Content Warning: The following discusses fire. We like fire (sometimes).
Today, we ponder fire, Walmart, and Tesla, and the specific relationship (of which we are only recently aware) between fire and the following rooftop technology:
Typical rooftop PV solar array (image courtesy of The World Bank).
Every youth who ever held a magnifying glass soon learned that SUN + TOOL = FIRE is a reliable equation for fun. But did you know that the tool could also be a photovoltaic (PV) panel in a rooftop solar installation? According to last week’s news buzz, Walmart knows this, and they know it quite well. Those links hit Reuters and Ars Technica, respectively, but if you prefer to hear it in a sardonic Australian accent, EEVBlog has got you covered via their YouTube channel. Since the TechReport has at least one member with a vested interest in rooftop solar, this one naturally got our attention.
Summary: Walmart experienced seven PV solar fires on retail store roofs from 2012 to 2018, including some that started after they demanded an electrical shutdown of the rogue firehawks nesting in their attics. This month, the company pursued the equipment vendor in court over arcane legal terms like “negligence” and “millions of dollars.” The twist is that the vendor was Solar City and Telsa purchased the company in 2016. Thus, Tesla Energy Operations is the named defendant in Walmart’s suit, adding a bit of intrigue to the story and handy SEO terms for thirsty parasites like us.
We did a little digging and learned the Walmart installations aren’t the only Solar City panels to go full Thich Quang Duc. Apparently a chap named Roe Osborn, an editor for The Journal of Light Construction, had a Solar City installation on his Rode Island home decide to take a smoking break in early spring, and his follow-up experience with Tesla’s support contractor doesn’t sound much better than what Walmart’s suit claims for their own service calls.
Complicating things further, Business Insider recently turned up details on a Tesla program named “Project Titan”. Tesla apparently recognized they had a fairly broad, fire-related problem with certain parts and pieces in their solar installs, and were attempting to quietly correct it. It’s not clear whether the equipment in the program was responsible for the Walmart fires, but it remains to be seen what the judge thinks of all this, assuming it goes that far.
Legal-eyed types can find the details of the Walmart suit at PlainSite. Our goal is to review why a solar panel might catch fire at all. The tale of a shiny, silicon-infused lifeform imbued with the sole mission of catching a 20-year suntan sounds more like a saccharine Lifetime Original set in Venice Beach than an origin point description in a fire investigator’s notepad. But since the Walmart lawsuit includes the following photographic evidence, we want to know why this might occur:
Ironically, made in America.
Yes, faithful caption readers, we went there. Deal with it.
Fear and Loathing in Las Standards
Okay, enough about Walmart and Tesla. Let’s move on to fire, which is why you all clicked through in the first place. First, if you don’t already know what the PV principle is or what a solar cell does more specifically, Wikipedia has you covered for both. Then, note that industry literature does recognize the possibility of a panel fire, and there are regulations. The catch is that PV solar is a hot market right now, and the regulations are still being developed. A new installation is probably a fair bit safer than one put in just five years ago, and that five year old install is probably somewhat safer than something from 10+ years ago. So goes life, of course, but it goes faster for a technology still in its high growth phase.
As an example, the US-based NFPA – publishers of the National Electrical Code (NEC), among others – provides extensive safety requirements for PV systems. They’re making the effort. But when I compared the section on PV systems in the three most recent NEC editions – 2011, 2014, and 2017 – there were a lot of additions and clarifications in just those six years. I would expect even more in the next three-year update in 2020. Moreover, not all jurisdictions enforce the latest NEC, often because they haven’t had time to review and incorporate it into law.
Meanwhile, if you want even more details on standards and licensing requirements, this Department of Energy publication should hold you for a couple hours. For the low, low price of whatever they’re charging these days, you can also consult UL Standard 1703-3.
Today, on “Will it Burn?”
Two questions. First, how can a solar panel burn? And second, what might cause it to ignite?
Regarding the first: in spite of the thick glass surface we associate with PV installs, a typical panel includes a lot of flammable material. Why? Well, the very best solar panels on the market today are about 22% efficient and most are in the 15-17% range. That means a lot of surface area is needed and it has to be supported somehow, but not be so massive as to require heavy installation methods.
Example: Assuming you spring for those best panels (because, according to L’Oreal, you’re worth it) and expose them to direct sunlight at 1050 W/m^2, plus a number of other simplifying assumptions…you get a situation where each square meter of solar panel assembly is rated for around 230W of DC power output. That’s for an hour or so at midday, southern exposure, with angels singing overhead. There are additional conversion losses before that DC power becomes AC power in the grid.
