Thermodynamic analysis of vapor-cooled shield with para-to-ortho hydrogen conversion in composite multilayer insulation structure for liquid hydrogen tank

Abstract

Reducing the heat leakage during liquid hydrogen storage (LH2) has been a critical issue for the long-distance transportation and large-scale application of hydrogen. A composite thermal insulation structure for LH2 tanks, which integrated the variable density composite multilayer insulation (FVD-MLI) and vapor-cooled shield (VCS) with para-to-ortho hydrogen conversion (P–O) catalysts, is used. Based on the “Layer by Layer” models, para-hydrogen equilibrium fractions with different VCS temperatures and P–O efficiency with different flow rates are combined. The thermal effects of P–O in VCS are considered. Experimental results prove the insulation performance calculation model's accuracy. The heat leakage and temperature profile after filling the P–O catalysts in single VCS and double VCSs are obtained. The optimal insulation positions of single VCS and double VCSs are determined, and the consequence of the vacuum level on the insulation performance is discussed in detail. Increasing the VCS number from single to double and adding P–O catalysts can effectively improve the LH2 tank's insulation performance. Compared with the single VCS, the heat flux through LH2 tanks with double VCSs is reduced by 25.6%. The heat leakages to the tanks can be further reduced by 5.08% and 7.45%, respectively, after adding P–O catalysts in single VCS and double VCSs. Moreover, the cooling effects of P–O shift the optimal position of VCS towards the cold boundary. The single VCS optimal position is moved from 58% of the insulation structure thickness to 50.6%. The optimal positions of double VCSs are moved from 38.9% to 75%–31.1% and 71.7%, respectively. It is found that adding P–O catalysts and increasing the VCS numbers can effectively increase the insulation effect regardless of the vacuum degree between the thermal insulation structure. However, considering the P–O efficiency, the insulation improvement effects after adding P–O catalysts decrease with interlayer vacuum deterioration.