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Comment by mensetmanusman | Hacker News Reader

An electricity-free technology to cool an area by reflecting all sunlight energy and simultaneously emitting black body radiation in the atmospheric mid IR window (unlike metals, which have low emissivity) is useful here.

They are an existing emerging technology. White paint helps somewhat but doesn’t work well in the IR. The current best performing film is made by 3M: https://www.3m.com/3M/en_US/energy-conservation-us/applicati...

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This part of the world needs something cheap and low tech. No one who is at risk has the money for cutting edge building materials.

Mirrors on every roof? Polished steel? Brick or concrete walls? At a city level, low tech could still make a difference.

Metal and mirrors retain heat due to the metal being near the air surface.

A real life example: think of your experience with a hot stone/glass surface and a hot steel surface. It’s much harder to tell the steel is hot until you touch it due to how it emits radiative heat.

Yeah, but maybe they can reflect enough back into the sky to be a net win. Ultimately you need something simple with the highest possible net energy rejection, whether by radiation or reflection or both.

white paint

Needs anti dust surface performance or else it absorbs soon, also doesn’t reflect on the mid far IR.

There are multiple "cool roof" products already sold in India. They should have lower performance than the 3M film, but also much lower cost.

I wonder if there's an low-cost additive that could give these existing products high emissivity in the atmospheric IR window?

https://www.youtube.com/watch?v=VeZDrFqJcj8

https://www.panachegreen.com/high-sri-coatings/cool-tops-pre...

https://thermacool.in/

Have to consider performance cost per unit time. Usually low cost options have to be replaced so quickly to actually have an impact that they are actually more expensive.

You still need to consider up-front affordability. A product could have high net present value, but if you don't have enough money to purchase it then the product might as well not exist.

>Usually low cost options have to be replaced so quickly to actually have an impact that they are actually more expensive.

I don't think that's what's happening here. You're comparing a product from 3M that's at the "fill out this form for a pilot project and we'll contact you" phase, whereas the Indian products can be bought by the bucketful, today.

Some of these coatings have real-world service lifetimes of 5-8 years.

How does this compare to your preferred 3M film?

Yeah, unfortunately it is so new that the information doesn’t exist for exactly this material:

https://arpa-e.energy.gov/technologies/projects/passive-radi... Describes the arpa-e funding

https://www.startribune.com/can-a-new-3m-technology-help-cli...

Shows the fruits of the research already reducing energy usage by 10% for supermarkets in a cold climate…

Anyway, exciting future!

A https://en.wikipedia.org/wiki/Windcatcher is electricity-free and the technology seems to have been invented in Iran some millennia ago. (But maybe itʼs not applicable everywhere and too costly for too many people.)

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