THERMODYNAMIC AND EXERGY ANALYSIS OF THERMAL SYSTEM

Abstract

In the thesis, the refrigeration and air-conditioning systems having low carbon emission and eco-friendly relationship have been examined and evaluated. An attempt has been made to synthesise the vapour compression and absorption cycles for the investigation of energy and exergy performance. The subcooling methods viz. liquid vapour heat exchanger incorporation to vapour compression refrigeration (VCR) system and dedicated mechanical subcooled VCR system have been investigated using R134a and alternate refrigerants R1234yf and R1234ze. An experimental VCR setup has also compared to simple theoretical VCR system in order to examine the performance and compatibility of alternative refrigerants. The results depict that the subcooling methods improve coefficient of performance (COP) and exergy efficiency of the VCR system. The performance of R1234ze is better than that of R1234yf and it competes with R134a. The low grade energy (waste heat) available at temperature (i.e. within 60-2000C) is utilized as source in the various cascade configurations of vapour compressionabsorption refrigeration cycles. The objectives of the present work are reduction in high grade energy (electrical energy) consumed by the compressor consequently, reduction in carbon emission and global warming potential. Simultaneously, the cascading effect produces lower temperature for the deep freezing and chilling applications. For achieving the dual objectives, the energy and exergy analysis of absorptioncompression cascade refrigeration cycles along with wide range of parametric analysis have been carried out by developing engineering equation solver (EES) software based computer programs. The waste heat available at higher temperature (165-2000C) has been utilized in compression-absorption (triple effect series flow H2O-LiBr) cascade refrigeration cycle while the low temperature (65-950C) waste heat has been utilized in compressionabsorption (single and half effect series flow H2O-LiBr) cascade refrigeration cycle. The low global warming potential (GWP) and zero ozone depleting potential (ODP), hydrofluoroolefin (HFO) refrigerant R1234yf has been proposed as a working fluid for vapour compressor refrigeration system. The various configurations of cascade refrigeration system have been compared to vapour absorption and compression systems separately to examine the thermodynamic performance. The results depict that COP and iv exergetic efficiency of single effect H2O-LiBr absorption-compression cascade refrigeration system (ACCRS) are highest and lowest for half effect ACCRS. The total exergy destruction is lowest for single effect ACCRS and highest for half effect ACCRS. It has also been observed that the energy and exergy performance of R1234ze is better than that of R1234yf in the VCR section of absorption-compression (single effect H2OLiBr) cascade refrigeration system. The elecricity consumption of half and triple effect cascade refrigeration systems is reduced by 54.29 % and 45.84% than that of convensional vapour compression referigeration cycle. The enhancement in COP by 85.26% and 118.7% in triple and half effect cascade refrigeration systems respectively. The enhancement in the exergetic efficiency of VCR cirucit of triple effect absorption compression cascade refrigerion is 85.28% and the reduction in tota exergy destruction is 70.8% and in half effect absorption compression cascade refrigeration system, enhancement in the exergetic efficiency of VCR cirucit is 118.74% and the reduction in tota exergy destruction is 80.66% . For simultaneous production of power, heat and refrigeration, tri-generation power plants having absorption compression cascade refrigeration system are examined and analysed for their performance improvement using energy and exergy principles. The results depict that the COP and exergetic efficiency of the cascade refrigeration system powered by micro-gas turbine improve over conventional VCR and vapour absorption refrigeration (VAR) system along with reduction in consumption of high grade energy (electricity). Additionally, retrofitted technologies viz. evaporative cooling (EVC) of inlet air to compressor and steam injection to gas turbine (STIG) are also investigated for performance improvement of gas turbine. The results highlight that the use of retrofitted techniques enhances power output by 60.79% and generation efficiency by 28.4% of simple gas turbine cycle. The current work is worthwhile to develop and design the energy efficient, eco-friendly advanced cooling cycles and thermal power plants.