Enthalpy
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!
Enthalpy
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.
StevenA
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.
Confused2
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.
Enthalpy
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.
wcelliott
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.
Enthalpy
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.
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
El_Machinae
What I like about this idea is that it gets the imagination working. It's "outside the box" thinking to the layman.
rethinker
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!!!

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.
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
Enthalpy
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.
rethinker
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 (->
 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.

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.
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
rethinker
Thank You Arthur
Confused2
As a quick and dirty estimate of the energy stored/m^3 at depth D I'd guess it would be about the same as the work required to push the bubble down to D. The bouyancy changes but I don't think that affects the result (um.. probably)

So .. upthrust on 1m^3 is 10,000 kgf or 100,000N. If sent down to 1000m we'd store 100 x 10^6 Joules. If we're looking at a smallish installation this would be 100kW for 20 mins so about 60m^3 would give 100kW for a day. At 1000 m depth the pressure is about 100 atmospheres so the actual bubble would be 0.6m^3 .. obviously not very big.

Scaling this for (say) 1MW for a day still makes for a pretty small (and manageable) bubble size.

(Always best to check my stuff before drawing any conclusion)

-C2.

(edit .. oops 60/100 = 0.6)

(Edit .. obviously ignoring efficiency etc .. but it gives a clue.)
Confused2
Another 'oops'..
It might be worth considering the possibility that water only weighs 1,000 kg /m^3
rethinker
Thanks Confused2
As I study it closer, I see every ones point of view including Mr.Garvey
Here is another link to understanding this idea.

Windbags under water
Enthalpy
Interesting link, with more figures than at the BBC.

Here is the energy density, when compressing and expanding at constant temperature, and without any kind of loss whatsoever:

W = PV*ln(P2/P1) where PV is the same at the surface or at depth.
With consistent units as usual... For instance Pa and m3 and J.

At -1000m or 10MPa (or 100b) : 46MJ/m3 where the m3 are at depth
At -3000m or 30MPa (or 300b) : 171MJ/m3

I will take -3000m (though the paper at CNN mentions 600m) in the following, because 300b is the usual pressure for hydraulics, and 3000m is the most common depth of the oceanic floor.

Now, imagine we want to store the electricity produced by the biggest wind turbines we have, or 4.5MW peak. At a very good site, the turbine delivers 1.5MW mean (often forgotten...): this is the power the storage must deliver, for about 5 days at a good site (Scotland), before wind blows again. This equates 650GJ. Then, wind power becomes a safe supplier.

An underwater bag of just 3800m3 stores this energy at -3000m. This is a hemisphere of D=24m.

If one wanted to hold the bubble with a ballast instead of anchors, he would need about 2000m3 stones. Imagine: this is 2m*31m*32m. Cheap. Even concrete could be possible.

As a comparison, we might store air at 300b in a 3800m3 vessel on the ground, but...
Using S700mc steel at 200MPa (regulations may well require <<200MPa), we would need 150mm thick walls for cylinders of D=2m totaling 1210m length.
The steel would weigh 9000t and cost maybe 20M€, more than the wind turbine.
And the energy of some 200t TNT would be stored nearer to humans.
rethinker
QUOTE (Enthalpy+Apr 6 2008, 01:13 PM)

As a comparison, we might store air at 300b in a 3800m3 vessel on the ground, but...
Using S700mc steel at 200MPa (regulations may well require <<200MPa), we would need 150mm thick walls for cylinders of D=2m totaling 1210m length.
The steel would weigh 9000t and cost maybe 20M€, more than the wind turbine.
And the energy of some 200t TNT would be stored nearer to humans.

Looks like the payoff would indeed be way above the land storage, if I understand you correctly.
Enthalpy
Land storage as pressurized air is just impossible. Underwater storage seems possible for the engineer, because basic physics figures are very good.

The fundamental difference is of course that water holds the 300b pressure, not the walls. The hemisphere with D=24m would need to hold only 1.2b pressure. Imagine: it's as difficult, or as easy, as holding a hemisphere of D=24m (about one tennis court length) full of water in the atmosphere. Far easier than a pressure vessel.
Confused2
Hi Enthalpy,

I was rather proud of my 'know nothing' method .. I still reckon it should be OK (ish) given the right data and the ability to perform simple arithmetic.

For 1m^3 we get an up force of 10kN .. pushing that down 1000m gives me 10MJ .. you've got 46MJ .. where's the extra coming from?

-C2.
Enthalpy
It's just the detailed subtle refinement that 1m3 at 1000m takes more volume at smaller depths, so it gives a greater up force there.

To account for that precisely, you have to assume a constant temperature (for instance), and then P*V is constant. Now, you can sum the work dW = V*dP (or p*dV, and put some signs ad libitum if you believe it sometimes improves something) because P*V = R*T (not completely true at 300b) for 1 mole.

Neglecting that moles aren't aquatic:

dW = R*T*dP/P gives W = R*T*ln(P2/P1) where P2/P1 is >1
Replace R*T by V1*P1 or V2*P2 (since they are equal) for convenience. Even better, it works now for any number of moles.

If you compress the air before sinking it, you still invest as much work in it, because this depends only on the thermodynamical state of the air.
Enthalpy
Just an example of hemispheric bag with D=24:

The up force is 38MN or 500kN/m circumference.

Because I'm disconfident with plastics and fibers under stress in sea water for 30 years, let's take steel. A good duplex steel, for instance a 22-5.

Since it is good rubbish, I'd trust it for use at 250MPa. Then, it just needs 2mm thickness, weighs 14t and costs maybe 100k€ - far less than the wind turbine.
yor_on
As us(u)all :)
Confused... I definitely am.

Enthalpy?

"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. "

So what is the difference between a submarine filled with air and those 'bags'
Are you suggesting that the pressure inside them will equal out the pressure from the outside.
You can't seriously mean that a plastic bag will whit stand the pressure better than steel.
How will one press down that over pressured air into the bags waiting down there?
I reckon another way would be to see to that all energy sources are connected all over the world as there will always be need for energy somewhere.
-----------

On second thoughts maybe it would.
It would allow for deformation much better than steel (without breaking)
Perhaps?

I don't really know...
anyone?

But you would still have the problem with getting it down.
Either you don't pressurize the bags and just sink them allowing deformation.
Or you over pressurize them?
But then you will need (air)pipes down to the bags, won't you?
As they might explode/expand otherwise on their way down?
Or
Confused2
QUOTE (Enthalpy+)
It's just the detailed subtle refinement

Yes... I like your answers :) . The cost of the pipe could be substantial .. it would need to be quite wide (any guesses?) .. this makes it bouyant .. and so on. There might be a benefit in just designing a sinking pipe ..once you know how to do this an extra 500 yards of low pressure stuff curled up on the bottom would be your storage zone.

