Energy harvesting aims to cut aircraft maintenance costs

Engineers at aerospace and defence giant EADS have developed a wireless sensor system for aircraft that powers itself by harvesting energy from its environment.

The aim of the project is to cut maintenance costs by fitting the aircraft with health sensors. Actively tracking the plane's condition means that maintenance engineers need only take action when there really is a fault, not on suspicion or at regular intervals.

The challenge for sensor networks is to avoid adding complexity and cost. Wireless communications avoid the need to add more wiring and enable sensors to go into hard-to-access places, but the sensors then need power for their radios, hence the use of energy-harvesting.

“A wireless sensor network that supplies itself with energy on location is a good solution to collecting maintenance-relevant data at a low cost,” said project team member Dominik Samson, who is a PhD student from EADS Innovation Works.

The energy harvesting team investigated several possible ways to power sensors, including solar cells, generators that produce energy out of vibrations, thermoelectric generators, and transmitting energy via radio waves, lasers or acoustics. Of all these options, the thermoelectric method - which converts heat flow into electrical power - was the most promising.

“This technology is particularly suitable, as major temperature differences occur in and around the aircraft,” said Samson. “There is the difference between the ambient air, with temperatures ranging from about minus 20 to minus 50 degrees Celsius, and the passenger cabin with a temperature of about plus 20 degrees, for example. Then there are also the strong temperature fluctuations on the outer skin after take-off or during landing.”

In addition, an artificial temperature difference can be created anywhere on an aircraft’s outer skin by connecting one side of a thermoelectric generator to a heat storage facility, while the other side is connected to the outer skin and cools down more quickly. The difference in temperature generates an electric current. The researchers decided to use small water-filled hemispheres stuck to the inside of the plane's skin as the heat storage medium, as water can store heat for a particularly long time.

The system also needed power management to generate a voltage suitable for the sensor, and an energy buffer for periods when no power is being generated.

The sensor system has been tested in a climate chamber, demonstrating that enough energy can be produced and stored during a flight to reliably operate a wireless sensor node requiring several mW. (By comparison, a typical LED consumes 20 to 50mW.) It is also enough for long-haul flights, as the 'health monitoring' sensors do not need to be active all the time and the sensor nodes are consumption-optimised.

The next step will be to test the technology in flight.

A different team from EADS Innovation Works is investigating ways of using the heat of an aircraft engine’s exhaust. An analogous technique is also being examined in the automotive industry. “Energy harvesting could conceivably be applied to other sectors, too,” noted Samson. “Industrial machines and domestic appliances are just two examples. Energy harvesting can give machines of all kinds an artificial nervous system.”

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