Pedal Powered Generator

What is it about?

Throughout the world there is always a need for cheap, renewable, non polluting energy sources. One of these sources is pedal powered generator. It is user controlled energy generation system that utilizes a physical energy of an individual to generate electricity.
A simplest system would consider connecting a bicycle mechanism with a DC generator in order to produce electricity.

In a gym with a lot of exercising devices and equipment, including many stationary bikes, and people training hard to improve their physical condition, a significant amount of electricity could be produced.

Why is it important?

The main importance of this system is that it only consumes human physical energy to generate electrical energy. This way, besides its main purpose, it also helps user to stay healthy and in a good physical shape.

How was it before and the advantages of this system?

Commonly used generators consume fossil fuels which are costly, limited and are polluting the environment.
Electricity generators can also be driven by using some other sources of renewable energy, like wind or water kinetic energy, but unfortunately these forms of energy are not always available.

So the advantages of pedal powered generator, compared with the other systems, are:

  • Free (besides the initial cost to build it)
  • Depending only on user availability
  • Doesn’t pollute the environment

The main disadvantage of this system is that it can generate enough power only for small appliances such as electric bulbs, mobile phones and lap top chargers, TV, etc.

How to build it?

A starting point for building a bicycle driven generator can be a permanent magnet DC motor, removed from a small industrial fan. A bicycle pedal mechanism should run DC motor as a generator, and charge a battery capable of powering lights and charging mobile phones and other small electrical devices.
First it is necessary to calculate the required gear ratio needed to achieve required output voltages at reasonable pedaling speeds. From there, there should be built a structural support for the generator, bike, and extra gears. The circuitry for the charge/discharge controllers and charge indicator should be then assembled.
The battery should provide power for at least 4 hours, without charging, from a full charge, and the system should be capable of powering components directly from generator in case of a dead battery.

The output capabilities of the permanent magnet DC motor are shown on the graph below:

As 18 to 20 volts would be sufficient to power the charge controller and provide the 13.5 - 15 volts necessary to charge the battery, from the diagram it can be found that the speed at which one should to spin the generator is roughly 625 RPM. This rotational speed is critical for determining the gear ratio needed on the bike stand.

The bicycle stand setup consists of the support structure, and the gear design. The support structure has a very simple design in which 2”x4” wood is used to create a stand on which the bicycle axel is mounted to angle brackets. The stand is made to approximately the same width as from pedal to pedal, thereby providing the necessary stability when climbing on/off and pedaling. The generator sits in drilled out pieces of plywood.

The gear design is based on the selected generator, and a gear ratio should be high enough to spin the generator sufficiently so as it charges the battery. In this case gear ratio should be at least 10:1.
To reach this, a chain is running from the largest 48 tooth gear at the pedals, to the smallest on the rear wheel axle, a 14 tooth gear. It is then mounted an additional, homemade gear of 52 teeth to the rear axle that is twice the rear cassette’s largest gear. A chain then runs from this added gear to a 14 tooth gear at the generator. The final gear ratio is 12.7:1. It is also used the original axle from the wheel, thus taking advantage of the preinstalled bearings.

The battery is selected based on the amount of time needed to operate the system at full load, and should be able to power the devices for about four to five hours. The exact battery selected is 12V, 18Ah.

The charge controller is the device that controls the voltage out of the generator, which is extremely volatile. The output voltage of the generator varies linearly with the rotational speed, and the speed that the user pedals at, is difficult to control though, so there is needed a switching circuit to maintain the voltage at a constant level. A schematic of the charge controller is shown below.

The heart of the charge controller is the buck chopper which takes the generator output, switches it with a pair of 2N3055 BJT’s in Darlington, and then smoothes out the chopped up voltage with D1, L1 and C1, which essentially act as a low pass filter.
D2 ensures that energy stored in the battery does not travel back to the generator and spin the generator as a motor.
There is also incorporated the ability to change the output voltage between the battery charging voltage (14.2 V) and 12 V which is suitable for running the loads directly from the bike generator. This switch is J2 in the schematic.

The discharge controller prevents the battery from entering deep discharge and unnecessarily shortening the life of the battery.

The discharge controller is set to limit discharging of the battery to a maximum of 50%,which helps ensure a longer battery life and better performance over the course of the battery’s life.

There also installed a charge indicator for checking the remaining battery capacity. This is done with a simple set of three LED indicators – green, yellow, and red. Green means full (or nearly full) charge, yellow indicates approximately 75% of the battery capacity remains, and red indicates that the battery is around 50% of capacity and should be charged before it enters deep discharge.

Since the battery capacity is based off the open circuit voltage, there is used a push button in order to ensure that the charge indicator is measuring VOC. This push button is normally closed to the rest of the circuit. However, when the button is pressed, all off the electronics are disconnected, except for the charge indicator. This ensures that we get the most accurate reading of the remaining battery capacity. The only time the charge indicator will not provide an accurate reading is immediately after charging or discharging. This is simply due to the nature of chemical energy storage in batteries. When the battery has been in operation, it needs time for the open circuit voltage to settle to its actual value.

Also, a flywheel could be incorporated in this system. The flywheel makes the pedaling smooth 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 user to avoid having to generate extreme pressure during the power stroke just to make it past the "dead" spots.

This generator should be capable of outputting about 60 watts total.

Another example of building pedal powered generator can be found here
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