Liquid hydrogen as prospective energy carrier: A brief review and discussion of underlying assumptions applied in value chain analysis

In the literature, different energy carriers are proposed in future long-distance hydrogen value chains. Hydrogen can be stored and transported in different forms, e.g. as compressed dense-phase hydrogen, liquefied hydrogen and in chemically bound forms as different chemical hydrides. Recently, different high-level value chain studies have made extrapolative investigations and compared such options with respect to energy efficiency and cost. Three recent journal papers overlap as the liquid hydrogen option has been considered in all three studies. The studies are not fully aligned in terms of underlying assumptions and battery limits. A comparison reveals partly vast differences in results for chain energy efficiency for long-distance liquid hydrogen transport, which are attributable to distinct differences in the set of assumptions. Our comparison pinpoints the boiloff ratio, i.e. evaporation losses due to heat ingress, in liquid hydrogen storage tanks as the main cause of the differences, and this assumption is further discussed. A review of spherical tank size and attributed boiloff ratios is presented, for existing tanks of different vintage as well as for recently proposed designs. Furthermore, the prospect for further extension of tanks size and reduction of boiloff ratio is discussed, with a complementary discussion about the use of economic assumptions in extrapolative and predictive studies. Finally, we discuss the impact of battery limits in hydrogen value chain studies and pinpoint knowledge needs and the need for a detailed bottom-up approach as a prerequisite for improving the understanding for pros and cons of the different hydrogen energy carriers.