Accelerated refueling of type IV hydrogen pressure tanks by passive means: Thermal material characterization and evaluation

Kurzfassung

The significant heat generation during refueling of hydrogen pressure tanks might exceed the permissible 85 °C temperature limit. This necessitates either a low filling rate or interruption of the process before reaching complete capacity. A common counter measure is energy consuming and costly precooling of the hydrogen. To address this issue, this study focuses on utilizing thermally efficient materials as a passive measure to facilitate heat dissipation through the tank. Type IV tanks, state of the art for mobile applications due to their lightweight, cost-effective, and fatigue resistance properties, are the focus. They consist of a thermoplastic liner completely overwrapped with carbon fiber reinforced thermoset plastic (CFRP). The liner serves as a barrier against diffusion and also used as tooling during tank manufacturing by fiber winding, while the overwrap carries the loads. The low thermal conductivity of these materials is a disadvantage compared to type III tanks with metallic liners. To mitigate this disadvantage materials with higher thermal conductivity and capacity were screened, characterized and finally evaluated in a thermal simulation where different material combinations were compared to the reference materials. A literature review revealed that refueling takes between 180-300 seconds, with the thermal load distribution being homogeneous or inhomogeneous depending on tank and nozzle geometry. Given the rapid filling rate and inherent heat release, both thermal conductivity and heat capacity are crucial for heat transport and energy buffering to limit temperature rise. As the liner is in direct contact with the filling gas, its properties have the highest impact on the occurring peak temperature. During the filling stage the temperature increase on the outer tank surface is minimal making the outer boundary conditions are negligible. To improve the thermoplastic liner's thermal properties, samples with various fillers and fill ratios were manufactured and tested. Since these may affect hydrogen diffusion resistance, permeability was also quantified. To improve thermal conductivity of the CFRP overwrap copper plated carbon fibers were used for sample preparation and then tested. The study further explored a hybrid Type IV/V tank design, where the thermoplastic liner is partially infused into the carbon fibers, potentially increasing thermal contact and conductivity. Using these findings, the most promising materials and designs were assessed in a transient thermal simulation to compare various application scenarios. One scenario is the globally homogeneous distributed thermal load at the inner liner surface, where almost exclusively out of plane thermal conductivity is predominant. In scenarios with localized thermal load higher in-plane thermal conductivity is beneficial.