Two-stage Transcritical Cycle Research Articles

Exergy analysis of a vortex tube expansion two-stage transcritical N2O refrigeration cycle

In the present article, two-stage transcritical cycle with expansion valve (TSTCEV) and two-stage transcritical cycle with vortex tube (TSTCVT) are compared mainly on the basis of the second law of thermodynamics. Natural refrigerant nitrous oxide (N2O) is used in both cycles for thermodynamic analysis. The gas cooler and evaporator temperatures are varied from 35°C to 55°C and −55°C to 5°C, respectively. The effects of various operating parameters on the optimum intercooler and gas cooler pressures, the coefficient of performance (COP), exergy destruction and the exergetic efficiency are presented. A component-wise exergy analysis of TSTCEV and TSTCVT shows that the use of the vortex tube instead of the expansion valve reduces the total exergy destruction and increases the exergetic efficiency as well as COP. The exergetic efficiency of TSTCVT is on average 9.17% to 11.68% higher than that of TSTCEV for considered operating conditions. Furthermore, the optimum intercooler and gas cooler pressures in TSTCVT are lower than those of TSTCEV. These results offer significant help for optimum design and the selection of appropriate operating conditions of TSTCVT using refrigerant N2O.

Effects of Intercooling and Inter-Stage Heat Recovery on the Performance of Two-Stage Transcritical CO2 Cycles for Residential Heating Applications

Due to the harmful effects of synthetic refrigerants, such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs)n the environment, natural refrigerants like carbon dioxide (CO2) have been attracting great interest. The higher inter-stage superheating of CO2 makes it difficult to predict the effects of the intercooling on heating performance of a two-stage transcritical CO2 cycle. In addition, very little is known about the potential of inter-stage heat rejection recovery in the heating performance enhancement of this cycle. In order to explore the effects of intercooling and inter-stage heat rejection recovery potential, three “sub-cycles”—(1) a sub-cycle with heat recovery, (2) a sub-cycle without heat recovery, and (3) a sub-cycle without intercooling—were modeled in Engineering Equation Solver (EES) software for three commonly-used two-stage transcritical cycles: (1) an intercooler cycle, (2) a flash cycle, and (3) a split cycle. Then, the discharge pressure and intermediate pressure were simultaneously optimized. Based on the optimization results, the heating performance of the sub-cycles for each cycle were compared. The results demonstrate that the incorporation of intercooling without heat recovery was detrimental to the heating performance in comparison to the absence of intercooling. It is also clear that there is a great potential for heating performance improvement through inter-stage heat recovery.

Performance characteristics of a two-stage transcritical N2O refrigeration cycle with vortex tube

ABSTRACTThe thermodynamic analysis has been presented in this article using nitrous oxide as the refrigerant in a two-stage transcritical cycle with the vortex tube (TSTCVT) instead of the expansion valve and its results are compared with the two-stage transcritical cycle with the expansion valve (TSTCEV). The evaporator and the gas cooler temperature ranges in both the cycles have been considered between −55°C to 5°C and 35°C to 60°C for the analysis. Gas cooler and intercooler pressures are simultaneously optimised to obtain the maximum cooling coefficient of performance (COP). The COP of the TSTCVT improves by 1.97–27.19% in comparison to TSTCEV. A decrease in evaporator temperature and an increase in gas cooler exit temperature reduce the COP of TSTCVT. The comparison of refrigerants N2O and CO2 in TSTCVT shows that N2O exhibits higher cooling COP, higher second law of efficiency and lower optimum gas cooler pressure under the considered operating conditions.