Gravity Based Electricity Generator

  • #1
kingamada
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2
TL;DR Summary
I am designing a gravity-based electricity generator using a 500 kg mass and exploring various mechanical configurations to convert gravitational energy into electrical energy efficiently. I am considering setups like pulley systems, piston mechanisms, compressed air systems, and piezoelectric approaches, with motor options ranging from 10 kW to 100 kW. I seek advice on the most effective setup and motor recommendations to maximize energy output.
Screenshot 2024-05-04 at 2.18.43 PM.png

Hello community,

I am currently designing a gravity-based electricity generator and exploring the feasibility of different setups using a 500 kg mass. The core of my project is to harness the potential energy from this mass as it is lowered from a significant height. I am considering several mechanical configurations to convert this gravitational force into electrical energy efficiently.

Here are the mechanisms I'm contemplating:

Pulley System: Utilizing a pulley attached to a generator, where the descending mass generates rotational energy. What would be the optimal gear ratio and mechanical setup to maximize efficiency in this configuration?

Piston Mechanism: Implementing a piston-like setup where the falling weight raises a lighter mass, creating a continuous cycle. How practical and efficient is this method for sustained energy generation?

Compressed Air System: Leveraging the weight to compress air, which could then drive a turbine as it decompresses. What are the potential energy losses and efficiencies expected with this method?

Piezoelectric Approach: Using piezoelectric materials to convert the mechanical stress from the weight's impact into electrical energy. Given the large scale of the mass, is this method viable?

Additionally, I'm evaluating the use of motors with different capabilities, ranging from 10 kW to 100 kW. Wind turbine motors that can generate 10 kW at 100 RPM are on my radar, but I am open to suggestions for other types of motors and methods that could be more effective by using 500kg of weight.
 
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  • #2
Welcome to PF.

How would this gravitational potential energy typically be stored in your system? Is it kind of like storing solar energy during the day in your weight storage system and extracting that energy back out at night?
 
  • #3
berkeman said:
Welcome to PF.

How would this gravitational potential energy typically be stored in your system? Is it kind of like storing solar energy during the day in your weight storage system and extracting that energy back out at night?
It would be used as a energy source, which means as the energy is generated it's being used, if the energy is in surplus then the remaining would be stored into a battery.
 
  • #4
kingamada said:
It would be used as a energy source, which means as the energy is generated it's being used, if the energy is in surplus then the remaining would be stored into a battery.
Um, how do you initially lift up those weights to store that energy? Or does it magically show up somehow?
 
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  • #5
Have you done the basic math yet? To get 10kW from 500kg falling it must be moving at about 2 m/sec. So you'll need 2 to 20 m/sec with perfect efficiency. That could add up to a lot of meters pretty quickly.
 
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  • #6
berkeman said:
Um, how do you initially lift up those weights to store that energy? Or does it magically show up somehow?
Well let's say moving the weight back up is not an issue, or the issue.
 
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  • #7
DaveE said:
Have you done the basic math yet? To get 10kW from 500kg falling it must be moving at about 2 m/sec. So you'll need 2 to 20 m/sec with perfect efficiency. That could add up to a lot of meters pretty quickly.
What if we're to use gears and also a high energy motor? For example a motor that is 30kw and rated for 3,000rpm. We then attached a pulley to it with gear high gear ratio that as the 500kg is coming down with each rotation it rotates 100 times, which for 30 rotation we have 3,000 rpm.
 
  • #8
kingamada said:
Well let's say moving the weight back up is not an issue, or the issue.
But it is an issue because conservation of energy requires there be a way of getting the weights up there in the first place. The mechanism that recharges the system will be powered by energy from some source, and it is that source that will decide the appropriate mechanism to handle the ebb and flow of energy in the system.

Your "Compressed Air System" is inefficient and neglects the possibility of pulling a vacuum in a chamber, which raises the external worldwide atmosphere slightly. A vacuum offers a constant force, which is better matched to a weight rising and falling. Vacuum does not suffer the reciprocal pressure and volume inefficiencies of air compression.
 
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  • #9
kingamada said:
Well let's say moving the weight back up is not an issue, or the issue.
But that is your source of energy!
 
