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.
Highlights
Due to the current global attention concerning the reduction of carbon dioxide (CO2) emissions, which is the largest contributor to global warming [1], the demand for renewable energy is increasing
The specific life cycle cost (SLCC) decreases as the R/G fraction increases for the same ship
This study proposed a partial boil-off hydrogen (BOH) re-liquefaction system based on the reverse Brayton helium cycles
Summary
Due to the current global attention concerning the reduction of carbon dioxide (CO2) emissions, which is the largest contributor to global warming [1], the demand for renewable energy is increasing. A large portion of CO2 emissions is attributed to the combustion of the fossil fuels, which provide approximately 80% of the total world energy supply [2]. From 2009 to 2018, global CO2 emissions increased by 16% [3]. To reduce CO2 emissions, power generation using renewable energy, which is not accompanied by CO2 emissions, has been increasing. According to the International Energy Agency, power generation achieved using renewable energy has increased by 57% from 2010 to 2018 [4]. The portion of renewable energy used for electricity generation has been projected to consistently increase in the future [5]
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