Mirrors are specular reflectors. A 100% white surface, by definition, is a surface that reflects back 100% of the light that hits it. That's what makes it white. Of course a 100% white surface only exists in theory but they have come very close
What is the best choice for reflective surface paint under Bi-facial panel(s)?
- Thread starter Tony S
- Start date
Warpspeed
Solar Wizard
Exposed surfaces have a lot more to contend with than just direct sun.
Ask anyone that has just repainted their house, how much better new paint looks than old paint.
Even white paint chips, peels and cracks over time.
Dust and pollution, plus ultraviolet, can degrade many types of surfaces over time.
A good grade of stainless steel (such as 316) lasts just about forever.
Ask anyone that has just repainted their house, how much better new paint looks than old paint.
Even white paint chips, peels and cracks over time.
Dust and pollution, plus ultraviolet, can degrade many types of surfaces over time.
A good grade of stainless steel (such as 316) lasts just about forever.
OffGridForGood
Catch, make or grow everything you can.
The incoming solar energy is a wide range of wavelenghs from long wave Infrared to short wave ultraviolet, a surface no matter the 'colour' (colours are very narrow part of the spectrum in the visable light range only, in the 400-700nm range) has little to do with the full range of light wavelengths.
It would not be surprising the specially developed "white" super-paint has components suspended within it to reflect short wavelengths well beyond the visable spectrum, and this increases it's efficiency at heat reduction.
Most heat gain from sunlight is from energy in the ultraviolet (short wavelengths) range, the problem with buildings generally is short wavelengths pass right through many building materials into the structure, where these wavelengths are absorbed into surfaces they strike, which warm and re-radiate longer wavelenghts (infrared) which do not pass easily through building materials and remain 'trapped' in the structure heating up the interior.
OffGridForGood
Catch, make or grow everything you can.
you could do the testing - would be interested to see the results.
But it is well known, solar PV uses only the 400-1100 nm range of the light spectrum, this overlaps generally with the visiable part 400-700 and a bit beyond what we can "see" with our eyes.
Some of the research into making better solar panels are just to find methods to capture additional parts of the full spectrum ie less than 400nm or higher than 1100nm.
When we wear sunscreen we are trying to block ultraviolet light not visable light.
Warpspeed
Solar Wizard
We are not talking about heat gain through a surface here, but reflecting usable solar radiation for photovoltaic solar cells.
That solar paint is designed for thermal applications, keeping buildings cool. A very different function.
I suspect a bright mirror reflector backed with a sheet of thermal insulation, would work better than a sheet of white painted wood. It would certainly not be cost effective though. The shiny aluminium foil used in the building industry is intended to reflect infrared radiation and act as a vapor barrier. Its quite effective at reflecting radiant heat transfer through a wall.
That solar paint is designed for thermal applications, keeping buildings cool. A very different function.
I suspect a bright mirror reflector backed with a sheet of thermal insulation, would work better than a sheet of white painted wood. It would certainly not be cost effective though. The shiny aluminium foil used in the building industry is intended to reflect infrared radiation and act as a vapor barrier. Its quite effective at reflecting radiant heat transfer through a wall.
Tony S
New Member
How would one attach it to the roof of a sleeper box, without having to put holes in the roof?
Warpspeed
Solar Wizard
Well right now there is 100% grey nasty cloud here, and its spitting rain.
Mid winter down here, so not much chance of doing any testing here.
I will certainly give it a go though if I get a chance.
The most reflective white surface I have here is probably some white melamine sheet. The stuff used to make the insides of kitchen cupboards.
And I have a suitably large mirror I could use.
Also have a small palm sized solar panel around here somewhere if I can find it, and adjustable resistive load, and a Turnigy power meter.
Should be an interesting thing to try.
Warpspeed
Solar Wizard
I suppose you already have some solid fixing points arranged for the actual solar panels.
With a bit of ingenuity, the reflective panel might be able to be supported from the same places.
OffGridForGood
Catch, make or grow everything you can.