-C2.

Edit .. yor_ on .. the end of the pipe would be open (and lower than the rest). We do it at (say) 1000m because the pressure of the water at that depth is what keeps the air compressed .. same pressure inside and out.
Enthalpy
The whole idea of storing the air deep underwater is that the water pressure pushes on the air, needing no hull to do it.

So yes, the bag would be open at its lower side, keeping the pressure of water and air equal at their interface, and also allowing water to flow out and in as we pump air in or consume it out.

The bag is still necessary just to keep the air down, but this is a much easier task, as the difference of pressure between air and water at the top of the bubble is just (with the hemisphere of D=24m) 12m * ~1000kg/m3 * 9,81m/s or 1.2b, as compared to 300b at -3000m depth.
QUOTE (yor_on+Apr 7 2008, 04:56 AM)
So what is the difference between a submarine filled with air and those 'bags'
Are you suggesting that the pressure inside them will equal out the pressure from the outside.
You can't seriously mean that a plastic bag will whit stand the pressure better than steel.
How will one press down that over pressured air into the bags waiting down there?
-----------

Well yes, the submarine is filled with air at surface pressure and is relying on the strength of its construction to avoid being crushed by the much higher outside pressures. The bag doesn't have this problem since it is filled with air that is at the SAME pressure as the water outside, there is no differential, there is only the bouyant force to deal with. This force is considerably smaller than the pressure at 600 to 3000m.

QUOTE
I reckon another way would be to see to that all energy sources are connected all over the world as there will always be need for energy somewhere.

It might eventually help, but electricity is still a regional resource and this is not likely to change anytime soon for a host of issues.

QUOTE (->
 QUOTE I reckon another way would be to see to that all energy sources are connected all over the world as there will always be need for energy somewhere.

It might eventually help, but electricity is still a regional resource and this is not likely to change anytime soon for a host of issues.

But you would still have the problem with getting it down.
Either you don't pressurize the bags and just sink them allowing deformation.
Or you over pressurize them?
But then you will need (air)pipes down to the bags, won't you?
As they might explode/expand otherwise on their way down?

The bags are sunk empty, then weighted or anchored (or both).
The air would be pressurized at the surface and pumped down or released up via one or more high pressure hoses.

Arthur
Enthalpy
Pipe diameter: nothing tragic.

If the full 4.5MW pump air compressed at 300b, it equates only 0.026m3/s.

Let's take an air speed of 10m/s. Why? No reason up to now. I feel it slow enough to lose little power through viscosity. Then 0.0026m2 is a pipe of 0.058m inner diameter.

At zero depth, it has 300b inner pressure and no outer pressure. 22-5 duplex steel used at 250MPa must be 4mm thick. 3000m weigh 18t or 100k€ again.

OK, optimizing it properly would be useful.
Enthalpy
Connecting far sites of wind energy would guarantee a constant supply, but transporting electricity far away is no easy task.

We would need wind farms located ideally at half the distance between two depressions, at northern latitudes, as would be Greenland and Scotland. This is quite a distance, and worse, over sea, as sites at the center of continents are little interesting.

The longest lines in operation are, for instance, between the Hudson Bay and cities in southern Canada. Or between Itaipu and São Paulo. About 800km, and already quite expensive.

The EU has plans to obtain solar electricity in Sahara and transport it to Spain. Also some 1000km to southern Spain, mostly over land, and already a challenge.

Take your look at the figures. Lines are already operated in DC at 1MV. Wires are made of aluminium (cheaper than copper): 27e-9 ohm-m and 2700kg/m3 and 3\$/kg. Oops!

http://www.lme.co.uk/aluminium.asp

Such power lines also make countries more vulnerable.

So it's not an easy solution. We would rely on many sites located too close toanother at meteorological scale - say, Spain and Scotland and Denmark - and this would maybe guarantee a minimum supply, but maybe not, and this minimum should be carefully checked.

On the other hand, I feel easier to guarantee a minimum wind over a period of time in Scotland or Spain, and undersea bags look cheaper than very long power lines.
Sandra doliak
QUOTE
Connecting far sites of wind energy would guarantee a constant supply, but transporting electricity far away is no easy task.

We would need wind farms located ideally at half the distance between two depressions, at northern latitudes, as would be Greenland and Scotland. This is quite a distance, and worse, over sea, as sites at the center of continents are little interesting.

The longest lines in operation are, for instance, between the Hudson Bay and cities in southern Canada. Or between Itaipu and São Paulo. About 800km, and already quite expensive.

The EU has plans to obtain solar electricity in Sahara and transport it to Spain. Also some 1000km to southern Spain, mostly over land, and already a challenge.

Take your look at the figures. Lines are already operated in DC at 1MV. Wires are made of aluminium (cheaper than copper): 27e-9 ohm-m and 2700kg/m3 and 3\$/kg. Oops!

http://www.lme.co.uk/aluminium.asp

Such power lines also make countries more vulnerable.

So it's not an easy solution. We would rely on many sites located too close toanother at meteorological scale - say, Spain and Scotland and Denmark - and this would maybe guarantee a minimum supply, but maybe not, and this minimum should be carefully checked.

On the other hand, I feel easier to guarantee a minimum wind over a period of time in Scotland or Spain, and undersea bags look cheaper than very long power lines.

No matter what we do, we cannot create energy. nor destroy it.

The energy required to build an energy saving or capturing device, is probably greater that the energy it will gather in ALL its lifetime.

Sandra
QUOTE (Sandra doliak+Apr 8 2008, 01:34 AM)
The energy required to build an energy saving or capturing device, is probably greater that the energy it will gather in ALL its lifetime.

Sandra

Sandra this can't possibly be true.

There are any number of simple examples which show it to be false.

Probably the most obvious examples were some of the earliest devices used to capture energy, like windmills that turned stone wheels to ground corn and water wheels that provided ample high torque rotational motion that helped to spark the industrial revolution.

These devices took relatively little material to build and lasted for many years, extracting an obvious excess of energy from moving wind and water over what was required to construct them.

From those early devices, we HAVE gotten even more efficient though.

Arthur
Sandra doliak
QUOTE (adoucette+Apr 8 2008, 02:05 PM)
...Probably the most obvious examples were some of the earliest devices used to capture energy, like windmills that turned stone wheels to ground corn and water wheels that provided ample high torque rotational motion that helped to spark the industrial revolution. ...

Well, the amount of wind pushing on the windmill (sails) equals the amount of wear and damage the windmill takes, the amount of energy the windmill produces is also equal to how much wind hits and wears the windmill.

This means the more energy you want, the more wear you'll get.

QUOTE
These devices took relatively little material to build and lasted for many years, extracting an obvious excess of energy from moving wind and water over what was required to construct them.

it is not the material that matters, it is the energy required to produce them let alone stick them together to build something that collects energy.

