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Harvesting Lunch for Free

I saw a presentation today at the Sensors Expo by Soobum Lee, Research Assistant Professor at Notre Dame, regarding an energy harvesting approach for tire pressure sensors. I should say at the outset that, while I’m going to do some academic nitpicking, my point isn’t to question his research; practical follow-up may confirm his ideas as useful. But it got me thinking.

The problem he was trying to solve was that of powering the sensors that detect whether your tire pressure is low using a system that doesn’t involve batteries. He presented a number of approaches that have been tried before, like using the deformation of the tires, and, for some reasonable-sounding reasons (which I won’t get into here), he explained why they were suboptimal.

His solution was almost obvious: put a magnet on the brake pad (or somewhere stationary), and then have a strap with a coil of wire on the wheel itself. If you take care of some details that he described (plus some suggested by the audience), then, every time the coil spins by the magnet, you get some currents that you can rectify and capture – in fact, his early work showed that it produced 12 times as much power as was required to power the sensor, in the low milliwatt range.

But, as anyone who has ever tried to start a lawnmower can attest, when a coil passes by a magnet, there’s some resistance. Now, he addressed the big potential issue of eddy currents in the metal wheel acting as an electric brake, so that’s not what I’m talking about. This is more subtle, and it becomes clear if you think of it in energy terms.

Using this solution, some of the kinetic energy of the rotating tire is transformed into electromagnetic energy to power the sensor. Because this kinetic energy is produced by the engine, there must be some extra resistance on the wheels that the engine has to compensate for, or else the harvested energy would be coming for free. So, in essence, the engine is having to work a bit harder to produce that extra energy and still keep the car moving at the same speed. It’s almost like a wireless transmitter of energy from the wheel to the sensor rather than a harvester.

Given that it’s only a few milliwatts extra load, in practical fact, you probably would never notice it, and, given the tradeoffs of alternative solutions, it may prove to be the best solution.

But in thinking about this, it occurred to me that you can divide the concept of “energy harvesting” into two different camps. The one we like to think of is one where we take some energy that’s not being used for anything and turn it into something useful. I’ll call this “energy scavenging.” By contrast, in other systems, we can tap into some useful energy that’s already there to do something else; I’ll call this “energy poaching,” at the risk of invoking a pejorative.

By this standard, this proposed tire pressure sensor harvesting approach is energy poaching, because it relies on the rotational kinetic energy of the tires, which is intentionally created to induce motion. Using the deformation of the tires, on the other hand, has an entirely different effect (discounting practical issues with the approach). When a tire deforms, it actually works against the effective rolling of the tire – energy is wasted in that deformation.

If some of that wasted energy is harvested instead for “good,” then, not only have we not borrowed any of the intentionally-created energy, but, in fact, if we end up with less tire deformation due to the slight extra stiffness of the wires or whatever is in the tire to capture that energy, then the wheels might actually stay rounder and roll better and contribute overall to the more efficient operation of the car – in addition to the harvested energy. It scavenges lost energy instead of poaching created energy. (Taking some license with the concept of “creating” energy, of course…)

To use another easy example, vibration is often summoned as a potential harvesting source. If you put a vibration harvester on a gasoline generator that’s buzzing noisily away, you can actually take some of the energy wasted in the form of vibration and generate yet more electricity. That would be energy scavenging. But if you took that same harvester and put it on a jackhammer, then each bit of energy used to create electricity is no longer available to crack concrete. That’s energy poaching.

The numbers may make this somewhat academic in the examples I’ve given, but it seems a productive exercise, when contemplating an energy harvesting approach, to ask the question: am I recovering energy that would otherwise go to waste? Or is my harvested energy coming at the expense of some other useful work I’m trying to do?

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