CO2 in A/C systems

Not a good thermal conductor conductor. Hydrocarbons are the way to go. Less impact on the environment than CFC's. We use propane.

I think you answered your own question. Its not used because it freezes when it expands. That tends to clog up the system since you cannot pump a solid. Ammonia and the various CFC's stay in their liquid state and the cooling is provded by the latent heat of vaporization when they expand. Its simply a matter of using a 2-phase system rather than a 3-phase.

CO2 can be used very successfully as a refrigerant and a lot of research has gone into CO2 refrigeration systems. Check out www.Modine.com to see the work that has been done on it. Also, see the SAE.org web site:

Alternative Refrigerant Systems Symposium
June 10-12, 2008, Resort Suites, Scottsdale, Arizona, USA

The main issues with CO2 refrigeration systems becoming a reality are:

infrastructure - everyone is heavily invested in systems for R134a and the CO2 systems are very different from the R134a systems, so there is a substantial investment that has to be made when going to CO2.

compressor life - CO2 systems run at much high pressures than other refrigerants, so compressor life is much more of an issue than with R134a or R22 systems.

Leak tightness of the systems - because CO2 molecules are much smaller than R134a molecules and the pressures are higher. System hoses, connections, pump seals, etc. must be capable of handling these higher system pressures and not leak - far more challenging than with R134a systems - but doable.

From an environmental impact standpoint, because CO2 is typically taken out of the environment to produce the gas that goes into these systems - any leakage as a zero environmental impact because the gas is going back to where it came from.

I think that CO2 systems would start coming on the market very fast if goverments started outlawing R134a and R22 which are the main refrigerants used today.

because you have operate at pressurres way over 2000 psi.