Everybody will have seen things like wind-up torches and of course Trevor Baylis is famous for his clockwork radio. So why canít the same technology power bigger things like power tools or vehicles? The answer is partly the technology and partly human considerations.
The mechanical springs that are used to power clockwork devices are not very good at storing large amounts of energy. They are fine for their original purpose which was to power a clock mechanism over a long period of time but they just canít store enough energy to run anything more powerful for very long. Trevorís original radio was powered by a heavy spring that could only store enough power to run the radio for about 20 minutes before it needed winding again. To overcome this, later versions of the radio and all later windup devices use an electrical battery that is charged by a generator to store the power needed. A small AA size battery can store much more energy than a metal spring weighing hundreds of grams.
So this raises the second point. If a battery can store a lot of energy, why canít they be used to run all sorts of appliances? Well of course they are; everything from mobile phones to power tools and of course electric vehicles. The problem is charging up the battery. If the battery can be charged from a power source like mains electricity then there is no problem. However if we try and do it using human effort we soon find that it is very difficult. This is because humans are not very good at generating large amounts of power.
To understand the problem we need to explain some of the basic principles of energy and power.
Energy is the capacity to do work and power is the rate at which that energy is used. So a battery can store energy and the bigger the battery the more energy it can store. A tool like a hand drill measures its power by how fast it can convert the energy in the battery into useful work. Energy that is stored in a battery is measured in Joules. We have a more detailed explanation on our page about energy. The power of a machine is measured in Watts and one Watt of power is delivered by converting one Joule of energy per second. So an electric motor that has a power of 100 watts is converting energy at the rate of 100 Joules every second. This means that to keep the motor running for 10 seconds we would need a battery that stored 10 x 100 or 1000 Joules of electrical energy. For comparison, a one horse-power motor is the same as a motor that can generate about 750 Watts of power. An AA torch battery can store about 3,000 Joules of energy but because of the chemistry and internal heating it can only deliver one or two Watts of power.
The Power consumption of various machines and devices is illustrated by this table.
A mobile phone uses about 2 Watts of power or put another way it uses two Joules of energy per second. To run the mobile phone for a minute needs 120 Joules of energy. The AA battery described above can run a mobile phone for about half an hour.
So how much power can a human generate to charge up a battery to run these devices? Unfortunately the answer is not very much because we humans are not very good at it and we get tired very quickly. The strongest muscles in our bodies are in our legs and using a machine like a bicycle it turns out that we can generate about 100 Watts of power more or less continuously if we are reasonably fit. This is roughly equivalent to riding a bicycle up a long gentle slope. A really fit Olympic standard cyclist might be able to generate about 400 Watts (about half of one horsepower) for short periods. An ordinary AA size battery can store approximately 3,000 Joules of energy although because of the chemistry and internal heating effects they typically can only deliver one or two Watts of power. This means that an AA battery can light a 1 Watt lamp for about an hour.
If we look at hand powered devices the figures are much lower. The most power that a human can generate for long periods with a simple hand generator, like a wind up torch, is about 5 Watts. Using a two handed generator we may be able to generate about 30 Watts for short periods.
So if we apply these capabilities to the power table above, how long would it take a human to create enough energy to run the devices for only one minute?
You can see from these figures that a couple of minutes of winding a charger like that in a windup torch can generate enough power to keep a mobile phone going for about two minutes. Good enough for use in an emergency or for occasional calls in remote locations.
However to run a small laptop computer for one minute would take ten minutes of winding with the same charger. If you try this you will find that it is almost impossible to wind continuously by hand for more than about five minutes. Most people give up after two or three minutes.
To give even a small vehicle like a scooter enough power to run for one minute you would have to wind continuously for nearly a day and a half, and to move a small car for a minute you would be winding day and night for nearly a week.
So this is why windup devices really donít scale up very well. We have successfully produced windup torches and radios, and even a windup mobile phone but a wind up laptop computer is a challenge that is still difficult to meet unless the engineers succeed in creating one that runs on much lower power than current units. The biggest challenge is the display and devices like eBook readers such as the Kindle have a very long battery life because the e-ink displays that they use require power only when they are changed.
We hope that this explanation has helped you to understand why human powered devices cannot be very powerful. For devices that consume more than a few Watts of power Solar Cells are probably a better way to go. Obviously they only work during the day time, but they have the advantage that the energy that they generate can be accumulated in a battery and then released over a short time to give the necessary power. However, before anyone asks a solar cell big enough to provide power for a vehicle is going to be very large. Some experimental solar powered vehicles do exist but they are not really practical devices.