Black vs Silver panels?

[SunEagle]

Many years ago, I measured light transmission through silicon wafers. It was quite substantial at many wavelengths, in many cases, 90%. PV cells are roughly 20% efficient. Some of the 80% lost must be going right through and either reflecting back or being absorbed. If it is being absorbed by the back sheet, then it is extra heat. The extra heat may not show up in STC measures. But if it is reflected back, it may have an additional opportunity to create electrical energy (standing waves, etc.) and then it would show up in STC.

That is why bimodal solar panels can generate more power from the reflected light from below. Unfortunately the cost of those panels doesn't seem to off set the efficiency increase.

[bcroe]

Many years ago, I measured light transmission through silicon wafers. It was quite substantial at many wavelengths, in many cases, 90%. PV cells are roughly 20% efficient. Some of the 80% lost must be going right through and either reflecting back or being absorbed. If it is being absorbed by the back sheet, then it is extra heat. The extra heat may not show up in STC measures. But if it is reflected back, it may have an additional opportunity to create electrical energy (standing waves, etc.) and then it would show up in STC.

If there is so much light shining through, why does not someone put a silver mirror coating
on the back so the light goes back again, raising panel efficiency approaching double?
Bruce Roe

[J.P.M.]

Many years ago, I measured light transmission through silicon wafers. It was quite substantial at many wavelengths, in many cases, 90%. PV cells are roughly 20% efficient. Some of the 80% lost must be going right through and either reflecting back or being absorbed. If it is being absorbed by the back sheet, then it is extra heat. The extra heat may not show up in STC measures. But if it is reflected back, it may have an additional opportunity to create electrical energy (standing waves, etc.) and then it would show up in STC.

Energy balance: What comes out = what goes in less what stays in. Assuming quasi steady state operation, if 1,000 W of energy hit a panel and 200 of those W turn into electricity, the remaining 800 W will be accounted for either as reflected from the panel (mostly from the glazing) or turned into heat and carried off by one of the 3 modes of heat transfer.

From lots of measurements and heat transfer correlations in the body of heeat transfer knowledge, I'm pretty sure not too much of what hits my array in the way of sunlight passes through the panels.

In my "you could just" younger days, I conjured up the idea that since only solar energy with wavelengths below 1.15 * EE-6 m (about 75 % or so of the total energy emitted by the sun is below that wavelength) is useful for silicon to convert sunlight to electricity, a surface coating that acted as a bandpass filter or as a selective reflector not unlike solar thermal selective absorber surface coatings, but sort of in reverse, might be able to reflect energy not useful to the solar conversion process before it got turned into heat and in so doing enable lower cell temps for the same POA irradiance.

[nwdiver]

Overall, I'm having trouble understanding how the color of a backsurface that sees VERY close to zero irradiance can have an effect on cell temp.

....... I'm just repeating what the chief technology officer of Mission Solar told me. Same cells, different STC, only difference was the back sheet...

[J.P.M.]

... I'm just repeating what the chief technology officer of Mission Solar told me. Same cells, different STC, only difference was the back sheet...

I understand what you write.

Except for why the published NOCT for the all back panels is slightly lower for the 320 W all black vs. the 315 W white back on the Mission Solar data sheets, which seems backwards to me, I'd put more faith in the differences discernible in the data sheets. I do note small differences in the dimensions and operating voltages and currents one to the other as is common.

[SunEagle]

If there is so much light shining through, why does not someone put a silver mirror coating
on the back so the light goes back again, raising panel efficiency approaching double?
Bruce Roe

Actually there are some panels that have a second set of cells below the first. What ever light goes through the first is harvested by the second. Again the cost of these panels are hard to justify when the additional harvesting is not much but does increase the overall efficiency of a panel.

[J.P.M.]

If there is so much light shining through, why does not someone put a silver mirror coating
on the back so the light goes back again, raising panel efficiency approaching double?
Bruce Roe

I believe what Bob-n writes, but there's not as much transmission through a PV cell as a casual read of Bob-n's post might lead a reader to infer.
Note the words "silicon wafers" and "at many wavelengths". Wafers may not be silicon PV cells, and if they are, what thickness and type ?
Also, in which wavelength (bands) did the silicon wafers transmit ? PV cells can't use use wavelengths longer than 1.15 microns. Is part of the reason such wavelengths are unusable due to transparency in some wavelengths and that may have something to do with what Bob-n reports ?
Bob, comment ?

If A photon wasn't snagged on the first pass through a cell due to the cell's transmission characteristics at that photon's frequency, why would it get picked up do so on a second pass ? And, how would the reflected photons find some n type silicon on the front side to free up an electron ?
Besides, seems to me the cell would need to be physically altered to be, in effect, two cells facing in opposite directions with a common p type layer.

I've no data on PV cell transmission or its wavelength response characteristics to support my doubts, but I kind of wonder if much of anything substantial would be gained by back reflection surfaces which, I'd rather suspect would also tend to increase the cell's operating temps.

Respectfully,

[bob-n]

J.P.M. Yes, I picked my words carefully because I don't know many of the important details, just enough to be dangerous.

My testing was done many years ago with a laser. I can't recall the laser wavelength, but we wore green goggles to block it, so it wasn't green. I don't even recall if the laser was visible. We never wanted to see the beam.

I do recall that the thickness of the wafer affected absorbtion, and we attributed it to standing waves in the silicon. It is possible that PV cell makers have engineered cell thickness for optimum efficiency. They certainly engineered the layers in the silicon sandwich (nitride, various oxides, metals, etc.) for efficiency as well as other unimportant things like reliability.

[GoingElectric]

Important discussion topic.

I had a proposal and was shown the Hanwa Q.PEAK DUO ML-G10 (non BLK) is listing about 5w more output compared the BLK version Q.PEAK DUO BLK ML-G10.

It is simple physics that the cosmetics to hide the circuits also decrease the power output.
This like putting a filter on a camera, you get better color range, but need to compensate in exposure time and/or lower focal length.

The funny thing is I am finding the mfg do offer the non-blacks, but they are harder to find in their online catalog.
PDF files more likely list all versions, such as this Q-cell catalog,

https://media.q-cells.com/v/eyEEJh2F/