Scientists design a new system that generates electricity from the wasted “cold energy” of stored liquefied natural gas

New regulations in maritime transport have made liquefied natural gas (LNG) an attractive eco-friendly alternative to fossil fuels. However, LNG is stored at low temperatures and heated before use, releasing “cold energy” in the process (which usually goes to waste). Now, scientists from Korea design a novel system that can use this cold energy to generate electricity in large ships. This system can lead to LNG-fueled ships becoming more sustainable as well as energy efficient. 

Environmental pollution and climate change have rightfully become a global priority over the last decades, and international agencies have now implemented stricter regulations across most fields of human activity. Recently, the International Maritime Organization issued a regulation to greatly reduce the amount of sulfur oxide emissions from ship fuel, making the use of liquefied natural gas (LNG) an attractive eco-friendly option because of its negligible sulfur emissions. But, LNG needs to be stored at very low temperatures and requires to be heated before use, which results in the dissipation of “cold energy.” Even though cold energy can be used to produce electricity, this technology has not been explored in ships so far. This is a major limitation of using LNG on ships, making the process energy efficient.

To this end, a team of scientists at Korea Maritime and Ocean University wondered: Would it be possible to make use of this “cold energy” beforehand? In a recent study published in Applied Thermal Engineering, the research team, including Prof Yong-Seok Choi and Prof Tae-Woo Lim, tackled this problem of exploiting the cold energy of LNG. Prof Choi explains, “We wanted to explore the full potential of using LNG to protect the marine environment as well as improve energy efficiency. Since the cold energy from LNG is rarely utilized in LNG-fueled ships, there is a large loss in terms of energy efficiency. Therefore, we conducted this study to find a way to recover the LNG cold energy and develop eco-friendly LNG-fueled ships.”

To achieve this, the scientists set out to design an organic Rankine cycle (ORC) suitable for large LNG-fueled ships. An ORC is essentially a closed circuit where a fluid is heated (vaporized) to drive the turbine of an electric power generator and then cooled (condensed) before being heated again. The scientists proposed a new system, in which the working fluid of the ORC is heated using hot water coming from the water-cooling jacket of the ship’s engine and cooled using LNG before it is heated and consumed as fuel. This means that the system generates electricity from temperature differences that would otherwise go to waste.

Unlike most previous studies that were conducted on small or land engines, in this study, the scientists performed simulations using the operating conditions of an actual large marine engine (12S90ME-GI). All this heating and cooling of different fluids, however, requires appropriate heat exchangers. Accordingly, a considerable part of the study also involved the optimal design of these crucial devices for the proposed ORC.

To top it off, the methodology used and the results obtained are relevant beyond maritime transport. Prof Choi concludes, “Though our work is focused on ships, it can be applied equally in other industries in general. The use of LNG cold energy is very useful way to increase energy efficiency, which translates to higher environmental protection.”

Further system designs to increase the efficiency of eco-friendly ships will surely be a step towards a sustainable future for our planet.

Reference

Authors: Tae-Woo Lim (1) and Yong-Seok Choi (2)

Title of original paper: Thermal design and performance evaluation of a shell-and-tube heat exchanger using LNG cold energy in LNG fuelled ship

Journal: Applied Thermal Engineering

DOI: 10.1016/j.applthermaleng.2020.115120

Affiliations:

(1) Division of Marine Engineering, Korea Maritime and Ocean University

(2) Division of Marine System Engineering, Korea Maritime and Ocean University