Establishment of a 4K Flat-plate Calorimeter Test Procedure and Evaluation of the Thermal Insulation Performance of MLI for Liquefied Hydrogen Storage Tank Applications

Liquefied hydrogen (LH₂) must be stored at an ultra-cryogenic temperature of approximately 20 K. If external heat ingress is not effectively minimized, boil-off gas (BOG) generation increases, leading to fuel loss, reduced system efficiency, and potential safety risks. Therefore, reliable experimental methods for evaluating the thermal insulation performance of multilayer insulation (MLI) used in cryogenic storage tanks are essential. Although liquid nitrogen is commonly used as a surrogate cryogen, it cannot sufficiently reproduce the operating conditions of LH₂ storage systems. In this study, a 4 K experimental procedure based on a flat-plate boil-off calorimeter was established in accordance with ASTM C1774. The proposed procedure consists of moisture removal through helium purging, stepwise vacuum conditioning, cryocooler cool-down, liquid helium filling, and BOG flow rate measurement. To verify the applicability of the procedure, an experiment was conducted using a 40-layer MLI specimen (Coolcat 2NW). A steady-state average BOG flow rate of 8,290 mL/min was measured, from which a heat load of Q=0.5059 W and an effective thermal conductivity of keff=0.3147 mW/(m·K) were derived. The established procedure provides a systematic basis for future repeatability studies and comparative evaluations of various insulation materials, thereby contributing to improved reliability in the thermal design of LH₂ storage tanks.