DC Coupling of Solar + Storage for C&I and Distributed Generation Projects: Opportunities and Challenges

Why DC coupling makes sense for C&I and DG solar plus storage and what some of the challenges in doing so are

At this point, it is safe to say that whenever we talk about a solar project, we’re really talking about a potential solar plus storage project. Pairing storage with solar of course offers several benefits, the primary of which is the ability to turn solar from an intermittent energy resource to a dispatchable one. Of the two methods of combining solar and battery energy storage, DC and AC coupling, the DC coupled approach holds unique promise for commercial and industrial (C&I) and distributed generation (DG) sized alternative energy projects – projects ranging in size from 500 KW and to 10 MW.

Why DC Coupled C&I and DG Solar + Storage Projects

DC coupling is particularly promising for the C&I and DG space for several reasons. DC coupling offers project owners and developers the ability to deploy solar plus storage with a single inverter. When AC coupling, two inverters are required. One of the problems of having to deploy two inverters (besides for the cost!) presents for C&I and DG level projects presents is that it makes these projects look twice as big to the interconnecting utility. Interconnects for C&I and DG projects are expensive and hard to come by, so maximizing their value and size is critical to the economics of a project.

When DC coupling solar plus storage, only a single inverter is required. Even better, all the excess DC solar generation over and above the name plate rating of the inverter (the DC overbuild) can be harvested. Unlike an AC coupled solar plus storage system, which clips excess PV production when it exceeds the name plate rating of the inverter, a DC coupled system allows PV power to be diverted to the battery during times of excess solar production. Of course, when PV production is lower or not available at all, i.e. night time, energy can be discharged from the battery. This unique dynamic of reducing interconnect size while maximizing the DC size of the project can have highly beneficial impacts on project returns and outcomes.

As such, a number of state level solar plus storage incentive programs, including Massachusetts’ SMART program, are particularly favorable to DC coupled topologies.

Technical Challenges for DC Coupling Solar + Storage for C&I and DG Projects

Despite the benefits it offers, DC coupling is still a relatively new technique for combining solar and storage at scale. Implementing this approach for C&I and DG scale projects presents some unique challenges. Perhaps the most significant challenge is that today, C&I and DG scale projects are typically deployed with string inverters. Developers who deploy PV plants between 500 KW and 10 MW have gotten used to modeling and building with distributed string inverters. Historically however, DC coupled solar plus storage plants have been designed around centralized topologies.

This however is where Alencon’s BOSS (bi-directional optimizers for storage systems) comes in to play. The BOSS offers system designers the ability to control a centralized battery, whether at the rack or the container level, and allows the battery to charge from or discharge into a number of individual string inverters. This unique feature will be particularly welcome news to developers who already have interconnect agreements approved for projects with specific string inverters as refiling interconnect applications can significantly delay a project’s deployment.

Just and solar plus storage offers the best of both worlds – turning solar into a clean, dispatchable energy resource – so too can the Alencon BOSS help developers deploy C&I and DG scale solar projects with string inverters while pairing them with storage on the DC side of the inverter