Thermodynamic optimization and comparative study of different ORC configurations utilizing the exergies of LNG and low grade heat of different temperatures

ORC is a good solution to recover the cold exergy of LNG (liquefied natural gas) and the thermal exergy of low grade heat simultaneously. The optimal working fluids and parameters change greatly with different configurations and heat source temperatures. In this paper, three different ORC configurations: SORC (single-stage ORC), PTORC (parallel two-stage ORC) and CTORC (cascade two-stage ORC), combined with DEC (direct expansion cycle) are analyzed and compared. Up to seven key parameters of sixty four fluid combinations under four heat source temperatures are optimized by the particle swarm optimization. The results show that PTORC is more suitable for lower heat source temperatures and CTORC performs better for higher heat source temperatures. PTORC+DEC with ammonia/ethane as the fluid combination yields the maximum efficiency of 17.36% for the heat source temperature of 50 °C. CTORCs without DEC, also with ammonia/ethane as the fluid combination, achieve the maximum efficiencies of 19.49%, 21.69% and 24.37% for the heat source temperatures of 100, 150 and 200 °C, respectively. The optimal turbine inlet temperatures are usually their upper bounds and the optimal condensation temperatures are the normal boiling temperatures of the fluids. There are optimum evaporation temperatures for all configurations, which increase with the heat source temperature.