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What Lies Beneath

What Lies BeneathIan Clover*

MLPE analysis: The growth of the module-level power electronics industry has tended to bring more questions than answers, chiefly concerning the cost-performance ratio when compared to traditional string inverter solution

MLPE analysis: The growth of the module-level power electronics industry has tended to bring more questions than answers, chiefly concerning the cost-performance ratio when compared to traditional string inverter solutions. And with all three types of solutions – DC optimizer, string inverter, and microinverter – each claiming to push the envelope on cost, safety and power output, is there a simple answer to the simple question: Which is best?

The weight of literature available online both extolling and downplaying the virtues of module-level power electronics (MLPE) is vast, and the effect can be dizzying for those not fully up to speed with latest industry best practice and opinion.

Residential and commercial solar customers that have conducted even the most cursory two minutes’ worth of research are likely to have read both the pros and cons of MLPE, but even then can wind up with no clearer answer as to what best suits their needs.

A power optimizer or microinverter offers greater energy harvest under shaded conditions, they may quickly learn. But then on a typical rooftop array, the multiple points of failure – compared to just one point of failure with a string inverter – could be problematic, they discover. Then again, an MLPE failure affects only one PV module at a time, not the entire string, further reading reveals, but is the initial up-front cost of an MLPE component worth it? And what’s the trade off? Do power optimizers and microinverters actually increase energy yield when compared to string inverters and, if so, by how much? Then there are questions of safety, rooftop architecture, warranty, and the bankability and reliability of the various suppliers that operate in this space.

It is enough to make the nervous solar newcomer eschew PV and plunge headfirst into the familiar embrace of a fossil fuel-supplied building. But that would be a mistake, because the very fact that there are conflicting opinions, analysis, and reports over which systems are best shows that the solar industry is pushing new boundaries with every new iteration – and competition breeds excellence, which in turn delivers increasingly lower costs.

Energy first

Amid the many publicized benefits offered by MLPE components comes the simple question: Will adding a DC power optimizer or a microinverter serve to increase the yield of a rooftop solar array compared to a typical string inverter-controlled system? The question may be simple, but the answer is not, given the many different testing approaches, architectures, and technologies available.

U.S. microinverter specialist Enphase Energy shared with pvmagazine the results of an independent test conducted by PV Evolution Labs (PVEL), itself a DNV GL facility, that showed how an Enphase microinverter was able to produce 1.2% more energy than an SMA string inverter under unshaded conditions.

The lab placed four residential PV systems side-by-side at its California testing site, two controlled by Enphase M215 microinverter systems, and two controlled by SMA’s SB3000HF-US string inverter systems of similar capacity. There were 12modules per system, and each module was flash tested prior to installation. DC capacity was also equally weighted across the four systems. Over the course of six months, the Enphase systems converted
1,340kWh/kWp of electricity, with the SMA string inverters converting 1,324kWh/kWp of electricity. As well as delivering 1.2% more energy, Enphase’s microinverters operated with a 1.7% higher efficiency than the string inverters, relative to each component’s weighted California Energy Commission (CEC) Efficiency Rating (which is calculated as an average value of DC-AC conversion efficiencies at six predefined outputs between 10% and 100% of the rated power), under normalized conditions. Further, under low irradiance and low temperature the energy harvest advantage offered by the Enphase microinverter was 10.8%.

The study also took into account the impact of module temperature, global irradiance, and direct and diffuse irradiation on the performance of each system. Over the course of a whole year, the lab extrapolated that the Enphase system would maintain its 1.2% energy yield advantage.

"Enphase microinverters can convert available energy of a solar PV module at very lower power levels with higher conversion efficiency due to a feature called ‘burst mode’,” Enphase CEO Paul Nahi explains to pvmagazine. "The microinverter captures the available energy from a PV module at low light levels and bursts this energy into the grid when enough energy is available for one or more full AC cycles.” According to Nahi, this feature takes advantage of the available PV module energy when the light conditions are too weak to operate the microinverter in the MPPT operating range. This means that a solar array fitted with Enphase microinverters begins to produce energy earlier in the morning and later into the evening compared to string (and central) inverter-controlled solutions, says the Enphase CEO.

Alternative efficiency

When an inverter reaches close to its maximum load, the closer to peak efficiency it is usually able to reach. However, peak efficiency data published in inverter and microinveter data sheets can often be unhelpful because most of the time inverters operate in the range of 20% to 30% of their rated power, wrote Stefan Krauter and Jörg Bendfeld of the University of Paderborn’s Electrical Energy Technology department in a recent paper studying the deviations of results for energy yield from efficiency rankings of microinverters.

They argue that to more accurately assess the power conversion efficiency of inverters, a more accurately weighted efficiency is required. In tests using the CEC-based efficiency rating that Enphase also used, the Paderborn University researchers also found that Enphase’s microinverters could boast a relative efficiency rating of 100% when based on a CEC efficiency of 95.6%, with SMA’s string inverter solutions delivering relative efficiency of 99.5% based on a CEC efficiency of 95.1%.

