Why don't we use sand to store energy?

I was thinking about how we currently store large quantities of energy - in the form of batteries, correct? - and then I got to thinking that wouldn't it be much more simple and efficient to just have turbines move buckets of sand into an elevated container and then when the energy is desired, allow the sand the fall and push turbines below it?

Theoretically the efficiency would be at least in the 90% range if it was designed well, possibly even approaching 99%.

I would have said doing this with water but in my opinion water is too valuable to store away as potential energy than is sand or pulverized rocks.

What do you guys think of this idea?

 

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Lots of friction. Water would be better, we store it anyway.

 

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Space-based solar power (SBSP) is the concept of collecting solar power in space for use on Earth. It has been in research since the early 1970s.

SBSP would differ from current solar collection methods in that the means used to collect energy would reside on an orbiting satellite instead of on Earth's surface. Some projected benefits of such a system are:
Higher collection rate: In space, transmission of solar energy is unaffected by the filtering effects of atmospheric gasses. Consequently, collection in orbit is approximately 144% of the maximum attainable on Earth's surface.
Longer collection period: Orbiting satellites can be exposed to a consistently high degree of solar radiation, generally for 24 hours per day, whereas surface panels can collect for 12 hours per day at most.[1]
Elimination of weather concerns, since the collecting satellite would reside well outside of any atmospheric gasses, cloud cover, wind, and other weather events.
Elimination of plant and wildlife interference.
Redirectable power transmission: A collecting satellite could possibly direct power on demand to different surface locations based on geographical baseload or peak load power needs.

SBSP also introduces several new hurdles, primarily the problem of transmitting energy from orbit to Earth's surface for use. Since wires extending from Earth's surface to an orbiting satellite are neither practical nor feasible with current technology, SBSP designs generally include the use of some manner of wireless power transmission. The collecting satellite would convert solar energy into electrical energy on-board, powering a microwave transmitter or laser emitter, and focus its beam toward a collector (rectenna) on the Earth's surface. Radiation and micrometeoroid damage could also become concerns for SBSP.

http://www.google.com/url?sa=t&rct=...Q_pHczpxxeSxdqKCQ&sig2=u-cRH85aD9OQwC5wsMsrvg

 

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Choose something that have a hi heat capacity , Do you know waht is the heat capacity of sand vs water ?

 

The reason I think sand would be better than water is because it won't evaporate and it isn't as necessary to life as water is. Salt water would be an obvious alternative but then we're dealing with a substance that has corrosive qualities.

I don't know the comparative heat capacity of the two. Why do you think high heat capacity is important?

Thanks for the response!

 

Originally Posted by arauca
Choose something that have a hi heat capacity , Do you know waht is the heat capacity of sand vs water ?

See: http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html, and
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html

 

Good post Cosmic! What I'm wondering now is why don't we all come together as a planet and litter space with solar energy collecting satellites that transmit electricity back to earth to power turbines that raise sand into elevated containers that can be stored indefinitely so we don't have to work anymore for basic sustenance??

 

spidergoat noted that water accomplishes the same purpose, in addition, it's a renewable resource, replenished by rainfall. cosmictraveler shows what may the most efficient method, since it collects a free renewable source with little or no environmental impact, for example, dams disturb aquatic life.

But one thing that is appealing to your idea is that it may be feasible for primitive living in arid locations. For example, you could construct a simple machine, using primitive tools and materials, and apply energy to lift the sand, store it at elevation, and tap into itas needed to develop power.

Here I'm imagining a crude turnstile, powered by a beast of burden, that lifts sandbags on ropes, through crudely improvised means (maybe old car wheels for pulleys) and then a stop to lock them in place, a clutch or gear that engages to connect them back to the driveshaft, and ouila, you get torque on demand.

Obviously there are many more elegant and efficient ways to do this.

I'm not suggesting this has anything to do with the global energy crisis, but it might solve some particular crisis. There may be some reason why a primitive person would need to store energy. So this would certainly be feasible.

