this post was submitted on 24 Aug 2024
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How is that clean energy, in any sense of the word? Any system that gains some energy from a passing car must necessarily decrease the (kinetic) energy of the car by an equal or greater amount. And the vast majority of cars get their kinetic energy by burning fossil fuels. Sounds like a more expensive, less direct, and less efficient version of a gasoline generator.
It depends. Is this energy the same energy that is already being burned?
Looking at an extremely simple example: Solar powered calculators (the real ones). They harness light from the light bulbs in a room which would be otherwise dark. The only time they are harnessing that power, is when the light is on. They add no extra draw to the light, they are 100% passive. The only time you’d really have to take into consideration how green that power is (explicitly for the calculator) is if you are turning on that light explicitly to power the calculator.
If the tech being talked about is just harnessing the “junk energy” of the vehicles in their normal operation, this would be 100% green energy. If it is adding a load, it is 100% dirty when powered by an ICE. If it’s somewhere in the middle… I hope you get my point.
You’re also ignoring the fact that not all vehicles are ICE.
Ehh it's still a rubbish idea, that money would be much better spent going after primary producers of energy, like solar, wind, geothermal, or nuclear.
Some napkin math and an equivalent area of solar, say over a road or parking lot would produce 3.5 million kwh in a year.
Those aren’t always a fit for everywhere. And getting energy from one place to another is an unsolved issue. Just because one option is cheaper than others doesn’t mean that particular option is the better choice. Diversity is very important.
Diversity is important, but it's still better to go after larger sources of energy first. There's just not much energy to be recovered from falling rain or waste from cars.
Make the cars waste less energy, or the transit system in general is much easier and will actually save money long term.
What kind of junk energy is there to harvest from a car (in meaningful amounts)? I guess breaking is the obvious answer, but that's already covered by regenerative breaking. Most car-based energy harvesting systems seem to employ speedbumps that clearly take useful (kinetic) energy away from the car (probably at a very poor efficiency).
That's a fair point, a device could theoretically harvest energy that would have otherwise been wasted, and that would be green energy. I imagine a wind system could work, though it might result in cars experiencing additional drag from slower wind speeds.
However, the piezoelectric generators mentioned in the article quite clearly do not use waste energy. They compress under the weight of the cars, turning a small amount of gravitational potential energy into electricity. That energy must be made up with extra fuel.
Finally, even if all of the vehicles on the road were powered by clean electricity, it would still be a useless system. Piezoelectrics are nowhere near 100% efficient, so you're just taking electricity from the vehicles at a loss.
I mean, I can think of many ideal places for such things. Intersections for instance, where cars will be wanting to slow down or come to a stop regardless. Speed limit changes, where you want cars to slow down. Even in place of speed bumps, assuming you can get enough of a drag on the car to effect a slow down.
Now that's an interesting idea; basically external regenerative braking. Not too helpful on a highway, but I suppose it would be useful in the situations you described.
How would a turbine that takes energy from the air current generated by a passing car decrease the energy of the car? The car has imparted the energy to the air, the air has already extracted energy from the car aerodynamic or directly Newtonian forces, and then the turbine would extract energy from the accelerated air from the passing vehicles.
IIRC there was a real world test of this idea some time ago and the results weren’t great. But imagine you do this in a tunnel- that will help improve the results and answer your question.
If you’ve ever been in a subway you’ve probably noticed the train pushes a wall of air ahead of itself. This is the energy you would need to capture. But by “capturing” it, you would inherently increase resistance on the airflow, and therefore the vehicle.
The reason you can’t imagine this being an issue in a typical roadside is the same reason the effect yields poor results. There is little in the way of channeling the air across the turbines, which also means there is little resistance on the passing vehicles. Of course the air in the atmosphere, pushing against the air your car moves ahead of itself, is the classic example of this overall effect.
I think the turbine would have to be close enough to slow the airflow boundary layer increasing drag on the moving train and/or cause tbe train to need to re-accelerate slowed air, and that would have to be uncomfortably close turbines. We’re not even getting into the weeds about the drag cause by the tunnel walls and any structure protruding from it, or how aerodynamic the train is in the first place yet.
Cars are as you point out a completely different animal, and the effect of any turbines would affect the next passing car only if a) the car were close enough to benefit from the “draft” of the preceding car, and b) if a is true, then it would lose energy re-accelerating air energy lost to the turbine.
I actually did give it more than a passing thought, and even if it’s just an esoteric discussion, the energy losses wouldn’t be worth calculating, and the gains minimal.
Not sure where you got that idea from, but how would that generate a meaningful amount of energy? It seems very unlikely that such a system would ever recover the energy spent on its construction.
I think it’s because you said, “Any system that gains some energy from a passing car,” not that anyone mentioned turbines explicitly.