The Top 10 Most Common Myths about Traditional DC Power Optimizers: Part 2 of 3
In part one of this blog post series we talked about how MLPE technologies impact performance and shared five common myths about power optimizers. In this second post, SMA expert Blair Reynolds continues providing clarification on five more of the most common misconceptions.
Myth #6) Traditional Power Optimizers help me design longer-strings… and that’s a good thing
Actually, this is one of the worst design tradeoffs one can make. In an effort to reduce copper wire and the labor with pulling a second string, there might be a temptation to push the string length over 20 modules (a maximum of 25 is technically allowed). But this is truly a terrible design decision. Let me explain: If we assume the native operative voltage for a single PV module of 40 Volts, one would need only about 10 modules in series to reach the 380/400V operating voltage of the inverter at Standard Test Conditions. If there are any more modules connected to the string, the power optimizers are forced to adjust the PV module voltage to a much lower voltage to match the inverter’s input voltage range. This is known as “bucking” the voltage by the optimizer. At this point the optimizer is operating at an efficiency far below the stated “peak” efficiency. The more bucking (i.e. the more voltage adjustment) the optimizer is forced to do, the more “work” it must perform, i.e. losses increase, and efficiency goes down. Therefore, adding “extra” PV modules to a string might save you some copper, but at the cost of additional losses incurred in the overall PV system, by forcing all optimizers into heavy buck mode.
Myth #7) Traditional Power Optimizers are inherently safer
While the optimizer does limit the output voltage to a safe level in the event of a fire (but only if the optimizer has not reached its end of life, been damaged prior to or during/by the fire) (even though it’s actually electrical current that is way more dangerous for first responders than voltage), by operating MLPE on the roof, you increase the risk of a fire occurring, which I admit is rather counter-intuitive. It all comes down to the law of total probability. By increasing the number of potential failure points within the DC circuit (be it connectors or capacitors or any other internal components), you increase the possibility that a component will fail. DC has a nasty habit of forming a persistent arc. The higher the module voltage, the less likely the arc will self-extinguish. AFCI circuits are (supposed to be) designed to help prevent this, but products which were certified to the UL 1699B Outline of Investigation (as opposed to the formal UL 1699B Standard available only since August 2018) don’t always work as expected Just ask Walmart. In any case, the AFCI circuit should always be considered the backup or secondary safety mechanism. Best practices for wire management and mating of connectors should always be considered the primary safety mechanism – to prevent arcs in the first place – not power optimizers, which increase the likelihood of occurring arcs.
Myth #8) Traditional Power Optimizers reduce my O&M costs
The cost of mobilizing to service a faulty module (or more likely a faulty power optimizer) can vary from a couple hundred dollars to a few thousand. The worst-case scenario I have heard of is in downtown San Francisco where a mechanical lift is required to access the roofs of row homes with rooftop arrays. No matter the situation, it is clear power electronics will eventually need replacement and the service risk for the installer offering a 10-year warranty (or even a 25-year warranty on a financed system) is significant. Ultimately, it is the thermal cycling (hot-cold-hot) that causes power optimizers to fail. The best independent studies I have seen on characterizing power optimizers in a climate chamber is 2,000 cycles (simulating about 5.5 years of continuous operation). Unfortunately, the devices don’t all fail at once either, making multiple truck rolls very probable.
The alternative is to delay a service visit until several failures have occurred, but this risks customer satisfaction by reducing energy production and could ultimately impact an installer’s reputation.
Myth #9) By standardizing on Power Optimizers I realize cost savings in my installation business
Upfront cost efficiencies can be realized by standardizing on a platform (sales and installer training, logistics, designs/permitting, etc.) But the undeniable fact is that by having a single source supplier strategy, your business has far more exposure to epidemic failures due to manufacturing defects or design quality problems or worse – the manufacturer going out of business, defaulting on all service and warranty promises, with no alternative supplier in sight due to persisting IP right restrictions for the entire platform. The lifetime service and O&M risks are significant (see Myth #8). You are also exposed to price fluctuations due to import tariffs, supply/demand, and scheduling risks due product availability and shortages. By choosing a platform which relies on proprietary communications rather than universal communications there is always the chance of future product incompatibility. Lastly, your opportunities to innovate and incorporate new technologies (like high efficiency, high voltage PV modules) are hostage to your single source supplier’s product roadmap.
Myth #10) Traditional Power Optimizers are good for the environment
Apart from the embedded energy in the devices themselves, there is the environmental impact of adding failure prone switches to nearly every single (or every other) rooftop PV module deployed in the USA.
As of June 2020, SolarEdge claims to have shipped over 50 Million power optimizers worldwide. These devices are fully potted and are therefore not recyclable. By the end of last decade, close to 115 Million pounds of non-recyclable material (containing high volumes of valuable materials such as aluminum and copper among others, which will eventually become scrap) will be deployed on rooftops around the world.
A more ecologically responsible mindset would dictate reducing material usage by focusing on product strategies that eliminate superfluous components. The very concept MLPE is at odds with the environmentally conscious value upon which the solar power industry was founded.
Discover a superior way to optimize PV systems and learn about all the resources available to you. Click here to read Part 3 of this blog post series.
Trackbacks & Pingbacks
[…] learn about the other myths of DC power optimizers check out part 1, part 2 and part 3 of our blog […]
Leave a ReplyWant to join the discussion?
Feel free to contribute!