Thermodynamic modelling of pressurised storage and transportation of liquid hydrogen for maritime export

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Abstract

There is interest in exporting carbon-neutral liquid hydrogen (LH2) as a replacement for fossil fuels. However, transporting LH2 for maritime export is made difficult by the requirement for high performance insulation to reduce pressure rise and cargo losses through venting. One method to reduce boil-off losses is to allow the tank to self-pressurise along the journey and delay venting by increasing the tank maximum allowable working pressure (MAWP). To assess this, an analytical model was developed, in Matlab, to predict pressure rise and boil-off gas (BOG) generation. Perlite insulation was modelled using analytical expressions for temperature-dependent specific heat capacity and thermal conductivity. The model was tuned over a range of data, including for pressure and boil-off rate. This model was used to assess an LH2 carrier carrying four 40,000 m3 spherical tanks insulated with evacuated perlite. At atmospheric pressure, a loss rate of 0.04 % per day was predicted for the laden voyage. Over a voyage duration of 15 days the model predicts that boil-off can be reduced by 31 % when the tank is allowed to self-pressurise to 5 kPaG. This is primarily due to a reduced rate of boil-off as the liquid transitions from a subcooled to a saturated state. At higher pressures, the delay in venting was observed to be a major contributor to reduction in boil-off, with a reduction of 91 % in losses at MAWP of 60 kPaG. Pressure rise was observed to be significantly greater than a saturated liquid model would suggest, particularly at high fill levels. Finally, an empirical equation for the pressure rise was fitted to 27 data points from the literature, allowing for quick estimates of the pressure rise based on the fill level, the Rayleigh number and the surface to volume ratio in the ullage. This study suggests very low loss storage of LH2 can be achieved in large double-wall tanks over a sufficiently short journey, with only marginal pressurisation. Additional investigations on the effect of ship motion on phase change within the tank will be required to fully understand design considerations for LH2 storage tanks.

Original languageEnglish
Pages (from-to)1273-1285
Number of pages13
JournalInternational Journal of Hydrogen Energy
Volume62
DOIs
Publication statusPublished - 10 Apr 2024

Keywords

  • Boil off
  • Cryogenics
  • Liquid hydrogen
  • Marine transportation
  • pressurisation
  • Shipping
  • Thermodynamic modelling

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