Author Topic: Why can't a gravity powered generator work?  (Read 25196 times)

Offline mountainmoma

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Re: Why can't a gravity powered generator work?
« Reply #30 on: January 09, 2014, 10:17:06 PM »
I have seen a light powered this way, found it, it was on idigogo, link here http://www.indiegogo.com/projects/gravitylight-lighting-for-developing-countries. A LED light does not use alot of power. SO, it just powers it direct, doesnt charge a battery first
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Offline mountainmoma

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Re: Why can't a gravity powered generator work?
« Reply #31 on: January 09, 2014, 10:24:13 PM »
Then, there is this one, which I dont think is being brought to market, the other one above will be. This floor lamp is more expensive and gives off 40 watt equivalent for 4 hours.

Of course, the hydroelectric dams, and realy all water wheels andmicroturbines, are gravity based power sources. A weight (solid or liquid) at elevation has potential that can be harnessed when it is released and under the pull of gravity
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Offline AngusBangus

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Re: Why can't a gravity powered generator work?
« Reply #32 on: January 27, 2014, 11:40:38 AM »
I was just reading an article about energy storage at the "large electric utility" scale, where batteries aren't feasible.  Many systems involve some sort of gravity/weight system, e.g. pumping water into a reservoir or tower when solar or wind energy is available, and then letting it run back through a turbine when the energy is needed.  One system even involves using railcars on a sloped railroad track.  So it's not an unrealistic idea, but I'm not familiar with homestead-scale systems using it.  I'm guessing the biggest challenge would be in not losing a bunch of energy to friction.


The Tennessee Valley Authority has reservoirs all over mountain tops for just that. They pump river water up there in high water times and then have huge reserves for hydro when water level is lower... also allows some level of river level moderation.

Regarding your clock idea, the concept would work but I can't imagine it's efficient and I bet it's WAY more expensive than just buying a bunch of battery chargers, building a gasifier (or other alternate fuel source for a generator). The weights would need to turn your mini generator rotor at significant speed (using gearing) inside a stator that generated the high frequency AC power which would then need to be converted into DC using a charger (or an extremely expensive piece of power electronics) that could handle NON-60hz power unless you could also govern the speed of your generator rotor. Then the DC would then charge your rechargeable batteries. I gotta think there are WAY easier ways to do this like hand pumping water to a tank and using gravity/micro-hydro.

Really fun to think about though...
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Offline drtcbear

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Re: Why can't a gravity powered generator work?
« Reply #33 on: July 09, 2017, 09:59:36 PM »
So I read the responses and followed some links, some bogus, some interesting.

But I think a lot of people are missing something. The original question didn't ask for free power, and it didn't ask for enough power to run a house. It just asked why gravity power can't work.

So the more relevant question is what do we want to power, when do we want to power it, for how long do we want to power it, and how will we capture the kinetic energy that gravity will produce. It doesn't have to be free, it doesn't have to be efficient, it doesn't have to be effort free.

I am not a hard core survivalist so I am not looking to live completely off the grid. But I live in FL so hurricanes and tornadoes are a definite concern. I am also a ham and involved with ARES, so my interest in this topic hasn't been to run a house, but to have a relatively low tech, no fail source of power for a sustained emergency. This is important because I run several digital repeaters, so I will have a couple of generators, and I have a lot of batteries, and I can store some gas, but at some point it would be nice to have something that didn't require gas.

So I happened to see the Africa light thing and my first thought was cool. I have one of those wind up flashlights which is great if you want to wind while using, but I thought suppose I just wanted a light on all night? So I thought to myself OK, I need to figure out how it can turn on its own without human intervention. A modest goal for sure, but it was just the starting point. I realized that I could do that easily and just use something heavier than a bag of sand or a small weight - like hoisting a concrete block up in a tree, and then using the descent of the block, through some gears that would capture energy and convert kinetic energy to electrical energy through a generator/alternator.

