Because structural integrity is so crucial to the safe operation of aircraft, their bodies are regularly inspected for signs of faults such as stress fractures. Some of these fractures can be virtually invisible to the human eye, so scientists are looking into the use of permanently-installed sensors, that would continuously provide information on the state of various parts of the aircraft. Given that one commercial airliner could potentially utilize hundreds of these sensors, however, running wiring to all of them could get quite complex. Using battery-operated sensors is one option, although ground crews would be constantly checking and changing batteries, plus it would be wasteful. Researchers from EADS Germany and the Vienna Institute of Technology now think they might have a better alternative - self-powered sensors that wirelessly transmit data.
The new sensors run on electricity produced by a thermoelectric energy harvester, built into each unit. At the heart of the harvester is a small water tank. The water will take on the temperature of the warmer air at ground level, and maintain that temperature as the plane ascends into the colder higher altitudes. The harvester is subjected to the difference in temperature between the warmer water and the colder outer wall of the sensor, and uses that temperature gradient to create electricity.
After spending some time aloft, the water will cool down. Electricity is then created on the way back down, based on the difference between the colder water and the warmer outer wall. During those times when there is no considerable temperature difference between the two, the sensors are able to run on power stored by the harvesters. One harvester could reportedly produce eight to ten milliwatt hours of energy on one flight, which would be enough to run one of the sensors.
Because the sensors transmit their data wirelessly, this means that holes wouldn't need to be drilled through the aircraft in order to run wires. Less holes would make for a stronger aircraft, while the elimination of wires and/or batteries would also make for a lighter one.
This research is reminiscent of work currently being done at New York's Rensselaer Polytechnic Institute. A student there is designing sensors that could be placed on the outside of submarines, wirelessly sending data and receiving power through the vessels' metal hulls.
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