Quite right, and that is what I already stated above, two separate things.
Incoming solar radiation is a wide range of wavelengths from short ultraviolet (invisible to our eyes) out to infrared (also invisible to our eyes) with a narrow band in the middle where we can 'see' the radiation. The solar panels also do not use the full spectrum of wavelengths (or not yet) and rely on only the part between 400 and 1100 nm (nanometers). we can't see 1100nm light it is out of the range we see.
For reflection of solar radiation from a roof to be useful to the bifacial PV panel the reflection should be the visiable part 400-700nm And the invisible part from 700 to 1100nm.
These are not the primary wavelengths that overheat your home, those problematic wavelengths are the ultraviolet range (short wavelengths) which we don't "see" we feel. These are also the wavelenghts that cause sun burn, skin cancer etc. Reflecting Ultraviolet light does not improve bi-facial solar panel energy collection, but will reduce heat gain in the building it was reflected from.
I think you are conflating reflectivity and emissivity
Tony S
New Member
I'm using a drill-free mount that adheres to the roof.
OffGridForGood
Catch, make or grow everything you can.
The solar panels out in the sun absorb ultraviolet light which does not pass well through the glass, this is why the PV panels heat up, and why your window glass also heats up.
The question posted was What is the best choice for reflective paint under bi-facial PV panel,
the clear answer for the bi-facial PV effectiveness is to reflect the part of the spectrum the PV panel can actually use, which is the visable light plus a bit beyond visable light into the longer wavelength infrared light used by PV 400nm - 1100nm. so any paint that reflects the visable and some infrared wavelengths would be effective.
since the OP has a sleeper bunk under this PV panel, reflection of UV light will be a nice side benefit if they are trying to keep the bunk cooler, the opposite if they are trying to warm it up.
The question posted was What is the best choice for reflective paint under bi-facial PV panel,
the clear answer for the bi-facial PV effectiveness is to reflect the part of the spectrum the PV panel can actually use, which is the visable light plus a bit beyond visable light into the longer wavelength infrared light used by PV 400nm - 1100nm. so any paint that reflects the visable and some infrared wavelengths would be effective.
since the OP has a sleeper bunk under this PV panel, reflection of UV light will be a nice side benefit if they are trying to keep the bunk cooler, the opposite if they are trying to warm it up.
Warpspeed
Solar Wizard
Yes spectral content and what is actually usable is a very important part of this. There are some very strong absorption bands in the atmosphere caused by dust, various gasses, and even the ozone layer.
And as you will also know, not all wavelengths are reflected equally. That is the whole basis of colour vision. What makes a red object red, or a green object green.
White is going to be excellent as a universal reflector, but a mirrored surface will be even better. A white surface scatters light, a mirror reflects light in a very precise direction. I think that is probably where the greatest gains from a mirror will be.
You can place your mirror at a distance and it will reflect the image of the sun along a direct path onto your solar panel.
A white surface will scatter the light, and unless its really close to the target, not much will reach the solar panel but diffuse away in all directions.
You could just try reflecting the sun's image onto a nearby wall. First with a mirror, and then with a white surface.
I have no doubt which will create the brightest spot on the wall.
And as you will also know, not all wavelengths are reflected equally. That is the whole basis of colour vision. What makes a red object red, or a green object green.
White is going to be excellent as a universal reflector, but a mirrored surface will be even better. A white surface scatters light, a mirror reflects light in a very precise direction. I think that is probably where the greatest gains from a mirror will be.
You can place your mirror at a distance and it will reflect the image of the sun along a direct path onto your solar panel.
A white surface will scatter the light, and unless its really close to the target, not much will reach the solar panel but diffuse away in all directions.
You could just try reflecting the sun's image onto a nearby wall. First with a mirror, and then with a white surface.
I have no doubt which will create the brightest spot on the wall.
OffGridForGood
Catch, make or grow everything you can.
Actually both Infrared and ultraviolet radiation are reflected by the foil.
