I love the Simpsons. Everyone who knows me knows this, and they know that I use many of the phrases from their televised episodes. One of them is “Crisistunity“. Homer mentions such term when Lisa tells him that “Germans have the same word for crisis as for opportunity.” To which Homer replies: “Yes, crisis-tunity.” And it is just that, the crisistunities that pop up in the engineering world when something unwanted surfaces that causes an unexpected improvement. A clear example of this was the so-called “moose test” in the Mercedes A-class and the electronic stability system, or ESP (1), as well as the electronic brake distribution system or EBD.
The aeronautical industry is witnessing a trend toward miniaturization of sensors, maximization of data and minimization of weights. Systems equipped with energy harvesting may be the key in the short term.
And what is the crisis-tunity in the aeronautical world today? Well, it relates to a specific term, energy harvesting, which is not particularly new. It has existed in technical dictionaries for more than two decades, but with the latest miniaturizations, IoT (Internet of Things) devices and the non-dependence of certain systems on the electricity network, it is picking up on a noticeable critical mass.
The concept is simple, and almost self-explanatory given its name: it entails extracting energy from the surrounding environment of the system. As simple as that. But there is a nuance. For example, is a solar panel an energy harvesting system? The answer is no since that is a power generation system. A system that includes a photovoltaic cell and a Raspberry Pi (2) to monitor CO2 in a city would be a more suitable example.
So, what is new about this challenge if it has already been used for decades? The answer is still simple, but the technical explanation and its practical development is what has taken time to take shape. Now we want to feed autonomous micro or nano systems through residual or parasitic energies of the system to which they are coupled. In plain words, it would be like imitating the behavior of the parasite (a mosquito) that draws its energy from our blood to continue flying.
And thanks to the advances in MEMS (Micro-ElectroMechanical Systems) technologies of the last decade, Energy Harvesting systems can now be applied to isolated micro-systems or sensors. MEMS are based on “carving” very diverse mechanical systems in silicon (the base material of 99.9% of the chips on the market) such as springs, diffusers, membranes, etc.
A typical and easy-to-understand example is that of a MEMS that, through the vibration of the host system, manages to oscillate a plate that excites a quartz crystal and, through these electrical impulses, charges a capacitor. It is very similar to the mechanism of automatic watches that do not require winding, since they extract energy from the movement of a toothed wheel on a set of rubies or sapphires.
So, where could these systems be used? In many applications. But we will focus on those of the aeronautical realm.
Vibrations: Aircraft wings are exposed to continuous vibrations due to the aerodynamics of flight, the engines, and gusts of air. They are the perfect place to place collector systems that transform them into usable energy. For example, fuel tanks are usually located on the wings, in such a way that if we place some isolated systems that are powered by vibrations and transmit the information to the cabin or to maintenance technicians on the ground via radio frequency, they can serve as an auxiliary fuel level measurement system or, can be equipped with cameras and LEDs and conduct internal inspections without having to access it.
Heat extraction: The old method of thermal or solar thermal power plants can also be leveraged. For example, in the post-combustion stage of turbines, temperatures can be extremely high, such that micro-systems can be designed so as to leverage the heat exchange to generate electricity and, for example, create a map of temperature measurements of the nozzle to perform predictive maintenance or failure analysis in new prototypes.
Fluid circulation: Aircraft fluids (hydraulics and fuel) will be increasingly replaced by electromechanical systems or batteries; but while this happens, they can be used to generate electricity. For example, a micro-turbine can be coupled to an auxiliary fuel return flow, which, as fluid passes through it, generates an electrical current that stimulates it and measures fuel consumption, redundantly to the main systems; or include a counter of metal particles or cross-contamination of fluids, in such a way that it reinforces early warning systems.
Airflow circulation: Much like fluid generation, airflows from NACA intakes, Pitot tubes, or other airflow inlets could be used to generate electricity through micro-mills. And just as in the previous case, they could be used to power and sense difficult-to-access systems that are excluded from the main electrical system, or as redundant measurement systems for flight variables that must be continuously monitored.
Radiofrequencies: This last method, although seemingly the most modern one, is technology from the Second World War. It is based on the theory of resonant electromagnetic fields and consists of an interrogator emitting a pattern at a frequency at which the receiver resonates, returning like a mirror reflection almost all the energy emitted, but including its signature (its stored data) in the response. It is very similar to the RFID tags that we find today in many everyday products. The difference lies in the fact that, on the labels that we attach or print directly on the surfaces of the plane, we can include, for example, micro-strain gauges that measure the deformations and tensions on the surface of the plane. After a normal or test flight, all the surfaces of the aircraft are scanned with an interrogator, and each label or tag will respond with the information of the maximum deformation experienced.
As you can see, the range of options and applications is vast. This is just a small sample of applications that have been developed or are under investigation. But what is certain is that they will be increasingly used, following the trend to miniaturize sensors, maximize data (more sensors per cubic meter) and minimize weight. And in these three areas, systems equipped with energy harvesting will clearly outperform.
Leading companies in aerospace technology, such as AERTEC, are already prepared to design and implement this new aeronautical resource. Want to join us?
- You can learn more about the ‘moose test’ in this link
- Raspberry Pi is a low-cost compact computer for the development of applications or preparation of prototypes to make technology accessible to all users.