Wireless sensors installed in an aircraft’s fuselage can supply data on its condition. This in turn simplifies and cheapens the vehicle’s maintenance. To ensure that the sensor network can be installed even in difficult-to-access places and that it doesn’t weigh too much, the energy required to record and transmit the data is generated on location. This is done by using the temperature differences between the surroundings and the interior of the aircraft to produce electrical energy. 

The aim of the project is to reduce maintenance costs, which account for up to 22% of an aircraft’s overall expenses per flight hour. Sensors can inform a plane’s operator about the vehicle’s condition. This is referred to as ‘health monitoring’. The system helps to save money, as the maintenance engineers only need to take action when there really is a fault, not on suspicion or at regular intervals. However, the savings effect can only work if the sensors’ installation and operation are not too costly in themselves.

“A wireless sensor network that supplies itself with energy on location is a good solution to collecting maintenance-relevant data at a low cost,” says Ph.D. student Dominik Samson from EADS Innovation Works, a member of the project team led by Prof. Dr. Thomas Becker and Martin Kluge. This form of local energy production is called ‘energy harvesting’. This is the name given by technical experts to the conversion of unused available energy into usable (electrical) energy. The best-known example of this principle is solar cells on the roofs of buildings. The researchers in the EADS team, however, are focusing on developing minimal amounts of energy for applications with very low consumption rates. “Saving energy is not our primary goal. What we want to achieve is energy autonomy. We will save money by making systems independent of central electricity sources,” says Samson.

Aircraft offer several possibilities for doing that. The energy harvesting team investigated various technologies, including solar cells, generators that produce energy out of vibrations, thermoelectric generators, radio waves,

“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 takeoff or during landing.” In addition, an artificial temperature difference can be created anywhere on an aircraft’s outer skin. This is done 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. In deciding what to use as the heat storage medium, the researchers identified a substance that has served humanity well for a very long time: water. This liquid can store heat for a particularly long period of time. Small hemispheres filled with water are adhered to the inside of the aircraft wall, forming the most noticeable part of a ‘health monitoring’ sensor node.

A special mechanism is also needed to transform the generated voltage into a value suitable for the sensor. This is known as ‘power management’. The system also has to buffer energy so that phases without energy production can be bridged. The system developed by the researchers for experimental purposes has now been tested in a climate chamber. The result was that, given an energy consumption of several milliwatts at the sensor node, the amount of energy produced and stored during the flight is sufficient to reliably operate the sensor node. 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. By comparison, a typical light-emitting diode consumes 20 to 50 milliwatts.

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

Source/Release: EADS