Thermodynamic improvements of LNG cold exergy power generation system by using supercritical ORC with unconventional condenser

Abstract Organic Rankine cycle (ORC) is the most commonly used power generation cycle that is adopted in this paper to make full use of the cold exergy of liquefied natural gas (LNG). As the heat exchangers are mainly responsible for the exergy destruction of the system, two improvement techniques: supercritical system and unconventional condenser, are proposed to decrease the exergy destructions in steam generator and condenser. The performances of ten preselected working fluids are tested by optimizing five key parameters at three heat source temperatures. The results show that using the supercritical system can improve the system efficiency for fluids with low critical temperatures by reducing the exergy destructions in steam generators. Using the unconventional condenser can greatly improve the system efficiency by decreasing the exergy destruction during the LNG regasification process of most fluids, barring a few wet expansion fluids. The improvements offered by both techniques become more obvious as the heat source temperature increases. However, combining the two techniques only improves the performance for fluids with low critical temperatures, and does not work for most fluids. The highest efficiency is always achieved by the subcritical system with an unconventional condenser at each heat source temperature. When the heat source temperatures are 100 °C and 150 °C, R290 obtains the highest efficiency values of 17.89% and 21.26%, respectively. At 200 °C, R600a achieves the highest efficiency of 25.35%.