The conclusion is clear.

Energy CANNOT be multiplied.

Kindest regards.

Sandra
rethinker
are we trying to multiply or capture/use the energy?
Enthalpy
My recurrent impression for a very long time:

When governmental agencies believe to have read something interesting in a forum, they try to hide it from other eyes behind pages of garbage.

Sandra Doliak doesn't believe what he writes nor doesn't he need any explanation, he just wants to spew trash and provoke more answers by writing obvious misconceptions.

The same happened in the discussion about the Pioneer Anomaly, and some more.

They manage to get paid for that, you know, and a few dozens Al Qaida members couldn't possibly justify tens of thousands of governmental agents.

But if somebody asks them where they found an innovative idea, they won't answer "a guy put it for free on a forum"... Rather "I won't answer, in order to protect my sources".
QUOTE (Sandra doliak+Apr 8 2008, 11:18 PM)
Well, the amount of wind pushing on the windmill (sails) equals the amount of wear and damage the windmill takes, the amount of energy the windmill produces is also equal to how much wind hits and wears the windmill.

This means the more energy you want, the more wear you'll get.

If that were true then there wouldn't be any old wind or water mills still in use:

But there are:

http://www.historicqac.org/sites/WMgristmill.htm

over 320 years old and producing the equiv of 26 horsepower around the clock.

Producing flour to feed the 1st Continental army under George Washington and it is still able to ground flour for you to take home and bake a batch of brownies.

Clearly, over its 326 years this mill has extreacted far more energy from the water than was used to consruct or maintain the mill.

or

http://www.outwoodwindmill.co.uk/

or

http://www.mapledurham.co.uk/history/watermill/

or

http://www.calbournewatermill.co.uk/

or

http://www.spab.org.uk/html/media-centre/p...ID=2c514b3df205

Arthur
Sandra doliak
QUOTE (adoucette+Apr 10 2008, 01:45 PM)
If that were true then there wouldn't be any old wind or water mills still in use:

Look at the bloody thing, it is lacquered, painted, and in god almighty god shape.

Is it not obvious that it has been given repair ever since that george Washington bloke?

My point was if it was not given constant repair, it would wither and tarter the more energy it collected.

Sandra
rethinker
QUOTE (Sandra doliak+Apr 15 2008, 01:51 AM)

Look at the bloody thing, it is lacquered, painted, and in god almighty god shape.

Is it not obvious that it has been given repair ever since that george Washington bloke?

My point was if it was not given constant repair, it would wither and tarter the more energy it collected.

Sandra

Writing that someone is not smart has nothing to do with the questions.

Repairs take little time to any generator, and run time is thousands of times longer than the down time for simple or complex repairs.

Would you suggest we stop wasting time building machines?
Enthalpy
A more precise evaluation of the pipe's diameter, since my post from 7th April 2008 01:55 on page 2. Found my old arcane text.

With a bag at -3000m, the air being compressed at the sea surface to 300b 300K. I take 367kg/m3 (perfect gas, this in unprecise at 300b) and a cinematic viscosity nu=5e-8m2/s under these conditions.

Storing 4.5MW peak minus 1.5MW mean needs 17.5dm3/s compressed air.

Then ID=63mm (tailored to our big needs) gives 5,6m/s and Re=7,1e6. From Prandtl's formula, lambda=1.7e-3. Then inject in Darcy's formula, get a loss of 470kPa or 4,7b or 1.6%.

More losses would probably be acceptable, as they improve at lower power, and this saves material costs.

Using 22-5 duplex steel at 250MPa as usual, the pipe needs 4mm thickness. It weighs 6.6kg/m or 20t. Taking 7€/kg (old figures but for smaller amounts), the pipe costs 140k€. Not very cheap, but definitely affordable. The steel can also be somewhat optimized.
Enthalpy
In my post of 2nd of April 2:49 (page 2), I suggested we could dilute the air as tiny bubbles in much water, and pump this mixture with a small overpressure at the surface. The density of the mixture would make most the the compression during the trip downwards, and water would keep the tiny air bubbles cool.

It looked interesting, but I've also computed the pipe in this case. Pity: we'd need about 3m diameter... Even though the pressure difference with the surrounding sea water is small and allows (uncomfortably) shallow walls, the pipe would then weigh some 400t and cost about 3M€. This is too much.

If I understand properly what Confused wrote on Apr 7th 2008, 10:23 AM, we might replace the too expensive pipe for water/air mixture by a diving bell that would transport the mixture in many trips. Interesting. It could deserve a more detailed study.

In case the diving bell is too expensive, then a standard pipe with air compressed at the surface is the answer. It works anyway, and uses existing technology.
Confused2
Hi Enthalpy,

I wasn't intending to suggest a diving bell.. though maybe it's a thought. My real concern was with the cost of consultants etc. to deal with a large(ish) structure 3,000 feet down. If the lower regions of the pipe were of standard section and negatively bouyant they would sit on the bottom by themselves .. (possibly) cheaper than a bubble. I had in mind something like the standard pipe that gas mains use .. using something like that you could hoist it up out of the water to check it was OK, replace it or increase the length/volume of it, without too much difficulty. I think there is a thing called a pig (possibly wrong) that you can put into the pipe to seal it .. the pig runs up and down inside the pipe as the pressure changes .. so the open end wouldn't need to be in any particular position (ie the lowest point).

-C2.
Enthalpy
Hi Confused2 and everybody!

I'm not too worried about the bag. Sure, it's somewhat new and will need engineering and experiments. But it's not that big neither: for the biggest available wind turbine, the bag would have the length of a tennis court as a diameter. I feel it reasonable.

And one should remember that we wouldn't build one wind turbine, but hundreds. Some 600 of today's big wind turbines are needed to replace just one standard power plant. So the engineering effort would be shared among many sites.

Cheaper than a bubble: this is difficult! A pipe for instance would use too much material as its walls are needlessly thick. Look how many km pipe total 3800m3.

About standard hardware: I don't feel a heavy need for it, as the underwater bag is big enough to justify special engineering. Welding some sheets to a hemisphere is cheap.
Enthalpy
Just an example of how to build the ballasted bag on the ocean floor:

Build it at the surface, and sink it.

That is, have the hemisphere filled with air float on the ocean to pull it to its final position. It floats, because only little of the ballast hangs under the hemisphere, for instance at three locations 120° apart.

[It could be nice to fix each ballast to several points of the hemisphere to stabilize the compound, like in a Serrurier truss for telescopes. Detail.]

Then, remove the air, and the bubble sinks. The hemisphere could work as a parachute: with 100t ballast, it would fall at just 2.3m/s. Alternately, take it gently down to a more precise locations by holding with three tugboats the ropes attached to the three ballast locations.

