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Using man-power to turn massive generator

by tmoney
Tags: generator, manpower, massive, turn
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tmoney
#1
Jul10-08, 03:03 PM
P: 8
Hi

I just wanted to run this hypothetical power plant idea past you physics guys to see what kind of feedback I could get.

My general question is "Could man-power through the use of massive gears, be used to turn a very large generator.?"
The generators used in the Three Gorges Dam(http://en.wikipedia.org/wiki/Three_Gorges_Dam) can generate 700MW of power and are about 75 feet in diameter. What kind of force would it take to get this thing spinning? My idea is to build a massive "outer" gear lined with manned pushing posts around the entire circumference. The generator would be in the center. Through the use of many gears that get smaller and smaller, enventually ending up as the rotor of the generator, you could gain massive mechanical advantage. The men would push at a walking pace while the center rotor spun at a great rate. Im picturing 100, 200, 500 men, not sure. Any initial thoughts? I realize there are several unknowns in this. I just want to gets some thoughts rolling. Thanks

Here is a pic of the generator.
http://www.nancarrow-webdesk.com/war...g.249065_t.jpg


OOps, I just realized this should be under the engineering section, sorry
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russ_watters
#2
Jul10-08, 04:35 PM
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P: 22,280
A reasonably athletic person can generate perhaps 200 W of mechanical power by pedaling a bike. So if you lined-up 3.5 million people on stationary bikes, you could run those generators with people. It is China!

I'm also not sure you understand what the term "mechanical advantage means". Whether it is a lever or a gear, when you multipy speed (distance) on one side, you have to multiply force on the other. So mechanical advantage yields no increase in power.

Welcome to PF!
tmoney
#3
Jul15-08, 12:22 PM
P: 8
By having a very large distance (the large outer gear), you would produce a larger force, right? This force would turn the generator rotor which creates electricity. I really don't know how generators work. Is there an increasing resistance as more power is being generated from the generator?

LURCH
#4
Jul15-08, 12:36 PM
Sci Advisor
P: 2,510
Using man-power to turn massive generator

Lareg force moving very slowly, yes. Or, you could reverse the gearing and have a very weak force moving fast. But the total energy at the output will be the same as the amount of energy put in (or less).
tmoney
#5
Jul15-08, 02:15 PM
P: 8
So, if a reasonably athletic person can generate 200W of power. There is no way I could, for example, get this 2400W generator going using just my power with the help of a very large and a very small gear.http://www.grainger.com/Grainger/items/6HJ87
russ_watters
#6
Jul15-08, 04:40 PM
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P: 22,280
No. Your problem isn't that you don't understand generators, it is that you don't understand what gears do: mechanical advantage. Read the wik link: http://en.wikipedia.org/wiki/Mechanical_advantage

In short, think about the example of a lever: one end moves a large distance with a small force and the other end moves a small distance with a large force. But the torque and work (or power or energy) done remains the same for both ends. Levers and gears multiply force, not energy.
tmoney
#7
Jul16-08, 12:10 PM
P: 8
Ok. Thanks for the info. These forums are very informative.
LURCH
#8
Jul17-08, 01:07 PM
Sci Advisor
P: 2,510
Quote Quote by tmoney View Post
So, if a reasonably athletic person can generate 200W of power. There is no way I could, for example, get this 2400W generator going using just my power with the help of a very large and a very small gear.http://www.grainger.com/Grainger/items/6HJ87
Not with gears, but you could possibly with a spring (or other energy storage and release mechanism). If you get on the treadmill and generate 200W fo energy for 10 hours, I suppose you could run a 2000W generator for 1 hour.
bigtruckdrive
#9
Sep12-08, 03:15 PM
P: 3
I have given thought to this concept as well, but from a bit of a different perspective....

Most of us pay taxes to keep prisoners locked up, and fed... so why not put them to work? Assemble a gearing system that links up dozens (maybe even hundreds) of bicycles, and connects them to a massive generator.

If you've got a prison of 800 men... you could divide them up into groups of 200, and schedule them to be always pedaling.... say
200 men * 200 watts/hr * 24 hours a day = just under 1 megawatt/day.....

this is just an example, and obviously not nearly enough power to power big cities... but surely there are some efficiencies to be gained in a large scale system like this.... maybe?

anyway, just a thought.
chroot
#10
Sep12-08, 03:33 PM
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bigtruckdrive,

The watt is a unit of power, defined as energy per unit time. The unit "watt/hr" makes no sense. Neither does the unit "megawatt/day."

Furthermore, if you want prisoners to produce mechanical energy, you'll need to provide them an equal amount of chemical energy, as food. (Actually, even more, since human muscles are not 100% efficient.) The big problem is that human food is considerably more expensive, per unit of energy, than many other forms of energy.

