Solar Panel Voc-MPPT?

BFL13 wrote:
I thought the panel temp was what counted, not the ambient temp. ISTR panel temps are usually 20C or whatever higher than ambient

It must take a while in the sun to heat up the panel though. When I took my readings soon after taking the panel outside to ambient 15C, the glass was not hot (yet)

Where we camp, even if it is hot out (90F) there is usually a cool breeze off the sea, plus the panel is propped up so there is no trapped air under it like a flat roof mounting.

I don't know how they can test at 25C. I guess they use a lamp for light that does not heat up the panel like the sun does.

your correct. It is the temperature of the solar cells themselves that is relevant in looking at output power/voltage vs. temperature. In fact the critcal temp is the cell PN junction temperature, somewhat hard to measure, but it is related to cell surface temperature. Unfortunetly silcon is a pretty good thermal insulator.

My experience is similar to yours, my panels get warmer than ambient, but not excessively given our relatively low ambient temps.

anything you do to lower panel temps will help. However the cells themselves are pretty good at absorbing energy and heat. But then some heat will be radiated from the backside of the array as well helping in cooling. In fact in a portable situation, I'd guess that could be a noticeable advantage over a flat roof mount. First a flat roof mount will trap heat, while a angled portable will at a minimum have convection cooling. Second the roof itself and surrounding area will radiate heat into the panel while a portable on the ground often won't get much heat radiated from the ground. (Unless your in arizona sand!!)

And it wouldn't suprise me if they either cool the panel to test or measure the temperature of the panel and then calculate what the Vmp, Imp Isc, Voc would be at whatever temp they spec. It's a pretty straightforward calculation since the device physics around silcon junctions is known to the Nth degree from research over the past 50 years.

Or the could put the panels in cooled chamber and use a defined light source and keep the cells at the defined temp.

My experience in the Pacific NW is that temp isn't that much an issue. First daytime peak temps are very often in the 80-85F when we camp, second, there is usually at least a slight breeze.

Now if I was in southern arizona in August with daytime temps from sunup to sundown above 100F another story!

so an interesting experiment would be to wait till you have a hot sunny day let the panel heat up, monitor output power or current, then using a spray bottle of water, spray ice cold water on the panel to cool it down, once cooled down wipe off the water and see what happens to output power and current. If panel temps really are a signficant factor in output power, you should be able to see a signficant change.

Hummm.. may need to try that, but that's going to have to wait till next summer.