By the way, if someone came up with a kit that could be deployed in impoverished parts of the world to exploit the availability of sand over water, I would propose one additional component: a solar collector, since these areas tend to be sunny. A primitive person might have no use for electricity, but steam, especially clean water, would address many a crisis. So I would devise a kit with both machines. Joined together, the primitive person gets his mechanical powered developed by sunlight. Put up enough of these, and conceivably you could drill a well, or pump water, or irrigate a field, or do some other essential task. As a real test of ingenuity, consider making all the components out of materials available in the local population, so that there is no reliance on outside help for spare parts. Make water the primary source of storage, and sand could be the backup if water is not available.

This may not at all be where you were headed, but it's the kind context in which it might be most useful.

 

The problem with water I have is two-fold. First it evaporates so it would need to be put into an enclosed vessel which makes getting it in there more tricky. The second problem is I'm pretty sure fresh water is non-renewable, or at least not without using considerable energy - and using too much fresh water would no doubt affect the ecosystem in a similar way that dams do. I think it's important to keep our drinking water just that - drinking water. To have such an essential part of our sustenance pent up for use as potential energy seems like a bad idea to me.

I completely agree with you that this would work very well in primitive systems. I think it's utility spans much further though. As far as I know the way we store large quantities of excess energy is through chemical batteries. These are inefficient in converting raw mechanical energy and they have a limited shelf life. Lifting sand into elevated containers produces very little waste of energy and can be stored almost indefinitely with little to no loss. Not to mention the ecosystem will probably not even notice that some of the sand is missing.

 

Except we already do it with water, and in the quantities we are discussing, evaportaion is not an issue.

http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

And the water is not harmed in the process.
It starts out as fresh water, it ends up as fresh water.

And Fresh Water (ie from rivers) is NOT the same as "Drinking Water".
Drinking water represents a small fraction of the daily flow of most streams/rivers from which it is removed and then treated to make it into drinking water, so for most places that's not an issue either.

 

There is no such thing as "free" energy. It would take more energy to raise the sand to the higher level than it would generate by falling. Even at 99% efficiency, you've simply invneted a machine that loses 1% of its energy with each cycle. You'd be far better served just directly using the energy you were going to use to raise the buckets, to power what you intended to power with the sand generator.

Can you imagine how long a power generating plant which cost 1 million dollars to operate for a day but only brought in 1.1 million dollars of revenue would last?

 

There are however advantages to having energy when you need it.

Electric companies can sell power in the middle of the day for much more than they can in the middle of the night.

So Pumped storage of water is done at night, during the times when power is in low demand, and sold for much more than it costs to pump it during times of high demand.

It's not ALL about efficiency.

 

I don't see why it wouldn't evaporate?

Also is this always done with a dam? I don't think dams are the greatest for the ecosystem. Would sand really be so much less efficient than water? I feel like it would be less hassle and maintenance overall with probably the same if not greater efficiency of energy transfer.

Am I wrong?

 

This question is completely arbitrary. You could create a scenario where one thing is more efficient that another, just based on whichever parameters you are attempting to optimize.

I don't think all hydoelectric power is harmful. But certainly there is concern about restricting 2-way traffic for aquatic life, such as salmon.

Your idea is essentially a good one, for a particular set of constraints. You are simply recognizing that lifting a mass is one useful way to store energy. You seem to be taking into account that barren sandy places are not a concern for impact. The question of efficiency spidergoat mentioned would invariably boil down to the low friction in transporting water. The actual method you have in mind, using a turbine, sounds like a system that will constantly be going through maintenance cycles to replace the parts that are polished down by the sand until they fail.

