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What is the smallest wind turbine available on the market that has been installed on a bicycle? and what about R&D or even conceptual wind turbine?

This question is a follow up to question why are regenerative brakes uncommon on e-bikes?

Since regenerative brakes are unlikely to appear on bicycles, maybe someone already tried to approach adding some aerodynamic drag to recharge the batteries ...

I apologize for not mentioning it clearly: I am interested to have aerodynamic battery charging as an alternative to regenerative braking, i.e. to use it when not powering the bicycle by motor/leg movement, to be more explicit, the micro-turbine should be acting while braking, going downhill, possibly coasting.

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    You shouldn’t trickle charge lithium batteries, in theory it’s possible with solar panels and investors, but the panels required and investors required to recharge something like a 48V cell would be bigger than the bike. You would be better harvesting the power and just charging your ebike normally – Dan K Aug 25 '20 at 14:09
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    Extracting energy from the air flow is no different than using a wheel-mounted dynamo. In both cases the energy comes from the cyclist's pedaling. – Andrew Henle Aug 25 '20 at 14:20
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    You are trying to make a perpetual motion machine. The energy to charge the battery is coming from the battery or the cyclist. If it comes from the cyclist, the battery is a net negative. – Ross Millikan Aug 25 '20 at 14:40
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    I guess if we’re discussing wind energy, why not just mount a mast to to bike, attach a sail and use the wind. Throw out the heavy battery and motor and just use wind as a back up energy source. – Dan K Aug 25 '20 at 14:59
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    @DanK - I did use a sail on my bike, back around 1962. Discarded it after about 2 days, though, since the wind was always blowing the wrong direction. – Daniel R Hicks Aug 25 '20 at 19:03
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Hilariously, such a product does exist. It comes from notorious crapgadget vendor Thanko. Note that it only purports to charge two AA batteries, and it's not an efficient way to do that. They make no estimate of how long it would take to fully charge those batteries.

I did some playing around with this calculator. Based on some guesses and estimates, it looks like the Thanko wind turbine would generate 0.001 kW, or 1 W of power with a wind speed of 20 km/h (this includes default estimates for efficiency, which are probably very optimistic). A single AA LiIon battery has about 3 watt-hours of capacity, so it would take 3 hours to charge (assuming a 100% efficient charger).

If you were trying to power a 250-W e-bike motor with that AA battery (and could magically convert its voltage at 100% efficiency) you'd get 43 seconds at full blast out of it.

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    Thanks. Rough estimates, power goes with the square of the length of the blade... so by having a 4 times larger blade, at 20 km/h you get 10 times power ... things start to add up and they became interesting. If you are going downhill, from Nepal mountains to Bangladesh seaside :) ! – EarlGrey Aug 26 '20 at 6:49
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    @EarlGrey However, that power is the drag it makes so it will be much harder to sustain those 20 km/h. And I do not believe you could easilly feather the prop. Even many small airplanes do not have this feature, it is pretty complicated. – Vladimir F Aug 26 '20 at 14:43
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    Wow, someone found a way to make something equivalent to a bottle dynamo, but much worse! – Chris H Aug 27 '20 at 14:56
  • @VladimirF is it feathering that's complicated, or variable pitch? I would think that variable pitch would be complicated, but feathering (simply having an engaged and disengaged position) would be simpler. – Duncan C Aug 27 '20 at 18:38
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    @DuncanC "Feathering" a propeller is changing the pitch so the blades are parallel to the airflow and the blades no longer turn - it can only be done with a variable-pitch propeller. Allowing the blades to turn freely when disengaged is "windmilling", and that still adds considerable drag. Which is why larger multi-engine propeller-driven planes tend to have variable-pitch propellers that can be feathered - when one engine dies the loss of thrust isn't compounded by the drag of a windmilling propeller. – Andrew Henle Aug 27 '20 at 19:34
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Ross's comment about a perpetual motion machine is bang-on. The second law of thermodynamics says that any transfer of energy from one form to another is wasteful.

This is a machine that tries to take the wind generated by forward motion, convert it to rotational energy, convert that to electrical energy, convert that to chemical energy (in the battery) and then convert chemical energy back to electrical, to rotational energy, and finally back to acceleration.

Trying to convert wind energy to electricity will increase the drag on the bicycle by more than the amount of rotational energy you would see in the turbine, and then you'd get more losses when you convert that rotational energy to electricity, and still more losses when you convert that electrical energy to chemical energy in the battery.

Any turbine and generator system will also increase the weight of the bike, meaning it requires more power to accelerate the bike.

The turbine will increase the drag on the bike, and require more power from the drive system (rider and/or motor) to compensate for it - quite a bit more power than the amount of power that would be collected in the battery.

Saying "It's a micro-turbine, the drag is small" is silly. If it generates a very small amount of drag, it generates an even smaller amount of power.

In theory you might be able to only deploy the turbine when the bike is coasting downhill and recover energy that would have been lost to braking. That means, however, that you would only generate electricity on steep downhills where braking would have been required - a small portion of most rides. Plus, any turbine large enough to collect meaningful amounts of energy is going to be both large and massive, and the simple volume of space it takes up is likely going to increase the total drag of the bike even when it is retracted. A mechanism to deploy and retract the turbine would also be relatively large, complex, and heavy, and require some power to operate.

All in all, it seems likely that any such system would reduce the range of the e-bike, not increase it, and it would certainly make it heavier.

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