Whole-Ocean Gravity Battery

Fri, 05/23/2008 - 05:05 — cbthiess

Because a Seastead is in deep water, it could use a different kind of gravity battery which might be very cheap.
First, sink a concrete dome/box right to the bottom. Connect it to the surface with a hose. Pump compressed air down to store energy, run it in reverse to generate electricity or mechanical energy. How is this a gravity battery? Because pumping air down the hose raises the whole world's sea level. :)
This should be very efficient, assuming you reheated the decompressed air.
How much energy could it store? At 1000m, a cubic meter of air would displace 1000*1000 kg of water 1m upward, so 1e7 joules, or 2.8 kWh. Since normal concrete weighs ~2400 kg/m^3 (~1400 kg/m^3 submerged), you could comfortably store 1 m^3 of air per 1 m^3 of concrete. It wouldn't need to be strong, just heavy.
What would it cost? Not sure. But concrete is cheap (~$85/m^3?), and forming it is cheap. You'd need 1000m of hose with the top at ~1500psi, whose required diameter would scale with power requirements, not energy storage. A heat exchanger to warm up the decompressed air, also scaling with power. And of course an air compressor/generator, also only scaling with power.
Doubling the weight after displacement of the ballast will double the storable energy, as will doubling the depth to which you sink the device. Halving will do the opposite.
Do you see any problems with this approach? Have any thoughts on a realistic price?

Infrastructure

Fri, 05/23/2008 - 06:47 — thebastidge

I'll throw the easy ones

I'll throw the easy ones first:

What is the energy source you're using to power your pump?
What are you using to power your air reheater?
How long do you think you hoses would last trying to pump tons of air down 1000 meters?
"Not strong, just heavy"- What happens when your "stored energy" (pressure) gets low- how does your dome, or even worse, your square box, stand up to the pressure at 1000m depth?
How long would your pump last and how big would it have to be?
Why do you think this would raise the entire sea level?

Not even the different oceans are at the same level. The Earth is not a perfect sphere, and the variances in its shape (and therefore gravitational attraction not being uniform across the entire surface of the planet) , the effect of the moon- all cause bulges in the level of water at various places and times. These are called tides. You know your device would only displace the volume of the box you put into the water, right? No matter much pressure you put into a concrete box, it stays the same size (until it breaks). What you're thinking of might get some of the physical effects your looking for if you tethered a balloon to sufficent ballast to hold it under water. But essentially if all you're looking for is a place to store air pressure to use later (and what would you use it in, some kind of turbine?) then you have much less problems simply pumping it into high pressure steel tanks on the surface..

Pumps are not 100% efficient devices. Neither are turbines to convert mechanical energy into other forms of mechanical or electrical energy. This sounds lilke a system that would take a whole lot of energy inputs and might put out some tiny fraction of what you put in (if anything), even if you could overcome the physical problems inherent in building such a system. Heat engines of some kind are usually the most effective at accomplishing work.

See: Possible-Effective-Elegant-Efficient-Economical

Tue, 05/27/2008 - 18:04 — Sundiver

Subsea surplus energy storage, air based

So here's the concept- store air under pressure with excess energy to utilize for peak requirements. The most practical energy solutions for a seastead are wind, wave, solar, currents, and otec. Otec is constant but may not be economically possible for the near term. Currents may be a constant energy source but are location dependend. Wind, wave, and solar are too easy to ignore. They are inconstant so storage is how to deal with the inconstant source and peak loads.
The mechanism is easy and relatively cheap. The storage device is a large bag that is designed like a lift-bag or high-altitude balloon. It is anchored to the bottom by a suction pile. A suction pile is simply a pipe with one end open. Put the open end on the bottom and pump out the water and it sucks itself down . They are used to moor rigs. A hose runs to the bag to conduct air both ways. When there is surplus energy pump air down, when energy is needed, use the air to power air-motor generators.
This is an order of magnitude cheaper and more reliable than hp storage tanks.
To adress the rest of the issues above-
I don't understand what the reheater is about. Not necessary.
Hoses- 1000 meters is 1446psi. That's not even considered high pressure in the industry, it's medium pressure. I'd incorporate the hoses in the platforms mooring lines (which would probably be synthetic anyway) and use the platform's anchors to anchor the storage units. This lowers the cost of energy storage to incidental btw. The hoses then would be designed to last 20 years.
“Prosperity is only an instrument to be used, not a deity to be worshiped.”