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  • #10
kingamada said:
What if we're to use gears and also a high energy motor? For example a motor that is 30kw and rated for 3,000rpm. We then attached a pulley to it with gear high gear ratio that as the 500kg is coming down with each rotation it rotates 100 times, which for 30 rotation we have 3,000 rpm.
Time to study some Newtonian physics, I think. The weight must fall at that rate to generate the power you specify. Gears and pulleys can alter velocity and force (or equivalent), but they don't change power (again assuming no losses).
 
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  • #11
kingamada said:
Well let's say moving the weight back up is not an issue, or the issue.
Nope. Thread is too close to you being a PMM advocate, so it is closed. If you have valid technical reasons for your responses, please send me PM. Otherwise, have a nice day.
 
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  • #12
Okay, based on the PM from @kingamada there is an external source of energy available to return the weights to the top. Thread is reopened. :smile:
 
  • #13
DaveE said:
Time to study some Newtonian physics, I think. The weight must fall at that rate to generate the power you specify. Gears and pulleys can alter velocity and force (or equivalent), but they don't change power (again assuming no losses).
That's where my major confusion is, if a motor is rated at 30kw at 3,000 rpm. And i attached a pulley system like the one above to the motor connected to a gear system with ratio 1:100, when the pulley rotates for the 30th time in 1 minute then the motor would have rotated at 3,000rpm thereby generating 30kw. But when at full load the torque will increase therefore the weight needed to rotate the pulley need to increase. That's why i want to find out if i have 500kg of weight, then that should be powerful to rotate a powerful motor at 3,000 rpm with by using a good gear ratio.
 
  • #14
Vanadium 50 said:
But that is your source of energy!
Let's say we find a cheaper way or people to take the weight back up, i explained that to Berkeman in PM, which makes it not a PMM thread.
 
  • #15
Baluncore said:
But it is an issue because conservation of energy requires there be a way of getting the weights up there in the first place. The mechanism that recharges the system will be powered by energy from some source, and it is that source that will decide the appropriate mechanism to handle the ebb and flow of energy in the system.

Your "Compressed Air System" is inefficient and neglects the possibility of pulling a vacuum in a chamber, which raises the external worldwide atmosphere slightly. A vacuum offers a constant force, which is better matched to a weight rising and falling. Vacuum does not suffer the reciprocal pressure and volume inefficiencies of air compression.
Thanks for your input, regarding the weight being lifted back up, let's say we're to use animals to take the loads back up to the top.
 
  • #16
kingamada said:
That's where my major confusion is, if a motor is rated at 30kw at 3,000 rpm. And i attached a pulley system like the one above to the motor connected to a gear system with ratio 1:100, when the pulley rotates for the 30th time in 1 minute then the motor would have rotated at 3,000rpm thereby generating 30kw. But when at full load the torque will increase therefore the weight needed to rotate the pulley need to increase. That's why i want to find out if i have 500kg of weight, then that should be powerful to rotate a powerful motor at 3,000 rpm with by using a good gear ratio.
You will need more knowledge of power, energy, and simple machines (gears, pulleys, levers, etc.) to understand our answers. PF isn't a great place to learn basic physics. It is a great place to ask questions about the parts that confuse you though.

A gear ratio of 1:100 can reduce the rotational speed by 100x, but it will increase the torque required by 100x also. The power is unchanged. This is why I could calculate the speed of your mass without knowing anything about the system down stream. You must make the power required from your mass falling first, then you could modify speed/torque as needed.

Khan academy has some good physics tutorials.
 
  • #17
The wiki might be enough. It's a pretty simple/straightforward concept:

https://en.m.wikipedia.org/wiki/Mechanical_advantage

The key is simply recognizing that conservation of energy always applies, so you can't just assume if you spin at nameplate rpm you'll generate nameplate power.

Regarding the question in the OP though: motor/generator is the most efficient method for this type of energy storage. And it's one that is done already with weights for energy storage.
 
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  • #18
BTW, I'm willing to humor the OP for a bit because they seem willing to learn but this still looks like the old classic pmm to me:

motor -> pulley -> generator -> Profit!
 
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  • #19
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  • #20
Fair enough, guys. Elevators too. What triggered me is that lack of understanding of mechanical advantage is the secret sauce in that classic pmm. @berkeman told me that he's in a pm discussion with OP and OP has something legit in mind for storing the energy.
 
  • #21
It's not unrelated to weight-driven clock mechanisms. In that case the clock owner winds the clock to lift large weights which then fall (at a low rate controlled by the pendulum and escapement) and provide energy for the clock. It's a GPE battery, in other words.
 