But then I thought, OK, I have the basic concept, what about powering the radio gear? It is mostly on receive status, with occasional 25/50 Watt transmissions, so charging it, or perhaps keeping it from total discharge,  wouldn't be that hard, and concrete blocks and hoisting them up a tree isn't all that hard and I am guessing that I can slow down the fall so that it would take several hours so it can run unattended at night.

I just happen to have a really cool, low friction, and efficient 120V DC motor which would be easy to use in such a system and any time the concrete block(s) get(s) too close to the ground all one has to do is hoist some more up in the tree. Beats the hell out of the bicycle generator I was thinking of because once the block(s) are up in the tree I can forget about it for a few hours.

So, it can absolutely be done, relatively easily, as long as we don't get distracted by constraints never posed in the original question.

Offline Carl

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Offline LVWood

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Re: Why can't a gravity powered generator work?
« Reply #35 on: July 10, 2017, 06:37:04 AM »
As the OP said, he's not looking for perpetual power or anything like that.
Sure gravity can work, you just have to figure out how much work needs to be done and size it accordingly.
There will obviously become a point where size and work required to move the energy uphill becomes counterproductive.
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Offline Carl

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Re: Why can't a gravity powered generator work?
« Reply #36 on: July 10, 2017, 07:40:05 AM »
As the OP said, he's not looking for perpetual power or anything like that.
Sure gravity can work, you just have to figure out how much work needs to be done and size it accordingly.
There will obviously become a point where size and work required to move the energy uphill becomes counterproductive.

I also feel that our power grid would be better if independent areas were developed as to prevent major issue from causing BIG problems.
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Offline bcksknr

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Re: Why can't a gravity powered generator work?
« Reply #37 on: July 10, 2017, 08:21:22 AM »
     I have one of the original Baygen radios. It has a clockwork-generator device that winds with a crank on the back. As the clockwork winds down it powers the dc generator to charge internal batteries that power the radio. Even when the clockwork is unwound, there is enough charge in the batteries to continue running the radio (and it had a small led light, if needed). This radio was originally designed in South Africa for villages in third world countries. For a little wind up you get about 30 minutes of radio, AM, FM and one broadcast shortwave band.
     I could see putting in a jack to connect to an external AA battery pack, for charging for other uses. I think this could also be "scaled up".
     I saw a cable sci-fi show in which were presented people going to the "gym". They spent hours on stationary bikes, before their "shift" was over and they returned "home". It was later revealed as the camera panned out, that the facility was huge, with floors of bikes, hundreds, thousands, tens of thousands. It turns out that this is how the future was being powered, and the labor of millions in producing energy was rewarded with food, shelter and existence.
     In a future with depleted, polluting fossil fuels; inadequate alternative energy development due to greedy "Big Energy" and a toxic external environment that may make closed habitation necessary, this could be an alternative. After all, how many educated people does the world really need? Our technological development requirements could be met with a small elite cadre of "smart" people, while the rest of humanity, not engaged in food or commodity production, could "earn their keep" by producing peddle power.
     Of course, this is ridiculous and could never support a global population, but I think properly scaled, human power could be feasible for some consumer devices; especially in an off-grid or a grid-down emergency. I have seen led light units that have a small weight driven generator that will illuminate for a while from raising a weight that drives the generator; sort of like a "cookoo clock", with out the cookoo.
     I'd like to see more development of devices like this. Oxen harnessed to wind up a giant clockwork engine (when they weren't plowing or otherwise engaged) could probably provide reasonable off-grid power for a small village?
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Offline LVWood

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Re: Why can't a gravity powered generator work?
« Reply #38 on: July 10, 2017, 08:33:23 AM »

     I saw a cable sci-fi show in which were presented people going to the "gym". They spent hours on stationary bikes, before their "shift" was over and they returned "home". It was later revealed as the camera panned out, that the facility was huge, with floors of bikes, hundreds, thousands, tens of thousands. It turns out that this is how the future was being powered, and the labor of millions in producing energy was rewarded with food, shelter and existence.
     In a future with depleted, polluting fossil fuels; inadequate alternative energy development due to greedy "Big Energy" and a toxic external environment that may make closed habitation necessary, this could be an alternative. After all, how many educated people does the world really need? Our technological development requirements could be met with a small elite cadre of "smart" people, while the rest of humanity, not engaged in food or commodity production, could "earn their keep" by producing peddle power.
     Of course, this is ridiculous and could never support a global population...

