In Europe, automotive air conditioners using CO2 as a working fluid may replace current equipment based on R134a within the next decade.
In this problem, you will carry out a simple thermodynamic analysis of this process, with the evaporator operating at 0C and the condenser operating at 40C (as in the previous problem using R134a), using the attached PH diagram (labeled "R744".) Assume a perfectly efficient compressor.
a) In contrast to R134a, it is not obvious how to choose the operating pressure for the condenser. Briefly explain this statement.
b) Analyze the process assuming condenser pressures of 1) 80 bar, 2) 100 bar, and 3) 150 bar. Draw all three processes on the PH diagram, and present PTHS tables for each process.
c) Which condenser pressure for CO2 gives the highest COP? How does this value compare to the COP computed for R134a in the previous problem (neglecting finite compressor efficiency)?
d) What is the major practical challenge in using CO2 as a refrigerant, compared to R134a?