# large scale energy storage?

by lfasn
Tags: energy, scale, storage
 P: 5 This idea is for an alternative to large scale energy storage. Sorry for the double and confusing post (http://www.physicsforums.com/showthread.php?t=179768), I should have posted here in the first place. As far as I can understand, pumped-storage hydroelectricity is the main technology used for grid energy storage. The efficiency of this procedure is about 70%-85%. Is it possible to use solid materials instead of water? I am not sure how, I am not an engineer. Lifting these objects with an elevator motor, seems the easy part. If these objects were let to fall down afterwards, wouldn't they turn the motor, so that it could produce some electricity back? Probably yes, because I read about regeneration (or 'four-quadrant motor operation') with counterweights and other stuff I cannot understand at http://www.aceee.org/buildings/coml_equp/elevators.pdf (page 5) which seems to be a similar concept. Could you make a rough estimation of the round-trip efficiency of this procedure? Does it worth to compare this with pumped-hydro and investigating it further?
 Sci Advisor P: 5,421 In principle you could do that, but for a typical size of pumped-water system you are talking about pumping millions of liters of water to a height of hundreds of meters. The nice thing about water is that it's easy to pump, and you don't have to do much to store it except find a piece of land that is a suitable shape and build a dam across one end of it. A solid (even a powder) would form a large heap which would probably be unstable (think landslides after heavy rainstorms, etc). To avoid that, you would have to build an energy-wasting system to spread the solid out, and then collect it together again. With water, gravity does the spreading and collecting for free.
 P: 5 Isn't it true that water evaporates from the upper reservoir of pumped-hydro systems? Wouldn't these losses compare to the energy wasting system that would be required to spread the solid material? You would not really have to spread and collect the solid material horizontally. Lifting thousands of tons of solid matter, would require many smaller elevators. I was thinking about it more like just putting them in shelves exactly at the position of their arrival.. Actually, the elevators could be spread around. And why powder? Wouldn't sealed iron boxes, filled with sand/rocks for example, solve the problem of losses after heavy rainstorms? As for landslides, certainly it is an issue. But, suitable places that are less prone to landslides could be found (rocky mountains perhaps?). Is it possible to construct elevators that would travel to greater heights than the height difference of current pumped-hydro systems? More than a kilometer perhaps? (I am totally out of my league here.. but what happens with elevators in mines?) Wouldn't be easier to find suitable places for such constructions than for a dam?
P: 5,421

## large scale energy storage?

To get some idea of the scale of the operation, lowering 1 tonne of material per hour through 500m could generate about 1 kWh (assuming about 70% efficiency).

So to run a 200MWh generating plant you would need to move 200,000 tonnes of material an hour from your shelving system, load it onto your multiple elevators, and restack it on shelves at the other end.

Personally, I would rather have a big water big pipe, open a valve, and let gravity do the rest. Maybe I'm getting lazy as I get older
 P: 5 Is it possible to construct elevators that can work for 2500 meters? Thus, in order to run a 1000 MWh generating plant, only 200,000 tonnes (i.e. 200 tons/MWh) of material would need to be moved every hour (assuming 70% efficiency ??? -I wish others could confirm this or not, that is the only thing I want to know right now). Compare this mass, with the mass of the water that is needed to drive a pumped-storage with a height difference of a 100 meters: it's 25 times less! (and probably about 50 times less in volume) Also, forget "my shelving system" and loading onto elevators. Even simpler, for storing, stack those boxes on top of each other. For loading, hook them with the wire that is connected to the motor, in order to eliminate the need for an elevator ramp altogether. But as I said, I am not an engineer and I cannot solve such problems. That's why I am asking questions here.. to resolve problems; not only to get older-lazy responses
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P: 8,962
 Quote by lfasn Isn't it true that water evaporates from the upper reservoir of pumped-hydro systems?
Evaporation from a reservoir is very small effect in most temperate regions. Plus you also get the same rainfall into a pumped storage scheme that you would get into a normal reservoir.
P: 5,421
 Quote by lfasn Also, forget "my shelving system" and loading onto elevators. Even simpler, for storing, stack those boxes on top of each other. For loading, hook them with the wire that is connected to the motor, in order to eliminate the need for an elevator ramp altogether.
You don't seem to have any idea about what 200,000 tonnes of material actually looks like.

Suppose you used 1-meter cube metal blocks (let's say iron). Each block would weigh about 7 tonnes. 200,000 tonnes is about 28,500 blocks. It's your choice how you stack them. In round numbers, you could have a 30 x 30 x 30 cube (which would need some pretty strong foundations to stop it sinking into the ground!) or (probably more practical) a stack 100 blocks square x 3 meters high.

Now, you need to get those 28,500 blocks onto your elevator in 3,600 seconds. That's about 8 blocks per second. Hope your rope doesn't break...

Actually, if you DO find a way to do this, forget about the elevators. You could make a fortune out of a high-speed loading and unloading system for container ships. Unloading a shipload of 20,000 tonnes of containers onto a dock in just 6 minutes would a quite a neat trick.

 That's why I am asking questions here.. to resolve problems; not only to get older-lazy responses
Well, YOU are the person who wants to design this system, not me
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