People are hungry for information. And when they want answers, they do web searches. Sales people know this, and a common approach to sales is to provide value by providing answers, hopefully building trust and leading to a sale. But how do you know if that sales site offering technical advice is really technically competent enough to offer technical advice? Can you trust everything you read?
No.
A potential customer just asked us about “hot spots.” This is an actual failure mode of a solar panel, where one cell’s resistance increases and generates internal heat, sometimes to the point of failure. Energy Sage, a well-regarded platform for solar information and quote comparisons, defines it like this:
“Hot spots, one of the most common issues with solar systems, occur when areas on a solar panel become overloaded and reach high temperatures relative to the rest of the panel. When current flows through solar cells, any resistance within the cells converts this current into heat losses. Imperfections in meetings, such as cracks, poor soldering, or the accumulation of dirt, can cause this resistance to increase in a concentrated area, causing a hot spot. It’s essential to deal with these immediately if they appear because, if left unchecked, they can cause degradation of your system or even render it irreparable.” https://www.energysage.com/solar/most-common-solar-panel-defects
Wikipedia has a great description and explanation that gets a little more down into the particle physics. They also provide a thermographic image of a hot spot. https://en.wikipedia.org/wiki/Hot_spot_(photovoltaics)
Now let’s look at another site (they call themselves “solar tech advisor”) that gets much deeper into the detail. This detail seems to imply a level of expertise beyond the norm. Certainly, they provide more detail than Energy Sage did. I’m picking on this one site because it was the first one I saw today. But they are far from alone.
The problem is, it’s wrong. Profoundly wrong.
Some of the errors stem from a misinterpretation of the literature. (It may have been the exact text provided by the Wikipedia entry that they paraphrased.) But others demonstrate a near-total misunderstanding of basic electricity. And a solar panel system generates enough power to kill, so anybody spouting electrical drivel and passing it off as fact is creating a public danger.
I’ll let the reader go through this section completely before I comment:
“Let’s assume a solar panel has 60 photovoltaic cells connected in series. Each cell has a rated output of 0.5 volts and 1.5 amperes. So, the total voltage of the solar panel would be: 60 cells x 0.5 volts/cell = 30 volts
And the total current of the solar panel would be: 60 cells x 1.5 amperes/cell = 90 amperes
“Now, let’s assume that a shadow covers 10 cells of the solar panel, which reduces the output of those cells to 0 volts and 0 amperes.
“This means that the remaining cells have to compensate for the power loss of the shaded cells. So, the remaining cells will have to produce the same amount of power that was being produced by all 60 cells before the shadow.”
No. No. A thousand times NO!
Let’s break it down:
Let’s assume a solar panel has 60 photovoltaic cells connected in series. Each cell has a rated output of 0.5 volts and 1.5 amperes.
OK, that’s reasonable. 60-cell panels are a staple of the residential solar market. Each cell is roughly a 0.5V device, since it is a common p-n junction. The max current, however, will depend on the cell efficiency and the surface area. 1.5A is in a reasonable range. That would provide a 0.75 Watt power output. (power is current times voltage, volts times amps, 1.5 x 0.5.) With 60 of those cells, total panel power would be 60 x 0.75, or 45 Watts. That’s fine, there are many applications for panels this small, but they likely aren’t on your roof.
So, the total voltage of the solar panel would be: 60 cells x 0.5 volts/cell = 30 volts
I agree. Let’s go on.
And the total current of the solar panel would be: 60 cells x 1.5 amperes/cell = 90 amperes
FALSE! This is basic series/parallel circuit stuff. In a series circuit, like cells in a solar panel, voltages add. In a parallel circuit, currents add. There is no world where both currents and voltages add. Remember how we determined that this is a 45 Watt panel? Using this description above, this solar panel puts out 2,700 Watts (30V x 90A). If the people at Solar Tech Advisor can provide this panel, we have solved the world’s energy issues.
But wait! There’s more!
“Now, let’s assume that a shadow covers 10 cells of the solar panel, which reduces the output of those cells to 0 volts and 0 amperes.
Close enough. Dense shadow will increase the impedance of each of these cells, which will reduce the amount of current it is able to pass and dissipate the resulting power loss as heat.
“This means that the remaining cells have to compensate for the power loss of the shaded cells. So, the remaining cells will have to produce the same amount of power that was being produced by all 60 cells before the shadow.”
Once again, NO. In what world do I reduce my power production by 1/6 and cause the rest of the system to generate more than its physical maximum to compensate? This is executive management logic; but it is NOT science!
To their credit, they get the basic actions correct. Shade on a panel is bad, make sure you choose your site wisely, make sure you perform regular maintenance. But then comes the sales pitch for microinverters and/or optimizers. While these module-level power electronics (MLPE) compensate for module-to-module variation and reduce the shading impact of one module on the rest of the string, they aren’t a preventive measure for hotspots. Hotspots can still form for all the same reasons. MLPE just does a thorough job of isolating the effects of that hotspot from the rest of the system performance. They can also make the under-performing module easier to identify and locate.
So, in conclusion, a sales approach that offers education as a service can be compelling. However, if you read technical information that worries you (“Will a hot spot catch fire? It all seems pretty terrible!”), rather than buying the proposed solution on the spot, look for a corroborating source, especially if you don’t understand the detailed description of the issue. Even if somebody sells you a reasonable product based on their faulty description of the problem it solves, the lack of ability to accurately describe the issue and its solution could be an indicator that support after the sale may not be of the caliber you need.
Chris Rohaly