Black-on-Black vs Black-on-White temp differences

[Jafoy]

Does anyone have any real numbers as to how much hotter - and as such how much less efficient - black-on-black panels are vs black-on-white, given an identical temp. coefficient? My specific area gets pretty hot (90-100+) in the summer for what it's worth.

I'm trying to decide if, say, a 295W black-on-white panel would outperform a 300W black-on-black. A 5W difference with a .4 %/C coefficient would seem to indicate that the black-on-whites would be more efficient if the black/blacks were > 12.5 degrees hotter, but I have no idea what actual difference to expect (or if I'm missing something else).

As always, thanks for the help.

[cebury]

I don't know the answer but if i may offer some advice.... if this is for a residential system < 10kw, your question essentially amounts to "how many more pennies will it add to the big pile of dollars it makes over the year". Once you've had the system a while you realize the small things weren't worth the worry or sweat or attempt to make the 100% perfect decision.

if you're asking for a different reason, such as purely academic, feel free to ignore.

[J.P.M.]

The blacker panels will be warmer, but probably not 12.5 deg. C. How much is a matter of irradiance, wind vector, tilt and, particularly if parallel to the mounting surface, distance to that surface, and other things. There are also other panel parameters that can also influence a panel's STC output. Actual temp, differences under side/side conditions may exist, but may be hard to come by.

However, I'd SWAG it and say that 12.5 deg. C,. warmer would be a stretch under just about any conditions.

A very rough 1st approx., based on STC Wattage differences alone :

295/300 = 0.9833. 1-09833 = 0.0166. .0166/.004 = 4.15. So, about a 4 deg. C diff. until measured seems a bit closer to what might be measured under 1,000 W/m^2 normal irradiance and no wind, and probably less under less irradiance/more wind site /measurement conditions.

Your conditions are not unusual. Also, and FWIW, your ambient air temp. on your roof is probably 3-8 C. warmer than the air temp. near the ground. In high summer, my panels temps. as measured run about 25-28 C. or so hotter than my roof ambient air temp.

[ButchDeal]

Generally from the same manufacturer the current models are BoW is 5 watts more than BoB and still the BoB costs a little more. So though the BoB will get a bit hotter it almost certainly will cost more.

[SunEagle]

Generally from the same manufacturer the current models are BoW is 5 watts more than BoB and still the BoB costs a little more. So though the BoB will get a bit hotter it almost certainly will cost more.

Butch

Do you think the higher price for the BoB is due to aesthetics which is similar to why the color of some appliances (like white or black) are more expensive then colors that don't sell fast (like blue or red)?

[ButchDeal]

knowing how the lines work. They put together the cells they have and make the modules then test them and sort them.
So the same big pile of cells come in one end and out the other end of the line comes :
280 BoW,
285 BoW,
290 BoW,
275 BoB,
280 BoB
285 BoB

and they get the majority of the modules in the 285 BoW and 280 BoB

[foo1bar]

knowing how the lines work. They put together the cells they have and make the modules then test them and sort them.

I would expect that they probably have sorted the cells beforehand.
(Or the cells were sorted at the supplier.)

That way they can make modules that are made up of cells that are fairly well matched. When all the cells are matched all the cells are going to have similar peak operating points so you maximize performance. The more the cells deviate the more spread out the peak operating points are. So one cell will be at it's peak while the one next to it will be 2% off. If half the cells are performing 2% less than they could you may have just turned what could have been a 286W module into a 283W module which means you sell it as a 280W.

But in general I agree that the manufacturer of the modules is going to assemble cells into the module then test them to determine what they can sell it as.
They'll have some formula that gets them from the test equipment data to what they can sell it as. And that formula I'm sure includes BoB vs. BoW.


Back to the OP's question.
Since the temp coefficient is the same for these panels there shouldn't be a difference in how their performance scales over different temperatures.
If you had 300W BoB and 300W BoW with identical temp coefficients I would expect them to track together very well. The BoB's cells are running at a higher temperature internally - but they're going to be higher performance cells in the module so that they can run at that higher temp and still produce the same watts. The temp coefficient is to *external* temperature. ie. air temp. Not the temp of the cell.