Fri, 05/23/2008 - 11:26 — Jesrad

Pressure delta impractical

Compressed air has a lousy efficiency in both storing and restituting energy. You waste a lot of energy as heat when compressing it in the first place, and you can't get that energy back from the environment when expanding it back because it would take eternity to heat the air back perfectly. You also store not much energy in compressed air, you need a lot of it and insane pressure just to get any decent amount of power from your system and not have a ridiculous autonomy. That's why the real world uses large heat differences or phase transitions instead of pressure differences in the thermodynamic cycles at the heart of the engines that power our stuff.

Tue, 05/27/2008 - 18:07 — Sundiver

Its being done

Pumping air for energy storage is not a new idea. The idea of energy storage is not governed by efficiency, efficiency is irrelevant in the sense you're using it. We are looking to store EXCESS energy so efficiency is farther down the list. Higher on the list is workability and costs.
There is no reason to waste heat on a floating platform. There is no need for an air reheater. However the heat of compression could be used for heating domestic water or space and the expansion cooling could assist air-condition and refrigeration.
At 1,000 ft depth the pressure is 500psi. That's pretty sane.
“Prosperity is only an instrument to be used, not a deity to be worshiped.”

Tue, 05/27/2008 - 22:20 — thebastidge

500 PSI is barely enought to

500 PSI is barely enought to power an impact wrench for a few minutes. Completely impractical for a storage battery, considering the cost, the possibility of failure requiring replacement of the system, when easier, cheapr, more maintainable solutions exist.

Wed, 05/28/2008 - 11:53 — vtoldude

The pressure doesn´t say

The pressure doesn´t say how long you can power stuff. You need to know the size of the pressure vessel for that.
Pressure-wise i´m sure you can drive power tools just fine on just 100psi or so. Forever, if your compressor tank is large enough and the tool doesn´t break.

Wed, 05/28/2008 - 13:42 — Jesrad

Power limit

The pressure may not define how long you can power things, but it places a hard limit on the power you can get. Pull too high a power from a given pressure, and your efficiency drops + the gas starts liquifying because you cannot heat it enough and cannot get the gas to move through the hose fast enough. It is a limit quickly attained, if I read my history of technology right. In fact this is a MAJOR issue in supersonic and hypersonic wind tunnels, see this website about NASA's wind tunnels.

Wed, 05/28/2008 - 15:07 — Sundiver

Wait a minute-

Thebastidge, I don't mean to flame but you seem to be speaking with authority and knowledge on this subject. Impact wrenchs run on 80-120psi. Most of the seals would be damaged in a few minutes at 500psi. You're confusing pressure with volume. It's like voltage and amperage.
Having spent some number of decades living and working in the oceans, depending on compressed air for my life and using it everyday in my work, I'm confident at this point that I can speak with authority on the subject. In this particular application (that of an exisitng, anchored platform in deep water), there is no easier, cheaper, more maintainable solution than deepwater air storage.
You hit on one of the most important factors for ocean operations- maintainability. It's a tough one. This concept uses off the shelf, extremely simple, industry standard equipment. It's scaleable and expandable. We may be talking about different ideas- I'm referring to my post "Subsea surplus energy storage, air based".
I'm not sure what Jesrad is talking about. I am sure it doesn't apply here.
It's likely that airmotor gensets would operate on 200-300psi. The air would be stored near the bottom to avoid too much stress on equipment. The deeper the storage bags, the more air they will hold (because of compression). If stored at 2000ft, the working pressure would be 890psi. It would be regulated down to the operating pressure of the airmotor genset.
"It is a mistake to think you can solve any major problems just with potatoes." -Douglas Adams

Wed, 05/28/2008 - 15:23 — thebastidge

I stand corrected

" Impact wrenchs run on 80-120psi. Most of the seals would be damaged in a few minutes at 500psi. You're confusing pressure with volume."

However, Jesrad's point stands. You quickly reach a point where the power stored isn't sufficient to do some things.

As an example, the Mars and Jupitr explorer probes have very efficient, super low thrust engines. they have to plan any vector changes far enough in advance to make a little push correct itself over a long distance. You simply cannot make a sharp vector change. sometimes you need a brute force approach even if it is not the most efficient. That's what I meant by my screed on efficiency, effectiveness, elegance, etc.

