Got a thougth 'bout that a few days ago while I was filling up a half-empty sub critic carbon dioxide system (most common) for a bigger grocery store. Here in sweden the CO2 systems are well used on bigger grocery stores freezers (both DX and as cold carrier), just for it's effiency, together with cold carrier for the refrigerators, and to chill the CO2-Tank. Some systems uses R404A or R407C to chill the CO2 as well.
As reciprociating compressors for CO2 DX were using Bitzer octagon series as they seem to stand for the high pressures.
Som places uses trans critic CO2 systems but i haven't seen those, yet.
Back to the question. How big is the use of CO2-systems ion the US?