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Interesting perspective, the idea isn't abhorrent to you, it's just unworkable.
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Offline Carl

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Re: Why can't a gravity powered generator work?
« Reply #39 on: July 10, 2017, 09:13:35 AM »
A progressives Elysium dream.
Interesting perspective, the idea isn't abhorrent to you, it's just unworkable.

Depends on who has to pedal.../.
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Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #40 on: July 10, 2017, 06:01:29 PM »
So I read the responses and followed some links, some bogus, some interesting.

But I think a lot of people are missing something. The original question didn't ask for free power, and it didn't ask for enough power to run a house. It just asked why gravity power can't work.
...
So, it can absolutely be done, relatively easily, as long as we don't get distracted by constraints never posed in the original question.

Welcome to the forum!

The original poster asked about charging a few batteries using gravity.  That is a very tough order because gravity is a very weak force compared to the other forces of nature.  Here is some quick napkin math (anyone, please feel free to check this).

Let's say we want to charge 4 rechargeable AA batteries with 2.5 Watt-Hour of capacity.  That is 10 Watt-Hours total.

In your proposed setup we would use the potential energy of a hanging weight to drive a DC motor into a charger.  This would require some sort of escapement as you just can't dump all the energy at once...it needs to charge the battery slowly (over say, about 8 hours).  If each of these is 80% efficient (very generous assumption esp. for the escapement) then we will need about 20 Watt-hours; that is 10/(.8X.8X.8 ).

So how much weight do we need for this?   It turns out that a 1 lb weight hanging 1 foot off the ground contains the equivalent of .000367 Watt-Hours (using PEgravity = mass*gravitational constant*height).  Lets assume that we are going to hang the weight from the tree 12 feet high and that we are going to lift it once every hour for the 8 hours.  Then we have:

Weight needed = 20 Watt-hours/(12 feet * 8 lifts* .000367) = 568 lbs. 

Net, if the math is correct that is quite an endeavor to charge 4 AA batteries.

Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #41 on: July 11, 2017, 05:17:57 PM »
For the OP, is this what you are looking for?
http://deciwatt.org/

GravityLight™ is an innovative device that generates light from gravity.

It takes only 3 seconds to lift the weight that powers GravityLight creating 25 minutes of light on its descent.

It can be used over and over again with no running costs

While the main purpose of the gravity light is, light, it could easily be set to charge batteries.  The problem, how many times would the system have to be reset to get a full charge on the batteries?

Might be good for keeping a charge on already charged batteries so they are ready for use.

Had a few minutes so looked into it.  They give the stats for it (http://deciwatt.global/technology/) so we can work out how many resets would be required.

Fully loaded and hung (27 lb bag at ~7 feet) it produces 0.085 Watts for 20 minutes (1/3 hour).  So it produces 0.0283 Watt-Hours per set (i.e. 0.085 * 1/3 Hours).

However, unlike the above example, this is net after escapement gears and dc motor/generator.  So we only need to take into account the loss due to charging, which means we need about 12.5 Watt-Hours from this device to charge the 4 AA battery pack.  This gives us:

Number of resets = 12.5 Watt-Hours/0.0283 Watt-Hours per set = 441 sets. 

Since each set runs for 20 minutes, this means that with one unit it will take 6 days and 3 hours of constant use to charge the 4 battery pack.

