[samch]

Reading the article helps. One of these tarps cooled a container of water underneath it by 8°C during an overnight test:

"We further demonstrate the effectiveness of radiative cooling for a relatively large thermal mass using water as a cold storage medium. A plastic water tank was placed underneath the radiative cooling glass-polymer hybrid metamaterial, putting water in close contact with the heat-conducting copper plate. Since the water is stationary in the experiment, its large heat capacity substantially slows down the cooling process. We therefore used a 10-µm-thick HDPE film on top of the Polystyrene foam box in this setup to reduce convective heat loss and improve thermal isolation. The water temperature continuously dropped, reaching more than 8° C below ambient after two hours of exposure."

[mrfusion]

That's amazing. You could actually cool a tank of water at night for free and pump it though your house during the day in place of AC.

[dreamcompiler]

Except it's not limited to nighttime use. Because of the tuned IR window, it works in daytime also, just not as well.

[Retric]

That means less than you would think as a container with the same setup and no meta material at night would also drop below ambient temperature.

[samch]

A sealed container? I'm not following the physics on this. I can only assume you're talking about evaporative cooling of some sort. This was not an open container.

[Retric]

No, you need to insulate the container from the ground. Then you have conductive and radiative heat gain and loss with the air. Works better on top of a hill, in the middle of a field, under a clear sky. The container has to have minimal thermal mass, and be a food conductor but that's it.

PS: An easy demo is a car roof with the windows open at night in a field.

[samch]

They did insulate the container from the ground.

[Retric]

Which is why I am saying this. The important thing at night is not delta from ambient temperate, but the delta with and without their covering.