A new string inverter and power optimizer architecture from SolarEdge Technologies shows how the competition in solar power electronics has moved to the commercial-scale space.

The fight is between an architecture that separates the DC optimizer from the inverter and one that combines them.

Distributed power electronics optimize direct current (DC) produced by a solar module and feed power to a companion inverter designed to convert the optimized DC into alternating current (AC) for transmission. Providers of traditional inverters such as SMA and ABB/Power One, and of power optimizers, such as Ampt and Tigo, are moving to seamlessly combine the two functions, either through in-house production of both or through partnerships.

SolarBridge, Enphase Energy (ENPH), Enecsys and other microinverter firms perform the power optimization and conversion at the panel.

 

Source: The Global PV Inverter Landscape 2013: Technologies, Markets and Survivors

“The hardware components and software by which the system adjusts voltage to find the PV system’s maximum power point is known as the maximum power point tracker (MPPT),” GTM Research senior solar analyst MJ Shiao explains in The Global PV Inverter Landscape 2013: Technologies, Markets and Survivors. “Traditionally, an inverter contained only one MPPT, and a PV system utilized only one inverter.”

Increasing the number of MPPT points produces a more optimized system, according to Shiao. By using module-level power electronics, it also becomes possible to connect more modules in a string and to obtain more efficient operation of the system. But, he added, it can come at a significant cost increase.

In the fight for dominance in the inverter space, there is a contentious debate about the contenders’ costs and benefits.

On one side are optimized string inverter topologies like SolarEdge's new 9-kilowatt, 10-kilowatt, and 20-kilowatt three-phase string inverters and its 600-watt dual module power optimizer, which enable strings of up to 50 modules instead of the more typical twelve to sixteen. Unlike the SolarEdge single-phase 3-, 4- and 5-kilowatt string inverters and single module optimizer architecture, this new equipment is designed for commercial-scale rooftop and ground-mount installations. 

On the other side are microinverter technologies that offer what SolarBridge calls an AC module, as its attached microinverter allows each panel to generate power-optimized AC electricity.

“An AC module,” SolarBridge Marketing VP Craig Lawrence noted, "eliminates the concept of DC strings altogether.” They provide the same module-level data and management of issues like shading, soiling, and module mismatch, but eliminate everything associated with string-length constraints, he added.

“We have those exact same benefits, but we do it a price point very close to a string inverter as opposed to a microinverter,” SolarEdge North America Sales VP Peter Mathews said. “Microinverters can be 200 percent to 300 percent higher on a cost-per-watt basis.”

“That is an extreme exaggeration, to say the least,” Lawrence said. “The all-in first cost of the two solutions is comparable.”

A DC optimizer and string inverter combination added a cost of $0.59 per watt to a commercial system in 2012, according to the GTM Research report, which also forecasted that the cost will drop to $0.31 per watt in 2016.

A microinverter added $0.62 per watt in 2012 and will add $0.37 per watt in 2016.

That is not, however, the end of the debate. 

Source: The Global PV Inverter Landscape 2013: Technologies, Markets and Survivors

Central inverters with optimizers can combine the module-level MPPT of an optimizer with the proven technology, and therefore increased bankability, of the central inverter, according to the GTM Research report. 

A system built around microinverters has increased costs and bankability risk, it said, but an increasingly good record of system reliability.

According to data in the GTM Research report from Westinghouse Solar, 318 out of 3,373 string inverter units failed in the study period, causing 464,000 lost panel hours, and 56.0 kilowatt-hours lost per failure. The report found 22 failures of 10,630 microinverter units, incurring 528 lost panel hours, or 0.9 kilowatt-hours lost per failure.

Advocates for both architectures claim cost advantages:

  • Lower levelized cost of energy (LCOE) with a differentiated and optimized solution is the key to winning in the commercial market, Mathews pointed out in advocating for the SolarEdge string inverter-DC optimizer architecture.
  • “AC module systems have the lowest LCOE for residential through medium commercial installations,” Lawrence said.


Whichever side is right, the sector is expected to rapidly expand. The GTM Research report provides a detailed cost benefit analysis for each technology in residential-, commercial- and utility-scale applications. It forecasts shipments of microinverters and DC optimizers to reach 7.5 gigawatts across all segments by 2016.