To build that output to a usable level, we’ll need at least several of these large devices, built using the best combination of low cost, light weight, moderate rigidity, high moisture resistance, and good electrical insulation. Say something like that and a smart product designer immediately thinks “plastic.” Modern PV solar panels are often built on a plastic substrate, laminated to a glass top and framed with aluminum.
The catch: any petrochemical plastic contains a lot of stored energy in search of an ignition source. If it finds one, it will burn merrily and spread melty flaming bits around the neighborhood. Less of a Lifetime plot and more of an ISIS plot, in other words. So either the plastic must be made flame retardant, with added environmental costs, or the panel must be specified and tested to interact with standard roofing materials in a way that resists excessive fire spread.
Which gets us to our second question: Ignition sources. These guys know a thing or two about the subject, and I recommend reading the whole thing. But if you want the Cliff’s Notes, the causes divide roughly as follows:
- Wiring fault on the power cables or conductors (poor installation practice, moisture incursion, animals, etc.). Bad wiring can overheat or begin arcing.
- Faulty ground connections (poor installation practice, poor maintenance). Bad or missing grounds can allow an overvoltage to occur in the panel array, causing a breakdown of electrical insulation in the array or surrounding conductors.
- Arcing fault in the panel (poor installation practice, panel deterioration). Damage to the panel or rapid deterioration of a poorly designed panel can lead to emerging electrical faults in the panel assemblies.
Of course, an inverter fault or other device downstream can also start a fire, but the above are the main reasons why the actual panel assemblies might light up on the roof.
Practical Panel Example
Let’s look at a real panel from a utility-scale installation. The following is an example of a popular PV panel purportedly produced by JinKO Solar, one of the larger Chinese design houses:
The juice is loose.
If you want to know more about the configuration, JinKO publishes the install guide online for your reading pleasure. Here’s a backside shot and nameplate sticker:
JinKO solar panel nameplate data.
To break down the data on that sticker, we have a company whose chosen name is phonetically similar to “jinx.” Suggestion: don’t do that. On further analysis, we also have a 1.94 m^2 panel rated 310W for a 1000 W/m^2 exposure test. Do the math and that comes out around 16% efficiency, which is squarely mid-pack for modern devices. We also have a Class ‘C’ fire rating, which is the most basic level of roof fire resistance offered in light-frame residential construction. This panel is installed on a tracker array in the desert, so that part doesn’t matter so much, but it becomes relevant if you think it would look good on your house.
So, the panel meets the basic requirements, and yet it will burn. There’s a fairly thick layer of glass on the top side, but it is sandwiched to a white plastic substrate which is obvious in these photos. The cells are bonded to it, the DC collector bus is bonded to it, and the main output block to the wiring harness is bonded to it on the back side (not pictured). And here’s the twist that affects this and all solar panels: there is no way to shut this thing off while exposed to light.
The PV panel will attempt to generate a maximum of 8 A at around 37 VDC, or about 9 A into a short circuit, as long as the sun is shining on it. It will do that regardless of whether it is delivering power into the inverter, or into a growing dark spot that is also starting to smell funny. The only thing that can practically stop it is a supervillain-scale sun blocker, nightfall, or the eventual loss of electrical performance that occurs in a nice hot fire. Two of those three can not be relied upon at the moment of need, and the third one is the moment of need.
Fortunately, PV panel fires really aren’t all that common, although US data collection is still problematic. What we can say is that if these things lit up every day, the other half of California would have already burned down and sealed itself in a tomb of re-solidified glass. Also, the Tesla/Solar City issues named in the Walmart lawsuit seem to be related to a specific vendor and timeframe, so either you’ve got an older Solar City install on your roof right now and should have it inspected, or you don’t.
That said, the safety precautions recommended by the installer – animal screening, routine inspections, electrical code compliance – are there for a reason and should be followed religiously. PV panels seem innocuous enough, something that parks on the roof and then does its thing, but they are only a fire-and-forget solution in that if you forget about them, you might get a fire.
Also, if you ever have issues with your array and need to put it in cold shutdown, do understand that even after disconnecting the inverter, the only way to actually stop the array itself is to cover it up using opaque materials recommended by your local solar installer or fire department. Fire resistance may be required for these materials, so easy materials like tarps or plywood sheets may not be acceptable to local regulators, let alone the HOA.
To our readers: Do you have any experience with PV installs, array fires, or the codes and standards that affect both of these? Please speak up in the comments! We would love to learn more about this topic from your personal or professional experiences.