Then, complete the ballast by letting it slide at the ropes. Bring the >38000t in several rotations. Secure the pipe to the bag, and release the tugboats.

That looks simple enough to work even at sea.

Just warn the inhabitants that the floating saucer is human-made.
Confused2
Hi Enthalpy and all,

Since windmills will seldom produce the power you want when you want it .. why not make 'em pump air instead. Lots of smallish compressors and one big generator. Any thoughts about a compressor on a pole?

-C2.
rethinker
QUOTE (Confused2+Apr 19 2008, 04:38 PM)
Hi Enthalpy and all,

Since windmills will seldom produce the power you want when you want it .. why not make 'em pump air instead. Lots of smallish compressors and one big generator. Any thoughts about a compressor on a pole?

-C2.

Good idea

Couldn't you actually have a long stroke piston using a vertical shaft up and down?

Even hooking up the windmill directly to a compressor seems to make sense.
If you used a reduction gear with a ratio of 1 to (whatever), you could have a continuous flow of compressed air.

Enthalpy
Hi!

I'm not completely sure I got the general direction of the suggestions, but...

The main quality of a wind turbine is: little maintainance. For that, manufacturers have replaced the gear which brought a convenient speed to an efficient, small and cheap generator by a slow, huge and less efficient generator that avoids the gear.

So modern wind turbines convert the mechanical power directly at the very slow speed and huge (really huge) torque.

Also, forget about compressing air to 300b or even 60b in one step, as:
- temperature would be too high
- we want rotating pumps for little maintainance. These require more steps.

Then, the easy combination with today's bits and pieces is a slow electric generator for the full 4.5MW and a fast 3MW electric motor that rotates multistage centrifugal pumps with intermediate coolers. Maybe the motor+pumps can be reverted as turbines+generator, or maybe better not.

The real drawback is that the huge slow generator looses some 10% of the power. Everything else has huge maintainance advantages over any piston design.

By the way, maybe some of you have never seen a modern wind turbine... It's simply huge and fascinating. Here some examples:
www.enercon.de/en/_home.htm
click on E-82 (only 2MW)
or on Downloads > Booklets > "Product overview" and "Technology and Service"
I like especially the annular generators.
rethinker
Enthalpy
Thanks for the link, very impressive.
I see your point. Pistons and rings scraping along steel, just don't come close to the efficiency of the generator.
Confused2
As usual .. I agree with Enthalpy.

With a view to stimulating novel ideas about ways of getting air to a reasonable depth.

Imagine a vertical pipe from the surface to a depth of say 100m. There is a valve at the bottom which prevents water from entering the pipe .. the pipe is full of air. At the top of the pipe we introduce a plug of water/entrained air and let go of it. In principle the plug of water falls to the bottom of the pipe .. smashes through the valve at the bottom .. the entrained air is then released into a collecting bubble. Actually you need two pipes .. one to allow air ahead of the plug to escape. I'm thinking 100m as an 'ish' depth where losses might be small enough to allow the plug to retain enough momentum to break through the valve against the pressure at that depth. There's something a bit perpetual-motion-ish about this so it probably doesn't work .. but I admit I can't see the flaw* .

Just a thought.

-C2.

Edit .. I'm thinking area under impulse .. hmm..
Confused2
From previous post..

Starting the plug of water off from a point rather higher than the surface might overcome some of the perpetual motion problems.

Enthalpy
Hi Rethinker and the others!

Piston engines could have an acceptable efficiency; the huge annular generator has a very bad one at some 90%, as electric machines of the power should have well over 99%. But the big and slow generator (introduced by Enercon's founder, widespread meanwhile) is the winning option today because it avoids maintaining a gear.

One should imagine a gear of well over 2m diameter... Not really cheap neither. And gears need grease, to be checked and replaced periodically because teeth rub directly against another.

Now, piston engines have drawbacks similar to gears: wear, grease monitoring... This is where a directly coupled generator is better. It "only" needs a bearing - seen some on a fair, 3m diameter! Just one single bearing per turbine, with crossed (+45° and -45°) cylinders to hold all moments. Because the cylinders roll instead of rubbing, they wear far less than a gear or a piston.

The same idea applies to pumps. If you consider having a wind farm offshore with 200 turbines, you really want to have centrifugal pumps coupled directly with electric motors, so that maintenance costs are simply affordable.
Enthalpy
Hello Confused2 and everybody!

A water hammer does work. It's used in discontinuous high fountains. Usually, engineers try hard to avoid this destructive effect, but with proper dimensioning, one can safely exploit the effect for useful results.

So are you suggesting to couple a noria to the wind turbine, to rise the water to be used in the water hammer? At least for the first part of the design, I doubt a clerk grants you a patent.

OK, just kidding.

Hydraulic gears (= centrifugal pumps +turbine) do exist. They could allow to increase the rpm to what the 300b pump needs, BUT I believe they have a bad efficiency, and I doubt they can work as slow as a wind turbine does.

Unless somebody finds a usable (really usable, over 30 years) way to accelerate water to the speed of the airblades' tips, which is some 100m/s: this would provide 50b. I saw odd patents willing to put generators there.

Some air turbines have a vertical axis, especially the Darrieus type:
http://en.wikipedia.org/wiki/Darrieus_wind_turbine
this couples much more easily with a centrifugal water pump.
Darrieus turbines are uncommon, I ignore why. They would allow bigger generators, with a better efficiency. Their bearings must be more expensive.

-------------------

I wanted to mix small air bubbles tightly with water in order to cool the air as we compress it (and warm it when it expands), but if cooling is done another way, we can separate air and water and still use the water's weight to compress the air.

That is, we would alternate (with some separator that doesn't wear, of course) 10m air with 100m water in the pipe and need only 1/10th of the pressure. Still, the pipe is probably too expensive.
Enthalpy
To reduce the cost of the 300b D=72mm pipe in the "mainstream" solution (I mean, Wind -> Horizontal rotation -> Big slow generator -> Fast motor -> Multistage air compressor with intercooler), I was just thinking of an exotic technology to save material.

The technology was developed for the casings of Ariane 5 's solid boosters. In Chermany, of course. They use an austenitic stainless steel (nice for our sea stuff) that hardens a lot (I mean: 2000MPa) when cold-drawn.

They start with a thick and low, turned cylindrical forging of annealed steel. I think it's Aisi 301, but the seaworth 316L reacts in a similar way.

With a big, purposely-built rotating machine, they roll the cylinder thin and high, by pressing the wall with several roller pairs, inside and outside.

As the walls get thin, they retain their strength, as a consequence of this material's behaviour. And a very nice feature is that the ends of the cylinder can stay thick, allowing to solder on a big section, so that the seam has a good strength despite being annealed with these steels.

