# Can you store off grid power with gravity?

Really busy recently taking on some extra work to help finance the desperately needed replacement battery bank.  I was thinking about other ways to store our power and had an idea on how we might be able to setup a mechanical system to store energy by lifting a large weight.  Might this complement the batteries substantially?  So, I decided to run the numbers on the physics involved to see.

## Stored Energy in Lifted Block

This system would convert electrical energy into stored potential energy by lifting a large weight  off the ground.  Whenever we had excess power from windy stretches the extra power could be diverted to this system.  The implementation would involve a small/medium size DC electric motor that would drive a reduction gear box, that in turn would rotate a spool of cable.  The cable would be attached to  a pulley on a sturdy large steel frame that could support a large weight or block  fastened to it.

When excess power is available, or we just want to “charge up” all our energy  systems,  we could apply the DC voltage to run the motor.  With the gearbox reduction and increase in torque, the cable would tighten and lift our block slowly.  Power is removed by a mechanical reed switch or something when the block reaches the top of our system.

To retrieve this  extra stored power, we simply unlock the hanging block and it slowly descends spinning the gearbox and DC motor which is now producing a current that we can charge deep cycle batteries with, or run the inverter for AC power.

## Test the concept

At this point, I’ll ignore many of the real world details like speed/voltage regulation when we extract power.  I just want to see if this could be useful to compliment my battery purchase.   I won’t consider heat loss and other mechanical inefficiencies yet, I just want to know if there is enough energy stored to start with.

A block weighing X kilograms lifted to a high of h meters will represent the stored energy in joules, which is also watt-seconds. 9.8 is the acceleration constant of gravity on earth.factor is the gravitational constant for earth.

Energy in joules = (weight kg)(9.8)(h meters)

Joules/3600 = watt-hours

Ok, lets test this out for a big system that would store a lot of power.   For the math, we’ll select a big concrete block that weighs 10,000 kg, or roughly 11 Tons.  We’ll lift it really high, say 10M, or about 33 feet.  The lossless raw energy stored in this system would be:

10,000kg * 9.8 * 10m / 3600 s = 272 watt-hours, or 22 amp-hours at 12V

Ker Flop!!  I guess this giant storage Idea would not yield any results worth pursuing any further!  However, when you think about it this way, it’s really amazing how much power can be packed in a deep cycle lead acid battery.

For example this result is less than the 30AH  we use from a marine deep cycle battery to run a little trolling motor!  Can you imagine?  Hooking up something the size of a car battery, and lifting 11 tons 33 feet in the air and only using half the charge??

## wait, it can be done with water

Storing your power in the form of potential energy is nothing new, this will work with water if you property has the right layout and terrain.  Lets take this system, remove the gearbox, the cable, and the weight.  Now we put an impeller on the motor shaft, or drive a pump.  Now lets say we have a storage pond that is 33 feet above the pump.  Remember water is heavy, about 8 pounds per gallon.

Lets take a small pond, say its 50 x 50 feet and 5 feet deep.  ( I know, no one has a perfect box pond, but this helps the math.)

1 cubic foot = 7.48 gallons,  and 1 gallon water = 8.35 pounds

pond volume = 50 x 50x 5 = 12,500 cu ft.

pond water weight = 2304* 8.32 = 104,000 pounds, or 47,270 kg

47,270kg * 9.8 * 10m / 3600s = 1287 watt-hours, or 107 amp-hours at 12V

Wow,  is amazing what you find running the numbers!  You really have to store a good deal of water for this to work.  This is how hydro projects are created.  Still probably not a practical off grid power storage technique, but if you have year round running water on your property, always consider micro-hydro power.

Did you know that if your pond was 50 x 50 feet and 50 feet deep that you could store 28,243 AH at 12V on this system?

What other ideas do you have for storing power?  Do you know of practical systems that are more efficient that current battery technology?   Please tell us about your experience in the comments below!!

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40 Responses to Can you store off grid power with gravity?
1. Arild Jensen
July 11, 2010 | 9:13 am

July 2010 National Geographics has an article on the smart grid and in the article mentions stored power such as water resevoirs and also mentions a pilot project in Alabama that stores energy using compressed air held in a deep cavern in a rock layer. Water seems like a more likely system for micro power generation

2. Jonathan Cole
July 11, 2010 | 10:14 am

Small scale pumped hydro is totally impractical. The pumping losses, the parasitic losses of friction in the small pipes and the sheer volume of water that must be pumped and stored would cost a fortune to set up. However if you have a watershed that feeds your upper reservoir, it becomes more practical since you don’t have to supply all of the elevated water that you need by pumping.

