Energy-saving coating without pigment keeps the surface beneath cool
npr.org
npr.org
I believe building regulations should require specific performance from paint on new buildings, and that commercial painters be required to replace any paint on a 'same or better' basis.
Performance would be measured on the basis of watts of absorption under sunlight, and watts of emissions to the night sky. It is possible to design paint that visually is any color, yet maximizes or minimizes each of those parameters independently - therefore allowing you to choose a coating to minimize energy use for the building in whatever climate surrounds it.
I think if all buildings were painted with optimal paint tomorrow, we could probably shave more off the worlds energy consumption than all EV's have saved so far.
The heat moves via light - infrared light usually. Cooler things emit less light overall, and the light they do emit tends to have a longer wavelength (thats why the embers of a fire might glow red, but a hotter fire might be orange or yellow).
For each color of light that something might emit or absorb, there is a 'shinyness' (technically known as reflectance spectra). A shiny surface reflects light, which means it will take longer to heat up, or longer to cool down.
By making a material which is shiny in the redder (ie. longer) wavelengths, yet not shiny in the shorter wavelengths, you can make a material which heats up fast when put next to a very hot item (like the sun), but doesn't cool down fast when put next to a very cold object (like the night sky). That would make a great paint for a building in a cold climate! This would be a visually 'black' material to our eyes, although you could design it to be any human color if you trade some performance.
Likewise, one can do the reverse. Reflective of short wavelengths, and non-reflective of long wavelengths. That will help it stay cool in the sun, and lose as much heat as possible at night to the sky. The ideal material for this would be visually white, but again, you can trade a little performance for any other color.
It is also possible to have panels which physically flip to reveal another coating either between summer and winter, or between day and night to get even better results - although obviously with a complexity cost. Shutters in some european countries are an early version of this idea.
Note - the above is a simple explanation. For the full explanation, see [1]
Among architects, I'm seeing a lot more hype about high thermal mass, e.g. sand-lime bricks and woodfiber insulation, to design houses that respond very slowly to outside temperatures.
But specifically on coatings, it's true that conduction and convection also play a role, and radiation is only part of the picture. However, it is theoretically possible to design a coating that can cool a material by 45C (~80F) compared to ambient at night, and practically a cooling of 4C (8F) has been achieved [1]. 4C is pretty substantial, considering it stacks on top of the effects of insulation, high thermal mass, and efficient heating/cooling appliances.
Remember that in the daytime the effect is far more substantial - the difference between the best and worst thermal coatings could easily be 100C - when people say 'that rock is so hot I could fry an egg on it', it's simply due to the absorption properties of the rock under sunlight.
Sounds like both the "keep warm" and "keep cold" coatings offer that possibility.
Nighttime electric power generation at a density of 50 mW/m2 via radiative cooling of a photovoltaic cell
https://aip.scitation.org/doi/full/10.1063/5.0085205
Sure, but it does provide some interesting opportunities for embedded technology (LEDs, security cameras, Bluetooth, etc.).
It sounds like this paint is a "heat shield" that modifies the rate of thermal transfer.
Shouldn't that mechanism improve the performance of the existing substructure and insulation?
The energy loss through foam depends on the temperature on each side of it. So if you can use a fancy coating to keep the outside of your house cool/hot in the day/night, then the energy losses through the styrofoam are reduced, and eventually your heating/cooling bill is reduced.
If a home is well-built, the wall assembly already includes a physical gap between the structural sheathing or exterior insulation (if present) and whatever cladding materials you're using.[0] That gap allows for airflow and drainage behind your cladding, so that the water that gets behind it--and it will--can drain out and everything can dry.
That gap would help create something of a thermal break (the cladding isn't attached directly to the house but to furring strips with a very low surface area), but it would still heat up the air in the gap itself. In that case, reflective paints would probably still be beneficial.
That said, I think their greatest benefit would probably be in roofing. Metal roofs, for example, also require an air gap[1] behind they can get extremely hot.
0. https://buildingscience.com/documents/enclosures-that-work/h...
Why wouldn't a surface that reflects heat stay cooler than something non-reflective that absorbs heat?
This doesn't exist.
Or is there a link to commercial paints that can do this? Or testing on real buildings not claims from a labs media department?
Here's a link disproving thermal paints which are ugly as well as don't work - https://www.sciencedirect.com/science/article/abs/pii/S03787...
Given money most people could better insulate their homes so even if these could exist they are pointless unless they are also reasonably priced.
For example, there's a reason so many buildings in the Mediterranean are painted white going back to antiquity.
What coatings should be used for roof and exterior walls for a well insulated building in in the mid forties latitude.
Snow has some insulating value so the heatloss is reduced. A highly absorbent roof might still be a plus. I'm currently trying to figure out the if I can put a thin layer of phase change material PCM on my exterior walls. The idea would be that the phase change would delay the reaction of the heat transmission through the walls. The building has a high thermal mass and it takes about 72 hours of a 15+ degree temperature difference to result in 4 degree temperature change of the interior of the building. Modern commercial buildings can't use open windows for ventilation. The ventilation has to be provided by a mechanical system. I'm trying to optimize the building to maintain a consistent overall temperature range of 72-78 degrees. The summer is not a problem the heat pumps and solar array mean I only get 5 electric bills a year. I think that I can do a lot of optimization of coatings and solar thermal to get to net zero.
>> choose a coating to minimize energy use for the building in whatever climate surrounds it
There's definitely something here, hopefully someone sells it pre-made, I'd love to try some.
3M sells film that does this. Advantage for film over paint is that it doesn’t build up, as it can be removed, recycled, and replaced. This is especially important for things that move so as not to gain additional weight.
Very cool material to experience in person. It’s black body emission is also tuned to exit the atmospheric IR window.
> Chanda uses a Boeing 747 as an example in his team's study. To cover these planes, approximately 500 kilograms of paint is needed. Chanda estimates that using structural paint, manufacturers could cover the plane with only 1.7 kilograms.
Can someone explain how this will look like in real life? I have a hard time imagining painting something that big with such tiny supply of paint. Brushing can not possibly work, but even if its sprayed over, how can so little spread over such large area?
It's cool tech though, but i'm not imagining how distracting driving along a street of iridescent houses will be!
> “The optical response of these artificially engineered nanostructures can be spectrally tuned across the entire visible spectrum to form a full-color gamut by controlling the gap plasmon dispersion via the geometrical parameters. In the proposed architecture, the subwavelength optical cavity ensures a large degree of angle insensitivity, while the stochastic nature of the self-assembled layer results in polarization independence and near 100% absorption at selected spectral bands.”
See https://news.ycombinator.com/item?id=35521006 for the link to the paper.
https://www.duluxprotectivecoatings.com.au/products/ranges/z...
https://www.scientificamerican.com/article/the-supercool-mat...
Ah, so this will never come to a home near you. Shame.