Sure, the same machine won't work on long narrow pipes, but maybe another machine? Cold-drawing the pipes excepted at the ends would save 3/4 of the steel amount. Maybe by introducing a kernel in the pipe, and pressing the wall against this kernel with rollers?
Enthalpy
Grouping several wind turbines to one pipe doesn't save much money. With figures:

For 10 wind turbines on one pipe, the inner diameter rises from 63mm to 176mm, the outer from 71mm to 197mm, and the price (at unjustified 7€/kg) from 140k€ to 1000k€ or 100k€ per turbine.

So this is not a very strong reason when making engineering choices.
Enthalpy
Centrifugal air compressor to 300b: nothing special.

Let's take a rotor speed of 240m/s (Mach 0.7) to limit the losses associated with pressure waves. This means a rotor of D=1.5m at 3000t/min, a common speed for electric motors, and a reasonable diameter for 3MW.

Depending on the design of the blades, one compressor stage gives a temperature increase of 29 to 58K, and multiplies the pressure by 1.38 to 1.86.

This is also a nice temperature increase, as we don't lose too much efficiency in our isothermal compression. Remember, we cool the air down to 300K between the stages, to avoid heating the air before it cools anyway at the storage bubble.

To achieve a compression factor of 300, only 9 to 18 stages are requested. Still reasonable - though 9 is better.

We may also consider a liquid ring compressor. Reliable, but is it as efficient as claimed?

Anyway, we wouldn't reduce much the number of stages, because the air temperature increase must be kept low for efficiency. But this compressor may allow a smaller diameter or a slower motor, for instance 1500t/min.

I don't believe the liquid ring itself can cool the air directly. However, spraying cooling water in the chamber could be an excellent method. This would permit to simplify the compressor: less stages, no heat exchanger more.

A small but cheap improvement could be to use deep (100m) cold sea water to cool the air, but warmer surface sea water to warm the expanding air at the turbine.
Enthalpy
Oops! At 300b, liquid ring compressors aren't far better than centrifugal compressors... Simply because water isn't that much denser than air. So diameter and angular speed can't be reduced that much - but the number of stages is lower.

However, we may spray cooling water directly in the chamber of a centrifugal compressor as well, especially if the water nozzles are on the rotor, to limit impact erosion. Less simple when it works as a turbine.
Enthalpy
As the stored energy is more or less the buoyancy of the air bubble, which is offset by a ballast (if we don't anchor the bag in the soil), I also had a look at an alternative solution:

Suppress the air bubble, and lift the ballast to store energy, sink it to recover the energy. Like in an old clock, just bigger.

This needs a somewhat heavier ballast, as it doesn't expand or get heavier at shallower depths, so we don't win the factor of 5.7 offered by air. Instead of 3800t, the ballast must weigh 21700t. This is about 12m * 30m * 30m stones, it would still be very affordable.

Also, an accident would harm seabed fish instead of surface humans.

What doesn't work here is the price of the cable (besides the price of the floating thing that lifts the ballast): It must pull 21700t over 3000m. Some 6000t of high-tech cable, completely unaffordable.

We might consider many smaller weights lifted or sunk one after the other with a single small cable... I feel it too insecure and complicated (fish a ballast automatically at -3000m), and this still needs the floating crane for the whole weight.
buttershug
QUOTE (Enthalpy+Apr 21 2008, 04:54 PM)
As the stored energy is more or less the buoyancy of the air bubble, which is offset by a ballast (if we don't anchor the bag in the soil), I also had a look at an alternative solution:

How do you figure that?
I would say it's the pressure of the air that provides the energy.

The deeper this project the less buoyancy.
Enthalpy
The stored energy is made by the compression of the air, as I put in my posts of Apr 6 2008, 06:13 PM and Apr 7 2008, 02:06 AM.

This should be clearer in the case we compress air at the surface and sink it later. The air gets enthalpy by compression as if it were in a machine surrounded by vacuum; then some water gets gravitational energy because it's lifted when we sink the air; and the atmosphere gives a bit of energy because the bubble has shrunk - but the position of the shrunk bubble doesn't change this contribution.

So to the PV*Log(P2/P1) I used, we shall:
- Add V2*(P2-P1) for lifting the Ocean
- And subtract P1*(V1-V2) because the atmosphere helps to compress the air.
Since P1*V1=P2*V2, we get just PV*Log(P2/P1).

If the process is different, for instance if the bubble is compressed by the water pressure when sinking, the absorbed energy is the same because the initial and final states are the same as in the former process. And in this new case, all the work is done by pulling the bubble to the seabed - in other words, fighting the buoyancy alone - provided one takes the actual volume of the bubble at each depth.

So both pressure and buoyancy energies are identical in this second case.

And as I don't plan to compress the ballast, I preferred to compare the bubble's buoyancy force and the ballast's weight, since they must be similar at full depth.

Between both, we still have the factor of log(P2/P1), or 5.7 for -3000m, which enfavours the expanding air over the stiff ballast.
buttershug
O so you are basically talking about some form of accumulated buoyancy.
And not the buoyancy of the outfit at the bottom.

I wonder at what depth it would lose all buoyancy.
Confused2
Hi buttershug,

Have a look at Boyle's law here :- http://en.wikipedia.org/wiki/Boyle's_law

If the volume at the surface is (say) 1 cubic metre .. that's where pressure (P) is one atmosphere. Since PV = k .. and we'll work with cubic metres and atmosphere's we get k = 1. The pressure increases by 1 atmosphere (roughly) every 10m of depth. So at 100m the pressure is 10 atmospheres and the volume is then 1/10 of a cubic metre. However far you go down the bouyancy just keeps decreasing with depth. At some point the air will weigh as much as the water it displaces .. I'm not sure I want to worry about that myself .. water is pretty heavy and air is pretty light.

-C2.
Enthalpy
Hello everybody!

Well, I've made all computations with air considered as a perfect biatomic gas, which is far from precise at 300 bar.

In fact, air molecules have their own volume, which is roughly the volume they occupy when liquefied. Liquid oxygen weighs some 1200kg/m3, liquid nitrogen a bit less.

PV=RT should better apply to the free void volume between the molecules, so when speaking of densities of 1.225kg/m3 *300bar or 370kg/m3, this is already 1/3 of the liquid's density, hence such correction would be needed. However, I don't care for a first intent.

More corrections would be useful. Most of them represent the attraction between molecules; they become more important at higher densities, but aren't very big because 300K is well over the critical points of nitrogen and oxygen.

Several semi-empirical laws are used; have a look at "Van der Waal's" and "Virial coefficients". Not very simple to use for hand computations...

Anyway, I've made more approximations here. For instance, sea water weighs rather 1015kg/m3 at the surface, mainly because of salt contents. This is also the value of seamen's "long ton": the buoyancy of 1m3 in mean sea water. Seawater's density varies - not little - with salinity, temperature, and also depth: take a compressibility of 3GPa, and 300b let the volume shrink by 1%.

