Design Article
Aircraft structures take advantage of energy harvesting implementations
Tony Armstrong, Linear Technology Corp.
5/11/2011 8:28 AM EDT
Energy harvesting basics
Transducers that create electricity from readily available physical sources such as temperature differentials (thermoelectric generators or thermopiles), mechanical vibration or strain (piezoelectric or electromechanical devices) and light (photovoltaic devices) are viable sources of power for many applications. Numerous wireless sensors, remote monitors, and other low-power applications are on track to become near “zero” power devices using only harvested energy.
Even though the concept of energy harvesting has been around for a number of years, the implementation of a system in a real world environment has been cumbersome, complex and costly. Nevertheless, examples of markets where an energy harvesting approach has been used include transportation infrastructure, wireless medical devices, tire pressure sensing, and building automation.
A typical energy scavenging configuration or system (represented by the four main circuit system blocks shown in Figure 1), usually consists of a free energy source. Examples of such sources include a thermoelectric generator (TEG) or thermopile attached to a heat-generating source such as an aircraft engine, or a piezoelectric transducer attached to a vibrating mechanical source such as an aircraft airframe or wing.
In the case of a heat source, a compact thermoelectric device can convert small temperature differences into electrical energy. And where vibration or strain is available, a piezoelectric device can convert these small vibrations or strain differences into electrical energy. In either case, the electrical energy produced can be converted by an energy harvesting circuit (the second block in Figure 1) and modified into a usable form to power downstream circuits. These downstream electronics usually consist of some kind of sensor, an analog-to-digital converter and an ultralow power microcontroller (the third block in Figure 1). These components can take this harvested energy, now in the form of an electric current, and wake up a sensor to take a reading or a measurement and then make this data available for transmission via an ultralow power wireless transceiver – represented by the fourth block in the circuit chain shown in Figure 1.
Figure 1: The four main blocks of a typical energy-scavenging system
Transducers that create electricity from readily available physical sources such as temperature differentials (thermoelectric generators or thermopiles), mechanical vibration or strain (piezoelectric or electromechanical devices) and light (photovoltaic devices) are viable sources of power for many applications. Numerous wireless sensors, remote monitors, and other low-power applications are on track to become near “zero” power devices using only harvested energy.
Even though the concept of energy harvesting has been around for a number of years, the implementation of a system in a real world environment has been cumbersome, complex and costly. Nevertheless, examples of markets where an energy harvesting approach has been used include transportation infrastructure, wireless medical devices, tire pressure sensing, and building automation.
A typical energy scavenging configuration or system (represented by the four main circuit system blocks shown in Figure 1), usually consists of a free energy source. Examples of such sources include a thermoelectric generator (TEG) or thermopile attached to a heat-generating source such as an aircraft engine, or a piezoelectric transducer attached to a vibrating mechanical source such as an aircraft airframe or wing.
In the case of a heat source, a compact thermoelectric device can convert small temperature differences into electrical energy. And where vibration or strain is available, a piezoelectric device can convert these small vibrations or strain differences into electrical energy. In either case, the electrical energy produced can be converted by an energy harvesting circuit (the second block in Figure 1) and modified into a usable form to power downstream circuits. These downstream electronics usually consist of some kind of sensor, an analog-to-digital converter and an ultralow power microcontroller (the third block in Figure 1). These components can take this harvested energy, now in the form of an electric current, and wake up a sensor to take a reading or a measurement and then make this data available for transmission via an ultralow power wireless transceiver – represented by the fourth block in the circuit chain shown in Figure 1.
Figure 1: The four main blocks of a typical energy-scavenging system
Each circuit system block in this chain, with the possible exception of the energy source itself, has had its own unique set of constraints that have impaired its economical viability until now. Low cost and low power sensors and microcontrollers have been available for a couple of years; however, it is only recently that ultralow power transceivers have become commercially available. Nevertheless, the laggard in this chain has been the energy harvester.
Existing implementations of the energy harvester block typically consist of low performing discrete configurations, usually comprising 30 or more components. Such designs have low conversion efficiency and high quiescent currents. Both of these deficiencies result in compromised performance in end-systems. The low conversion efficiency increases the amount of time required to power up a system, which in turn increases the time interval between taking a sensor reading and transmitting the data. A high quiescent current limits how low the output of the energy-harvesting source can be, since it must first overcome the current level needed for its own operation before it can supply any excess power to the output.
