As hydrogen continues to dominate the global transition to clean, zero-emissions transportation, including powering anything from small electronic devices to vehicles, aircraft and even whole buildings, hydrogen storage is emerging as an essential challenge.
Hydrogen can be obtained by electrolysis from electricity produced from surplus renewables. It can be stored in large quantities for extended periods of time. Unlike with batteries, this energy is not lost over time and can therefore be produced and stored on an industrial scale as part of a green energy mix. This stored hydrogen can then be retrieved as a backup energy supply when needed.
Hydrogen can be stored in three different ways:
- As a gas under high pressures
- In liquid form under cryogenic temperatures
- On the surface of or within solid and liquid materials
Each of these storage techniques has its own requirements and challenges. Safety is of paramount importance when storing hydrogen in highly pressurized tanks.
Green aircraft are a priority for aircraft manufacturers. Arguably, the only feasible long-term option is hydrogen, with hydrogen-powered aircraft due to come into service in the mid-2030s. But the storage of hydrogen will not be possible without a waterproof, leak-proof, light and space-saving tank suitable for service at cryogenic temperatures. The reason for storing hydrogen at cryogenic temperature is its density, which more energy to be stored in a smaller volume.
Most designs for storing LH2 have centered around metallic tanks, which are relatively heavy. Many steels exhibit embrittlement at such low temperatures, exacerbated by hydrogen, although some special austenitic stainless steels have shown good resistance.
Aluminium is a lightweight material with, compared to steel, improved mechanical properties at low temperatures and is increasingly used. It is cheaper than composites and lighter than steel. But there are challenges, most notably 100% leak-proof weldability.
As a result, the Netherlands, UK, France, Germany and the US are all building up test capability, looking at thermoset and thermoplastic composite materials as alternatives.
Composite tanks developed in the space industry tend to be suited only for single-use (low-cycle) applications. However, commercial hydrogen aircraft will need fuel tanks to be as light as possible, filled and emptied numerous times (high cycle), and must last for several decades.
In December 2021, Toray Advanced Composites (TAC, Nijverdal, Netherlands) announced a Dutch liquid hydrogen (LH2) composite tank consortium to develop a long-life, fully thermoplastic composite LH2 tank for civil aviation.
In February 2022, a new type of large, fully composite, linerless cryogenic fuel tank, designed and manufactured by Boeing (Chicago, US) and managed by NASA, passed a critical series of tests.
In Germany, BMW and Cryomotive have participated in an automotive development program using an insulated carbon fibre-reinforced polymer (CFRP) tank system. Also in the automotive sector, part of Forvia’s (Nanterre, France) developments and strategy for enabling the clean mobility transition include cryogenic hydrogen storage tanks using thermoset and thermoplastic composites. Forvia, in collaboration with Paris-based Air Liquide is targeting heavy-duty vehicle transportation with the manufacture of a sub-cooled liquid hydrogen cryogenic tank.
In the UK, the Bristol-based National Composites Centre (NCC) is testing composite cryogenic storage tank demonstrators with increasing complexity, to support the transition to the hydrogen economy.
LH2 requires well-insulated cryogenic storage vessels to maintain it at a temperature of -253°C, and handling requires specialist knowledge and equipment. The testing program will validate and accelerate design, manufacture and test capabilities, starting with a linerless carbon fibre tank for storing LH2 (Fig 1).
The NCC has designed and manufactured five linerless carbon fibre demonstrator tanks, using a mix of automated fibre placement (AFP), tape winding, and hand lay-up composite processes. These tanks will be used for the liquid hydrogen testing program, which started at the end of April 2023, with results expected summer 2023.
The NCC team of specialist engineers has also created and built a range of comprehensive design tools to help organizations overcome the engineering challenges critical to accelerating hydrogen development.
With no clear existing industry standards for aerospace cryogenic tanks, the ability to analyze a range of designs is essential. This led to the development of a range of concepting tools covering tank design space exploration, permeability, microcracking, thermal and mechanical stresses. These tools form a baseline toolset that can be used by the industry for cryogenic tank projects, enabling detailed design and manufacturing risk identification and analysis. Understanding cryogenics at this fundamental level will enable the NCC to support a wide range of industries and applications, each facing separate but overlapping challenges.
As part of the process, the NCC has partnered with Filton Systems Engineering (FSE), a Bristol-based SME specializing in fluid system engineering, that owns and operates a hydrogen test facility capable of both gaseous and liquid hydrogen testing. Working in partnership with FSE, the NCC has developed a tank-testing program that uses an LH2 vacuum test chamber and cryo-rated testing instrumentation.
Daniel Galpin, Advanced Research Engineer, NCC, says, “The cross-sector capability we’ve developed will enable the industry to accelerate and advance engineering expertise in composite cryogenic storage.”
VACNT composites
At the recent international composites exhibition, JEC World, held annually in Paris, NAWA Technologies, a nanotechnology pioneer headquartered in Aix-en-Provence, France with a second location in Dayton, USA, demonstrated carbon-fibre based composites based on the patented Vertically Aligned Carbon Nano Tubes (VACNT) technology (Fig 2).
Originally conceived in 2016, vertically aligned carbon nanotubes (VACNTs) are one-dimensional carbon objects which are anchored atop a solid substrate. In contrast to randomly oriented carbon nanotubes (CNTs), VACNTs are therefore geometrically and result in stronger, lighter, more advanced materials, which, combined with electrical conductivity, provides a new class of functional materials with novel prospects, across a range of areas, including hydrogen storage, sporting goods, and the aerospace and automotive sectors.
VACNTs may be combined with another NAWA technology, NAWAStitch, described as ‘nano-velcro’, which aims to solve two of the biggest issues of composites – interlaminar cracking and delamination. According to NAWA CEO Antoine Saucier (Fig 3), the combination yields virtually unbreakable materials, with a 900% increase in impact damage tolerance and up to 30% reduction in weight.
Applied as a universal or localized interlaminar reinforcement of prepreg carbon fibre systems, NAWAStitch prevents cracks from bridging across the interface of composites, so achieving consistent behavior within a ply.
For hydrogen storage tanks, NAWAStitch can provide stronger, but crucially lighter tanks, contributing to weight savings on zero-emission vehicles. NAWA’s first proof-of-concept (POC) testing of a liquid hydrogen tank with VACNTs in cryogenic conditions has demonstrated 20-50% less microcracking, compared with typical composites.
Other applications
Consumers of sports goods that are typically subjected to extreme impacts, such as golf club heads, hockey sticks, or mountain bike wheels can benefit from NAWAStitch, while cost-effective and efficient mass production of VACNT composites means there is a short time-to-market.
NAWAStitch’s versatility can also be applied within the aviation sector. Not only can VACNT be used to dramatically strengthen the composites used in aerospace, but the nano-material can also carry and store electrical energy, with benefits in anti-icing and de-icing, structural health monitoring, or even structural energy storage, giving a composite component multi-functional capabilities.
The nano-material’s innovative combination of greater strength, combined with the ability to carry electric current can be also applied to next-generation composite electric vehicle battery casings, that are not only stronger and safer, but by applying a current to the nano-layer, can keep the battery at a constant temperature, avoiding the requirement for additional thermal management, and helping to prolong the performance and life of the cells inside.
References
- Vertically Aligned Carbon Nanotubes as Platform for Biomimetically Inspired Mechanical Sensing, Bioactive Surfaces, and Electrical Cell Interfacing.
- What is hydrogen storage and how does it work?
- FAQ Guide – Cryogenic Hydrogen Storage & Cryogenic Cooling
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