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> Storing Energy In Underwater Bags
Enthalpy
Posted: Mar 28 2008, 04:00 PM


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Hello everybody!

I just can't resist showing you these news:
http://news.bbc.co.uk/2/hi/uk_news/england...ire/7315059.stm

In short, they would accumulate energy when it's abundant (that is, when wind blows or Sun shines) by pumping air to deep undersea bags.

Sure, the bags wouldn't be small to accumulate a significant amount of energy: if they're 3000m deep, they will store about as much energy per volume unit as a dam with a 3000m head - but volume is just plentiful at sea. 1km3 there disturbs us less than on land, as land beings, and it can be taken anywhere.

Another nice feature is that the envelope doesn't need to withstand the storage pressure: it's basically the water that pushes on the air. The envelope only needs to withstand the pressure difference linked to the thickness of the air bubble, not to the storage depth and pressure.

Even better, the envelope works under tension, not compression like in a dam, so it can use a strong thin material.

Dams are already used to store energy in an economic context. Even from Spring to Autumn, not just within a week - though you get good wind once a week in Scotland.

Do we finally have a solution where we pay little material to store big amounts of energy?

I like it!
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Enthalpy
Posted: Apr 1 2008, 02:58 AM


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Hopefully we get results soon, with nice pictures of well-sized balloons!

I would just avoid navigating over such a bag, in case it breaks its ropes. Imagine a 1hm3 or 1km3 bubble ascending under your hull: it can become uncomfortable.
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StevenA
Posted: Apr 1 2008, 03:12 AM


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A friend of mine mentioned an idea similar to this not long ago ... maybe he got the idea from this. There are definitely some interesting ideas with this.
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Confused2
Posted: Apr 1 2008, 04:33 PM


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If we have (say) a 500kW compressor from (say) 14 psi to (say) 140 psi ( 'bubble' is 300 feet down) .. doesn't the air get rather hot? .. obviously 'P' changes but I'm not quite sure what to do with 'V' to apply the gas laws to this .. any ideas?

-C2.
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Enthalpy
Posted: Apr 2 2008, 02:49 AM


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Air getting hot: yes.

And during storage, it would cool down, losing a good part of the work we put in it. When we would use the cooled air, it would become really cold, and we'd get moist condensation in the turbine instead of work.

The best answer I figure would to both compress and expand the air at ambient temperature instead of adiabatically.

For such a pressure ratio (I rather figure out 3000m to save bag volume), the best way could be to have many compressor and turbine stages and insert heat exchangers with the atmosphere or the sea between the stages.

A long time ago, as tramways ran on compressed air, people spayed water in the air to reduce the temperature rise when compressing. Simple, but I guess inefficient for 300 bar.

Suggestion: Instead of a pipe full of air between the bag and the surface, fill a bigger pipe with much water and tiny air bubbles, and pump the mixture. Solves the temperature issue. Compressors and pumps work with a denser fluid, more volume and less pressure, so speeds are more civilized. It needs a couple of mixers-separators down and at the surface, but this technology must already exist.
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wcelliott
Posted: Apr 3 2008, 03:12 AM


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Echoing others' points, you'd lose energy when pumping hot air into the cold ocean that you wouldn't get back, and you'd need a lot of ballast to keep that huge bubble of air down there, it'd want to float to the surface. A simple "bag" wouldn't do the job, you'd need a submarine bigger than the Titanic (and heavier - remember, when it had air in it, it floated).

If you want clean energy, think geothermal.

Dig a hole a few meters deep, and you can keep your house from getting any colder than the year's average temperature in your area, dig a deeper hole in Yellowstone National Park, make a artificial "Old Faithful", and you can have as much energy as you'll ever need, if you make the "New Faithful" big enough.


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Enthalpy
Posted: Apr 3 2008, 11:20 PM


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WCElliott, you posted again an objection that is already addressed, about temperature differences. Giving a sight to the previous posts (not even a page long here) would help you make a good impression.

And by the way, there are already hundreds of solutions, apart from what I suggested, which have been well known for a long time by good engineers.

Professor Seamus Garvey suggested bags because it is the best choice indeed. I feel it extremely presumptuous to look half a minute at a proposal and answer "impossible because I don't see how". And yes, you may reasonably suppose that some of the best-educated among other people are aware that air buoys in water.

Ballasting the bag with rocks found locally is certainly cheaper than building a hull with man-made materials. Bolting anchors in a soil is also quite a common engineering task, even underwater.
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adoucette
Posted: Apr 4 2008, 03:14 AM


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QUOTE (wcelliott @ Apr 2 2008, 10:12 PM)
Echoing others' points, you'd lose energy when pumping hot air into the cold ocean that you wouldn't get back, and you'd need a lot of ballast to keep that huge bubble of air down there, it'd want to float to the surface.  A simple "bag" wouldn't do the job, you'd need a submarine bigger than the Titanic (and heavier - remember, when it had air in it, it floated).


Its not a question of this vs geothermal.

The issue is more along the lines of how to store energy produced by Wind Turbines operating in the ocean when that energy isn't needed.

