Making infrared cooling paint from grocery store items [video]
youtube.com
youtube.com
Here is a video showing radiative cooling being used for interior cooling: https://youtu.be/CVginpnALL8
The downside is that if the dew point (and thus wet bulb temperature) gets too high, the panels shut down, so when you need them most they don't work and the building becomes a sauna.
The paint reflects radiation from the sun, but heat can get into the building by Conduction (air touching the paint) and Convection (air getting into the building) which stays more or less the same clouds or not. Is there another reason for the reduced efficiency when it's cloudy?
And really it works by radiating heat into the open sky, space. Clouds and buildings reflect a non trivial portion of the heat back compared to the open sky
The second is heat moving inside the material, think of pot's handles becoming hot- the heat "creeps" through the material. This rate this happens depends on characteristics of the material, metal will conduct heat better than air for example. When referring to containers you are in the worst place- the metal walls conduct heat very efficiently. How can you solve it? by adding an external layer not connected (as much as possible) to the metal- think of double glazed windows.
And then comes the third way- radiation. Think of the heat you feel from fire, even if nothing is physically connecting the fire and your hand. Heat travels from the sun as infra red radiation, it's simply a shorter electromagnetic weave length, which becomes heat when it hits an absorbing surface. I don't know the exact numbers, but putting something in the shade, hence less radiation, will make it as hot as the environment- not comfortable but not terrible. Exposing it to radiation can drastically increase the temperature, cars for example can reach 50C or more in the sun even when it's only 30C outside.
Painting white, or using this special paint, can reflect the radiation and reduce the heat. Shading by trees or something else has basically the same affect.
Reality is a little more complex though, hot countries have traditions of airing the house at night when it's colder and shading, for example using shades, during the day. Other tricks are having a bit thermal mass to hold the "cold" during the day and dampen sharp temperature changes, for example by building in stone.
Highly insulated houses tend to not function well in hot places. Heat, for example through windows, gets trapped inside and then it is hard to release it back.
But I am with you that using lime especially on roofs/sun exposed walls (assuming lime does not eat up some materials) before summer is sensible if done correctly. Otherwise we will have a spike in hospital admissions of ppl splashing lime over themselves.
https://www.sciencedirect.com/science/article/abs/pii/S23524...
Is this any better than regular white paint?
In my personal experience (and in the FLIR application guides) the main factor affecting paint emissivity is the glossiness (matte paint is much more emissive). I would have liked to see that test done with matte white paint, or at least have them mention that factor. From the video it was hard to tell, he said super bright Rustoleum, which isn't the same as matte. I would imagine emissivity to be more important that reflectivity given the result with aluminum (super reflective but still got super hot due to the extremely low emissivity).
I like the use of the thermocouples, but unfortunately it didn't go quite far enough...
The insulator is just to prevent the plywood side that is measured to be heated by the ambient temp.
https://www.goldenpaints.com/products/medium-gels-pastes/spe...
Best possible outcome would be something that can be sprayed and something that can be rolled on or applied with a paintbrush.
Apparently one of the problems here is that the paint has to be "very white", which I assume means that any dirt reduces the effectiveness. So that would be another challenge on the boat.
If there were a way to suspend this pigment in gelcoat in a way that preserved its properties, this would be killer for boats.
[In fact, I just noticed the author of the OP video posted on this one]
Plenty of governments make building energy efficiency laws, because they realise that the commercial incentives of house-builders and house-owners and the national economy aren't well aligned. Yet these laws tend to disallow 'clever paint design' as a technique to reduce building energy consumption, despite plenty of academic papers demonstrating effectiveness.