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Seeing the advances of things like GravityLight, is it possible to have a power plant where people work pulling ropes for oversized verisons of GravityLight to generate 120v mains electricity? I imagine a room with a couple dozen workers and a couple hundred ropes. Each rope is connected through a pully system to a 200kg bag. Maybe a 1:8 pully ratio for the ropes, so each worker feels like they're pulling 25kg. The power plant would have a height of maybe 50 meters, which is how high the 200kg bags are raised. When one bag is brought to the top by a worker pulling the rope, they move over to another and keep pulling.

Is this a viable system? Are there any major issues that I'm missing? What effect would this have on the cost of electricity? How much would these workers be paid?

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    $\begingroup$ Any reason why not to use oxen, donkeys, horses etc? And why to fund a power plant where you still have to pay wages instead of funding regular one that burns things? $\endgroup$
    – Mołot
    Commented Jun 8, 2017 at 23:18
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    $\begingroup$ Not super relevant but: Superman and Yoda $\endgroup$
    – Nicolai
    Commented Jun 9, 2017 at 3:38
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    $\begingroup$ It wouldn't be workable in itself, as it's a very inefficient way of converting food to electricity, but I have sometimes wondered about the feasibility of placing small electric generators on slot machine handles. $\endgroup$
    – jamesqf
    Commented Jun 9, 2017 at 5:02
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    $\begingroup$ Relevant video: youtube.com/watch?v=S4O5voOCqAQ CBA to watch? An elite cyclist is put on an exercise bike connected to a generator to power a toaster. By the time he has toasted one sandwich, he is worn out. Also GravityLight is not an "advance". Using gravity and weights to power things is not new or particularly inventive. It is just very ineffective. The only reason this is resurfacing is that now we finally have devices that require very little power to be useful. $\endgroup$
    – MichaelK
    Commented Jun 9, 2017 at 9:36
  • $\begingroup$ Could you include a sentence or two to explain the significance of GravityLight? A quick look at their web page does not. $\endgroup$
    – JDługosz
    Commented Jun 9, 2017 at 10:26

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I'd say No.

But that also depends on your settings, your technology level, culture, and such.

In current time, on earth, we already passed the era when brute strength is provided by human - trireme, child labor, etc. If you are to build a power plant using similar mechanism, the most logical move is to have animal (cow, horse) do it for you instead.

GravityLight is designed to bring light to poor households, not being a substitute to power generator. I haven't read all specifications for it, but I doubt it can provide power to a computer, for instance. There are a lot of other more efficient power generators, like water-mill (albeit has their own limits) that can provide even more power.

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    $\begingroup$ It can provide enough energy for a led barely brighter than kerosene lamp. Really dim by our standards. $\endgroup$
    – Mołot
    Commented Jun 9, 2017 at 8:02
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NO. It's not worth the sandwich.

Simple back-of-the-envelope calculation : I remember eating a sandwich worth 2 mega joules of energy (2,000,000 J) the other day. 2 MJ is about 0.5 kilowatt-hour (1 kWh = 3.6 MJ) and, according to this site, 1kWh is worth 41 cents in 2011 in Denwark (which is the highest price displayed here).

So, assuming I use all the energy of a sandwich to produced electricity via your system (I would not, since my body itself requires part of this energy to maintain itself alive), I would produce around 20 cents worth of energy.

My sandwich cost me 3$.

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A horsepower is larger than a manpower and costs (usually) less.

For efficient uplifting of weight based on human power oversized critter-wheels are better suited and were actually used in practice (e.g.: in Mt. Saint Michael elevators).

Having systems based on GravityLight-like systems makes sense if you need to generate relatively small power for a long time, so you lift a sand-bag some meters and then go doing your business while the bag comes back to ground. If You need a lot of energy you would connect an array of critter wheels (or other devices like spiked hubs) directly to the generator.

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I remember looking into something similar to this a long time ago - what I was specifically looking into was whether it was worthwhile using an exercise bike to charge electricity (for the sake of charging electricity, not simply as an added benefit of using an exercise bike) - the answer is no.

The reason is very simple - the mechanical energy produced by something like a bike (or what you're proposing) doesn't get converted to electrical energy with perfect efficiency - in fact the efficiency is pretty low.

Where this becomes extra problematic is if you're powering devices with mechanical parts, which use a lot of energy- e.g. power tools, washing machines, etc. You have two points of inefficency, because energy is lost both converting the mecahnical energy to electricity, and then again when it is converted back to mechanical energy.

If you do have systems like this, they're much more efficient if they power mechanical devices directly, rather than considering electricity at all. This isn't exactly a novel concept - this is how traditional mills work, whether they're powered by wind, animals, people (e.g. hand cranked mills) or otherwise.

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  • $\begingroup$ There's also lots of inefficiency turning the chemical energy of the worker's metabolism to mechanical energy - most of it is just heat. $\endgroup$
    – Spencer
    Commented Jun 9, 2017 at 9:46
  • $\begingroup$ @Spencer this is true - but not a lot can be done about that. Human, animal, wind, steam engine etc are pretty much the base level in terms of the efficiency you can control (ok, steam engine probably isn't true - you can build a more efficient steam engine). At this point it's really more important whether it's cheaper. If it cost massively more to use more efficient forms of energy production for a particular application, you'd just stick to using animals anyway - at least if it was an option. $\endgroup$
    – danl
    Commented Jun 9, 2017 at 9:54
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As ZioByte mentioned everything is better than human work. In force produced, energy (food) needed to live, things that people with thumbs could do instead.

I don't really see reason for those 200kg bags. The weight in Gravity lights is for the sole reason of not working on chain all the time. If you plan to have workers work full time then they could just apply the force at the axle or rotor. You would loose lees power. Like in this movie below making power at the core

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