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.
Highlights
Received: 7 February 2022Traditionally, energy carrier ships transport fossil fuels over long distances for utilisation in locations far from their origin [1]
These quantities are shown at Beaufort numbers (BN) of 0 to 12, a value which we use to quantify the weather conditions through which the ship is travelling
The model is based on thermodynamic modelling of the fuel tank and empirically derived trends for required engine power, ship motion, and heat transfer enhancement due to sloshing at a range of ship velocities and weather conditions represented by Beaufort number (BN)
Summary
Energy carrier ships transport fossil fuels over long distances for utilisation in locations far from their origin [1]. This paper introduces an approach that predicts the natural and sloshing induced BOR properties of cryogenic energy carrier ships, and considers various practical implications relevant to the tank properties, weather conditions, and operating velocities during a laden voyage. The approach is employed to explore the effects of (i) weather conditions, (ii) operating velocity, (iii) sloshing, and (iv) ship design on the BOR properties of a conceptual LH2 ship. The approach is calibrated using data from a conventional LNG ship, conceptual studies on LH2 ships, reliquefaction units, and electric component data This is followed by an investigation of the suitability of using LH2 ships as fuel carriers, focusing on the effects of insulation thickness, the presence or absence of reliquefaction, and propulsion type. Conclusions and recommendations for future work close the paper
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