[sidcypher]

I suppose if the air is compressed by the bottles, the inlet is on the outer side of the cardboard membrane and discharge is on the inner side.

Excessive heat is then released into the bottle and stored in the plastic until it radiates away - mosly by the large, voluminous bottle part.

[kazinator]

According to the DIY build instructions, the bottles are cut in half, so they provide horn-like flares, open on both ends.

(Not that this adds any plausibility.)

The technical claim is summarized in this image:

http://i.dailymail.co.uk/i/pix/2016/06/10/18/3521DBE60000057...

[Nomentatus]

Compressing, then expanding air, so this is the same principle as a fridge uses with freon (ok, not freon any more), on a much gentler scale, with much-lesser-but-still-useful results.

[kazinator]

Sorry about my earlier comment; re-reading yours now, it's clear you're writing about the same thing.