Perpetual motion?

[Larry Parker]

I recently purchased an indoor “Smart Trainer”  and have been having a lot of fun with it while the roads are in their “iffy” mode with ice here and there.  Winters around here are not much fun. Rarely enough snow to even think about XC skiing, but enough freeze and thaw to create dangerous ice.  ANYWAY . . .

This thing plugs into the wall to power the sensors, Bluetooth, ANT+, and the ‘brake’ that creates the drag to feel like climbing a hill, etc. All works well but . . .  I was thinking, WHY a cord and utility line power?  I am pedaling this thing, it is eating my watts, how hard would it be to include a generator to eat some of those watts and charge a battery to power everything?  Including a Charge port (USB?) to make sure the battery starts out at full charge would be a backup, if needed.

 I wouldn’t think it would take that much, but maybe creating the resistance to make me put out 100 watts takes 100 watts?  I think that might be true, but maybe not?

It just would be great if it could be set up to do SOMETHING with the power I put into it.  Not that I produce all that much, but there are DIY designs to charge a car type battery for assistance in off grid living.     Any engineering knowledge out there to set me straight on this?  My father was an EE and would have LOVED this kind of problem, if he was still alive ( & able to tinker at 110.)

Larry Parker

[L. D. Barnes KE8QBI]

It's not a probable concept without a lot to consider. To produce Watts, the formula is Amps X Volts. A small generator like you might find on a motorcycle is roughly 10 amps and 12 volts depending on the size of the motorcycle etc. If you were able to pedal fast enough our might be able to produce enough current to power a florescent bulb. To alleviate that problem you would need a way to store your electricity such as a battery . Your battery would supply a consistent power source until it was low. You'd then have to keep pedaling to replenish your battery. That's a lot of pedaling, just get 600 hamsters hooked inline. 

[Steve Palincsar]

There are plenty of fine 6v3w dynamo hubs available for bicycles to power headlights and tail lights.  There are also connectors to allow a rider to charge a cell phone or GPS from dynamo hub power.  These hubs are very low resistance, certainly not enough to provide the drag needed to simulate hill climbing, etc., required for the indoor trainer experience. 

Note that the OP is not asking for anything like "perpetual motion," which we all know is impossible.  He's merely asking is it possible to do something with the power produced by a rider.  Dynamo hubs for bicycles show that something is indeed possible. But the amount of power available is small, as is the resistance, probably not enough to power the electronics on a "smart trainer."

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Steve Palincsar
Alexandria, Virginia 
USA

[Peter J Leiss]

The Tacx Neo will function with or without the power brick plugged in. It just won’t assist going down virtual hills. The Cold Play a band here in Canada would have people ride stationary bikes to help power their concerts. So it is possible….practical may be a different story

Peter

"John Muir: Of all the paths you take in life, make sure a few of them are dirt.”

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[Lparker_0254]

Okay. I have bikes with SON hubs & lights. Not a big deal, but the front wheel does not rotate on the trainer. I can sit and crank out enough watts to light a couple of incadescent light bulbs all day.

The question is:

If the trainer had a generator & battery, could I supply a surplus of power and not need to power from the wall?  Or will the Bluetooth/ANT+, LED indicators, and (the biggie) the resistance control eat up more than I put into them? 

Does it take 200 watts of power to resist my 200 watts of pedaling?

Larry

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[Cesar Villegas]

Larry-  Trainers "waste" your input energy into heat. You didn't mention what type of trainer you have but almost undoubtedly it has a way to dissipate heat. On my Tacx Flux, you can touch the flywheel at the end of a workout and its quite hot. Its quite easy to dissipate 200W into the surrounding air. So, NO, it does not take 200W to counter your 200W! If you were pulling 200W from the wall, then your 400W would have to be dissipated somehow. 

Yes, if your trainer had a generator hooked up to it, it could charge a battery and that energy would be available for use on whatever you want. Part of the problem is that power electronics can be a bit pricey. On a smart trainer, you're mostly likely running an ERG (the trainer has a target power, regardless of cadence). To replicate this level of controllability and reaction time on a generator set up, it would take a somewhat modern controller. All of this COULD be a little more manual and accomplished with a cheaper electronic components but you would lose a lot of the "seamlessness" of a modern trainer. 

 the electronics in your trainer (bluetooth, powermeter, leds) do not take anywhere near the power youre putting in. its probably a fraction of a watt (pulled out of my bum but based on how seldom i have to change batteries on my power meter). 

