Researchers at the Fraunhofer Institute for Telecommunications, Heinrich-Hertz Institute, HHI, are developing fiber-optic-based sensors that can detect hydrogen and outperform traditional sensors in several ways.
Appropriate safety measures must be followed whenever hydrogen is utilized, stored, transported, or transferred. Despite the fact that hydrogen is non-toxic and weighs less than air, harmful scenarios might arise: Indeed, if the hydrogen concentration in the air surpasses a four percent threshold, which can happen fast if a hydrogen tank is under pressure or if a space is not adequately ventilated, a modest ignition source, such as a single spark, is enough to cause an explosion.
Small, simple to use, and without any inherent safety risks
To avoid this, a proactive strategy is required, and Dr. Günter Flachenecker, the Senior Scientist at Fraunhofer HHI, understands how to do so. The physicist and his colleagues at Fraunhofer HHI’s Fiber Optical Sensor Systems branch lab in Goslar are working on new technical techniques to detect hydrogen using glass fiber sensors: “Traditional, commercially available hydrogen safety sensors, such as catalytic heat tone sensors or electrochemical cells, require an electrical power source. If the device or the electrical supply cables have a malfunction, both types might operate as a source of ignition and create the explosion that they are designed to avoid,” Flachenecker argues. “This is not a problem with our fiber optic sensors.” At the same time, they don’t require complicated wiring, are compact, and can be simply incorporated into a wide range of structures in the plant or vehicle under observation.”
Because optical fibers are strong and have a tiny diameter of roughly a quarter of a millimeter, they are practically predestined for sensing applications in a safety-related setting. An optical fiber must be changed in many locations before it can be used as a hydrogen sensor. To do this, a laser is first used to imprint particular patterns into the fiber optic core, resulting in a fiber Bragg grating, which is a periodic refractive index modulation that assures light is reflected at a certain wavelength.
The glass fiber sensing portion is then coated with a particular functional coating to guarantee that it reacts precisely to hydrogen: Flachenecker explains, “We deal with catalytic coatings, such as palladium or palladium alloys.” “Palladium, like a sponge, has the potential to absorb hydrogen. The hydrogen disintegrates into atomic fragments as soon as the two substances collide, and the hydrogen atoms that are liberated penetrate the palladium crystal structure. This generates elongation in the optical fiber, which can be monitored instantly via the built-in fiber Bragg grating as a change in the reflected light signals. The hydrogen is released from the palladium as soon as the hydrogen concentration in the air lowers again.” This implies that the coating is unharmed and the sensor may be reused. Flachenecker continues, “The procedure described here only works because hydrogen atoms are so tiny.” Other chemicals are unable to pass through the palladium layer in this manner.
A wide range of applications is possible
However, this isn’t the only way that the researchers have tried. For example, hydrogen may be detected using glass fibers with their fiber claddings scraped away or a very thin coating of nanoparticles placed on the glass fiber surface. “It’s a large playing field,” Flachenecker adds, “and there are a lot of things we want to test out.” “It’s vital that we develop methods for detecting hydrogen that is both rapid enough to avoid mishaps and reliable within the needed sensitivity range.” In that aspect, we are now making excellent progress.”
The new fiber optic sensors might, for example, become an intrinsic element of hydrogen-powered automobiles and be used to monitor hydrogen filling stations, auto repair shops, or electrolyzers in practice. Based on this technique, a bigger sensor network that monitors hydrogen infrastructure at several sites concurrently may be simply scaled up. The electronics for capturing measurement data, such as a spectrometer for optical assessment of fiber optic sensors, might be placed in a secure location apart from the sensors. If a predetermined hydrogen concentration is surpassed and the sensor detects it, the alarm management system for the relevant application is triggered, and certain actions, such as an audio warning signal, valve closures, or window openings, can be taken in a matter of seconds.
The German Federal Ministry of Economic Affairs and Climate Action is funding a study project conducted by Günter Flachenecker, which is being carried out in collaboration with a local fire protection firm. It began two years ago and will culminate this summer with the completion of an ongoing field test in which fiber optic sensors will be installed in vehicles. A follow-up project is being planned, in which the new sensors will be examined in greater detail and further preparations for certification and commercialization will be made. The objective is clear: to be able to operate with hydrogen in a safer manner that does not result in any mishaps.