Regarding maintaining batteries, the voltage and current specs (2.7 volts, 0.031 Amps) are mighty close to what would be needed to maintain NiMH battery packs.  So, it would potentially work for that purpose with something like a 12 hour on/12 hour off pattern. 
« Last Edit: July 11, 2017, 05:26:57 PM by iam4liberty »

Offline Carl

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Re: Why can't a gravity powered generator work?
« Reply #42 on: July 11, 2017, 05:37:26 PM »
Had a few minutes so looked into it.  They give the stats for it (http://deciwatt.global/technology/) so we can work out how many resets would be required.

Fully loaded and hung (27 lb bag at ~7 feet) it produces 0.085 Watts for 20 minutes (1/3 hour).  So it produces 0.0283 Watt-Hours per set (i.e. 0.085 * 1/3 Hours).

However, unlike the above example, this is net after escapement gears and dc motor/generator.  So we only need to take into account the loss due to charging, which means we need about 12.5 Watt-Hours from this device to charge the 4 AA battery pack.  This gives us:

Number of resets = 12.5 Watt-Hours/0.0283 Watt-Hours per set = 441 sets. 

Since each set runs for 20 minutes, this means that with one unit it will take 6 days and 3 hours of constant use to charge the 4 battery pack.

Regarding maintaining batteries, the voltage and current specs (2.7 volts, 0.031 Amps) are mighty close to what would be needed to maintain NiMH battery packs.  So, it would potentially work for that purpose with something like a 12 hour on/12 hour off pattern.

But you can make the chore more fun by using a rocking chair or make it operate off a heavy ,long swinging pendulum ...
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Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #43 on: July 11, 2017, 05:54:33 PM »
But you can make the chore more fun by using a rocking chair or make it operate off a heavy ,long swinging pendulum ...


Offline Carl

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Re: Why can't a gravity powered generator work?
« Reply #44 on: July 11, 2017, 05:57:40 PM »
  They can build a man's idea before he finishes saying it :)

Maybe Tesla can make a car that runs on a cat and a hairbrush?
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Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #45 on: July 11, 2017, 08:39:02 PM »
  They can build a man's idea before he finishes saying it :)

Maybe Tesla can make a car that runs on a cat and a hairbrush?

:)  I have it on good authority that a cat and hairbrush is the secret behind John Galt's engine.

Speaking of building it, apparently some people have made a hanging weight to electric system.  This one uses a heavier weight than above (1000 lbs) but they winch it to a much lower height (4 feet): https://www.youtube.com/watch?v=VsJ7m2VzfRA

They don't appear to be having too much fun trying to keep up with the system.  Which reminds me of a favorite saying of one of my old physics teachers: "There is no gravity, the earth just sucks!"

Offline drtcbear

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Re: Why can't a gravity powered generator work?
« Reply #46 on: July 12, 2017, 09:30:08 AM »

IamLiberty said: "Fully loaded and hung (27 lb bag at ~7 feet) it produces 0.085 Watts for 20 minutes (1/3 hour).  So it produces 0.0283 Watt-Hours per set (i.e. 0.085 * 1/3 Hours).

However, unlike the above example, this is net after escapement gears and dc motor/generator.  So we only need to take into account the loss due to charging, which means we need about 12.5 Watt-Hours from this device to charge the 4 AA battery pack.  This gives us:

Number of resets = 12.5 Watt-Hours/0.0283 Watt-Hours per set = 441 sets. 

Since each set runs for 20 minutes, this means that with one unit it will take 6 days and 3 hours of constant use to charge the 4 battery pack.

Regarding maintaining batteries, the voltage and current specs (2.7 volts, 0.031 Amps) are mighty close to what would be needed to maintain NiMH battery packs.  So, it would potentially work for that purpose with something like a 12 hour on/12 hour off pattern."

You anticipated some of my concerns regarding your original response, that I don't think I ever posted. But I still tend to think that you are looking for ways to make it seem as though this can't work vs collaborating on ways to make it seem that it can work.

So, again, the question was: Why can't it work" and the answer is "It can work, but it may be inconvenient".

Still not sure about your numbers, but let's assume that they are correct and see how we can modify the situation to make it more feasible.

