[bluemax]

Several companies are developing thermo acoustic heatpumps that will have some advantages over regular heatpumps. They should produce less noise, don't use greenhouse gasses and work in a wider range of temperatures.

https://www.blueheartenergy.com/ https://www.equium.fr/en/home

[rihegher]

Equium is using helium, a non renewable gas. Not a greenhouse gas but not really sustainable either.

[bill38]

They fill the core of their heat pump with helium at 30+ bars and seal it. That's all. It's not used continuously. So not a big quantity used.

[DrScientist]

Anybody got any actual experience of these yet?

[hasmanean]

LBNL had a solid state thermoacoustic resonator the size of a coin, which I read about but never saw it in the market. It was supposed to be able to extract heat from anywhere.