While it is very common for solar installers in the U.S. to install so-called power optimizers in PV systems to meet the local electrical code requirements, these devices are subject to considerable controversy in the rest of the world.
Five Myths About DC Power Optimizers
Hannes Knopf | SMA America
Sweden, for example, has even issued a partial ban on the sale and installation of power optimizers from one major manufacturer because they do not meet the applicable electromagnetic compatibility (EMC) requirements.* So what’s the story with MLPE? What is really technically relevant, and what is just clever marketing? We resolve five common myths about power optimizers.
To establish whether a PV system really needs so-called power optimizers (optimizers or module level power electronics – MLPE – for short), let’s start with a brief explanation of what they actually are. A MLPE device is basically just a voltage converter with built-in communication capability. More precisely, it is a DC-to-DC converter whose output voltage can be adjusted to be either higher or lower than the input voltage.
In modern string inverters, this function – known as a Maximum Power Point Tracking (MPPT) input – is often built-in.
System design: string inverter vs. MLPE
Whether equipped with MLPE or string inverters, PV systems feature PV modules that are connected in series to form strings. The major difference in design is that in systems with MLPE, the DC–DC stage of the inverter is not integrated into the inverter but instead is distributed across the entire PV array (all the PV modules in the system) in the form of many small devices. These additional devices “optimize” the string voltage to match the design input voltage of the inverter (e.g., 380 V DC in single-phase systems).
Myth #1) Power optimizers generate more energy for shaded PV arrays
This myth is based on the claim that power optimizers produce more power when one or more of the PV modules are shaded. While in theory, some aspects of this claim may have merit, in a real PV system, slightly more power from time to time does not necessarily result in a higher annual energy yield. So let’s explain what’s happening here: Like the maximum power point tracker of a modern string inverter, the power optimizer adjusts the operating voltage of the PV modules to match the total DC string voltage required to operate the inverter. If one or more of the PV modules becomes heavily shaded relative to the other modules in the string, the power optimizer behaves like any other MPP tracker. This is because the current path through the shaded portions of the PV modules is bypassed – not by the power optimizer but by the PV module bypass diodes. The power optimizer will therefore operate 1/3 or 2/3 of the PV module at a maximum power point based on the reduced output voltage of the shaded module.
Power optimizers are beneficial only when there is heavy shading
Shadows are the longest early in the morning and late in the afternoon. But these are also the times of day, when solar irradiation levels are the lowest. Therefore, a power gain of maybe 4% to 5% early in the morning does not translate into much additional energy yield, if at all.
In fact, a number of studies including a recent publication by the University of Southern Denmark (SDU) suggest that the internal power consumption of the power optimizers bucking and boosting voltage all day outweighs the additional energy yield from the early mornings and late evenings.
Power optimizers thus might only contribute a benefit in terms of energy yield in cases where sections of the PV system are heavily shaded during the middle of the day. But let’s be honest: Who builds a PV system in the shade? The economics of such a system would be questionable.
Incidentally, if the entire system is shaded evenly (like from a cloud), power optimizers do not contribute any additional yield compared with systems that use string inverters.
Intelligent yield optimization: The patented ShadeFix function integrated into SMA’s inverter software optimizes the yield of PV systems in every situation – even where there is partial shading.
Myth #2) Power optimizers generate more additional energy than they consume
Power optimizers are subject to standby consumption losses associated with running the integrated power electronics and communications. These devices require power, which they draw from the PV modules whenever they are in operation. On top of that, power optimizers are almost constantly bucking or boosting voltage all day every day. Remember, their job is to operate the module at a voltage that allows the aggregate group in a string to match the design input voltage of the inverter (e.g., 380 VDC). Under suboptimal operating conditions, such as when PV modules are mismatched (combination of PV modules with different power outputs), or have different tilt angles, or if there is shading, the power optimizers will be forced to adjust their operating voltage, which, in turn, reduces their efficiency. As a rule, the worse the operating conditions become, the lower the efficiency will be, because the devices are forced to work harder to adjust the voltage.
Power loss due to additional cabling
The increased voltage drop – something that is not accounted for on the power optimizer datasheets – also has to be factored in. A power optimizer needs around 2.7 m of extra connecting cable (input and output cables) for every PV module to which it is connected. This extra cabling alone results in a voltage drop of around 0.27 V per power optimizer in full sun. In a 12 kWp system, that’s more than 145 W of lost power. The power loss caused simply by the extra cabling and by the added resistive losses associated with the additional four connectors per power optimizer can quickly add up over the course of a year.