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cdhmanning
5/16/2011 2:34 PM EDT
Don't forget turbulence!
Has this actually been tested on aircraft or is it just theoretical?
It seems almost pointless to do thermal harvesting around the engine since this area already has wiring. I can see some sense in doing it lesewhere though.
It also seems challenging stepping up from low-bandwidth measurement of slow-changing signals like building stress measurement and tyre pressure measurement to monitoring an airframe. I would have expected that airframe measurements would require high frequency measurements to be analysed via FFT etc.
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mysterylectricity
5/18/2011 8:58 AM EDT
Very educational article, but seriously: is aircraft the proper testbed for this nascent technology? Whereas such self-powered sensors might seem reasonable during flight, can these systems be fully checked for operation before takeoff? Not without a blowtorch or some highly trained elephants in the hangar. Whereas loss of signal indicates a hard fault in most systems, such a fault in an energy-harvesting sensor leaves considerable ambiguity: is it the sensor proper, or is it simply a lack of harvestable power under the circumstances? I for one would not want to hear, "The pilot is waiting for the Sun to heat up the wings to the point where the sensors become active" while waiting for takeoff. C'mon. This must be a joke.
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docdivakar
6/19/2011 1:02 PM EDT
This is article starts on a good premise but makes the case for acoustic emission techniques in fatigue crack propagation rather poorly. Also, there are many inaccurate statements.
There are two major types of repetitive stressors in an aircraft's life: the high cycle fatigue (flutter in wings, aelerons and control structures are good examples) and low cycle fatigue (examples include de/pressurization of the cabin which expands & contracts the fuselage, landing gears...). The fatigue crack behavior is different for HCF & LCF, described in simple terms by the Paris equation.
The optimal monitoring methodologies and tecniques for LCF & HCF effects assessment can be different and the article doesn't distinguish that at all. Secondly, the behavior of defects 'detected' by such sensor nets have to be strongly correlated between multiple states of the aircraft -while in flight, while on the ground, etc. The behavior of defects will be signicantly different in these states.
The existence of defects in any assembly process is a given and the industry has found ways to keep their damaging effects to a minimum.
@mysterylectricity: I see your point! Safety critical systems should never ever be trusted to a system without redundancy! I am for energy harvesting as a backup for structural integrity monitoring thru sensor nets, not as the primary source. Better yet, use energy harvesting for powering things like cabin lights, fans, even laptop chargers...
Dr. MP Divakar
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docdivakar
7/7/2011 2:53 PM EDT
Subsequent to my comment above, I ran into this article at IDTechEx's Energy Harvesting Journal:
Energy harvesting sensors for aircraft
http://www.energyharvestingjournal.com/articles/energy-harvesting-sensors-for-aircraft-00003545.asp?sessionid=1
One of the approaches discussed in the Energy Harvesting Journal is to tap into the temperature differentials between the outside (coooold!) and inside of the aircraft and use thermoelectric materials that can be used as heaters, coolers and generators.
This article also leaves out details on how exactly the cracks are detected. Now a days, in addition to Aluminum / Titanium alloys, Carbon composites are used also in aircraft structures. The fracture mechanisms and the propagation of cracks are different in these materials and techniques suitable for one may not work for the others.
Needless to say, there is a lot to be done here before 'systems' evolve where energy harvested (or powered) sensor nodes are used to provide real time data on the integrity of the aircraft while in operation or at rest. In short, an area of opportunity!
Dr. MP Divakar
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docdivakar
7/7/2011 8:00 PM EDT
Another useful article to read, related to the topic and discussion:
Thermoelectric Energy Harvesting
http://www.sensorsmag.com/networking-communications/energy-harvesting/thermoelectric-energy-harvesting-8708
With a 15degK temperature differential, it is possible to generate 1mW of power.
MP Divakar
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David1975
12/15/2011 8:51 AM EST
Regarding the Aloha Airlines Flight 243 incident, in addition to the injuries, a flight attendeant was was blown out of the airplane and killed.
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