PERFORMANCE ANALYSIS AND MULTI OBJECTIVE OPTIMIZATION OF MULTI STAGE VAPOUR COMPRESSION REFRIGERATION SYSTEMS

Abstract

Finding optimum operating conditions for maximum performance, minimum cost and energy consumption using environment-friendly refrigerants is now-a-days an important objective to be reached in the area of refrigeration. Energy, exergy, economic and exergoeconomic analyses play vital role in selection of refrigeration systems and suitable refrigerant. Multi-objective optimization paves the way to achieve optimal

thermodynamic performances with reasonable cost. This thesis explores the thermo- economic behaviour of basic and hybrid multi-stage refrigeration systems through

analysis and multi-objective optimization approach. The systems under analysis are: A two-stage refrigeration system with flash intercooler cum indirect sub-cooler, A cascade refrigeration system and A hybrid cascade refrigeration system which is a combination of two-stage and cascade system. Keeping in view the environmental hazards caused by refrigerants, only natural refrigerants have been included in the study. First multi-objective optimization of an ammonia based two-stage vapour compression refrigeration system incorporated with a flash intercooler cum indirect sub-cooler has been done. The study is carried out via thermo-economic optimization of the system in order to maximize the exergetic efficiency and minimize the total capital cost of the

system. Evaporator temperature, condenser temperature, subcooling parameter and de- superheating parameter are considered as the four design variables of optimization

problem. Multi-objective Genetic Algorithm (MOGA) is employed to carry out the optimization using MATLAB. TOPSIS decision making technique has been used to find unique solutions for five different weights of exergetic efficiency and total cost. The results reveal that exergetic efficiency and total capital cost of the system are

vi 41.76% and 223717.6 USD respectively at thermo-economic optimal operating conditions. Thereafter a comparative analysis based on multi-objective optimization of a cascade refrigeration system using NH3-CO2 and NH3-N2O refrigerant pairs has been done. Exergetic effciciency and overall cost rate has been taken as the two conflicting objective functions and evaporator temperature, condenser temperature, LTC condenser temperature and cascade heat exchanger temperature difference are the four design variables. The optimization work has been carried out using genetic algorithm

which results in non-dominated optimal solution represented in the form of pareto- optimal curve and TOPSIS method is used to select a unique solution. The results show

that NH3-N2O is a better alternative as compared to NH3-CO2 refrigerant pair from multi-objective optimization point of view. The corresponding optimal operating conditions has also been suggested for both the refrigerant pairs. The above work is followed by comparative energy, exergy and economic analysis of natural refrigerant couples working in a cascade refrigeration system incorporated with

a flash tank in its higher temperature cycle and a flash intercooler with indirect sub- cooler in its lower temperature cycle (Hybrid Cascade Refrigeration System). The

analysis is conducted using seventeen refrigerant couples namely R717, R290, R600a, R744, R744a, R170, R1150 and R1270. A comparison based on COP, exergy efficiency, system cost rates, and exergy destruction rate, to identify the best alternative refrigerant couple, is performed. Thermodynamic optimization has also been carried out with evaporator temperature, condenser temperature, LTC condenser temperature, cascade temperature difference, de-superheating and subcooling parameter of LTC intercooler, as six design variables. To evaluate the economic accountability of refrigerant couples a simple payback analysis has been done at the thermodynamic

vii optimal conditions of system. Results of analysis and optimization show that R717- R290 is the best refrigerant couple for this system from thermodynamic as well as economic point of views. The maximum COP and exergy efficiency obtained are 1.917 and 39.14% respectively and the corresponding total annualized system cost is 836395 $/yr for R717-R290 refrigerant couple. From the study is can be concluded that R717- R290 (ammonia-propane), R290-R1270 (propane-propylene) and R600a-R290 (isobutane-propane) are the best refrigerant pairs from thermo-economic point of view. Therefore an exergoeconomic analysis is done on a Hybrid Cascade Refrigeration

System (HCRS) using the three natural refrigerant pairs- ammonia-propane, propane- propylene and isobutane-propane. Thermodynamic optimization of system is carried

out with exergy efficiency as the objective function and evaporator temperature, condenser temperature, subcooling parameter, de-superheating parameter, LTC condenser temperature and cascade temperature difference as the six design variables. Overall system cost rate, component-wise exergy destruction rates, exergy efficiencies and exergo-economic factors are investigated for the three refrigerant pairs at

thermodynamic optimal operating conditions. The study concludes that ammonia- propane is the best performing refrigerant pair from exergetic and economic point of

view with an exergy efficiency of 39.14% and total cost rate of 836395 $/yr. Propane- propylene and Isobutane-propane give moderate and worst thermo-economic results

with 38.03% and 37.06%`of exergy efficiency and 1178000 $/yr and 2189000 $/yr of total system cost rate respectively. The exergo-economic factor is maximum in flash tank and minimum in EV II for all the three refrigerant pairs. Lastly a thermo-economic optimization and comparative analysis of HCRS using different natural refrigerant pairs is performed. Thermo-economic optimization is carried out to maximize the exergetic efficiency and minimize the overall cost rate. The

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optimization model involves six design variables which include subcooling and de- superheating parameters, LTC evaporation and condensation temperatures, HTC

condenser temperature and cascade temperature difference. The comparative analysis of twenty-two natural refrigerant pairs based on results of thermodynamic and economic optimizations reveal that R717-R290 is most efficient pair and R290-R1150 is least efficient refrigerant pair thermodynamically whereas R717-R1270 is the best and R600a-R290 is the worst pair economically. Seven potential refrigerant pairs are chosen via thermodynamic and economic optimization results and they are further compared based on their performances obtained through multi-objective optimization

(maximization of exergetic efficiency and minimization of total cost rate). Multi- Objective Genetic Algorithm (MOGA) is used for optimization which results in seventy

nondominated Pareto optimal solutions where TOPSIS method is used to select a unique solution for each refrigerant pair. A comparison of refrigerant pairs using these unique solutions shows that R717-R1270 is the best refrigerant pair for the cascade system under consideration. It is also found that R717-R1270 results in 7.77% rise in COP and 5.32 % reduction in overall cost when compared with NH3-CO2 refrigerant pair working under identical operating conditions.