Integration of a three-stage expander into a CO2 refrigeration system

The inclusion of an expander with work recovery provides two advantages for transcritical CO2 refrigeration cycles: the COP is improved and the exhaust pressure of the main compressor is lowered. Several designs of expanders have been proposed for this application and some prototypes have been tested already. In our laboratory a three-stage expander has been developed, which replaces the throttle valve of the normal refrigeration cycle and expands into the two-phase region. For optimum integration into the overall system it is proposed to install a vapour-liquid separator between the second and third stage of expansion. The vapour is guided back to the third expander stage whereas the liquid is supplied to the cooling stations via thermostatic or electronic expansion valves.