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Energy Options => Other Energy Sources => Topic started by: mangyhyena on June 16, 2012, 07:31:08 PM

Title: Why can't a gravity powered generator work?
Post by: mangyhyena on June 16, 2012, 07:31:08 PM
Right out of the gate, I realize this probably isn't viable.  For the physisists here, I'm NOT talking about free energy.  What I want to know is, why can't we use gravity, or I guess suspended weight, as our energy storage medium?

We have a cuckoo clock with a swinging pendilum at my home.  I pull the chain, raising two weights, then it keeps time for about a day, keeping the pendilum swinging the whole time.

Could a few rechargable AA batteries be charged in a day or two if the pendilum were a magnet and, say, 2 coils were placed at each side of the pendilum?  Or would it not produce enough electricity for even that?  Could a capacitor be loaded and periodically discharged into the batteries to overcome the small amounts of electricity produced during each swing?

Next question:  if a larger pendilum were set up with much heavier weights, could it produce a respectable amount of power in a 24 hour period, enough for a small battery bank?  Could it charge a a small battery bank in a week's time, assuming the weights were reset each day?  No, it would not produce more energy than it takes to raise the weights, so using power equipment to raise the weights would result in a net loss of energy.  However, those weights could be raised using human or animal power.  If I can lift my vehicle with a jack, I can reset the weights for the pendilum, even if they weigh 1,000 pounds each.  Pain in the butt, I'm sure, but I could manage it.

I guess the question isn't whether or not this setup would produce electricity.  Rather, would it produce a usable amount of electricity?  How much or how little?

I'm assuming solar and wind are more productive.  It's just curiosity that drove me to ask about this.
Title: Re: Why can't a gravity powered generator work?
Post by: cmxterra on June 16, 2012, 07:59:35 PM
Someone smarter than I will come along and make more sense.

My understanding of the way energy generation works is that the energy density of whatever power source you are using has to have enough potential energy before it is converted to electricity to be a viable energy source. Be that fuel, water, sunlight or wind.

With a gravity driven system the amount of energy you would need to exert to set your weight up would at BEST equal what you might get out of it (and that would never happen as you can not have a perfect conversion of energy)

So to get enough energy to do any useful work with a gravity system you would have to put more energy into it than you would get out of it. At least with solar, wind and hydro nature is putting in the work for you.   
Title: Re: Why can't a gravity powered generator work?
Post by: Entity on June 17, 2012, 01:42:29 AM
taking energy from the pendulum would damp and eventually stop the pendulum. The gravitational potential energy that is converted to kinetic by the bottom of the swing is re-converted to potential to get it up the other side.

taking energy of the falling weight (and simply modulating the rate at which that weight falls), is ... exactly how your clock works
Title: Re: Why can't a gravity powered generator work?
Post by: Jeff NH on June 17, 2012, 06:16:27 AM
Note that you might also be using the wrong terms here. What you are describing (both in terms of desired device and your example device) is a energy storage device (like a battery) and really less of a generator. You pull the weight to the top, and it falls down slowly. The pendulum is there to keep time and is getting energy needed to overcome friction from the energy you stored in the weight system.

Given a significant enough weight, you could store energy in a system like this as an alternative to a battery. Not sure what the efficiency of such storage would be. It would be a fun project. To get any significant/useable amount of electricity, you'd need much more weight than the clock.
Title: Re: Why can't a gravity powered generator work?
Post by: mangyhyena on June 17, 2012, 07:29:19 AM
The much-more-weight challenge could be met, I think.  My real question is can that weight be converted to a usable amount of electricity.  If it can, great.  It'll be time to start figuring out how to elevate the weights.  If not, why bother?

If I wanted a sidnifigant amount of weight raised, so when it falls it generates electricity, I'd have horses or cows walk up onto a raised platform, where they would stay until the platform came back down.  4 horses or more would seem to present a respectable amount of weight.  And, the animals could walk themselves up to the correct elevation at feeding time.

I've got 2 horses, but neither is in any shape to do work.  One is lame with ringbone, the other is old.  Both, however, could easily walk up to a raised platform at feeding time and stay there as the platform "falls" (slowly) and produces electricity.  We would take in and care for more so-called useless animals for this purpose, if it could work to produce a usable amount of electricity for my home.

