Small scale mechanical energy battery

[charleymcdowell]

Hi there. I am a novice but I was wondering what you all think is the most applicable mechanical energy storage device for small scale off grid applications is.
I just read the thread on pump storage and it seems like a good idea but I thought I would keep it broad.
Thanks!

 

[berkeman]

Welcome to the PF.

Here are a few previous threads related to your question...

https://www.physicsforums.com/threa...storage-seeking-comments.166679/#post-1308922

https://www.physicsforums.com/threa...stored-in-compressed-air.922350/#post-5819041

https://www.physicsforums.com/threads/controlled-energy-release-mechanism.899546/#post-5662805

 

[CWatters]

Might help if you told us what the source of the mechanical energy would be? People pedalling? Wind turbine?

There is a plan to trial gravitational storage in the UK using weights in old mine shafts.

 

[russ_watters]

Might help if you told us what the source of the mechanical energy would be? People pedalling? Wind turbine?

And how much you want to store.

 

[charleymcdowell]

And how much you want to store.

My main question is what would be the best way to find out how much energy a 10000lb weight elevated at 20’ would produced if attached to a power generator.
This is vague but I am just wondering what it would be.
All that work and it doesn’t even power a refrigerator?
Or oh

 

[charleymcdowell]

It does power a whole house

 

[russ_watters]

My main question is what would be the best way to find out how much energy a 10000lb weight elevated at 20’ would produced if attached to a power generator.

Well this is an easy one; Do you know the equation for gravitational potential energy? And a decent generator is about 90% efficient. And my refrigerator uses 140 W when on, and runs half the time. So can you calculate from that how long your system could run my refrigerator?

 

[charleymcdowell]

Yes!
My first thought was well what is the calculation for potential energy and then convert it to joules and then see how many joules refrigerator uses.
But alas,
I am an idiot and cannot find this on the internet.
Sound like the term would be
Gravitational potential energy equation

 

[CWatters]

Pe=mgh

Pe in joules
m in kg
h in meters

 

[charleymcdowell]

So I calculated 414,000 J of energy for potential energy of a 10,000 pound weight at 30 feet high, so would I then convert this to watts?

 

[berkeman]

So I calculated 414,000 J of energy for potential energy of a 10,000 pound weight at 30 feet high, so would I then convert this to watts?

1 Watt = 1 Joule/second. So you can have about half a Megawatt for a second, or 100 Watts for how long? (ignoring generator efficiency)

 

[charleymcdowell]

66hours?

 

[berkeman]

66hours?

Something like that, yeah. How long could you run Russ' refrigerator?

 

[charleymcdowell]

I think the average refrigerator use about 200 W /second? Which would mean if it ran half of the day you were just multiply 200 W Times 36,000 seconds?

 

[charleymcdowell]

Or no just Cut the previous 66 our number and half and you have how long you could run something at 200 W for 33 hours

 

[charleymcdowell]

Can anybody confirm this for real? Because if so it seems like a small scale potential energy using gravitational force and a 10,000 pound weight rigged up in the air at 30 feet would run a refrigerator basically forever if giving the components are maintained.
Because comparison would have to be made with offgrid cost of providing power i.e. buying a new set of batteries every five years or so.
This seems like it might be a viable option to me but maybe my math is totally wrong and it probably is

All you have to do is rig up another system of the same value and then that could run basically your household appliances and all your lights

I would estimate the system taking up about 40‘ x 40‘ of flat ground
But I don’t want to get too ahead of myself because I don’t know if I got these numbers correct

 

[berkeman]

I think the average refrigerator use about 200 W /second? Which would mean if it ran half of the day you were just multiply 200 W Times 36,000 seconds?

No, remember 1 Watt = 1 Joule/second. Trying to come up with a unit "Watts/second" makes no sense.

Or no just Cut the previous 66 our number and half and you have how long you could run something at 200 W for 33 hours

Yes, although I got a number more like 69 hours at 100 Watts. Can you show your calculation in detail?

