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# Do Giant Compressed Air Underwater Balloons Store Energy Well?

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OK, here are a list of the claims for the energy storage, used to make wind power baseload.

The basic outline:

* A new approach for wind: these wind turbines will float far off the coast and not be visible from land.

* They will compress air, not generate electricity.

* The compressed air is stored in large rubber balloons deep under water, about the size of your house.

* These balloons use the pressure of deeper sea water to maximise the pressure that the air is stored at, making the rubber materials cheaper than trying to store all that air in steel strong enough to take compressed air on land.

* With good wind, the turbines blow the compressed air straight into generating electricity. When the wind is low, the balloons take over supplying the compressed air to move the turbines.

* It’s cheaper than any storage so far: Batteries are at about $500 thousand per mWh, Pumped hydro is about$80 thousand per mWh of storage, but these compressed balloons are only about \$1 thousand per mWh!

* Claims that the whole UK could run on wind without Brits even seeing the turbines because they are all so far off-shore!

But a pro-nuclear guy I respect as level headed and very technical replied:

"I would like see just how they intend to expand the air isothermally, and still make it do work. Look this is hand waving around the thermodynamic issues that plague all CASE systems. When you calculate the amount of heat involved it turns out to be huge.

This is the other issue, trafficking in large amounts of heat is not trivial, it has to be dumped out in the compression stage, which means some sort of cooling, and it has to be added back during expansion very quickly, and in huge volumes. One way or the other that is going to involve burning natural gas, or some other fuel. There is no other way around this problem.

Thus this idea is just another way renewables are being used as a Trojan horse for natural gas"

Seamus responds — in another discussion with another person raising the same argument — that:

To Rich (July 23). You calculation for pressure is wrong. 750m of water gives you 7.5 MPa, not 20 MPa. The stored energy density at that level is between 33 MJ/m3 and 65 MJ/m3 depending on how much you reheat the air recovered from store and whether you reheat it between expansion stages. The lower figure is extremely conservative and it involves expanding the air “isothermally” at about ambient temperature. Let’s take 40 MJ/m3 as a reasonable energy density. I agree with your approximate volume calculation so the stored energy in one bag would be around 160 GJ. That is equivalent to over 45 MWh – about 380 times more than the 120 kWh which you suggest !

(from second comment down, under the show description and transcript)

http://www.abc.net.au/rn/scienceshow/stories/2010/2952227.htm

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But a pro-nuclear guy I respect as level headed and very technical replied:
When I read the title, I immediately gave a bit of thought to the thermodynamic issues but I don't quite agree with those objections. If the deep water is no warmer than the environment at the surface, good design could make it efficient. Further, if the temperature is significantly lower down below, the difference could even be exploited to draw a bit of thermal energy from the environment, while you're at it. It all depends on how feasible a good design would be.
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according to this (i can't vouch for its accuracy though, it just seemed reasonable to me), the temperature at depth is between 3 and 6 degrees, and on the surface between 4 and 28 degrees. so if done on the equator, there could be some potential to that idea. http://residualanalysis.blogspot.com/2010/02/temperature-of-ocean-water-at-given.html

personally i'd be more worried about friction and viscosity losses over such a large length of piped air, i think.

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I like the idea. Obviously, pumping the balloons will result in heat generated at the compression phase, but being in the sea, this could very easily be shed into the ocean itself. Vanes and radiators in the water where the pipe enters the balloon should shed enough heat so as to not be a problem. When the compressed air is used, i.e. the balloon deflates, the same system of vanes and radiators will extract enough heat from the ocean to warm the pipes and vents so as not to freeze over. The ocean is a vast thermal store, and the greenies should be satisfied that all the thermal energy you dump into the sea at the compression phase will be removed at the decompression phase.

I don't really see a problem here, except for why the balloons should be underwater. The best scheme would be to inflate them on the surface, and then physically sink them and deflate them for extra oomph when they're on the bottom. But this implies attaching, lifting and lowering weights that will probably eat up any energy you could generate with such a scheme. This will only be beneficial if you have enough overflow energy (when it's windy) to be used to lift and raise any weights you might have attached to your balloons.

I think a much better scheme will be to sink huge turbines into the ocean, anchor them with cables to concrete anchors so that they float at a depth where the strongest streams are and with weights, balances and steering planes have them face stable currents face-on. The Benguela current off the South African coast goes like stink every hour of every day, regardless of time or season. And it's millions of tons of water, generating much more torque than mere wind can. The Gulf Stream is another potential candidate. I think it will take many generators like these to have any effect on the streams at all.