If you want to know how to live well and off the grid, with solar, even in a cloudy place, you should visit my web site where there is factual information about how to accomplish it. http://www.lightontheearth.org/

I have been living off-grid for the better part of 30 years and am trying to provide in the depth, factual information that people need to accomplish this.

• marshall
July 12, 2010 | 10:08 am

Thats kind of my conclusion, gravity plus real world losses that I ignored in the calculations make these ideas impractical for most Individuals.

However, full size hydro projects work well because of scale and the large group of people (customers) behind the utilities.

3. elnav
July 11, 2010 | 12:25 pm

Jonathan, Can you suggest how solar can be made cost effective if the location has 16 hours of darkness in winter and only about 4.5 hours of effective sunlight in summer?
Right now out hydro-electric power amounts to 8 cents per kilowatt. The best solar panels only run about 23% efficiency when tested under ideal conditions. When you factor in low sun angle and cloud cover running 30% on average I can’t get the cost /benefit ratio to work out as being very good.

4. Jonathan Cole
July 11, 2010 | 12:39 pm

ElNav, solar requires light. If you live where there is little light then solar will not work for you. However, I live in a place that is quite cloudy and often of the dark gray type. Yet I have all the amenities and rarely have to use my backup generator. However if you live far enough north (or south), then winter days can be very short and very dark. Solar is not the best clean energy technology in that event. However, those short winter days are often cold winter days which further complicates hydropower. Frozen water doesn’t work in hydropower systems. People who want to live with clean energy systems have to live where the energy is available and practical to harness. Otherwise it is a never-ending uphill battle against nature. The trick is to work with nature. If you are in a place where nature does not cooperate, go someplace where it does! Or else be prepared to suffer!

5. elnav
July 11, 2010 | 12:51 pm

Sage advice – except politics intervene. I cannot move far enough south to get into a solar zone. There is a political border that prevents me from moving there.

6. elnav
July 11, 2010 | 12:55 pm

Jonathan wrote: “Frozen water doesn’t work in hydropower systems”
Well gee nobody told Mother Nature that. The ice may be 12″ thick but the water keeps flowing under that. Even the creek behind the house runs all year. You can see the flowing water when snow doesn’t cover the clear ice.

7. elnav
July 11, 2010 | 1:10 pm

Storage of energy is partially based on having excess generating capacity which can be diverted to doing pumping work. When the solar, wind, or water power has fully charged a battery bank what do you do with the surplus? Most controllers either divert the excess power to a dump load or disconnect the solar panels or simply let the excesss water run by in a diversion channel. The storage concept relies on the notion thei suplus energy can drive pumps no matter if they are somewhat inefficient and store water or weight as potential energy instead of kinetic energy.
Properties in mountaineous regions often have excess wind so a wind mill can be put to use to fill a resevoir. And these locations often have elevation changes that make it easy to employ gravity driven falling water to drive a turbine. This doesn’t work for flat lands.

8. Jonathan Cole
July 11, 2010 | 1:12 pm

Elnav,
of course water that is bound up as ice is not available to power your generator. Of even more importance are frozen pipes that prevent water from being pumped. Of course, You could bury the pipes deep below the frost line. In cold climates it might be worth investigating ground based heat pumps and using the temperature differential between the deep earth and the surface to drive a turbine operating on a low evaporation temperature gas.

9. elnav
July 11, 2010 | 1:40 pm

Marshall has already explained th4e physics as to why his big water tank doen’t freeze in winter nd moving water seldom freezes. Ground based heat pumps are so expensive it would be cheaper to just run a generator. When you factor in the cost of dynamiting hundreds of trench feet through rock or drilling a borehole at \$20/ foot ground based heat pumps isn’t always as cheap as the salesman would have you believe. In such cases a Stirling engine driven by wood heat is a beeter bet. A swiss company already has such a product.

10. marshall
July 12, 2010 | 10:13 am

Received an email comment from Steve M. about an upcoming Energy Storage Conference. Our readers may be interested in the link.

http://www.fullpowerinc.com/AES2010.html

11. Bob @ JuicyMaters.com
October 3, 2010 | 5:05 pm

I don’t know enough about the details, but it seems the best way to “store” energy is to do away with the need for it, and a friend has done away with ALL energy needs to supply water by using a ram pump.

True, you must have a LOT of waterflow available sincve a ram pump at a good efficiency level uses 8 gallons of water to produce 1 gallon of water into his household, but it makes his potable water totally free after the 110 dollars he spent in the plumbing section of HD for ram pump parts…and a good scavenger could do it for far less.