Long and short of it - 300W is 300W. BoB vs. BoW is a aesthetic decision, not a performance/economic one.

[ButchDeal]

yes they sort cells then again sort the modules. I just was starting at the module step.

So a module that performs at 284.9 watts is not tossed it as underperforming the 285 watt line, it is just labeled and sold as a 280 watt module.

[J.P.M.]

The temp coefficient is to *external* temperature. ie. air temp. Not the temp of the cell.

Unless I misunderstand what's being said, and as long as the coefficients of Voltage, current and power are assumed to be linear with respect to temperature, that is an incorrect statement.

Assuming the temp. coefficients of Voltage, current and power are linear with respect to temperature, they are not commonly referenced to any particular temperature.

A panel's STC efficiency is done when the cell/panel temp. is held at a nominal 25 C. That's probably close to the ambient air temp. around the test rig.

The temperature needed to calculate a cell's or a panel's instantaneous efficiency either for STC efficiency determination or in the field is the cell temperature.

The ambient air temp. is a necessary but not sufficient piece of information for finding or estimating the cell temperature at other than STC conditions.

If the external (ambient) air temp. was all that was needed there would be no need for the extensive and varied methods to calculate cell temperature, nor would cell efficiency change when things like wind velocity changed resulting in a change in cell temp. and a change in instantaneous efficiency.

The ambient air temp. around the cell/panel is added to the result of : (POA irradiance- power output - reflection losses)/(thermal loss coeff. per degree of (panel temp-ambient temp)) .

If ambient temp. was all that was necessary, panel instantaneous efficiency would not vary, for example as (wind vector), or as f(POA irradiance), as the POA irradiance affects cell or panel balance temp.

[ButchDeal]

Useing SolarWorld as an example they have a BoB 285 and a BoW 285 both at 17% efficient and all the electrical numbers are the same except the thermal characteristics and the Performance at 800W/M^2

At 800W/M^2 the BoB has a Pmax of 211.1W while the BoW has 213.1 W

[J.P.M.]

Useing SolarWorld as an example they have a BoB 285 and a BoW 285 both at 17% efficient and all the electrical numbers are the same except the thermal characteristics and the Performance at 800W/M^2

At 800W/M^2 the BoB has a Pmax of 211.1W while the BoW has 213.1 W

FWIW, I'd also note that the SolarWorld spec sheets I' looking at show that the BoB 285 has an NOCT of 48 C., while the BoW 285 has a lower NOCT of 46 C. which seems to make some sense. I've also noticed over the years that specs often seem to change slightly for nominally the same panel from time/time for most mfgs., and probably to be expected. Some of this is separating fly specs from pepper.

[foo1bar]

If the external (ambient) air temp. was all that was needed there would be no need for the extensive and varied methods to calculate cell temperature, nor would cell efficiency change when things like wind velocity changed resulting in a change in cell temp. and a change in instantaneous efficiency.

I disagree.

I would expect there to still be many methods to calculate cell temperature and would still expect the wind velocity to change the efficiency.

And there would be one standard method for how to document the number on the label for the module.

In fact that's what we have now.

You think the temperature coefficient calculation methodology doesn't include effects from light hitting the backsheet.
I think it does.
Certainly one of us is mistaken.
But quite possibly it's a small enough effect that the impact can't be seen in the numbers that are published.

In any case - We know at least the PTC values should include the self-heating since they specify an air temp, wind speed, and irradiance.

That might be the most useful thing for the OP to look at.
PTC is a value that's probably fairly reasonable to use to make a guess on what real production differences will be - it's a 68'F day with a small (1m/s) breeze.

[Jafoy]

I don't know the answer but if i may offer some advice.... if this is for a residential system < 10kw, your question essentially amounts to "how many more pennies will it add to the big pile of dollars it makes over the year". Once you've had the system a while you realize the small things weren't worth the worry or sweat or attempt to make the 100% perfect decision.

if you're asking for a different reason, such as purely academic, feel free to ignore.

