Hydrogen storage is a key enabling technology for the use of hydrogen as an energy vector. To improve volumetric and gravimetric performance, carbon fiber composite cylinders are currently being developed. However, current standards governing the design, qualification and in-service inspection of carbon fiber composite cylinders do not allow cylinder design to be optimised. In particular, safety factors for cycle life and burst pressure ratios appear to be conservative, which results in the cylinders being overdesigned and thus costly. Furthermore, the requirements in these standards are often not based on degradation processes in composite materials but have been adapted from standards covering metallic cylinders.
To address these issues, HyCOMP will conduct pre-normative research on high-pressure type III and type IV composite cylinders for hydrogen storage and transport for automotive, stationary and transportable applications. The project will generate all the data necessary to develop a comprehensive scientific and technical basis for fully justifying as well as improving the full set of requirements defined for ensuring the structural integrity of the cylinders throughout their service life, covering design type approval, manufacturing quality assurance, and in-service inspection.
The outcome of the project will be recommendations gathering broad support for improving the applicable European and international standards and regulation on high-pressure hydrogen cylinders for automotive, transport and stationary applications, as well as defining a strategy for implementing these changes. These recommendations will include performance-based design requirements, and improved procedures for type testing, batch testing and in-service inspections.
The European project HyCOMP, funded by the Hydrogen and Fuel Cell Joint Undertaking (FCH-JU), started in January 2011 and finished in March 2014 (total duration: 39 months). HyCOMP whose full title is “Enhanced design requirements and testing procedures composite cylinders intended for the safe storage of hydrogen”, had for objective to generate all the scientific data necessary to improve the full set of existing requirements defined for ensuring the structural integrity of composite cylinders throughout their service life. These requirements are related to cylinder design, but also to testing procedures for type approval, manufacturing quality assurance and in-service inspection. Outcomes of HyCOMP are recommendations intended to Regulations, Codes and Standards, but also to the Industry.
HyCOMP is based on an experimental and numerical study. Damage mechanisms are first identified at a micro scale (plate specimens). Acoustic emission is the preferred technique to characterize the type of damage and its accumulation over time. A fine analysis of acoustic signals enables the classification of damage type usually encountered in composite materials: fibre breaks, delaminations, microcracks of the resin, etc. Damage accumulation is then quantified as a function of pressure loads (nature (static or cyclic) and level). Effects of environmental conditions like temperature and humidity are also assessed. These experimental results are used to feed a numerical model, able to simulate damage accumulation in composite materials, accordingly with experimental results. Both numerical model and experimental results are used to evaluate an intrinsic safety factor (iSF), covering intrinsic properties of composite materials.
Another part of the experimental work is conducted at a macro scale (cylinder structure). The effect of preconditioning on the residual strength of the cylinder is assessed. Preconditionning is a pressure test (static or cyclic) performed in controlled conditions to produce damage in the cylinder. Then residual strength is evaluated by a final destructive test (burst or cycling). Different types of preconditioning have been evaluated on both Type 3 and Type 4 cylinders: static pressure loads, cyclic pressure load (with different cycle amplitude), combination of both (and importance of the order), gaseous pressure loads, etc… It has been demonstrated that monitoring the evolution of mean value of the key parameter characterizing cylinder strength is not sufficient. Evolution of scattering is also of high importance. A probabilistic assessment of the key parameter is then proposed.
Different Non Destructive Testing techniques have been used in the project to monitor damage level on cylinder: Acoustic Emission and Optical Fiber. Both are promising techniques but require further research to be fully operational and reliable.
Another part of the experimental test program was to evaluate the influence of manufacturing process parameters on cylinder strength, on a short and long term. Different parameters were selected and implemented with big variations on cylinders. For type 3 and type 4 cylinders, initial performance is evaluated by a burst test, whereas long-term performance is assessed by a cycling test for type 3 cylinders and a burst test for type 4 cylinders after a preconditioning (sustained load simulating service life). Acoustic Emission is also used here to monitor cylinder during their first pressurization, in order to identify cylinders that deviate too much from a reference batch supposed to behave as expected.
Finally, a deep analysis of all the experimental results and existing requirements in current standards, allowed the identification of issues that needs to be improved and/or justified. A list of recommendations for design requirements and testing procedures, based on a scientific rationale, is then proposed. HyCOMP outcomes were presented during a public dissemination workshop organized on March 5th 2014 in AFNOR facilities in Paris. Audience gathered mainly people involved in standardization working groups, but also people from cylinder testing laboratories and manufacturers. Positive feedbacks and constructive comments were received from the audience.
Next step is the implementation of HyCOMP recommendations into existing standards.