Performance Analysis of Irreversible Combined Refrigeration Cycles Based on Finite Time Thermodynamic Theory

This paper provides a performance analysis of combined refrigeration cycles. A theoretical survey based on finite time thermodynamic (FTT) analysis was presented to evaluate the operating performance of two different combined irreversible vapour compression refrigeration cycles: the conventional cascade (CC) and the integrated cascade (IC). The purpose here is to appreciate a new FTT investigation by stage (FTTS) for complicated architecture systems like the IC system that can’t be divided into separate cycles. In this FTTS approach, the system will be divided into n stages where each stage is defined by two consecutive heat exchangers. We began by detailing finite time thermodynamic analyses by cycle (FTTC) and by stage (FTTS) for the CC system. Then FTTS analysis was applied for the IC systems to derive a simple expression for the coefficient of performances (COP). Numerical examples are given to evaluate effects of internal irreversibility and intermediate heat-exchanger temperatures on the coefficient of performance of the CC and IC systems. It was found that at fixed condensing and evaporating temperatures and for same intermediate heat-exchanger temperature ratio, the IC is more efficient than the CC system. The coefficient of performance of the IC system can be more enhanced under the condition of minimum intermediate heat exchanger irreversibilities (less glide between the two counter-flow intermediate heat exchanger)