Thermal Efficiency and Economics of a Boil-Off Hydrogen Re-Liquefaction System Considering the Energy Efficiency Design Index for Liquid Hydrogen Carriers

Abstract

This study analyzes the thermodynamic, economic, and regulatory aspects of boil-off hydrogen (BOH) in liquid hydrogen (LH2) carriers that can be re-liquefied using a proposed re-liquefaction system or used as fuel in a fuel cell stack. Five LH2 carriers sailing between two designated ports are considered in a case study. The specific energy consumption of the proposed re-liquefaction system varies from 8.22 to 10.80 kWh/kg as the re-liquefaction-to-generation fraction (R/G fraction) is varied. The economic evaluation results show that the cost of re-liquefaction decreases as the re-liquefied flow rate increases and converges to 1.5 $/kg at an adequately large flow rate. Three energy efficient design index (EEDI) candidates are proposed to determine feasible R/G fractions: an EEDI equivalent to that of LNG carriers, an EEDI that considers the energy density of LH2, and no EEDI restrictions. The first EEDI candidate is so strict that the majority of the BOH should be used as fuel. In the case of the second EEDI candidate, the permittable R/G fraction is between 25% and 33%. If the EEDI is not applied for LH2 carriers, as in the third candidate, the specific life-cycle cost decreases to 67% compared with the first EEDI regulation.

Deep Reinforcement Learning-Based Energy Management for Liquid Hydrogen-Fueled Hybrid Electric Ship Propulsion System

“…However, when ship capacity increases or bunkering intervals are extended, the volume of fuel tanks needed to store gaseous hydrogen becomes very large. In contrast, when storing hydrogen fuel in a liquid state and using it as fuel by vaporization, it is expected that liquid hydrogen (LH 2 ) can reduce the volume of fuel tanks, as it has a higher volumetric energy density than gaseous hydrogen (approximately twice as high as 700 bar gaseous hydrogen) [20,21]. Furthermore, the individual volume of fuel tanks required for storing high-pressure gaseous hydrogen is not higher than that of LH 2 fuel tanks.…”

Strategies to recover and minimize boil-off losses during liquid hydrogen storage

“…H 2 liquefaction can be divided into several steps, namely H 2 compression, pre-cooling, cryocooling and liquefaction [80]. The H 2 liquefaction process includes two cooling stages [81] as can be appreciated graphically in Figure 6a. In the first cooling stage, also called precooling, the H 2 feed gas is cooled to a temperature of around 80 K. In the second stage, the pre-cooled H 2 passes through a heat exchanger to reach 30 K through a cryocooling system, which operates with refrigeration cycles such as the helium reverse Brayton cycle (possibly with H 2 as well, but there is a minimum temperature limitation [80]) or the H 2 Claude cycle [79].…”

“…This can result in substantial energy savings, as it is estimated that LN 2 production for the pre-cooling phase accounts for about 30 % of the energy consumption in H 2 liquefaction [128]. Another benefit is that the H 2 is already in the para isomer form, so catalytic conversion of o-to p-H 2 will not be necessary [81]. However, two issues that need to be considered in an LH 2 storage-reliquefaction facility to take advantage of the low BOH temperature.…”

“…The SEC was estimated at 3.30 kWh/kg LH2 [129], a value lower than the characteristic SECs for liquefaction processes from ambient conditions. Choi et al [81] also performed a study of partial reliquefaction of LH 2 with a reverse Brayton cycle. Similar to the work of Lee at al., the non-liquefied H 2 fraction went into a fuel cell to generate electricity.…”