Reflected light from sand to increase PV output?

zeuspaul · September 2010

If I tilt mount a pair of 135 watt solar panels on the ground and place white sand in front of them will the output go up? How much?

Would the increased light damage the PV panel?

Thanks

Zeuspaul

Cariboocoot · September 2010

Re: Reflected light from sand to increase PV output?

zeuspaul wrote: »

If I tilt mount a pair of 135 watt solar panels on the ground and place white sand in front of them will the output go up? How much?

Would the increased light damage the PV panel?

Thanks

Zeuspaul

Short answers:
Yes; the more light you can deliver to the panel the more it will produce (up to a limit).
Probably not enough to off-set losses from what would likely be high ambient temperatures in such an environment.
No; can't imagine there could ever be enough light on Earth to damage a PV. It'd damage a lot else first! Remember: it's reflected light; it can never be brighter than the source.

This works better with snow, which reflects quite nicely on a bright Winter's day - when it's also cold enough to keep panel temperatures down in the high-efficiency range.

BB. · September 2010

Re: Reflected light from sand to increase PV output?

If you wish to experiment---Take any solar cell/panel and connect it to a amp meter (digital multi-meter, etc.)... The output of a solar cell is basically proportional to the amount of light energy hitting the face.

Point the panel at the sun with dark material in front--measure the current. Do the same thing with sand in front and measure the current again. That will tell you how much, if any, increase sun power you will collect.

Note, depending on the "down stream hardware", the temperature of the panel may also have an effect.

As the panel temperature rises, the Vmp falls--Remembering that Power=Volts*Amps, falling voltage means less power (maybe 20% less with really hot panels). A PWM charge controller does not use the "extra" Vmp--so there is not really any effect. MPPT (Maximum Power Point Tracking) charge controllers (and Grid Tied Inverters) do very much work with Volts*Amps from the solar panels, so a panel near the ground with reflected sunlight may run hotter than its counter part mounted on a pole over grass--Which setup will produce more power--your guess is as good as mine.

The current for a solar cell is not affected very much by temperature (it does increase a tiny bit) so it will "ignore" any temperature effects with respect to the panels.

You can get a simple kit, pre-built, or even follow the instructions on building your own Solar Irradiance Meter from here

-Bill

zeuspaul · September 2010

Re: Reflected light from sand to increase PV output?

Thanks for the responses.

I had done a little research before posting. I found cautions about mirrors due to too much light. I also found references to benefits from snow reflection.

What I don't understand is if there is a max cut-off. If you can double the light with a mirror do you double the output?

The Kyocera 135 watt panel in question indicates about 500 watts per square meter yields about 4.3 amps. 1000 watts per square meter is about 8.3 amps or about double with twice the irradiance. That is the max on the chart and I assume it is for full sunshine.

If I double the max with a mirror I assume there may be problems and that I would not get double the output.

I found some reflective numbers for snow and sand. My recollection is snow was at least double that of sand.

I guess my question is...If the panel is in full sunshine and I can increase the light by 10 percent with sand reflection will the output go up by about 10 percent? Would the % effect be different if the panel were in reduced morning light vs full sunshine?

In an attempt to limit the thread to one issue I did not ask the other half of the question. However Cariboocoot referenced heat. I have considered mounting copper water pipe on the back as a heat sink.

The question seems to have two components. Increased output from increased light and heat for sand. And increased output for increased light in cool conditions.

For snow. If the light is increased 20 percent over a full sun condition is the output increased 20 percent?

And for sand. If the light is increased 10 percent and the panel is cooled with water pipe backing is the output increased 10 percent?

Is 20 percent for snow and 10 percent for sand a reasonable estimate?

Thanks

Zeuspaul

Cariboocoot · September 2010

Re: Reflected light from sand to increase PV output?

That's a very interesting question. Perhaps Solar Guppy could answer it?

For instance, with the snow scenario; how much improvement in output is due to the low temps and how much to the increased lumens?

In either case, there must be an upper limit to how much the P-N junctions can be driven. Certainly heat, whether ambient or from the increased output, would eventually slow the pathways so that there was no further gain. There would probably be a logarithm function; lessening % output as % light exposure increases.

The mirrors would concentrate heat as well as light, and to some extent so would the sand or snow. Different materials reflect different amounts of the spectrum, including infrared. So it would be a matter of how much does a given material reflect the most useful (for PV) segment of the visible spectrum and not the infrared?

Definitely not my area of expertise (as anyone can tell from reading this). I just make 'em work.

zeuspaul · September 2010

Re: Reflected light from sand to increase PV output?

Some numbers

From wikipedia

The albedo of an object is a measure of how strongly it reflects light from light sources such as the Sun.

and some of the albedo the numbers

worn asphalt 0.12
conifer forest 0.09-.15
bare soil 0.17
green grass 0.25
New concrete 0.55

Desert sand 0.40
Fresh snow 0.8 - 0.9

Bottom line for me is with a ground mount system do I leave some horizontal space beyond what is necessary to avoid shadows from in-front panels?