OK, N2 and O2 can be packed at some 1200kg/m3 when liquid, so achieving 1000kg/m3 with gases might be possible, but not at 300b.
Enthalpy
And here are some considerations about how the centrifugal compressor-turbine may look like.

At 240m/s, it could be built with just 10 stages, but...

Multiplying the pressure adiabatically by 1.77 at each stage, the temperature rises from 300K to 353K, and we squander 53kJ/kg air, while the isothermal compression needs only 47kJ/kg. This means we lose 13% of the energy at compression, and again 13% when expanding the stored air. Plus all technological losses. Not satisfactory.

So we should either spray water in the air before compressing/expanding it - but this erodes the turbine - or rather use a smaller compression per stage.

Let's take 18 stages. Not very convenient, but at least this corresponds to a stable compressor (pressure-throughput relation). Then, each adiabatic stage multiplies the pressure by 1.37 and heats the air from 300K to 328K. We put 28kJ/kg in it where 26kJ/kg would be needed, so we now lose 8% twice. Better.

Designing the intercooler is easy, especially because we don't need to separate the air from the water; this just means that the cooling or warming water must be pumped to the air pressure. However, a classical design with pipes certainly is possible. Each stage should exchange 13.3kW with few K drop.

First design: we use many metal discs, spaced by 6mm air. They rotate slowly, their bottom is in water, and the air flows between them above the water. Then, each stage needs some 1m3 total volume with 3K drop.

Second design: we let air flow as bubbles through the cooling water. Smaller bubbles would be even far better, but I took D=3mm. Even if the air didn't move in the bubble like a smoke ring, each bubble would exchange 7mW with water at 3K drop, so we need 2 million bubbles. With 200*200 holes in 2m*2m, this makes a stack of 50 bubbles, or 1m height if they're 20mm apart. Take more surface and less height to augment the throughput. Looks cheap.

Letting water rain through the air would give comparable results. Also cheap.
rethinker
Enthalpy
Wonderful science topic.
How much deeper can your math go!

What about staged separate pumps at different depths to help with cooling? Because space is not as critical, pumps could have large heat sinks of fins to reach out to cooler water temperature's.

Would this use more or less energy?

How much pressure can the bags hold.
If you set off a small explosive after the bags were at max pressure from pumps, could this add more pressure?

Have you seen the latest hart pumps? The latest hart pump may hold features that can be adapted to your ideas. They are extremely long lasting,and very powerful.

edit: I see you were posting while I was writing and you address the pumps.
Enthalpy
Just to have a reference, I've compared the underwater bags with a flywheel, which is one of the least bad ways of storing energy.

Let's take a disk of steel (a torus would be equivalent). We need high strength for quite a massive piece and at low cost, which precludes both precipitation hardening steel and normal tempering steel. A good choice would be a spring steel, with some 1.8% silicon to stabilize the martensite at any depth. Also good for alternate and long-duration stress.

It can be used at 1000MPa with a reasonable safety margin. This equates 504m/s. The whole disk then stores 64kJ/kg and must weigh 10100t to store the same 650GJ as the underwater bag.

While the volume of 1290m3 is still conceivable (diameter = 20m and thickness = 4.1m), manufacturing it is difficult, and it's too expensive: at 3€/kg, just the steel would cost 30M€.

So the underwater bags look way cheaper than flywheels.
Enthalpy
Also compared with lead batteries. Forgetting about the very limited number of cycles and concentrating just on purchasing costs.

Just a small extrapolation from a 12V 80Ah battery weighing 27kg to the 650GJ used for reference: this is 205,000 times more, so expect 5500t batteries.

just the lead amount would cost 15M Usd or 9M€.

Also quite more expensive than the underwater bags. Other types of batteries have better performances but cost more per J than lead batteries, as far as I know.
Confused2
Hi Enthalpy et al,

I don't know anything about suitable electrodes for hydrolysis .. are there cheap ones? Rather than using compressed air you could make (and store) hydrogen at (say) 3000 m (saves pumping it down) and have a little pipe to let it up to the surface (I'm guessing hydrogen is more nutritious than compressed air). To get the electricity down thin wires you'd want to use a high voltage feed with a transformer on the sea-floor. Overall a nice combination of water, high voltages, explosive gasses and a little bit of engineering.

-C2.

Edit.. releasing oxygen at that depth would probably encourage the evolution of interesting new lifeforms .
Montec
Hello Confused2, et al.

Interesting, so it would be essentially a combination of this technology with this technology that would operate at depth in the oceans.

Edit: 5,000 psi equals about 3,515 meters of water depth. From this site.
rethinker
Montec and all
Reading Montec's links,I wonder if anyone can explain how this process takes place.

Quote:
>Water electrolysis systems
employing proton exchange membrane (PEM) cell technology are sufficiently mature and
able to generate high-pressure oxygen without the use of pumps or complex pressure control
systems. A single cell operating at 12.9 Mpa (1850 psid) oxygen, ambient pressure hydrogen,
has demonstrated over 50,000 hours of continuous operation with a volumetric energy
consumption of approximately 10 kW-hr/m3 of the net oxygen output. A high-pressure
oxygen generating assembly (HPOGA) is examined as a potential replacement for the ISS
Oxygen Recharge Compressor Assembly (ORCA). Operational and performance
characteristics of the HPOGA that would support the development of an oxygen recharge
capability for lunar and Martian bases utilizing in-situ resources are also identified.<

Is it causing expansion within the assembly?
Montec
Hello all

For those that are interested here is an Introduction to Fuel Cell Technology.

The operating pressure of a cell has a direct correlation to the cell's operating voltage.

Enthalpy
Electrolysis works. Its energy density is so much higher than underwater bags that nobody wants to combine them.

What still doesn't work perfectly is the production of electricity from hydrogen, on the long term and at low cost. This is where underwater bags are interesting, because they just assemble existing technology.

So as both technologies will probably never mix, I would consider them separately. Hydrogen is worth its own separated discussion, to my opinion.
rethinker
QUOTE (Montec+Apr 24 2008, 09:00 PM)
Hello all

For those that are interested here is an Introduction to Fuel Cell Technology.

The operating pressure of a cell has a direct correlation to the cell's operating voltage.

Thanks Montec, that helps tremendously.

Enthalpy, No plans to change topics here, just an interest in all aspects of energy.
Enthalpy
I've had a look at a diving bell, and I'm not convinced.

I considered two pairs of bells. Each pair hangs under a cable moved by a pulley, where one bell rises when the other sinks. The bells carry enough ballast to sink even when full of air at the lowest pressure. Power is fed and recovered mechanically on the pulley, as water pressure compresses and expands the air.

