As I mentioned elsewhere: https://news.ycombinator.com/item?id=31791936 there is a company that claims to use thermoacoustics for general heat-pump duty. I believe they are targeting European style hydronic heating. Aiming to be efficient whilst still producing the 80c water that many older hydronic systems here in Europe still require.

Interestingly, they aren't that loud because any sound lost is energy lost, so they try very hard to 'keep it quiet'.

Thermoacoustic... love to hear about others :) yes pun intended.