The pendilum approach came from me looking at that cuckoo clock, but if there is a better way to harness the energy from the falling weight, I'd be all for it.
Title: Re: Why can't a gravity powered generator work?
Post by: Mr. Bill on June 17, 2012, 09:42:12 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.
Title: Re: Why can't a gravity powered generator work?
Post by: mangyhyena on June 17, 2012, 04:18:36 PM
There is another thread I got going about animal power, which was more about using the animal's muscles to generate power, be that a generator or a water pump.  Running both these threads at the same time made me wonder if they could be combined.  Using the animal's weight for power generation seems doable, since the animal is mobile and can put itself in a position to "fall" (slowly/humanely) without extra energy input from people or oil/coal.  Build a gravity motor & use the animals as the "resettable" weights.

Sounds good, but a setup would need to produce a usable amount of energy and for a long enough period of time for it to be feasabile.  And that is where this whole idea could be all wet.

But, just think of the multiple benefits toward self sufficiency if it could be made to work.  Animals produce power, and/or pump water, using their muscles, weight on a gravity motor, or both.  Manure from animals goes to a methane digester first, then to fertilize crops.  If pumping water for energy storage, the water goes to irrigation and watering animals after producing power via hydro generator. At some point, the animal winds up on the dinner table.

If you have the land for it, you could board other people's animals, for a fee, and have their animals generate power; in essence, having other people pay for your energy production.

Again, depends on whether or not a usable amount of energy can be produced and stored in this manner.  That is a big IF.
Title: Re: Why can't a gravity powered generator work?
Post by: Philip on July 15, 2012, 09:02:06 AM
I don't know how accurate this would be but if you took the calculations from a theoretical hydro setup www.powerspout.com (use their calculator)  and figure the weight of the water over time at x-fall you might get a rough estimate.
Title: Re: Why can't a gravity powered generator work?
Post by: fritz_monroe on July 15, 2012, 10:01:54 AM
Back to your weights.  How about pull the weights to the top and as they slowly fall, they turn a gear.  Step this up to the point where it is turning an alternator fast enough to charge your batteries.  I guess that would be a generator using weight as the fuel source.  I have no idea how long it would run for on each lift of the weight or any other details.  Someone much smarter than I am would need to figure that stuff out.
Title: Re: Why can't a gravity powered generator work?
Post by: LdMorgan on July 15, 2012, 02:58:22 PM
The problem with animal power is that it takes much more feed to keep the animal at its normal weight when it's put to work. Just standing around the stable doesn't require much energy, or much feed to provide it.

You don't need to invent a gravity motor for  animals--just check out some of the existing technology.

Example: http://www.fao.org/wairdocs/ilri/x5455b/x5455b0z.htm

That's a pretty poor engine, BTW. Even at a glance I can see a much better & cheaper way to get the job done.

For gravity power, the trick is just to find something heavy uphill and send it downhill.

You could throw a 5 lb rock off a 100-ft cliff--tied to a rope that spins a generator as it pays out. Wind it back up empty, and then drop another rock.

When you cut a tree down, why waste the energy of position? A 50 lb limb falling 10 ft would do the same job as the 5 lb rock.

If thinking outside the box is fun, go the other way: Send a passive hot air balloon up on that same rope. All you need is a balloon and a campfire. 5lbs of lift and 100 ft of rope--voila! Then just reel it back in when it cools and starts to drop. Better yet, 50 lbs of lift and a long enough rope to use it all.

Or send up a kite and let the wind pay out the rope. Then untrim the kite, reel it back in, and repeat.

Of course, if you have a 100-ft cliff in the first place, why not just drop a bare copper wire over the edge and pull a perpetual voltage out of the bottom end? Cheap, no moving parts, and you can double the power just by adding another wire.



