If keypresses could generate electricity?

[raid517]

Hi, given that it should be relatively simple to design a computer keyboard that generated electricity via keypresses, maybe through some kind of induction coil arrangement of some kind for example, if you had a nearly unlimited (but not infinite) supply of monkeys, how many monkeys would you need and how many keypresses and with what frequency might be reasonable (say in words per minute, or key strokes per minute) to power an average 40 Watt light bulb for 20 minutes?

 

I know it sounds like a dumb question, but it's for a young people's training event and we would like to show them that science can be fun and interesting, as well as useful and serious. As a biologist however, this is not my specialist area at all. I know some assumptions will have to be made, for example in estimating the potential efficiency of a computer keyboard as a generator of electricity, but that's purely an engineering question (and is thus again beyond my capacity), so that's fine. I'm basically looking for a formula, with the full explanation of the formula for non-scientists too.

[studiot]

Whilst performing tasks the human body consumes around 100 watts.

 

http://www.physicsfo...t=145043&page=2

 

However most of this goes to keeping the body and brain running rather than to external tasks.

 

So the output whilst sitting typing at a keyboard will be at most 10 watts.

 

If you convert all of this to power your light bulb you would never actually operate any keys so say you operate at 50% efficiency you can maybe extract 5 watts.

 

Degrade this by the conversion efficiency to say 3 watts.

 

However I think this to be a fatuous example which only goes to show proper engineering in disrepute and have the opposite effect to the desired one.

Edited August 4, 2012 by studiot

[raid517]

Oh it's just trivial and fun. It isn't meant to be serious at all.

[swansont]

While walking on a level treadmill at 3 mph I burn about 360 Calories/hour which is 360,000 calories/hr, or 100 cal/sec. There are 4.18 Joules/cal, but your body is about 25% efficient, so this represents about 100 Watts of mechanical output. That's walking, i.e. using my large leg muscles. 40 Watts with your fingers would not be realistic; I think studiot's estimate is in the ballpark — I'd be surprised if you could get more than a few Watts.

[Joatmon]

Building on Swansont's comments and thinking of making something to think about you could do something like this:-

Use a bicycle frame and pedals to generate electricity to run the computer. If you wanted to make one compact assembly you could use a hand operated generator to charge the battery - not novel, but it would work.

[raid517]

Yes I know you guys seem to prefer classical examples, but none of this really answers the original question. The point of the question is just meant to be a trivial but funny anecdote, derived from the often stated fable of an infinite number of monkeys over an infinite amount of time, typing the complete works of Shakespeare. In this case none of the variables are infinite, it's just meant to estimate how much energy typing on a keyboard uses up (when young people use Facebook etc.) over a given period of time.

 

"5 Watts" isn't a very useful answer. Over what time period and with what frequency of typing and how many typing monkeys might you need to sustain this over a 20 minute period? Also what would the formula look like? Some assumptions may have to be made (regarding the efficiency of the keyboard as a generator of electricity and the speed of the person/monkey typing), but that's fine, it's just something that is intended to make a class of 8 to 12 year olds smile, before moving on to more serious matters.

Edited August 4, 2012 by raid517

[InigoMontoya]

I'll take a stab at it....

 

Assume: Keyboard press takes 0.5 N of force and the stroke is 0.01 m.

 

Work performed pushing key is in therefore 0.5 * 0.01 = .005 J.

 

Assume: Magical keyboard converts 10% of that energy into "output" energy. So for each keystroke we get 0.0005 J of useable energy.

 

When someone says they type at 40 wpm, that classically means 200 keystrokes per minute, or 3.33 keystrokes per second. So each monkey is generating 0.0005*3.33 = 0.00167 W of useable energy.

 

Therefore a 40 watt light bulb would require 40 / 0.00167 = 24,000 monkeys to operate.

Edited August 4, 2012 by InigoMontoya

[CaptainPanic]

I think that studiot came close to the right answer. Your body (or that of a monkey) will have an output of only a few watts on a keyboard. Probably actually less than 1 watt. If we assume it's 1 Watt, then you need 40 monkeys to power your lightbulb.

[John Cuthber]

A couple of experiments show that my keyboard takes about 60g ie about 0.6N to press a key (but the force isn't constant throughout the key's travel) and that the distance is about 0.5 cm so Inigo's estimate is about the right ballpark but (at least for my keyboard, it's an overestimate of the energy and so an underestimate of the number of monkeys).

 

With any plausible efficiency of energy recovery, hundreds of thousands of typists would be needed to power a lightbulb.

[swansont]

I'll take a stab at it....

 

Assume: Keyboard press takes 0.5 N of force and the stroke is 0.01 m.

 

Work performed pushing key is in therefore 0.5 * 0.01 = .005 J.

 

Assume: Magical keyboard converts 10% of that energy into "output" energy. So for each keystroke we get 0.0005 J of useable energy.

 

When someone says they type at 40 wpm, that classically means 200 keystrokes per minute, or 3.33 keystrokes per second. So each monkey is generating 0.0005*3.33 = 0.00167 W of useable energy.

 

Therefore a 40 watt light bulb would require 40 / 0.00167 = 24,000 monkeys to operate.

 

If you're going for energy generation, I think that you want to use a mechanical keyboard as an example for energy output. Computer keyboards are designed to reduce effort, and require far less force and range of motion. I wouldn't be surprised if a manual keystroke required 10x the force, and the travel would be a few cm.

 

I don't know where I could find an old typewriter to check this, though. What I do have is a small scale with a readout, and exerting a few N of force is trivial (several hundred grams indicated, i.e ~0.3 kg, so that's ~ 3N and I wasn't pressing that hard). You probably do this anyway on a keyboard, because the key stops moving — you're pressing harder than you need to to simply move the key.

 

I think you can also tick the rate up, since accuracy of what's being typed is not an issue. So 5N* 0.03 m * 5 kps is 0.75 Watts expended, which you have to modify by the efficiency (I think you can do better than 10%). So maybe half a Watt, using these numbers.

[John Cuthber]

Many years ago, when electric typewriters were new, someone pointed out (probably in an "end of article filler" in the reader's digest I think) that a secretary who changed from a mechanical to an electric typewriter would do less physical work and would- if they made no other changes to their lifestyle and diet- gain weight. If I remember rightly the gain was of the order of 10 pounds a year.

The energy density of fats is pretty consistent. My bottle of cooking oil says it's 3400 KJ per 100 ml.

That's about 3.7MJ/ 100g or 37Mj/Kg.

About 170 MJ of energy is stored in the secretary's 10 Lbs of extra fat.

For a typical working year of 200 days or so with 8 hrs a day that's 1,600 days or 138,000,000 seconds.

 

That comes out at pretty close to 1 Watt for a mechanical typewriter (assuming my memory of something from 30 years ago is correct).

So we need about 400 old secretaries (at 10% conversion) or about 100,000 new ones to power a light bulb.

 

Overall, most of the typists would be working in the dark so they wouldn't be able to read what they were typing.