Low angle solar performance

I find that hard to believe. Do you have a source for that?

Panel efficiency is around 14%. This test reduced reflection by 75% while increasing efficiency by only 3%. In the overall picture, reflection is a minor issue.

HiTech wrote:
Well in reading solar studies and standards, it seems that as the sun angle moves away from perpendicular, the dominant loss of efficiency in typical solar cells is reflection from the top glass surface rather than the technology used.
Jim

Exactly. But the numbers above are pure light reflection losses *before* applying the cosine power reduction for angle to solar production. These losses are before any light hits the photovoltaic cells (when mounted under smooth glass as is typical).

Many have probably noticed this effect looking through a window from the side with much more light, say out at night, or looking into a house on a sunny day. Looking straight it is pretty easy to see in. Looking nearly parallel down the length of the glass, you see the reflection instead.

Jim

If azimuth is directly at the sun, the output of a panel changes with the cosine of the incident angle. If the sun is hitting the panel at an angle of 0 deg, the cosine of 0 is 1. That means you multiply the rating of the panel by 1 to get its output.

If the angle is 45 deg, the cosine of 45 is .7, so the output is 70%.

And so forth. A panel puts out its rated power under ideal conditions, but his gives you an idea of the importance of tilt. Tilting doubles my output in December.

Well in reading solar studies and standards, it seems that as the sun angle moves away from perpendicular, the dominant loss of efficiency in typical solar cells is reflection from the top glass surface rather than the technology used.

Head on, as in the standard testing method used to rate the watts of a panel, the reflection loss is only 4%. But as the sun gets lower in the sky this becomes a larger factor than loss of light intensity, due to the high angle of incidence of the light against the glass.

At 70 degrees you are already eliminating 30% of the light hitting the solar panel, due to reflection from the surface glass. This is before you compute the lower light levels actually hitting the photovoltaic cells (you can look that up on the power intensity tables, after you subtract out the glass reflection losses off the top).

At 80 degrees you are reflecting over half the light from the glass. Part of why you see the tail drop off so fast in flat mount panels at the beginning and end of the day.

At 85 degrees (where many studies ended) 73 percent of the light is lost before it even gets through the glass.

I'm still looking for an idea of what the Tefzel coating on the front of the UniSolars refractive index is, to compute similar numbers for that technology.

If I had a sheet of glass large enough, it would be interesting to see how much additional low angle current would drop from reflection losses by mounting a unisolar and standard solar panel under it. Not the same as native, but it would show the increased loss which might be instructive to know.
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Wikipedia article on watts per m^2 at various angles of sunlight

Solar intensity vs zenith angle and airmass coefficient AM
AM range due to pollution[11] formula (I.1) ASTM G-173[10]
degree W/m2 W/m2 W/m2
- 0 1367 1353 1347.9[15]
0° 1 840 .. 1130 = 990 ± 15% 1040
23° 1.09 800 .. 1110 = 960 ± 16% 1020
30° 1.15 780 .. 1100 = 940 ± 17% 1010
45° 1.41 710 .. 1060 = 880 ± 20% 950
48.2° 1.5 680 .. 1050 = 870 ± 21% 930 1000.4[17]
60° 2 560 .. 970 = 770 ± 27% 840
70° 2.9 430 .. 880 = 650 ± 34% 710
75° 3.8 330 .. 800 = 560 ± 41% 620
80° 5.6 200 .. 660 = 430 ± 53% 470
85° 10 85 .. 480 = 280 ± 70% 270
90° 38 20

This illustrates that significant power is available at only a few degrees above the horizon.
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Jim