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fml87 t1_iv8udpl wrote

Reply to comment by Scullvine in Researchers designed a transparent window coating that could lower the temperature inside buildings, without expending a single watt of energy. This cooler may lead to an annual energy saving of up to 86.3 MJ/m² or 24 kWh/m² in hot climates by mossadnik

That doesn’t really make sense. Once the light has entered the space the energy is there. It doesn’t matter if that energy is spread over a large surface area or small, it’s the same total energy.

Curtains do make a difference because they do reflect light back out of the window even if they aren’t mirror-like.

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HKei t1_iv9c0dh wrote

They also absorb and re-emit heat, some of which will go into the room, though some will go right back outside too.

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DogsSureAreSwell t1_iva0al9 wrote

It matters if some of the delivered energy goes right back out the window.

Inside the room, heat the floor and you'll have convection and such pushing heat everywhere.

Inside a hot curtain even if it had 0 insulation value, half (ish) of the heat is going to head towards the room, and half (ish) is going to head back towards the window. The hotter the pocket between the window and the curtain, the more is going to head towards the room, the better the curtain is insulated, the more is trapped against the window to be transmitted back outside.

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fml87 t1_ivak9lj wrote

In theory maybe but not in practice. The vast majority of the heat will be carried into the room via convective current. You’re also not going to get 50% heat transfer back out of an insulated window vs a sheet of fabric.

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