The two pairs allow to operate at constant power, as one pair accelerates when the other brakes. And some kind of heat exchanger keeps the temperature of the air in the bell equal to the water's one, not the biggest difficulty here.

Well, such bells have a really big drag, which is reduced by huge dimensions and lower speed.

In fact, we'd better build a staged lift. The first stage, with big bells for 1 bar, would go just to -30m for instance. In this way, the up-down cycles could be faster, allowing smaller bells. A second stage could go from -30m to, say, -100m: since the air takes already less room, this stage can use smaller and faster bells. Two or three more stages would reach -3000m.

Not really simple, is it? Now have a look at the dimensions...

The upper stage will move at 3m/s, then it has a cycle time of 10s+10s, so that the throughput of 5.3m3/s @1b needs a bell capacity of 53m3 each. This is for instance D=3.7m and h=5m. I pretend to achieve Cx=0.25, then each bell drags 12kN, consuming 36kW each, or 2*1.2% of the stored power.

The lower stages look a bit better.

Among the many complications of such bells: one shouldn't recover the compressed air at the top of the bell, or he loses >5m from the painfully obtained 30m depth, so tilting the bell would be better. And so on and so forth.

In fact, an inverted watermill wheel (immersed, moving bubbles) would be far better for the first 30m depth, but is inadequate for the last 2970m.

Coupling the ropes with the wind turbine is best done by electric machines, again. I can't imagine a clutch, and even less a speed variator, for such torques.

Altogether, it looks unreliable, complicated, expensive. Without any further development, I clearly prefer an 18-stages centrifugal compressor-pump with intercoolers.
Enthalpy
A less bad variant of the diving bells:

We use many more bells. So many that they compose a cylinder, or rather a cone, wide at the surface, narrow at the bottom. One big improvement is that the hydrodynamic drag drops a lot.

They are still individual bells within the cones: they are separated by ceilings within the cones, and have openings at their bases to let water in and out.

Two cones in a pair have opposite movements: one rises when the other sinks. Additionally, there are valves between the cones, yes - also a vertical oscillating movement driven from the top. The combination makes the air flow from one cone to the other at each half-cycle, so that the air sinks over many cycles when pumping, and rises when recovering power.

The air flow between the cones can be rather gentle, as we open the valves when the air-water interfaces are at the same height. Also, the vertical stroke is now smaller than with bells moving 30m under a cable, and cycle times can be shorter, allowing smaller volumes.

Among the non-obvious issues...
Transport or assemble locally cones of 3km length. OK, oilfield operations do it.
Place the valves at the right depths after 3km
Clean the valves at 3km depth

You know what? I'm still not convinced...
Enthalpy
And maybe an even less bad variant, because it works continuously, and rotates instead of oscillating.

It's an Archimedes' screw. Excepted that it's inverted: it works beyond water, and moves air bubbles among much water instead of water volumes among much air.

http://en.wikipedia.org/wiki/Archimedes'_screw

The movements would be as in the animation in Wiki, but the construction needs to be adapted a bit.

For our needs, the outer shell rotates. It is conical as usual, as we need less volume at depth. The screw's wall are soldered on the outer shell, for instance through holes in the shell filled by solder, or because the shell is composed of a helically rolled sheet. And the screw has no kernel at all - in other words, its center is open through the whole depth.

In operation, the screw moves water as well as air, in the same direction. But as the diameter shrinks at depth, the water throughput decreases as well, needing the center hole.

Also, water has different densities at the surface and at depth, and the pressure difference would easily be 10m at mid-height, or much more than a screw pitch. To avoid this, water holes (tubes) between the center of the screw and the outside are necessary from place to place.

One very nice feature is that the metal is alternately in contact with water and air, working as an excellent heat exchanger. It needs improvement only at the shallowest depths; put more screw walls there, or additional hairs that let water spurt, or something similar.

Now, this screw can have a rotation period quite faster than the cycle time of the diving bells, reducing the volume. The limit is just the water's friction on both sides of the hull and on the helix.

Among the nontrivial issues:
- The amount of metal needed. And the screw must be tilted, pity.
- How to catch the bending forces on the screw. I can't imagine holding it just at both ends. Maybe we need several sections, and one or two solid towers to hold them.
- Assemble a thing 1.5*3km long at sea.
- Torques are big again.

This one looks like the least bad method with undersea movement, especially as it could achieve a reasonable reliability, but I still prefer the centrifugal compressor and a pipe.
Enthalpy
But for compressing air at the surface and feeding it through a pipe, here is a method I prefer to the centrifugal compressor-turbine: it's a rotary screw compressor.

Unfortunately, there is no animation nor drawing at Wiki
http://en.wikipedia.org/wiki/Rotary_screw_compressor
so those who haven't seen one must try to imagine it, not really obvious:

It comprises two screws, with little metal and much air. These screws have opposite pitches, parallel axis, and are close enough that their threads fully interpenetrate, up to contact. Also, a precise casing closes the volume around both threads - so the casing has two overlapping holes, one for each screw.

Still there? Each thread step is closed, mostly by the casing, and by the interpenetration by the other screw.

No, the screws rotate in opposite directions, synchronously, so that the volumes enclosed in each thread step move parallel to the axis. This is a pump.

Small refinement for a gas compressor: the volumes decrease at the high-pressure end. There, the diameter is smaller, the thread shallower, and the pitch can be smaller. Not easy to manufacture, but we have CNC machines nowadays.

Such compressors run often at moderate speeds, but I would run them Allegro so that leaks are less critical - though not at 240m/s.

One very nice feature is that we can spray cooling water in the compressor itself, so that no exchanger is needed, and compression ratios can be high - maybe 2 steps for 300 bar. Erosion by water is reasonable, as we don't use turbine speeds.

I would really use immoderate amounts of cooling water, I mean, a volume not negligible compared to the air. Combined with the fast rotation, it helps keeping the compressor airtight. And let it spurt and spray the big way, as we must evacuate 240kW in a reduced volume.

I know there are hairsplitters among you (where is A.? I haven't read him for weeks, how is he doing?), who will suggest that a rotary screw compressor needs a gear to synchronize both screws and transmit half of the power from the motor to the opposite screw, and gears wear out, and all that stuff. But here comes the answer (in fact, for every rotary screw compressor):

Use TWO motors, one on each screw shaft, so that little torque must be transferred through the gear, which won't wear out then. Even better, put sensors on the gear, and regulate the power among both motors so that no torque at all is transferred. The gear works only as a synchronizer then, and as a passive safety.

One very nice feature is that this compressor works as an engine as well. Reuse also the cooling circuit as warming and the electric motors as generators.

