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 m 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.

中文翻译：

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海运出口液氢加压储存和运输的热力学模型

人们有兴趣出口碳中性液氢（LH2）作为化石燃料的替代品。然而，由于需要高性能隔热材料来减少压力上升和通风造成的货物损失，因此海运出口运输 LH2 变得困难。减少蒸发损失的一种方法是让储罐在旅途中自我加压，并通过增加储罐最大允许工作压力 (MAWP) 来延迟排气。为了评估这一点，在 Matlab 中开发了一个分析模型来预测压力上升和蒸发气体 (BOG) 的产生。使用与温度相关的比热容和导热系数的解析表达式对珍珠岩隔热层进行建模。该模型根据一系列数据进行了调整，包括压力和蒸发率。该模型用于评估一艘 LH2 运输船，该运输船携带四个 40,000 m 的球形储罐，并用真空珍珠岩隔热。在大气压下，预计满载航次的损失率为每天 0.04%。在 15 天的航程中，模型预测，当储罐自行加压至 5 kPaG 时，蒸发量可减少 31%。这主要是由于液体从过冷状态转变为饱和状态时蒸发速率降低。在较高压力下，观察到排气延迟是减少蒸发的主要因素，在 MAWP 为 60 kPaG 时，损失减少了 91%。据观察，压力上升明显大于饱和液体模型所显示的压力上升，特别是在高填充水平时。最后，将压力上升的经验方程拟合到文献中的 27 个数据点，从而可以根据填充水平、瑞利数和空缺中的表面与体积比快速估计压力上升。这项研究表明，在大型双壁储罐中，只需边际加压，即可在足够短的行程内实现非常低损失的 LH2 存储。需要对船舶运动对储罐内相变的影响进行更多研究，以充分了解 LH2 储罐的设计考虑因素。