Bicycle Powered Generator

During my time as a volunteer and advisor with STEP, I was involved with the bicycle generator project. The goal is to design a demonstration system that uses bicycles to power AC devices. While similar concepts have been done before, the STEP bike generator was specifically designed to require almost continuous pedaling with only a small amount of energy storage. This is so it can be used as an effective demonstration of the amount of physical effort required to power common devices. Once you stop pedaling, the system turns off within a few seconds. There are many approaches to building a bicycle generator but I think the STEP system has some unique goals and features that I think are worth recounting. The picture below shows it powering lights during Earth Hour 2010:

To follow the progress as STEP builds an improved version of the system, have a look at the STEP bicycle generator project page.

The purpose of the bicycle generator is to advance STEP's goal of education about sustainable technology. While STEP has deployed fixed demonstrations of alternative energy (e.g. solar PV and an in-progress solar thermal installation), the group needed a smaller, more hands on demonstration. It had to be portable and attention getting. Therefore, the following goals were established:

1. Demonstrates the value of electrical energy in physically understandable terms. Thus, the system has only minimal energy storage so that if you want to keep the light on, you need to continuously provide power. Physical effort is a great way to demonstrate the value of electricity; it has been said that if modern society were ran on human power alone, each one of us has the equivalent of dozens of human slaves at our disposal.
2. Generates AC power so that any electrical device can be powered. This allows for maximum flexibility and utility when demonstrating the project.
3. Any bike can be put into the system and be used to immediately generate power. This allows the generator to be set up anywhere and demonstrated without requiring a specific bike.
4. It should be able to combine the power from multiple bicycles to power larger loads. This is so it can be used as a fundraising tool by enabling pledge drives for a bike powered event like a concert. STEP has invested in a number of components to enable this large bicycle generator to be built.

STEP built a prototype bicycle generator as a proof of concept. So far, these are the types of things that have been powered by the system:

• Charging my laptop and running two strings of rope lights:
  
  This is probably about 80W to 100W of load and takes a lot of effort. While people are perfectly capable of generating this 80W to 100W, there are inefficiencies in the system. This means that to power 80W to 100W of load, the person needs to be expending much more than that.
• 1 small house fan, 1 compact fluorescent light bulb and a guitar amp hooked up to an MP3 player.
• The STEP bubble-blowing machine (a small AC fan and small AC motor).
• Rope lights and Christmas lights for bicycle powered Earth Hour lighting.

The prototype is fairly simple. A bicycle training stand is used to hold the back wheel of a bicycle off the ground so it can be freely turned. An electric scooter motor (24VDC, 2600 RPM, 250W permanent magnet DC motor, model number MY1016) is used as the generator. A round nylon rod is fitted to the motor's drive shaft and used as a roller on the rear wheel of the bike (i.e. the rear tire is pressed against the round piece of nylon and as the tire turns, the motor is driven). This results in some friction losses due to slipping between the tire and the nylon rod so some power is lost. However, with this system, the bike's gears and the nylon rod diameter can be chosen to get the right RPM range for the motor to generate 12V DC. The entire motor / nylon rod assembly is mounted so that the rod is pressed up against the tire with some pressure. The mounting method depends greatly on the bicycle training stand and required some machining and improvisation.

The picture below shows the STEP bike generator functional prototype, where the bicycle stand is a used bike trainer that was modified to mount the generator / motor instead of the original magnetic resistance device. The electrical components are in the plastic box to the right.

STEP volunteers also designed and constructed a custom bike stand, where the motor is mounted on a hinged board that is then tensioned into position by old bike tubes. The custom stand is a little bit wobbly due to an inexact fit around the axle of the bike but is otherwise a fully working bike stand. The first picture below shows how the motor is mounted but the nylon roller (used for contacting the rear tire of the bike) was removed when I took it. The second picture is a CG composite of this same motor mounting method, but shown for another bike trainer.

