I really like this guys videos. I have been thinking about this passive cooling concept for the last few years ever since I learned about skycool which is an attempt to commercialize a similar concept (radiative cooling). The main issue for most of the US is that you only want to cool for part of the year. one idea I had would be to paint blinds for windows or the louvered roof of a pergola with this type of paint so that you could open it and let the sun shine through when it is cold, but reject heat when it is hot. I’ve also tried to do some calculations for building a panel with blinds that could flip between black and cooling paint that could be hooked up to a heat pump. Both of these ideas exist on their own (see skycool panels and solar heat panels), but would be cool if you could combine them by having blinds that flip inside the panel. One of the main difficulties is that if it is cloudy outside the amount of heat the panels can reject or absorb is greatly diminished. So it may be that you need some way to store heat or cool. I think there has been some progress on this front using phase changing materials and there are now a few products that are commercially available. In any case, I think radiative cooling is an extremely cool concept and I’m excited to see what ways people come up with to harness it.
I am dubious of that interior radiant cooling design... I suspect the panels could easily end up below the dew point, especially in a room with lots of people in, and end up dripping water all over your kids birthday party.
It's used in commercial buildings, combined with a dew point monitoring system and some actual A/C to cool and dehumidify supply air.
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.
These don't work at all IN a room. They need to have a direct live of sight to the sky, or else you're just radiating the IR back into the room and you achieve no net cooling.
> One of the main difficulties is that if it is cloudy outside the amount of heat the panels can reject or absorb is greatly diminished
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?
The paint works by radiating in the infrared while reflecting the shorter wavelengths. On a moonless night a surface painted with this would be cooler than one coated with normal paint, or a mirror.
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
Do you mind expanding on that? I think this paint has a solid use case for refurbishing shipping containers into dwellings, as currently their main drawback is thermal insulation
Heat (well, energy) moves from place to place in three ways- first is hot material moving. for example hot air getting into your house or cold air "escaping" your fridge. When referring to houses this means air tightening the building.
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.
Which is just bad construction? You can add coolant reservoirs to buildings. The tiles in Mediterranen housing come to mind. You could make temperature creep highways, like moving the water of a night cool pool into the cellar. It's just rarely done because moving part and lots of space. Finally AC, a gets your air cold, thus the materials in contact with air, but it's way to late a defense line. Sunlight does not have to reach your house. Plant trees or have masts with sunsails on them.
Seems like a lot of work. Greenhouses sometime need shade so we coat them in 50lb bags of Hydrated lime (Calcium Hydroxide) which can be purchases for really cheap. While I enjoyed the video when I have thousands of squarefeet that need coverage hydrated lime works great.
The input substrate to produce CaO is CaCO3. This requires a lot of energy, often obtained by burning coal and the reaction releases CO2.
Absorbing CaO/Ca(OH)2 makes sense only in systems like submarines/bunkers with no external air supply.
As a CO2 capture method on a large scale is makes no sense. Even if we think about using CaO as some "CO2 capture and release" material with who knows, storing pure CO2 underground, I think even theoretical no CO2 emitting energy needed for baking CaCO3 has a better use.
If the lifetime energy savings are greater than this energy cost then it's still a good idea. You only need a thin coating so it sounds plausible to me.
A well applied lime wash is something more people should be aware of in the modern world. If you live in an old stone building it is what you should be using for exterior stonework and not modern paints!
It is not the problem with lime vs another white paint outside of the buildings. Modern paints based often containing TiO2 are more durable.
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.
Seems like a lot of work to do yourself but if made commercially there’s nothing in the process that would make the end product very inexpensive either.
- absorption and radiation are correlated (a black surface would radiate more than a white surface when heated to 6000K (assuming they survive the temperature unchanged))
- a solid polished surface reflects better than a powder surface (thats imho a flaw in the article)
- basically you want a surface that is white in solar and black in atmospheric
I would like to see a comparison using a thermocouple. I'm not convinced with the measurements taken with a thermal camera. Further, the use of an insulator doesn't make for a great comparison.
You can check out Tech Ingredients video [0] on the same topic, which will give an answer to your question. It is: Yes, it is 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 fact that he measures the back of the plywood controls for quite a bit I think. At 3m27s into the vid you see the plywood that has the 'paint' on the _other_ side is even cooler than the square that had been in the shade all along.
The insulator is just to prevent the plywood side that is measured to be heated by the ambient temp.
Since the last video I have been seriously thinking about making these powders as a hobby. The problem is that I want to mix them with some paint or resin to get a more permanent application. I know this needs some research but any ideas about where to start?
He makes something like that with acetone dissolved acrylic. It would be interesting to see what happens if you dispersed the microsphere material in a commercial paint. I think it would work?
He specifically engineered his particles to be as densely packed as possible. I assume the pigments in commercial paint would limit the cooling capacity. Probably that's why he went with a clear acrylic base (with bubbles to make it reflective).
As others mentioned the nighthawk guy has solved most of these problems, but what I do think he needs help with is improving the application. Currently his best method is just squeezing it out of a syringe.
Best possible outcome would be something that can be sprayed and something that can be rolled on or applied with a paintbrush.
I thought it was odd that he got good coverage spraying with a hand sprayer bottle but then went through the effort of squeezing it out of a large syringe.
Sailboats are an interesting use case. What percentage of the deck do you think you could apply it to? Assuming you want to keep your non-skid, and a bunch of the deck space is shaded by lines and sails.
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.
Yes but the plastic layer also needs to be kept very transparent. I assume the vid was talking about a cheap replaceable plastic coating, although plastic itself is probably also just more washable. Still, the top of boats gets dirty and most cruisers don’t keep it boat-show clean year-round!
Has anybody ever applied this to a passive cooling box (that only uses insulation)? It would be interesting to see what temperatures can be achieved inside the box using the described effect.
I don't think anyone is talking about painting the desert with this stuff, so it isn't geohacking in any real sense.
What you CAN do with this stuff is paint your roof and external walls with it, which will reduce the need for active cooling in your building. This both saves money for the purchaser (making it commercially viable) and reduces energy usage, which in turn reduces fossil fuel usage.
Seems like it would be a lot quicker / easier to pick up a few large sunshades? Coolaroo makes roller shades up to 10ft / 3m wide which are easy to mount under eaves (no association, except I installed one on a west-facing balcony and it works nicely), if your structure has a roof with those. Then you can block solar heat in the summer, but let it do what it can for you in the winter.
While your comment is interesting discussion, I'm not sure the posted video is arguing for geohacking. I watched it last night and got more of a "here's a cool physics trick that seems like it has some real-world applications outside of a lab, and I did a lot of work to make it achievable outside of a research lab."
I'm disappointed that governments haven't more aggressively forced this tech to be deployed commercially.
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.
Here is a video showing radiative cooling being used for interior cooling: https://youtu.be/CVginpnALL8