This Small Business Technology Transfer (STTR) Phase II project will develop a multi-functional active fiber Bragg grating sensor technology for the monitoring and management of cryogenic fuel such as liquid hydrogen and liquefied natural gas. The proposed technology uses in-fiber light to actively adjust sensor temperature, which will drastically improve responsivity and sensitivity of fiber sensors in the cryogenic environment. By coating fiber Bragg grating sensors with functional films, liquid fuel levels, spatial distribution, hydrogen concentration, and temperature can be simultaneously measured at cryogenic temperatures. Active sensors to be developed in this program are immune to electromagnetic interference and can be multiplexed in a single fiber, which allows a one-fiber and one-fiber-feedthrough solution for the cryogenic fuel management on the ground and in space.
The broader impact/commercial potential of this project will be the development of a prudent sensing technology and system to improve the safety and reliability of the use of both liquid hydrogen and liquefied natural gas fuels. As major alternative fuels to power the U.S. economy for decades to come, they share a high economic value that requires accurate and reliable metering and management. Having a flexible, multi-use system available that can be installed with absolute confidence to monitor and manage these fuels, as well as the health of installed systems, will have a major impact on the acceptance of these volatile fuels as safe alternative energy sources. The ability to multiplex many sensors on a single fiber will enable safer and more economical penetrations in cryogenic walls and the low corrosion potential of the fibers will enable sensors to be placed along piping underground. The same basic active fiber sensor technology has the potential to be extended to fuel flow and other economically useful functions.
This STTR project was in cooperation with the University of Pittsburgh. Its objective was to develop a manufacturable optical hydrogen gas sensor and a liquid level sensor for cryogenic fuels, including hydrogen and liquid natural gas. Fiber optic sensors are of interest for these applications because optical sensors do not conduct electricity, so they are immune to electrostatic discharge, electromagnetic interference, lightning, high voltages and other sources of sparking that might cause ignition or explosions in hydrogen or natural gas storage and transmission facilities. Further, several types of fiber optic sensors can be placed on the same fiber, making multiple measurements possible in a simpler and cheaper installation than can be obtained with electrical sensors. The outcomes of the project were the demonstration of two fiber optic hydrogen gas sensors and a continuous liquid level sensing method. The hydrogen gas sensors were built on discrete sensors contained within the fiber call Fiber Bragg gratings. The continuous level sensor worked on phenomena occurring normally inside the glass optical fiber and thus did not require any other sensing mechanism, but the optical reading instrument was much more expensive than that for the Bragg grating sensors. Three hydrogen sensor types were investigated that depended on the interaction of hydrogen with palladium metal that was applied to the outside of the fiber by physical vapor deposition methods. One sensor worked by the strain caused on the fiber by the swelling of the palladium when it absorbs hydrogen and forms a larger palladium hydride molecule. This sensor worked well from about 0.2% hydrogen in air to above 5% (the explosion limit of hydrogen in air is 4%). This sensor could be used both as a quantitative sensor and an alarm sensor. The disadvantage is that vapor deposition is an inefficient process for coating the small optical fibers, so it is expensive. The second type of sensor was intended to work by a type of Bragg grating that was tilted, or blazed, to reflect some light from inside the fiber out to the fiber surface to interact with the palladium and its hydride as it transformed due to the absorption of hydrogen. The disadvantages are that this technique requires a very broad spectrum light source and is polarization dependent. This made the sensor hard to measure and it was judged unsuitable. The third type of sensor was based on a long period, or stretched-out grating, that also reflects or scatters light from the inside of the fiber to the surface. The interaction in this case is that the absorption of the light by the palladium varies with the content of hydride. This sensor was sensitive from 3% to 5% hydrogen in air and could be used as an inexpensive alarm sensor, but not as a quantitative sensor. The continuous level sensor depends on slightly overheating the optical fiber, so when the fiber is in the liquid it is colder than when it is in the space above the liquid. As the liquid moves up or down the fiber, the position of this temperature transition can be detected within a few millimeters. This is a superior method to the discrete sensors usually used that do not yield continuous information.
