Power peripherals/Bicycle Powered Generator

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.

Bicycle Generator Basics

Looking at a bicycle generator from a system level we have the following:

A human powered bicycle provides rotational motion that is then connected to a generator through some sort of transmission (through a chain, belt, gearing or frictional contact) to produce electrical energy. The power output of the generator is dependent on the type of generator used as well as the RPMs of the generator, where a higher RPM will produce a higher current and thus more power. DC motors can produce power at low RPMs, but the power output is unregulated and the motor can be less efficient than an alternator when at high RPMs. An alternator has voltage regulation to maintain a voltage output of between 13.5V and 14.5V but typically requires higher RPMs to produce power, sometimes as high as 8,000 to 11,000 RPMs, though some low speed alternators can generate power with as little as 600 RPMs. As the generator requires a higher rotational rate than what’s considered comfortable pedaling speed for humans (70-80 RPMs), some sort of gearing will be required to effectively power the generator. This can be done by utilizing the existing gearing on a bicycle or by adding an additional gearbox. A flywheel can also be added to the crank of a bicycle to both make the bicycle easier to pedal by storing rotational energy and smooth out current spikes on the generator’s output which occurs whenever the rotational speed changes dramatically (such as during startup of the generator).

To ensure than the output current is DC and to prevent power surges from harming circuitry a diode or full bridge rectifier should be added to the generator output followed by a fuse rated for no higher than 20A. From this point, the output will go through a regulator which will either divide the output power between several XOs so that not one laptop receives more than 17W at a time, or it will divert 17W of power to a single XO and the rest to a battery. Current and voltage regulation is needed to ensure that the output power to the XO is between 11V and 25V and that the output current is less than 1.5A. For power going to the battery, a charge controller is needed to ensure that the battery doesn’t overcharge and that the current and voltage is acceptable for the battery being used. When the generator stops running, the battery can then discharge to the XO until it’s fully charged. However, charging the battery and then later discharging it retains only about 80% of the power, so there are some efficiency losses through this method (Appropriate Technology; Tools, Choices, and Implications (Hazeltine 1999)). A lead acid battery is an inexpensive and attractive option, though these batteries should never be deep cycled and the charge going into the battery should be monitored carefully.

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.

Design

Bicycle Stand

[Image:Bike_Stand.PDF] [Image:Bike_Stand.PNG]

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.