Thermal Analysis and Multi-Objective Optimization of Cascaded and Advanced Absorption Refrigeration Technologies

Abstract

This thesis presents an in-depth analysis of advanced modifications of absorption refrigeration systems, with the primary aim of enabling these systems to operate at reduced evaporator temperatures while achieving higher performance. This detailed study marks a significant advancement in refrigeration technology, specifically in the realm of cascade compression absorption refrigeration systems and the advancement of standalone ARC. The goals of this research are to collectively address critical challenges faced by traditional refrigeration cycles, such as energy inefficiency, high compressor power requirements, and environmental concerns, through a comprehensive approach.

The research encompasses the development and simulation of sophisticated cascade compression-absorption refrigeration setups and novel stand-alone absorption system frameworks. Initially, the study focuses on the integration of modified ARC (Absorption Refrigeration Cycle) and advanced RAC (Recompression Absorption Cycle) with enhanced VCRs, incorporated with an ejector to develop advanced proposed novel cascaded configurations: Ejector Compression Absorption Cycle (ECAC), Ejector Injection Compression Absorption Cycle (EICAC), Ejector-Compression Recompression Absorption Cycle (E-CRAC) And Ejector enhanced vapor-Injection Compression Recompression Absorption Cycle (EI-CRAC). Furthermore, the study pioneers the adaptation of novel stand-alone absorption system frameworks, incorporating ejector-injection and recompression technologies to develop Refrigerant Ejector enhanced Recompression Absorption Cycle (RE-RAC) and Vapor Injection enhanced Recompression Absorption Cycle (VI-RAC). Both the advanced cascaded and stand-alone configurations undergo extensive analysis from energy and exergy perspectives, coupled with multi-objective optimization. Utilizing Artificial Neural Network (ANN)-based predictive models, the research meticulously assesses thermal performance, establishing optimal operating conditions and identifying operational limits. This comprehensive evaluation offers profound insights into the systems' behaviors across a spectrum of conditions, enriching our understanding of their potential and constraints in various application scenarios.

The findings reveal that the proposed systems significantly outperform traditional systems in terms of Coefficient of Performance (COP) and exergy efficiency. Specifically, ECAC and EICAC systems achieve approximately 15% and 6% higher COP, respectively, compared to conventional cascade systems when using the R41-LiBr/H2O refrigerant. Additionally, EICAC and ECAC show significant improvements in exergy efficiency, up to 20% and 10%, respectively, with optimal performance around 77℃ generator temperature. Furthermore, the research explores RAC based proposed cascaded systems: one basic CRAC and two advanced configurations: E-CRAC and EI-CRAC. They significantly outperform the traditional CARC system, with the COP being nearly three times higher. EI-CRAC and E-CRAC show a COP enhancement of about 10% and 20%, respectively, along with an increase in exergy efficiency of 15% and 25% over CRAC, indicating superior efficiency in cooling operations. Finally, this research introduces novel stand-alone recompression absorption refrigeration systems integrating ejector-injection setup to replace expansion valves (RE-RAC and VI-RAC). RE-RAC and VI-RAC significantly outperform conventional ARC and RAC systems. The COP of RE-RAC and VI-RAC is 76% and 63% higher than the conventional RAC system, respectively, despite RE-RAC requiring more external heat generation due to VI-RAC’s additional compressor demands.

This research contributes novel insights into the field of refrigeration by analyzing the integration of advanced absorption and compression technologies, providing a pathway for the development of more efficient and environmentally friendly refrigeration systems. The comprehensive analysis from both energetic and exergetic perspectives offers valuable guidance for future improvement and optimization, potentially revolutionizing cooling applications with lower environmental impact. Implementing these systems in real-life scenarios, such as power plants and various industries (e.g., textile, manufacturing, steel), can enhance waste heat utilization by achieving lower evaporator and generator temperatures with higher performance, making them suitable for efficiently using low-grade energy.