Irradiance Decreases VS Efficiency Increases

[MUHAMMAD FAIZAN]

Dear All,
as we can see in data sheets of renesola,there is one table of irradiance vs effciency,
Please guide me some concept about this table as how the efficiency can increase as irradiance goes down.

[inetdog]

One possible explanation is that the temperature increase of the panel above ambient temperature (which decreases efficiency by lowering voltage) decreases with decreasing insolation.

[MUHAMMAD FAIZAN]

Irradiance VS efficiency

first thank you very much for your reply,
next is that the same thing were in my mind but one of my student told me that sir, there are places where irradiance is present but temperature is below ambient temperature or equal to it.

[J.P.M.]

While temp. is probably the biggest single factor one of the other things that may influence perceived efficiency is the frequency distribution of the irradiance. Also, PERHAPS one other subtle, smaller, more variable and harder to nail down effect somewhat related to that frequency dist. effect may lie in the instrument measuring the irradiance. Silicon cell measuring devices - this includes most pyranometers - have a frequency response more similar to that of PV panels than, say, some of the Eppley pyranometers, or cavity instruments usually called pyrheliometers. See Duffie & Beckman, chap. 2.

[inetdog]

first thank you very much for your reply,
next is that the same thing were in my mind but one of my student told me that sir, there are places where irradiance is present but temperature is below ambient temperature or equal to it.

That could be possible with a very clear cloudless sky (and maybe high altitude.)
The panel will radiate into the "empty" sky except for that small angle occupied by the sun. If you somehow have a very dark (clear night) sky but the sun is still up, I imagine that you could have a radiation equilibrium temperature which is lower than the ambient air temperature. But that would be a very extreme condition, IMHO.

The full sun insolation of 1000w/m² is likely to swamp out any effect from radiation into a dark sky.

[J.P.M.]

That could be possible with a very clear cloudless sky (and maybe high altitude.) The panel will radiate into the "empty" sky except for that small angle occupied by the sun. If you somehow have a very dark (clear night) sky but the sun is still up, I imagine that you could have a radiation equilibrium temperature which is lower than the ambient air temperature. But that would be a very extreme condition, IMHO. The full sun insolation of 1000w/m² is likely to swamp out any effect from radiation into a dark sky.

And also with concentration of solar radiation - large or small. .................On earth, or in outer space, the solution for a quasi steady state temp. of a body is mostly a problem in heat transfer - convective and radiation heat transfer on or near the earth, and effectively pure radiation heat transfer above the atmosphere. The radiation portion of the transfer is a relatively easy one to get a 1st approx. for in outer space. Did it as an undergraduate . Pretty common heat transfer text problem sometimes done in the context of radiant heat transfer to/from a satellite as it rotates, sometimes as part of a problem to highlight cyclic stress from thermal expansion of parts of a satellite, etc............... The terrestrial situation is much more complicated, making most any approach to a solution no more than an approx., but at the end of the day, still involving an energy balance on an element, usually in a control volume. Also, in near earth orbit, the irradiance will be of a different character and intensity, all beam and close to the solar constant with the sun radiating with a temp. and frequency/wavelength distribution close to that of a blackbody at 5,777 deg. K. , and space treated as a blackbody at close to absolute zero.

[J.P.M.]

If you somehow have a very dark (clear night) sky but the sun is still up, I imagine that you could have a radiation equilibrium temperature which is lower than the ambient air temperature. But that would be a very extreme condition, IMHO.

Actually, it's not that unusual, sometimes when the sun in still above the horizon, but usually with the sun below the horizon, and/or often around sunrise, that usually being the coolest part of the day................ A frost at ground level or a surface such as a low angled roof when the air temp. is above freezing is pretty common in many parts of the world. Usually, that can occur more easily under conditions of low wind, and, as noted, under very clear skies. The ground or other surface temp. will be lower than the air temp. because that surface is radiating to a night sky that has a temp. below the ambient air temp. A heat balance on an "element" of ground or surface will show more heat leaving by radiation to a cold sky than heat transferred to that element by convection from the (warmer) air to the ground for a (mostly) windless atmosphere. If the heat transfer continues, and the ground or surface temp. gets below freezing, result is frost. Not usually a hard frost, often because a clear atmosphere with a low parameter that is often called the " effective radiant sky temperature" will usually have a low dew point, but it can and does occur, as anyone who has seen an early morning frost on the ground with a coincident above freezing air temp. will attest. Estimates of an "effective radiant sky temperature" are available in the literature, and usually run from a few degrees K. below ambient to about 20 deg. K or more lower than ambient. Empirical correlations often use dew point, precipitable water vapor, solar time time and or other variables to estimate the sky temp.

[inetdog]

Actually, it's not that unusual, sometimes when the sun in still above the horizon, but usually with the sun below the horizon, and/or often around sunrise, that usually being the coolest part of the day................
...

I agree, and it is one component cause of unexpectedly high panel voltage in winter, even more than justified by the ambient air temperature.
And I appreciate the additional information and details.

But the OP seemed to be talking about a time when the panels were actually getting significant insolation. Possibly early morning while the panels are still warming up?
And under some conditions (closer to space ) even during the middle of the day.

[J.P.M.]

I agree, and it is one component cause of unexpectedly high panel voltage in winter, even more than justified by the ambient air temperature. And I appreciate the additional information and details. But the OP seemed to be talking about a time when the panels were actually getting significant insolation. Possibly early morning while the panels are still warming up? And under some conditions (closer to space ) even during the middle of the day.

Understood and agreed. One extreme example would be a panel at the top of mount Everest aligned for normal incidence at 1200 hrs. solar time, with booster mirrors on either side and with wind blowing at a constant rate of 100km/hr., with an ambient temp. of -40 deg. for the noon measurement. Chances are the P.O.A. would be pretty high and an energy balance on the panel would show a temp. quite a bit below the 25 deg. C. S.T.C. rating, maybe even close to ambient temp. As a less extreme case that probably wouldn't fry the panel quite as quickly, I'd imagine places such as in the high(er) Rockies in winter may see higher levels of irradiance due to elevation above M.S,L and atm. clearness and ambient conditions that can keep irradiance levels high, panel temps low, and voltages and output high - conditions that would be pretty off design in a lot of locations that must be accounted for in the Rockies for the referenced application. As for efficiency going up with decreased irradiance, as I wrote in a prior post to this thread, and in agreement with what you wrote, it's probably mostly due to decreased panel temp., but as also mentioned, perhaps not entirely. If so, then perhaps the next logical avenue of investigation, assuming for the moment we're talking steady or perhaps more accurately quasi steady state conditions, might be to see just what it is that influences the panel temp., and in what fashion, and to what degree. One tool to do that is to use the concept of a control surface around the device and try to figure out all the ways energy can cross that boundary. It seems likely that under some conditions, what the OP's referenced spec sheet may be showing is at least possible, even likely, but there may be other things going on and cumulative effects from multiple sources that are unknown, or not considered, or acting in perhaps complicated and synergistic ways. ...................... Bottom line, as the OP's questioning student suggests, it's possible, and maybe, so it would seem, probably somewhat likely under conditions that may occur more often in some places than others. How common and where more common is a matter for some discussion.