Performance comparison of multilayer insulation coupled with vapor cooled shield and different para-ortho hydrogen conversion types

•The optimal VDMLI configuration and VCS position are obtained by Genetic Algorithm.•The optimal position of the VCS with para-ortho hydrogen conversion is 40.41 %.•12.99 % heat leakage can be reduced after the continuous conversion is introduced. Application of para-ortho hydrogen conversion to a vapor cooled shield (VCS) in a multilayer insulation system can utilize more cooling capacity of cryogenic hydrogen vapor and achieve better thermal protection for liquid hydrogen storage. Based on energy conservation and layer-by-layer model, a thermodynamic model was developed to study a variable density multilayer insulation (VDMLI) structure coupled with one VCS and para-ortho hydrogen conversion. The effects of introducing the VCS and different para-ortho hydrogen conversion types were investigated. The maximum heat leakage reduction was 56.44 % after the VCS was installed at the middle of the 43-layer VDMLI with an optimal configuration. The ratio of the distance between the optimal VCS position and the cold boundary to the VDMLI thickness changed from 50 % to 40.41 % in the existence of a continuous para-ortho hydrogen conversion. The heat leakage of the VDMLI with one VCS could be further reduced by 12.99 %, 12.99 % and 10.09 % with a continuous conversion, single-stage isothermal conversion and single-stage adiabatic conversion, respectively. Notably, the adiabatic conversion could be implemented for its relative installation convenience. The heat leakage of the VDMLI could be reduced by 60.44 % after one VCS with an adiabatic conversion is adopted.