PV Micro Inverters and Optimizers: Not Just for Lazy Designers

by Joseph McCabe, PE

More and more solar electric installations are using AC micro inverters and DC to DC optimizer electrical balance of systems (BOS) components. This BOS gear goes directly on the back sides of PV modules providing higher valued electricity than output from the PV cells alone.

Two years ago I considered micro inverters as only necessary for lazy designs or bad installation practices.  I’ve changed my attitude towards these approaches after organizing two years of forums as the American Solar Energy Society (ASES) Solar Electric Division Chairperson. These forums brought together experts who compared and contrasted AC micro inverters and DC to DC optimizer BOS equipment.

PV panels previously could not be installed in partially shaded locations because shade over a small area of the panel would drastically reduce the power production of the entire PV system. Now, shaded systems can benefit from AC micro inverters because each PV module can operate independently, instead of at an aggregated system level. Miss-matched PV modules were previously binned before installations so that each string had similar performing modules. Now the new electrical BOS gear eliminates problems with under, or over performing modules.  More recently, I have learned about the cost reduction and performance enhancing promises of these distributed technologies.

These electrical BOS approaches have evolved substantially in the last few years, and have come a long way since the first failed introduction of micro AC inverters in the late 90’s.

Micro-BOS Approaches

Micro electrical BOS components promise easier designs, lower installed costs, along with improving annual performance. Module level electrical BOS solutions for PV have many different flavors. All strategies promise to reduce the impact of individually miss-matched PV module performance over time, possibly reducing wiring and installation labor costs. Some products have communication strategies which help owners understand real-time performance and maintenance opportunities. Depending upon the project specifics, the levelized cost of energy could be reduced 20% or more.

AC micro inverters attempt to optimize efficiency by converting the DC voltage from PV modules into AC voltage that match the electrical grid’s specifications. This enables AC wires to be used, along with widely available AC electricians. Some DC to DC optimizers strategies boost the DC voltage to an optimal level. Others boost and / or buck (reduce) to maintain a specific DC voltage. There are parallel connections that add amperage, and series connections that add voltages. DC to DC optimizers raise the system voltage, lowering the wiring costs, but still need a box to invert the higher DC voltage to AC. Standard AC inverters are being optimized to work with DC to DC equipment.

Project specifics will determine which micro approach, if any, would be most appropriate. The trends are for residential systems to have AC micro inverters, and large systems to have DC to DC optimization.

The Shoot-Off Forums

At last year’s ASES Shoot-Off Forum, we had AC micro inverter companies in the same room with DC to DC optimizers comparing and contrasting their gear. This year we separated the forums into one AC micro inverter and one DC to DC optimizer group. Next year we will likely further divide the forums into companies that are shipping and companies that hope to ship.

This year’s forum included a presentation from the leading company shipping these types of solutions, Enphase Energy. Founded in 2006, they have shipped over 750,000 AC Micro Inverter units, with 25,000 installations in North America in the last 30 months. They have a 13% market share for US residential installations below 10 kW.  According to Enphase, micro inverters will be 11% of all world wide inverters by 2014, which means we need to keep a close eye on these market trends reshaping the PV industry.

For the first time in public, Ampt LLC presented their large-scale PV systems approach with their DC to DC optimizer technology. Ampt’s roots are intertwined with Advanced Energy Industries Inc. (stock symbol AEIS), which makes thin film deposition power conversion and thermal instrumentation equipment as well as PV power inverters. On May 3, 2010, Advanced Energy (AE) acquired all of the outstanding common stock of PV Powered providing AE with a full line of DC to AC Power Inverters. The Co-founder and Chairman of AE is Douglas S. Schatz. He is listed as an inventor on Ampt patents and is Chairman of Abound Solar (previously AVA Solar). A nice central station thin film PV solution is evolving from this AE / Abound Solar and Ampt relationship. In my option, thin films can benefit from these micro technologies because of the soft shape of the power curves and immaturity of thin film technologies in comparison to crystalline PV.

At the forum, SolarBridge Technologies announced volume production of their AC micro inverter including strategic partnerships with PV module manufactures. They are offering a 25-year warranty through their PV module panel integrators. This makes for a central warranty location, as long as the PV module companies stay in business. Matching module warranty with the micro gear is a very good marketing strategy. Very long mean time between failure (MTBF) numbers were presented by various companies, in the 400 to 500 year ranges. The high operating temperatures of this gear exposed to the heat of the sun make these MTBF’s highly questionable. The PV industry will surely become more savvy in estimating and marketing MTBF in the future.  

Other unique strategies were presented at the forum. eIQ Energy presented their parallel DC to DC optimizer including an integrated wiring harness solution made by Shoals Technologies Group. Tigo Energy explained how their DC to DC optimizer solution uses a combination of real-time module and string-level information to compute the optimal operating state of each PV module. There are many more micro approaches and business models being promoted in today’s micro electrical BOS space.

Future Competition

Be on the look out for two international leaders in traditional PV AC inverters to introduce micro inverters; Power-One (stock symbol PWER) and SMA Solar Technology AG (stock symbol SMTGF.PK/S92.DE).  In September 2009, SMA purchased OK4U, one of the original micro AC inverter technologies. Kaco New Energy Inc’s transformer-less inverter was shown as a partner for the DC to DC strategies in the forum, and like other existing inverter companies, will have good opportunities to customize their grid interactive technologies with micro technologies.

Beware, these micro technologies are highly duplicate-able. This means they will probably be championed by very intelligent electrical engineers from developing nations. I heard a rumour from this year’s Solarexpo conference in Verona that there was an Enphase knock-off from China, everything the same, except the very important aspect of quality.

Copycat designs will be enabled by National Semiconductor’s May 2011 announcement of the availabilit
y of their integrated circuits (IC’s) for use in the design of PV system micro inverters, power optimizers, and charge controllers.  National Semiconductor ended its original June of 2008 SolarMagic business of selling complete micro components and calling it a “per-panel electronics solution that maximizes power output of multi-panel installations”. Now, they are backing up the supply chain to supply IC’s  instead of BOS components. Texas Instruments has been marketing PV power IC’s for a few years.  

The largest inverter companies, and the smallest companies enabled with computer chips from National Semiconductor and Texas Instruments are creating an exciting playing field for micro PV BOS solutions. All these approaches continue to put pressure on lowering installed PV system costs,  increasing the annual performance and increasing the market for less than optimal installations.  We will be seeing increased innovations from electronics integrated directly on the back of DC PV modules. It is all very exciting; the innovations, and our learning how they fit into the PV industry has just begun.

For more in formation on the American Solar Energy Society please visit ases.org and plan on attending the annual conference held in Denver May 13th 2012.

Joseph McCabe is a solar industry expert with over 20 years in the business. He is an American Solar Energy Society Fellow, a Professional Engineer, and is internationally recognized as an expert in thin film PV, BIPV and Photovoltaic/Thermal solar industry activities. McCabe has a Masters Degree in Nuclear and Energy Engineering. Joe is a Contributing Editor to altenergystocks and can be reached at energy [no space] ideas at gmail dotcom.

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