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Just for fun - human-powered, bicycle-based generator

Old 02-14-21, 02:43 PM
  #1  
andy_p
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Just for fun - human-powered, bicycle-based generator

On my list of speculative DIY projects is "human-powered bicycle generator". Ideally, it would be made from repurposed parts, especially bike parts (as I have a lot of those). I've finally got around to thinking about this a little more, and wonder if it is even feasible as a DIY'er with very little budget (frankly, it's not worth spending much on!). I'm an electronic engineer, not a mechanical or electrical engineer, and I get a feeling I'm a little too far out on a limb here.

Starting with the generator itself, I understood that a car alternator is not a good idea, due to the very high rpm needed (1000's of rpm). I happen to have a scrap battery scooter in my junk pile, and did some calculations on the wheel and pulley size, to determine the motor rpm at estimated max power output (which I assumed would be around 12mph). It came out as 4,600rpm! That tells me that high rpms are just part of the operating conditions of a DC generator. Are there other types of motors or generators that will run at lower rpm?

Are there any feasible ways to get this kind of rpm from a human leg-powered input? It's about a 60x multiplication! It's beyond what a chain drive can easily handle, especially one made from old bike parts. I could use an initial chain-drive stage, and a 10x planetary gearbox, but I have no idea if one of those of the right spec can be sourced cheaply.

Any musings on this topic appreciated!
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Old 02-14-21, 02:57 PM
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Originally Posted by andy_p View Post
On my list of speculative DIY projects is "human-powered bicycle generator". Ideally, it would be made from repurposed parts, especially bike parts (as I have a lot of those). I've finally got around to thinking about this a little more, and wonder if it is even feasible as a DIY'er with very little budget (frankly, it's not worth spending much on!). I'm an electronic engineer, not a mechanical or electrical engineer, and I get a feeling I'm a little too far out on a limb here.

Starting with the generator itself, I understood that a car alternator is not a good idea, due to the very high rpm needed (1000's of rpm). I happen to have a scrap battery scooter in my junk pile, and did some calculations on the wheel and pulley size, to determine the motor rpm at estimated max power output (which I assumed would be around 12mph). It came out as 4,600rpm! That tells me that high rpms are just part of the operating conditions of a DC generator. Are there other types of motors or generators that will run at lower rpm?

Are there any feasible ways to get this kind of rpm from a human leg-powered input? It's about a 60x multiplication! It's beyond what a chain drive can easily handle, especially one made from old bike parts. I could use an initial chain-drive stage, and a 10x planetary gearbox, but I have no idea if one of those of the right spec can be sourced cheaply.

Any musings on this topic appreciated!
even a 50t chainring with a 10t sprocket at a cadence of 100 rpm will yield only 500 rpm. You’d need to link that to another 10:1 drive to get to 5000 rpm - I suppose you could get up to that rpm if there wasn’t too much resistance, but how long could you sustain it? Years ago as an undergrad I pondered the “people-powered centrifuge” - I even had a name for it , the “SweatSpin” - get a heavy but low friction rotor up to ~3k for 10-20 min
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Old 02-14-21, 03:15 PM
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746 Watts = 1 HP.
How much power do you plan to produce?
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Old 02-14-21, 03:18 PM
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You realize a fairly athletic youthful human is good for maybe 200-250 watts to the pedals for maybe an hour or two?
Pro level outputs are on the order of 400+ watts for 30-60 minutes at the top end, ie best 10% of the peloton.

As to the generator you might want to burrow into the characteristics of small wind power generators, ie what makes
them work at sub 100 Rpm ratings. They seem to be similar to car alternators, ie 3phase low voltage output but at much
lower rpm.

Last edited by sch; 02-14-21 at 03:29 PM.
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Old 02-14-21, 03:20 PM
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If you’re doing it for the fun factor rather than trying to harvest useful amounts of energy, a car alternator will begin charging at RPMs within bicycle range. I helped build a setup years ago. A 2” pulley on the alternator, then we ran the belt around the rim or the rear wheel. Done.
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Old 02-14-21, 03:34 PM
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Definitely just buy a windmill, set it up without the tall post (or with it, it might look cool to be pedaling up over your neighbors houses) replace the blades with a chainring and hook your bike up to it. Bingo bango.
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Old 02-14-21, 04:16 PM
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Thanks for the replies. Yes, I'm aware that even a fit amateur cyclist doesn't produce much power in the grand scheme of things :-) Hence why I'm not investing much money or resources into this project!

