James Montgomery, Associate Editor, RenewableEnergyWorld.com
December 23, 2013 | 7 Comments
Solar panels on a building for the San Diego Unified School District. Credit: Enphase.
SolarBridge's Lawrence argues in favor of microinverters on an operations & maintenance basis. Central inverters account for half of an operations & maintenance budget and it's the single highest failure component in a solar PV system; that's why there's been a shift from those to string inverters on commercial-scale solar. "All the reasons you'd do that, are the exact same reasons to go from string inverters to microinverters," he said. "You want as much redundancy and granularity as you can possibly get, to maximize your rooftop utilization and simplify your O&M." Factoring in replacement costs, labor savings in not having work with high-voltage DC, "for most of our customers that alone is enough to justify the additional [price] premium." With a microinverter you'll know when (and which) one panel is underperforming, and it might be tolerable to just leave it alone; on a string inverter you might not know where the problem is while you lose power over the entire string, he pointed out.
Scott Wiater, president of installer Standard Solar, acknowledges that microinverter technologies and reliability have improved over the past couple of years, but he's not convinced this is an argument in their favor vs. string inverters. "I have concerns over the long term," he said. "If you truly believe you're going to get 25 years out of a microinverter with no maintenance, that might hold true, but we haven't had that experience." In fact he advises that any residential or commercial system should plan to replace whatever inverter it uses at least once over a 20-year lifetime.
Commercial-Scale Adoption: Yes or No?
Talking with both inverter vendors and solar installers, the choice of microinverters vs. string inverters for commercial solar settings is making some initial inroads into light commercial applications, but might not be quite ready to move up in scale at that commercial level.
"For projects under 50kW, we have found that microinverters can be positive for the project LCOE on an 'all-in' basis," explained Jeremy Jones, CTO of SoCore Energy, an early adopter of microinverters, including commercial solar projects into the hundreds of kilowatts in size. In general the technology's "high granularity of real time data is very useful in the ongoing asset management," and SoCore's projects with microinverters "have consistently outperformed our other string inverter and central inverter sites." The technology stacks up favorably to central and string inverters (especially for three-phase 208-volt systems) in terms of added costs, he said: warranty extensions, third-party monitoring, and other balance-of-systems costs. Microinverters' performance and low-cost warranties also benefit longer-term finance deals, he added.
However, above 50kW "we have had a harder time making microinverters 'pencil' on typical projects," Jones added. Until costs come down, those larger-sized projects where microinverters can make sense tend to be unique cases where there's a higher value per kilowatt-hour (higher electric rates or SREC values), or sites that can maximize kWh per kW due to high balance-of-systems costs, such as parking canopies, he explained.
SMA's Marshall is "bullish on the commercial market, that's where the volume will be" for inverters in general, but he doesn't see it as a big boon for microinverters because of what he calculates as a 25-30 cents/Watt cost delta from residential string inverters. In the residential space there are ways to knock prices down to mitigate that difference, but in the commercial space that gap is too big for the average buyer, he said. "As a mainstream option? We don't see it today." Microinverters may have a play for "some unique projects" such as campuses or municipalities spanning multiple buildings, but the big growth in commercial solar will be in large retailers, "big flat open roofs, and big flat structures like carports," he said, and there a three-phase inverter "blows the door off in terms of raw economics."
SolarBridge's Lawrence is "seeing a lot of activity" in smaller commercial settings (100-kw or less), tallying to 15-20 percent of the company's product installations. But while the company is bidding into projects ranging up to 1-MW, it's "harder to make the case above 250-kW," he acknowledged; "those don't pencil out for us right now."
"Anything below around 1 megawatt, we are shifting from a central to more of a string inverter, but we're certainly not going to the microinverter level yet -- nor do we think we will anytime soon," said Standard Solar's Wiater. "The economics behind the projects and having it pencil out, microinverters just can't compete with string or central inverters on a larger scale." While microinverters can help on some rooftop applications where shading might be an issue (close to elevator shafts, vents, HVAC units), a more tightly-designed system with an efficient string inverter "can have a much better return for the customer," he said.
Jeff Jankiewicz, project/logistics manager at Renewable Energy Corporation in Maryland, "definitely considers" microinverters as part of a system design; "we like the performance and efficiency they provide." But for his company it's really only for residential and small commercial projects; the largest they've done is a 20-kW system out in Maryland's horse country. Any bigger than that and it's a case-by-case comparison, specifically looking at shading and energy conversion.
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