Power peripherals/Bicycle Powered Generator

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Bicycle Generator

Introduction

A bicycle generator can be a very efficient, inexpensive and easy to construct method for powering the XO laptop. Under such a setup, a stationary bike is connected to either a permanent magnet DC motor or a car or truck alternator which outputs a certain amount of power to the laptop. A human being pedaling at a comfortable cadence can produce more than 50W continuously for over an hour without tiring, and wattage output much higher than this can easily be achieved. A trained cyclist can put out as many as 200W for a sustained period of time and outputs as high at 450W for brief periods of time are possible (Bicycling Science). Considering that the XO can only accept a maximum of 17W at one time when charging, directing the entire output of the bicycle generator to the laptop, will still mean pedaling for about 2 hrs (the average time to fully charge the XO battery at 17W) to charge the laptop. Since consistent power outputs of 50-100W will likely be possible for a bicycle generator, the generator can either connect to multiple XO to charge several at once, or store excess power in a battery, where it can later be discharged to fully charge an XO. By using a battery, pedaling on the bicycle for as little as 30 minutes should be sufficient to fully charge an XO thus allowing human power to a be a very attractive power option for the XO.

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ILXO Proposed Bicycle Generator

This summer, the ILXO team brainstormed several generator concepts and constructed one setup to test the feasibility of a bicycle generator. The following plans are what we believe to be the best way to construct a bicycle generator inexpensively than can accommodate a range of different bicycle as well as DC motor and alternator sizes. To accomplish this, we decided on the following characteristics for the bicycle generator.

Generator Characteristics

Bicycle Connection

A key accomplishment of our design is the ability to quickly connect bicycles of various sizes to the generator quickly without having to cannibalize these bicycles. To do this, the generator is driven directly using the rear wheel of the bicycle through frictional contact without the use of a belt or additional transmission. Though this method is less efficient than other connections, this allows bicycles to easily be switched in and out of the generator without much work. The bike stand is modeled after a common bicycle trainer where a welded frame of pipe provides the structural support for the rear wheel. The frame is held in place by a simple clamp that secures to the hub and elevates the rear wheel so that it can spin freely. The rear wheel comes in direct contact with the driving wheel of the alternator or DC motor to transfer power. To ensure that there is enough frictional contact between the generator and bicycle wheel, rubber or sand can be bonded to the exterior of the generator wheel to allow it to more easily turn with the bicycle wheel.

We decided to forgo using a flywheel in this setup as it adds additional expense and complexity and serves as a potential hazard for children when in use. The regulator located farther downstream should be able to account for current spikes in the generator and a fuse can help protect against usually high currents.

Adjustable Generator Setup

For this setup to effectively work, the generator wheel must be in close contact with the rear wheel of the bicycle or power will be lost due to poor contact. Additionally, bicycle wheels vary in size anywhere from 408mm to 740mm in diameter [[1]] so the generator must be able to be adjusted so it can have the proper contact with a range of bicycle wheels. This setup uses track system for adjusting the location of the generator so that it can have to most contact with the rear wheel of the bicycle. This is done using two sizes of C channel struts with one channel only slightly larger than the other, allowing an inexpensive track to be made by nestling the smaller C channel in the larger channel. An additional set of tracks also allows different sized DC motors and alternators to easily be attached to the setup so that the system can accommodate whatever is available to generate power. Considering that there are a range of different alternators and permanent magnet DC motors, that there lacks a current sizing standard for how these motors are constructed and how we have no way of determining which motors would be most available in regions where bicycle generators might be used, having a system that can easily incorporate motors of various sizes is imperative for building the generator.

There are several ways that these tracks can be secured in place to ensure that the alternator doesn't move during operation. A press screw or C clamp can be attached to each movable C channel to allow for fine adjustment of the generator or the channels can be locked in place by drilling into the sides of the large channel and securing the smaller channel using thumb screws.

Gearing

Considering that the generator will be powered through the rear wheel of the bicycle, the gearing of the bicycle itself can likely increase the output RPMs enough to power most alternators or DC motors. Though it's difficult to determine what the exact gear ratios are for most bicycles, a bicycle will typically have between 22 and 53 teeth in the crankshaft and 11-32 teeth in the cassette [[2]], meaning that gear ratios as high as almost 5:1 are possible with many bicycles.

