Abstract:

In order to decrease the energy consumption of the R404A／CO2 cascade refrigeration system, exergy analysis was applied to this system. An exergy analysis method was adopted to obtain  the influence of the design and operating parameters on the coefficient of performance ε, the exergy destruction X and the exergetic efficiency ηe of this system. The design and operating parameters include the evaporating temperature Te, the condensing temperature Tk and the temperature difference ΔT in the cascade-condenser. The results indicate that there were a maximum ε, ηe and a minimum Xtot at the same optimal condensing temperature of the cascade-condenser T4opt when Te, Tk and ΔT were constant in the system. It is helpful to reduce X and improve ηe  and ε through increasing Te, decreasing Tk and ΔT. The total exergy destructions of the throttling device and the compressor of the R404A circuit, the cascade-condenser and the compressor of the CO2 circuit were  80％ of the total exergy destruction of the system. A Multiple linear regression analysis was employed in term of  Te, Tk and ΔT to develop mathematics expressions for t4opt, εmaxand ηe,max.

Key words: CO2, cascade refrigeration system, exergy destruction, exergetic efficiency