58595 - Cryogenic Fuel Storage Modelling and Optimisation for Aircraft Applications 

Designing commercial aircraft to use liquid hydrogen (LH2) is one way to substantially reduce their life-cycle CO2 emissions. The merits of hydrogen as an aviation fuel have long been recognized. However, liquefaction of hydrogen is necessary for all but very short-range aircraft because of the large weight and volume of the tanks needed to store hydrogen as a compressed gas. The handling of a cryogenic fuel adds complexity to aircraft and engine systems, operations, maintenance and storage. The fuel system and fuel tanks could account for 8-10% of an aircraft’s operating empty weight.

For the same energy content, LH2 saves almost two-thirds of fuel mass compared to Jet-A, potentially enabling reduced take-off weight, if the tanks can have high gravimetric efficiency (the ratio of the useful hydrogen mass to the mass of a full fuel tank). However, even when the LH2 is stored at temperatures as low as 20 K, the tanks still need a volume four times greater than for Jet-A, implying bigger and heavier airfames. The fuel system must prevent contact of the LH2 with air, which requires pressurised tanks with walls that will withstand hydrogen permeation. Adequate insulation must be provided to minimise heat transfer to the fuel and to avoid excessive pressure build-up inside the tank, or the need to vent hydrogen gas. A double-wall vacuum-insulated tank with multi-layer insulation (MLI) offers the lowest boil-off rates, but rigid polyurethane foam insulation can offer a lighter solution for large fuel tanks.

This paper describes the heat transfer model developed in the EU Horizon 2020 project that is used to predict heat ingress to a tank with external insulation. It accounts for heat transfer according to the tank contents, the insulation material properties, the environment, and the dimensions of a cylindrical tank. The model is validated against previous experiments on hydrogen storage reported in the literature. It estimates the rate of pressure change according to the state of the fuel and the rate at which fuel is withdrawn from the tank. An example is given to show how pressure may change throughout a flight, taking account of the varying ambient conditions. Finally, it shows how optimal tank dimensions and insulation thickness can be selected to minimise tank weight and any need for venting.