Firefighters are a brave sort. They routinely risk their lives so that others may live. They are well trained and take great pride in their service. As solar has grown in popularity over the years, they have received specialized training on how to handle burning structures which have rooftop solar, but sometimes that training is suspect. The myth we cover in this episode: Are solar panels energized and dangerous when a flashlight is shined on them?

A few years ago I was at a party, talking with a couple of firemen over a beer. The discussion became extra interesting when they learned that I was in the solar industry. We had a great exchange. I provided to them lots of information about the actual operation of solar systems. They provided me their views and concerns about solar, how they operate as a crew, and bits about their strategies for handling fires.

I learned about how they are trained to deal with burning houses which have energized rooftop solar systems. They said one thing in particular that shocked me. No pun intended.

One of the firefighters said that covering solar system risks, his trainer told the audience that solar is “hot” or “electrified” and therefore dangerous even if a flashlight is shone upon a module. I was shocked. No pun intended.

The Myth:
“Solar panels are dangerous even if you shine a flashlight on them”

Solar Mythbuster Investigation & Response…

Lets investigate…

Consider a high powered LED flashlight. Wait, no, lets consider a really high powered pistol grip spotlight… A 36 watt 10 Million Candlepower bruiser. Like this (https://www.amazon.com/10-Million-Candlepower-LED-Spotlight/dp/B01KQQ5DB8):

Great… now lets dig some more…

Solar panels convert light energy to electrical energy based on an absorption spectrum. That is, certain “colors” (wavelengths) of light cause them to produce electricity. Of course flashlights have an emission spectrum – electricity causes them to produce specific colors of light (wavelengths) in various intensities.

So the first step is to understand how the emission spectrum of an LED bulb compared to the spectral response of a typical cSi (crystalline silicon) solar cell. I found that for all intents and purposes, LED bulbs produce light in the same wavelength range that solar cells absorb. That means LED bulbs can energize solar modules (see https://www.pveducation.org/pvcdrom/solar-cell-operation/spectral-response and https://www.comsol.com/blogs/calculating-the-emission-spectra-from-common-light-sources/).

The next step is to understand what kind of voltages a standard 300W solar module could reach being blasted by 36W of LED light.

Lets assume two scenarios:

Scenario 1: A firefighter points the beam from standing height down onto a module. We must start by framing the technicals of the situation:

  • The electronic circuitry in a solar module. Typical 60 cell modules are wired such that there are 3 groups of 20 cells, per the diagram below. 60 cell modules are approximately 65” x 40”. So that means there are 3 cell groups (or substrings), each approximately 1/3 of the 40” width of the module, which is about 13”. Each group of 20 passes over a bypass diode to introduce intra-module shade tolerance; that is, the bypass diodes allow the substrings to operate independently of one another in the case of partial module shading. SolarEdge has a nice paper on this here: https://www.solaredge.com/sites/default/files/se_technical_bypass_diode_effect_in_shading.pdf.
  • The area of the light beam projected from standing height. Lets assume a medium beam angle of 30 degrees (see lightbulbs-direct’s page here: https://www.lightbulbs-direct.com/article/spotlight-beam-angles/). I estimate that from standing height the distance between the spotlight and the module to be about 4’ (firefighter’s arm extended downwards). A 30 beam angle over 4 feet will produce a circle of light approximately 2’ in diameter – 48” x tan(30), which is about 28”.

Wow! These technicals are great, but why are they relevant?

Because the amount of electrical energy that a solar module can output is heavily dependent on what part of the module is illuminated. Based on the above and the link to SolarEdge’s excellent technical paper, it is UNLIKELY that a solar module will produce any appreciable electrical energy being hit by a light beam 28” in diameter – none of the substrings can possibly be fully illuminated due to the geometries of the scenario.

Scenario 2: A firefighter points the beam down from high enough to illuminate the whole module (as though they are on a truck ladder). If we assume that the flashlight evenly distributes photons across the beam spread, then a good portion of the beam is not hitting the module (and the portions that are not hitting the module are not hitting other modules in a meaningful way – not enough of neighboring modules would be illuminated for electricity production). How much of the flashlight’s power is actually illuminating the module?

According to some calculations, I figure the beam would have to be about 76” in diameter. Assuming a circular beam distribution, thats 31.5 square feet of beam area. A 65” x 40” module is about 18 square feet in area. That means the module can absorb about 57% of the beam’s power (18/31.5 = 0.573).

Great, how much power is 57.3% of that big flashlight’s output? Well, according to the Amazon product description, the flashlight draws 36 watts of electrical power to generate its beam. Lets assume zero inefficiencies inherent to the flashlight’s production of light. From conservation of energy, we get 36 Watts of light over the previously touched on spectrum.

57.3% of 36 Watts is 20.6 Watts

Now, lets frame this based on the typical characteristics of a module. Decent 60 cell modules are about 20% efficient these days, and they operate at about 37 volts.

20% Efficiency x 20.6 Watts is about 4 Watts.
4 Watts at 37 Volts implies 0.11 Amps or 110mA

How dangerous is this voltage and current (110mA at 37VDC)?
The resistance of adult skin should be high enough such that touching 37VDC leads is no problem.
Basically the only way this scenario is dangerous is if the firefighter finds bare leads from the module and sticks them into their flesh, breaking their skin.

Given the personal protective gear and general hazard preparedness of firefighters, what are the chances of that happening?

Solar Mythbuster Conclusion: Sure, a sufficiently powerful flashlight that can illuminate an entire solar module can cause that module to produce electrical energy. However, extenuating circumstances would be required for a first responder or anyone in general to be at risk from such a scenario.

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