Mystery solar panels appearing on existing street light poles in Los Angeles

[SunEagle]

If the area reflecting is large in relation to the area receiving the power, it doesn't matter how far it is. It could be infinitely far away, and as long as it was also infinitely large, you'd get the same amount of power as an infinitely large reflecting plane that was ten meters away.

So if it's over something that's large, like a freeway (and the freeway is doing most of the reflecting) it shouldn't matter. But if it's small, like a section of sidewalk, then it would be important to have it the 'right' distance.

While I am still learning I might disagree based on my understanding of how light energy is reflected.

At certain distances the amount of lumens or light energy drops off and dissipates. So I presume that a bifacial panel would not get enough of those lumens to make the electrons in the cell material to jump out of their "level" to the surface to be collected.

If the panel is too close then there will not be enough sunlight that gets to the surface below to be reflected back to the panel. But at some distance (maybe that 0.8 meters) there is enough solar energy or lumens that gets to the surface and is reflected back to energize the electrons to be motivated out of the semi-conductor cell material and become dc electricity.

Sure light will continue to travel over millions of miles but at what energy level? A solar panel will not generate power if the energy is not high enough to motivate the electrons in the cell material.

[jflorey2]

At certain distances the amount of lumens or light energy drops off and dissipates.

That's definitely true of a point source; the light falls off at a rate of 1/R^2.

But for an infinite plane it's always the same. Think about looking at a perfectly white, featureless plane that is reflecting light. How would you be able to tell if you were a meter from it or a thousand meters? It looks the same. No matter where you are, you are getting light from the same directions in the same amount.

Of course there's no such thing as an infinite plane in the real world. But if the reflecting surface is large compared to the panel then it functions the same way. A good example of this would be snow on a field. If the field is large, then the power will not fall off noticeably as you increase the elevation of the panels.

(As you mention, as you lower the panel, eventually it blocks its own light, and that reduces power.)

That would seem to suggest that the higher the panel the better - and that's largely true. Of course, rack costs quickly go through the roof (pun intended.)

Sure light will continue to travel over millions of miles but at what energy level? A solar panel will not generate power if the energy is not high enough to motivate the electrons in the cell material.

Same energy level it starts with. The only thing that can reduce the energy of a photon is relativity - the source moving away rapidly, or gravity slowing it down. Then you get redshift and that reduces the energy of the photon. But that doesn't apply to anything on the scale we are dealing with.

The other things that can reduce overall energy of the light are diffusion (i.e. 1/R^2 from a point or small source) or blockage (i.e dust or scattering.) But at the distances we're talking about, dust and scattering isn't going to reduce power significantly.

[SunEagle]

That's definitely true of a point source; the light falls off at a rate of 1/R^2.

But for an infinite plane it's always the same. Think about looking at a perfectly white, featureless plane that is reflecting light. How would you be able to tell if you were a meter from it or a thousand meters? It looks the same. No matter where you are, you are getting light from the same directions in the same amount.

Of course there's no such thing as an infinite plane in the real world. But if the reflecting surface is large compared to the panel then it functions the same way. A good example of this would be snow on a field. If the field is large, then the power will not fall off noticeably as you increase the elevation of the panels.

(As you mention, as you lower the panel, eventually it blocks its own light, and that reduces power.)

That would seem to suggest that the higher the panel the better - and that's largely true. Of course, rack costs quickly go through the roof (pun intended.)

Same energy level it starts with. The only thing that can reduce the energy of a photon is relativity - the source moving away rapidly, or gravity slowing it down. Then you get redshift and that reduces the energy of the photon. But that doesn't apply to anything on the scale we are dealing with.

The other things that can reduce overall energy of the light are diffusion (i.e. 1/R^2 from a point or small source) or blockage (i.e dust or scattering.) But at the distances we're talking about, dust and scattering isn't going to reduce power significantly.

True that distance will not reduce the energy of the photon but it will increase the chance of something like dust or other solid object getting in between to block the photon. At some point the increase of distance also increase the % chance of blockage and reduces the chances or % output. Theoretically speaking.

[Sunking]

Around here lightpole-mounted solar panels are commonly used to power traffic cameras, weather monitors and wireless signal relays.

Done everywhere like that where no power is available, but not where power is available.

Just another worthless post by Dan.

[jflorey2]

True that distance will not reduce the energy of the photon but it will increase the chance of something like dust or other solid object getting in between to block the photon. At some point the increase of distance also increase the % chance of blockage and reduces the chances or % output. Theoretically speaking.

I'll agree with you there. If the array is so high that clouds or dust storms can get between you and the ground, the array might be too high to get the benefit of reflections!

[DanKegel]

True that distance will not reduce the energy of the photon but it will increase the chance of something like dust or other solid object getting in between to block the photon. At some point the increase of distance also increase the % chance of blockage and reduces the chances or % output. Theoretically speaking.

SunEagle, are things cleared up now? No lenses, no focusing, pretty much all that matters is whether the area under the panel is well-illuminated and has good diffuse reflectivity. So ground mounted panels over a snowy field would get enough light on the bottom to make a difference, as long as they're up high enough / spaced enough for the sun to get under the darn things and light up the snow. Once you get that, it's easy to recognize what's a possibly good application of bifacial panels, and what's not.