You suggest lifting a 27# weight 7' in the air? Why? Why not lift our 27# weight 70' in the air? That suggests we can bring the reps down to 44.

You suggest lifting one 27# object at a time and letting it fall before lifting it again?  Why? Why not lift 1 27# object on our device for every 10' of height, or 10 weights running at a time, which would allow it to run, unattended, longer? That suggests we can get down to 4.4 reps pretty easily.

Still, something seems a little off in all this and my hunch is that there is a mismatch in units somewhere. But it seems abundantly clear that while inconvenient, this approach is far from infeasible.

As to the 1,000 # device - I am sure we both agree that those guys are not going to win any awards for efficiency in design, fabrication, or implementation.

Offline bcksknr

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Re: Why can't a gravity powered generator work?
« Reply #47 on: July 12, 2017, 06:14:45 PM »
     There are many ways to generate electricity. The problem is that not all are practical for A. the location B. the power demand. C. scaling up or down D. cost etc. It all depends on what you want to do. I've made batteries out of pint mason jars using cheap magnesium fire starter bars and flattened copper pipe as the cathode and anode They use plain old water as the electrolyte. Four of them in series will light a bright LED...practically forever, or at least until the metal is used up.
     The voltage is there, but the current is very low. It would probably take hundreds or thousands in series parallel circuit to run anything bigger than an LED. However, that LED will provide minimal light for hours, days, weeks...who knows?
     As I understand it, electric car battery packs are made up of many individual cells. Individually, they couldn't do much, but together they can power a car. That's what I mean by scaling up a power source. So I wouldn't dismiss low output generating devices. In a SHTF future, we may be running things on potatoes with electrodes stuck in them.
     Using falling weight devices (gravity) has been around for centuries. Example: The "treadwheel" crane was used during Roman times and the Middle Ages to lift and move very heavy loads. It made use of the weight of people walking inside a large wheel (think their body weight and gravity), turning a spindle and winding a rope, using gravity and mechanical advantage to lift stone blocks for castles and cathedrals. Don't forget Middle Ages "artillery": the Trebuchet, A weight driven" catapult" that could throw hundreds of pounds, hundreds of yards.
     I don't see why creative thinking couldn't use gravity to power electrical devices. 
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Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #48 on: July 12, 2017, 06:35:18 PM »
you are looking for ways to make it seem as though this can't work vs collaborating on ways to make it seem that it can work.

Not at all.

So, again, the question was: Why can't it work" and the answer is "It can work, but it may be inconvenient".

I would put it another way.  You can use gravity to store energy but it is useful in only a very small number of very specialized cases.  Man moveable masses are only practical for powering things of very low wattage, like a few LEDs.  As others pointed out, extremely large masses do have use for storing excess power plant energy.  Examples would be water pumped storage or the new ARES rail system which uses train cars and regenerative braking technology.

Still not sure about your numbers, but let's assume that they are correct...Still, something seems a little off in all this and my hunch is that there is a mismatch in units somewhere

No need to assume as it is easy enough to verify.  To keep it simple we can use the SI system units and just convert Joules to Watt Hours at the end.

Potential Energy of hanging weight = Mass X Height X gravitational constant

Mass = 1 pound = 0.4536 kilograms
Height = 1 foot = 0.3048 meters
Gravitation constant = g = 9.8 meter / seconds2

Therefore potential energy of 1 lb mass at 1 foot is:

PE = 0.4536 kg * 0.3048 m * 9.8 m/s2 = 1.357 kg m2/s2 = 1.3549 Joules

1 Watt-Hour = 3600 Joules so:

1.3549 Joules/(3600 J/W-H) = 0.000376 W-H

This is very close to what I calculated before.  Originally I used the imperial system and used 32 f/s2 for g where I would have needed use 32.152 f/s2 to correspond more precisely to the 9.8 m/s2 used here.

Net, I see no error in my calculation.