Clearly my preferred one.
Enthalpy
I've tried to put some figures on the rotary screw compressor, it looks feasible for a pressure of 60b, corresponding to a depth of 600m. Then the throughput must be 0.12m3/s at 60b and 7.3m3/s at 1b.

Though common use it to have screws with parallel axes, I considered putting them nearer at the high-pressure side. This gives more room to put two electric motors side by side, and helps making the screws airtight.

Assuming that 30% of the section of the screws contains air, we can rotate them at 50Hz, have a big diameter of 0.8m and a small one of 0.2m, with pitch varying from 0.48m to 0.13m.

This means peripheral speeds of 126m/s and 31m/s. They guarantee that compact water will gather at the casing, but are low enough to limit erosion - in fact, some Pelton turbines work with higher water speeds.

With pressures of 1b, 2b+, 5b-, 10b, 20b, 30b, 40b, 50b, 60b at 1 turn intervals, and with 4-lobes screws, the maximum pressure drop per lobe is 2.5b. Having enough water in the chamber, all clearances will leak water instead of air, at 23m/s - limited by water's inertia, not viscosity.

Leaks are bigger at the high-pressure end. With a clearance of 0.2mm at the radius (not much, if you remember the complicated form) to allow some thermal expansion, leaks have about 2cm2 available and amount 4.5dm3/s: this is <4% of the pumped volume and power.

For higher depths and pressures, leaks become more difficult for the screw compressor. I evaluated them to some 13% at 300b and 0.12mm radius clearance: uncomfortable. One would need tighter tolerances, better controlled thermal expansion and deformations by pressure, which is not obvious, or some kind of non-wearing gasket, which isn't neither.

One nice feature of these water leaks is that, with 23m/s, they can serve to produce the cooling spray. The pressure drop at a screw lobe is nearly the one available at a water tap, which makes nice jets. In case leaks have a bad form, one can drill small holes at the screws' periphery.

All put together, I'm not quite sure the rotary screw compressor is better than a centrifugal one. It does eliminate the separate coolers, but it needs big precise parts, a gear, two motors instead of one, puts heavy axial loads on the bearings, and looks more difficult at 300b.
Enthalpy
"Small" improvement to my estimation of the interstage coolers, over the figures I posted Apr 24 2008, 01:45 AM...

We don't need to exchange 13.3kW heat between each of the 18 centrifugal compressor stage, but 167kW.

So multiply all surfaces by 12.6 or use metal disks put closer toanother or smaller air bubbles or rain drops. By chance, we had some headroom left.
Confused2
Nit-picking ..

We have to allow allow for the floating bit (I assume it floats since I'm thinking of vertically above 300ft of water) goes up and down .. partly tidal and (worse) waves. This precision rotating thing probably wouldn't like that .. also it will get driven off station by x metres .. can your design handle that?

-C2.

Enthalpy
These are nontrivial issues, but fortunately, the oil industry has already solved them. Ask them, I'm not in mood to reinvent it.

Marine wind turbines are built on the ground of shallow waters up to now, some 30m deep.

The underwater bag would need deep water, 600..3000m.

We can either let the wind turbine float atop the underwater bag on deep waters, and take advantage of stronger winds there, or rather transport the electricity. This technology is required anyway, and already exists; they make it in DC under rather high voltages, with electronics at both ends of course.

DC high-voltage lines were already built for the Itaipu dam 30 years ago, as only thyristors were available. With today's Igbt, it's a small effort.

==========

Did you mean: the air compressor-turbine wouldn't like movements? No worry, movements are much worse in a jet fighter.
Enthalpy
And here is a cooler adapted to 167kW, for use with the centrifugal compressor-turbine.

Still with metal disks that rotate between air and water - but more disks with a narrower gap.

The airgap between the disks is only 1mm. The disks could be 1mm thick as well. Spacers stabilize the gap.

The disks have OD=1.2m and a big hole ID=0.8m at the center. Air flows from the center outwards. There are 450 disks, totaling 0.9m length.

With that surface (over 500m2) and gap (1/6mm distance in laminar flow), the air is just 2K warmer than the disks when exchanging 167kW, not bad.

And in laminar flow at 3.8m/s, the air pressure drop is only 165Pa. This drop would allow an even narrower gap and smaller exchanger, but capillary action will limit the gap somewhere.

This is for the lowest pressure stage. Higher pressures make the exchangers easier.

The air should be filtered before entering the compressor. I suppose any residual air dirt collecting on the disks will be washed away by the water, especially if water is circulated in the direction opposed to air (inwards then). However, I ignore how to reduce dirt, salt and scale deposit from sea water, but other people know it.

Bubbles and rain coolers still look very possible alternatives.

And I still ignore if I prefer the centrifugal compressor+cooler or the screw compressor with spurting water inside.
dextercath96
I used underwater bags and I think it is good to use. I charge it with the solar power. I think it stored energy.
ziyue2010
oh...........
marchse
underwater storage has two advantages over underground facilities. Underwater storage is not as limited by a location and the pressure in the undersea Energy Bags is constant, which means that turbines can be used to convert the air back into electricity in a relatively efficient way.

Power Plant Development
Enthalpy
QUOTE (panbrian+Jun 17 2010, 01:19 AM)
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.

This is precisely what I suggested several pages ago.
Not too tired after this effort?
Enthalpy
Prof. Seamus Garvey's underwater bags have been prototyped.

Here by the supplier of Vectran, used to anchor the balloon at the seabed:
http://www.vectranfiber.com/NewsInformatio...gy-Storage.aspx
which is a May 6, 2011 reprint from the Energy Harvesting Journal:
http://www.energyharvestingjournal.com/art...ge-00003358.asp

40m3 at 600m depth offshore Scotland, with the bag made by Canadian's Thin Red Line, the prototype being supported by the electricity supplier E.ON. Plans exist for 6000m3 bags.

Looks like this idea has momentum.
My enthusiast and most sincere best wishes!
El_Machinae
I really like the feel of this concept. It intuitively makes sense. And, I love it because it just never would have occurred to me.
Robittybob1
QUOTE (Enthalpy+Nov 9 2011, 02:12 PM)
Prof. Seamus Garvey's underwater bags have been prototyped.

Here by the supplier of Vectran, used to anchor the balloon at the seabed:
http://www.vectranfiber.com/NewsInformatio...gy-Storage.aspx
which is a May 6, 2011 reprint from the Energy Harvesting Journal:
http://www.energyharvestingjournal.com/art...ge-00003358.asp

40m3 at 600m depth offshore Scotland, with the bag made by Canadian's Thin Red Line, the prototype being supported by the electricity supplier E.ON. Plans exist for 6000m3 bags.

Looks like this idea has momentum.
My enthusiast and most sincere best wishes!

Large plastic tubes buried in the sea floor might work better. Less wear and tear and the weight of the sediment above them will keep them fastened in place.
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