Effect of IR blocking nano coating atop solar reflectors....

[shane2]

I'm needing to reflect additional solar into a greenhouse to increase plant beneficial PAR light (400-700nm) while restricting and minimizing excessive IR heat also building up inside, too.

I'll be using 400 sq ft of flat plate reflectors (40'X10') at ground level horizontally positioned along and outside the southern facing vertical clear poly walls of the greenhouse.

These rigid flat reflector surfaces will be topped with a specular mirror finished poly film, specifically http://www.reflectechsolar.com/technical.html

If compatibility tests first confirm no issues (manufacturer said should be OK), I intend to then next topcoat apply onto reflective film this liquid nano IR blocker product here... https://drywired.com/wp-content/uploads/2015/06/Liquid-NanoTint-PDS.pdf

Which claims to block/absorb a significant portion of the NIR and Far IR heat producing portion of the solar spectrum when applied to windows.

My question is, of the light then reflected and exiting this reflective combo, what might specular degradation and transmittance %'s and IR absorption %'s all look like after those incident solar rays had passed through that nano top coating twice?

Is it reasonable to just cut performance values in graph above in half for rough idea of the effect to be expected, or is there likely other and additional effects to also be considered here now? (Besides expecting absorbed IR heat should be greater on treated reflector and its substrate than a window that only gets a single pass.)

Thanks for any comments or thoughts!

- Shane

 

[shane2]

This might be a better way to ask what I'm inquiring about here...

How do you feel the two columns of values on the right in the following chart of the IR absorber NanoTint would change with incident solar reflecting off of 94% mirror film causing then two passes through the NanoTint instead of single pass that's being represented here...

Thank you for any thoughts.

- Shane

 

[LURCH]

Since your IR blocker is a coating, and non-directional, I would think that your initial idea was right, and the IR gets rejected twice; 90% on the first pass, and some significant amount of the remaining 10% on the way back. What happens to this second set of IR waves is difficult to predict, since I don't know exactly how the coating works. They may get reflected back down at your mirror, which reflects them back up again and so on. This would result in the IR bouncing back and forth between the two layers with the nano coating letting 10% through every time, until it all gets passed. Not a big deal, since it's only 10% of the original value, to begin with.

However, the temperature inside your greenhouse is still likely to rise significantly. You might have more knowledge than I about the way greenhouses work, but as I understand it, the clear walls already block most IR and allow light in the visible and UV ranges to pass freely. These frequencies are then absorbed by the plants and other objects inside, which convert them to IR. That IR is then blocked from escaping. It is this heat energy, radiating from objects within the greenhouse, that is chiefly responsible for the increased temperature inside. So anything you do that increases the amount of visible light entering the greenhouse will also cause an increase in temperature.

If this is true , then you might be better off skipping the IR blocker, and looking for ways to vent the excessive heat. Just something to consider.

 

[shane2]

Since your IR blocker is a coating, and non-directional, I would think that your initial idea was right, and the IR gets rejected twice; 90% on the first pass, and some significant amount of the remaining 10% on the way back. What happens to this second set of IR waves is difficult to predict, since I don't know exactly how the coating works. They may get reflected back down at your mirror, which reflects them back up again and so on. This would result in the IR bouncing back and forth between the two layers with the nano coating letting 10% through every time, until it all gets passed. Not a big deal, since it's only 10% of the original value, to begin with.

However, the temperature inside your greenhouse is still likely to rise significantly. You might have more knowledge than I about the way greenhouses work, but as I understand it, the clear walls already block most IR and allow light in the visible and UV ranges to pass freely. These frequencies are then absorbed by the plants and other objects inside, which convert them to IR. That IR is then blocked from escaping. It is this heat energy, radiating from objects within the greenhouse, that is chiefly responsible for the increased temperature inside. So anything you do that increases the amount of visible light entering the greenhouse will also cause an increase in temperature.

If this is true , then you might be better off skipping the IR blocker, and looking for ways to vent the excessive heat. Just something to consider.

Thank you for your comments, Lurch.

I'm in conversation with tech guys from NanoTint and awaiting more feedback from them as they had not explored using it this way, essentially as a top coat to a mirror then generating two passes, but they did confirm it works in either direction the same as it's via absorption, not reflection. I'm hoping to hear from them what they think it'll do to the numbers overall and to the specular nature of the incident rays striking the mirror finish, how much we'd lose to diffusion scattering and all.

As far as the greenhouse poly film walls, it's like a car with windows up on a sunny day, shortwave solar radiant heat readily passes through them, then it's absorbed by everything inside to the degree that it's non reflective and they then re-radiate as long waves that the windows or poly film then won't let so readily pass back out. Those heated up materials in there in turn heats up the air inside.

Minimizing that initial IR getting in, while still maximizing plant needed PAR 400-700nm, is job #1.

Next is coating everything not growing inside with reflective material, something like this http://whiteoptics.com/portfolio-item/technology-benefits/

And, third step is whatever internal air still did get heated up too much needs to then be conditioned to cool it back down and/or exchanged with cooler outside air via exhaust ventilation (vents & fans) of it. (Evaporative cooling can also be employed in many areas which can help with either.) Which, if job #1 & 2 has been done as well as it can, should be a lesser hurdle.

Once all of above has been maximized, if air heat gain still rising too fast and too high inside, then time to start shutting off some portion of the solar gain by closing up reflectors, shading, etc.

That's the plan, anyways, subject to reality revisions & adaptations.

