Best solar panel for cloudy days...

[svetz]

Rainy and cloudy outside, my 340W LGs are pulling in about 47W.

Yesterday, in another thread, I wondered if different panels types (e.g., poly or mono, high or low efficiency) get more power than others for hazy or cloudy days? I'll start off by saying, I don't know!

We know at STC two 100W panels should produce the same power regardless of their efficiency or whether they are mono or polycrystalline, or thin film.

Does Efficiency Matter?
If watts-out = watts-in/area * efficiency * area, then efficiency doesn't matter as it's a constant. That is at 80% light each would still produce the same amount of power.

It's a matter of Wavelength
But, is efficiency truly a constant? Consider that different solar panel types are all different in terms of the amount of energy they absorb from different wavelengths. Super high efficiency panels have three layers, the top layer converts energy from the highest wavelengths, the bottom layer from the lowest.

The energy converted from the various wavelengths might look like:

Humidity, air mass, cloud type and thickness will all have an effect on energy absorption, and not all wavelengths are absorbed equally. For example, clouds typically block UV and can reflect IR.

I suspect that different panels do indeed have different efficiencies for varying cloud cover, but also that it would be really hard to quantify since no two clouds are the same. Still, with raw data it might be possible to get some generalities (e.g., mono, vs poly, vs thin) or manufacturer. What do you think?

Update: Ran across this article...

 

[cass3825]

Researching PERC technology may also be worthwhile.

PERC cell explained

Learn what is PERC cell technology and why it is becoming the new favourite cell platform to manufacture high efficiency solar panels

www.aleo-solar.com

 

[svetz]

Found some raw panel data in 5 minute intervals at a farm that had a few types of panels (e.g., mono and poly). Then I looked for a day where the output around noon had a dip, that should mean there was something shading the sun. Then, for all those different panels I took the ratio of the max power to the dim power to see which panel types generated the most power during the dip. Results: no significant difference. The data and link to source is in the attached file.

 

[JoeHam]

So your tag line is correct!

Works in theory! Practice? that’s something else

 

[tictag]

I could be wrong here but the PV technology you are referring to in your OP is not yet mainstream (probably on military satellites and stuff, but not $100 eBay panels!) You are correct though in that a PV panel will have a power conversion profile because photons have varying energy potentials and the material properties of silicon are well established.

The thing is, at a consumer level, most PV cells (not PV panels) will likely be all very similar i.e. the cheapest possible technology / materials (go capitalism!) so the actual efficiency of consumer-oriented panels probably won't vary that much between panel manufacturer's. This is born out by Will's practical tests - they all produce around the same.

I think the biggest efficiency modulator will be the SCC. PWM SCCs are known to be relatively inefficient, MPPTs are better, better MPPTs are better than that! Some MPPTs track maximum power in second intervals, some tracking in milli-second intervals. Some have 92% efficient energy transfer some closer to 99%.

My argument would be, how much energy would a $1 investment return if spent on PV arrays compared to a SCC? My guess would be that the best return would come from investing in the SCC.

 

[svetz]

Great points TicTag! I hadn't considered inverters... but certainly the amperage will vary as the panels are more/less shaded, and inverter efficiency would probably vary because of it.