Cascaded Organic Rankine Cycles (ORCs) for Simultaneous Utilization of Liquified Natural Gas (LNG) Cold Energy and Low-Temperature Waste Heat

Abstract

Liquified Natural Gas (LNG) is a good way to transport natural gas from suppliers to end consumers. LNG contains a huge amount of cold energy due to the energy consumed in the liquefaction process. Generally, the LNG cold energy is lost during the regasification process at the receiving terminal. Power generation with LNG as the heat sink is an energy-efficient and environment-friendly way to regasify LNG. Among different kinds of power generation technologies, Organic Rankine Cycle (ORC) is the most promising power cycle to recover LNG cold energy. ORC has been widely used to convert low-temperature heat into electricity. If low-temperature waste heat and LNG cold energy utilization are utilized simultaneously, the efficiency of the whole system can be improved significantly. However, due to the large temperature difference between the low-temperature waste heat source and LNG, one stage ORC cannot exploit the waste heat and LNG cold energy efficiently. Therefore, a cascaded ORC system is proposed in this study. The optimization of the integrated system is challenging due to the non-convexity and non-linearity of flowsheet and the thermodynamic properties of the working fluids. A simulation-based optimization framework with Particle Swarm Optimization algorithm is adopted to determine the optimal operating conditions of the integrated system. The maximum unit net power output of the integrated system can reach 0.096 kWh per kilogram LNG based on the optimal results.