Enhancing generation of green power from the cold of vaporizing LNG at 30 bar by optimising heat exchanger surface area in a multi-staged organic Rankine cycle

Abstract

Liquefied natural gas (LNG) is warmed by seawater in a regasification terminal and the cold energy is usually wasted. Organic Rankine cycle (ORC) has emerged as the preferred thermodynamic cycle to extract green power from LNG using the temperature difference existing between LNG and the environment. The design of ORC based on a fixed temperature difference for all heat exchangers is proposed to be replaced with optimum temperature differences. Simulation with Aspen HYSYS 8.6® shows that the total surface area as obtained with a fixed temperature difference can be optimally redistributed among the heat exchangers to increase extraction of power. A three-stage cascaded ORC with ethane, ethane and propane as the working fluids generated an additional power of 123 kW with LNG vaporising at 30 kg per second without any additional surface area of heat exchangers. The paper further shows that for a delivery pressure at 6 bar(a), additional power of 630 kW can be generated from LNG vaporising at 30 bar(a) and thereafter expanded to 6 bar(a) in a turbine, instead of extracting power from 6 bar(a) LNG. The analysis shows that the third stage generates only 10% and the other two stages contribute equally to power generation.