Also, thanks for the windmill tip-off. I'll take a look at how those guys do it.



Originally Posted by dabac View Post
If you’re doing it for the fun factor rather than trying to harvest useful amounts of energy, a car alternator will begin charging at RPMs within bicycle range. I helped build a setup years ago. A 2” pulley on the alternator, then we ran the belt around the rim or the rear wheel. Done.
Do you know roughly what kind of power you got out of that setup?

Last edited by andy_p; 02-14-21 at 04:20 PM.
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Old 02-14-21, 05:39 PM
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Originally Posted by dabac View Post
If you’re doing it for the fun factor rather than trying to harvest useful amounts of energy, a car alternator will begin charging at RPMs within bicycle range. I helped build a setup years ago. A 2” pulley on the alternator, then we ran the belt around the rim or the rear wheel. Done.
I believe that this is the third-world approach.
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Old 02-14-21, 05:48 PM
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Originally Posted by sch View Post
You realize a fairly athletic youthful human is good for maybe 200-250 watts to the pedals for maybe an hour or two?
Pro level outputs are on the order of 400+ watts for 30-60 minutes at the top end, ie best 10% of the peloton.

As to the generator you might want to burrow into the characteristics of small wind power generators, ie what makes
them work at sub 100 Rpm ratings. They seem to be similar to car alternators, ie 3phase low voltage output but at much
lower rpm.
250 to 500 watt-hours will fully charge a good-sized battery. This is plenty to run a small computer (e.g. Raspberry Pi or One Laptop per Child) for a full work day or light a small house all evening with LEDs. A 500 watt-hour battery runs my small ham radio station for in excess of a day.
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Old 02-14-21, 07:55 PM
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The bicycle would need to be connected to a torque multiplier, such as a torque converter or planetary gears. Couple that to a transfer case with the appropriate gear ratio. Have the output shaft from the transfer case direct drive a differential gear assembly that is equipped with an axle output shaft. From there, affix a pulley to the axle's output shaft & either cog belt or chain drive the generator unit.
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Old 02-14-21, 08:16 PM
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How about these gadgets?
Human Powered Generators For Sale to Generate Your Own Electricity | K-TOR (k-tor.com)
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Comcast nuked my web page. It will return soon..
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Old 02-14-21, 10:05 PM
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Years ago Buhl Planetarium here in Pittsburgh had, as one of its scientific displays, a bicycle powered generator that had the rear tire driving a friction roller. It was wired to a panel board with a series of household light bulbs from 25 to 150 watts. As you pedaled faster the bulbs lit in succession. The thing must have had huge drag losses since both the effort and cadence needed to light the 150 watt bulb were very high. Most of the kids who tried it could get the 100 and maybe the 150 watt bulb to light for, at best, a few seconds.
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Old 02-15-21, 09:14 AM
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You might find this interesting: David Butcher: Pedal Power Generator - DIY Plans

Here's the key text:

My Pedal Power History: 35 Years Researching the Power of Human Energy

The 12 Volt DC Pedal Generator you see on this site is a completely original invention. I built the first version of the 12v Pedal Generator in 1976. As an improvement over rudimentary bicycle generator and bicycle dynamo designs, I focused on efficiency and versatility. While a 12v bike generator is an alternative to my design, pedaling will be less efficient, and powering non-electric equipment may be difficult. A unique feature in my design was a 36" particle board disk with a groove routed in the edge that served as the flywheel and crankshaft for the permanent magnet 36 volt DC motor ( 1 2 ) seen at the upper right edge of the device. A small-pitch chain provided the power transfer system. The groove around the outer edge was lined with "rim strips" - thin rubber straps that prevented the chain from slipping and digging into the particle board. They are standard bicycle parts. The motor was obtained around 1980 from Northern Hydraulic, now known as Northern Tool and Equipment Company. It is a General Electric Permanent Magnet Motor, model 5BPA34NAA44, a very nice heavy-duty, ball bearing unit. I paid USD $29 for it if I remember correctly, and I still have it. The bottom frame of the Pedal Generator was welded steel plate and channel, the crankset was an American Schwinn ball bearing set, a cotterless crank conversion spindle, alloy cranks and inexpensive pedals with toe clips.

