Hydrogen is the most important chemical energy carrier in the transition from fossil fuels to climateneutral, regenerative energies. Already today 55 terawatt hours (TWh) to 60 TWh of hydrogen are produced and consumed only in Germany. However, this is mostly “grey” hydrogen from natural gas; only about five percent is “green” hydrogen and the trend is rising. Although the gas is not toxic, leaks quickly lead to critical situations.
Explosive mixtures are formed if air contains more than 4% hydrogen. This can happen, for example, in poorly ventilated rooms. Even a spark can trigger severe oxyhydrogen explosions. For engineers, this means that they have to set significantly higher safety standards than with hydrocarbons.
Detecting hydrogen in the air even at low concentrations with fiber optic sensors.
There have been measuring devices for detecting hydrogen in gas mixtures for years. "Conventional safety sensors that are currently commercially available for detecting hydrogen these are usually catalytic catalytic bead sensors or electrochemical cells require an electrical power supply," explains Günter Flachenecker from Fraunhofer HHI. "In the worst case, if the device or the electrical supply lines are defective, both variants could trigger the explosion themselves as an ignition source, which they were actually supposed to prevent."
These are also not difficult to wire; they can be integrated relatively easily into different applications. This includes stationary systems, but also vehicles for transporting hydrogen.
According to Flachenecker, however, lightconducting glass fibers have several desirable properties. They are robust and have a diameter of around a quarter of a millimeter, which enables them to be used in relevant applications. But first, the engineers prepared fiber optics for the planned use. Fine structures, socalled fiber Bragg gratings, were embossed into the core using a laser. These are interference filters, i.e. optical components that reflect light depending on the frequency. In the next step, the glass fibers were coated with palladium or special alloys of this metal.
"Palladium has the property that it absorbs hydrogen, like a sponge"
"As soon as the two substances meet, the hydrogen breaks down into its atomic fragments and the released hydrogen atoms penetrate the crystal structure of the palladium."
This leads to a stretching of the glass fiber and fiber Bragg gratings can be used to measure changes in the light pulses immediately. The good thing about it: If the hydrogen concentration in the surrounding air drops again, the gas escapes from the palladium and the glass fibers return to their normal geometry. The sensor can be used for a long time. Other gases, such as hydrocarbons, do not interfere with the measurement. Only small geometry molecules can interact with palladium; larger ones, such as nitrogen, oxygen or hydrocarbons, do not lead to any optical effects. The detectors are not only structurally robust, but also hardly susceptible to interference from other gases.