Exergoeconomic and environmental analyses of CO2/NH3 cascade refrigeration systems equipped with different types of flash tank intercoolers

Abstract

Research Highlights• CO 2 /NH 3 cascade refrigeration cycles with flash intercoolers are investigated.• Exergoeconomic factors of components are determined to assess their relative significances.• An environmental analysis is applied to determine the penalty cost of GHG emission.• The effects of operating parameters on COP, exergy efficiency and total cost rate are investigated.• An optimization is applied based on the maximum COP and the minimum total cost rate. 1© 2016 This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Abstract: 11Exergoeconomic and environmental analyses are presented for two CO 2 /NH 3 cascade 12 refrigeration systems equipped with 1) two flash tanks, and 2) a flash tank along with a flash 13 intercooler with indirect subcooler. A comparative study is performed for the proposed 14 systems, and optimal values of operating parameters of the system are determined that 15 maximize the coefficient of performance (COP) and exergy efficiency and minimize the total 16 annual cost. The operating parameters considered include condensing temperatures of NH 3 in 17 the condenser and CO 2 in the cascade heat exchanger, the evaporating temperature of CO 2 in 18 the evaporator, the temperature difference in the cascade heat exchanger, the intermediate 19 pressure of the flash tank in the CO 2 low-temperature circuit, the mass flow rate ratio in the 20 flash intercooler and the degree of superheating of the CO 2 at the evaporator outlet. The total 21 annual cost includes the capital, operating and maintenance costs and the penalty cost of 22 GHG emission. The results show that, the total annual cost rate for system 1 is 11.2% and 23 2 11.9% lower than that for system 2 referring to thermodynamic and economic optimizations, 24 respectively. For thermodynamic and cost optimal design condition the COP and exergy 25 efficiency of both systems are almost the same. Finally, in order to obtain the best balance 26 between exergy destruction cost and capital cost, the exergoeconomic factor is defined for 27 each component of proposed systems, for cases in which the system operates at the best 28 performance conditions. 29 Keywords

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