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| Composite cryogenic demonstration tank (courtesy NASA) |
Storage vessels for liquid hydrogen (LH2) can be broadly classified as single-wall tanks or double-wall dewars. In both cases, operating pressures are generally kept relatively low. As mentioned in my previous post, stainless steel and aluminum alloys are the most commonly used material for these vessels. Metal liners with a composite overwrap have also been used in some LH2 applications.
A very active area of ongoing development are vessels comprised only of composite material for applications where weight reduction is critical (see photo of an example above). Composite vessels can also potentially withstand higher pressures with reasonable wall thicknesses providing increased storage density and greater operational flexibility.
Single-Wall Tanks
Single-wall tanks are used in applications where storage times are relatively short and consumption rate is very high. The most common example are rocket stages that are loaded on the launch pad and consume most of the LH2 during the several minutes required to reach orbit.
Spray-on foam insulation (SOFI) is the most common option historically used for thermal protection of a single-wall LH2 tank. Key design considerations include: foam thickness; micro-cracking due to large temperature differentials; water uptake from the environment; repair and maintenance; and other factors.
Aerogel blankets are another potential option for single-wall LH2 tanks. While this option mitigates some of the issues with foam mentioned above, other design considerations come into play (e.g. cost, installation, total insulation mass, etc.).
Double-Wall Dewars
Dewars are comprised of an inner wall that contains the LH2 and an outer wall exposed to the environment. The space between the walls is evacuated and generally contains insulating materials. These vessels are sometimes referred to as vacuum jacketed and are based on the same principle as "vacuum flasks" used to store hot or cold beverages.
Dewars are heavier than a single-wall design with comparable storage capacity because of the additional containment wall. However, their thermal performance is far superior due to the minimization of conduction and elimination of convection heat transfer within the vacuum jacket. For this reason, virtually all current stationary and transportation LH2 vessels are dewars.
Insulation options inside the vacuum jacket to further improve thermal performance include: reflective surfaces, perlite, glass bubbles, aerogel beads, multi-layer insulation (MLI), and others. MLI is the highest performing practical insulation option within a vacuum jacket. Double aluminized mylar with dacron netting spacers is a common MLI configuration.
Design parameters that effect MLI performance include: materials used, number of layers, layer density, boundary temperatures, compression load, and degradation factors. Degradation due to seams and penetrations must be minimized with proper design and installation techniques to ensure acceptable storage performance.
Penetrations
Penetrations refer to any solid conduction path that is in thermal connection with the inner tank wall and its contents. In well-insulated tanks, penetrations and their associated thermal conduction often impose the largest heat load into the LH2. Some key penetrations for LH2 vessels of any type include:
- Supports, flanges, ports and similar structural components
- Fill line for loading LH2 into the vessel
- Drain line for removing LH2 (a single fill/drain line is sometimes used)
- Feed line for high LH2 consumption rate applications
- Pressurization or other pressure building subsystem
- Vent line for pressure relief and fluid conditioning (sometimes tied into the pressurization line with appropriate isolation valving)
- Sensors for temperature, pressure, and mass gauging or fill level monitoring
In the next post I'll talk about at how LH2 is transferred in and out of a storage vessel.
Reference
