THERMAL OPTIMIZATION OF REFRIGERATION SYSTEMS

Abstract

This study examines the thermodynamic assessment and multi-objective optimisation of a modified vapour compression refrigeration cycle (VCR) in both subcritical and transcritical modes. The cycle modification is achieved through the use of a vortex tube in the VCR, with the objective of enhancing system performance. The efficiency of a standalone vortex tube is low; however, its integration into a thermodynamic cycle or VCR system enhances the coefficient of performance (COP) and reduces exergy losses, particularly when the cycle operates within the transcritical zone. An economic analysis of the modified VCR cycle with a vortex tube has been conducted to compare it with the base case. Moreover, this research conducted a computational fluid dynamics (CFD) study of a standalone vortex tube to enhance its temperature separation phenomenon. In this research, thermodynamic performance of a vortex tube integrated single-stage vapour compression refrigeration cycle (VTC) has been evaluated in a subcritical region. The thermodynamic evaluation consists of an energy and exergy analyses for VTC in order to determine the effect of various design and operating parameters on its performance. Moreover, the results of VTC are compared with those of simple VCR for the considered range of evaporator and condenser temperatures using R1234yf as the refrigerant. The analysis reveals that the cooling capacity of VTC is 14.5% to 49.7% higher than that of VCR for the considered range of condenser temperature. Also, the maximum COP of VTC is 5.6% to 27.3% higher than that of VCR. The exergetic efficiency of VTC is 5.6% to 27.3% higher than that of VCR. A multi-objective optimization using a genetic algorithm has been conducted, which suggests that evaporator temperature is the major decisive parameter to find the suitability of various applications of the system based on VTC. Further, this research deals with the thermodynamic investigation of vortex tube coupled with trans-critical vapour compression refrigeration cycle (TVTC) using Carbon-Dioxide, followed by environmental analysis and multi-objective optimization. In this study, effect of various operating and design parameters is studied on the performance of TVTC. Furthermore, a comparison is made between the outcomes of TVTC and simple trans-critical vapour compression refrigeration cycle vi (TVCR). Results show that the optimum gascooler pressure for TVTC is observed to be lower than that of TVCR. Also, the cooling capacity and COP of TVTC are observed to be 10.1% to 21.1% and 2.3% to 11.3%, respectively, greater than those of TVCR. Moreover, the exergetic efficiency of TVTC is 2.3% to 11.3% higher than that of TVCR for the investigated range of evaporator and gascooler exit temperatures. The environmental penalty cost (per unit cooling capacity) of TVTC is 3.5% to 12.2% lower than that of TVCR. Furthermore, the coefficient of structural bond is calculated in order to choose the most sensitive parameters for system’s performance. Additionally, genetic algorithm-based multi-objective optimization has been performed, with the evaporator temperature serving as the primary determining factor in establishing the optimal solution. This finding can guide the development of TVTC- based systems for a wide range of applications. Nevertheless, this study conducts an economic analysis of TVTC and compares it with the base case, TVCR. For the economic analysis, three cost components have been considered: capital and maintenance cost, operational cost, and environmental penalty cost. The sum of all three components constitutes the total plant cost. The influence of parameters including evaporator temperature, gas cooler exit temperature, and cooling load on the individual components of cost and the overall plant cost rate has been analysed. Results indicate that the plant cost rate of TVCR is 6.8% to 7.3% higher than that of TVTC and plant cost rate of VCR is 7.5% to 17% higher than that of VTC. This supports the use of vortex tube in transcritical vapour compression refrigeration cycles from an economic standpoint. While the application of vortex tubes in refrigeration cycles shows benefits in thermal performance, standalone vortex tubes are noted for their low efficiency in temperature separation. Therefore, efforts have been undertaken to analyse the thermal performance of a standalone vortex tube through a CFD study utilising compressed air as the working fluid. This study examines the impact of geometrical modifications on the temperature drop at the cold exit of a vortex tube. The influence of varying air entry angles (ranging from 0⁰ to 5⁰) on the cold exit temperature of the vortex tube has been examined. A three-dimensional solid model of a vortex tube has been created, and the vii standard k-epsilon model is employed to conduct the simulation in ANSYS FLUENT 2022 R1. The maximum temperature reduction occurs at the cold exit when air enters the VT at angles of 2° and 3°. The minimum cold end temperature achieved is approximately 262 K, representing an improvement of 2 K to 3 K compared to the base case (i.e., air entry angle at 0°).