Therefore, according to Lee et al. (2006), a system with natural
refrigerants NH3 and CO2 satisfies these requirements. Many
efforts have been dedicated to increase the performance of
these cycles focusing totally on the reduction of the exergy
losses through the expansion valve. Getu and Bansal (2007)
theoretically investigated a cascade cycle with CO2 as the
refrigerant of a low temperature unit and different materials
such as R717, R290, R127,R404 as the agent of the high temperature
unit. They concluded that the condensation temperature
of the low temperature part plays an important role
on the efficiency of cycle. Getu and Bansal (2008) evaluated the
effects of three design parameters on the performance of the
cascade cycle with NH3 and CO2 as the refrigerants. Yuhala
and Wang (2004) stated that a compressed refrigeration
cycle had embedded ejector and CO2 as the refrigerant experiences
25% increase in COP as compared to conventional
cycle. Deng et al. (2007) presented a theoretical analysis of the
transcritical CO2 ejector expansion refrigeration cycle that
used the ejector as the main expansion device instead of an
expansion valve.