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Possibility of a Mechanical Generator

  1. Dec 14, 2012 #1
    Like the title says is it possible to build a generator which will make around 5 watts of power for an hour? or even 2.5 watts in 30 minutes without needing constant human interaction like a hand cranked generator?

    I have been looking at things like the grandfather clock mechanism, hanging a mass and letting gravity produce electricity by pulling it towards the ground and even liquid powered such as water flow through a dynamo.

    Is there some way that this can be done?

    For gravity power I have been using the potential energy equation where:
    potential energy = mass x gravity x height

    This is the equation I tried:
    pe= 20 x 9.81 x 1.5 which comes to 294 joules.

    Because I wanted to produce 5 watts in an hour I used the equation:

    watts= joules / seconds

    W = 294 / 60 which gives 4.9w

    This is basically 5 watts but I haven't taken into account the efficiency losses which I don't know how to calculate but if I say that the efficiency is 50% then according to the equation if I have the gearing correct I could make 9.8 watts in 30 minutes before efficiency losses.

    Are my equations correct?

    I want to say that this is not a homework question, this is just something that I am interested in building as my final project for my Design Technology class.
  2. jcsd
  3. Dec 14, 2012 #2
    It's possible. I don't know if you could get the duration you want out of a pendulum unless it were extremely heavy with a very long arm. Keep in mind that the energy you get out can't be equal to or more than the energy you put in, so to get 5Wh out you have to put at least that much in. If you know (I know you don't) the losses from the mechanism itself plus any others, especially from the generator (coils resist magnets moving through/by them, etc) then you could figure out what you need.

    I think it would be easier to do with a crank (hand or bicycle) and either using it to charge a battery, or spin up a flywheel.

    Water flow through a turbine or over a water wheel would work as well. The higher the reservoir the better.
  4. Dec 14, 2012 #3


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    That's OK, except 60 seconds is one minute not one hour. You would only get 0.08W for an hour.
  5. Dec 15, 2012 #4
    Wow, that was a really silly mistake on my part; it should have been divided by 3600.

    So the possibility still exists that 5 watts of electricity can be generated by dropping a 20kg mass in 30 seconds right?

    Also at the rate of generating nearly 5 watts in 30 seconds I assume that you won't be able to charge a battery so could a capacitor (or even a flywheel) be used to store the energy and then use it over the hour?

    I did consider using a hand crank to charge a battery or to spin up a flywheel but I thought that it would take a while for a person (i.e. me) to do it and I wanted the process to take at maximum a minute of human work to produce the electricity needed which is why I'm looking at these relatively automated methods (if I'm mistaken about the time it would take please let me know).

    Regarding the water flow generator method, do the same potential energy equations apply and how about calculating the amount of flow and the duration?
    For example, how much water will be needed to produce 5 watts of electricity?

    Please feel free to mention any methods which I may not have given any thought about.
  6. Dec 15, 2012 #5
    You want to put 18kj (5w power for 3600s) into storage in less than a minute, and have it be "easy" for a person. Carrying a 20kg weight to a height of ~196m (18000 = 20 * 9.81 * 196) does not sound easy no matter how you trade off height vs. mass!

    It will take the same amount of work no matter what, you only have control over how long it takes, and some control over the losses incurred turning that work into electricity.

    I think personally that the bicycle method is the easiest (if you don't have a river nearby or a cliff to put a hose on) because of the mechanical advantage miracle known as gearing ;) Pedaling up that 200m hill with a 20kg load isn't so hard if you gear down and take your time. I only mention flywheels because they're cool. :)

    What exactly are you after that needs a 5w draw for an hour, anyway? Grandfather clocks use very large weights that slowly fall (or a wound spring) via an escapement to keep the pendulum swinging from the right height while energy in the pendulum is lost moving the gears of the clock. Once a week or so you have to raise the weight back up or wind the spring.
  7. Dec 16, 2012 #6
    I guess I wasn't understanding energy calculations properly until now, I didn't realise that there is a pretty big difference between 5 watts and 5 watt hours. I'm still at the stage of schooling where the affect of gravity is 10 newtons on a kg and some of my friends are having a hard time in realising that light and microwaves are part of the same spectrum.

    The aim of the DT project is to build a renewable energy power source which can power one of the lights in our classroom which is a 5 watt led light for an hour. For extra marks (much more) it has to be something which can produce electricity at any time of the day be it rain or shine and have a reset time of less than 2 minutes.

    This is the reason that I'm looking at gravity based power because that is the only source of energy that is around constantly.

    Like I said in my previous post if there is anything that I'm missing out on please let me know.
  8. Dec 16, 2012 #7
    That's a pretty interesting project for sure, and thanks for the context. I suppose massive flywheels, huge pendulums, waterfalls and rivers are *all* out. :)

    By "reset" do you mean the energy store has to be recharged? For example if the batteries are dead, you have to be able to recharge them to give another 5Wh of power in 2 minutes or less?

    If so, that puts solar and wind out of reach as well.
  9. Dec 16, 2012 #8
    Yeah it is interesting but I have a weird feeling that it might not be possible to get the higher marks because what needs to be done sounds improbable.

