Gravity Based Electricity Generator

[kingamada]

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.

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.

 

[berkeman]

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?

 

[kingamada]

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.

 

[berkeman]

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?

 

[DaveE]

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.

 

[kingamada]

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.

 

[kingamada]

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.

 

[Baluncore]

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.

 

[Vanadium 50]

Well let's say moving the weight back up is not an issue, or the issue.

But that is your source of energy!

 

[DaveE]

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).

 

[berkeman]

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.

 

[berkeman]

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.

 

[kingamada]

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.

 

[kingamada]

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.

 

[kingamada]

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.

 

[DaveE]

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.

 

[russ_watters]

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.

 

[russ_watters]

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!

 

[DaveE]

I don't see the PMM connection. The OP never mentioned anything about recycling power. These are real systems.
https://aresnorthamerica.com/our-company/

 

[russ_watters]

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.

 

[Ibix]

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.

 

[kingamada]

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.

 

[kingamada]

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.

 

[Ibix]

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.

 

[DaveE]

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.

 

[kingamada]

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.

 

[DaveE]

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.

 

[russ_watters]

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.

 

[Rive]

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.

 

[berkeman]

or very
small.

That's cute! I think I want one...

 

[isquaredr]

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.

 

[jrmichler]

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.

 

[sophiecentaur]

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.

 

[Ripcrow]

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.

 

[DaveE]

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.

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.

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.

 

[sophiecentaur]

Typical of this forum. Everyone projects their own limits onto others thoughts.

That happens when all the parameters aren't specified - including the skill level of the questioner.

Unless the OP is a practised project. builder and DIYer there will be serious limits to any project. Looking around, I can only think of one domestic level mechanical energy storage system and that is the very successful Wind Up Radio. That should indicate the feasibility of the general idea. Inventing outside the box risks a shed full of unfinishedpart projects and no results. Good fun, on the way, though.

 

[Ripcrow]

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.



Yes! But there are limits from the laws of physics that should be understood and evaluated.



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.

The op specified 10 -100 kw. We don’t know long it can fall as we don’t know the height. Yes we are limited by physics but we are not limited by existing techniques if we are open to ideas. Read any post in this forum that questions existing ideas or has a different approach and you’ll see the post get wrecked by responses that attack ideas or the responses are merely projections of the responders own thoughts. There are plenty of posts where the op gets accused of trying to create PMM when it’s clear they are not.

 

[Ripcrow]

That happens when all the parameters aren't specified - including the skill level of the questioner.

Unless the OP is a practised project. builder and DIYer there will be serious limits to any project. Looking around, I can only think of one domestic level mechanical energy storage system and that is the very successful Wind Up Radio. That should indicate the feasibility of the general idea. Inventing outside the box risks a shed full of unfinishedpart projects and no results. Good fun, on the way, though.

So that inventiveness should be encouraged and instead of self projected limitations questions should be asked.

 

[Vanadium 50]

or the responses are merely projections of the responders own thoughts.

People post their own thoughts here? The fiends!

 

[Ripcrow]

People post their own thoughts here? The fiends!

And that’s fine except when the projections are biased

 

[kingamada]

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.

Thank you so much, you really did grasp my idea. Maybe I didn't ask the right questions, as you know sometimes it's about the quality of the question. Though I'm still trying to see how mechanical advantage does not alter the amount of power. Before we answer that, let me see if this rephrasing of my question will lead to a helpful answer and guide. Let's say I have a motor that has the below specification

MMA size (shaft height, mm)	Rated Speed, rpm	Max Speed, rpm	Rated Power, kW	Rated Torque, N•m	Peak Torque, N•m
SH80	580…2600	1400…6000	0.6…8.6	10.5…31.5	21…88
SH100	580…2600	1400…6000	2.3…21.1	35.6…94.6	80…200
SH132	580…2600	1400…6000	6.4…48.4	90.7…217	200…400

If we attach 500kg of weight to the pulley weight to each of the motor and we attach a pulley with multiplying gears such that each rotation of the pulley in 2 seconds results to 100 rotation of the motor, which would translate to 30 rotation = 3,000 rpm. Now knowing the rpm, power output of the motor, peak torque and the height is 5 meters. Wouldn't it produce it produce the rated power as it's coming down?

 

[Rive]

500kg of weight ... height is 5 meters...

You have ~ 24.5kJ energy in that (height*mass*g). That much can feed 1kW load for 24.5 seconds.

If done right, then the rated power of the motor/generator will determine only the maximal load the system can handle.

