Reheating unlocks more power from LNG cold energy

The study found that a two-stage Rankine cycle using hexafluoroethane (R116) in the upper cycle and ethane (R170) in the lower cycle could generate 7.5 MW of net power with 24.1% thermal efficiency. When reheating was added, the best configuration increased output to 9.2 MW, representing a 22% improvement over the optimal single-fluid design. These findings provide a practical pathway for LNG terminals to convert otherwise wasted cryogenic energy into useful power.

LNG is transported at extremely low temperatures, around −162 °C, allowing natural gas to be stored and shipped efficiently across long distances. However, before it enters pipeline networks, LNG must be warmed and regasified. In conventional terminals, much of this valuable cold energy is released to seawater or ambient air, resulting in a major loss of usable exergy. Previous studies have explored direct expansion, Rankine, Brayton, and mixed-fluid cycles, but many designs either show limited efficiency, rely on narrow operating conditions, or lack systematic optimization across both working fluids and cycle structures. Therefore, improving working-fluid matching and cycle configuration remains essential for efficient LNG cold energy recovery.

A study (DOI: 10.48130/een-0026-0007) published in Energy & Environment Nexus on 11 May 2026 by Shing-hon Wong’s team, The University of Western Australia, reports that reheated two-stage Rankine cycles offer the most effective configuration for maximizing LNG cold energy power generation.

The researchers developed a systematic process simulation and optimization framework to evaluate LNG cold energy recovery under representative terminal conditions, using an LNG receiving capacity of 216 t·h−1. They first constructed a baseline two-stage Rankine cycle, in which the upper and lower cycles each operated with independent working fluids. The upper cycle was heated by seawater, the lower cycle was cooled by LNG, and an intermediate heat exchanger linked the two stages. This two-stage arrangement reduced temperature mismatch across the wide span between LNG and ambient conditions. To identify optimal working fluids, the team screened 30 single-fluid combinations and 49 binary-mixture combinations. Genetic algorithms were coupled with Aspen HYSYS simulations to optimize evaporation pressure, condensation pressure, intermediate temperature, mixture composition, and other cycle parameters. For single fluids, R116 consistently performed best in the upper cycle because its dry-fluid behavior and non-isothermal heat rejection improved heat transfer to the lower cycle. R170 and R1150 were strong lower-cycle candidates, with R116–R170 delivering 7.5 MW net power. The researchers then assessed mixed working fluids, which can evaporate and condense over a temperature range, better matching LNG’s non-isothermal warming curve. Binary mixtures improved performance consistency, with R116-based upper-cycle combinations showing less than 5% variation among leading candidates. The best mixed-fluid baseline produced 7.7 MW, only modestly higher than the single-fluid case. Finally, four advanced configurations were tested: Rankine-regeneration, Rankine-reheating, Kalina-regeneration, and Kalina-reheating. Reheating delivered the strongest improvement because it allowed higher pressure expansion in the upper cycle while maintaining favorable exhaust temperatures for the lower cycle. The optimal Rankine-reheating configuration, using R116 in the upper cycle and an R1150–R170 mixture in the lower cycle, generated 9.2 MW. In contrast, regeneration and Kalina integration offered little or no performance benefit.

Overall, the study shows that the efficiency of LNG cold energy recovery depends not only on selecting high-performing working fluids, but also on integrating them into the right cycle architecture. While binary mixtures can improve thermal matching, the largest gain came from reheating, which enhanced both upper- and lower-cycle power production. The findings suggest that reheated two-stage Rankine systems may offer LNG terminals a technically feasible strategy to recover wasted cold energy, reduce energy losses during regasification, and support cleaner power generation from existing LNG infrastructure.

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References

DOI

10.48130/een-0026-0007

Original Source URL

https://doi.org/10.48130/een-0026-0007

Funding Information

The first author (Shing-hon Wong) received a PhD stipend scholarship from the Future Energy Exports CRC (www.fenex.org.au). This work has received partial support from the Australian Research Council under the Discovery Projects Scheme (DP210103766 and DP220100116). FEnEx CRC Document 2025/21.RP1.0072.PHD-FNX-MILE0861.

About Energy & Environment Nexus

Energy & Environment Nexus is a multidisciplinary journal for communicating advances in the science, technology and engineering of energy, environment and their Nexus.