CO2 for closed cycle compression refrigeration has great thermal properties and is particularly applicable at temperatures from -25 F. to maybe -65 Deg. F. Below that temperature, the triple point is encountered. In large central systems, it has been applied as the low temperature end of a cascade system, often with R-717 as the high temperature side of a 2-temperature plant. There have probably been 40 such systems constructed in the last 5-years.
CO2 at higher evaporating temps involves some pretty high pressures; (45 deg. F. and Satn is around 600 Psia) and direct to ambient condensing results in a transcritical system: rejected heat is all sensible heat, none of the advantages of Phase Change available. This last condition leads to low COP's on systems condensing over 88 deg.F.; and the equipment for 88 deg. F. would have to withstand about 1300 Psig to achieve "normal" conformity to codes, etc.
Sanyo has designed a suitable compound compression circuit for lower capacities and transcriticalapplications; it inherently supports a gas displacement overcirculation arrangement. Other players include Linde, Nestle, and Star Refrigeration in the UK.
By comparison with R-717, major driving forces for design with CO2 include: Smaller Vessels, compressors and piping; easier tranportation for oil and liquid in an overfeed arrangement; and lack of toxicity should a leak or discharge develop. There is an inclination with "compact" CO2 circuits to simply leave out a number of the valves that would nominally be on an industrial system, simply because the easiest way to manage charge during servicing is to dump it and renew it.
Thanks for your insight!...specifically were looking at the IT application. This sounds like a viable option for a high power density application and avoids the mixing of electrical and CHW inside the cabinets:
The chiller system appears to be designed around standard air conditioning parameters. The primary refrigeration system: R134a chiller supplying 43degF CHW absorbs about 10degF worth of heat from the secondary refrigerant-CO2. The secondary CO2 refrigerant system is self contained except for the interconnecting piping. One drawback as you mention is the high operating pressures ie. 50bar or 735psi. I suspect this turns the project into a hybrid type of HVAC piping project with the additon of process piping (hi pressure welded SS tubing) not to mention additional costs.