A gallon of gasoline contains 35,000 Calories and costs about $4. A pint of Ben & Jerry's ice cream contains about 1,200 Calories and costs almost as much. You'd be much better off sparing the prisoners and just burning gasoline.

- Warren
bigtruckdrive
#11
Sep12-08, 03:48 PM
P: 3
hah, i guess that is a good point. hadn't thought of that.

I guess you'd have the prisoners putting out a decent amount of CO2 as well....

Although I wonder, if the energy they are burning is from a low cost food source, like rice or pasta, this might be more feasible?

It seems to me that this is a resource we should take advantage of! lol
russ_watters
#12
Sep12-08, 07:21 PM
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P: 22,280
Your calculation produced 1 megawatt-hour per day of energy or 40 kilowatts of power. That's not very much. Just as a benchmark, commercial office space runs around 5 watts per square foot in the summer. So your 40 kW gives you enough power to run 8,000 square feet of office space.
bigtruckdrive
#13
Sep13-08, 12:03 PM
P: 3
Ya, seems like the power produced would be negligible. If anything I supposed this generated power could be use to power the prison.....
LURCH
#14
Sep13-08, 02:37 PM
Sci Advisor
P: 2,510
I think it's a good concept. I have long held the position that there should be no such thing as "hard labor" as a seperate sentence for convicted criminals. It should be understood as part of their sentence that they will work rather than sit around living off of taxpayer's money. But, I suppose that putting them on bicycles to power an air compressor so a jack hammer can split rocks is inherently less effecient than putting a sledge hammer in their hands and having them whack away.

Still, the comparrison to gasoline does not take into account the fact that, if these men do not use their energies to do work, they are still going to burn those callories, so you don't save that energy by not putting them to work. There must be some practical way of using all this manpower to defray some of the expense of operating the prison.
tmoney
#15
Jan21-09, 09:07 AM
P: 8
I had thought of the prison idea as well, and these guys would already be eating and burning calories bench pressing or beating up their cell mates. You might as well harness those calories.

I like the idea of man-generated electricity. The human animal doesn't need electricity to survive. Humans have produced a society that requires electricity. It should be Human's that do the work to produce it instead of harnessing various forces of nature.
QuantumPion
#16
Jan21-09, 09:29 AM
P: 772
Quote Quote by tmoney View Post
I had thought of the prison idea as well, and these guys would already be eating and burning calories bench pressing or beating up their cell mates. You might as well harness those calories.

I like the idea of man-generated electricity. The human animal doesn't need electricity to survive. Humans have produced a society that requires electricity. It should be Human's that do the work to produce it instead of harnessing various forces of nature.
Humans are very weak an inefficient at producing energy mechanically. But with just a few watts of electrical energy, our brains can devise ways of harnessing other sources of energy far greater.

Instead of putting a thousand prisoners to work moving boulders, you should stuff them in the library and force them to learn physics & engineering. :)
tmoney
#17
Jan21-09, 09:41 AM
P: 8
Hmm, good observation. I guess we have created a monster that we can no longer feed, and we are took weak to do what it does for us.
robotbender
#18
May23-09, 11:55 AM
P: 1
Hi guys, my first post! I really enjoyed reading this thread, it's funny and intelligent - a great combination.

I found this site trying to work out the power a person can generate (not just consume).. you know how it is, ideas, nothing to do, calculator itching for numbers..

My physics is a little rusty so I really enjoyed reading this post. It looks like a few keen minds are as rusty as me and the perhaps improper use of some of the terms used is caused by not fully understanding them. Things like what power, leverage or a KW/h actually is.

What roused my interest is the 200W a 'reasonably fit' person can produce. Am I right to think you mean this in a calorie/joule - enery ratio per hour?

An average althetic male will use about 1700 cals a second (612Kcal hour) playing squash - intense physical exersion. A watt is the SI derived unit of power, equal to one joule of energy per second. Ergo 1W = 1J/s. 1 calorie = 4.18400 joules (4.2W) which equates to 7112.8W or about 7Kw second for the squash player. As Watts and Joules are time oriented, we can't multiply this out to infer a human will generate 25MW/h! I understand this healthy person is capped out at 7Kw/h (I said I was rusty).

I can only understand from tmoney's original question (also with reference to Giesick's Force Needed to Turn Generator at Maximum Output post) that from 6 hours (hard labour a day) would equate to (7Kw/h * 6 hours) 42KW/h day.

I see one obvious flaw (aside from my maths) in the fact that we would expell 7112.8W a second just to do the work so any device used to capture the energy would be less efficient. Some of the most efficient alternators are about 80%, but it's interesting to imagine how much real-power can be produced from pure grunt. In honesty my interest comes from thinking the gym in itself is a waste of time and quite literally - energy. It would be interesting to save it up and use it at home squashing the electricity bill, staying fit, or do I just need to get out more?


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