Thinking energy storage when you’re thinking 1500vdc PV

More than half of the battery storage capacity added in the U.S. in the next three years will be paired with a solar photovoltaic (PV) power plant, bucking the historical trend of majority standalone sites.

by Allen Austin, ABB Inc.

The energy storage market is booming. Wood Mackenzie estimates that the segment will grow from $1.2 billion in 2020 to $4.3 billion in 2025. That’s a 258% increase over just five years.

Much of that growth is expected to come from utility-scale solar plants that incorporate storage, typically battery energy storage systems (BESS). Most larger inverters and converters operate at 1500vdc now, and there are also choices to make on the storage side—here’s a quick take on both.

What’s so great about 1500vdc?

In two words: efficiency and cost. Higher voltage means lower losses on the collection system cables. It also implies running at lower current (but the same power), which equates to reduced heat levels in inverters and smaller gauge wire between inverter and transformer.

Where 1500vdc really saves, though is in balance-of-system components. So, in addition to fewer—but longer—strings, there are fewer connectors, fewer combiner boxes, and reduced cabling requirements. The combined savings can be significant. In fact, some research estimates that a 10-MW solar plant designed to operate at 1500vdc will save the plant owner around $400,000 in deployment costs over a 1000vdc system of the same capacity.

Suppliers haven’t quite caught up to the demand for 1500vdc components; there are still more available for 1000vdc systems. There are also design accommodations that must be made for larger, higher-voltage equipment.

Back to storage…

In terms of energy storage, the design choices relate not only to capacity but how the BESS is connected to the PV array. A DC-coupled system offers greater efficiency than a comparable AC-coupled approach, as explained in the following excerpt of an October 2019 article (recently presented at Energy Storage USA Virtual Summit 2021):

In AC-coupled solar-plus-storage installations there are two inverters, one for the PV array and another for the battery energy storage system. With this system configuration, both the battery and solar array can be discharged at maximum power and they can be dispatched independently or together, providing the operator with more flexibility in terms of how they operate and dispatch the asset. Located at the same site, the solar array and energy storage facility can either share a single point of interconnection to the grid or have two separate interconnections. In DC coupling, the co-located solar and energy storage assets share the same interconnection, are connected on the same DC bus, and use the same inverter. They are dispatched together as a single facility. DC coupling reduces efficiency losses, which occur when electricity current is converted, such as from DC to AC.

DC-coupled BESS systems have been shown to be slightly less reliable than AC-coupled systems, at least in one case study of a utility-scale solar plant. However, use of advanced materials like silicon carbide in advanced power devices could mitigate that. Redundancy in design can also address the reliability issue, but the authors are quick to point out the “importance to design for reliability at the system level.”

While it’s always necessary to evaluate any technology in the context of the application where it will be used, there does seem to be a strong case not only for 1500vdc within the solar array and collection system but for DC-coupled energy storage operating at the same voltage. It will be interesting to see how the PV-plus-storage market evolves over the next few years.

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