The subject of testing solar products is quite interesting (for those not intimidated by the technical aspects) and is frankly one of our greatest challenges with selling products to consumers.
The issue is that typical customers can't give a fair evaluation of a solar panel in the same way they would if they'd purchased another product. If they buy a tape measure, they can easily compare it to other tape measures and quickly know whether the manufacturer is producing a quality product.
With solar panels, many manufacturers produce lower-quality panels and sell them as rated much higher than reality. Sadly, these companies rely on the fact that consumers have no way of testing (or don't desire to test).
One of our concerns is that some customers will come away from the experience thinking "solar simply doesn't work," which taints the entire industry.
At PowerFilm, we have some different dynamics at play. First, we're completely making our products now in the US (Ames, Iowa). Second, we produce many products for the US Military - who require reliable/high-quality products that will deliver what is promised.
We also produce custom solar products for industrial applications (for example, GPS-asset tracking solar modules for the trucking industry). So, unlike many of our competitors, we don't repackage low-quality cells and sell them with questionable power ratings.
With that backdrop, let's dive into testing.
Things to know about solar testing & simulation
Light intensity primarily affects current (which affects power)
Different types of panels will be impacted differently by increases in temperature
Temperature impacts power output
Shading impacts power output. For example, bird poop, or anything else covering portions of a panel, will impact the overall product's output. If the module's architecture is such that large cells are wired together in series, and one of the cells is shaded, the overall impact will likely be reduced voltage (significant power drop).
The light spectrum will greatly affect output, and different solar technologies respond differently to different sources. This is critical.
It would be best if you considered both sides of the equation:
Each light source is different
The spectrum from each source will vary wildly. For example, the light intensity from a florescent bulb may look to your eyes as very bright. But when you place a solar panel under it, very little power is produced. The bulb produces an inferior representation of sunlight, and the panel responds to certain frequencies of light, but not necessarily the frequencies produced by the bulb.
Each PV type responds to a given spectrum differently
This is known as "quantum efficiency."
You might think of this as a person's hearing. One person may say, "that loud, high-pitched noise is really annoying," and their 80-year-old grandfather (who has lost his high-frequency hearing) would say, "what noise?"
In the same way, different PV types under a simulator require special testing considerations because of frequency differences (QE). In other words, unless you've managed to reproduce the spectrum of outdoor sunlight perfectly, you can't simply throw different PV types (a-Si, monocrystalline, polycrystalline, CIGS, etc.) under a simulator and expect to do an "apples to apples" comparison.
The QE differences in the solar types and the spectral mismatch of your light source will distort your results. Also, your reference cell (the cell in your meter to show you the intensity) must be of the same type of PV as the module under test.
The sun's spectrum varies
This is just another wrinkle that we must accept. For example, we noticed a sizable change when testing foldable solar panels for the military one year. When we checked the outdoor spectrum, we found that the light had changed. We could correlate that to forest fires in the West that had slightly tinted the spectrum toward the red wavelengths.
We have now built several of these simulators, and we have battled issues such as temperature stability, light uniformity, and spectrum to perfect our simulator.
Many try to make a simple "poor man's" tester that can accurately compare the various modules found on the market. While that's a great idea, it's something that maybe can't be done.
We've tried doing that ourselves with similar LEDs and, in the end, decided that the only way to do it was to design a custom LED system (hand-picking each LED, spending many hours considering spectrum matches, comparing indoors/outdoors, etc.).
The bottom line
Tackling the subject of testing solar panels to compare units in a market full of misinformation is incredibly difficult. We are happy to help.
There are some gotchas with using indoor lights for simulation. We developed our own systems, which are dimmable, good spectrum matches to simulate outdoor conditions for our PV, uniform, temperature controlled, etc.
Consider that your reference cell should be of the same type as the unit under test. It should probably be temperature compensated as well.
Interested in learning more about our testing procedures or PowerFilm in general? Contact Us or leave a comment below.