I've been gradually drifting towards this mindset, so thanks for the nudge. I am looking at probably a ~13kw residential system, and have been trying to accumulate as much knowledge as possible before committing to anything. I wanted to make sure there wasn't a drastic difference here, and this thread seems to confirm that there is not (and in hindsight if it WAS more drastic, there'd already be more discussion about it). The resulting thread has been very interesting regardless, so thanks all for the input and education.

[J.P.M.]

I disagree.

I would expect there to still be many methods to calculate cell temperature and would still expect the wind velocity to change the efficiency.

And there would be one standard method for how to document the number on the label for the module.

In fact that's what we have now.

You think the temperature coefficient calculation methodology doesn't include effects from light hitting the backsheet.
I think it does.
Certainly one of us is mistaken.
But quite possibly it's a small enough effect that the impact can't be seen in the numbers that are published.

In any case - We know at least the PTC values should include the self-heating since they specify an air temp, wind speed, and irradiance.

That might be the most useful thing for the OP to look at.
PTC is a value that's probably fairly reasonable to use to make a guess on what real production differences will be - it's a 68'F day with a small (1m/s) breeze.

OK, let's have a reset.

Starting with: Your statement: " the temp. coefficient is to *external* . ie. air temp. Not the temp of the cell."

I take that to mean, with some confusion on my part as to your exact meaning, that the temperature coefficient (of Voltage ,current or power, or all three ?) is referenced to the air temperature around the cell or panel or array.

Maybe I'm showing off my ignorance here, but any way I interpret that statement, on the face of it, I believe it's incorrect because at best, it's an incorrect statement that the temp. coefficients - that is some properties that describe how Voltage, current or power changes are dependent on the ambient temp. around the cell, panel or array for their values.

My understanding of the temperature coefficients for solar PV devices is that, unless otherwise noted, the temp. coefficients are assumed and are in fact stated as constant on the data sheets for most panels.

I do note however, that the coefff. of Voltage does change in a noticeable and somewhat measureable way (as I've done) when the electronics are working properly and rooting around looking for an MPP as can be seen from a current-Voltage graph showing different irradiance levels. I don't believe that's what where discussing here.

For example, coeff. of power = 0.5 % /deg. C., etc., that is, not f(any temperature). Nor is their rate of change with temp. an f(ambient temperature).

The way I understand it, the effect that cell temperature has on cell/panel/array efficiency is an inverse function of the of cell temperature, with higher cell temperatures meaning reduced cell/panel/array instantaneous efficiency, with that value being constant and the same for all values of cell temperatures.

The (assumed, unless otherwise noted) constant value of those properties makes the instantaneous efficiency of a solar device as f(cell temperature).

Now, second point: There is, in principle, one simple in concept but more complicated in execution way to determine the cell temperature of a solar device, or any object for that matter. It's called an energy balance.

I alluded to a simplistic energy balance in a prior post to this thread. I apologize if, in my efforts at brevity, I left you with the impression I was ignoring, or perhaps unaware of all the energy inputs to a solar device, not only from the solar disk, but also reflected radiation for any and all directions as well as thermal radiation contributions and losses both to and from a solar device.

I'll attempt to clarify.

There may also be some contribution from internal generation, although I can't think of one, and in any case I'd expect it to be small. I assume that's what you refer to as self heating.

All the rest of the heating of a solar device is from external inputs - directly from the sun, circumsolar diffuse solar radiation, sky (not circumsolar) diffuse solar radiation, reflected beam solar radiation from the surroundings, reflected diffuse solar radiation from the surroundings, and thermal radiation from objects that the solar device "sees" that are hotter than the solar device. There are a few others. Anyway, all those inputs become part of the energy input portion and need to be accounted for.

