Daytime Passive Solar Cooling

I just saw this video HERE:



It describes how one can utilize an easy-to-make "paint" that will actually reduce the painted surface to below ambient temperatures while sitting in direct sunlight.  The principle behind this effect is twofold:  First, the paint is ultra-white, whiter than freshly fallen snow.  Secondly, the paint re-radiates heat at a wavelength that only minimally interacts with the atmosphere and therefore radiates away its heat into space.  I have seen a couple of videos on this principle using a couple of different chemical applications--both cheap and completely non-toxic, in fact food-grade.

I would love to know what you think.

Eric

I have seen this video (and likely the others you allude to). This one in particular excited me, since it is quite literally garage-level chemistry, with common materials. One caveat on "food-grade" seems to be the choice of binder to adhere it to a thing.

I'm curious about the "few degrees below ambient temperature" part... The test situations were exposed to the ambient air, or close to it behind a thin foam insulation board.
What if the coating were on the surface of a "solar hot water collector" which was multiply glazed as described in the video, but in a very well insulated enclosure on the other 5 sides?
Add a circulating pump, and equally well insulated reservoir of water. How much cooler than the ambient temperature could it make the water?

Would it be limited to achieving just a few degrees below a nighttime low on a clear night? Or, since the heat sink is outer space (essentially limitless capacity for cooling) is it limited by isolating the device (insulation) from ambient temperatures?

Eric, I'm super excited to find someone else who's super excited about this topic... my partner would like to hear more about accomplished old plans than shiny new ideas!
I so very much want to try it out! (lots to clear off my plate first) It may dovetail well with another shiny idea I want to build, a low-temperature differential Stirling engine.

Kenneth,

Fair points about the binding agent, but you are correct in that I was referring to the dye itself.

I just read an article regarding a similar formulation of the same product.  On clear, sunny days the painted surface was 19 degrees F cooler than the ambient temperature!  I would say that this qualifies for more than a bit better than a few degrees.

Eric

I have seen reports of other 'heat reflecting ' paint that is fraudulent.
But I see the real issue is to keep surface temperatures lower than ambient.
If that is the case a double roof skin I call a safari roof may work in many cases.
I use it on roofs and walls where it can be installed.

Eric, thank you for sharing this cool idea.

I asked Mr. Google about this and he was very helpful:

https://www.purdue.edu/newsroom/releases/2021/Q2/the-whitest-paint-is-here-and-its-the-coolest.-literally..HTML

To Help Cool a Hot Planet, the Whitest of White Coats

https://www.nytimes.com/2023/07/12/climate/white-paint-climate-cooling.html

https://www.science.org/content/article/cooling-paint-drops-temperature-any-surface

John C Daley,

I partially agree with you.  The commercially available ultra-white paints don’t have the cooling properties that the home-made stuff does.  Part of the reason has to do with the wavelength of IR radiation that is emitted from the paint.  That wavelength has to be one of a couple of wavelengths that interact only minimally with the atmosphere.  Somewhere I saw a video that compared the effect of cooling by painting with a commercial paint, polished aluminum, and a solution of barium sulfate (which works even better than calcium carbonate).  The commercial ultra-white paint and aluminum both heated up plenty while the barium sulfate cooled by several degrees.

Eric

I am deeply curious about this as well. I have an idea for building a "refrigerated" root cellar. Either using solar power to run a condenser refrigerator system to cool it.. but if there was a way to use this kind of paint to cool a room for long-term food storage that would be amazing...

John C Daley, your "safari roof" is known as a "cool roof" here in the USA, made with air channels between the roof materials/deck and the structure. The stack effect helps to exhaust the heat accumulated by the roof , and the separation from either an attic space or living space below means less of the heat infiltrates the living space. It is a decent improvement using available materials and skills with only a modest cost bump.

The IR cooling coating could reduce the "heat island effect" of the built environment, possibly even more than planting trees (although without all the other ecosystem services that trees provide), since it could be implemented on/above surfaces where trees would be impractical. Of course this is a "techno-fix", where other methods such as planting trees, green roofs, de-paving, etc. are working more with nature. Even still, it could reduce cooling loads in buildings, or improve efficiency of cooling systems, provide shelter like a bus stop or a pavilion in a park...

Kenneth,

I did actually see an application where the painted surface was applied to a heat sink.  The fins were then placed upside down into a container of water as a way of cooling that water.

On a different note, apparently the ancient Persians used a variant of this technique to make ice in the desert during nighttime.  Water was poured into a large, shallow depression which was extremely white (probably it was coated in calcium carbonate).  On cold but not freezing nights, the pools of water actually radiated enough heat that a film of ice formed on the surface.  

A desert would be about the perfect place for this application as nighttime temperatures drop significantly from daytime temperatures and the low humidity blocks little radiated heat from the ground.  For years it was known that the ancient Persians could make ice, but until recently it was not known how.

Eric

Eric, the coated heatsink with fins in the water would be similar to my solar hot water flat plate collector conversion idea. Your Persian nighttime ice-making story does make me wonder... is  "Daytime Passive Solar Cooling" a misnomer? sure, it's a nice time to do that thing, but it's more like "Clear View of Outer Space Passive Cooling" , no? In fact, I think it's pretty awesome that it works in the shade, so long as the surface is skyward facing, and that the best times for solar collecting are also good times for rejecting heat to space!

It reminds me a bit of the (now defunct) A/C supplement device which made a giant block of ice using off-peak electricity, to be able to not run the A/C compressor and fan during peak-electricity, just the air handler. Not only could you do the same making ice (at night) you could cool the air passing over the condenser coils. Cooling panels could be placed on roof surfaces unsuitable for solar (shaded, North-facing {in the northern hemisphere}, custom shapes for surrounding solar panels or other mechanicals)