Thermoeconomic Assessment of Zeotropic Mixtures in Hybrid Organic Rankine Cycle and Direct Expansion Systems for Power Generation from LNG Cold Energy Recovery

The regasification of liquefied natural gas (LNG) releases 750 kJ/kg of cold thermal energy, evaluated for LNG heated from storage conditions (−162 °C) to ambient temperature (15 °C) at the 75 bar send-out pressure. Extending previous work on single-, two-, and three-stage Organic Rankine Cycles (ORC) using pure working fluids, this study optimizes zeotropic working fluid mixtures and introduces a hybrid configuration coupling two-stage ORC and direct expansion (DE). Unlike pure working fluids, zeotropic mixtures improve the thermal matching between the condensation and the sensible heat LNG stream and reduces heat transfer irreversibility. Binary candidates such as R32/R161, R41/R161 and ethane/propane were screened under environmental (GWP, ODP), safety (toxicity, flammability), and operational constraints, including a minimum condenser pressure of 0.15 bar and a pinch-point temperature difference of 10 K. Across LNG regasification capacities of 200,000–800,000 Sm³/h and a baseline seawater temperature of 20 °C, two-stage ORCs achieve net energy efficiencies of 12.8–15.1% and net exergy efficiencies of 29.0–32.5%. The optimum mixture is R32/R161 at 0.55/0.45 mass fractions delivering a specific net power of 96 kW/kg_LNG/s), a levelized cost of electricity (LCOE) of 80 €/MWh, and a specific investment cost (SIC) of 6000 €/kW. Costs follow the Turton bare module method with Chemical Engineering Plant Cost Index (CEPCI) correlations, a 4% discount rate, 25-year lifetime, 7000 operating hours per year, and 130 €/kW annual operation and maintenance (O&M) costs. The hybrid ORC + DE system, with LNG pumped to 95 bar and expanded to the 70 bar send-out pressure, raises the specific net power to 112–125 kW/(kg_LNG/s), combined efficiency to 15.6–17.4%, and peak power to 17.5 MW at 800,000 Sm³/h. A seawater sensitivity analysis (10–30 °C) shows up to 30% variation in net power. Overall, two-stage ORC + DE offers the best thermoeconomic performance.