The crankset had a steel chainwheel on it. I drilled some larger holes in the chainwheel and bolted the particle board disk to it. It was strong enough (fine Schwinn steel!) to hold the weight of the particle board disk and run true. I routed an oblong hole through the particle board disk for the "arm" of the crankset.

The seatpost and handlebar tube were standard galvanized water pipe. The generator/motor was mounted on a piece of 3/4 plywood visible in the motor pictures seen above, which was then bolted to the water-pipe frame.

The particle board disk was a key feature of this unit. The weight of the disk served as an excellent flywheel. Human legs and pedals create an extremely "peaky" torque curve, resulting in jerky motion and lots of stress on parts. The flywheel smoothes this all out by absorbing part of the energy on the power stroke, lowering peak torque, and releasing it on the "dead" part of the stroke, creating torque where Human legs/pedals cannot generate any. Another thing to remember is that Human legs do not like extreme stress. The flywheel allows the Human to avoid having to generate extreme pressure during the power stroke just to make it past the "dead" spots. Many "bicycle converters" lack the flywheel characteristic because tires/rims are designed to be so light.

Noisy but extremely efficient, I have powered 12v CHAIN SAWS directly (yes, while someone else cut wood with them) with this unit.(1) Pedaling position was similar to a bicycle. The seat is barely visible at the upper left of the photo, and the handlebars (dropped, as on a ten speed road bike) are at the upper right.

Burst pedal power output: 25 amps at 17 volts (425 Watts) at 25 years old, and 265 Watts at 52 years old, and 301 Watts at 55 years old! Yes, I am in better shape than I was three years ago!

30 minute average output (back when I was in shape) 150 Watts

Accessories:

A drill chuck threaded into the end of the motor shaft provided power for a flexible shaft drive. Drilling 1/2" holes through 2x4 fir with this arrangement was easy. The flex-shaft was rated at 1/2 HP (a commercial unit, about 3/4 in. thick - not a "dremel" type!!) and I was still worried that the torque would be too much for it.

For immediate electrical use, cigarette lighter outlets provided direct access to the generator output. I even had a small 12v toaster oven, and pedaled bread to toast more than once. For electricity storage I would charge a 12v 100Ah fork-lift battery. I could approximate the output of a small 10 amp battery charger.

Instrumentation consisted of a voltmeter and an ammeter, which together provided me with state of battery charge, output watts and somewhat of a "speedometer." The math needed to determine power output was easy: VOLTS x AMPS = WATTS. A 50 amp silicon stud diode mounted to a four inch square piece of aluminum sheet metal prevented reverse current flows (which would cause the motor to turn the flywheel, instead of the other way around!), and became satisfyingly warm after long sprints. It was mounted in the center of the aluminum plate visible in the first motor picture. For top efficiency (and safety), a switch was also installed to completely isolate the diode and motor/generator from the battery.

I had to be careful - I burned out several expensive 12v halogen bulbs powering them directly. If there was no voltage control, exuberant pedaling would fry the bulbs in short order. When the storage battery was connected, this was less of a problem because the battery tended to even out the voltage, but sprinting would still raise the voltage to the danger level.

I experimented with various non-electrical devices, connected directly to the chain with their own sprockets. I substituted a ball-bearing 3600 GPH Labawco type P pump for the generator, resulting in amazing water pumping capacity. The suction from the pump was strong enough to collapse the heavy wall 1 inch vinyl tubing used for the intake (radiator hose would have been better, with the wire reinforcement) and the output shot a stream of water about 25 feet across the street. A 5 gallon bucket was emptied using this pump in less than half the time it took a garden hose to fill it. I believe the pump was driven to capacity (1 gallon per second, emptying the bucket in five seconds) in sprints.

I also tried smaller pumps, including a MATEX rotary vane pump, with great success. I have had difficulty locating that brand recently (30 years later!), but Northern Tool & Equipment carries a pump that appears to be identical. And what a great price!

I never had a chance to determine how efficient the Pedal Generator was in converting mechanical energy to electrical energy, but I believe it was probably quite good. When it was running, only 4 ball bearings were turning, the only high-speed part was the armature of the motor, and I know from research that chains can be as high as 98+% efficient in power transfer. The permanent magnet motor was probably better then average at power generation, because it was designed to be efficient as a motor. In "reverse" tests, with the motor driving the unit with no load, the power consumed was less than an amp at 12 volts. This is negligible, and much of it was resistance loss in the motor windings, since the motor drew half an amp with no load connected to it.

