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Comment by upofadown | Hacker News Reader

Solar panels are only 10-20% efficient. The rest of the sunlight ends up as heat which presumably might end up in the building. It would be difficult to cover an entire sunward side of a building with solar panels anyway which is what you would want to do with this technology should it ever be made practical.

My question would be: why this over insulation?

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You wouldn't just cover the sunward side of a building with the new material. You would cover every surface that has an unobstructed view of the sky, from any angle.

Furthermore, the surface could be corrugated instead of flat, to increase the radiating surface area. The cooling would work even when radiating at a reflection of the sky.

This would likely be used in addition to insulation. Your radiators would be thermally connected to the building through a managed, thermally conductive channel, so that the cooling effect would not continue throughout the winter, for instance.

The outside surface would also be fairly strongly coupled to the ambient environment due to convection. I suspect that coupling the outside surface past the insulation would be a loss in any case where it was hot enough outside to make people care about air conditioning.

Couldn't you just put the whole thing behind a layer that does not absorb IR or visible light, but is also a poor thermal conductor?

The article mentioned that they did that using an enclosed box to test the material. It wouldn't be much additional effort to add a heat pipe that transfers heat from the object to be cooled, through the box, to the radiator-reflector. That way, you are cooling only what you want to cool rather than the whole environment surrounding the device.

I was thinking about that too. You would need one or more sheets of something transparent to infrared 8-13 microns. Such materials apparently do exist:

http://www.infraredtraininginstitute.com/infrared-transparen...

Such a material would also have to not absorb too much sunlight.

I think with a material like this you would use both. Based on the article, it sounds like this material has a fixed cooling effect for a given temperature and incident radiation flux. So if you want to maintain 75F in a building and outside it's 110F -- this material would reduce the cooling effort to the same as a normally insulated building where it's only 100F outside. A substantial energy savings. (I made up the 10F cooling effect, pick your favorite constant)

With insulation, you are insulating your ability radiate away heat as well.

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