Bob

12. Arild Jensen
October 3, 2010 | 5:58 pm

Each situation and location differs. Ram pums are not of much use in the flat desert of the South west where you rarely find running water. But there solar power is fine. Energy is in effect stored in wood and other biomass that can be released later on as fire and heat. To say the solution is to do away completely with using energy as Bob @ JuicyMaters.com suggest is not really practical. This would mean all activity stops at sundown. Marshall’s original idea of using mass as a form of energy storage has been tested in spinning flywheels or gyroscopes with limited amounts of sucess. researchers continue to seek ways of recovering energy from various places. Energy that was originally put ther as sunlight.

13. Chris_TX
March 25, 2011 | 11:40 am

Looks like your math might be off a bit? A 50x50x5 ft pond holds 12,500 cubic feet which would be about 93,500 gallons. At 3.77 kg per gallon, that would weigh 352,858 kg. Using your formula, 352,858kg * 9.8 * 10m / 3600s = 9606 watt-hours instead of 1287 watt-hours.

14. Chris
November 30, 2011 | 11:42 am

Just kinda thinking in the abstract, but the gravity storage probably loses less energy from heat, etc. than many other forms of storage. Li ion is pretty efficient but obviously on a large scale is prohibitively expensive. Hydro storage has pretty poor efficiency as well but fits the large scale model well especially considering mother nature donates a lot of potential energy in the form of rain and snowmelt. I really like solid weight idea and while you did calculations with a rather small weight, imagine if you stored all the potential energy of dirt/rock removed from an industrial pit-mine. You could pretty much store all the energy for a small city. The actual construction of a project like that would suffer from some insane engineering problems but I like abstract thinking sometimes. So storing say wind energy which is notoriously unpredictable as gravity energy(don’t really know what to call it) would be an environmentally friendly idea. Wind energy makes sense because often the grid can’t handle the excesses of wind energy esp. during night when demand is smaller.

• Bill Egge
May 22, 2013 | 8:35 am

Hey Chris, imagine this: There is no reason this has to be above ground, shorten the distance and increase the mass – if you dig a 6x6x6 hole in the ground then put some kind of container in there that can move up and down by 1 foot then fill the container with the dirt you dug up. If the outer part was water proof you could “charge” the system by pumping in water to lift the container then discharge by draining the water.

15. derek hunter
December 16, 2011 | 5:36 am

Like most good inventions the obvious sometimes escapes you.
I have invented a prototype machine that if expanded to commercial dimensions, is capable of storing over 1Mwhr using only gravity (but not as we know it Jim)To understand this concept you really do have to think outside the box and I am not likely to tell you the answer unless you have the willingness or connections needed to pursue this project to its conclusion.

16. Wilfred Barrett
December 20, 2011 | 3:38 pm

There is problem with the equation. It wouldn’t take an hour for 11 ton block to fall 10m, it would only take a second. So the real power stored in the device would be 980,000 watts. Which if you use 500 watts an hour would last you 1,960 hours or 81 days, which is a long time without wind or sun.

For comparison a deep cycle solar battery has about 6000 watts in it, so you would need 163 batteries to equal the 11ton block, 10m tower device, which if you compare size is much smaller. The advantage of the device would be a constant discharge as opposed to a battery which has decreasing voltage, you could discharge the device completely with harm to it’s life cycle and it would have nearly infinite life cycles depending on how you build the tower and quality of motor.

I use a mechanical spring to power a bike light, you would be surprised at the power density of mechanical force. If you wanted to replace a 8 battery bank you would only need a 500kg block and a 10m tower, which if you mounted a wind turbine and solar panels would be a pretty nice set up. Cheers!

• Wilfred Barrett
December 22, 2011 | 7:52 pm

Also another common misconception, watt-hours are the amount of watts a device uses in an hour, not watts per second. A watt equals a joule/second, so to get the watts per second of a watt-hour you would divide by 3600. To get what a device uses in joules you have to convert the watt-hour to seconds so your second equation should be; “joules/second = watt-hours/3600″

17. Eric
December 31, 2011 | 2:56 pm

My take on your gravity idea is that it wouldn’t matter how much the item weighed as long as it was sufficient the spin the motor/generator. The true problem would be getting it to travel at the correct speed to spin the motor at the optimal speed for as long as possible. The only advantage to the added weight would be that you could add many motors and produce an extreme amount of energy, but unless you can slow the blocks travel speed you would need a lot of battery’s to store it all in the second or two it spun the motors (before it mashed into the ground). If you can make a 10 pound block take 1 hour to reach the ground and have a motor that’s peak performance speed was very slow (considering you only have 33 feet of rope) then you could achieve optimal charging for one hour at what ever the output rating is of the motor. One of the small ways I add power on not so good days is a motor with several spoons attached under my rain gutters. Its not much but it helps when its raining.

