ReVision Offers Opinion on Microinverters in Solar Pro Magazine
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We love Solar Pro magazine – we started with our anthem calling to be on the Cover of this seminal industry publication, and a few months later then we were, after sharing some insights on complying with new code requirements. We were honored to be asked to return to the magazine this past month to share our view on module-level electronics.
Module-level electronics encompass a class of devices generally designed to help solar panels optimize their individual performance. Microinverters were introduced as an alternative to central (one or more) string inverters in 2008. The key selling features at the time were greater shade tolerance and adaptability to unusual design conditions such as an array split across multiple roofs.
Seven years on, the technology has had revealed both benefits and downsides, as well as matured a great deal. We have been working with Enphase and other microinverters since they were first introduced, and ReVision co-founder and engineer, Fortunat Mueller, was asked for his opinion in Solar Pro’s article Integrator Perpectives on Module-Level Power Electronics.
Here is an excerpt from the article, featuring just Fortunat’s comments.
How would you characterize your experience with MLPE systems?
We have been installing systems using MLPE for about the last 5 years. We have projects with Enphase microinverters and SolarEdge optimizers and inverters, as well as LG and SunPower ac modules. We have roughly 400–500 MLPE systems installed, the majority of which are Enphase, although SolarEdge is quickly catching up. Our largest MLPE system is approximately 20 kW.
In terms of annual installed system capacity, approximately what percentage of your installations use MLPE? How has this changed over time?
Historically, we used MLPE only when there was a compelling, site-specific reason, such as substantial shading, multiple roof planes or a system that we expected to expand in small increments over time. These projects represented about 20%–25% of our residential volume in 2013. In 2014, with the introduction of small string inverters with wide input-voltage windows, better shade tolerance and dual MPPT inputs (SMA America’s Sunny Boy TL-US models in particular), the percentage of ReVision’s projects using MLPE fell to 10%–15%. However, in 2015, due to the adoption of NEC 2014 in one of our markets, the number of systems using MLPE increased rapidly to 26%–35% of our overall residential volume and close to 100% of our residential systems in New Hampshire and Massachusetts. Almost all of these systems use SolarEdge equipment.
What version of the NEC is in effect in the regions your company serves? Have Code changes, such as Section 690.12, impacted your use of MLPE?
We are on NEC 2014 in Maine, Massachusetts and New Hampshire. Massachusetts and New Hampshire are starting to enforce 690.12, which forced a move to MLPE. So far, Maine is not enforcing the rapid-shutdown requirements. The state is waiting for more products to come to market that are listed and work effectively with popular string inverters.
How do MLPE products impact the cost of systems? Is there an effect on your cost of doing business? For example, do you realize increased sales, system design, inventory, installation or O&M efficiencies?
Using MLPE definitely increases the installation cost (both labor and hardware), but the difference isn’t huge. I don’t think we’ve seen any savings related to increased sales, design or inventory savings. Frankly, our O&M experience with MLPE systems has not been great. First-generation Enphase inverters failed at an intolerably high rate in our coastal climate. Our experience with the first-generation SunPower ac modules was equally bad, though we installed significantly fewer of those. The cost and customer disruption for microinverter replacements are why we have been less than enthusiastic adopters of MLPE, and why we’ve moved back into that realm only due to regulatory pressure. So far, our experience with the newer generation of products from Enphase and SolarEdge has been substantially better. We are cautiously optimistic that the MLPE vendors have resolved some of the early design and manufacturing issues.
What are some primary sales, design or installation advantages of MLPE systems?
The biggest advantage is that MLPE opens up areas of the roof that would historically have been unusable, such as areas partially behind a chimney or a dormer adjoining the main roof. Being able to utilize more of the rooftop helps us meet our customers’ growing desire to meet more of their energy needs with a PV system. MLPE simplifies design somewhat, but, considering the current generation of string inverters with dual MPPT inputs and wide voltage windows, residential system design is already incredibly flexible and straightforward.
What are some primary sales, design or installation limitations of MLPE systems?
Because our MLPE systems are currently mostly SolarEdge, we are limited somewhat by the choice of inverter sizes and voltages. In particular, there are not great options for smaller projects connecting to 3-phase power systems, but that is an issue with both microinverters and string inverters.
What has been your experience with the reliability of MLPE components? How does it compare to non-MLPE components or systems? Has your experience with MLPE reliability changed over time?
Our experience with first-generation microinverters was not positive, and that has made us fairly conservative about adopting this technology. As a result, string inverters tend to be our default choice unless a compelling reason, such as site conditions or Code requirements, pushes us to MLPE. In the last few years, we have had better experiences, although we still have more issues overall with MLPE systems than with string inverter systems.
What advancements in MLPE components or systems would increase your use of these technologies?
Most important, we need increased reliability and decreased cost. A distant third would be continued improvements in the robustness of communications and ease of setup. However, MLPE communications are now much more reliable and user friendly than they were just a few years ago.
—Fortunat Mueller, co-founder, ReVision Energy, Portland, Maine
Read full article Integrator Perpectives on Module-Level Power Electronics