All these factors conspire to reduce the efficiency of the PV system. Ultimately, PV system operators care most about reducing electricity costs and purchasing less electricity from the utility grid. And to achieve this, they need their energy yield to be as high as possible.
Myth #3) Power optimizers are 99% efficient
While inverters exhibit a European weighted efficiency of up to almost 99%, there is no standardized method for testing or verification of the European weighted efficiency of a module-level power optimizer. The datasheets, however, may list a “Weighted Efficiency” for the power optimizer as a simple marketing term without any common definition or standardized testing procedure. While there might be one power optimizer operating point where it operates at 99% efficiency, most operating points will likely be running at much less than 99%, leading to additional PV system losses.
Scientific analysis of MLPE efficiencies
As a matter of fact, in a laboratory at ZHAW Zurich University of Applied Sciences in Winterthur, Switzerland, PV systems with and without MLPE were measured and analyzed – the findings were made public in 2021. The studies revealed, that the actual efficiencies of MLPE very much depend on their operating point (input-output voltage ratio and power) and realistically would range between 96% and 97,5%. This offers another important insight into why the annual yield difference between a MLPE based vs. a string inverter-based PV-system is much less than some bloated double-digit marketing claims by power optimizer manufacturers try to suggest. This also aligns with the findings of a field study by the University of Southern Denmark (SDU), published in 2019.
The research work continues and could potentially enable a systematic performance comparison of MLPE and string inverters. The aim of the research is to ensure that the manufacturers’ datasheets contain reliable and comparable performance data. Only then will it be possible for PV-system planners to perform a realistic and economic comparison of expected PV yields, especially in cases of partial shading.
Myth #4) Power optimizers make PV systems safer
Power optimizer manufacturers like to sell their additional hardware by saying that they make PV systems safer. They claim that MLPE technologies featuring rapid shutdown capability would make life easier for firefighters and could even help to prevent fires.
The MLPE device is supposed to limit the PV system output voltage to a safe voltage level, but this works only if, in the event of a fire, the device has not yet reached the end of its service life or was not damaged before or during the fire. A firefighter in action will have no means to verify the reliable functionality of any rooftop electronics and will still have to treat the entire PV system like any other electrical system with the required safety measures and distances for their fire-fighting operations. Also, it should be noted that protection against electric current is much more relevant for the personal safety of first responders than the advertised protection against voltage.
Remember, PV systems equipped with MLPE devices have an increased number of potential failure points within the DC circuit – be it connectors, transistors, capacitors or any other internal components – which increases the probability that the PV system will fail. So operating MLPE devices on the roof actually may increase the risk of accidents occurring in the first place.
Effectively preventing electric arcs
If electrical devices and components such as connectors in the PV system are defective, a persistent and extremely hot electric arc can occur in the defective area. AFCI circuits are designed to prevent electric arcs. But products that were not certified to the latest international standards (UL 1699B Ed.1, latest revision and final draft of standard IEC 63027 – for more information, see SMA document), don’t always work as expected – just ask retail giant Walmart in the U.S.
In order to offer our customers worldwide the most up-to-date safety standards, the modern AFCI solution SMA ArcFix will be integrated into all SMA string inverters. However, the AFCI function should always be considered the backup or secondary safety mechanism.
To prevent electric arcs, the focus must therefore remain on proper cable routing, rather than increasing the risk of fire with additional power electronics components (MLPE devices). The following rule of thumb holds true: The fewer the connections, the safer the system.
The benefit of SMA SafeSolar: PV systems equipped with MLPE devices require three times as many DC connectors as those equipped with SMA string technology. More connectors mean a higher risk of dangerous electric arcs and fires.
Myth #5) Power optimizers are good for the environment
By the end of 2020, power optimizer manufacturer SolarEdge claims to have shipped more than 65 million of its devices worldwide. These devices are fully potted but contain valuable materials such as aluminum and copper. So what about recycling? Can these valuable materials be recycled when the service life of the device comes to an end, or do they just end up in the landfill? If it’s the latter, this would equate to around 68,000 metric tons of non-recyclable material for the period mentioned above – packaging not included. To illustrate this, this roughly equates to the payload of over 600 fully loaded Boeing 747-400F all-cargo aircraft. In comparison, if you were to interconnect all these power optimizers together to form a chain, it would reach around our planet four times.
Any ecologically responsible mindset would strive to reduce material usage and eliminate superfluous components in the design of PV systems. As you can obviously see, the very concept of MLPE systems is completely at odds with the solar power industry’s mission to achieve a sustainable generation of electricity.
The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag
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