A novel system based on a series of ORCs to recover cryogenic energy from the LNG regasification process for power and freshwater production

Abstract

Seawater desalination emerges as a feasible solution to solve the issues related to the freshwater scarcity generated by climate change. In the meantime, the energy transition promotes using Liquid Natural Gas (LNG) as a cleaner fuel. Usually, it is stored near marine areas at 113 K, therefore the recovery of part of the exergetic cold potential for freshwater production appears very appealing. In light of this, this study evaluates the energy and exergy performances of an innovative seawater desalination plant supplied by the cryogenic power released in the LNG regasification process. Specifically, three Organic Rankine Cycles (ORCs) arranged in series, operating between the seawater (hot source) and LNG (cold source) thermal levels, are investigated. Seawater releases heat to promote the evaporation of proper low-boiling fluids, cooling down to −5.62 °C and forming a two-phase mixture consisting of 50 wt% of pure ice and brine, so that the same liquid phase is employed for ice transportation in post-treatment devices. Simultaneously, the LNG is preheated by the low-boiling fluids’ condensation heat, allowing for the limitation of the thermal energy required for the phase change. Power generation is obtained by turboexpanders located between vaporizers and condensers. Eighteen different ORC configurations using diverse working fluids were analysed assuming steady-state conditions and ideal transformations. Considering a regasification terminal with a capacity of 8 billion Sm³ per year, the maximum power production amounts to 11.24 MW, recovering 13.33% of thermal exergy and producing 137.5 kg/s of freshwater. When desalination is privileged, 149.9 kg/s of freshwater can be produced, generating a power output of 7.02 MW with an exergetic recovery efficiency of 8.32%.