A human being can comfortable pedal at 70-80 RPMs without quickly tiring so let's use this cadence when calculating the resulting RPMs going to the alternator.Assuming that we're using a bicycle with wheel diameter of 740mm and that the generator wheel is the same size as the one we tested or 65mm in diameter,we have a gear reduction of 1:11.38 going from the wheel to the generator. Assuming where on the lowest gear of 53 going to 11 teeth, we have a total reduction of 1:54.85. Assuming a comfortable riding speed and 100% conversion of rotations (which would likely not be the case in practice), RPMs as high as 3850-4350 are entirely possible on this generator,though this serves as the upper limit for the operation of the generator. Thus it's best to aim for using an alternator or DC motor that can output power consistently below about 3500 RPMS. The alternator that we tested only produced power high above this amount meaning that the generator only output about 1V at 3000 RPMs and around 3V during bursts of about 6000 RPMs.

Power Transmission

Those this wasn't investigated in our prototype generator, we plan on using the generator to charge a single XO while also charging one or more batteries which can store power and later discharge it into the charging XO so that the generator can potentially be used for as little as 30 minutes to fully charge an XO. The generator would be connected to a full bridge rectifier to provide DC power and a fuse rated for 20A to prevent current from surging to the XO. The output power would be regulated to a single charging XO at 12V and 1.5A and any additional power will be bled to one or more batteries monitored by a charge controller.

Design

Bicycle Stand

Image:Bike_Stand.PNG

Shop Drawings: http://wiki.laptop.org/go/Image:Bike_Stand.PDF

The bicycle stand is constructed from several pieces of metal piping welded together to form a sufficient frame for supporting a bicycle. Either aluminum or steel can be used for the frame depending on what's readily available. A 33.7mm x 4mm thick pipe was used in this model and this provided sufficient structural strength to allow a 250 lb person to ride on a bicycle attached to the stand with a factor of safety of 2.8. Provided that pipe is at least 1" or about 25mm, it should be sufficiently sturdy enough for daily use. The frame is constructed by welding two 18" (457mm) pipes together to a perpendicular 2" (51mm) pipe in a upright triangle shape so that the top of the frame is approximately 15" (381mm) above the ground and that the two 18" pipes tilt to the side 15 degrees from the vertical forming one side of the bike stand.The other side can be constructed by repeating this setup,this time welding the 18" pipes to a perpendicular 3" pipe where 2" of the pipe stick toward the center of the bike stand. The sides are then joined by welding 2 23.25" (592mm) pipes to the base of the two sides to form the two upright triangular structures seen above. The 3" pipe is then capped at the inner end using a small metal plate.

For constructing the first part of the track used to adjust the alternator, two 14" (356mm) long aluminum or steel c channels are welded onto the end of the bike stand to create this track and the frame is squared away by welding the ends of the two channels to another length of pipe. The channels used in this model are 1"x1" (25.4mm x 25.4mm) with 1/8" thickness (3.1mm) though various sized channels may be used.

Track System

Hub Connectors

System Level Design

Additional Notes

This article is a stub. You can help the OLPC project by expanding it.

Documentation of tests/work by ILXO

August 14, 2008

  • Switched bike (small kid one) to a larger one to achieve higher rpms
  • No belt -- tire is in direct contact with alternator
  • ND alternator, serial number -- 4340202037314 -- wish there was some way to find out more about it
    • we don't know what rpms it operates on
  • Read output of voltage from B terminal and something sticking out near it - only two terminals that provided any sort of reading. Presumably, this is the right output, and not some regulated value.


Pedal:Wheel = 42:11

Wheel circumference: 80.11 in
Alternator wheel: 8 in


  • Test 1:
    • Measured output: ~1 V
    • 70 pedals/min
    • theoretical - 2676.4 rpms
  • Test 2:
    • Measured output: ~3 V
    • 27 pedals/10 sec
    • theoretical - 6193.96 rpms


We're assuming either losses in rpm through the transfers. However, adding rubber bands to minimize slippage had little measurable effect. To ensure that the alternator was being run in the correct direction, we flipped our set-up, also with little measurable effect.

Other possibilities may be that the alternator was an RV alternator -- requiring high rpms (up to 8000) -- or, it might even be dead. Uh oh.

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