(Berlin) - If the hydrogen concentration in the air exceeds a threshold of four percent, this gas mixture is highly explosive. If there is enough pressure in a hydrogen tank or if there is insufficient ventilation in a room, this can be achieved quickly - "a small ignition source, a single spark, is enough to trigger an explosion," explain researchers at the Fraunhofer Heinrich Hertz Institute (HHI).
There are commercially available safety sensors. However, these required an electrical power supply. If the devices or the electrical supply lines were defective, “in the worst case, they could act as an ignition source and trigger the explosion that they were actually supposed to prevent,” says Günter Flachenecker, Senior Scientist at Fraunhofer HHI.
Glass fiber sensors
In the Fiber Optic Sensor Systems Department at Fraunhofer HHI in Goslar, physicists are researching ways to detect hydrogen using sensors made of glass fibers that do not require electricity. “At the same time, they do not require complex wiring, are small and can be easily integrated into a wide variety of structures in the system or vehicle to be monitored,” says Flachenecker.
According to the researchers, light-conducting glass fibers are “predestined for sensory applications in a safety-relevant environment” due to their small diameter of around a quarter of a millimeter and their robustness. However, it would have to be modified for use as a hydrogen sensor.
Palladium layer
To ensure that the glass fiber reacts specifically to hydrogen, a functional coating is applied around the glass fiber jacket: “We work with catalytic layers, for example palladium or palladium alloys,” says Flachenecker. “Palladium has the property of absorbing hydrogen, similar to a sponge. As soon as the two substances meet, the hydrogen breaks down into its atomic fragments.”
The released hydrogen atoms would then penetrate the palladium crystal structure. The previously modified fiber optic is stretched, which leads to a change in the reported light pulses and can be measured. “As soon as the hydrogen concentration in the air decreases again, the hydrogen dissolves again from the palladium,” say the scientists. The coating and the sensor are reusable.
Other methods are also being investigated to develop current-free sensors using glass fibers. Flachenecker: “It is crucial for us to find options for hydrogen detection that are fast enough to prevent accidents and that react reliably in the required sensitivity range.” We are “currently on a very good path.”
Dangers require reactions within seconds
The electronics for recording measurement data can be “installed as far away as you like in a safe location”. If a certain hydrogen concentration is exceeded and the sensor goes off, various safety measures can be initiated and triggered within seconds, such as an acoustic warning signal, closing valves or opening windows.
The new sensors could become part of hydrogen-powered vehicles and, for example, monitor hydrogen filling stations, car repair shops or electrolysers. In addition, “building a larger sensor network that monitors a hydrogen infrastructure in many places at the same time” is easy to implement, explain the HHI researchers
The research project is funded by the Federal Ministry of Economics and Climate Protection and takes place in cooperation with a fire protection company. It ends in the summer after fiber optic sensors are installed in trucks for a practical test. A follow-up project is planned.
Photos
Scientists are working on sensors to prevent hydrogen and oxygen from combining into an explosive mixture. / © DLR, Thomas Ernsting
