The solar industry has shown a much broader interest in combining energy storage with solar installations as grid-integration benefits have increased and battery costs have decreased. Key system design considerations can help projects be more cost-effective, reliable, efficient and bankable. For utility-scale solar-plus-storage projects, one of the most critical considerations is choosing between AC and DC coupling architectures. 
As each name implies, this architecture defines the way solar PV and storage are coupled together. In AC coupling (Figure 1), the outputs of the PV and battery are coupled on the alternating-current side of the system. In DC coupling, as shown in Figure 2, the outputs of the PV and battery are coupled on the direct-current side of the system.
Implementation of both of these approaches is predicated on the inverter technology applied. In AC coupling, the inverter directly converts DC to AC. Such inverters are single stage, and in this implementation, separate inverters are used for batteries and PV. In this architecture, conventional PV inverters are used. Given the large established market for this type of inverter, this equipment is often considered reliable, efficient and low cost. In AC coupling, the batteries are connected with bi-directional inverters with full four-quadrant capability. Like PV inverters, these bi-directional inverters are single stage and, therefore, also efficient, reliable and low cost.
DC coupling requires a different approach and applies a dramatically different inverter topology. With DC coupling, the DC sources are coupled together; however, in practice, this is more complicated and even potentially dangerous. When combining PV and batteries, installers must take care to never directly combine the DC buses. PV voltage is regulated by a maximum power point tracking algorithm and changes throughout the day based on irradiance levels and module temperature. 
On the other hand, battery voltage is regulated based on state of charge. Perhaps more importantly, batteries both accept power and provide power, while PV panels only provide power. Erroneous attempts to “charge” a PV array could be both dangerous and damaging. Similarly, applying the incorrect DC voltage to a battery could also be both dangerous and damaging to the batteries. Due to these issues, separate DC/DC converters on the different DC sources are used (Figure 2). These converters provide both the proper voltage regulation and the electrical isolation needed by the PV array and the batteries. When more than one DC input (or port) is provided, it is common to call it a “multi-port” inverter. Because the ports require isolation, they are also multi-stage inverters. This refers to the concept that power is processed multiple times before an output AC waveform is provided.
The multi-stage aspect of the multi-port inverter is one of its key weaknesses. When power is processed twice, there are greater losses. This directly results in lower efficiency, which results in greater heat generated in the converter, greater component stress and higher failure rates.
The impact of this double processing on efficiency is shown in Figure 3. In a single-stage AC coupled inverter, power is only processed once (Figure 4). This yields inverters that are more efficient.
The impacts of efficiency alone on a project can be substantial, particularly when combining PV with storage. In many locations, the value of just 1% greater efficiency is easily $1,500/MW per year. With storage, efficiency is critical because the power is processed both when the battery is charged and then when it is discharged. Such efficiency losses are effectively doubled.
When comparing AC and DC coupling in large-scale systems, it is hard to ignore the substantial “head start” that AC coupling has. Solar-plus-storage projects that employ AC coupling tap inverter technology that is similar to solar and wind inverters. In fact, some vendors have their storage inverter platforms based on a common platform with wind or solar. Given the many years and many gigawatts that have been installed in wind and solar, such inverter types have evolved and progressed. Because of this, these inverter platforms have proven field experience and high reliability, and they are often considered “bankable” by debt providers. Typically, vendors that promote AC coupling are large, public and global electrical equipment providers – further enhancing bankability. In contrast, multi-port inverters tend to come from private start-ups or other small companies.
Design flexibility is another key consideration for large projects combining PV and storage. With AC coupled systems, it is straightforward to adjust the PV/storage ratio. For example, 10 MW of PV (using 2 MW inverters) and 2 MW of storage (using one inverter) is shown in Figure 5. This provides a 20% storage ratio and is easily adjusted to 40% by adding an additional storage inverter see Figure 6.
Now contrast this approach with a multi-port or DC coupled design, as seen in Figure 7.
With DC coupling, the PV and batteries are paired on each inverter. This is inherently convenient for 1:1 ratios of PV to storage, but less so when optimizing designs around other ratios. The other attribute that becomes obvious when reviewing these two approaches is the battery arrangement. AC coupling is inherently better because it is optimized around a smaller number of larger battery packs; this is less costly and easier to deploy and manage than having multiple distributed battery packs. For example, using five smaller battery packs rather than one large one will require five fire suppression systems and five air conditioning systems. 
This adds both a significant upfront cost and significant ongoing operations and maintenance costs.
As previously mentioned, when designing large solar-plus-storage systems, there are multiple benefits with AC coupling vs. DC coupling – in terms of reduced costs and increases in efficiency, reliability, bankability and design flexibility. However, it is important to note that this comparison is specifically under the context of high-power, multi-megawatt projects. With residential or commercial projects, many of these benefits become less significant. In fact, on such projects, the benefits of using a single multi-port inverter rather than two AC coupled inverters should be considered. In these cases, the multi-port inverter approach can save installation cost and space.
When an installer makes any design trade-off decisions, one approach is typically not the best for all applications. In the case of utility-scale projects, however, there are significant benefits to AC coupling over DC coupling.
Chris Thompson is the grid-power business unit manager at power management company Eaton, where he manages product lines for inverters that connect both solar and storage systems to the grid. All charts contained in this article are courtesy of Eaton.
