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Curious, can this be used in reverse, to keep sand cool and use it to cool down houses?

Would bring down a lot of AC usage in tropical countries.

A major problem would he that the temperature delta for storage will be way smaller. There isn't really space for the same 500C below ambient in that direction :)

Normally they just pump sea water since that's usually below 20 C anyway.

https://en.wikipedia.org/wiki/District_cooling

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