Analysis and optimization of cascade Rankine cycle for liquefied natural gas cold energy recovery

This study proposes a new concept called the cascade Rankine cycle, which recovers LNG (liquefied natural gas) cold energy for power generation, optimizes the cycle by investigating the effects of key parameters on its performance, and compares its thermal efficiency, exergy efficiency and economic evaluation to those of the conventional alternatives. The cascade Rankine cycle consists of multiple stages of the organic Rankine cycle in a layered structure in which the first stage encompasses the second one that, in turn, encompasses the next. Due to its layered configuration, optimization of the cycle is straightforward and involves sequentially optimizing the individual stages. Optimization of the subsequent stages, however, required process simulation considering the equipment efficiency and the thermodynamic properties of the working fluid. Process simulation indicated that the indicators such as net power output, thermal efficiency, and exergy efficiency generally increase as the number of stages increases. These indicators were, however, significantly affected by the thermodynamic properties of the working fluids. The proposed cycles demonstrated significantly better performance in these indicators than the conventional cycles. The three-stage cascade Rankine cycle with propane as the working fluid exhibited the highest net power output, thermal efficiency and exergy efficiency within the set.