However, when tested using the European conversion efficiency rating to measure performance, the leadership roles are switched, with SMA’s string inverter approach coming out on top (100% relative efficiency based on the European efficiency rating of 95.4%), with Enphase second (on 99.8% relative efficiency based on the European efficiency rating of 95.2%). Essentially, Enphase products are designed to maximize Californian sunshine, while SMA’s string inverters handle better European irradiance conditions.

System plant owners interested in getting technical can dig even deeper on this point to find the solution that best suits their array’s distinct details, such as the voltage and type of modules used, the amount of shading on any of these modules, and even the type of weather patterns expected at their site. Many other system owners, however, simply want to know which solution is likely to feed the most AC energy into the grid or the home, and at the most attractive cost point.

One of the most dynamic portions of the MLPE space is DC power optimizers, which are DC/DC converters that sit on the back of each solar module and use MPPT to deliver the highest DC power yield to the inverter. In some conditions, such as cloudy days, optimizer-controlled arrays have been shown to offer as much as a 3% energy yield increase over microinverter-controlled and normal string arrays, whereas in perfect weather conditions other tests show that a simple string solution can outperform MLPE. During the winter months of longer shadows, string inverter-based systems will have a higher risk of delivering lower output during the earlier and later parts of the day because some of the modules on an array will be shaded for longer, thus bringing down the overall power produced. This effect is less pronounced when using MLPE, often by as much as 8%, some tests show.

Cost and flexibility

It is becoming increasingly important for solar installers to understand the benefits of flexible rooftop array design. As discussed, on an optimal rooftop with few shading problems and nestled under blue skies, a simple string inverter design can do the job admirably. However, the real world offers real challenges such as cramped roof space and unpredictable weather (including soiling) that can hamper energy harvest.

It is under these conditions where the benefits of MLPE shine through – provided installers are given the tools and know-how to install a flexibly designed array. With this in mind, leading MLPE companies SolarEdge and Enphase have introduced new models to their ranges that have been designed to make installation easier, faster, and cheaper. SolarEdge’s HD Wave, for example, is far lighter than previous iterations of its inverters, and this expedites the installation process.

Enphase’s new IQ6 and 6+ Micro are not only more highly powered than previous microinverters, but are also lighter and come equipped with lighter cabling systems. "This translates to logistical savings throughout the supply chain,” says Nahi. "For example, we have increased the number of microinverters within a shipping box from 12 to 16.” These seemingly small changes lower Enphase’s warehouse density, which lowers up-front costs.

Up on the roof, it also means there are fewer items to carry, which further brings down labor costs. "With the addition of the Q Cabling system we are reducing the amount of copper on the roof by 50%,” Nahi explains. "The Q Aggregator further eliminates sources of unreliability in the junction boxes, and an increase of up to 10conductors in the conduit to just three conductors in the home-run brings more copper savings and enables some installs to use half-inch conduit.” The impacts of this are reduced electrical labor, improved aesthetics and greater design flexibility on the roof, meaning that the modules can be placed in such a way as to maximize output efficiency.

Another cost-reduction strategy being employed by MLPE providers is vertical integration: building in more off-the-shelf solar AC and smart modules into their supply chain. In a recent IHS Markit PV Microinverter and Power Optimizer Report, the analysts forecast that revenue for solar modules pre-embedded with either microinverters or DC power optimizers could rise to almost $500million by 2020, from a baseline of around $100million last year.

One of the main benefits of this approach is to streamline sales and logistical channels, which IHS Markit believes will help MLPE providers capture new markets more easily. Although accounting for only 200MW of global module shipments in 2015, by 2020 smart (DC-optimized) and AC modules could comprise 4GW a year by 2020.

Enphase is already well along the path to this integration embrace, Nahi says. "Through mock installations conducted with four installation partners, the installers observed time savings of two to five minutes per microinverter,” states Nahi. Time savings were mostly low-tech: quicker unpacking, fewer components to carry, and swifter waste management, while the "hard cost” savings included a reduction in the number of bolts, nuts, washers, and panel mounts required, the study showed. At a more technical level, new policies supporting the smarter, safer home will aid the adoption of MLPE in the coming years, says IHS Technology solar analyst Camron Barati in a recent report. "Policies and standards will be a significant driver of MLPE adoption and associated module integrated solutions over the next five years, particularly in the U.S. The 2017 update to the U.S. National Electric Code (NEC) will standardize safety requirement applications in state markets.

"This will ultimately incentivize the use of microinverters and power optimizers for rooftop applications.” With increasing opportunities for MLPE suppliers to engage with a wider audience, greater cost reduction is the next goal in many companies’ sights.

Systems based on MLPE components can still cost up to 20% higher than power plants controlled by string inverters, the Paderborn University researchers said, and so further cost reduction strategies will continue to keep the R&D units of MLPE companies busy for some years yet.

* Ian Clover joined the pv magazine team in 2013 and specializes in power electronics (inverters) and battery storage. Ian also reports on the UK solar market, having worked as a print and web journalist in Britain for various multimedia companies, covering topics ranging from renewable energy and sustainability to real estate, sport and film.

(pv-magazine.com)

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