Technische Universitat Dresden (TUD) has developed a level sensor for liquid hydrogen based on Magnesium Diboride (MgB2) superconducting wire. The sensor is expected to be suitable for alternative fuel applications. If so, superconductivity will provide the basis for perhaps the most reliable, inexpensive, safe, and accurate sensing technology required to use hydrogen as an automotive fuel.
TUD is collaborating with the research center Forschungszentrum Karlsruhe, the gas industry, and with potential commercial manufacturers, but declined to name any specific companies, citing confidentiality agreements. In collaboration with a German automotive manufacturer, a number of liquid hydrogen automotive dewar tanks (effectively high tech thermos bottles for keeping the hydrogen fuel cool) will be equipped with superconducting level meters supplied by TUD, with realistic laboratory tests beginning in the middle of this year.
“This level sensor could potentially be used in all liquid hydrogen-fueled vehicles, as well as in hydrogen fuel infrastructure such as fuel stations, trailers, and liquefier plants,” stated Christoph Haberstroh, Assistant Professor at TUD. “The patent for the sensor is pending, and the end goal is to reach a license agreement with commercial manufacturers.”
Advances in MgB2 Key to Sensor Development
Liquid hydrogen is difficult to measure, compared to other cryogenic fluids, due to its low density and the small difference in the dielectric constant between the liquid and vapor phases. Capacitance-based level sensors are currently used in automotive applications, but these have a poor signal-to-noise ratio, with limited resolution and reliability, according to Haberstroh. TUD’s new superconducting MgB2 level sensor is based on the design of the NbTi level sensors that are used with liquid helium.
Until the discovery of MgB2 in 2001, a superconducting material with a suitable transition temperature to be used with liquid hydrogen did not exist. Neither low- nor high- Tc superconductors are appropriate for hydrogen’s boiling temperatures between 20-29K, which corresponds to a saturation pressure between 0.1 and 0.7MPa. Haberstroh commented, “The recent availability of MgB2, which is superconducting below 39K, in a wire form, has made the hydrogen level sensor possible.”
More information on how the MgB2 sensor works, issues surrounding manufacturing and commercializing the devices, and project status, see the coming issue of Superconductor Week (Volume 20, Number 20).
Superconducting level detectors work by measuring the location of the vapor/liquid interface. A superconducting filament, typically clamped inside a protective tube, is arranged vertically inside a dewar, and is connected to a current source and a voltage meter. A heating element is mounted near the top of the superconducting wire. When it is not heated, the whole filament (both in the vapor and in the liquid region) will be in a superconductive state, and no voltage drop will occur. When the heater is energized, a resistive section is generated at the top of the filament, which causes ohmic losses and thus propagates downwards. When the liquid interface is reached the propagation stops, due to better cooling inside the liquid. The length of the resistive part, and thus the location of the interface, can be derived from the voltage measurement.
Norway Developing Hydrogen-Fuel Infrastructure
Norway provides insight into how hydrogen fuel infrastructures may take shape around the world. The Norwegian national HyNor project is planning a series of hydrogen filling stations, with the initial goal of making it possible to drive a hydrogen fueled car between Stavanger and Oslo. The first filling station will be opening this month near Stavanger, and the second is planned to open in Greenland in spring 2007. HyNor is a collaboration between more than 30 industry, government, and academic partners in Norway working to promote hydrogen as an alternative fuel.
Coinciding with the opening of the Stavanger station, Mazda will show its RX-8 Hydrogen RE car, for the first time outside of Japan, at the ONS2006 energy exhibition being held in Stavanger. Mazda began leasing the dual-fuel, hydrogen and petrol powered rotary-engine vehicle to companies in Japan earlier this year.