You suggest lifting a 27# weight 7' in the air? Why? Why not lift our 27# weight 70' in the air?

kckndrgn asked the specific question of how many times the deciwatt gravitylight would need to be reset.  If you look at the specs, you will see that there are maximum design limits set by the manufacturer.  One is a maximum weight of 27 lbs.  Another is a maximum height of about 7 feet.  Why does it have these limits? There are several probable reasons on which we can speculate:

One is that the device uses an electrical vs. mechanical escapement.  Specifically, it is the LED circuit itself which slows the drop of the weight.  This is actually quite clever as it keeps the mechanism very simple.  Despite how much weight you put in the device, it falls at about the same rate and maintains about the same voltage.  What changes is the current flowing through it.  Eventually this current will rise to the point where the circuit can't handle it.  If it is fused, it will blow.  Otherwise it goes *poof*.

Another issue with the weight is the gears.  The gears are plastic.  This has manufacturing and performance advantages (esp. not needing lubrication).  But it also means the teeth will break if over stressed.  The manufacturer made use of a planetary gear system to push the limit of this.  This setup allows multiple teeth to be engaged with the main gear splitting the force among them.  But again you can only go so far with such a setup.  Similarly, the plastic chain also causes a weight limit as it will stretch or snap if the weight is too great.

Regarding height this is limited in any hanging weight clockwork system like this.  Chains have mass.  So as the clock runs, the chain moves from one side of the clockwork to the other.  This changes the ratio of mass disrupting the movement.

The manufacturer of the gravitylight spent nearly a million dollars optimizing these pieces to come up with the best device they could.  It is actually quite impressive piece of engineering.  But the real brilliance (Ah, a pun worthy of Carl!) is the LED.  It is amazing just how efficient LEDs are at creating light.  We have to remember that their creation was worthy of a noble prize in physics. 

I've made batteries out of pint mason jars using cheap magnesium fire starter bars and flattened copper pipe as the cathode and anode They use plain old water as the electrolyte. Four of them in series will light a bright LED...practically forever, or at least until the metal is used up.
   ...
I don't see why creative thinking couldn't use gravity to power electrical devices. 

The natural forces within chemical storage are much, much stronger than gravity.  Gravity is very weak.  Think of it this way.  We live on a big rock with a mass of about 6,000,000,000,000,000,000,000,000 Kg.  Yet, with just a little thought we can send a tiny current into our muscles and completely overcome the gravity to propel ourselves.  Or how easy it is to lift a small magnet off a wood table (gravity only) vs. a metal table (+ magnetism).  The number calculated above shows the potential energy per pound per foot.  That is what we have to work with in gravity systems. 
« Last Edit: July 12, 2017, 06:41:01 PM by iam4liberty »

Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #49 on: July 12, 2017, 07:23:03 PM »
PE = 0.4536 kg * 0.3048 m * 9.8 m/s2 = 1.357 kg m2/s2 = 1.3549 Joules

Oops.  Saw a typo. Sorry for any confusion. This should read:

PE = 0.4536 kg * 0.3048 m * 9.8 m/s2 = 1.3549 kg m2/s2 = 1.3549 Joules

Offline drtcbear

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Re: Why can't a gravity powered generator work?
« Reply #50 on: July 12, 2017, 09:11:10 PM »
I am new here, so I could be wrong, but the original poster seems to have been:

mangyhyena
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Why can't a gravity powered generator work?
« on: June 16, 2012, 07:31:08 PM »

and the question was not about the gravity light so that portion of your post is still puzzling to me. Concrete blocks weigh 28 pounds and were one, among many things that I would consider using as a weight and raising to a much higher level than you assume.

But, here's a challenge for you, since you say you want to collaborate - How would you make this work? Say you got caught in a flood, and all you could salvage was a DC motor and someone you cared about needed some small, but life-sustaining, battery operated, machine to keep running for an indeterminate period?

Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #51 on: July 12, 2017, 10:27:03 PM »
and the question was not about the gravity light so that portion of your post is still puzzling to me. Concrete blocks weigh 28 pounds and were one, among many things that I would consider using as a weight and raising to a much higher level than you assume.