Status: The Original Pedal Generator never broke down, and never wore out. I still have the motor and flex-shaft, but several job-related moves finally forced me to dismantle the unit, even though it was still in perfect working condition.

(1) Three things about the MINIBRUTE 12 Volt DC chain saw. One, I was in great shape and probably was generating over one horsepower in the sprint. Two, the branch/log was about three inches in diameter - not anything near the 10 inch bar length. And three, the saw was a 12 volt saw, so it was designed to be efficient. The literature from the saw said that the motor was a permanent magnet Bosch electric winch motor, which was a good match for the maximum output of the Pedal Generator. It was great to see the chips fly!
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Old 02-15-21, 09:26 AM
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Just Google "how to make a bicycle powered generator" & same search on YouTube will get lots of info.
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Old 02-15-21, 10:50 AM
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Old 02-15-21, 02:23 PM
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Originally Posted by Jeff Wills View Post
Interesting. Not much power output for the $350 price, which tells me something about how much of a difficult problem this is.

Originally Posted by AeroGut View Post
You might find this interesting: David Butcher: Pedal Power Generator - DIY Plans
I did find that very interesting - thanks very much! I completely overlooked the issue of irregular torque over one pedal stroke. That guy is a hero, although judging by his efforts on that project, it might take me a while if I decide to try and replicate it.

The link didn't work for me, though, until I took out the "s" in "https".
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Old 02-15-21, 02:33 PM
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Originally Posted by Crankycrank View Post
Just Google "how to make a bicycle powered generator" & same search on YouTube will get lots of info.
I certainly found lots of poorly-engineered "we slung a belt around a bicycle back wheel and it kinda started charging a battery" type solutions, but that seems like a waste of effort all around.
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Old 02-15-21, 02:36 PM
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Wouldn’t it be simpler to start with an old magnetic trainer?
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Old 02-15-21, 02:40 PM
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Originally Posted by caloso View Post
Wouldn’t it be simpler to start with an old magnetic trainer?
I don't think so - they don't have any coils, or involve electricity in any way.
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Old 02-15-21, 02:58 PM
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I guess I was thinking more about the mechanical aspects of already having a machine that holds a bike without needing to modify it, that readily transmits the wheel rotation to a flywheel.
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Old 02-15-21, 03:11 PM
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I'm not an expert with these, but as I understand it, an alternator is a synchronous AC machine with a rectifier and regulator (usually on board). The regulator keeps a narrow voltage profile by varying the field current to the rotor. If you're handy enough to modify the field excitation, you might get usable DC at lower voltages at lower rpms. A great advantage of an alternator, besides the low cost of salvaging one, is the pulley mount, and you might get a better low speed torque profile.

DC motors are excellent for low-speed torque, but I think the inverse of that is difficulty in starting them in generator mode, especially with a belt drive.
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Old 02-15-21, 03:16 PM
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Originally Posted by dabac View Post
If you’re doing it for the fun factor rather than trying to harvest useful amounts of energy, a car alternator will begin charging at RPMs within bicycle range. I helped build a setup years ago. A 2” pulley on the alternator, then we ran the belt around the rim or the rear wheel. Done.
Originally Posted by andy_p View Post

Do you know roughly what kind of power you got out of that setup?
Sorry, no.
The guy I helped build it used it to charge a car battery. Then he ran a tv intended for an RV off the battery. He had to earn his screen time!
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Old 02-16-21, 11:40 AM
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Originally Posted by dsbrantjr View Post
I believe that this is the third-world approach.
The Third World is perfectly aware of how miserable the human body is at power generation. All Old World civilizations used draft animals and New World aboriginals, who didn’t, were delighted to steal horses from the Spaniards. Today, the Third-World approach is coal-fired steam generators capitalized on predatory terms under the Belt and Road initiative. Coal will be keeping the world’s lights on, and its vaccine freezers running, for a long, long, time. “Windmills for show, coal for go.”
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Old 02-16-21, 01:05 PM
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Geared DC motors are very common. The fact is that electric motors like high RPM, in all configurations. Most applications use a integrated gear box, even Tesla uses an 8:1 reduction. A 250W hub motor run in reverse would probably be a good starting place.
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Old 02-17-21, 01:42 PM
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