Title: Re: Why can't a gravity powered generator work?
Post by: tamo42 on July 16, 2012, 12:39:02 PM
The easiest way that I'm aware of to do this kind of thing is to pump water uphill into a holding tank/pond, which would then be available for hydroelectric (or hydrothermal or whatever) applications. Depending on your available resources, you might use solar power, wind power, water power (ram pumps for example) to get the water uphill in a "free" way.
Title: Re: Why can't a gravity powered generator work?
Post by: ToMegaTherion666 on July 16, 2012, 01:12:46 PM
You can hook the clock up the cubbord, and wind it everytime you eat. Id imagine there are a hundred little things you could do if you wanted to generate a small charge.

a pull rope in teh bathroom that works a dynamo on the way down and on a geared/weighed release (you could likely make a working modle with the guts of a hand crank flashlight, some rope, and an old bicycle) - so you pull the rope a little and the light comes on and because of the mechanical amplification the light will continue to run and charge for some preset amount of time.

which might be a more efficient solution than using several methods to maintain battery across a grid.

on the high tech end.. floor pressure plates are being tested to take energy from people walking across them to generate light etc

Title: Re: Why can't a gravity powered generator work?
Post by: markl32 on July 16, 2012, 06:17:18 PM

http://www.activepower.com/ (http://www.activepower.com/)

Fly wheel.  Power keeps the flywheel spinning.  Power goes down, flywheel becomes a generator, for a while. 

Title: Re: Why can't a gravity powered generator work?
Post by: mangyhyena on July 18, 2012, 07:28:19 PM
I knew other people wondered about strange contraptions, too. 

Isn't the flywheel a power storage option?  I thought I read they're using them to supply immediate power to computer servers at the onset of a power outage, giving the generators time to kick in.  I could easily have that wrong, though.

The hot air idea is a fun one.  Who wouldn't want to play with fire while flying a balloon?
Title: Re: Why can't a gravity powered generator work?
Post by: Bradbn4 on July 18, 2012, 07:57:30 PM
The easiest way that I'm aware of to do this kind of thing is to pump water uphill into a holding tank/pond, which would then be available for hydroelectric (or hydrothermal or whatever) applications. Depending on your available resources, you might use solar power, wind power, water power (ram pumps for example) to get the water uphill in a "free" way.

A quote from an old Professor when I was going to school and asked this question:

Pushing water up hill is the best way to store energy. 

When I was over seas on my big European vacation, we took a tour of a man made "lake" at the top of a good size hill.  During the day they would use water to help support peak electric loads while at night they used excess energy to pump the water back up the hill. 
Title: Re: Why can't a gravity powered generator work?
Post by: joeinwv on July 18, 2012, 08:47:30 PM
As long as the weight, water, animals, etc magically finds its way to the top - it all works. By the time you factor in the time,  work and materials it takes to set all this crap up, you almost certainly end up with a negative gain. Especially when you factor in the crappy battery technology we have now.

Your clock works because you spend energy winding it - the flywheel and springs slow down the release of that energy, but it's not making any power. It's storing / releasing the power you put into it.
Title: Re: Why can't a gravity powered generator work?
Post by: Jerry D Young on July 19, 2012, 12:16:43 PM
I think a gravity engine will work. I have a design for one, but I just haven't had the time or money to follow up on it.

Title: Re: Why can't a gravity powered generator work?
Post by: fritz_monroe on July 20, 2012, 07:33:58 AM
As long as the weight, water, animals, etc magically finds its way to the top - it all works. By the time you factor in the time,  work and materials it takes to set all this crap up, you almost certainly end up with a negative gain. Especially when you factor in the crappy battery technology we have now.

Your clock works because you spend energy winding it - the flywheel and springs slow down the release of that energy, but it's not making any power. It's storing / releasing the power you put into it.
All true, but the OP seems to want to be able to charge some batteries in event of a catastrophe.  Can't run a radio, flashlight or any other small electrical item off the muscle power of an animal.  But convert that to an electrical charge by having that animal run a pump to pump water uphill to run a micro hydro set up and now you're cooking.

Just because it's a negative gain doesn't make it useless.  In event of a major power outage, that animal could just be wasting that energy wandering around the paddock.  Putting it to work to run the pump, generator, etc is generating an energy source that is usable to the OP.
Title: Re: Why can't a gravity powered generator work?
Post by: donaldj on July 20, 2012, 07:56:48 AM
Yes. One of the biggest misunderstandings of conservation of energy is people assume things are working with 100% efficiency to begin with, and any new load on the system means more power.