Hope this helps. 

Source: I worked in design for electric drivetrains of heavy machinery.

[Ken Freeman]

Ok, I'm an electronics engineer with about 18 years in power electronics: power converters for aircraft and spacecraft equipment, solar arrays, batteries and conditioners for spacecraft, and little motor controls systems, battery regulators and sensors.

First, there's no perpetual motion, and I think the 200 watts from your body is transmitted to back to the power line. Not 400 back to teh line, because the original 200 came from your breakfast. 

So you're assuming your pedaling dumps 200 watts into the rear wheel, which turns a roller due to non-slipping rear wheel friction.  If you drive the wheel at constant speed, then the load is, rather than the wind and work against gravity, is the gadgetry inside the electronics box in your trainer.  Together the trainer makes up a "torque generator" (probably a DC motor or a brushless DC motor) which delivers the same 200 watts to the roller that the tire is driving.  If you are putting 200 watts (sourced from your breakfast, remember!) into the bike wheel, the torque generator is delivering another 200 watts.  That 200 watts draws more than that from the house power system, because the flywheel or roller is fed power from the power line to generate resistance torque (your legs generate propulsive torque).  But that is done via a controlled power source (the power electronics), which controls that power rather precisely.  There are many ways to make one of them, but all of them involve some energy lost in the circuitry, about 25%.  So if you are delivering 200W to climb your imaginary hill, the wheel is also generating 200 watts with a loss of perhaps another 25% or 50 watts of added heating.  The trainer therefore draws 250 watts of power from your local power company and it dumps 250 into the air, from the electronics. 

So your leg power goes into the wheel, then into the roller, then into the motor and power circuits which generate resistance torque.  That circuit generates waste heat (nominally 50 watts, though it could be more or it could be less depending on the design).  

There are certainly holes in this picture, but I don't see how you can gain anything by storing the energy from your legs, short of having a large stand-alone battery.  Once that battery is full enough, you might be able to use it to operate other house equipment.

[Cesar Villegas]

Ken,

Your answer is simply wrong, and in so many ways, that I am unsure where to start. It does not take 250W from the wall (or 200W at 100% efficiency, for arguments sake) to counter a rider input of 200W. 

One can envision a system like you described with an input roller,  a dc motor and a controller. If the roller spins the motor mechanically (on the same shaft or via belt/gears), you could hook up a resistor bank across the power leads of the motor. By varying the resistance of said bank, you can vary the power required to spin the motor at a specific speed. NO EXTERNAL POWER REQUIRED. As stated before, other smart trainers already dont require external power and actually use the riders input to power its internal electronics (sensors, controllers, coms).  And if you go one step further, old school "dumb" trainers were not hooked up to the wall at all. they had a "fluid" or magnetic resistance that dissipated input power as heat.

[Ken Freeman]

Cesar, 

Overall this is what I get for trying to answer a question on which I'm rusty when it's late at night, wife is trying to talk to me, and the TV cannot be turned off. For the simplest power profiles you are correct, no energy input is needed.  If the complexity of the use case is a lot higher, active braking is needed to simulate the effects of hills with varying slopes.  I was thinking in terms of a programmable resistance system like on my Wahoo, which can add resistance based on electronic command.  It contains a power amplifier, therefore, but it processes the kinetic energy collected from the wheel by the roller. 

The 200 watts from the rider (the breakfast)  is mechanically input to the electric motor, which acts as a high power transducer to convert the kinetic energy to electrical power.  That converted 200 watts can be routed to a fixed or variable resistor.  If the electrical component is a power converter (switchmode, presumably) you have a dynamic braking system.  Now the converter is modulated to control the rate of energy absorption from the bike wheel, as would a hill, brake, or headwind. The inverter dissippates based on conduction losses, but that is the 25% loss I assumed, a very pessimistic academic assumption.  Assuming the converter is a DC/DC converter delivering power to a battery, it is actually a regenerative braking system.  Overall, the power extracted from the bicycle wheel can be dissippated mechanically or electrically, or stored electrically, or bussed to operate other electric stuff.  

I apologize for the confusion!

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Ken Freeman
Ann Arbor, MI USA