The study of a novel two-stage combined rankine cycle utilizing cold energy of liquefied natural gas

Abstract

In order to save energy and protect the environment, the cold energy of liquefied natural gas and industrial waste heat are utilized in the Rankine cycle. In this study, a novel two-stage Rankine cycle that combines the advantages of the cascade two-stage Rankine cycle, parallel two-stage Rankine cycle, and regenerative Rankine cycle was proposed, in which the cold energy of LNG is utilized step by step. Ethane + R32, ethane + propylene, R23 + propylene, and R23 + R32 are selected as the working fluid combinations. The performance of working fluid is compared by studying the cycle performance under different parameters, and the cycle parameters are optimized through a genetic algorithm. The results show that Ethane + R32 is the best combination, with a higher exergy and thermal efficiency in most cases; further, ethane + propylene achieved the maximum exergy and thermal efficiency of 31% and 20.8%, respectively. When the temperature of the heat source is changed from 303 K to 323 K, the exergy efficiency of the cycle varies by approximately 16%. Compared with the parallel two-stage Rankine cycle + direct expansion cycle and the cascade two-stage Rankine cycle + direct expansion cycle, the exergy efficiency of the novel cycle increases by 104% and 29.76%, respectively.