Combined cascaded Rankine and direct expander based power units using LNG (liquefied natural gas) cold as heat sink in LNG regasification

• Efficient electric power at ambient temperature in a LNG (liquefied natural gas) regasification plant.
• Using the LNG regasification task as the power cycle heat sink.
• Two cascade Rankine cycles combined with a direct expansion turbine.
• Argon and methane for the cascaded Rankine cycles and NG (natural gas) for the direct expansion turbine as working fluids.

The main objective of this research work is to generate electric power efficiently using the cold exergy from the regasification task of LNG (liquefied natural gas) regasification plants. Thus, the article aims to propose and analyse an unconventional efficient power plant composed by cascade Rankine cycles, combined with a direct expansion power unit. The rejected heat from each cascade power unit is used to heat the liquefied natural gas in a regasification plant. The analysis of the proposed cycle is carried out by comparing some recent contributions with the proposed cascade Rankine based power plants in which argon and methane have been used due to their inherent condensation characteristics concerning the quasi-critical condensation temperatures. As the result of the optimisation of an objective function, the cascaded Rankine cycles operating with argon and methane, followed by a direct expander unit working with regasified LNG, yield a high performance index based on the ratio of the attained power to the mass flow rate of regasified LNG (approaching 235 kW/kg-LNG) with pinch point of 2 °C at 30 bar, and 145.6 kW/kg-LNG with pinch point of 6 °C at 70 bar, when compared with the most recent contributions carried out in this field.