__High power Pedal generator
(300 watts) __15/04/10

Click image for Hi res version

This is my latest attempt at building a highly efficient powerful pedal generator.

Pedalling a stationary bicycle can at times be a little tiresome therefore I wanted to make an as efficient as possible pedal generator so for every joule of energy I exert I then harvest the maximum amount of power out. Some of the pedal generators I have made in the past have been rather inefficient with a typical output of around 100 watts. This latest project has yielded 3 times that figure.

My earlier pedal generator projects here

At the heart of this project is the bicycle hub motor, the one I chose was a 24v 500 watt brushless unit, my thinking was the lower voltage version (36 and 48v versions are available) would have heavier gauge coil windings thus coil resistance being lower and efficiency better (0.3 ohms per phase in my case) of course the lower voltage hub motors would need to turn faster to generate a charging voltage into a 12v battery, this was not a problem as I would use gearing to achieve this. My hub motor produced 1 volt per 18 rpm. Therefore a 12v 'cut-in' would be achieved at 12.5 x 18 = 225 rpm.

An excellent article on the various types of hub motors available here

These hub motors can be purchased for around £50-£100 for a second-hand unit, they occasionally appear on eBay, and local bike shops. China reportedly manufactured 12 million such motors in 2009, so more and more second-hand units should become available soon. It is important to choose a non internally geared motor to reduce losses in additional friction.

The hub motor of course needs to be a rear wheel type ideally with a multi gear freewheel sprocket attached. Once a comfortable chosen gear ratio is found, mine turned out to be a 44t chain wheel driving a 13t rear sprocket (or 3.38:1 ratio) you could replace the multi sprocket freewheel with a BMX single speed freewheel and remove the derailleur to increase efficiency further.

Wiring up is pretty straight forward, simply a 3 phase bridge rectifier and suitable heat sink, an optional watts up power meter can be used and is very useful for monitoring performance. The inclusion of a 30 amp fuse connected just before a suitably sized (60-100 A/h) lead acid battery is essential.

The video below proves how I, not an athlete by any means can generate up to 300 watts of power in bursts, 150 watts was achievable for long periods, typically 30 minutes.

I suspect a very fit person could sustain 300 watts for possibly 10 minutes, to give that some real world comparison that amount of power would run a 17" screen (30 watt) laptop for 1 hr 40 minutes.

For further discussion on this subject we have a very active forum with a human power generator section here