Cryogenic energy-assisted Organic Rankine Cycles with low-flammability refrigerants: System charge effects and environmental performance

Cryogenic carbon-neutral fuels offer a dual opportunity for energy recovery, releasing thermal energy during combustion and cold energy during regasification. This study presents a dynamic model of a cryogenically enhanced Organic Rankine Cycle (ORC) that simultaneously utilizes both heat sources while addressing safety challenges associated with conventional hydrocarbon working fluids. Low-flammability hydrofluorocarbons (HFCs) were introduced as alternative working fluids to improve system safety without compromising performance. Parametric simulations were conducted to evaluate the influence of evaporation temperature and working fluid charge on energy, exergy, and cryogenic efficiency. The results revealed that energy efficiency decreases with higher evaporation temperature, while power output increases. The system exhibited resilience to undercharging up to a certain liquid-to-volume ratio, but overcharging caused significant performance degradation. Using R452B as the working fluid achieved a peak specific work output of 45.64 kJ/kg, thermal efficiency of 10.43%, exergy efficiency of 12.75%, and cryogenic energy efficiency of 11.8%. Scenario-based environmental analysis, aligned with Marine Classification Society criteria, demonstrated that employing low-flammability fluids can reduce greenhouse gas emissions by up to 85% compared to conventional marine engines. The findings confirm that low-flammability HFCs are viable and safer alternatives for cryogenic energy-assisted ORC systems in marine and low-temperature power applications. • Cryogenic-assisted ORC modeled using low-flammability HFC working fluids. • Evaporation temperature raises power output but lowers thermal efficiency. • System shows strong tolerance to undercharging but degrades when overcharged. • R452B delivers peak work of 45.64 kJ/kg and highest overall efficiency. • Low-flammability fluids cut greenhouse emissions by up to 85% in marine use.