This works both to keep out unwanted UV heat gain,
and retain long wave radiant heat within from traditional radiant heating systems.
A good way to understand the differences from UV and IR radiation:
A silver/aluminum backed piece of glass - a mirror - will get hot in direct sun. This is because the glass absorbs the high energy UV in both directions.
A layer of foil over a piece of the same glass will get less heat gain. This is because the aluminum foil will reflect most of the UV before it can reach the glass.
This is an easy experiment with a mirror and a sheet of cooking aluminum foil,
Put the mirror in direct sun wait a few minutes and touch it - the glass will be hot.
Cover the same mirror with aluminum cooking foil and put in direct sun wait a few minutes- it will be cooler.
That's exactly emissivity, not reflectivity. You need an air gap for emissivity to work. That's why the mirror gets hot. Placing glass directly on the emissive surface kills emissivity. Place foil over the glass and the highly emissive foil, with the air gap, does it's work. Now, place a piece of glass directly on top of that foil, and you've killed emissivity, again
OffGridForGood
Catch, make or grow everything you can.
So back to the question of the material for under the PV on a sleeping bunk,
The special white paint that was developed for keeping buildings cool, likely contains additives to assist with reflection of the UV radiation, that although will assist with keeping the bunk cooler in the sun, will do little to assist the bi-facial PV panel, and may be more of a negative since the reflected UV will be absorbed by the glass layer in the PV panel and become heat.
Reflection of the visible light - ie White - should be effective at aiding the bi-facial PV panel.
Reflection of IR (infrared) wavelengths in the 700-1100nm range would also be beneficial to the bi-facial PV panel, ie silver foil would reflect a high percentage of the IR radiation.
A foil surface may not be easily attached to the bunk, while paint would be easily applied. There are silver paints that use tiny flakes of aluminum suspended in a paint mixture. Likely this would be effective for the bi-facial PV and easily applied, and would still have the side benefit of keeping the sleeping bunk cooler from UV heat gain too.
The special white paint that was developed for keeping buildings cool, likely contains additives to assist with reflection of the UV radiation, that although will assist with keeping the bunk cooler in the sun, will do little to assist the bi-facial PV panel, and may be more of a negative since the reflected UV will be absorbed by the glass layer in the PV panel and become heat.
Reflection of the visible light - ie White - should be effective at aiding the bi-facial PV panel.
Reflection of IR (infrared) wavelengths in the 700-1100nm range would also be beneficial to the bi-facial PV panel, ie silver foil would reflect a high percentage of the IR radiation.
A foil surface may not be easily attached to the bunk, while paint would be easily applied. There are silver paints that use tiny flakes of aluminum suspended in a paint mixture. Likely this would be effective for the bi-facial PV and easily applied, and would still have the side benefit of keeping the sleeping bunk cooler from UV heat gain too.
OffGridForGood
Catch, make or grow everything you can.
The glass will stop a high percentage of UV energy and absorb it, it will release this energy as heat to the surrounding air/surfaces it touches by conduction and convection, some as long wave radiation (we feel as heat). A piece of plain clear glass will heat up in sunlight.
The same piece of glass with aluminum foil blocking the sun - touching the glass or not - will not heat up.
underdog5004
New Member
I have a 8x 240 watt panels mounted at a steep angle on the side of my house. the angle is not optimal so to increase my catchment I spread out a big sheet of white poly tarp in front of the panels. This increased power catchment from 800 watts to 900 watts when I laid it out I'm not sure that overall Improvement.
These panels are at a steep angle for a couple reasons. one, I want to increase winter power catchment and also if they were at a less steep angle they would interfere with my sidewalk walkway
These panels are at a steep angle for a couple reasons. one, I want to increase winter power catchment and also if they were at a less steep angle they would interfere with my sidewalk walkway
OffGridForGood
Catch, make or grow everything you can.
Higher gloss will have more reflectivity, ceiling paint is matt-finish to hide imperfections, not for light reflection.