If you like playing with numbers, you can imagine how much sand it would take, and at what height, to store a given amount of energy. I'll walk through this, in case you're interested:

This is from the DOE:

Suppose you want to store a 3 day supply for 100 homes; we can scale this later for a city. That's the same as 9580 kWh. Convert to seconds = 34.5 GW-s = 34.5 GJ. Now you can divide by g (9.8 m/s) and you get 3.52G kg-m. So pick any height, let's say ten meters. That tells you you need 352 million kg of sand. Dry loose sand is around 1442 kg/m[sup]3[/sup]. So that says you need 245k m[sup]3[/sup] (call it 250k). So far we haven't accounted for losses due to friction, etc., so this is an ideal scenario. So all you need to do is scale this. Say you think you can get 50% efficiency lifting and 50% efficiency in converting falling sand to electricity. Then you would need to lift 1M m[sup]3[/sup] a height of 10 stories, and you could potentially store a 3 day supply for 100 homes.

Note, 1M m[sup]3[/sup] fits in a cube 100 m on a side, or the length of a football field on each edge. And all you need to do is lift it 10 m and recover the energy as it falls over a period of 3 days, and you've covered 10 homes.

As you see it's quite a feat. But certainly not impossible. It really turns into a big project when you try to cover an entire city. This is another reason dams are practical, sense they span such immense volumes. Also, water is self-leveling, and doesn't pack down like sand. As for evaporation, they replenish from the watershed at the head end where gravity is working its magic.

You probably can't move sand with blades, as they will quickly wear and break. Conveyors would have problems with abrasion on all their moving parts. Elevators might work, where the sand is piled into a box, hoisted by a motor, and harnessed by the motor, acting as a generator, on descent. You would just need a structure the size of the Treasury Building, only much taller, crammed full of these elevators. To account for the sluggish response of the elevators, you would need a battery bay, to smooth the curve as surges occur (so lights don't dim and PC's don't reboot, as an elevator begins to accelerate and match the power demand).

 

You mean like a giant hour glass connected to a turbine.

Reusable.
Renewable.
Clean.
Not weather dependent.

Let's do this.

 

Excellent post Aqueous Id! Your elevator idea is a good one. Originally I did have blades in mind but I realize now they would be worn to dust in no time. I thought about your elevator idea but didn't like the fact that it would require a motor to operate. What if the sand was filtered into buckets that are fixed to a large, vertical belt? There wouldn't be much friction then, ay?

Of course this is still a crude outline of what would be required. As far as smoothing out the curve goes, why not just use the infrastructure that's already available with the power lines? I don't think the ac generators that are currently being used are what literally smooth out the voltage/current. Isn't that done at the location where the electricity is being used with inductors or capacitors?

Again, great post!!!

Also, does "GW-s" = gigawatt/second? And what is "1 M m(cubed)" mean? I was a little confused by those.

 

First of all a good question & a good thinking & people are working on it & the question is fully answered in the above post of cosmictraveler.

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regards.

 

Hydraulics work with water because water flows. Sand flows only in the loosest sense - its coefficient of friction is astronomical compared to water.

Hydroelectric power only works because the water is already at a higher level. If we had to pump seawater up to all the damns with hydroelectric generators, we'd have nothing. The cost would be a thousand orders of magnitude higher than it is now. And on that sort of scale, every kw we could generate would be consumed by the power requirements to pump all the water above the generating plants.

Only in a few very limited cases is pumping water up to a storage reservoir during the night very efficient - the Mohawk Electric Generating station is one of the few, and that it workable only because it has a great deal of excess generating capacity - it generates far more than the demand of its customers (who pay damn near nothing for the power anyway!)


Rich

 

Rich, the thread is titled "Why don't we use sand to store energy?". I hope that clears up the confusion for you!

 

It would - but very, very slowly. Heck, sand blows away when it gets windy. Nothing's perfect.

If they are in an area that doesn't already have a river they are no worse than a pile of sand. We are planning to build one near Lake Elsinore that doesn't use a river.

Much worse. Dealing with it incurs more friction losses if used as a "working fluid."