I still say, that for less cost, you can store more pressure, and probably larger volumes of compressed air on the surface, and maintain them better and recycle the materials later.

Fri, 05/23/2008 - 15:19 — cbthiess

Answers to Objections

What energy source powers the pump? Any mechanical or electrical source you have on hand, like windmills or solar. Cheap energy storage is only really useful and necessary when you have an unreliable primary power source.
What would power the air reheater? Ocean water with a heat exchanger. Essentially, some more pipe. Fouling might be the biggest concern.
The 'hose' might just be a narrow-gauge steel pipe. Again, you have fouling and corrosion issues, but a pipe strong enough for 1500 psi is actually pretty common.
The dome or box wouldn't have to be strong because it would have an open bottom. It would just need to withstand the bouyant force of the air. The pressure at that depth is provided by the weight of the water above.
Not sure how big the pump would have to be, but apparently small, efficient, multi-kW motors/compressors exist for compressed air cars. Just need to attach an electrical generator/motor to the shaft. How long would it last? Probably longer than a high-heat diesel engine.
Why does it raise the whole ocean? Because when you displace water the sea level rises. Adding air displaces more water (again: the dome/box is open-bottomed).
I don't know exactly what the fixed cost of hose/pipe, compressor, heat exchanger, and generator would be, but they all look pretty simple to me, and could probably be purchased off the shelf. The only incremental cost for more energy storage is the concrete shell, and that should only have material costs around $30/kWh at 1000m depth. That's very cheap compared to batteries.
Finally, turns out this approach has precendent. http://en.wikipedia.org/wiki/Compressed_air_energy_storage The energy storage and recovery sound well solved and efficient. The problem has been storing the compressed air. The article actually points out something much like this very method (using balloons, not shells), but then goes on to say the deep ocean is usually not close at hand. Seasteads, though, do have it close at hand.

Fri, 05/23/2008 - 15:24 — thebastidge

again

Windmills or solar are going to pump air 100m down underwater to a highly compressed state?

"Ocean water with a heat exchanger. Essentially, some more pipe. Fouling might be the biggest concern. "

You're planning to compress a bunch of air into a very cold environment. That might be counteracted a bit by the temperature rise due to compression. But then you want to release that pressure, on air that has bled its heat into the bottom of the ocean. By the time you get that air really moving on the way back out, you'll probably freeze CO2.

"The 'hose' might just be a narrow-gauge steel pipe. [...] a pipe strong enough for 1500 psi is actually pretty common."

Internal pressure yes- that's tensile strength. How about in crushing compression? What lengths do they make it in and how will you connect those lengths?

"The dome or box wouldn't have to be strong because it would have an open bottom"

Is it resting on a completely level, flat surface? What keeps it stable from tilting sideways in a current or due to an uneven ocean floor?

"Not sure how big the pump would have to be, but apparently small, efficient, multi-kW motors/compressors exist for compressed air cars. Just need to attach an electrical generator/motor to the shaft. How long would it last? Probably longer than a high-heat diesel engine."

For compressed air cars that pump tons of air down a kilometer-long pipe? This electrical motor is not going to generate a lot of heat? Never going to have brushings wear out? It's not uncommon for a diesel engine to surpass 300k miles of pushing a truck around.
This does not seem at all practical.

Fri, 05/23/2008 - 15:53 — cbthiess

Practical?

This is very practical. It's so practical it's incidental. cbthiess' answers are pretty right on.
The pressure- normal stuff for industry. Scuba and paintball compressors run 3000-5000psi. Efficiency is pretty high now. The actual type of compressor would probably be a screw compressor because they're more efficient for high volume, medium pressure than reciprocating compressors. We pump air down to divers at depths of 2000 (1000psi). Ordinary industry stuf.
Temperature- I can't see the issue here. You can utilize the heat of compression/expansion for energy savings, but otherwise there's no issue.
Hose- no issue. It can be compressible or not. The physics allow either.
A dome or box is not needed. Current lift bag technology is sufficient.
Swash-plate type air motors are very efficient. Coupling to a generator is normal industry stuff.
What makes this incidental is that you already have anchors on the bottom and anchor lines running to them.
“Prosperity is only an instrument to be used, not a deity to be worshiped.”