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  • #22
DaveE said:
I don't see the PMM connection. The OP never mentioned anything about recycling power. These are real systems.
https://aresnorthamerica.com/our-company/
Yes, something like this, my major concern is knowing the power that i can generate from 500kg weight, would using a high capacity motor yield more energy? If so which motor(if i connect it to small 50watt motor or 30kw motor to a 500kg wight, would energy that would be generated be the same?) and setup would i need to effectively use 500kg of weight. In the case of Ares North America, they use a car cable method.
 
  • #23
Ibix said:
It's not unrelated to weight-driven clock mechanisms. In that case the clock owner winds the clock to lift large weights which then fall (at a low rate controlled by the pendulum and escapement) and provide energy for the clock. It's a GPE battery, in other words.
Technically, but my system would be used to generate the electricity, meaning we would keep on winding the clock to lift the object and releasing. I'm working on sketches that i might likely share here, or privately.
 
  • #24
kingamada said:
Technically, but my system would be used to generate the electricity, meaning we would keep on winding the clock to lift the object and releasing. I'm working on sketches that i might likely share here, or privately.
I would say that whatever is lifting the weight is the energy source. This is a battery (if you store the input energy for later) or a rate smoothing device (if the energy input is spiky and you output at the average rate) or a mix.
 
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  • #25
kingamada said:
Yes, something like this, my major concern is knowing the power that i can generate from 500kg weight, would using a high capacity motor yield more energy? If so which motor(if i connect it to small 50watt motor or 30kw motor to a 500kg wight, would energy that would be generated be the same?) and setup would i need to effectively use 500kg of weight. In the case of Ares North America, they use a car cable method.
Sorry, this question just doesn't make much sense. You seem to not really understand the difference between mass, energy, power, etc.

I think you need to pick a power level to operate at, which will in turn determine a bunch of things, like generator size. There are multiple choices at play, none of which can be made well if you don't understand the basic concepts connecting them.
 
  • #26
DaveE said:
Sorry, this question just doesn't make much sense. You seem to not really understand the difference between mass, energy, power, etc.

I think you need to pick a power level to operate at, which will in turn determine a bunch of things, like generator size. There are multiple choices at play, none of which can be made well if you don't understand the basic concepts connecting them.

I think the below picture will give a clear understanding of my question, stepping on this device below generates a little electricity. My question is that, since a human weighing on average 80kg, stepping on this device will generate small watt of electricty, what if we have a heavy object of 500kg stepping this device and connected to a bigger motor. Let's work with 30kw, 500kg weight, height depends on the setup. I hope i convey the question clearly now.
Screenshot 2024-05-06 at 4.19.53 PM.png
 
  • #27
kingamada said:
I hope i convey the question clearly now.
Nope. Sorry, weight isn't the same thing as power. More weight doesn't necessarily mean more power.
You need to study high school physics.
 
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  • #28
kingamada said:
Yes, something like this, my major concern is knowing the power that i can generate from 500kg weight, would using a high capacity motor yield more energy? If so which motor(if i connect it to small 50watt motor or 30kw motor to a 500kg wight, would energy that would be generated be the same?) and setup would i need to effectively use 500kg of weight. In the case of Ares North America, they use a car cable method.
You're still approaching the problem backwards. You need to figure out or specify how much mechanical energy/power you have available, and then you simply match the generator to it. @DaveE gave you most of the answer and how to figure that out in Post #5.

For the generator selection; there is no benefit to over- or under-sizing the generator: too much and you waste money and lose a bit of efficiency. Too little and you'll either generate less power or damage the generator. If you have/want 50W you use a 50W generator. If you have/want 30kW you use a 30kW generator. That's it.
 
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  • #29
kingamada said:
if i connect it to small 50watt motor or 30kw motor to a 500kg wight, would energy that would be generated be the same?
The difference is in the speed your weight falls (descends) with. With the same weight you can keep an 50W generator/motor running longer, while an 30kW one will un(down)wind it very fast.

It's not trivial to have this work properly. You need some kind of (lossless) brakes to have the weight descend according to the actual electrical load. That makes the practical size of this kind of thing either big (commercial energy storage) or very
small.
 