 

[Ibix]

Though I'm still trying to see how mechanical advantage does not alter the amount of power.

The energy available is the gravitational potential energy of your mass. The maximum power output of your device is thus the rate of change of that energy, or $mgv$, where $v$ is the speed of the weight as it falls. So if you want 10kW out of a 500kg mass it must fall at 2m/s (in fact, you will need it to fall faster because losses will reduce the output power far below the input). Nothing you do can improve on this power output because there is no more energy to be had. If you have a 5m drop then you can have this power for 2.5s before you need to recharge. Note also that it will take a minimum of 0.2s to spin up to 10kW from nothing.

These are basic physics limitations. Once you get into the engineering and have to deal with losses the numbers will be far worse.

You asked about mechanical advantage. Essentially the generator acts as a brake on the falling mass, with a force that depends on speed. Changing the gearing between the generator and the mass changes the advantage of the braking force on the mass, so changes the speed at which balance between weight and braking occurs. So it will change the power output of the generator, but it will also change the power input to the generator by slowing or speeding the mass.

 

[sophiecentaur]

If we attach 500kg of weight to the pulley weight to each of the motor and we attach a pulley with multiplying gears such that each rotation of the pulley in 2 seconds results to 100 rotation of the motor, which would translate to 30 rotation = 3,000 rpm. Now knowing the rpm, power output of the motor, peak torque and the height is 5 meters. Wouldn't it produce it produce the rated power as it's coming down?

You have ~ 24.5kJ energy in that (height*mass*g). That much can feed 1kW load for 24.5 seconds.

The details of power, force and torque are much less relevant than the basic Energy equation. That's where the money lies. The mechanics are easy to adjust.

 

[Ibix]

The details of power, force and torque are much less relevant than the basic Energy equation. That's where the money lies. The mechanics are easy to adjust.

Just for comparison purposes, this 5m tall half ton device can provide 10kW for 2.45s, which is a storage capacity of about 0.007kWh. A Tesla model S battery typically stores on the order of 100kWh and the entire vehicle weighs only four-to-five times what this device does. The energy density here is much, much, lower than chemical batteries, which is why this is only done in old (or pretend-old) devices like mechanical clocks or on huge scale like pumped storage. In either case it's the only option.

None of this is a reason not to do this, as long as your "why" takes account of it.

 

[kingamada]

I mean i knew the potential energy is 24.5kJ. I'm just finding it difficult to see how if the motor is rotating at the rated rpm and torque and it wouldn't produce the rated kw of more than 10kw.
Though this is the vision, still refining the idea for the best approach using animal weight to generate the electricity.

 

[Rive]

Overthinking.
You only need a big treadwheel.

But honestly ... this just won't go anywhere.

 

[russ_watters]

Just for comparison purposes, this 5m tall half ton device can provide 10kW for 2.45s, which is a storage capacity of about 0.007kWh. A Tesla model S battery typically stores on the order of 100kWh and the entire vehicle weighs only four-to-five times what this device does.

Or going the other way, it's about the capacity of a cell phone battery.

 

[russ_watters]

Maybe I didn't ask the right questions, as you know sometimes it's about the quality of the question. Though I'm still trying to see how mechanical advantage does not alter the amount of power. ?

I mean i knew the potential energy is 24.5kJ. I'm just finding it difficult to see how if the motor is rotating at the rated rpm and torque and it wouldn't produce the rated kw of more than 10kw.

It will. You're just not connecting all the dots here. Energy and power are related by time. Do the math and figure out how long it runs, if you don't believe what people are telling you. Again: you can gear it to get the power you want, and in doing so you'll reduce the amount of time it will run.

I stand by the vibe I got earlier. You're using your instincts/gut because they are telling you what you want to hear, and avoiding the exceptionally easy math because you don't like the answer it gives. That's the wrong road you are going down.

 

[Ibix]

I'm just finding it difficult to see how if the motor is rotating at the rated rpm and torque and it wouldn't produce the rated kw of more than 10kw.

It would, minus losses in the motor and assuming a motor running as a generator is anything like a good generator. But there isn't time to spin up to 10kW output before you're out of energy. In free fall it would take 2s to reach that power output, in which time the weight would fall 20m. You can't accelerate it faster than that, so you can't achieve that power output.

If you gear the generator so it spins fast even when the weight is moving slowly the braking from the generator will have the mechanical advantage on its side and the weight will accelerate very slowly and you'll never get much power out. There's no way of escaping the rather slow initial energy release of this device.