There is a capacitor for energy storage. Most bicycle generators use batteries but the STEP system was designed to need continuous pedaling in order to be an effective demonstration. Batteries would take too long to charge and provide power for far too long in order to meet this goal. On the other hand, some energy storage is needed to accommodate spikes in the load and/or pauses in the pedaling. The capacitor provides a small buffer of energy for this. The STEP system uses a 58F, 15V capacitor from Maxwell. The 58F capacitance provided a very comfortable level of storage - enough to keep loads of up to 80W running while we switched riders; this was important for uninterrupted operation during longer events. Audio capacitors did not work for the STEP system. A used 1F audio capacitor failed and vented after prolonged use and some new 2F audio capacitors failed to work at all. The 2F capacitors behaved as if they had no capacitance when hooked up to the generator but they could be charged by a DC power source in series with a resistor. If anyone knows why audio capacitors do not work for this, let me know either by leaving a comment below or contacting me.

A blocking diode is required to prevent current flowing back from the capacitor into the motor. The blocking diode represents a source of inefficiency in the system. The forward voltage times the current flowing through it is the wasted energy through the diode; therefore, the forward bias voltage should be as low as possible.

Since the capacitor needs to be kept under 15V, a voltmeter is connected to the capacitor terminals and then attached to the bicycle so that the rider can see what the voltage level is. If it gets too high, the rider must stop pedaling or slow down. The voltmeter also allows the rider to see if the voltage is getting too low (the system needs at least 11.5V to run properly), in which case the rider must pedal harder.

In addition to the voltmeter, a dump load control circuit, which consists of an op amp comparator with hysteresis, is used to turn a dump load on or off depending on the voltage level in the capacitor. The dump load's job is to use up excess energy if the capacitor is charged up to 14V already. If the rider continues to pedal when the capacitor is at 14V, then the dump load will take up some of the excess energy rather than letting it go into the capacitor. This gives the rider more time to notice the rising voltage and slow down. The dump load control circuit detects the voltage level in the capacitor and then triggers a relay to turn the dump load on at 14V. The dump load remains on until the voltage drops back to 11.5V. While the control circuit was built by another STEP member, I put together this analysis of the op amp comparator with hysteresis circuit. The analysis was used to configure the circuit to switch the dump load on and off at the right voltage levels. The dump load itself needs to be chosen properly: it should be able to consume a substantial amount of the power that a person can generate with the bike generator. The current STEP system uses a 50W 12V DC light bulb (for RV use) as a dump load. This configuration is fairly easy to keep below the 15V maximum working voltage as long as the person riding the bike keeps an eye on the voltmeter.

Finally, a DC to AC inverter is used to convert the DC energy supplied by the generator and capacitor into usable AC power. Although the DC to AC inverter is designed for 12V DC operation, the off-the-shelf model that STEP uses accepts a small range of voltage levels around 12V (approximately 11V to 14V). STEP has another more expensive inverter that accepts a slightly different operating voltage range. These specifications are usually listed in the inverter product manual. The bottom line is there was some tolerance for voltage variation in the capacitor which made operating the whole system much easier.

The system diagram of the STEP bicycle generator is shown below:

The relay bypass switch is there to allow the capacitor to be discharged through the dump load independently of the dump load control circuit. This is to empty the capacitor before the system is put away after each use.

Multiple bicycles can be attached to this system and STEP has briefly experimented with this. All that was required was to attach another voltmeter, generator and blocking diode in parallel with the capacitor. With two bikes, the capacitor charges up faster and 100W loads are easier to power. On the other hand, there is less margin for error for keeping the system voltage under 15V since the dump load can only use up a smaller fraction of the total energy that the bicycle riders can put out (hence it is easier to push the voltage up to 15V). The planned multi-bike generator system will use a much larger dump load along with a much more sophisticated control circuit.

The components that were used in the prototype STEP bicycle generator are listed below.

• Voltmeter: Equus 8V - 18V Voltmeter Gauge from Canadian Tire.
• Generator: 24VDC, 2600 RPM, 250W permanent magnet DC electric scooter motor, model number MY1016. Ordered online.
• Blocking Diode: Two 6A rectifier diodes in parallel, each with peak inverse voltage of 50V. We were originally going to use one diode rated at 15A but we wanted axial diodes for ease of attaching them inline with the wires but we could not find any axial diodes rated at 15A.
• Main Capacitor: 58F, 15V capacitor from Maxwell.
• Dump Load Control Circuit: Op amp comparator with hysteresis.
• 12V Relay: 30A 12VDC relay.
• Dump Load: 50W, 12V DC light bulb (meant for RV use).
• Relay Bypass Switch: 15A DC rocker switch
• DC to AC Inverter: Off the shelf 300W 12V DC to 120V AC power inverter, with operating DC input between 11V to 14V.