Kevin Warnock

Entrepreneurship, ideas and more

Store meaningful amounts of electricity in rocks or sand? Engineers, please help me run some numbers.

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Compressed Air Energy Storage. Graphic from

Compressed Air Energy Storage. Graphic from

Today solar and wind plants sometimes provide round the clock output by using daytime power from the sun to pump water up hill to a resevoir, which is then allowed to flow downhill through turbines at night to generate electricity. This takes space and water, which probably makes it unworkable at home. This form of energy storage is called Pumped Hydro Energy Storage.

There is also another storage method called Compressed Air Energy Storage, which works like it sounds — air is compressed with solar energy and the compressed air is used later to run a turbine to generate electricity. I can see that working for industrial scale systems, but for home use I see it as being too noisy, given the racket my Harbor Freight air compressor makes filling an eight gallon tank. I wonder how noisy the residential Honda natural gas compressors are to fill your compressed natural gas Civic automobile in your home garage? I bet they’re not silent.

What I propose is to lift rocks or sand with solar or wind generated electricity. Then, when the sun is down or the wind is calm, lower the rocks or sand back down and have the cable spin a highly geared generator to make electricity. The mechanics of this would seem to be similar to what’s been developed for electric vehicles with their regenerative braking systems. In such systems, a heavy car is brought up to speed at great cost in energy, and when the driver steps on the brakes, that energy is harvested and put back into the on board batteries where the energy was earlier, before the driver stepped on the accelerator. An elevator of weights in the backyard or basement would work similarly, and would be far less complex to design and build than an electric vehicle, I suspect. The most complicated part would probably be figuring out how to pull large irregular amounts of electricity out of the system, to run a hair dryer for a couple of minutes, for example. Since houses draw power almost all the time, the weight would always be in motion, but ever so slowly if the only load was a few electric clocks at night during moderate weather when neither heat or air conditioning is needed.

My friend Jim Castelaz over at the very cool startup Motiv Power Systems has a lot of very smart engineers working on electric power systems for buses, and I suspect any of them could figure out in minutes if this blog post has any merit. I simply don’t know how to calculate how big a weight would be needed to store enough energy to be practical. What I do strongly suspect however is that this is not far fetched. Sure the weight may need to be 100 tons, but so what? That’s not going to dent the earth, and since sand can be used, the weight can be carried in via wheel barrow if needed, by one person.

The big benefit of this is that the efficiency would not decline like with batteries, and such a system could last for decades with proper maintenance, like elevators do in buildings. Also, there are few environmental hazards with such a system. No explosive gasses emitted during ‘charging.’ No chance of explosion. No chance of getting acid burns. No worry if the system freezes. Easy to understand and repair. Easy to manufacture. I love it.

There is also no issue with an idle system losing power just sitting there. So this could also work for backup power for data centers. Seems far easier to engineer than huge concrete flywheels on the roof like I’ve heard about.

Maybe ocean shipping conainers could be used as the energy storage elevator cars. How much would a container with sand weigh, anyway? Would it burst from the load if you tried to lift it by the lifting points? How much trouble would it be to reinforce a container to handle the weight if it were filled with sand?

I love this idea, which I had while falling asleep November 23, 2011 at 1:45am and I wrote up minutes later on my blog so I would not forget it. I finished refining and editing this post today, December 20, 2011, when I made it public to my readers.

It turns out I’m not the first to think of storing electricity via lifting weights. Have a look at this educational piece from a website that describes itself as about primitive Christianity. Here’s a discussion about using weights hanging from an ocean seasteading platform. Here’s an even more thoughtful analysis from someone who also independently thought of doing this. Finally, here is a presentation by Gravity Power Storage which is proposing to build utility scale versions of what I describe here, with 2,000 ton weights storing megawatts. I came up with this idea independently. While I could have discarded this post upon discovering lots of other prior work in this field, as shown by these links, I am publishing it anyway because I am proud to have independently developed this idea, and I think it deserves more attention. It looks like Gravity Power Storage has concluded I am not nuts. Thank you.

The idea of storing energy in heavy weights is so elementary I suspect this idea dates to soon after the harnessing of electricity by man. What I want to know is whether my idea is practical. If it is, why has it not been implemented more widely?

Yes, in much of the United States, the power utilities are required to buy any electricity you make and then sell it back to you when and if you need it. This makes the power company into nearly a perfect battery that sidesteps the energy losses inherent in chemical battery storage.

But what if we could get rid of electricity companies over time? Do we really need to have electricity companies? If we cover the roofs of our homes with solar photovoltaic panels and switch to electric bicycles and live close to our jobs, perhaps there would be no need to import electricity from afar.

Sure, my system as described would cost over USD $10,000, even in quantity. But so does a swimming pool, and there are plenty of those around, with many sitting idle most of the time. A pool also requires a big hole in the ground, so there are lots of hole digging experts around. A pool of water probably weighs 100 tons, so such a huge weight at home has been routinely handled for centuries. The alternators and inverters required by my idea are commodities. Elevator mechanisms are commodities. Tying everything together will take some expertise that needs to be developed, but I don’t see any insurmountable challenges.

Again, a system like I describe will have a long and relatively trouble free life span. Such systems probably can store enough power to last a week provided electricity use is cut to what an off grid solar house uses today. All houses ought to cut down their usage to such levels, even without adding the on site storage component. There are very efficient appliances out there already. Heating can be done with solar hot water collectors. Everything needed has been invented and is already made at scale.

If what I write about is practical, let’s just implement it already. Cheap and efficient electricity storage is a market likely to be worth tens of billions of dollars. This is exciting enough that I could be persuaded to work on this full time. If you’re an engineer, please run some numbers and let me know what you conclude.

Written by Kevin Warnock

December 20th, 2011 at 9:28 pm

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