# I Another 'Gravity Battery' Question

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1. Aug 5, 2015

### CBWB

Hi All, this question has been thrown around before but never fully answered so I'll put it very simply and cut out the ponderings:

A 'gravity battery' could be created by using an energy source to slowly lift a large weight (using gearing or pulleys), then discharged by lowering the weight to run a generator when needed. Kind of like a giant grandfather clock.

The effect would be the same as using a Tesla Power Wall but with cheap materials, no heavy metals, unlimited lifetime, and the ability to be infinitely reproduced inexpensively- making it economically viable.

But what about physical viability: The gravity battery would not have to power the entire house at current U.S. electrical consumption rates (a previous debate in this forum - answer is it can't), just as the Power Wall is not designed to do this. The Power Wall is designed to assist by storing 10kWh of electricity, diffusing the power grid and helping to solve the biggest current problem with renewables - the fluctuation in production.

I am not a physicist, so:

Assuming a 5m drop length (about right for a house) what mass would a weight have to be to store 10kWh of electricity?

More importantly, when you factor in likely energy losses and efficiency, could a gravity battery compete with a Power Wall as an economically viable power storage device??

Please approach this with an open mind. If we could somehow work our way through the practicalities just think of what this approach could mean for us, for developing nations, and for our energy future. I'm looking forward to some great minds and optimistic solutions here. Thanks.

2. Aug 5, 2015

### Staff: Mentor

Let's see, 10 kWh is 36 million joules.
That means that the gravitational potential energy is:
U = mgh
36,000,000 = m(9.81)(5)
m = 734,000 kg.

That's 734 tons.

My mind is open, but my wallet is closed.

3. Aug 5, 2015

### Nidum

This is done on a large scale with pumped water storage . Off peak and surplus power generation is used to pump water to a high level reservoir . In times of greater demand water is discharged through low level turbo generators .

Using raised weights in a domestic system is possible but probably impractical for anything except very small scale power storage .

Before coming up with specific ideas on power storage it is helpful to have clear ideas as to why domestic power storage might be needed .

Most developed countries now have well developed power generation systems and a national grid . Apart from rare local breakdowns the systems are extremely reliable . So a ordinary domestic property does not need to store power under normal circumstances .

There are places where small scale local power generation is the only option available . This is where storage of power for domestic use becomes important .

For very small scale systems like one house or farm with a wind generator batteries usually seem to be best option

Where there is a group of domestic dwellings and possible small industry dependent on local power generation there is more scope to be imaginative and realistically better opportunity to make decent investments in equipment .

Most enthusiasts for DIY power seem to get locked in mentally to one technology to exclusion of all others . Real solutions to small scale generation and storage of power use several different technologies at the same time .

4. Aug 5, 2015

### sophiecentaur

You can DIY:
1. What's 10kWh in Joules? (=Energy E); a lot!!
2. GPE is mgh
3. h = 5m
4. g = 10 (ish)
5. Gives and equation E = 50m (Energy in =energy out)
then re-arrange that to give you your required Mass; really quite a lot!!
It just shows you how Heat (what accounts for nearly all our Energy consumption) is the equivalent of a lot of Mechanical Energy.

I see my answer came too late. Time, tide and PF posts wait for no man.

5. Aug 5, 2015

### Nidum

There are multiple variants on 'mechanical' energy storage schemes . One that seems to work but is very expensive to actually do is the 'big flywheel in the basement' concept .

Many large and small scale energy storage schemes have been considered in the past but there have been very few ideas developed into practical systems .

6. Aug 5, 2015

### sophiecentaur

In many applications, the best way to store Energy - if that energy is required in the form of heat, is by thermal storage (big insulated tank of hot water).
If the requirements or the OP are for typical domestic applications then probably the requirement for driving motors would be a fraction of the 10kWh. A 70 tonne falling mass might be a better engineering proposition (1kWh) and could deal with that and the rest could be by thermal storage. These days, 'Hybrid' arrangements seem to crop up everywhere.
If the energy requirement is for Air Con, then a tank of cold water could store the required 'coolth'. No limit to the ingenuity of a dedicated environmentalist engineer.

