Modelling of Boil-Off and Sloshing Relevant to Future Liquid Hydrogen Carriers

Abstract

This study presents an approach for estimating fuel boil-off behaviour in cryogenic energy carrier ships, such as future liquid hydrogen (LH2) carriers. By relying on thermodynamic modelling and empirical formulas for ship motion and propulsion, the approach can be used to investigate boil-off as a function of tank properties, weather conditions, and operating velocities during a laden voyage. The model is first calibrated against data from a liquefied natural gas (LNG) carrier and is consequently used to investigate various design configurations of an LH2 ship. Results indicate that an LH2 ship with the same tank volume and glass wool insulation thickness as a conventional LNG carrier stores 40% of the fuel energy and is characterised by a boil-off rate nine times higher and twice as sensitive to sloshing. Adding a reliquefaction unit can reduce the LH2 fuel depletion rate by at least 38.7% but can increase its variability regarding velocity and weather conditions. In calm weather, LH2 boil-off rates can only meet LNG carrier standards by utilising at least 6.6 times the insulation thickness. By adopting fuel cell propulsion in an LH2 ship, a 1.1% increase in fuel delivery is expected. An LH2 ship with fuel cells and reliquefaction is required to be at least 1.7 times larger than an existing LNG carrier to deliver the same energy. Further comparison of alternative scenarios indicates that LH2 carriers necessitate significant redesigns if LNG carrier standards are desired. The present approach can assist future feasibility studies featuring other vessels and propulsion technologies, and can be seen as an extendable framework that can predict boil-off in real-time.

Boil-Off Gas Generation in Vacuum-Jacketed Valve Used in Liquid Hydrogen Storage Tank

“…The Lee model is composed of the mass of liquid or gas within a control volume, the difference between the fluid temperature and the saturation temperature T sat , and the empirical constant known as the relaxation coefficient R. T sat can be expressed as a function of the absolute pressure P, presenting the temperature at which the vaporization of the liquid hydrogen begins, as shown in the following equation: logT sat = 1.497 + 0.209(log P) + 0.021(logP) 2 (5)…”

“…Smith et al [2] investigated the rate of BOG generation for a conceptual liquid hydrogen fuel carrier with a capacity of 173,600 m 3 using numerical and empirical formulas based on thermodynamics. The rate of BOG generation was influenced by the external weather conditions, speed of the fuel carrier, insulation thickness of the storage tank, and sloshing and re-liquefaction unit installed in the fuel carrier.…”

Life Cycle Global Warming Impact of Long-Distance Liquid Hydrogen Transport from Africa to Germany

“…The cryogenic LH 2 is transported in heat-insulated, cryogenic spherical tanks [39]. The boil-off was not estimated to be reliquefied [40] or used as a marine fuel [41]. The LH 2 supply chain involved some infrastructural measures.…”

An Extensive Review of Liquid Hydrogen in Transportation with Focus on the Maritime Sector

“…The authors rightly claimed that accurate temperature measurements are required also for commercial transactions since the LH 2 density can vary consistently in a small range of temperature and pressure. Finally, in 2022, Smith et al [148] proposed a model to simulate both sloshing and BOG formation within the cryogenic tanks of LH 2 carrier ships.…”