
Concept tire could generate electricity - wamatt
http://bizbeatblog.dallasnews.com/2015/03/concept-tire-could-generate-electricity.html/
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gus_massa
The article has almost no technical details. The idea of harvesting heat is
not impossible, but I think it's not practical. Does anyone have a back of the
envelope calculation? Let's try:

The problem with heat is that you can't transform it completely to
electricity. By the Second law of thermodynamics (
[http://en.wikipedia.org/wiki/Second_law_of_thermodynamics](http://en.wikipedia.org/wiki/Second_law_of_thermodynamics)
), the maximal transformation ratio is (Tmax-Tenv) / Tmax, where Tenv is the
environment temperature and Tmax is the maximal temperature. And this is an
upper bound, real cases are even smaller.

So, if you want to transform a lot of heat to electricity, you'd better have a
very hot spot. That's why the concentrated solar power plants concentrate the
light in a small place to get a high temperature. The same idea applies to
other form of work, so this is the same reason the car engine is hot (and it's
hotter inside the cylinders).

Back to the tires, I don't know the temperature of a tire, but I never heard
of someone getting burned by touching a car tire. I'd guess that the maximal
temperature is 50°C(122°F) and let's assume a cool day environment temperature
of 10°C (50°F). The efficiency calculation must be done in the Kelvin scale,
so the hot temperature is 323K and the cold temperature is 283K, do the
maximal possible efficiency is (323K-283K)/323K ~=13%.

So, at most the 13% of the heat of the tires can be transformed to
electricity, but not all the energy of the fuel goes to heat the tires. At
high speed most of the energy goes to moving the air, some energy goes to the
air conditioner system, some is dissipated in the radiator and with the
exhaust fumes, ... So the maximal theoretical efficiency is 13% of a small
part of the fuel energy, probably less than 1% overall.

~~~
maxerickson
This article is somewhat better, it is at least more measured:

[http://blog.caranddriver.com/goodyears-concept-tires-
shift-s...](http://blog.caranddriver.com/goodyears-concept-tires-shift-shapes-
and-generate-electricity/)

There are places in a car where you need resistance, tires are one of them.
Trying to make that resistance electrical in nature isn't completely nuts, but
I think it is hard to predict how worthwhile it is in each case.

(brakes are the obvious one, harvesting that energy is already a pretty big
win for vehicles, the article I link mentions harvesting energy from shocks)

The Car and Driver article also mentions piezoelectric materials in the tires,
which at least sounds more sensible than just harvesting heat.

~~~
gus_massa
I agree, this article it's more skeptical

> _Goodyear gives no indication as to exactly how much electricity the BHO3
> could generate, but it does make the rather bold claim that “this visionary
> tire technology could eliminate the vehicle-range anxiety motorists may have
> with electric cars.”_

Brakes are and interesting case. (I guess that you understand all of this, I
only wanted to make the details more explicit.)

One important detail is that in brakes you convert the kinetic energy to
electric energy directly. They didn't plug an electric generator to harvest
the heat of and old disk brake.

Another detail is that the car has a lot of energy stored as kinetic energy,
so you have a lot of energy to convert when you try to stop the car. (I'm too
lazy to do the calculations now.)

I agree that harvesting energy from the deformation looks more promising that
harvesting heat.