7. Aug 5, 2015

### CBWB

Nidum - the concept is to create a 'gravity battery' competitor to the Tesla power wall: same reasons, same motivations. The Tesla concept intrigues me because it shows the way to a more robust power grid with diffuse generation and storage, but the material costs are high.

I am not a DIY enthusiast, I'm trying to look at the larger picture for alternative solutions to this end of the renewable energy problem. Yes I have considered water and I know this is done in some places already, but there are losses to evaporation and I wanted to explore a low-tech solid weight solution with minimal losses instead. Don't worry, I won't be rushing out and building one (or starting a company and asking you to invest - relax guys) so I'm not locked in mentally to it.

Flywheel may well work in some applications and I would be interested in any comments regarding efficiency and viability as a Tesla alternative. Are we anywhere near that as a reality?

Calculation of the potential energy for the gravity battery concept seems like the straightforward kind of physics problem I would have failed in high school, thanks for the help Drakkith. A quick google shows average weight of concrete is 2,400kg/m3, meaning the volume of a concrete weight for 10kWh would have to be 306m3. A big chunk indeed. Any materials scientists out there with better ideas for a higher-density low-cost alternative to normal concrete?

When I'm not asking dumb physics questions I'm actually the captain of a ship and I have void spaces on board that are around 300m3, so I can picture the substantial size of that. Not easy to hide on the side of a house, even if you build it into a wall or something. So where else could you put it...

How about downwards? In areas away from groundwater you could dig a hole under the house (in a new construction at least) that is perhaps 10m deep instead of five, get lucky and find a cheap material with a higher density of perhaps 3,000kg/m3 (?) and thereby reduce the mass of your weight to 122m3. 8m x 8m x 2m horizontal slab would do the trick. Now we're getting somewhere (a little) more realistic...

Perhaps we decrease our energy consumption or pair it with other aspects of a smart grid and reduce our need from 10kWh to 7? Now you only need a 6m x 6m x 2.5 slab. I'm not tied to any of these figures, just searching out new ideas because it interests me. Throw 'em out there.

Better thoughts along these lines, anyone?

8. Aug 5, 2015

### CBWB

Sophicentaur, I really like your ideas!

Personally I live in a 150 year old house in the U.S. South that solved heating and cooling issues a different way (solid walls, shade trees and french doors perpendicular to the prevailing breeze) but I realize we aren't that smart anymore.

In the future I think a/c is out, sorry modern man. Heating has always been a necessity of course, in many areas this can (and has always) been solved by closed-loop cycling through enhanced rapid production of carbon-rich combustibles. Also known as coppicing. Every town should have some.

Throw away clothes dryers as a modern American absurdity. Change all light sources to LED. Insulate the heck out of your roof and add solar panels & water heaters as an extra shade layer. Now what are we down to for household consumption, and could a gravity battery help cope with that level of fluctuation?

9. Aug 5, 2015

### Nidum

Flywheel storage is quite a mature technology . Flywheels can store large amounts of energy in relatively small spaces . Flywheels have been successfully used to drive commuter trains and buses .

Generally mechanical energy storage systems which use kinetic energy will be smaller and use less weight of materials than systems that use potential energy .

10. Aug 5, 2015

### CBWB

Very good point, but aren't kinetic energy systems much more liable to losses? Perhaps not an issue for short term local storage...

11. Aug 5, 2015

### CBWB

Perhaps too simple a way to put it...

As I understand flywheel technology you are either limited by friction losses through a mechanical bearing or hampered by production costs for a magnetic/superconducting bearing. Then there may be energy loss through counteraction of precession. With these production expenses to overcome, can a flywheel beat the cost of a lithium-ion battery for the same energy storage potential? Perhaps the answer is as simple as 'not yet'.