18. William Egge
January 16, 2012 | 8:43 am

How about a very very heavy spinning thing that you could charge up by spinning it faster?

The details are just

* how heavy it is (what about a circular container made of Plexiglas that uses water for its mass instead of metal – water is much easier and cheaper to obtain)

* the friction (maybe a small motor run off solar to compensate for the friction)

* and how fast it needs to spin (the heavier the slower it needs to spin).

• William Egge
January 16, 2012 | 4:16 pm
• William Egge
January 16, 2012 | 4:55 pm
• Wilfred
February 25, 2013 | 6:52 am

The big drawback with flywheel storage is that there is a fairly quick loss of energy due to friction, we’ll call it self-discharge for the sake of argument. ” Flywheel energy storage systems using mechanical bearings can lose 20% to 50% of their energy in 2 hours.” While chem-elec batteries its about 2-30% per month. http://en.wikipedia.org/wiki/Self-discharge That being said short of structural collapse I can’t see where a gravity battery would self-discharge at all in a given time-frame. So it may be the best at ultra long term storage. That being said flywheels can be used for short term storage and energy smoothing, but capacitor are smaller and less prone to mechanical failure for energy smoothing applications.

19. Russell Lee
February 24, 2012 | 12:17 pm

Hello,
Here is a machine design that uses gravity to turn a generator: The Pinwheel Generator http://www.squidoo.com/pinwheelgenerator
Thank you,
Russ

20. William Egge
February 26, 2012 | 9:02 am

Curious question, suppose you used a weather balloon for lift. How much energy would it take to deflate the balloon and store the hydrogen for re-use? Could something be made to constantly go up and down?

21. Mariner
March 20, 2012 | 2:54 pm

You created a simple system to store energy. The 11 ton block would need a reduction gearset and I would also raise it using a gearset and shaft. Additionally, keeping this a constant rate should be fairly simple through braking the gear set at the right rate of discharge. William added the FES, which can be a very highly efficient device, especially if you have near frictionless bearings (magnetic). Water, as everyone pointed out, has too much friction loss, especially in smaller diameter pipe. Finally, you could create the energy in the first place through a stirling engine, rather than solar panels. Mechanical stuff is way underutilized. The world is ruled by EE’s when it should be ruled by MEs.

22. enstorman
June 17, 2012 | 9:06 am

How about a solar concentrator on a tank of water (or molten salt as some big utilities are using) in which a stirling engine is embedded. I don’t understand the stirling engine enough to know if you could get it to produce any useful power… But I think the fact that it is an ‘external combustion’ engine makes it sound plausible.

OR

How about an air compressor filling a tank that runs a compressed air generator at night. I think I can do the calculations on this once I find the efficiency of these compressed air engines. But I would not want the noise of a compressor all the time.

23. Steve Anson
October 19, 2012 | 2:36 am

Hi Marshall

What about using hydraulics to store surplus energy?

Unused solar or wind energy could be stored in a device that compresses air. The air is released to power a generator when required.

I’m exploring renewable energy options for our new home.

Steve

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January 30, 2013 | 10:38 pm

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how can I contact you?

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27. Bill Egge
May 22, 2013 | 8:16 am

Maybe someone here can do the math:

Suppose you had a heavy weight (W) that you let sink down into very deep water (D) and while it dropped you used something to harness the energy during the drop then once the object has reached the bottom you inflate a balloon to make it rise again and during the rise you again accumulate the energy and once the object is at the top you deflate the balloon.

The deeper the drop the more energy you can get, this sounds like a perpetual motion machine because it appears (the big assumption) that the energy you get from the up down motion will be more than enough to inflate and deflate the balloon.

Can someone do the math on this?

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34. Lee Surber
April 30, 2014 | 11:17 am

Someone earlier mentioned hydraulics! I had that idea too! I am planning a system right now using externally rectified 48v pma wind turbines to power a programmable 10kw golf cart motor! This would turn a generator head at the required 3600 rpm. A load transfer switch would then handle switching back to grid when no power is available. The second part of my system is to use a secondary array to drive a hydraulic pump/ram system to charge a freight car spring array! Once the springs are charged they can be released back through a flow controlled hydraulic motor coupled to another generator head. I do believe it would work!

Lee

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