The development of hydrogen as a reliable energy vector is strongly connected to the performance and level of safety of the components of the supply chain. In this respect, achieving an efficient storage is crucial to address transition markets and automotive markets. For near term, compressed hydrogen storage is currently the most promising technology. To achieve required performance in terms of autonomy and weight efficiency, hydrogen must be stored at pressure up to 700 bar in carbon fibers composites cylinders with a polymer inner shell called liner. Recent observations show that the polymer liner can collapse and deform permanently during cylinder gas emptying. The extend of the liner collapse depends on the pressure in the cylinder, the remaining pressure after emptying, emptying flow rate and the nature of the gas. During hydrogen cycling, the liner is subjected to mechanical and thermal (due to gas heating during filling) loads. The effect of such gas cycling on a liner presenting collapse is currently unknown. A liner collapse could crack or the barrier properties could be affected after some gas cycles. The induced tightness loss could have dramatic consequences. The lack of knowledge on the impact of gas cycles on the lifetime of a composite tank appears as an issue to the development of the hydrogen energy from economic, safety and regulatory standpoints. It is thus necessary to address it in a research project to gain knowledge on the impact of gas cycles on the lifetime of a tank presenting a liner collapse and on the identification of the initiation mechanism and its occurrence conditions in term of pressure and emptying flow rate. These are the main objectives of the project COLLINE, submitted to the French call “AAP ANR MATETPRO 2013. The project gathers an industrial partner, Air Liquide, end-user of composites cylinders with experience on the cylinder supply chain and on tests in gas and Institut Pprime/ISAE-ENSMA, acknowledged for its competencies in experimental, theoretical and numerical mechanics to address multi-physics items (thermo-mechanical and diffuse-mechanical loads in particular) and composites structure durability. In the framework of this project, the effect of pressure cycles on a liner collapse will be studied thanks to thermo-mechanical fatigue tests on representative specimens. The analysis of such aged specimens will lead to the definition of an end of life and/or a loss of performance resulting from cycling criteria. In parallel, tests in hydrogen will be carried out on representative samples of a liner-composite assembly to identify the mechanism at the origin of the liner collapse. These approaches on specimens will be validated by comparison with tests on composite tanks. This will allow the development of predictive models of liner collapse occurrence and fatigue lifetime of a tank with a liner collapse. Finally, the project will lead to recommendations for the industry on the design of cylinders and their operating conditions to optimize lifetime, as well as on qualification tests of future cylinders and possible critical collapse size for withdrawal of cylinders in service.

Acetylene is a fuel gas largely used in industry and the craft industry for operations of thermal cutting and assembly. It is conditioned in steel cylinders garnished with a porous mass. This latter contains a solvent in which acetylene is dissolved. Recent evolutions appeared in the market and the applications of acetylene (laboratory analyses, precursor in fine chemistry, electronics). Moreover the use of acetylene tends to spread itself for leisure/do-it-yourself applications. These markets require new packagings (reduction in cylinder size), more safety, and the respect of more drastic quality specifications. There is also a need for differentiation compared to traditional and cheaper packagings, often coming from the Central European or Asian countries. These latter however, do not always guarantee the required quality, safety, or environmental features (asbestos-based porous mass). Air Liquide and its partners identified important possibilities for improvements related to: - characteristics and performances of the porous mass - the manufacturing process of this porous mass - the elimination of the harmful effects and the reduction of consumption of energy and water. A series of investigations is necessary to confirm these possibilities and to make them industrial. The main objectives of the OPTIMPAD project are: - lead to an innovative process for porous mass production based on the comprehension/optimization/simplification of the existing process - increase safety performances through a perfectly reproducible quality, conform with new safety requirements imposed by the new markets - increase flow rate performances of the small current acetylene cylinders through an increase of the flow rate/cylinder volume ratio - acquire on the European market a decisive competitive advantage thanks to the development of this new process, to be protected in terms of intellectual property and exploitation. Three R&D sub-projects are thus defined: 1. Understand/control the influence of the raw materials properties (limestone, quicklime, water…) on the manufacturing process. 2. Define and model the physicochemical properties of the porous ceramic material and carry out the correlation with the expected performances of the cylinders in terms of safety, durability and flow rate. 3. Understand/optimize the hydrothermal synthesis parameters of the porous mass. Collaboration between public and private teams will bring the essential academic knowledge to the comprehension of the formation mechanisms of the porous mass during the manufacturing process. This scientific approach associated with the industrial constraints experience (safety, costs, reliability) will lead to a new porous mass manufacturing process for acetylene cylinders, new evaluation criteria, schedules of conditions and working procedures. All innovations and improvements will be tested on an industrial scale on the Air Liquide production site in Villeneuve-sur-Yonne, the last site of this type in France.