Conventional cooling is all about moving heat from a place where you don’t want it to a place that you care about slightly less. Your refrigerator, for example, cools itself by pumping heat into your house. Your house cools itself by pumping heat into the outdoors.
It takes a significant amount of energy to keep this up — 15 percent of the energy consumption of most buildings is spent just on air conditioning—meaning that the work put into transferring the heat generates even more heat.
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And then it’s not like the heat just vanishes when it gets outside: in urban areas, all of this waste heat builds up to increase local temperatures as part of the urban heat island effect.
In Nature this week, Stanford researchers describe a passive radiator system that can lower the temperature of anything that it’s placed on by up to five degrees Celsius by absorbing heat and sending it directly into outer space, and it even works in direct sunlight.
Radiative cooling is a way of passively moving heat from one place to another through thermal radiation, without the need for any additional energy (like electricity). If you have a hot thing, it will radiate its heat into whatever cooler thing is most convenient. In your house, this is probably the air outside, and in your car, it’s also the air outside, by way of the water in your radiator.
Since the general approach here is to use the atmosphere as the final heat sink, radiative cooling doesn’t work if you’re trying to end up at a temperature lower than the ambient temperature outside, which is why completely passive air conditioning isn’t a thing.
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The clever thing about the passive radiative cooling system that Stanford came with is that it skips the atmosphere completely, and uses the entire universe as a place to dump heat.
The entire Universe, being mostly empty space, has an average temperature of just under three Kelvin, meaning that it’ll happily absorb just about as much heat as you can possibly throw at it, making it a heat sink that’s nearly, you know, universal.