 

[charleymcdowell]

I’m sure your calculation is more correct since I just google searched calculators and used online tools to get it .

 

[charleymcdowell]

So if Our refrigerator uses 200 W then technically speaking if not taking into account efficiencies our system would run it for 33 hours?

 

[berkeman]

Can anybody confirm this for real? Because if so it seems like a small scale potential energy using gravitational force and a 10,000 pound weight rigged up in the air at 30 feet would run a refrigerator basically forever if giving the components are maintained.

How do you go from about 30-60 hours to "forever"?

 

[charleymcdowell]

I just meant that if you build the system so that could be repeated using some sort of a renewable source of Energy lifting the weight up and down then it being such a simple system maintenance on it would be very little and the parts would be easy to come by there for making it a system that could run forever with very little energy put into it once it’s built
Forever was in adequate I would expect you could get at least 30 to 40 years out of the system without any major issues

 

[charleymcdowell]

But like I said I was getting ahead of myself I just thought I wanted to make sure the numbers were even feasible before entertaining the idea

 

[berkeman]

making it a system that could run forever with very little energy put into it once it’s built

The energy you put in (store in it) is the energy you get out (minus inefficiencies). So if you have a 500W solar panel with a good Maximum Power Point converter/controller to run your lifting motor, you can store about 500W multiplied by however many seconds = however many Joules per day, and then extract that energy out of the falling weight at night, for example.

 

[CWatters]

So if Our refrigerator uses 200 W then technically speaking if not taking into account efficiencies our system would run it for 33 hours?

I made it about 33 mins not hours...

4536kg * 9.81m/s/s * 9.14m = 407,000J

407,000/200 = 2036 Seconds

2036/60 = 34mins

 

[charleymcdowell]

OK so you’re saying work backwards
I was pretty confident that solar panels could provide enough energy I just wasn’t sure how large the wait would need to be if the wait would need to be 50,000 pounds that’s pretty much a no go for somebody trying to put together a small scale system but a 10,000 pound weight is a little more doable there’s some large trucks that are that heavy and most wenches that are worth their salt can pull 10,000 pounds so that was kind of why I started with that number

 

[charleymcdowell]

I made it about 33 mins not hours...

4536kg * 9.81m/s/s * 9.14m = 407,000J

407,000/200 = 2036 Seconds

2036/60 = 34mins

OK I figured as much would’ve been a little too easy to solve that problem like that would probably be needing something Iike 300,000 pound weight to run a refrigerator

Oh well I’m sure we humans will figure something out you guys I mean not me I’m just a Carpenter I really appreciate everybody’s responses though

 

[berkeman]

I made it about 33 mins not hours...

4536kg * 9.81m/s/s * 9.14m = 407,000J

407,000/200 = 2036 Seconds

2036/60 = 34mins

Oops, thanks for fixing that!

 

[Dr.D]

How do you go from about 30-60 hours to "forever"?

This just requires an infinitely deep hole in the earth; where's the problem?

 

[phyzguy]

Look at it this way. Your 414,000 Joules is about 0.1 kWh. Electricity costs about $0.10/KWh, so the energy stored in your device is worth about 1 cent. How much do you think it will cost you to build your contraption, counting digging the mine shaft, buying the weight and the motor/generator? Even scrap iron costs about $0.10/pound, so the weight alone will cost at least $1000. Would you spend $1000 plus the cost of the motor-generator to save a penny a day on electricity cost? Or we can look at it another way. A car battery stores about 50 Amp-hours of current at 12V, so 600 Wh = 2 MJ, which is about 5 times what your device will store. And a car battery costs less than $100. Hmmm, decisions, decisions...

 

[Tom.G]

some large trucks that are that heavy and most wenches[/size] that are worth their salt can pull 10,000 pounds

wench (plural wenches)
(archaic) A young woman, especially a servant.
(from: https://en.wiktionary.org/wiki/wench)

You best treat her well!