Comparison of working fluids and cycle optimization for heat recovery ORCs from large internal combustion engines

This paper addresses the optimal working fluid selection for organic Rankine cycle recovering heat from heavy-duty internal combustion engines. Four cases are considered featuring two different engine exhaust temperatures (245 °C vs 354 °C) and two scenarios (maximum recovery of mechanical power vs. cogeneration of low-temperature heat). The analysis includes both pure fluids, including recently synthesized refrigerants, and binary mixtures. To perform a fair comparison between the different fluids, a computationally efficient cycle optimization approach, able to determine the maximum achievable efficiency for each working fluid, is adopted. The approach combines the evolutionary optimization algorithm PGS-COM with a rigorous heat integration methodology. The most efficient fluids are HCFO-1233zde, HFE-245fa2, HFO-1336mzz, HFE-347mcc, HFE-245cb2 and Novec 649 for the engine with lower temperature exhausts (reaching an ORC mechanical efficiency of 18.6–19.9%), and cyclopentane, ammonia, HCFO-1233zde, HFE-245fa2, HFO-1366mzz for the engine with higher temperature (reaching 23.76–22.70% efficiency). Compared to pure fluids, the use of optimized binary mixtures does not appear to lead a considerable efficiency gain.