Ah, i see where that could have been confusing.  if you look through the responses you will see this one from kckndrgn where there was a question of how many times a gravity light would need to be reset to charge a battery pack:

For the OP, is this what you are looking for?
http://deciwatt.org/

GravityLight™ is an innovative device that generates light from gravity.

It takes only 3 seconds to lift the weight that powers GravityLight creating 25 minutes of light on its descent.

It can be used over and over again with no running costs

While the main purpose of the gravity light is, light, it could easily be set to charge batteries.  The problem, how many times would the system have to be reset to get a full charge on the batteries?

Might be good for keeping a charge on already charged batteries so they are ready for use.

I was offering an answer to that question.

But, here's a challenge for you, since you say you want to collaborate - How would you make this work? Say you got caught in a flood, and all you could salvage was a DC motor and someone you cared about needed some small, but life-sustaining, battery operated, machine to keep running for an indeterminate period?

Well, that isnt much to go on from a scenario point of view.  How much power are we talking?  I definitely wouldnt use gravity storage as an intermediate step as it would be much more efficient to charge the battery directly for example with a hand crank.

Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #52 on: July 12, 2017, 10:45:17 PM »
PLEASE, check my numbers... I babble a lot....

Turning generators with moving water, caused by the sun (natural, and artificially induced means to move water to a higher location, or from a pressurized container.) Power can be constant and regulated.  Most naturally occurring cases of water in a high gravity location have already been exploited.

Where tanks can be positioned at significant differences in altitude (i.e. 100'+) water pumped by windmill to the higher tank can bank the energy (serve as a battery) for later expenditure by turning a generator when dropped again thru a turbine. Think outside the box… Can you modify a turbocharger from a car to serve as the driving turbine in a micro-hydro generator?   

Factors:

1kw = 1.3 hp
Water flow in cubic feet/second x height difference in feet divided by 8.8 = hp
1 cubic foot = 7.48 gallon
Assume two 10,000 gallon tank, one 100' higher than the other.  To generate 1kw of power
1kw = 1.3hp = flow/second x 100 / 8.8
1.3 x 8.8 = flow x 100
11.44  = flow x 100
11.44 / 100 = flow
.1144 cubic feet = flow
.1144 cubic feet = .856 gallon/second
10,000 gallon tank / .856 = 11,682 seconds / 60 / 60 = 3.24 kilowatthours for this "battery".

Each of the above tanks is only about the size of a modest “above ground” swimming pool.  Consider a well where the water level is more than 100 feet below the surface. A small windmill could easily during the day fill the pool, providing the evenings power for light and electronics.

Hi fred.greek.  We can use the 0.000376 W-H per lb per foot number calculated above to quickly check your thought.  A gallon of water weighs 8.34 lbs so:

Potential Energy = 10,000 gallons * 8.34 lbs/gal * 100 feet * 0.000376 Watt-Hour/foot-lb = 3,136 Watt-Hour or 3.1 KWH.

This is really close to the 3.2 KWH you calculated so it looks good to go.  The slight difference is probably rounding in using 1.3 hp per kilowatt vs 1.341 hp per kilowatt.
« Last Edit: July 12, 2017, 10:57:56 PM by iam4liberty »

Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #53 on: July 15, 2017, 01:50:00 PM »
Using falling weight devices (gravity) has been around for centuries. Example: The "treadwheel" crane was used during Roman times and the Middle Ages to lift and move very heavy loads. It made use of the weight of people walking inside a large wheel (think their body weight and gravity), turning a spindle and winding a rope, using gravity and mechanical advantage to lift stone blocks for castles and cathedrals. Don't forget Middle Ages "artillery": the Trebuchet, A weight driven" catapult" that could throw hundreds of pounds, hundreds of yards.
     I don't see why creative thinking couldn't use gravity to power electrical devices.

You are onto something here. 

