3 ms·

I didn't, but this is just after sunrise in the Bay Area, so the temperature is pretty low - not in the zone where solar panel efficiency starts to decline.

by danans 6y ago

dmoy 6y ago
Right, wouldn't lower morning temperature be better for solar cell efficiency? It's been a long time since I took semiconductors, but I vaguely recall a decrease in voltage at higher temperature. You'd need to measure the temp of the actual cell, because it gets wonky. You can't just look at air temp.
- danans 6y ago
  It would be surprising to see such huge difference in cell temperature in sun vs fog when the ambient temperature itself is about only about 60F, which is typical, or maybe even high for 7-9AM on the type of days I'm describing. Even if the cells were 80F on the sunny mornings vs 60F on the foggy ones (or a difference of roughly 10 Kelvin) that would result in about .5% to 1% drop [1] in efficiency, so I should see only slightly lower output on sunny vs foggy mornings. Instead what I see is that initial production on foggy days is several times higher than on sunny days. The only explanation I have for this is some kind of local effect where the fog reflects light that would have otherwise gone straight west in the morning down to my solar panels. 1. https://i2.wp.com/synergyfiles.com/wp-content/uploads/2017/01/Efficiency_PV_vs_Temperature2.png https://i2.wp.com/synergyfiles.com/wp-content/uploads/2017/0...
  - hinkley 6y ago
    Morning fog means the dew point is being crossed. If you control for temperature, as you have here, then the humidity is the deciding factor. Can a hot solar panel shed heat faster into humid air than dry air? I'll bet if we look it up that the answer is 'yes'.
    - danans 6y ago
      Enough to cause a 2-3x increase in panel efficiency? Seems unlikely. Instead the panels must be getting more light somehow.
hinkley 6y ago
I think you'd have to measure the panel temperatures to know when it gets too hot, rather than just tracking air temperature, true. All objects lag behind changes in ambient temperature. I don't expect solar panels have a lot of thermal mass, but whatever they sit in front of might...
- saeranv 6y ago
  Yes, the panel temperature can be hugely different due to absorption of radiation (I don't think it's the thermal mass, which as you say is going to be negligible). Ambient air temperature is not a good way to approximate the panel's temperature. There's a huge difference between ambient air temperature and an object's surface temperature due to (among other things) the latter's absorption/emittance of radiation. As I mentioned in another comment, I think what's happening is the dew (fog) on the surface of the panel is evaporating and the panel is cooling due to the evaporative cooling impact. Specifically, the surface gets cooler as latent heat is used to phase change liquid to vapour, and degrades the air film on the surface (which also contributes to thermal resistance of panel). This would also explain why you're getting such a huge peak, since the latent heat impact is nonlinear. So the temperature of the solar panel is increasing linearly as the air/sun rises, then the dew starts evaporating and the temperature increase will stop.

I didn't, but this is just after sunrise in the Bay Area, so the temperature is pretty low - not in the zone where solar panel efficiency starts to decline.

by danans 6y ago

I didn't, but this is just after sunrise in the Bay Area, so the temperature is pretty low - not in the zone where solar panel efficiency starts to decline.

dmoy 6y ago

Right, wouldn't lower morning temperature be better for solar cell efficiency? It's been a long time since I took semiconductors, but I vaguely recall a decrease in voltage at higher temperature. You'd need to measure the temp of the actual cell, because it gets wonky. You can't just look at air temp.

danans 6y ago
It would be surprising to see such huge difference in cell temperature in sun vs fog when the ambient temperature itself is about only about 60F, which is typical, or maybe even high for 7-9AM on the type of days I'm describing. Even if the cells were 80F on the sunny mornings vs 60F on the foggy ones (or a difference of roughly 10 Kelvin) that would result in about .5% to 1% drop [1] in efficiency, so I should see only slightly lower output on sunny vs foggy mornings. Instead what I see is that initial production on foggy days is several times higher than on sunny days. The only explanation I have for this is some kind of local effect where the fog reflects light that would have otherwise gone straight west in the morning down to my solar panels. 1. https://i2.wp.com/synergyfiles.com/wp-content/uploads/2017/01/Efficiency_PV_vs_Temperature2.png https://i2.wp.com/synergyfiles.com/wp-content/uploads/2017/0...
- hinkley 6y ago
  Morning fog means the dew point is being crossed. If you control for temperature, as you have here, then the humidity is the deciding factor. Can a hot solar panel shed heat faster into humid air than dry air? I'll bet if we look it up that the answer is 'yes'.
  - danans 6y ago
    Enough to cause a 2-3x increase in panel efficiency? Seems unlikely. Instead the panels must be getting more light somehow.

hinkley 6y ago

I think you'd have to measure the panel temperatures to know when it gets too hot, rather than just tracking air temperature, true. All objects lag behind changes in ambient temperature. I don't expect solar panels have a lot of thermal mass, but whatever they sit in front of might...

saeranv 6y ago
Yes, the panel temperature can be hugely different due to absorption of radiation (I don't think it's the thermal mass, which as you say is going to be negligible). Ambient air temperature is not a good way to approximate the panel's temperature. There's a huge difference between ambient air temperature and an object's surface temperature due to (among other things) the latter's absorption/emittance of radiation. As I mentioned in another comment, I think what's happening is the dew (fog) on the surface of the panel is evaporating and the panel is cooling due to the evaporative cooling impact. Specifically, the surface gets cooler as latent heat is used to phase change liquid to vapour, and degrades the air film on the surface (which also contributes to thermal resistance of panel). This would also explain why you're getting such a huge peak, since the latent heat impact is nonlinear. So the temperature of the solar panel is increasing linearly as the air/sun rises, then the dew starts evaporating and the temperature increase will stop.