(Darmstadt) - The Fraunhofer Institute for Structural Durability and System Reliability LBF wants to develop a “sensor-based on-board monitoring” that enables permanent control of hydrogen pressure tanks. This is intended to ensure a “high level of safety for hydrogen vehicles”.
Hydrogen is currently stored in gaseous form under high pressure of up to 700 bar in containers made of fiber composite materials (FRP). Before their first use, pressure tanks are subjected to extensive tests to ensure safe operation over their service life, according to a statement from the LBF. In addition, “the integrity of the container must be ensured in the event of recurring loads caused by refueling and removal of hydrogen, as well as in the event of damage”. So far, tests of high-pressure storage systems have been required every two years, but only as an “external visual inspection”. In this way, damage inside the tank cannot be detected.
Permanent condition monitoring
In the HyMon joint project, scientists are now researching “Structural Health Monitoring” (SHM). According to the LBF, this is an “intelligent system for permanent condition monitoring” and damage detection.
“Thanks to our technology, TÜV inspectors receive objective information about the load on the tank after an accident, for example, and can therefore objectively decide whether it is still usable or needs to be replaced,” says LBF scientist Johannes Käsgen. On the other hand, it should “help reduce maintenance costs and ensure safe utilization of the tanks over their entire service life”.
Fiber breaks are measured acoustically
If a single carbon fiber breaks in the pressure tank, a sound wave is created that travels through the fibers, the researchers explain. Acoustic emission sensors detected this sound wave and could therefore determine the number of broken fibers. “Special load cases, such as rear-end collisions, can cause local damage to the tanks, causing many fibers to break within a very short space of time,” explains Käsgen. The measurement signals are processed and thus provide information about the “health status” of the tank.
HyMon research: Measurement data allows conclusions to be drawn about the state of fatigue of the material in the hydrogen tank. © Hexagon
The technologies for detecting fiber breaks are being developed at Fraunhofer LBF. “Sensors on the tank record the high-frequency sound waves in the event of a fiber break, the algorithms detect the fiber breaks, which are counted.” If the rate of fiber breaks suddenly increases, “this is an indication that the hydrogen tank is at the end of its useful life,” sums up the researcher.
Continuous structural monitoring guarantees an increased level of safety for hydrogen vehicles, as possible damage even from minor impacts - for example from hitting a bollard - and the remaining service life of the tank can be estimated. “Through comprehensive quality assurance, unnecessary replacement of the hydrogen tanks can be avoided.”
Glass fibers monitor stretching
Visual monitoring could also detect dangers. If “fiber-optic strain sensors” were wrapped in the FRP layer directly during the production of the tank or subsequently applied to the surface, “continuous or periodic automated monitoring of strains around the hydrogen tank” would be possible.
Light-guiding glass fibers are more suitable than conventional strain sensors due to their robustness against high material strains and load cycles. With the measurement data “the calculation models of the pressure tanks are verified and, on the other hand, insights are gained about how the material behavior changes over the service life of the tank in order to draw conclusions about the fatigue state of the material.” At the end of the research work, a test vehicle will be equipped with the technology for validation. Ultimately, a complete system will then monitor the condition of the hydrogen tanks as standard.
The consortium
The research consortium includes the Aachen-based FEV Europe GmbH, a designer and developer of conventional and alternative drives, the Darmstadt-based Fraunhofer Institute for Structural Durability and System Reliability LBF, Hexagon Purus GmbH, among others a manufacturer of battery packs for fuel cells, the developer of software for the design of High-pressure containers made of fiber composite material, Mefex GmbH, as well as the RWTH Aachen and the Cologne University of Technology.
The research is already underway and the project will be funded by the Federal Ministry of Transport and Digital Infrastructure with around 2024 million euros until the end of August 1,46.
Photos
LBF scientist Johannes Käsgen at the HyMon pressure tank: The tank is damaged, sensors detect the damage and provide data for calculation models. © Fraunhofer LBF / Ursula Raapke