Animals are absorbing energy just living. Getting them to expend some more energy may get you a lot of usable energy.

A single duck flying south for the winder uses a lot of energy. All his buddies making the duck formation are NOT adding their 'weight' to the first duck's load, they're using the benefits the first duck already created (high pressure zone) more efficiently.

A truck going down the road has to bludgeon it's way through the air (wind resistance) to get from point A to B. The myriad of other trucks and cars that draft the truck are not increasing the load on the first truck, they are using the already expended energy (the low pressure zone behind the truck).

A car's turbocharger is using already created exhaust gas flow to spin a compressor. For only minor reverse pressure, the car can make additional horsepower from this energy. It's not sapping the system, it's using already expended energy more effectively.


As for the water thing, isn't there a model that uses high tides?  High tide creates a reservoir and turbines are run as the water enters, and low tides run the turbines as the water leaves.

Title: Re: Why can't a gravity powered generator work?
Post by: mangyhyena on July 20, 2012, 07:36:50 PM
I think that if teotwawki actually happens, the survivors will figure out what works best for them in their particular circumstances, in terms of generating and storing power.  The survivors will not give a crap about what academics say is inefficient, impractical, ect.  They will care only about getting results to power their lives.  They'll do what works.

Title: Re: Why can't a gravity powered generator work?
Post by: fred.greek on July 20, 2012, 08:57:19 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.
Title: Re: Why can't a gravity powered generator work?
Post by: tamo42 on July 21, 2012, 12:01:34 PM
Seems workable if you have the height.
Title: Re: Why can't a gravity powered generator work?
Post by: mangyhyena on July 21, 2012, 08:28:07 PM
Did someone say to think outside the box?  All right, I'll try.

I've read that a buoyancy/gravity motor is not possible due to water pressure.  (inserting a boyant float into the bottom of a tank will require more energy than gets released as float rises, due to water pressure working against insertion, if I understood correctly.)

What if the float also acted as the weight for the gravity motor?

Float is heavy and shaped like a boat for water displacement, so it will float.  It sits in a container.  When water from a higher location flows in, the boat/weight rises, generating energy.  When water is released from the container, the boat/weight falls, generating energy.  Perhaps the container empties into another container set up the same way.  Height would dictate how many containers could be set up in series.

I don't know if this setup would equal the efficiency of a regular hydro setup.  Maybe in a location where there isn't much in the way of head, it might work out.

Watching the way the ships are "lifted" through locks so they can continue onto a higher body of water gave me this idea.  Ship enters lock, water is released into lock, ship rises as lock fills, then ship leaves lock at a higher elevation than when it entered.

Title: Re: Why can't a gravity powered generator work?
Post by: LdMorgan on July 23, 2012, 02:44:06 PM
Yup--that's the basic tidal generator, in a nutshell.

A person could anchor a barge, and have the top end of the anchor cable wrapped around a windlass.

When the tide rise, cable pays out, turning a generator. A low tide, just reel in the slack.

Or do the same thing with smaller floats.

If the reel windlass was ratcheted, it could also be a wave generator at the same time.
Title: Re: Why can't a gravity powered generator work?
Post by: cottonpatchenergy on January 09, 2014, 09:36:37 AM
ok I'm new to this but the short answer is yes. A suspended weight is an excellent way to store potential energy. Gravity driven generators are the same thing as hydroelectric generation. Now for the purpose of this forum pertaining to your own power generation you will have to compile all the stuff you've heard and fix the broken ideas. The falling weight has been used since they built the ancient cities of the past. Here's the basic problem with gravity driven generators you have to reset them in order to complete a cycle. Energy in energy out so you have to have energy to start with but since we are talking about storage to start with the medium is fine. Now that being said there is I believe an idea of making the unit reset or reload the falling weight. One of the post makes me laugh because it is so close to the truth without seeing it, the classical buoyancy drive. The problem is they see the advantage of lifting the weight with buoyancy but just cant seem to figure out how. The key is not to force a buoyant object into a tank of water, it is much easier to just move the air and create the displacement. So without going into great detail here you reset the weight back to the top by floating back up there. You can lift a weight 19 feet in the air with just 8.5 lbs of pressure. That is air pressure. The volume of the air at this pressure will equal the weight in your falling object that you will couple to your gear box and turn your generator. Now here is the kicker the power needed to create the displacement that will lift the weight back to the top of your grandfather clock gravity drive is suspected to be less than the power generated by the falling weight. Here is an example to think about: lets say you have a 400lb weight and your gona lift it to a height of 10 ft. The rules say that the potential energy the weight represents is exactly equal to the amount of energy that can be extracted from the falling object. I agree and it applies in a normal gravitational field but when you throw in another force which negates a portion of the gravitational constant then you end up with a different result. It's a direction of thought that will blow your mind once you get your head around it. Any change in elevation represents energy.
Title: Re: Why can't a gravity powered generator work?
Post by: MillenniumMan on January 09, 2014, 10:42:42 AM
There's always Heron's fountain. The only problem with that is water eventually evaporates.

