Design and Analysis of LH2 tank structures for aircraft retrofit applications

Abstract

This thesis investigates the design and analysis of cryogenic liquid hydrogen storage tanks in the context of aircraft retrofit. Hydrogen propulsion offers significant potential for achieving zero emissions in aviation, but its adoption introduces challenges related to safe and efficient storage.
The research focuses on comparing single-wall and double-wall tank architectures, assessing their ability to meet stringent operational, thermal, and structural performance requirements. In the preliminary assessment of a tank design’s viability, two key performance requirements are cruise time and dormancy time. A minimum cruise time of 20 minutes ensures the tank can support basic flight operations, while a dormancy time of 1 day ensures no hydrogen loss occurs if the aircraft remains stationary for an extended period, accounting for potential delays.
The methodology to calculate cruise time involves determining the maximum time the aircraft can remain in the cruise phase based on the inner tank dimensions, fill ratio, and mission profile. The dormancy time is the time required for the tank pressure to reach the venting pressure, at which point hydrogen must be released, and is calculated by implementing a thermodynamic model that simulates the tank's dynamic behavior over time, accounting for heat inflow from the external environment.
The evaluation of the single-wall tank reveals its simplicity and potential cost-effectiveness, but also exposes considerable limitations in terms of thermal insulation for the specific retrofit case study. This design approach is a viable option for larger-scale applications, where a lower surface area-to-volume ratio reduces heat transfer and, consequently, hydrogen boil-off. However, the compact dimensions of the tanks required for aircraft retrofitting present a significant challenge due to the inherently higher surface area-to-volume ratio, which leads to increased thermal losses and prevents the single-wall architecture from meeting the performance requirements imposed by this specific case study.
In contrast, the double-wall tank, equipped with a vacuum layer and multi-layer insulation (MLI), offers improved thermal performance. The heat transfer from the external environment is significantly reduced, allowing to preserve the cryogenic temperature of the hydrogen fuel. However, the added complexity introduces new challenges, particularly regarding the design of the inner vessel support system which must maintain the inner vessel’s position while accommodating thermal displacements and managing structural loads. Assessing the heat leakage budget for the support structure is the final critical step, as it determines the maximum allowable heat inflow through the support system, ensuring the tank meets its dormancy time requirements while allowing for design optimization.
The thesis develops a design methodology for the inner vessel support system, balancing the need for flexibility (to accommodate thermal contraction experienced during the first filling of the tank) with sufficient stiffness (to withstand operational loads, including emergency landing conditions). This approach involves selecting suitable materials and geometries that meet thermal requirements, while accurately determining the support structure’s stiffness properties. Different loading scenarios, such as normal operations and emergency landing conditions, are evaluated to analyze stresses and displacements in both the tank and support system. Adjustments to the design are made if stress or displacement exceed safe limits. The analysis reveals that optimizing the support structure is critical for the double-wall tank’s overall feasibility. While the double-wall design is technically viable and meets the thermal performance requirements, its success depends on further refinement of the support system to minimize heat leakage and ensure structural integrity.
The results of this study suggest that, although single-wall tanks are not suitable for this application, double-wall tanks offer a promising solution for retrofitting aircraft with cryogenic liquid hydrogen storage. Nonetheless, significant challenges remain, particularly in designing efficient support structures that can handle the operational demands without compromising thermal performance. Future work should focus on optimizing the support system design, exploring flexible materials, and considering additional factors such as sloshing loads to further improve tank reliability and performance.
In general, this thesis contributes to the development of a robust methodology for the preliminary design of cryogenic hydrogen storage tanks, providing a foundation for further advancements in hydrogen-powered aviation.