perhaps the momentum can propel something - but would be complex mechanical drive - if one exists in the 1-st place ... the gravity vector and Earth's magnetic field are also present along with background EM radiation . . .
some starting point (chk related mov.-s)

 

Tyre pressure monitors pretty much all use batteries, is this not an option for you? How much juice are you looking for, or what are you trying to do with it?

I have a small device in the tire that is consuming around 20µA and is using a XXXmAh battery. Depending on the size of the battery this lasts very long, but at some point, I'll still have to open the tire to charge the battery. I would like to have it run by itself "forever".

There is no such thing as "free energy" ...That said, if wanting to generate electricity on a bike. there are lighting generators the have been around since before I could ride a bike, and I'm 71.

Nobody said anything about "free energy". Nobody said the electricity is needed for a light or outside the tire at all.

The second component will contribute nothing, since it is constant (piezo accelerometers output a current only during a change in acceleration).

Maybe I am using the wrong word "acceleration sensor" for it. It's actually not about acceleration (positiv or negativ) ... rather ... in my eyes it is more like a "constant" force ("made" by the turning wheel). There must be some way to use this force to gain electricity, don't you think?

 

I have a small device in the tire that is consuming around 20µA and is using a XXXmAh battery. Depending on the size of the battery this lasts very long, but at some point, I'll still have to open the tire to charge the battery. I would like to have it run by itself "forever".

You could use one of the wireless charger circuits, receiver coil inside the tyre with its position marked on the outside of the tyre so you know where to put the transmitter circuit.
For instance: https://www.seeedstudio.com/Wireless-Charging-Module-5V-1A-p-1912.html

 

Here are some links of interest:

https://www.fierceelectronics.com/embedded/powering-tire-pressure-sensors
https://www.sema.org/sema-enews/2007/21/no-batteries-needed-new-tpms-sensor
Batteryless using RFID:
https://www.researchgate.net/publication/224504617_A_Battery-Less_Tire_Pressure_Monitoring_System

A piezo (vibration based) generator has been used, as have passive transponder (RFID) technology and inductive recharging.

 

it is more like a "constant" force ("made" by the turning wheel). There must be some way to use this force to gain electricity, don't you think?

I don't think so. You want continuous energy. Energy = force x distance. Unless either the force or the distance changes continuously where's your energy coming from?

 

Any thing and especially the coil and magnet is also going to increase the effort to move the bicycle too. There is no such thing as "free energy". Then with something in the tire you will also need a slip ring and brush to transfer the energy, another type of resistance to movement.

That said, if wanting to generate electricity on a bike. there are lighting generators the have been around since before I could ride a bike, and I'm 71.

True, but TS is not asking for a huge amount of energy. So the bicycle rider won't need to see that much of a load. He's using the energy on/in the wheel, so no need for slip rings.

Two possibilities come to mind regarding coils inside the wheel. One would be a flex PCB that fits inside the tire. It would have etched coils along it's length such that when it is placed inside the tire it will form a series of coils inside the circumference of the tire. A few diodes to direct the current into the battery and there you have it. Flex circuits are designed to take constant flexing and it would be simple to install. A good strong neodymium magnet can be placed on the frame of the bike.

The other is finely wired enamel coils, each with a lead that carries the current to the battery. Again diodes will rectify the current and direct it into the battery. This one seems far more involved and I'm not sure one could make a coil robust enough to take the punishment of being inside a rubber tire.

 

...till you ran over a nail...#$@!&%#@!!!

 

A good strong neodymium magnet can be placed on the frame of the bike.

Already rejected by TS. It all needs to be inside the tyre.

 

Maybe I am using the wrong word "acceleration sensor" for it. It's actually not about acceleration (positiv or negativ) ... rather ... in my eyes it is more like a "constant" force ("made" by the turning wheel). There must be some way to use this force to gain electricity, don't you think?

No, there is not.

Think about it a little bit: if there were some device that could generate electricity from a constant force, I could just put one on my table top and stick a weight on it, and get free power. Obviously not possible; and it isn't possible with the centrifugal force on a rotating bicycle tire, either.

 

Hmm, maybe I will try it with piezo-pressure sensors instead. Multiple of them should be able to generate enough power.
But then I will still need a "collector & converter" like the mide-ehe which doesn't seem to be available anymore ... does anyone have an idea?

 

Hmm, maybe I will try it with piezo-pressure sensors instead. Multiple of them should be able to generate enough power.

Under fixed pressure they will generate voltage, but no significant power.

 

Could a peltier thermoelecric cooler module work? cooling side to the rim and the hot side facing tire tube.