Tsougranis, E.-L.

Wu, Dawei

Liquefied natural gas (LNG) is natural gas – predominantly methane (CH4) that has been cooled down to liquid form, about 1/600th of the volume of natural gas, for ease and safety of storage and transport. In most applications, LNG needs to be re-gasified before it can be further utilised, e.g. injected into the gas grid, combusted in an engine or boiler. A significant amount of LNG latent heat of vaporization, 5.1 X 105 J/kg, in cryogenic energy form, is released from regasification processes. Global regasification capacity at land-based LNG terminals has risen to 851 Million Tonnes Per Annum (MTPA) in 2018. The capacity of floating terminals, known as Floating Storage Regasification Units (FSRU), have increased to a total capacity of 84 MTPA in 2018. In the United Kingdom alone, 5.44 MTPA of imported LNG in 2017 released 771.3 GWh cold energy at its four terminals. On the other hand, LNG is an emerging clean marine fuel. An increasing part of the global LNG carrier fleet (478 vessels in 2017) have started to use LNG as an alternative fuel. Since 2000, other ship types, firstly passenger ferries and public service vessels, now merchant vessels and cruise liners, have started to use LNG as fuel, including 121 in operation, 126 on order and 111 LNG-ready vessels. The wide use of regasification at LNG terminals and on-board LNG fuelled ships provides a growing potential market for waste cryogenic energy re-utilisation. The cryogenic energy released from regasification has garnered interests from both industry and academia since 1970s. Various power generation cycles are proposed by researchers and industry stakeholders, including Organic Rankine Cycle, Brayton Cycle, Direct-expansion Open Rankine Cycle, and their cascaded combinations, for exploiting waste cryogenic energy to achieve higher overall thermal efficiency. As well as conventional power generation cycles, a novel technology, Thermoelectric Generator (TEG), has been investigated to expand its application range to include cryogenic temperature level. In addition, LNG waste cryogenic energy can be utilised in gas separation or desalination for producing some valuable by-products, e.g. liquified nitrogen, liquified oxygen, dry ice, hydrogen, and fresh water, although cost effective power generation from LNG waste cryogenic energy is the foremost consideration in most potential applications. The chapter incorporates a comprehensive review of all the technologies to reutilise waste cryogenic energy from LNG regasification process at terminals and on marine vessels. Hopefully, it offers a resourceful reference of LNG cryogenic energy technologies for researchers, industry stakeholders, policy makers and general public.<br/

LNG Cryogenic Energy Technologies

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