They perform best when they’re in it and angled appropriately, but what happens when these ideal conditions aren’t achievable?
What happens to a solar panel when it’s partially shaded for a substantial portion of a day?
After asking ourselves these questions, we wanted to learn more, so we set up a test.
For this particular test, our focus was on comparing the effects of partial shade on amorphous silicon (a-Si) and crystalline silicon (c-Si) solar panels.
Date: July 29th
Location: Ames, Iowa
Time: 8:00 AM - 4:00 PM
45 Watt Amorphous Silicon Solar Panel (5-7% efficient)
50 Watt Crystalline Silicon Solar Panel (18-22% efficient)
Charge Controllers: 4.5A PWM Charge Controller (PowerFilm part number RA-9)
Data Loggers: PWR Check
Time-Lapse Photos Interval: 5 minutes
Watch the time-lapse video below and see the results of partially shading an amorphous silicon solar panel vs. a crystalline silicon solar panel.
The above graph clearly illustrates how devastating partial shading over the course of a day can be to a series-connected crystalline panel.
Total power generated:
45 Watt a-Si: 238.3 Watts
50 Watt c-Si: 118.8 Watts
Looking at the above data begs the question, “How can an amorphous silicon panel produce 100% more power compared to a “more efficient” crystalline silicon panel?”
Let’s dig into that and discover why amorphous performs better in partial shading and why label efficiency doesn’t always translate into real-world effectiveness.
The main reason that amorphous silicon solar panels perform much better in the shade has to do with the way cells are laid out. Crystalline silicon solar panels feature square compact cells whereas amorphous silicon panels feature larger area cells. This physical difference makes it far more difficult to shade an entire cell.
Another reason amorphous silicon solar panels have superior partial shade performance is they require more area than crystalline silicon panels to produce the same power. While this is often a downside, in the case of partial shading, it is more difficult to shade significant portions of an amorphous panel compared to the smaller area crystalline panel.
Additionally, crystalline silicon’s interconnection system limits the output to the lowest-performing portion of the panel. Bypass diodes can compensate for partial shading, but only to a point and you will still lose the output of the area that is shaded.
Finally, the type of charge controller you use matters. Solar panel systems with PWM controllers experience more significant losses when partially shaded because they don’t adjust to changing conditions. Systems with an MPPT controller will adjust to whatever the new maximum power point is. MPPT controllers can’t eliminate the loss that the shade caused, but they help ensure you get the most power out of the un-shaded portion (learn more about different charge controller options).
We set out to see what would happen if you partially shade two solar panels for a substantial portion of a day.
Our test results, as you saw above, demonstrate that while NREL efficiency is a standard, and something that all solar technologies are judged by, those efficiencies don’t always illustrate a solar panel’s actual capability to perform in real-world conditions.
Rather than merely selecting the highest efficiency or cheapest option, it’s essential to drill down into your application.
What does your operating environment look like?
Is the temperature consistent year-round?
Will your panel be in perfect sunshine?
Is partial shade something your panel will experience?
These are the questions you must ask and research before selecting the solar panel that’s right for your application.
We would love to help you on that research path.
We can suggest the best technology for your specific use case and explain how to mitigate potential issues.
With over 30 years of experience and 200+ engineering years in the solar industry, we are ready to help you design your next solar product.