Opportunities, Strategies & Best Practices for Electrical Balance of Systems Optimization: Page 4 of 5
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SP: To what extent does your approach to eBOS optimization vary based on application specifics?
SSI: It makes all the difference in the world. On ground-mounted projects where we mount portrait-oriented modules side by side in a long linear row, for example, we like to use a process we call skip stringing that takes advantage of generous module wire whip lengths. This wiring method connects every other module in the row in series so that the source circuit is wired as follows: 1 ▶ 3 ▶ 5 ▶ 7 ▶ 9 ▶ 11 ▶ 13 ▶ 15 ▶ 17 ▶ 19 ▶ 18 ▶ 16 ▶ 14 ▶ 12 ▶ 10 ▶ 8 ▶ 6 ▶ 4 ▶ 2. When we wire a source circuit in this manner, the positive and negative homerun wires end up at adjacent modules rather than on opposite ends of the row. [See Figure 2] In this scenario, skip stringing eliminates a length of wire equivalent to the width of 18 modules or approximately 58 feet. When considered across a 2 MWac site, this results in wire savings of more than 25,000 feet of #10 PV Wire, a premium-priced conductor, plus the labor and materials to handle and secure that extra wire. Project size also has a significant impact on the eBOS optimization strategy. With a smaller rooftop system in the 300 kW range, for example, all we really need is a single ac accumulation panel. For a larger 2.5 MW ground mount, we need to have several ac accumulation panels feeding a master accumulation panel.
eBOS Vendor Perspectives
Equipment vendors have a unique perspective on eBOS optimization based on how customer orders and product requests change over time. I reached out to leading combiner box and wire harness suppliers to find out what products and solutions are in high demand and to get vendors’ perspectives on eBOS optimization strategies.
What are the most effective strategies to achieve eBOS cost reductions for dc and ac circuit aggregation on commercial and utility-scale PV systems?
I firmly believe that a system-level approach is the best strategy. Engineers and integrators should work with their suppliers to fine-tune ways to make all of the components work well together and fit in with their construction practices. This might sound obvious, but you would be surprised how many people are still making assumptions about what you can and cannot do with eBOS. Involving the eBOS supplier or manufacturer early in the process can save lots of time and money down the road.
—Jason Schripsema, CEO, SolarBOS
Optimizing the distribution and aggregation of strings can help reduce eBOS costs. For example, on a crystalline-based system, combining two strings can yield significant cost reductions. However, combining a large number of strings does not yield the same results if the installation then requires oversized cable.
—Tony Gulrajani, solar marketing manager, Eaton
Beyond considering string aggregation on the dc side, customers are increasingly interested in coordinated combiner box and wire harness assemblies. We design these project-specific assemblies with the appropriate connections to support easier and faster installation, reduce labor costs and improve reliability.
—John Vernacchia, RE global segment manager, Eaton
For large utility-scale projects, our customers are quickly moving from 1,000 Vdc to 1,500 Vdc. Bentek has already shipped more than 200 MW of complete UL 1741–listed 1,500 Vdc solutions.
—John Buckley, executive sales and marketing, Bentek
Most of today’s jobs are systems that have higher-voltage ratings than in the past—1,000 V systems are becoming the industry standard. We are also seeing a lot of demand for 1,500 V combiner boxes and switch cabinets. We expect that trend to continue as it reduces the number of combiner boxes and strings required as well as the necessary wire diameter, which ultimately reduces the total cost of the project. About 90% of our customers are using aluminum for the feeders exiting the combiner boxes to the inverter. Combiners with prewired and preconfigured whips or pigtails that have watertight connectors are another standard solution for saving labor in the field. These whips come with string ID labels, inside and out, and are available in black, red or white.
—Tom Willis, director of sales, AMtec Solar