Historically, if a problem occurs with a traditional string power inverter that links several solar panels together in series, or even simple shade, that issue can bring down an entire solar photovoltaic (PV) array. However, a microinverter installed on each panel can entirely eliminate these problems because the requisite conversion from direct current (DC) to alternating current (AC) happens at each panel, rather than in a central inverter.
According to Navigant Research, the microinverter is contributing to the strong growth of U.S. distributed solar power. In its recent report on microinverters and DC optimizers, Navigant stated that a growing list of solar PV module manufacture are integrating these technologies at their own production plants. In addition, large power electronics companies and incumbent manufacturers are making strategic partnerships and acquisitions, a trend that will accelerate in the next few years. Navigant forecasts a total of 52.7 gigawatts (GW) of module-level power electronics (MLPEs), such as microinverters, will be installed between 2013 and 2020.
What is a microinverter?
First, a little history. According to John Williams, chief operations officer of Solar Panels Plus, Chesapeake, Va., a standard solar inverter is a power-conversion device that connects to a string of solar panels wired in series at up to 1,000 volts and converts DC power into AC. Standard inverters are called string inverters because they connect to a string of electrical panels, or subpanels, in a building. They convert DC to AC and match that AC frequency to the frequency provided by the utility. These inverters become disabled when the frequencies don’t match as a safety feature.
Microinverters are the first technology to efficiently perform DC-to-AC power conversion directly at the solar-panel level, eliminating the issue of a single point of failure. Furthermore, they enable a more efficient adaptation to environmental conditions, according to Brian Korgoankar, senior product manager for Enphase Energy Inc., Petaluma, Calif.
Even though microinverters have been around in one form or another since the beginning of the modern solar industry, they did not begin to achieve commercial success until recently, when micro-electronic manufacturing processes made it possible to cost-effectively produce a fully regulatory-compliant device that fits with a single (or a few) solar panels within an array, according to Christopher R. Jones, co-founder of Chilicon Power LLC, Los Angeles.
Enphase Energy was the first manufacturer to successfully mass-produce microinverters.
“Rather than trying to overcome industry doubts and create another small version of a central inverter, Enphase rebuilt the inverter from the ground up by developing new system-level electronic topology based on a digital networked architecture, featuring strong communications capabilities,” Korgoankar said.
This systems approach allowed for a high degree of semiconductor and software integration, improving efficiency and reliability while reducing costs.
“Although still the industry leader, Enphase has been joined by dozens of manufacturers producing microinverters,” Williams said.
Microinverters make the most sense for several applications, including small solar PV systems less than 1-kilowatt, any size application where shading is a problem, PV systems where panels are installed at varying azimuths, and PV systems that will be expanded over time in small increments.
“Microinverters are mostly used in residential and commercial rooftop-solar arrays where they are most advantageous,” Jones said.
However, as prices continue to decrease, microinverters are being considered more frequently for larger scale solar installations and are among the most advanced, easy-to-install, grid-ready, distributed-power generation products on the market.
Advantages and limitations
Generally, microinverter technology advantages include better performance in shady areas, higher levels of safety, better system reliability, the capability to add panels to a PV system, and installations on complex roof designs. Due to their distributed, semiconductor-based, software-defined architecture, microinverters can optimize the energy produced at the solar module level, according to Korgoankar.
“For solar contractors, microinverter systems offer design flexibility and ease of installation, reducing both labor and materials costs,” he said.
In addition, intelligent cloud-based monitoring systems provide a real-time window into the system’s operational efficiency and performance, streamlining and simplifying the system’s upkeep and maintenance.
Because microinverters are individually attached to each solar panel, any performance problem affects only that panel. With a string inverter, the entire string of panels is degraded to the performance level of the lowest producing panel.
“This is particularly important when there are transient shade conditions that affect a small portion of the array at different times of the day,” Williams said.
Some may consider that having multiple microinverters increases the chance of failure; however, when string inverters fail, the entire array could be down for days or weeks, as opposed to a single panel not operating.
“At a cost of around $140 or so, an extra microinverter can be affordably kept on hand as an immediate replacement,” Williams said.
The design flexibility microinverters offer is enhanced by the fact that they do not require the same high levels of voltage planning as string inverters.
“Less planning means lower overhead costs during the design process,” Jones said, adding that, since microinverters require familiarity with AC power, the learning curve for installing them is short. “In addition, the flexibility offered by microinverters enables each solar module to be installed at different angles and orientations without forcing all of the modules in the array to adopt the same power point, as required by string inverters.”
Of course, microinverters still face limitations.
“Microinverters still need to provide significant new benefits that legacy or rival technologies cannot match as well as be absolutely reliable, even under the most extreme conditions,” Korgoankar said.
No new technology, particularly in the energy sector, can earn a mainstream global customer base if it doesn’t perform as expected without downtime or excessive maintenance.
Also, the upfront cost of microinverters is still about 30 percent higher than for string inverters.
“About 20 percent of that higher cost is made up for in average increased productive life of the microinverter,” Jones said.
According to Williams, improper panel selection creates another potential drawback. Currently, most of the largest microinverters are only a good choice for panels in the 240- to 250-watt (W) range. While using larger panels is possible, most of today’s microinverters cannot use all of the power of a larger capacity panel, even though they can be connected to the larger panel.
“It is not unusual for a 250W continuous-power-capable microinverter to be rated for use with larger wattage panels,” Williams said.
This approach may be acceptable when attempting to generate lower power in lower solar conditions, such as early morning and late afternoon, but during periods of strong insolation, any power potentially generated by the panel beyond 250W is lost.
Finally, some microinverter brands require propriety trunk cables, which installers often find hard to work with and are cost prohibitive. However, some brands use nonproprietary standard components, do not require trunks cables and allow for a simple daisy-chain configuration, according to Williams.
Need to know
Advancements, such as microinverters, are breaking down the barriers of who can install solar PV systems. In addition, since all wiring is standard AC, significant balance-of-systems costs for expensive DC electrical components and large gauge wires are eliminated.
“Although installing solar-energy systems is still largely a specialized trade, a growing number of general contractors, roofers and electrical contractors have started offering solar as a complement to their services,” Korgoankar said.
Targeted programs, such as North American Board of Certified Energy Practitioners (NABCEP) coursework and accreditation can train electricians in solar installation practices.
“Contractors need to know that microinverters will provide their customers with high visibility into the renewable-energy installation,” Jones said.
Contractors should also be aware that their own cost of installing microinverters will be potentially lower than the cost of string inverters because of their shorter learning curve and the fact that a DC-wiring planning stage is not required.
“Microinverters’ decreasing cost structure is increasingly making them a more cost-effective choice for both present day and next generation solar installations, and contractors that understand that fact are in a good position to offer them,” he said.
Contractors also need to understand microinverters’ potential short return-on-investment period, so they can demonstrate the advantages to their customers.
“Depending on the customer’s energy consumption, local rate structures, utility or state subsidies, type of financing and other factors, the return on the original capital investment can be recouped in as little as a few years,” Korgoankar said.
The microinverters market share will grow as prices fall closer to string inverters.
“One concept that is sure to eventually succeed will be microinverters that are built into the panel, creating an AC solar panel,” Williams said.
In addition, the power density of microinverters will continue to increase from 250W to 500W, and even 1,000W, and will be able to be used by multiple solar panels without significantly increasing costs.
“Whereas string inverter technology is mature and stable, microinverter technology is still advancing, improving its value over time to both the installer and end-user,” Jones said.