    The 'reset' means that the electricity generating method should be able to be restarted in under 2 minutes after it has produced the 5 watts in an hour and then no more human contact, so every 60 minutes you would spend 2 minutes moving mass back up. For example in your post you say that you need the 20kg mass at a height of 196m to generate the 5wh and in this case the 'reset' means that after the 20kg has come to ground level you have to get it back up to 196m in less than 2 minutes so it can start dropping again and then forget about the thing until you need to get the 20kg back up again.
    (I hope that makes it clear because I'm having some trouble explaining it properly)

    For the higher marks solar and wind can't be considered because they are not constant, the former not because of the day and night cycle, but because of cloudy days and the latter because wind just isn't reliable.

    It doesn't give us a lot to work with. The only form of clean energy I can think of that is constant is nuclear and I highly doubt that I have the skills to build a reactor or even if I did I would think the local authorities would want to have a word with me.
  10. Dec 17, 2012 #9
    I spoke to my teacher today and he said that everyone can get the higher marks if instead of trying to make 5 watts in an hour you can make a minimum of 15 watts in 12 hours without having to depend on solar or wind.

    Today I was thinking of using a thermoelectric generator like this one and have it be heated up by a small bio-ethanol flame. This would provide constant power and can be easily reset. I also considered using a stirling engine because of it's higher efficiency but due to moving parts and the price I feel like a thermoelectric panel will be better suited.
    Let me know what you think.
  11. Dec 17, 2012 #10
    I'm assuming you mean 15Wh and not 15W? That's not bad at all in 12 hours. You could still do something gear driven now that you have hours and not just a minute or two. The peltier or stirling engine would both be cool but I'm not sure which one is going to be easier.

    The junction only generates power when there's a good temperature differential. So you'll have to keep the heat away from one side, and actively cooling it if you can would be best, perhaps with a little heatsink and fan.

    You'll need more than one of them as well, 5w of output is at the extreme end of the range on those. I'd shoot for 1-2W output each and use several together.
  12. Dec 17, 2012 #11


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    Staff: Mentor

    That grammar issue is making it hard. Praanan, these are the correct way to say the two possible things you might be looking for:

    XX Watts FOR an hour.
    XX Wh IN an hour.

    It is never 'watts in an hour'.
  13. Dec 18, 2012 #12
    Apologies, I knew there was a difference between the two but I didn't understand it properly (and may be I still don't)

    First example:
    If I have a 50 watt light bulb and run it for 1 and 3 hours would it have consumed 50wh and 150wh respectively? and the 50 watts represents that the bulb needs 50 joules every second as seen in the joules to watt conversion equation.

    Second example (more related to my project):
    To power the 5 watt bulb for 3 hours (or even a 3 watt bulb for 3 hours and a 1 watt bulb for 6 hours) I would need 15wh.

    Something I'm still confused about:
    If my above 50 watt light bulb was only lit for 17 minutes and 30 minutes would the watt hour rating be 14.17wh and 25wh respectively?

    Anyway back on to the topic, now that the rules have been loosened a little anyone got any ideas?
  14. Dec 18, 2012 #13
    You are basically correct.

    Watts tell you how fast water is flowing out of the faucet, watt-hours tell you how long that faucet is left on. You need the faucet open a minimum amount in order to fill your container as fast or faster than it's leaking (being consumed by a load).

    To get into the nitty gritty details you're going to have to design around the bulb, meaning your power supply needs to match it's voltage as well as draw. What are the specifications of the bulb you need to light up?
  15. Dec 19, 2012 #14


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    Not how long the faucet is on, but how much water comes out.
  16. Dec 19, 2012 #15
    The writing on the bulb says 12 volt 5 watt (is there anything else that is needed?) and because the necessary power will be generated over a period of hours I also found a 12 volt 4 amp-hour which can be borrowed if the need arises.
  17. Dec 19, 2012 #16
    a 12v 4Ah "what?"
  18. Dec 20, 2012 #17
    whoops, a 12v 4ah battery
  19. Dec 26, 2012 #18
    Just giving this thread a bump to see if anyone can give an idea on how to generate 15wh in 12 hours.

    Ideally it would be really great if it's something mechanical but at the moment I'm looking at going thermoelectric by heating one side with an ethanol flame and attaching a cpu heatsink with thermal paste on the other side.

    One thing I would like to know is if the colour of a flame shows it's temperature. I ask because kerosene burns in a yellow/orange colour while a high percentage ethanol flame is blue and when it comes to stars you can tell their temperature by looking at their colour so I was wondering if it is the same with flames.

    Lastly, Happy Holidays to everyone.
  20. Jan 17, 2013 #19
    One more bump to see if anyone can help me.
  21. Jan 17, 2013 #20


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    Gold Member

    Hi PraAnan,
    The bulb you have is pretty modest, 12V and 5watts means it uses 5/12 = approx 0.4 amperes when running, so your 4amp hr battery will run the bulb for around 10 hours.
    The battery is almost enough to run the bulb for 12 hrs by itself.
    That suggests that what you need is a trickle charger that generates about half an amp at 12 volts.
    Most homebrew electric motors or generators don't get that much voltage, but hopefully you could find an old bicycle generator that would fit the bill.
    Of course that does not meet the spec of a 2 min reset, someone would be pedaling pretty steadily, but it would work.
    The meaning of the 15 watt in 12 hours challenge is still not clear. Watts are a measure of power, volts x amps, so does he mean achieve a 15 watt peak sometime in a 12 hour period or maintain a 15 watt output over a 12 hr period?
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