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 - 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).
Generating electricity is very inefficient
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
Furthermore, there will be an additional slight loss as the battery stands idle, and the charge efficiency (also known as “charge acceptance” or “coulombic efficiency”) of the battery will deteriorate over time. And to make the calculation complete, you should actually also include the energy loss in the electrical device that you are powering (we won’t do that here).
An energy loss of 42 to 67.5 percent of naturally means that it takes 42 to 67.5 percent more effort or time to power a device (say, a blender) via electricity compared to powering the same device mechanically. This can be considered an acceptable loss if you are using solar panels or a wind turbine connected to a battery as an energy source, but it becomes rather problematic when you have to deliver the energy yourself.
If you produce 100 watts of power and 42 to 67.5 percent is lost in the conversion, there is only 32.5 to 58 watts left to power the device. If you power the same device mechanically, you deliver 100 watts straight to it. You thus have to pedal 2 to 3 times as hard or as long if you choose to take the intermediate step of generating electricity and storing it in a battery.
Traditional bicycles were not made to generate stationary power
It does not stop here. The second problem with the present approach to pedal power is that it uses a traditional bicycle on a training stand instead of a pedal powered machine built from scratch - as was the case at the end of the 19th century. Of course, using a traditional bicycle has its advantages, but again it should be realized that this approach is considerably less efficient.
One reason is the use of a so-called friction drive - the rear bicycle wheel acts upon the small roller of the motor/generator. While chain and belt drives (used in late 19th century pedal powered machines) have an efficiency of up to 98 percent, a friction drive is only 80 to 90 percent efficient (and wears much faster). This energy loss should be added to the 42 to 67.5 percent efficiency loss calculated above, which rises to 48 to 73.5 percent. Low tyre pressure will further decrease efficiency.
It should be noted that there is also energy loss in the bicycle itself: your pedals are not attached to the rear wheel itself. You turn a sprocket, which turns a chain, which turns a sprocket, which turns the rear wheel. So, on top of the efficiency loss of the friction drive should be added the efficiency loss of a chain drive (plus the energy loss in the derailleur, if your bike has one).
Additional energy losses occur when using a racing bike or a mountain bike
Connecting a bike chain directly to the generator would prevent the energy loss of the friction drive, but it implies that you have to adapt the bicycle - destroying the whole concept of today’s commercially available pedal generators.
Additional energy losses can occur when using a road bicycle to generate electricity. For example, the picture accompanying the Windstream generator shows a racing bicycle. This is a very bad choice, because the position of a rider on a racing bike is aimed to reduce wind resistance. Tests on ergometers (stationary bikes used to measure the power output of cyclists) have shown that pedalling in such a position is only about 80 percent as effective compared to a normal upright position, again resulting in considerable energy loss.
On the road the rider position on a racing bicycle is beneficial because of the large importance of air resistance. However, on a stationary pedalling machine this position has no advantage whatsoever. The popular mountain bike is equally disadvantageous because of the corrugated tyres, which of course lower the efficiency of the friction drive. In short, while using a road bicycle to generate electricity has the advantage that you can use your own bike, this does not mean you can use just any bike.
Another important drawback of using a common road bicycle is the absence of a flywheel - a heavy disc made of concrete, wood or steel that continues to generate power after it has been put in motion. In a pedal powered machine built from scratch, like the ones used at the turn of the 20th century, the flywheel applies the function of the rear bicycle wheel in the training stand (although the flywheel is mostly placed at the front of the machine). The pedaller powers the flywheel, and the flywheel powers the machine (which can be a mechanical device or a motor/generator to produce electricity).
Why is a flywheel advantageous? Because there is an important difference between riding a bicycle on the road and pedalling a stationary machine. If we are pedalling, the power exerted by our feet on the pedals is inconsistent. It peaks every 180 degrees of crank rotation, and because the two cranks are placed 180 degrees out of phase this results in two power peaks per turn of the crank. Similarly, there are dead spots in between at the top and bottom position of the pedals (to be correct this minimum torque is not zero but about one third of the maximum).
On a stationary bicycle without a flywheel, the natural pedalling rhythm results in jerky motion, limiting the energy output of the rider
On a bicycle, this uneven exertion has little effect because of the inertia of both bike and rider. But on a stationary pedal powered machine, this natural pedalling rhythm results in jerky motion and additional stress on parts.
Because of its large mass and rotational speed, the flywheel evens out the difference between power peaks and dead spots. Evening out the power input means that the rider tires less quickly and can thus generate more power. The obvious disadvantage of a flywheel is that it is heavy - from 10 to 80 kg for stationary pedal powered machines - and thus not exactly mobile.
Generating electricity is not eco-friendly
Generating electricity is not only ineffiicient, it also makes pedal power less sustainable, less robust and more costly. To begin with, batteries have to be manufactured, and they have to be replaced regularly. This requires energy, which can completely negate the ecological advantage of pedal power.