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1. Would vanes like that really bring the heat back quickly enough to decompress the air and produce enough high pressure to run an electric turbine?

2. I understand the attraction of high-torque sea currents and gulf streams, but the corrosion and sheer forces involved are proving a challenge for the engineering. If materials science increase to that point, we've got it made. But if not, then GenIII reactors are good enough for me until GenIV comes along.

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As to corrosion on machinery built to capture energy from ocean streams, I can't see that the demands on such materials should be any greater than on a balloon system. You can use fibreglass turbine blades, for instance, and seal off anything remotely metallic. I don't know. I think in both situations your biggest problem will probably be to properly insulate your transmission lines back to land.

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1. Would vanes like that really bring the heat back quickly enough to decompress the air and produce enough high pressure to run an electric turbine?
What I would do is to have it absorbing back heat as it gets up to the top and have it reach the highest available temperature before entering the turbine. Likewise on the way down, but have it absorbing the heat uncompressed and design the compressor to be as isothermal as possible.

I forgot to say, in my first post, if the temperature down below varies predictably it is more advantageous to increase air storage when cold and use it more when warmer.

The best scheme would be to inflate them on the surface, and then physically sink them and deflate them for extra oomph when they're on the bottom. But...
Indeed: But. Enthalpy is enthalphy and there's no such thing as a free lunch unless there is a free source to exploit. Temperature difference is one such source, so are harnessable currents and winds. Even using these to draw balloons down, as a way of compressing the air, wouldn't be as good as isothermally compressing the air when already cold.
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Well, the idea is to hoist them when the wind is blowing and you have surplus energy, and to sink them when the energy demand goes up. Nobody's expecting a free lunch - but there are times when the energy production exceeds demand.

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hoist them when the wind is blowing and you have surplus energy, and to sink them when the energy demand goes up
It seemed to be the other way around, in order to exploit the compression; I only pointed out that isothermal compression is better.
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Well, you're obviously gonna have to have some kick-*** weights tied to your balloons to make them sink in water. So, when the energy demand goes down and you have surplus, you can hoist them up to sea-level (together with their weights - that'll take some juice) and then once inflated, you can just let them sink again to let the increasing water pressure provide you with the necessary compression. Doing it the other way around will imply hoisting the attached weights while there is an energy demand.

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Well, you're obviously gonna have to have some kick-*** weights tied to your balloons to make them sink in water.
It isn't inevitable if you are exploiting the wind source to drag them down.

Doing it the other way around will imply hoisting the attached weights while there is an energy demand.
Not if buoyancy is greater than weight.

Now, if you aren't exploiting a temperature difference, you're only making storage, then the weight only partly offsets the storage effect of compression and, vice versa, buoyancy offsets gravitational storage. Therefore I don't see the point in added weight if you want to compress by dragging balloons down, nor of the water if you are hoisting weights for storage. It only seems to complicate things without an improvement.

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To your first point, it isn't inevitable, no. But you don't want to use your primary energy source for mundane things like hoisting weights while you've got an energy demand from the grid. That's to be done when you've got surplus.

To your second point, I wouldn't bank too much on any sort of temperature gradient between the surface and the deep ocean. It's not much. Somewhere in the region of only about 20 degrees Celcius. The whole point in blowing the balloon up on the surface is that it will be a low-pressure operation (low-tech/cheap) and only has to be sunk with a weight system that can overcome the buoyancy of an inflated balloon of given size (low-tech/cheap). The increasing water pressure will compress the balloon and lower the buoyancy considerably, until right at the bottom of the circuit (depending how big your ballon is and how much you've pumped into it) the balloon will merely be a thin sausage hanging in the water. Consider this the inverse of weakly-inflated weather balloons, expanding as they rise. The alternative is to have your balloons on the bottom with a pipe leading from the surface to inflate them. This pump will have to overcome the kind of pressures that kill submarines (high-tech/expensive). For optimum pressure, your balloon should let go of its air cargo whilst on the bottom, implying that they will have to be lifted manually with no buoyancy at all, which can be done with sufficient gearing and energy when there is low demand from the grid.

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The problem with submarines is that they gotta have living quarters for folks inside them, also they can't have their outer shell just permanently solid, once and a while they surface and a bunch of things have to work. Sturdy tubes for high pressure differences OTOH are commonplace in industry and pressure difference is the very purpose of compressors and turbines.

But you don't want to use your primary energy source for mundane things like hoisting weights while you've got an energy demand from the grid.
You seem to be losing track of the two opposite things here.

