How much energy would be released if 2L of (relatively pure) water compressed to 1L at room temperature was suddenly decompressed to atmospheric pressure? Even a first-order approximation would be helpful.
As far as I can tell, said water would end up sitting on the border between Ice-VII and Ice-X, as according to here ice-VII has a density of ~1.50g/cc and ice-X has a density of ~2.51 g/cc.
I am aware that increasing the density of water by 2x would require an rather high pressure (to put it mildly). It's not impossible, though - based on the chart here it's "only" about 7*10^10 Pa - and we've achieved higher sustained pressures in a laboratory setting before.
I'm not sure how to calculate this, though, as I have little data on either the density of the various phases as a function of pressure, or the energy of each phase change.