The use of CO2 in supermarket refrigeration started with applying CO2 as the working
fluid in indirect systems for freezing applications. Then it has been applied in cascade
solutions mainly with R404A in the high temperature stage. After gaining experience
and the availability of components CO2 has been used in trans-critical system solutions,
mainly in northern Europe. The two main trans-critical system solutions applied are the
parallel and the booster, the later is the latest in CO2 systems development series and
has been applied in most of the installations in Sweden.