Saurenergy Explains: AC Block vs DC Block – Battle of BESS

Renewable energy sources struggled to become a reliable alternative to fossil-based power for long. The Utility-scale battery energy storage systems (BESS) emerged as a panacea to renewable woes, with new renewable projects now even accommodating BESS as a part of the project. The landscape of utility-scale BESS is also now witnessing a dynamic evolution, with a notable shift from traditional DC coupled systems to integrated AC block configurations. 

The transition is driven by technological advancements and the diverse needs of modern energy infrastructures. Understanding the distinctions between AC and DC coupling is essential for stakeholders aiming to optimize energy storage solutions.​

What are AC and DC Blocks of Energy Storage?

The main job of energy storage systems is to store energy and release it when needed. The electric grid operates on Alternating Current (AC), while the storage systems store energy in Direct Current (DC). Thus, BESS requires the ability to convert electric current from DC to AC for the grids.

In AC block configuration, string instruments are internalised in each energy storage unit which convert DC power from the batteries to AC, allowing the energy storage enclosure to directly interface with the grid. 

On the other hand, DC block configuration uses the same inverter as the solar field to convert the DC power stored in the BESS into usable AC output to the grid. 

Historical Context: The Evolution of BESS Configurations

Until very recently, BESS designs were predominantly DC-coupled. In these setups, batteries store energy as Direct Current (DC), necessitating external Power Conversion Systems (PCS) to convert DC to Alternating Current (AC) for grid compatibility. This configuration offered flexibility in PCS sizing, allowing customization based on specific project requirements.

The dominance of DC-block was challenged in 2019 for the first time when Tesla pioneered  the ‘AC-block’ design as it introduced Megapack that integrated the inverter within the enclosure simplifying installation and operation. Since then, several firms have forayed into the technology paving the way for the competition for hegemony between the AC and DC blocks of BESS.

Advantages and Disadvantages of AC Coupling

AC-block systems offer several key advantages for energy storage project developers. One of the primary benefits is the use of string inverters, which enable decentralized and granular control. This level of control, when combined with advanced software, allows for optimized energy throughput, enhancing overall system performance.

Another significant advantage of AC-block systems is their higher availability, or the percentage of time an energy storage system remains operational at its designed capacity. Additionally, both solar panels and the grid charge batteries in AC coupled systems. Being grid-tied means that if your solar PV system isn’t generating enough electricity to charge your battery at any point fully, you can still rely on the grid as a charger.

In DC-block configurations, a failure in the central inverter can lead to a large portion of the system being taken offline for repairs. In contrast, AC-block systems make it easier to replace individual components, such as faulty string inverters, without shutting down a significant portion of the system. This modular approach minimizes downtime and improves reliability.

Additionally, AC-block systems can accelerate project development timelines. With fewer components and a more streamlined installation process, these systems require fewer equipment shipments, reducing logistical complexity. 

AC blocks also connect directly to medium-voltage transformers, simplifying the grid interconnection process and further shortening deployment timelines.

However, a major drawback of AC coupling is efficiency loss due to multiple energy conversions. In this case, the DC block is better compared to its AC counterpart. 

Since AC coupled systems require electricity to be inverted between AC and DC multiple times—once when storing energy in the battery and again when supplying it to appliances—there are small energy losses at each stage. This additional conversion process makes AC coupled systems slightly less efficient compared to DC-coupled alternatives. 

In addition, DC block products are also lower cost options when compared to AC block even after accounting for the additional inverter, controls, and engineering costs. 

Surging Market of AC Block

The advantages of AC blocks like the ones mentioned above present a strong case for the suitability of technology in the market known for DC coupled systems. Several firms are now promoting AC coupled systems Sungrow and Fluence being one of the notable names.

Sungrow’s PowerTitan 2.0, an AC block system, is rapidly gaining global traction, with the product having expanded its presence from Europe to the Middle East and Asia.. Sungrow recently announced the commercial operation of the 100 MW/331MWh Bramley BESS installed with its BESS 3-hour PowerTitan 2.0. 

Fluence also joined the club with the launch of its AC block product Smartstack, a recent grid-scale BESS solution. The launch of Wärtsilä made Quantum3 AC block solution is another notable example showcasing the accelerated adoption of AC coupled BESS worldwide.

While the champions of DC BESS systems include some of the heavyweights like BYD and Contemporary Amperex Technology Co., Limited (CATL), the emergence of AC block and new products wired on the technology now present options to the utility scale project developers, each offering their own sets of perks. 

Despite AC block gaining momentum, DC block is not slowing down either with newcomers like Canadian Solar popping up offering traditional DC block solutions. Others like EVE Energy, American Energy Storage Innovations, REPT BATTERO, Trina Storage, Chint Power Systems, and LG Energy Solution Vertech are also the champions of DC coupled BESS.

 

 

Interestingly, the hybrid solutions are also lurking in silence that involves offering both AC and DC versions. HyperStrong’s introduction of the HyperBlock is offering both AC and DC solutions, a strategy that may become disruptive for the energy storage market. 

The Path Forward

The decision between AC and DC-coupled BESS configurations hinges on various factors, including project scale, budget constraints, desired efficiency levels, and integration requirements. 

AC block systems offer streamlined installation and enhanced reliability, making them attractive for projects prioritizing these aspects. Conversely, DC coupled systems provide flexibility and cost advantages, appealing to projects where customization and budget are critical considerations.​

As the energy storage industry continues to evolve, the coexistence of AC and DC configurations is likely, with each serving distinct market needs. Advancements in technology and design will further blur the lines between these configurations, leading to more versatile and efficient energy storage solutions.