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Commercially Available Bike Generators Are Not Sustainable (2011)
The present approach to pedal power results in highly inefficient machines.
Pedalling a modern stationary bicycle to produce electricity might be a great work-out, but in many cases, it is not sustainable. While humans are rather inefficient engines converting food into work, this is not the problem we want to address here; people have to move in order to stay healthy, so we might as well use that energy to operate machinery. The trouble is that the present approach to pedal power results in highly inefficient machines.
When operating a bicycle generator you are basically pedalling to produce the energy required to manufacture the battery.
There are two ways to power a device by pedalling. You can power it directly through a mechanical connection - as was the case with all pedal powered machines for sale at the turn of the 20th century. Or, you can pedal to generate electricity, which is then used to power the device. In the 1970s, most research was aimed at direct mechanical power transmission. Today, the interest in pedal powered machines is almost exclusively aimed at generating electricity, for instance for charging cell phones and laptops - products that did not even exist in the 1970s.
With one exception (the ‘Fender Blender’, a pedalled powered machine to make smoothies), the only pedal powered machinery that is now commercially available in the western world (offered by Windstream, Convergence Tech and Magnificent Revolution) are stands to fit your bike to, connected to an electric motor/generator and a battery. That is a combination that can quickly convert your regular road bicycle into an electricity generator. These are also the pedal powered machines which are used for educational and arts projects, like powering a music concert, a cinema
projection
or a supercomputer, or teaching kids
the difference in energy use between, for instance, an incandescent light bulb and an energy saving lamp.
In an effort to raise awareness about energy use and global warming, the BBC even made a TV-programme in which an entire household was powered via these generators, with 80 cyclists generating up to 14 kW. These multi-person pedal power generators were pioneered in the 1970s by the Campus Center for Appropriate Technology (CCAT).
There are several problems with the present-day approach to pedal power. First of all, it is important to know that generating electricity is far from the most efficient way to apply pedal power, due to the internal energy losses in the battery, the battery management system, other electronic parts, and the motor/generator.
These energy losses add up quickly: 10 to 35 percent in the battery, 10
to 20 percent in the motor/generator and 5 to 15 percent in the converter (which converts direct current to alternate current). (Sources:
1/2/3). The energy loss in the voltage regulator (or DC to DC converter, which prevents you from blowing up the battery) is about 25 percent (sources:
1/2).
This means that the total energy loss in a pedal powered generator will be 42 to 67.5 percent (calculation example for highest loss: 100 watt input = 80 watt after 20% loss in motor/generator = 57.5 watts after 25%
energy loss in voltage regulator = 37.5 watts after 35% loss in battery = 32.5 watts after 15% loss in converter = 32.5 watts output = efficiency of 32.5% or energy loss of 67.5%).
You have to pedal 2 to 3 times as hard or as long if you choose to power a device via electricity compared to powering the same device mechanically