Zeuspaul

BB. · September 2010

Re: Reflected light from sand to increase PV output?

There have been people who have done concentrating solar arrays. If I recall correctly, they do ten times sun and produce around five times the output.

All this is done with lots of heat sinks on the back of the panels.

So far,I have not seen any panel companies survive to production (not that I follow that closely).

We also have talked about water cooling solar pv panels too--the costs both in power and hardware costs just does not seem to make it worth while.

-Bill

niel · September 2010

Re: Reflected light from sand to increase PV output?

in general it is ok to have some reflections hit the pvs, but you just don't want the pv to heat up too much. mirrors are not recommended because they work too well at increasing the light hitting the pv. remember that these pvs were not rated for purposely increased concentrated light and they will degrade or age the pv some. unfortunately one will have difficulty limiting the infrared portion while increasing the other spectral areas. btw, infrared light is partly converted to electricity too in a pv. look at it this way, if nature can do it then it is fine. this means snow or chopped water reflections that occur in nature will yield an acceptable increase to the pvs. the little bit of increase from the white sand in my opinion is fine. sand grains are not flat like a mirror and, although it reflects light, it reflects it in a scatter pattern meaning the reflectiveness is not all going to one place.
i am adding that a wall painted white will also be acceptable as the light is reflected more randomly. if that same wall were painted with a highly reflective metallic paint i would say you are pushing it a bit as it is starting to be more directive in its reflection, but may still be chopped up enough depending on the surface painted.

Cariboocoot · September 2010

Re: Reflected light from sand to increase PV output?

One of the problems with cooling panels is that they are basically made of glass, which doesn't conduct heat well (as opposed to copper or aluminium). Putting metallic fins on the back brings up the trouble of poor heat conduction between dissimilar materials (use lots of heat sink grease?) and the different expansion coefficients.

I wonder if manufacturers have ever toyed with adding glass ribs to the underside? It would add strength and a least a bit of radiance, but probably not enough improvement to justify the cost. They've spent a lot of research on making the front glass ideal for transmitting visible light and standing up to the elements, but what about adjusting the rear piece for purposes of losing heat?

To that end we have seen that most mechanical contrivances to remove heat from panels end up using more power than is gained from the cooling effect (no surprise there; no such thing as perpetual motion) so natural convection cooling would seem the only viable solution left. Simply difficult to accomplish. Probably end up with some complicated evacuated tube assembly molded in to the rear panel ... and then run in to trouble with a temp difference between the front and back side.

Don't mind me: just thinking out loud.

mike95490 · September 2010

Re: Reflected light from sand to increase PV output?

I've noticed an effect of overcast days, of the sky is bright hazey, my noon peak production goes down, but my overall daily production goes UP. I get a fairly flat power output over the whole day, not the typical bell curve of a clear day. I've often wondered about a frosted glass dome over the PV panel, like was done over some solar water heaters ????

Dave Angelini · September 2010

Re: Reflected light from sand to increase PV output?

zeuspaul wrote: »

If I tilt mount a pair of 135 watt solar panels on the ground and place white sand in front of them will the output go up? How much?

Would the increased light damage the PV panel?

Thanks

Zeuspaul

With the Sanyo HIT "doubles" you can get a 30% gain. I have customer with them and in snow he had peaks and longer time periods on the array that were very close to that. These panels are designed for it and they are very impressive. They are also incredible in high temperature with very little negative voltage coefficient. My last on these panels is the WOW factor! If there was ever a solar panel that was like a very beautiful woman and not as expensive as the ones in space this is it!

solar_dave · September 2010

Re: Reflected light from sand to increase PV output?

Dave Sparks wrote: »

If there was ever a solar panel that was like a very beautiful woman and not as expensive as the ones in space this is it!

Dave you need to get out more, and all beautiful women are expensive and high maintenance! (except for my wife of course)

I read some stuff on those panels a while back for a covered walk way, those are the ones with a clear/no back correct? They sound great and just what the walk way needed, some shading without a full shade.

zeuspaul · September 2010

Re: Reflected light from sand to increase PV output?

If you wish to experiment---Take any solar cell/panel and connect it to a amp meter (digital multi-meter, etc.)... The output of a solar cell is basically proportional to the amount of light energy hitting the face.

The Kyocera 135 watt panel is rated 7.63A maximum power current and 8.37A short circuit current. My HF multimeter is rated 10 amps DC.

Do I just put the meter in series between the two leads directly from the panel? Is it ok to short circuit the panel? Can I leave it short circuited for ten or fifteen minutes?

I tried putting the meter in series with a 48 watt fan rated 0.12 amps. I then placed and removed a white sheet on the ground in front of the panel. No discernable difference in the current reading.

Thanks

Zeuspaul

Cariboocoot · September 2010

Re: Reflected light from sand to increase PV output?

