Low angle solar performance

[HiTech]

Has my one seen data on performance by specific panel or general PV type for different incidence angles of solar radiation? All I can find so far is generic info.

Jim

[HiTech]

vermilye wrote:
I wonder what the effect of adding low transmission loss diffusion to replace the glass cover would have? Since the solar industry generally tilts the panels for permanent installs, I doubt it would be a commercially available product, but for flat mounting on the roofs of a RV would it improve low angle performance?

They do play with a lot of coatings and etching for special situations. I cannot find standards and studies for anything but plate glass. BFL what range of power vs mid day do you see with panels aimed roughly at the sun at 5 degrees before sunset?

Jim

[BFL13]

Understand the low angle and the glass. The low sun in the "haze" is a separate thing. Play with a portable panel when the sun is low and aim it right on, you still can't get full amps like you can with it aimed when the sun is higher.

To get just the glass effect you would have to try low angle when the sun is up higher

[vermilye]

I wonder what the effect of adding low transmission loss diffusion to replace the glass cover would have? Since the solar industry generally tilts the panels for permanent installs, I doubt it would be a commercially available product, but for flat mounting on the roofs of a RV would it improve low angle performance?

[HiTech]

BFL13 wrote:
Don't forget that low angle sun is when it is in atmospherics up to say 15 degrees altitude, so even if your panel is great at low angle stuff, it doesn't matter because the light is going through haze etc.

That is why they usually define the solar day as from mid-morning to mid afternoon.

Looks like it is not the haze BFL. It's the losses from having glass in front of the photovoltaic cells. The first and last 20 degrees of daylight are drastically attenuated on flat panels by having a sheet of glass in front of the solar cells. Even just 5 degrees above sunset there are still 270 watts per square meter to catch compared to a nominal of 1000.

Jim

[HiTech]

"Test results show nearly identical relative light transmission plots for all five test modules with glass superstrate regardless of the type of PV cell technology. (This project tested monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide). This indicates that the reflective losses are governed almost exclusively by the air-glass interface of the PV modules.

The relative light transmission plots obtained using the IEC 61853-2 model were in good agreement with the plots obtained using the theoretical air-glass interface models and the empirical model developed by Sandia National Laboratories for the glass superstrate PV modules.

The standard states that “for the flat glass superstrate modules, the AOI test does not need to be performed, rather, the data of a flat glass air interface can be used.”The results obtained in the current study validate this statement."

[HiTech]

Yes. Plus or minus 30 or even 45 degrees is not the huge hit that being flat is.

Light refraction calculator. Air is 1.0 and smooth glass is 1.5. Pop in any angle you like.

Light refraction and reflection calculator

Solar angle of incidence effect where glass refraction dominates results for Glass-covered photovoltaics.

Jim

[BFL13]

Don't forget that low angle sun is when it is in atmospherics up to say 15 degrees altitude, so even if your panel is great at low angle stuff, it doesn't matter because the light is going through haze etc.

That is why they usually define the solar day as from mid-morning to mid afternoon.

You can do a bit more if you can squeeze out every little bit, but it doesn't come to many AHs out of your whole day.

Another example is with my "tracking" contraptions. It isn't worth the bother of tracking all day. Three moves a day gets almost as many AH due to the panels being able to get fairly good input off angle between moves.

[HiTech]

Already baked in. Some play with coating styles and other mitigation, but mostly it seems if they are worried about the loss they just use tracking, which keeps it to 4%. It's just interesting to take apart the studies into the pieces that make them up. Mostly early and late day production is written off unless you have a solar tracking system.

Jim

[full_mosey]

HiTech wrote:

I wonder how different they are for panels not under glass?

-Jim

If you STFW for macslab you will find that there is an enormous academic study behind that simple implementation.

IMHO, if glass reflection or thickness had any significant difference, I can't believe these folks weren't aware of that and baked it in.

I do have a program that calculates the exact angle for any date and any N Latitude at solar noon. Plus, I calculate the length of the tilt bars. I could cut the bars beforehand. I don't know enough math to do time of day. I tried tilting but it is totally incompatible with travelling. Ok for sitting still.

If you get the tilt within 15deg you will be ok. You can do that with a toy suction dart at any time of day.

Some issues just get lost in the rounding/noise.

HTH;
John

[HiTech]

Very handy.

If we merge the the two together, there are some interesting results. The calculator suggests that the overall rule of thumb at 85 degrees is that there is a 91% loss of energy vs 0 degrees. The Wikipedia data on reflection says that of that loss, 63% of it (91% * 70%) is due to reflection of sunlight back off the glass at the extreme angle, and only 27% of that power loss is due the less light hitting the photovoltaic itself at a low angle.

In reality there are other factors baked into these handy numbers like spectrum shift in the sunlight near the horizon, and the air mass coefficient going up considerably (more air for the light to go through)...etc.

I wonder how different they are for panels not under glass?

-Jim

[full_mosey]

Solar tilt calculator

[Salvo]

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

[HiTech]

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

[bill_h]

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.