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  • #30
Rive said:
or very
small.
That's cute! I think I want one... :smile:
 
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  • #31
OP if you are interested, there is an existing technology common in industrial cranes where when the load is lowered down, the rotational force it produces to the motor is converted back to electricity so that it will help in braking the lowering of a load to a stop. Its called regenerative braking, however, this is really not a feasible idea for any scale of power generation that you want to achieve.
 
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  • #32
kingamada said:
I'm evaluating the use of motors with different capabilities, ranging from 10 kW to 100 kW.
Industrial servomotors are available in that size range. One such manufacturer (not the only one) is Allen-Bradley. Check out this link: https://www.rockwellautomation.com/...ol/rotary-servo-motors/mma-series-motors.html. Click into the technical literature, and look for references to regenerative buss supplies. A quote from the technical literature:

Kinetix 5700 servo drive systems consist of these required components:
• 2198-Pxxx DC-bus power supplies (up to three modules wired in parallel are possible)
• 2198-RPxxx regenerative bus supplies (provides full-line motoring and regenerative power)


If you use one of these servomotor/drive systems to raise and lower a load, it automatically draws power from the line to raise the load and pumps power back into the line when lowering the load. You program the system to move the load, and the drive takes or pumps power as needed to make it happen. Do not expect a cheap system, and do expect a learning curve. But it will do what you want.
 
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  • #33
kingamada said:
I think the below picture will give a clear understanding of my question, stepping on this device below generates a little electricity. My question is that, since a human weighing on average 80kg, stepping on this device will generate small watt of electricty, what if we have a heavy object of 500kg stepping this device and connected to a bigger motor. Let's work with 30kw, 500kg weight, height depends on the setup. I hope i convey the question clearly now.
This is a dreaded Energy Harvesting Scheme. You must realise that the only systems that harvest successfully are where you have a shed load of energy that you feel you have to get rid of. Significant Energy from people walking will wear those walkers out very quick. They will avoid that particular route on their walk.
 
  • #34
Typical of this forum. Everyone projects their own limits onto others thoughts.
Your idea is good and can be worked. You’ll need an external energy source to lift your weight though. Even lifting a smaller weight as your 500kg falls will decrease the power available to be generated. The option is to gear it so that your weight takes a long time to fall so the gearing would be stepped up to drive a generator. I’d be thinking something like an alternator where you could alter the generating field by altering the current and thereby increasing the magnetic drag and generation output. If you take a 12 volt alternator and supply the rotor with 6 volts instead of 12 you’ll find it doesn’t output 12 volts. But it will require less power to rotate. If you supply 15 volts to the rotor coils it’s output will exceed 12 volts and require higher power input to cause rotation.

Using this you could design an alternator that outputs your required voltage and amperage and your designed rate of drop to match without the issue of interrupting the rate of fall. Use the rotor voltage to control fall. The potential energy stored in your 500 kg weight will not change so it’s only how you can harvest it and lift that weight again that decides if it’s a good solution or not. Mechanical advantage does not alter the amount of power only how that power is distributed between torque and rotation speed.

Don’t let the limits of others stop you.
 
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  • #35
Ripcrow said:
The option is to gear it so that your weight takes a long time to fall
The OP specified 500kg and >10KW. The only pertinent unspecified variable is how far the weight gets to fall; i.e. how long do you get 10KW. You don't get to choose the minimum speed of the fall if you've specified the other stuff.

Once you've achieved the minimum speed, then you would want to control the rate the weight falls with gears and such, or store the extra power produced. But this will happen naturally with whatever energy extraction system you have. There will be an equilibrium speed that is determined by the power extracted. Of course there are kinetic energy issues with the beginning and end of the fall.

The basic point is that you don't have to know a lot about the details of energy extraction (alternators, gears, etc. ) to determine some significant limitations on what the falling weight has to do. I would strongly suggest a "black box" approach first.


Ripcrow said:
Don’t let the limits of others stop you.
Yes! But there are limits from the laws of physics that should be understood and evaluated.


Ripcrow said:
Typical of this forum. Everyone projects their own limits onto others thoughts.
I respectfully disagree. But we do believe in physics here. Maybe we are "projecting" reality. No one's saying this can't be done. Gravity batteries are a thing. People have built them, and they work. But, they do have to be designed within the limitations of physical laws.

Honestly, I think we are trying to help. When I say "study high school physics" I don't mean it as an insult, I'm trying my best to point the way to a successful outcome.
 
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