Don't forget about angles.
The angle of reflection is equal to the angle of incidence.
Up here in the frozen North, the Winter angle of panels is extreme (72 degrees for me) which means reflected light off snow is more likely to actually strike the panel. In Summer, the panel angle is low (29 degrees) so any reflecting surface would have to be floating in the sky to aim additional sunlight on the panel.

Maybe they should make panels with a slight parabolic curve to them. :roll:

Dave Angelini · September 2010

Re: Reflected light from sand to increase PV output?

solar_dave wrote: »

Dave you need to get out more, and all beautiful women are expensive and high maintenance! (except for my wife of course)

I read some stuff on those panels a while back for a covered walk way, those are the ones with a clear/no back correct? They sound great and just what the walk way needed, some shading without a full shade.

Solar Dave,
Are you talking about beautiful women in space or beautiful solar panels in space?

solar_dave · September 2010

Re: Reflected light from sand to increase PV output?

Dave Sparks wrote: »

Solar Dave,
Are you talking about beautiful women in space or beautiful solar panels in space?

hehe, both are high cost!

zeuspaul · September 2010

Re: Reflected light from sand to increase PV output?

I finally got some meaningful results.

The test panel is a 135 watt Kyocera KD135GX-LPU rated 7.65A Maximum Power Current and 8.37A Short Circuit Current at 1000 Watts per sq meter.

At 3:30 PM I figured the sun was low enough to provide significant reflected light. I wired an ammeter directly between the two outputs on the solar panel using wire nuts to connect the probes to the wire on the panel. I laid the 5 ft long panel on its side and pointed it directly at the sun and tilted it back.

The ammeter reading was 8.2 amps.

Then I placed an 8 ft x 8 ft white sheet in the foreground.

The ammeter reading jumped to 8.63 amps.

Then I placed white styrofoam packing sheets on both sides of the white sheet in the foreground. Direct reflections from these foreground extensions would not intersect the panel. However the amps went up again most likely from scattered light reflections.

The ammeter reading went up to 8.7 amps.

I repeated several times always with the same results.

My conclusion was about a six percent increase in amps (and watts?) with a white foreground under my testing conditions.

One might assume the effect is larger with lower sun angles. Maybe I'll try a few more tests now that I have a working testing system.

It's about 95 F degrees which is about 15 degrees hotter than recent past. My neighbors harvest has gone from about 45 KWhr per day to about 40 KWhr per day presumably from the heat.

The panel was hot to the touch during the testing. Our skies are consistantly blue.

Zeuspaul

Cariboocoot · September 2010

Re: Reflected light from sand to increase PV output?

This is good results. Do you think it's worth the effort?

Surely some of it is along the lines of Mike's revelation: defused light may be less intense than direct sun, but can deliver a more consistent exposure over the course of a day.

zeuspaul · September 2010

Re: Reflected light from sand to increase PV output?

Do you think it's worth the effort?

If you can save five percent with inverter selection and another five percent with the way you mount your panels it starts to add up. If you pay attention to detail you may end up saving 10 percent or more vs losing 10 percent or more if you ignore the little things.

It may not be effort as much as it is design. I was conisidering mounting my panels long ways in several rows, one in front of the other with the spacing dependent on the shadow cast by the infront rows.

Now because of what I have learned about reflected light I am considering one row with the panels upright so that they can capture unobstructed reflected light from the foreground.

I think an inverter that draws 20 watts is a big deal. Increasing the output of a 400 watt array by 20 watts might also be considerd a big deal.

Zeuspaul

rollandelliott · February 2011

Re: Reflected light from sand to increase PV output?

a pretty cheap and durable white material that is often available for free after elections and at sign shops is white 4mm coroplast (plastic cardboard)
it is about $8 a sheet new, very light and fairly durable outside in the elements.

it is also made in thicker 6mm and 10mm sheets. Typically Costco/Sams club signs hung up in the air are made out of this material.

A sign shop might be able to get you discards for next to nothing.

ggunn · February 2011

Re: Reflected light from sand to increase PV output?

Cariboocoot wrote: »

That's a very interesting question. Perhaps Solar Guppy could answer it?

For instance, with the snow scenario; how much improvement in output is due to the low temps and how much to the increased lumens?

In either case, there must be an upper limit to how much the P-N junctions can be driven. Certainly heat, whether ambient or from the increased output, would eventually slow the pathways so that there was no further gain. There would probably be a logarithm function; lessening % output as % light exposure increases.

The mirrors would concentrate heat as well as light, and to some extent so would the sand or snow. Different materials reflect different amounts of the spectrum, including infrared. So it would be a matter of how much does a given material reflect the most useful (for PV) segment of the visible spectrum and not the infrared?

Definitely not my area of expertise (as anyone can tell from reading this). I just make 'em work.

A problem with mirrors, of course, is that they are unidirectional in their reflection, so any use of mirrors would of necessity include some sort of tracking system to keep them aligned.

Reflected light from sand to increase PV output?

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