BESS?

• Limited flexibility: DC-coupled BESS may be less flexible and complicated to integrate with an existing Solar PV or grid infrastructure than with an AC-coupled BESS, as the inverter needs to be located close to the battery.
• Less resiliency: With a single inverter in a DC-coupled system, if the inverter fails, the solar power as well as the battery capacity is lost. They are dependent on the Solar PV voltage levels and the characteristics of the solar PV system, which can limit their versatility as compared to AC-coupled BESS.
• Grid Support: As compared to AC-coupled BESS, grid support services may be more limited in DC-coupled BESS.

?Summary:

• The choice between selecting AC/DC coupled BESS depends on the specific application scenarios and system design. And the project requirements. DC-coupled BESS is often more efficient and cost-effective for solar PV-specific installations. At the same time, AC-coupled BESS is more versatile and can be integrated easily into an existing grid or DER infrastructure. The choice between AC-coupled BESS and DC-coupled BESS should be based on the priorities and constraints of the particular BESS project.
• AC-coupled BESS systems are best if it is required to integrate BESS into an existing solar PV system, making them easy to install.
• Energy must be converted three times in AC systems, making them less efficient.
• In DC systems, the energy only needs to be converted once, making them more efficient and less expensive.