The largest trebuchet in the world (actually the largest siege weapon period) is the one at Warwick Castle.  It holds the world record for energy in siege engines launching projectiles.  It uses a crew of six in giant 'hamster wheels' to set the counterweight (crew of four to charge and another two to unwind ropes so it doesn't self destruct). 



It is likely the most substantial human power device ever weighing 50,000 lbs with its arm rising to 59 feet above the ground.  Amazing machine.





So how much energy does this thing produce?  It hurls ~30 lb balls at 121 mph (13.5 kg @ 54.1 m/s). 



So we can use that as a basis of its output energy using the following formula:

Kinetic Energy = 1/2*mass*velocity2 = 1/2 * 13.5 kg * (54.1 m/s)2 = 19,756 Joules = 5.49 Watt-Hours

However, this is the energy that it outputs in throwing a ball.  This is quite an inefficient process as you are accelerating a large, heavy arm, not just the projectile itself.  So let's calculate the energy it actually stores in its counterweight. 

From the published materials the counterweight itself weighs a whopping 13,440 lbs! But it doesn't tell us how much this weight is displaced by the 'wheelers' lifting it up with the hamster wheels.  The weight itself doesn't start on the ground but rather hangs several feet above.  Looking at the photos we can find where it hangs in each position and therefore estimate its rise using references within the photo.



I estimate this to be 6.1 feet.  So the amount of energy stored is:

Potential Energy = 13,440 lbs * 6.1 feet * 0.000376 Watt-Hour/foot-lb = 30.8 Watt-Hours

So with the proper escapement and charging arrangement this should be able to charge 6 AA rechargeable batteries!
« Last Edit: July 15, 2017, 02:10:54 PM by iam4liberty »

Offline Alan Georges

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Re: Why can't a gravity powered generator work?
« Reply #54 on: July 15, 2017, 03:33:01 PM »
Nice job of number-crunching, I4L.  8)

So how much energy does this thing produce?  It hurls ~30 lb balls at 121 mph (13.5 kg @ 54.1 m/s). 
...
So we can use that as a basis of its output energy using the following formula:

Kinetic Energy = 1/2*mass*velocity2 = 1/2 * 13.5 kg * (54.1 m/s)2 = 19,756 Joules = 5.49 Watt-Hours
In terms of more familiar ballistic energy units, that's 14,571 ft-lbs – about the same as a modern .50 BMG.  That's a cool coincidence.  Of course, a trebuchet stone ball is much more massive, moves slower, and will carry a wallop more in momentum.  Don't want to get hit by either!

Quote
So with the proper escapement and charging arrangement this should be able to charge 6 AA rechargeable batteries!
Makes one appreciate modern technology.  A little $100-ish Goal Zero kit can do the same job, fits into a backpack, and doesn't need a half-dozen serfs to turn the hamster wheels.

Artes sunt magis quam instrumenta.

Offline iam4liberty

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Re: Why can't a gravity powered generator work?
« Reply #55 on: July 15, 2017, 09:01:49 PM »
In terms of more familiar ballistic energy units, that's 14,571 ft-lbs – about the same as a modern .50 BMG.  That's a cool coincidence.  Of course, a trebuchet stone ball is much more massive, moves slower, and will carry a wallop more in momentum.  Don't want to get hit by either!
Makes one appreciate modern technology.

It is really interesting to look at 'ball throwers' over time.  There seems to be a sweet spot for 25 pound projectiles.  We saw that the best medieval technology from the 1500s using gravity can throw such a projectile at about 100 mph. Fast forward to the 1800s and with 3lbs of blackpowder (ie chemical energy) a cannon like the Parrott Rifle can throw a 24 lb projectile to 1100 mph. 



Fast forward to today and an electromagnetic railgun can throw a 24 lb projectile at 4600 mph.

http://www.popularmechanics.com/military/weapons/a21174/navy-electromagnetic-railgun/

« Last Edit: July 15, 2017, 09:14:14 PM by iam4liberty »