According to this research paper (pdf), the embodied energy of a 150Wh lead-acid battery (like the one offered with the Windstream pedal power generator) is at least 37,500 Wh, which equals 250 full charges of the battery (more sources: 1/2). In other words: if you can deliver 75 watts of power to the battery, you have to pedal for 500 hours in order to generate the energy that was needed to manufacture the battery. Because the life expectancy of a lead-acid battery can be as low as 300 discharge/charge cycles (sources: 1/2), you are basically pedalling to produce the energy required to manufacture the battery. If you also factor in the embodied energy of other electronics and parts, the ecological advantage of a pedal powered generator connected to a battery becomes rather doubtful. It might costs more energy than it delivers.
A pedal powered generator might cost more energy than it delivers
Of course, it also takes energy to manufacture a pedal powered machine that does not take the intermediate step of generating electricity. This concern lies mainly with the production of steel, and quite a lot of it. The commercially available Fender Blender mentioned earlier weighs 25kg (55 pounds).
If made from recycled steel, and using these figures to calculate the embodied energy of steel, this comes down to an energy cost of at least 41,625 Wh, slightly more than the battery needed for the electricity generator. If freshly made steel is used, the embodied energy is at least 138,750 Wh (3.7 times the embodied energy of a single battery). However, these machines can last at least 100 years (pedal powered machines surviving from the late 19th century are still in use), while the battery of the electricity generator has to be replaced every few years.
If we ignore the embodied energy of other parts than the battery (both training stand and electronics), and take a life expectancy of 4 years for the battery (rather optimistic), a pedalled powered generator would require an embodied energy of 937,500 Wh over the course of 100 years - 6.7 to 22.5 more than a mechanical unit. Moreover, it is easy to make the frame for a mechanical pedal powered machine from scavenged materials, bringing the embodied energy down to almost zero, while this is an impossibility for the batteries. Never mind that in addition, the toxicity of the materials is another thing to consider.
Generating electricity is less robust and more expensive
While a pedal powered machine is the most robust and resilient energy source around if you power devices mechanically, this advantage is lost when you start generating electricity. Few people can manufacture batteries themselves, so you remain dependent on a regular supply of replacement batteries.
Furthermore, the electronic parts of the machine (voltage regulator, motor/generator, converter) can break down and are not easy to make or repair yourself either - contrary to old-fashioned pedal powered machines, which can be fixed yourself with readily available materials. Mechanical pedal powered machines are generally even easier to repair and maintain than bicycles.
The extra components also make pedal generators more expensive. The commercially available models sell for \$700 to more than \$1000, not including the necessary replacements of the battery over time. Even if you make your own pedal power generator, the costs add up. The 2008 book ‘The Human-Powered Home: Choosing Muscles Over Motors‘, which has plans for several kinds of pedal powered machines, estimates the costs of a DIY generator at about \$50 (using scavenged parts) to \$350 (using new parts), not including a bicycle stand and replacement batteries. Another source estimates the cost at \$600.
The mechanical pedal powered machines in the book can be built for \$10 to \$50 (the washing machine being more expensive at \$100), everything included. While the only commercially available mechanical pedal powered machine today is very expensive too (the Fender Blender sells for \$1,700), the high cost is almost entirely due to the steel frame - which, as mentioned, could easily be replaced by the frame of an old exercise bike, or built oneself from scavenged materials. Moreover, there are no additional costs for replacement batteries and the machine is built to last for a very long time.
One way to solve the large energy losses of pedal power generators is not to produce electricity at all and power devices mechanically, whenever possible. Another way - the only way for devices that cannot be powered via a direct mechanical connection because they do not rely on rotary motion - is to make the generation of electricity more efficient.
This can be done by building a pedal powered generator from scratch instead of using a road bicycle, and/or by ditching one or several electronic components in the power transmission chain. All approaches can be combined, resulting in a pedal power unit that can power a multitude of mechanical devices and generate electricity comparatively efficiently. Read more.
Kris De Decker (edited by Shameez Joubert)
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Sources (in order of importance)
“Pedal Power in Work, Leisure and Transportation“, edited by James McCullagh, Rodale Press, 1977. Still the best resource on pedal powered machines.
“The use of pedal power for agriculture and transport in developing countries” (pdf), David Weightman, Lanchester Polytechnic, 1976
“Design of a human-powered utility vehicle for developing communities“, Timothy J. Cyders, 2008
“Supplement, Energy for rural development“, National Research Council, 1981
“Bicycles and tricycles“, Archibald Sharp, 1896
“In search of the massless flywheel” (pdf), John S. Allen, Human Power (Fall/Winter 1991-1992)
“Design and development of a human-powered machine for the manufacture of lime-flyash-sand bricks“, J.P.Modak & S.D.Moghe, Human Power (Spring 1998)
“Human Powered Flywheel Motor: concept, design, dynamics and applications“, J.P.Modak, 2007
“Make electricity while you exercise“, Mother Earth News, 2008
“Science & civilisation in China, Vol.5, Part 9“, Joseph Needham, 1987