The theory is however that if you have a water wheel at the point just below the apex of the fountain, you could use that to constantly run a hydro-electric dynamo. Something like that exists on rivers, but so far no one has bothered implementing it on the fountain.
Title: Re: Why can't a gravity powered generator work?
Post by: kckndrgn on January 09, 2014, 10:56:53 AM
For the OP, is this what you are looking for?
http://deciwatt.org/ (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.
Title: Re: Why can't a gravity powered generator work?
Post by: SloSheepdog on January 09, 2014, 11:03:35 AM
I like seeing neat mechanical contraptions, but I gotta think that this is the way to go if you're using animal power, or some variation of it. Gear up or down using pullies or gears, connect to generator to battery bank or directly to work load to be done, and pow! Energy!

Wiki says it's a limestone quarry, but the principle remains and can be used for anything.
(http://i1064.photobucket.com/albums/u370/85huhwhat/TSP/800px-Chatillon_treuil_carriere_Auboin_2.jpg) (http://s1064.photobucket.com/user/85huhwhat/media/TSP/800px-Chatillon_treuil_carriere_Auboin_2.jpg.html)


Title: Re: Why can't a gravity powered generator work?
Post by: AvenueQ on January 09, 2014, 02:11:34 PM
My physics degree has some use in the prepping world! Though I can only speak from a theoretical, not design standpoint (that's for the engineers  ;) ). If you really want me to explain how gravity and potential energy work I will...but it might be irrelevant to what you're trying to do.
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty on January 09, 2014, 07:08:58 PM
Gravity is a very weak force so it typically takes a lot of mass to store usable amounts of energy. Historically, it has been easier to use other physical means (for example air pressure, rope tension) or chemical means (e.g. battery, fuel) for energy storage.

If you are really interested in this, check out punkin chunkin (www.punkinchunkin.com).  You can see practical gravity (trebuchet), tension (catapult), and air (pressure) storage systems.  Plus it is just so cool   ;D
Title: Re: Why can't a gravity powered generator work?
Post by: mountainmoma 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
Title: Re: Why can't a gravity powered generator work?
Post by: mountainmoma 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
Title: Re: Why can't a gravity powered generator work?
Post by: AngusBangus 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...
Title: Re: Why can't a gravity powered generator work?
Post by: drtcbear 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.
Title: Re: Why can't a gravity powered generator work?
Post by: Carl on July 10, 2017, 05:12:44 AM
Gravity can work just fine .

https://www.indiegogo.com/projects/gravitylight-lighting-for-developing-countries#/

https://gravitylight.org/
Title: Re: Why can't a gravity powered generator work?
Post by: LVWood 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.
Title: Re: Why can't a gravity powered generator work?
Post by: Carl 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.
Title: Re: Why can't a gravity powered generator work?
Post by: bcksknr 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?
Title: Re: Why can't a gravity powered generator work?
Post by: LVWood 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...

A progressives Elysium dream.
Interesting perspective, the idea isn't abhorrent to you, it's just unworkable.
Title: Re: Why can't a gravity powered generator work?
Post by: Carl 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.../.
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty 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.
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty on July 11, 2017, 05:17:57 PM
For the OP, is this what you are looking for?
http://deciwatt.org/ (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/ (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. 
Title: Re: Why can't a gravity powered generator work?
Post by: Carl 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/ (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 ...
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty 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 ...