[SunEagle]

SunEagle, are things cleared up now? No lenses, no focusing, pretty much all that matters is whether the area under the panel is well-illuminated and has good diffuse reflectivity. So ground mounted panels over a snowy field would get enough light on the bottom to make a difference, as long as they're up high enough / spaced enough for the sun to get under the darn things and light up the snow. Once you get that, it's easy to recognize what's a possibly good application of bifacial panels, and what's not.

Yep pretty much. I guess the last issue would be that if the panel is mounted very high there is always the chance of shadows creeping in to reduce the amount of reflected light getting back up to the panel. But otherwise the technology is interesting.

[J.P.M.]

If the area reflecting is large in relation to the area receiving the power, it doesn't matter how far it is. It could be infinitely far away, and as long as it was also infinitely large, you'd get the same amount of power as an infinitely large reflecting plane that was ten meters away.

So if it's over something that's large, like a freeway (and the freeway is doing most of the reflecting) it shouldn't matter. But if it's small, like a section of sidewalk, then it would be important to have it the 'right' distance.

What you refer to is handled by something called radiation shape factors or exchange factors. See: Rosenow & Hartnett, "Handbook of Heat Transfer", or Siegal & Howell : "Thermal Radiation Heat Transfer" for details. Reflected radiation is not the same as thermal radiation, but the two share many properties with respect to energy transfer from one surface to another, particularly with respect to the geometry involved and some of the controlling surface characteristics.

[J.P.M.]

That's definitely true of a point source; the light falls off at a rate of 1/R^2.

But for an infinite plane it's always the same. Think about looking at a perfectly white, featureless plane that is reflecting light. How would you be able to tell if you were a meter from it or a thousand meters? It looks the same. No matter where you are, you are getting light from the same directions in the same amount.

Of course there's no such thing as an infinite plane in the real world. But if the reflecting surface is large compared to the panel then it functions the same way. A good example of this would be snow on a field. If the field is large, then the power will not fall off noticeably as you increase the elevation of the panels.

(As you mention, as you lower the panel, eventually it blocks its own light, and that reduces power.)

That would seem to suggest that the higher the panel the better - and that's largely true. Of course, rack costs quickly go through the roof (pun intended.)

Same energy level it starts with. The only thing that can reduce the energy of a photon is relativity - the source moving away rapidly, or gravity slowing it down. Then you get redshift and that reduces the energy of the photon. But that doesn't apply to anything on the scale we are dealing with.

The other things that can reduce overall energy of the light are diffusion (i.e. 1/R^2 from a point or small source) or blockage (i.e dust or scattering.) But at the distances we're talking about, dust and scattering isn't going to reduce power significantly.

Assuming for a moment, a flat non specular reflecting plane, and ignoring different reflectance and absorbance characteristics between the sky, the plane and the receiver as well, for an infinite (flat) plane and a finite receiver, there will be a view factor between the receiving surface and the plane and the sky that's f(relative orientations), with somewhat directional (and probably mostly forward scattered) diffuse radiation that will influence the energy transfer to (and from) the receiving surface that's f(relative surface orientations) and also, to a smaller extent, the distance of the receiver from the plane. For an "upward" facing (flat) receiver oriented at some angle to the horizontal, the view factor to the sky will be greater than the view factor to the (flat and horizontal) ground. As the receiver get "closer" to the ground, the view factor between the ground and the (upward facing) receiver will increase. For the (downward facing) backside of the receiver, the view factor between the ground and receiver will increase slightly as the distance to ground decreases with self shading by the receiver becoming somewhat more important, but that self shading too is f(solar zenith angle) , and may not account for as much as thought over a full day. For example, at, say, 1600 hrs. solar time, the shadow the receiver casts will probably, again, depending on surface orientation, become less of a factor because the view factor will be less.

As for attenuation of sunlight, as it passes through a medium (say the earth's atmosphere) it will transfer energy to the atmosphere and also dissipate energy that's manifested as diffuse sunlight. Shorter wavelengths (higher energy levels) are diffused more - hence blue skies. A "clear" atmosphere will diffuse (scatter) or absorb about 25% of the incoming radiant energy down to sea level. Solar energy at the top of the earth's atmosphere is greater than at sea level (1,367 W/m^2 vs. ~ 1,000 W/m^2 Normal irradiance levels). Dust, mixed gasses, H2O vapor as well as other things can have a significant and measureable effect of irradiance levels, and of course clouds will absorb, reflect and diffuse more energy and reduce the ground level irradiance even more.

The atmosphere also slows the speed of the sunlight as it traverses the atmosphere - hence refraction. See Snell's Law and Fresnel's Law.

Albedo is an interesting subject, but gets complicated in a hurry as the above very simple examples may point out. That's why generalizations are often used and the subject is not generally well understood, and why it's easy for the uneducated to think the back side of bifacial panels see an appreciable amount of solar radiation, and that radiation is in an easily useable form. It is usually neither. Also, with respect to solar energy applications, relativistic effects have little influence and can be ignored for all practical purposes.

See Duffie & Beckman for a decent treatment of solar energy and how it interacts with the atmosphere.

[DanKegel]

A possible clue:

I see in http://www.lamayor.org/sites/g/files...The%20pLAn.pdf
on page 71 two things that might be related:

Ensure Los Angeles’s preparedness for all natural disasters
- Develop citywide WiFi for emergency use powered by backup solar power
- Increase electricity-based preparedness (e.g., electrical grid and
grid upgrades, micro-grids, grid-tied backup solar and streetlight
solar-to-grid)

I pinged the city guy again to see if it's related to one of those.
No idea if he'll reply, though.
- Dan