 

[sophiecentaur]

I am trying to confirm what exactly you want from this exercise. Is the following right?
My take on it is that your reflecting panels need to reflect wavelengths needed for photosynthesis ( 450-700 nm) and not the longer wavelengths. The effect of transmission of the liquid nano tint will be 'squared' because everything goes through both ways. That implies that only around 50% ( 70% of 70%) of wanted light wavelengths arriving at your greenhouse. (That's a disadvantage because it requires more area of reflectors) If the filters get hot, the re radiated IR is radiated in all directions and there is no specular reflection so that isn't so bad. I am not sure that the nano tint is what's needed. But you could have a different arrangement with the filter suspended overhead and the reflectors underneath, pointing at the greenhouse wall. The wavelengths that you want would be attenuated by only 30% because the light would only go through once. You would need less reflecting area, that way. Is this nano tint expensive?
If it were up to me, ideally I would look for a reflective coating (interference films) that reflects the wanted wavelengths and passes the unwanted wavelengths. Having reflectors, behind, to deflect this unwanted radiation back up to the sky would reduce the heating effect. But that would involve finding a different form of reflecting film which I couldn't find in a quick search. That sort of filter is available in quantities of a few cm² (optical instrument applications).
I have SMARTGLASS^TM on my new conservatory, which has a G value of 40% and light transmission of 52%. It may be worth looking into. Pilkington also do the same sort of stuff, I believe. (Here is a brief spec)

 

[shane2]

I am trying to confirm what exactly you want from this exercise. Is the following right?
My take on it is that your reflecting panels need to reflect wavelengths needed for photosynthesis ( 450-700 nm) and not the longer wavelengths. The effect of transmission of the liquid nano tint will be 'squared' because everything goes through both ways. That implies that only around 50% ( 70% of 70%) of wanted light wavelengths arriving at your greenhouse. (That's a disadvantage because it requires more area of reflectors) If the filters get hot, the re radiated IR is radiated in all directions and there is no specular reflection so that isn't so bad. I am not sure that the nano tint is what's needed. But you could have a different arrangement with the filter suspended overhead and the reflectors underneath, pointing at the greenhouse wall. The wavelengths that you want would be attenuated by only 30% because the light would only go through once. You would need less reflecting area, that way. Is this nano tint expensive?
If it were up to me, ideally I would look for a reflective coating (interference films) that reflects the wanted wavelengths and passes the unwanted wavelengths. Having reflectors, behind, to deflect this unwanted radiation back up to the sky would reduce the heating effect. But that would involve finding a different form of reflecting film which I couldn't find in a quick search. That sort of filter is available in quantities of a few cm² (optical instrument applications).
I have SMARTGLASS^TM on my new conservatory, which has a G value of 40% and light transmission of 52%. It may be worth looking into. Pilkington also do the same sort of stuff, I believe. (Here is a brief spec)

Sophiecentaur, thank you for your comments.

Yes, just trying to strip what IR out I can without losing too much of the PAR 400-700nm wavelength.

A 'hot mirror' I've played with, got deal on 2' diameter edge chipped 'second', with dreams of positioning it on greenhouse wall with bank of heliostats all aiming at it like a bullseye, but wasn't cost effective when talking 100's of square feet worth of tracking heliostats.

NanoTint isn't cheap, but I'm also looking at their clear version that on single pass claims 87% VLT and 60% IR rejection. Double pass numbers through that then don't look too bad at 75% VLT. BTW, NanoTint is liquid, so it's not a filter that could be suspended elsewhere. I'd first thought about applying it directly to the poly film of the greenhouse itself, and while it might stop the IR there fine, it'd heat up that poly and then conduct a goodly portion of that heat inside defeating the purpose. An interference film coating applied there, though, reflecting, instead of absorbing, the IR, would be great, if it affordably exists to be had.

BTW, link for brief spec you'd indicated at end of your comment was to the NanoTint product.

 

[sophiecentaur]

BTW, link for brief spec you'd indicated at end of your comment was to the NanoTint product.

Haha. It's a small world. I hope it works as well as is claimed or I'll be needing blinds for the hot weather!

bank of heliostats all aiming at it like a bullseye,

No need for "bullseye' or for high accuracy heliostats. If you look at the philosophy for PV panels (same basic requirement), you find that almost no one bothers to steer them. If you are not pushed for building space, a fixed set of reflectors would be far cheaper than any mechanism.
PS if you want to add motorisation, I should think that a thermostat controlled fan would be cheapest, most reliable and very much off-the shelf. If you do it that way, the nano tint could be applied to a much smaller area of glass - the windows of the greenhouse itself. Reflectors could be simple reflective mylar sheet- fairly easy to erect and to replace.

 

[shane2]

No need for "bullseye' or for high accuracy heliostats. If you look at the philosophy for PV panels (same basic requirement), you find that almost no one bothers to steer them. If you are not pushed for building space, a fixed set of reflectors would be far cheaper than any mechanism.
PS if you want to add motorisation, I should think that a thermostat controlled fan would be cheapest, most reliable and very much off-the shelf. If you do it that way, the nano tint could be applied to a much smaller area of glass - the windows of the greenhouse itself. Reflectors could be simple reflective mylar sheet- fairly easy to erect and to replace.

I should clarify, when I was talking about heliostats aiming at bullseye, that was all about if I'd used a hot mirror, I'd be aiming at it. That was before coming across the NanoTint, as I agree with you and KISS principle of simple reflectors. Also, yes, ventilation is way to go after exhausting all other means to minimize heat gain first.