I wouldn't bank too much on any sort of temperature gradient between the surface and the deep ocean. It's not much. Somewhere in the region of only about 20 degrees Celcius.
That's quite exploitable. We're talking about a great thermal capacity environment, even without high efficiency one can gain quite a bit of extra power.

For optimum pressure, your balloon should let go of its air cargo whilst on the bottom, implying that they will have to be lifted manually with no buoyancy at all, which can be done with sufficient gearing and energy when there is low demand from the grid.
You want to bring it up to the surface, reaching the turbine without having decreased pressure and exiting it at atmospheric pressure. To do this for each ballon drawn down strikes me a bit more complicated.
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Some misunderstandings, NIMROD Energy vs. the Solar Grand Plan

I think this thread shows some misunderstandings of Seamus Garvey’s NIMROD Energy Ltd. wind turbine + “energy bag” system.

That system doesn’t store energy by submerging buoyant air bags in the ocean, then generate it by allowing them to rise. Rather, flexible neoprene rubber bags affixed to the sea floor are pumped up with high pressure air by piston pumps incorporated into very large horizontal axis wind turbines (vertical axis turbines aren’t allowed), and a compressed air jet, either from the turbine or from the bags, is used to spin a (presumably enclosed) turbine connected to an electric generator. This 23 Mar 2010 article has the best pictures, both of prototype air bags and drawings of his unusual (and huge – 300+ m) “bicycle pumps like spokes on a wheel” wind turbine/compressor.

The advantage of using submerged air bags instead of conventional compressed air tanks on the surface is that they can be light and cheap, relying on the pressure of the surrounding water rather than the strength of tanks.

Garvey acknowledges that underwater bags aren’t the ideal choice for compressed air energy storage – airtight natural underground formations, typically leached-out salt deposits, are better, and have be a popular candidate for such use for many years. Such formations are not found everywhere you’d like them to be, however, so a solution like NIMROD energy bags, which can be installed anywhere there’s deep enough water and good underwater anchor points finds its niche.

The heat problem is one of efficiency. Since the bag walls will have limited insulating ability, they will loose a lot of heat into the surrounding cold water, wasting energy. Whether this inefficiency is offset by the low cost of the system is a detail that I’ve not, but presumably Garvey and other proponents of the system have, explored in detail.

Garvey’s plan has interesting similarities to Zwiebel et al’s “Solar Grand Plan” (1.3 MB PDF). The SGP, however, relies primarily on reflector-concentrated photovoltaic collectors connected by efficient DC power lines to compressor/generator plants at large natural underground air storage sites.

My bottom-line, intuitive judgment is that the SGP’s

solar cell [imath]\to[/imath] electric line [imath]\to[/imath] compressor [imath]\to[/imath] compressed air storage [imath]\to[/imath] compressed air generator [imath]\to[/imath] electric line [imath]\to[/imath] consumer

scheme is better, on a large scale, than Garvey’s

windmill/pump [imath]\to[/imath] short air hose [imath]\to[/imath] underwater bag [imath]\to[/imath] short air hose [imath]\to[/imath] compressed air generator [imath]\to[/imath] electric line [imath]\to[/imath] consumer

one.

A big concern I have about a system that relies on wind power from a small region is that wind vs. a solar one is often very slow for long periods, while the sun rises and sets as surely as ... well, as the sun rises and sets.

However Garvey’s has the advantage that it can be implemented on a small scale – a single turbine/bag/generator unit to power, say, a private residence or remote resort hotel, while the SGP is, well, grand, and needs the cooperation of many private companies and state governments.

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I think this thread shows some misunderstandings of Seamus Garvey’s NIMROD Energy Ltd. wind turbine + “energy bag” system.

B hadn't misunderstood, he simply voiced favour of an alternative. I also think Garvey’s idea can be improved, only in a different way.

(vertical axis turbines aren’t allowed)
? I don't get this.
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(vertical axis turbines aren’t allowed)
? I don't get this.

Garvey puts it like this:

My turbine is dramatically different: a horizontal-axis machine with eight blades — four long and four short. A floating framework replaces the tower, and it converts wind power internally within the blades. Think of a bicycle wheel rotating slowly, and a loose bead on each spoke. The beads represent pistons travelling back and forth inside tubes in the blades, compressing air as they do so.

(from another times online article, this one by Garvey:
)

So the wheel must be rotating on a horizontal axis, so its compressor pistons can be driven by their own weight.

It’s easy to get fixated on his system’s Energy BagsTM, and fail to notice its big departure from conventional generator-at-the-hub wind turbine design. Garvey writes briefly on this in the article I linked. In short, his system has big, slow-turning turbines, while the current industry standard is smaller, fast-turning ones.

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