Comparative study of Rankine cycle configurations utilizing LNG cold energy under different NG distribution pressures

NG (Natural gas) distribution pressure varies in a wide range in different applications and has significant effect on system exergy efficiency. RC (Rankine cycle) and DEC (direct expansion cycle) can utilize the cold and pressure exergies of LNG (liquefied natural gas), respectively. In this paper, SRC (single-stage RC), PTRC (parallel two-stage RC) and CTRC (cascade two-stage RC) with and without DEC are optimized by Particle Swarm Optimization and compared to obtain the optimal configurations for different NG distribution pressures. Propane, propylene, ethane and ethylene are adopted as the working fluids. The results show that the addition of DEC could increase the exergy efficiency by lifting the evaporation temperature of LNG. CTRC + DEC with propane/ethylene and CTRC + DEC with propane/ethane as the working fluid achieve the largest exergy efficiencies of 23.89% and 18.25% at NG distribution pressures of 1 MPa and 2 MPa, respectively. PTRC + DEC with propane/ethane is the optimal system with maximum exergy efficiencies of 15.18%, 13.24%, 11.7% and 10.4% at NG distribution pressures of 3, 4, 5 and 6 MPa, respectively. PTRC + DEC with ethane as the working fluid in both cycles is a generally suitable system for all NG distribution pressures with high efficiency and only one type of working fluid.