I have been researching the shade issue with solar panels as I am going to have to deal with that in my new build. After looking at how panels deal with shade using bypass diodes. I am wondering if it makes more sense when constructing a system that has to deal with shade to use 100 watt panels? I was going to use 400 watt panels, but it looks to me that 100watt panels typically have 2 by-pass diodes and 400 watt panels have 3 diodes. I will need about 4k in panels. Based on this theory if I used 100 watt panels they would have 80 diodes. Using the 400 watt panels I would have 20 diodes. It would seem when dealing with mottled shade and processive shading that the more bypass diodes the better??? I am sure I am missing something in this construct. Please explain...
Shade 100watt panels vs 400watt. More diodes?
- Thread starter squarpeg
- Start date
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Excluding all other factors, your analysis is correct. However, when it comes down to the detailed design, things are likely to be very different between 100W and 400W panels.
Backing up a bit, the bypass diodes work best if you have a single string. With a single string, the string voltage may change when the bypass diode kicks in but the current will stay at what the pannels 'like'. The MPPT controller can then find the best new voltage for the string.
If you have multiple strings, when the voltage on one string drops the MPPT has to find an average voltage that gives the best power (None of the strings are operating at the optimum Current-Voltage point.
So....Assuming you want a single string, you have to worry about keeping the string voltage down to something your controller can handle... or you need to use multiple controllers. I have no idea what panels you are looking at, but I am willing to bet the 400W panels have less than 4x Voc of the 100W panels. Therefore, your total Voc is going to be lower with the 400W panels, and that will make it easier to find a controller that will work.
The other thing about the modern 400W panels is that they tend to have a very low-temperature coefficient for the Voc. That means your string will have an even lower temperature-adjusted Voc.
On the flip side, since the 100W panels will have a smaller Voc, it is easier to closely match the temp adjusted Voc with the spec of a particular MPPT controller.
The bottom line is this: There is too many variables to make a blanket statement that one is better than the other. You have to play with different designs and trade-offs to decide what works best for any given situation.
I had not taken into the account any temperature coefficient. They will be ground mounted in Vermont. It is always cold here. The shade will only be an issue in Nov. Dec, Jan with the sun so low. Going to be freezing then. Still considering how to wire them. I was thinking of using something along the lines of 3-4 strings. Running the panels in series on each string. so you might have a voltage of 120 -150 volts on each string depending on the solar panel used. Probably be better to run them all in parallel for shade but location is at least 100ft from the house. Obviously I am just trying to form a general plan here so everything is open to change. I have no experience at this just trying to flesh things out through videos and reading on the forum.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Adjusting Solar Panel Voc for temperature
When designing you solar panel system, it is important to adjust you solar panel Voc for temperature in order to ensure you do not over-voltage the PV inputs of your solar charge controller. This paper shows how to calculate the Temperature...
diysolarforum.com
Pi Curio
Rim Tim Tagi Dim
System voltage? Picture of the shaded area?
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