I love the Simpsons. Everyone who knows me knows that, and they know that I use a lot of phrases that appear in their episodes. One of them is "Christianity". Homer mentions it when Lisa comments that "Germans have the same word for crisis as they do for opportunity". To which Homer replies: "Yes, crisis-unity". And it is just that, the crisistunities that appear in the engineering world when there are something unwanted that causes an unexpected improvement. A clear example of this was the so-called "elk test" in the Mercedes A-class and the electronic stability system, or ESP (1), as well as the electronic brake distribution, or EBD.
In the aeronautics industry, there is a trend towards miniaturisation of sensors, maximisation of data and minimisation of weight. Systems equipped with energy harvesting may be the key in the short term.
And what is the crisis-unit in the aeronautical world today? Well, it is related to a term, energy harvesting (energy harvesting)which is not particularly new. It has been around in technical dictionaries for more than two decades, but with the latest miniaturisations, IoT devices (Internet of Things) and non-reliance on the grid for certain systems is taking on an appreciable critical mass.
The concept is simple, and almost self-explanatory with its name: It's all about extract energy from the system's surrounding environment. It is as simple as that. But there is a nuance. For example, is a solar panel an energy harvesting system? The answer is no, as that is an energy generation system. A system that includes a photovoltaic cell and a Raspberry Pi (2) to monitor CO2 in a city would be more appropriate as an exemplification.
Well, what is new about this challenge if it has 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 powering autonomous micro- or nano-systems by means of residual or parasitic system energies where they are attached. To put it more simply, it would be like imitating the behaviour of the parasite (a mosquito) that draws its energy from our blood to continue flying.
And thanks to advances in MEMS technologies (Micro-ElectroMechanical Systems) of the last decade, it is now possible to apply the systems of Energy Harvesting to micro-systems or isolated sensors. MEMS are based on "carving" very diverse mechanical systems into silicon (the base material of 99.9% of the chips on the market): springs, diffusers, membranes...
A typical and simple to understand example is that of a MEMS which, by means of the vibration of the host system, manages to oscillate a foil 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 need winding, as they extract energy from the movement of a toothed wheel on a set of rubies or sapphires.
And where could these systems be used? In a multitude of applications. But we will focus on those in the aeronautical world.
VibrationsAircraft wings are exposed to continuous vibrations due to the aerodynamics of flight, engines and gusts of air. They are the perfect place to install systems that transform these vibrations into usable energy. For example, fuel tanks are usually located on the wings, so if we place isolated systems that are powered by vibrations and transmit information by radio frequency to the cockpit or to the maintenance technicians on the ground, they can serve as an auxiliary system for measuring the fuel level, or equip them with cameras and LEDs and carry out internal inspections without having to access the aircraft.
Heat extractionThe old method of thermal or solar thermal power plants can also be exploited. For example, in the post-combustion stage of turbines, temperatures can be extremely high, so that micro-systems can be designed to take advantage of the heat exchange to generate electricity and, for example, create a map of nozzle temperature measurements for predictive maintenance or failure analysis in new prototypes.
Fluid circulation: Increasingly, aircraft fluids (hydraulics and fuel) will be replaced by electromechanical systems or batteries, but in the meantime, they can be used to generate electricity. For example, a micro-turbine can be attached to an auxiliary fuel return flow, which, as the fluid passes through it, generates an electric current that stimulates it and measures, redundantly to the main systems, the fuel consumption. Or include a metal particle or fluid cross-contamination counter to reinforce the early warning system.
Air flow circulationIn much the same way as fluid generation, airflows from NACA intakes, Pitot tubes, or other airflow inlets could be used to generate electricity by means of micro-windmills. And as above, they could be used to power and sense hard-to-reach systems that are excluded from the main electrical system, or as redundant measurement systems for flight variables that must be continuously monitored.
Radio frequenciesThe latter method, while appearing to be the most modern, is a World War II technology. 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 almost all the energy emitted, but including its signature (its stored data) in that response. It is very similar to the RFID tags that we find today in a multitude of everyday products. The difference is that the tags that we attach or print directly on the surfaces of the aircraft, we can include, for example, micro-strain gauges, which measure the deformations and stresses on the surface of the aircraft. After a normal or test flight, all surfaces of the aircraft are scanned with an interrogator, and every label or tag shall respond with information on the maximum deformation experienced.
As can be seen, the range of options and applications is vast. This is only a small sample of applications that have been developed or are under investigation. What is certain, however, is that they will be used more and more, given that the trend is towards miniaturisation of sensors, maximisation of data (more sensors per cubic metre) and minimisation of weights. And on these three points, systems equipped with energy harvestinghave a lot of points in their favour.
Leading companies in aerospace technology, such as AERTEC, are already prepared to design and implement this new aeronautical resource. Will you join us?
- You can find out more about the "moose test" at this link
- A Raspberry Pi is a low-cost compact computer designed for application development or prototyping to make computing accessible to all users.
