Comment by 0x53 | Hacker News Reader

Comment by 0x53 | Hacker News Reader

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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.

Here is a video showing radiative cooling being used for interior cooling: https://youtu.be/CVginpnALL8

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.

Watch the video. They are used in a room to absorb IR radiation given off by people in the room.

> 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.

Well, the extreme is Geothermal Heating & Cooling although I rarely saw cooling being used.

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