Consider for instance in the North Sea, as more and more wind farms at sea are developed, you will get to the point where a LOT of excess electricity will be produced at periods of very low demand.

So the question becomes, how do you store it?

This method is one, and since we are dealing with excess electricity, the payback doesn't have to be that great as we can turn around and use it to fill in at peak periods and to smooth out times when the wind isn't blowing.

The savings, vs having to have alternate production capacity in standby, can easily make the ROI relatively good, even for rather "wasteful" methods of energy storage, since without any storge method then ALL the electricity produced greater than demand is wasted.

Note, this also shows how expensive battery storage is.

When you are dealing with large quantities of energy, some form of physical storage (as in pumped storage at Niagra) is the only economical method.

Arthur

This post has been edited by adoucette on Apr 4 2008, 03:15 AM


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El_Machinae
Posted: Apr 4 2008, 02:51 PM


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What I like about this idea is that it gets the imagination working. It's "outside the box" thinking to the layman.


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rethinker
Posted: Apr 4 2008, 03:17 PM


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seekers of energy
I think getting the idea into our minds is a good idea also,however

I may be under the wrong impression by thinking that converting air pressure to mechanical energy and then to electrical energy has been studied and is extremely inefficient?

First you must use the wind turbine to turn some type of shaft to run an air compressor.

Second you must continue to pressurize the holding tanks/bladders etc. This would have to be done by 2-4-6 or 8 stages of compression. What that means is more energy loss to mechanical mechanisms.

Third air in any form of a chamber wants to float.

Forth, air to mechanical work is very inefficient, check with the pneumatics industry who have studied this for a long time.

Air compressors are for convenience not that you get good work from compressed air.

Five, Now we want to go back to another shaft of a generator!!!
blink.gif

Then OK if it all worked when do you use it? When the peek is high? That's when the generators are working already!

I think this idea has too many stages of energy transfer before it gets back to the lines going to shore. Which by the way have a tremendous resistance to voltage.(voltage drop).

The list can go on and on for this idea.

The best way is always direct.

This post has been edited by rethinker on Apr 4 2008, 03:18 PM


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adoucette
Posted: Apr 4 2008, 05:54 PM


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QUOTE (rethinker @ Apr 4 2008, 10:17 AM)
I may be under the wrong impression by thinking that converting air pressure to mechanical energy and then to electrical energy has been studied and is extremely inefficient?


Efficiency isn't necessarily that chief concern when the input energy is "free".

The OVERALL cost per kw of energy retrieved is the primary consideration.

If you can store large amounts of energy at relatively low material cost per kwh, then you are likely to have a good solution.

Meaning, that a wind turbine has a fixed cost and it produces energy regardless of demand, so if you don't come up with a method of storing the excess energy produced when the demand is low then the efficiency is ZERO.

Arthur


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Enthalpy
Posted: Apr 4 2008, 06:58 PM


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Air compressors and turbines have a good efficiency. Even, quite good. Think of >95% for large machines.

Part of the discussion about efficiency was that air gets hot when compressed, but we would lose this heat as the compressed air cools down in the underwater bag.

However, we speak of a couple of 10% here, and this limitation has been solved over a century ago. Many solutions exist.
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rethinker
Posted: Apr 4 2008, 10:09 PM


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QUOTE
Air compressors and turbines have a good efficiency. Even, quite good. Think of >95% for large machines.

Yes I agree. The actual compressed air is not efficient. The volume of air to do work, is very large.

QUOTE
Efficiency isn't necessarily that chief concern when the input energy is "free".

I do see this point.

We only need to build the machinery, and everyone will get free electricity.
I think I have an old newspaper that said the reactor power would make everyone have cheep or free electricity.

Would the bags hold low pressure, but large volumes of air, or would they hold high pressure?

Sounds like blowing a marble with a straw to me, but I'm open minded.

What about using high pressure hose,flexible enough to withstand necessary movements,and supply compressed air direct to need. Maybe smaller storage tanks(no less than needed).
I know industry can build very large high pressure hose.

If you use compressed air and propane, you make the propane more efficient, and it helps if it is hot, so no need for cooling the compressed air.

This post has been edited by rethinker on Apr 4 2008, 10:10 PM


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adoucette
Posted: Apr 5 2008, 01:25 AM


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Rethinker,

People consume energy primarily during their "waking" hours.

Our energy needs go down considerably when the majority of the population is asleep.

But wind turbines don't go by this schedule and if you build a large quantity of turbines to produce power for typical needs during the waking hours you will also get a lot more electricity than you can use during the night time.

The bags are one potential way to store this energy until it is needed.

The bags, in this scenario are said to be at 3,000 meters depth, so you would be storing a the air at very high pressures.

Arthur

This post has been edited by adoucette on Apr 5 2008, 01:26 AM


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"We cannot prove that those are in error who tell us that society has reached a turning point; that we have seen our best days. But so said all before us, and with just as much apparent reason. On what principle is it that, when we see nothing but improvement behind us, we are to expect nothing but deterioration before us?"

Thomas B. Macaulay
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rethinker
Posted: Apr 5 2008, 02:25 AM


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Thank You Arthur


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