Solar panels can be coupled, or linked, to a battery either through alternating current (AC) coupling or direct current (DC) coupling. AC current flows rapidly on electricity grids both forward and backward. DC current, on the other hand, flows only in one direction.

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DC current is what is generated by solar panels and stored by batteries, but because appliances use AC current (just look at the AC circuit board you have in your home) the current must be converted via inverters.

In the past, AC-coupled BESSs were most often used with residential and commercial solar installations, and DC-coupled systems were used for remote and off-grid installations, but more options for DC-coupled systems have become available. Equipment manufacturers are developing streamlined and standardized power electronics equipment for DC-coupled BESSs.

Over the past decade, inverter technology has advanced and resulted in the development of new AC-coupled and DC-coupled systems. Let&#;s find out more.

What are AC-coupled systems?

In AC-coupled systems, there are separate inverters for the solar panels and the battery. Both the solar panels and the battery module can be discharged at full power and they can either be dispatched together or independently, creating flexibility in how the system operates. The solar panels and battery can either share an interconnection to the grid or run on separate interconnections.

AC-coupled battery energy storage system diagram.

AC BESSs comprise a lithium-ion battery module, inverters/chargers, and a battery management system (BMS). These compact units are easy to install and a popular choice for upgrading energy systems and the systems are used for grid-connected sites as the inverters tend not to be powerful enough to run off-grid.

It&#;s worth noting that because both the solar panel and the battery are DC-current compatible, the current will need to be converted three times in an AC-coupled system.

What are DC-coupled systems?

DC-coupled systems typically use solar charge controllers, or regulators, to charge the battery from the solar panels, along with a battery inverter to convert the electricity flow to AC.

DC-coupled battery energy storage system.

The solar panels and battery module use the same inverter and share the grid interconnection, reducing the cost of equipment. This also reduces power losses from inverting the current and running separate interconnection lines to the grid, as the solar array and battery are dispatched as a single facility. But this offers less flexibility than an AC system.

What are the advantages of AC-coupled battery systems?

There are several benefits to using an AC-coupled BESS for your solar plant, including:

• Retrofitting: AC-coupled batteries are easy to install on an existing solar panel system, 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: The flexibility to install multiple inverters and batteries in different locations helps risk of an outage if an inverter fails. Having multiple inverters provides more combined power and battery faults do not have an impact on power generation.
• Versatility: AC-coupled systems enable batteries to charge from the grid as well as the solar panels and the grid, so if the solar panels are not generating enough electricity, the battery can still charge from the grid.

What are the advantages of a DC-coupled system?

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

• Affordability: 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 such as voltage transformers and switchgear.
• Higher efficiency: Unlike AC systems which convert the current multiple times, DC BESSs only convert the current once, reducing energy losses and making them more efficient.
• Oversizing: DC-coupled systems allow solar panels to generate more electricity than the inverter rating. The excess energy can be used to charge the battery, an EV charger or a water heating system, whereas in an AC-coupled system the energy is lost.

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What are the disadvantages of a DC-coupled system?

• Limited flexibility: Installers have less flexibility than with an AC system, 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.

Should I use an AC- or DC-coupled system for solar plant?

AC-coupled systems are the preferred option for larger and utility-scale plants. That&#;s because while AC-coupled systems are slightly less efficient at charging batteries (90-94% vs 98% achieved by DC-coupled), they are far easier to install, especially into an existing system.

That said, whether AC-coupled or DC-coupled is the best solution for your PV plant design will be project specific. You can use a PV plant software solution to run several simulations and identify the best option.

AC Coupled Systems

AC Coupled &#; Off Grid

Slightly less efficient than their DC counterparts they do have other advantages. Off Grid AC Coupled Systems have cost efficiencies at scale and are more effective at powering high AC loads throughout the day.

Generally used for larger scale off grid systems they use a string inverter alongside a hybrid or multi mode inverter. These are typically easier to set up with the potential to be expanded using multiple solar inverters to become micro grids.

Pro&#;s

• When used through the day, they achieve higher efficiency to power AC appliances such as air conditioning, pool pumps and hot water systems.
• Cost efficiencies for large scale systems over 6kW
• Ability to use multiple string solar systems across different locations (AC coupled microgrids)
• When used above 3kW with multi-MPPT inputs, panels can be installed at differing orientations and tilt angles.

Con&#;s

• Lower efficiency for battery charging &#; usually around 92%
• Solar inverters can be expensive to replace for smaller systems
• Will achieve lower efficiency if powering direct DC loads throughout day

 

DC Represented in Orange / AC Represented in Blue

 

 

AC Coupled &#; Grid Tied

Historically used in larger off grid systems, advancements in technology in recent years has seen a new evolution in battery storage with AC Coupled Batteries used in many grid connected homes. This technology allows those with existing solar the opportunity to retro fit an AC coupled battery alongside their new or existing Solar. 

Don&#;t be fooled though. They are both AC but designed for very different purposes. On grid, rather than off grid, AC coupled batteries of this type are not typically powerful enough for off grid applications or to run a home in its entirety. Some may even struggle to handle multiple appliances so thorough research is always recommended. However, as they are smaller and compact in design, Systems tend to be modular and very easy to expand if storage needs grow.

Pro&#;s

• Can be retrofitted to homes with existing solar systems
• Simple to install
• Low cost &#; economical way to add energy storage
• Expandable and modular &#; system can be expanded If required

Con&#;s

• Not usually suitable for off grid applications
• Lower efficiency due to conversions from DC to AC to DC at around 90%
• Some AC Batteries cannot provide back-up power supply during grid outages

 

DC Represented in Orange / AC Represented in Blue

 

Read our article which covers all the main Solar Battery Systems types

Soltaro offer both AC or DC Coupled systems to suit your needs &#; Explore our AC and DC Coupled Systems by visiting our products page

Soltaro AIO2

We dare to be different!

In addition to utilizing a PV-Battery DC charger, the AIO2 is also able to be AC coupled to existing solar. This means that the battery can be charged, not only from your new Soltaro solar system, but also from any excess power generated from your existing solar system. 

Read more about the benefits of the Soltaro AIO2 ESS.

 

If you are looking for more details, kindly visit lithium storage battery for Hybrid energy storage systems .