CO2 is less common today, but has hardly been "superceded." I specialize in energy modeling for industrial refrigeration systems, and have studied several new i

CO2 is less common today, but has hardly been "superceded." I specialize in energy modeling for industrial refrigeration systems, and have studied several new industrial cold storage projects comparing CO2 to freon and ammonia. Their efficiency is on par with ammonia systems, within a few percentage points. I'll agree that the equipment is heavy-duty, but disagree if you mean "massive." The energy density of CO2 is so high that suction lines can be 2"-4", 6"-8" is common for ammonia. Modern systems use many (a dozen+) small recip compressors instead of larger HP screw compressors. When all is said and done, CO2 systems are small enough that they are frequently contained to a single rack and placed on the roof of a building, whereas a comparable ammonia system requires its own engine room and a significantly larger footprint. The biggest opportunity for CO2 to outperform ammonia is in heat recovery and reuse. I had a customer who was exploring selling his (waste) heat as a utility to a neighboring food processor.