On the outgoing side of the energy balance, there is of course, the electrical energy produced. There is also what some may argue to not be a loss to the solar device at all since it never gets past the outer boundary of a solar device - the reflection, primarily from the panel front glazing, but also and to a lesser degree reflection of sunlight, whose origins are either directly from the sun or reflected from the surroundings and then reflected off panel frames, and shiny/light colored panels backings, etc. Such reflection "losses" (or reduced inputs), even with ARC's, commonly account for 5 % or more of the total energy reaching the solar device over a sunny day.

The other losses are thermal in nature. Those include thermal radiation losses to the surroundings that the solar device "sees" and which are lower in temperature than the solar device. Such heat transfer is often poorly understood and/or ignored, even though thermal radiation losses can account for roughly 20-40 % or so of the thermal losses from a solar device. Losses also include conduction losses through the panel frames to the racking and support structure.

Now, the last big loss component of the loss portion of the energy balance on a solar device is the convection loss.

That phenomenon happens in two ways: When thermal energy on the surface of something - for our purposes a solar panel or array - is transferred to the (usually) cooler ambient air, either via natural convection (in still air, and in a gravity field) where the air movement is induced by a buoyancy difference in the air as it comes in contact with the (assumed) hotter surface, or, the other way, called forced convection, so named because the air motion is provided by something other than gravity. In this case, the driving force being the wind.

The convective heat loss from a solar device is usually a combination of the natural and forced convection film coefficients, with that heat transfer taking place from all surfaces, top, side and back. The film coefficients are usually treated as linear with respect to temperature, but are in fact, highly non linear with respect to wind velocity. there are about a dozen or so commonly used and highly empirical correlations in use convection coeff. calcs. for solar devices, and literally hundreds of such correlations in the literature and the stuff of many master's theses.

So, the deal with the energy balance is, for instantaneous efficiency calcs anyway, and assuming "quasi steady state" operation where things like gains and losses are assumed to be constant for a few minutes or seconds, and so no energy changes (meaning mostly steady temps. here), is that energy gains == energy losses.

If you're still with me, I'll ask you to take my word for it that for our calculations, we can manipulate the energy balance equation and ALL it's various terms as described above, including a reasonable estimate of the POA irradiance by conversion of pyranometric GHI data (and including the ambient air temp. around the solar device that you seem to say is what determines temperature coefficients - again, that temp. being necessary but not sufficient information, and having nothing that I can see determines a power, Voltage of current panel coefficient), and form all that get a cell temperature for any given set of conditions, in the lab, on the testing bench or in the field. I've done it on my roof. Several hundred times. Measuring all the variables is a bit like herding cats, and there are some correlations I've found that are better than others in the sense they fit my measured values better. There are also shortcuts - some other time.

From those cell temperatures, and some assumed value of the published STC efficiency, it's possible to estimate an instantaneous device efficiency. The ambient air temp. is only a piece of the puzzle.

BTW, given all the above data, it's possible to get an estimate of array fouling as f( weather, time between cleanings. Hence the several hundred measurements I wrote of above.

FWIW, while I respect your opinion on PTC values, I believe they mostly do little more than muddy the waters, and also give the usual homeowner a phony impression that PTC outputs are something to take to the bank for reliability, and that an array will always produce the same output, regardless of input - just like STC values. One standard is enough. More can be a cause of unnecessary confusion.

Take what you want of the above. Scrap the rest.

[cebury]

I've been gradually drifting towards this mindset, so thanks for the nudge. I am looking at probably a ~13kw residential system, and have been trying to accumulate as much knowledge as possible before committing to anything. I wanted to make sure there wasn't a drastic difference here, and this thread seems to confirm that there is not (and in hindsight if it WAS more drastic, there'd already be more discussion about it). The resulting thread has been very interesting regardless, so thanks all for the input and education.

In these days with information available to those who seek it and for those witg the ability to quickly learn, we fall into time sinks trying to make the perfect decision; which easily leads to analysis-paralysis. I'm as guilty as the next guy. Once you own the PV system a while you look back and hindsight shows where you were sweating the small stuff. Good luck to you..