Thermodynamic analysis for working fluids comparison in Rankine-type cycles exploiting the cryogenic exergy in Liquefied Natural Gas (LNG) regasification

Most regasifying plants waste the cold exergy stored in Liquefied Natural Gas (LNG), which could otherwise be utilized for power production with the advantage of displacing fossil-fuel. Although thermodynamic assessments of appropriate cycles using LNG as heat sink can be found in the literature and for various working fluids, their direct comparison is difficult or impossible as they obey to different constraints. This paper contributes to fill this gap. It considers simple Rankine type cycles and direct expansion, as well as the combination of both. These cycles are thermodynamically modelled and systematically computed under the same boundary conditions, for a set of working fluids. A multi-objective optimisation by using genetic algorithms was carried out seeking both the maximum net electric power and the minimum heat exchanger capacity. Plotting the optimised Pareto front curves for each fluid helps in reaching a customised design from compromises between power production and cost. The case study coastal plant (Sines, Portugal) currently consumes around 1.2 MW. Upgrading with just a direct expansion would save 830 kW, while a Rankine cycle could produce up to 2 MW of net power. Adding a direct expansion to a Rankine cycle will only marginally improve the edging power production.