I am guessing the energy storage lifespan per unit of energy (time before stored energy dissipates for one reason or another) in flywheel and battery technology are reasonably similar. Perhaps there is another technology - no more efficient but with characteristics advantageous to the relatively short timescale required to even out daily fluctuation in a renewable energy system? In this line of thinking my gravity battery is the exact wrong direction - you could keep the weight hanging up for years without losing potential energy.

...Is there anything else with a better trade-off of cost-effective energy storage capacity vs. storage lifespan?

Maybe I'm getting away from my own topic here, just trying to follow ideas to their logical ends as we go along.

12. Aug 5, 2015

### Staff: Mentor

Yeah, and the cost to build one compared to other energy storage systems is enormous. I mean how much would it cost to build a structure even 15 feet high that's capable of holding several hundred tons? It's got to be way more than the few thousand you'd spend on batteries.

13. Aug 5, 2015

### CBWB

Frankly a good point, Drakkith. Does that put the final nail in the coffin for the gravity battery idea forever?

Any possible alternatives from anyone before this thread ends??

Frankly if there's one thing I've learned it's the enormous power even a hashed-together product like the power wall can store, I guess there's no beating manipulation at the atomic level. Maybe I should invest in Tesla now?

14. Aug 5, 2015

### Staff: Mentor

Unless you're dealing with hydroelectric power, probably.

15. Aug 5, 2015

### sophiecentaur

Interestingly, I was on holiday near Granada, a couple of years ago and I noticed there were a number of hydro schemes in the Sierra Nevada. I talked to our (well informed) landlord about them and asked why there were not many more (free electricity and all that.) He said that several sites had been surveyed but the cost of damming those valleys was estimated to be too great. I really hadn't thought that there could be a problem if you've got mountains and loads of winter snow but the economics are not always favourable. The same situation must apply to pump storage schemes. You need high valleys with the right shaped natural constriction, I guess, to keep the construction cost reasonable. Same must apply to Pump Storage schemes. You're dammed if you do and damned if you don't, I suppose.

16. Aug 5, 2015

### CWatters

Way back in 2008 this design for a gravity powered lamp won a design prize. It works like an egg timer, you turn it over when the electricity runs out.

http://www.sciencedaily.com/releases/2008/02/080220221822.htm

The organisers didn't spot the design flaw..

They say it produces 800L for four hours. Lets say the LED is 100% efficient at 683 Lumens/W so that 800L for fours hours needs 1.17W for 4 hours. So energy required = 1.17 x 4 x 60 x 60 = 16848 Joules

Energy stored energy in the weight is..

PE = mgh

m = mass in Kg
h = 1.2 meters (four foot)
g = 9.8

so m= 16848/(9.8 x 1.2) = 1432 Kg

1.4 Tons

17. Aug 5, 2015

### sophiecentaur

HAHA
Green doesn't always mean common sense.

18. Aug 5, 2015

### nasu

Later on they say that it is just a design solution and it will work only wit future LEDs, requiring much less energy.
And even mention that with current technology will require tons of mass.
Maybe that part was added later. :)
But even with the best LED's (100% efficient) you still need the energy carried by the light and this requires still tons, according to Cwatters above.

It is more like science fiction: admitting some "slight" change in the natural laws, you create new gadgets.
I wonder what won the first prize. An interesting way to paint a faster than light spaceship maybe.:)

19. Aug 5, 2015

### CWatters

The gravity powered lamp concept reappeared in 2012..

http://gravitylight.org/tech-specs/ [Broken]

This time with a 12kg weight producing 0.1W aimed at third world countries with no grid.

Last edited by a moderator: May 7, 2017
20. Aug 5, 2015

### sophiecentaur

Problem is that it isn't 'gravity powered', is it? It's powered by the guy who provides the movement with his muscles. He could be pedalling or turning a handle and achieving the same power output.

Last edited by a moderator: May 7, 2017