(https://img.newatlas.com/murakami-rocking-chair-0.png?auto=format%2Ccompress&fit=max&h=670&q=60&w=1000&s=9b38be0a0f0edcc0951bea9949c8a95d)
Title: Re: Why can't a gravity powered generator work?
Post by: Carl 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?
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty 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 (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!"
Title: Re: Why can't a gravity powered generator work?
Post by: drtcbear 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.
Title: Re: Why can't a gravity powered generator work?
Post by: bcksknr 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. 
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty 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. 
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty 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
Title: Re: Why can't a gravity powered generator work?
Post by: drtcbear 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?
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty 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/ (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.
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty 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.
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty 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). 

(http://simpson.edu/wp-content/uploads/2012/10/May-Term-2011-Trebuchet-Warwick.jpg)

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.

(https://howdymanchester.files.wordpress.com/2014/06/warwick-castle-064.jpg)

(https://i.ytimg.com/vi/b3Eq59StKUY/maxresdefault.jpg)

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

(http://www.balettie.com/zenphoto/albums/vacation-photos/england-2006/18-july-2006-warwick-castle/Trebuchet%20projectiles.jpg)

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.

(http://www.libertyassociate.com/survival_podcast/Warwick_Trebuchet.png)

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!
Title: Re: Why can't a gravity powered generator work?
Post by: Alan Georges 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.
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty 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. 

(http://civilwarwiki.net/w/images/8/89/30pdr-parrott.JPG)

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/ (http://www.popularmechanics.com/military/weapons/a21174/navy-electromagnetic-railgun/)
(http://pop.h-cdn.co/assets/16/22/768x554/gallery-1464989154-railgun-usnavy-2008.jpg)
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty on May 19, 2018, 07:48:55 PM
Just a follow-up. The new version of the gravity-light is out.  The designers pretty much did a 180 on the gravity storage aspect.  They ended up incorporating a rechargeable, battery, solar, and direct hand crank (by pulling cord) to make it practical.  They re-dubbed it the "nowlight".

https://www.theregister.co.uk/2018/05/01/nowlight_launch/ (https://www.theregister.co.uk/2018/05/01/nowlight_launch/)

The second generation adds solar power and a rechargeable battery. The latter may be surprising – co-designer Jim Reeves said short-lived and costly rechargeable batteries were far from ideal. But things change, and the ability to store the energy is useful.

(https://c1.iggcdn.com/indiegogo-media-prod-cld/image/upload/c_limit,w_695/v1525940983/tdz5ehm1e7jc4cmviltm.jpg)

Apples-to-apples this pales in capability to the k-tor human power and basic solar options.  IMO it is pretty much a marketing gimmick now and from the indiegogo struggle to raise money it looks like people who supported it as the gravity-light the first time around are agreeing.  They raised $400K from people first time around and now seem doubtful to reach $75 k this time:

 https://www.indiegogo.com/projects/nowlight-renewable-energy-on-demand-camping-design#/ (https://www.indiegogo.com/projects/nowlight-renewable-energy-on-demand-camping-design#/)
Title: Re: Why can't a gravity powered generator work?
Post by: iam4liberty on May 19, 2018, 07:59:19 PM
Just saw that within a week after the previous article on the nowlight broke they closed down the gravitylight foundation:

https://gravitylight.org/gravitylight-blog/2018/4/11/conclusions-from-our-gravitylight-pilot-in-kenya (https://gravitylight.org/gravitylight-blog/2018/4/11/conclusions-from-our-gravitylight-pilot-in-kenya)

Through our pilot we have reached over 30,000 people in Kenya, with 3,000 families now owning a GravityLight.
...
we also heard about the significant brightness and duration increases needed to keep up with the rapidly advancing solar industry. And with mobile phones being used by 90% of the population, being able to charge a mobile was an essential addition.
...
we do not think that The GravityLight Foundation is able to tackle the challenge of energy poverty in the most scalable and sustainable way. We want to use this final blog to share with you that, we have taken the extremely hard decision, to close The GravityLight Foundation.