Tue, 05/27/2008 - 21:54 — cbthiess

Very cool to hear some

Very cool to hear some specifics about what sorts of materials and parts could be used. Never heard of suction piles: very elegant!
I proposed domes/boxes because they could be so cheap and locally produced - just need brute mass. How cheaply can you get a lift bag + suction pile for 1 cubic meter of air?
Heat exchange is an issue because when you compress a gas, it will increase in temperature by the ratio of its compression, and cool on expansion by the same. So when you compress the air, it will heat up, then that heat energy will dissipate into the surrounding ocean water. The air that comes back up will be cool to begin with, then cool a lot more when it's expanded. To get the same energy out that you put in you either have to heat it up prior to expansion, or after expansion. After is cheaper since you can use a heat exchanger with surface water to do it. To be explicit: 5C 1500psi air arrives at the surface. Expand 1L while reheating with ocean water to 15L of ocean-temperature 100 psi air. Use that to run the generator.

Wed, 05/28/2008 - 15:33 — Sundiver

I don't know. . .

I don't know how much it would cost on a practical scale. It's not a problem to price out but too much of a time requirement for this forum. For one cubic meter? $300.
Suction piles are extremely elegant. The concept is a bit counterintuitive but it's a common anchor system for large floating items like tension leg platforms. Here's a link- www.sptoffshore.com
A recent variation is SEA anchor, a Suction [pile] Embedded Anchor. see the video on the spt site.
Here's the really cool thing, the same anchors that anchor the seastead can be used to anchor the storage bags. The anchor lines can be composite (like kevlar) and hollow and double as the vertical risers (air hoses). The bags themselves are simple, think of the design of a high altitude balloon, with no need to consider weight (you can use stronger materials). That's why I say "almost incidental".
Heat exchange is not really an issue. What you say about the heat of compression and expansion is true but can be ignored. But the heating and cooling are energy sources that should be conserved. Like for domestic water heating and air-conditioning and refrigeration.
"It is a mistake to think you can solve any major problems just with potatoes." -Douglas Adams

Thu, 05/29/2008 - 15:04 — cbthiess

That's a good price - about

That's a good price - about $120/kWh at 1000 meters. But concrete goes for $85/yard, 1 yard is enough to anchor 1 cubic meter of air, and turning it into a dome requires just a few 2x4's and plywood for a form. So that's about $35/kWh. It also doesn't require additional pile capacity. Of course, concrete might cost more on the ocean, and it is less portable if you plan to move around a lot.
And you may be able to find do-it-yourself materials for lift bags that are a lot cheaper than $300.
I hadn't thought of using the heat/cool directly - good point. But it would depend on whether you have need for hot and cold. If your goal is high total system efficiency for electrical storage, or cheapest electrical storage, you would have to address the issue.

Fri, 06/06/2008 - 00:04 — Sundiver

That's not a scaleable cost

You asked how much for one cubic yard. That's how much a piece of pipe, some hose, and a 500lb lift bag costs. It doesn't scale.

You may be able to find cheaper materials but isn't serviability more important?

Right on the heating cooling thing. It's not a problem, it's a source of heating/cooling energy.

Concrete may go for $85/yard but an undersea concrete dome does not. That's like saying AL goes for $1.32 a pound and that relates to a 757. Concrete is not very heavy underwater. never mind.

Fri, 06/06/2008 - 14:23 — cbthiess

It is a scalable cost

It is a scalable cost because one hose could service any number of 1 cubic meter compressed air stores.
Servicibility is only important if it reduces expected lifetime cost. If moving around a lot, a lift bag would certainly be cheaper. If staying in the same spot, a fixed dome would probably be cheaper. Wouldn't take much or any maintenance, either, since it has no moving/flexing parts.
Making a concrete dome is actually very simple. All you need to turn a yard of concrete into an air-tight dome is a reusable form made of about $50 of plywood and 2x4's. The final product doesn't have to be pretty. It just has contain slightly less than its submerged weight in air. Concrete having a density of 2.4x that of water, using 1 yard of concrete for every yard of air would give you a nice safety margin.

Fri, 05/23/2008 - 16:11 — vtoldude

I´m a little confused about

I´m a little confused about where you store the compressed air. Is the concrete thing on the bottom a pressure tank? I understand that the balloon version would sort of push the ocean up, but not the other concrete thing.

Fri, 05/23/2008 - 16:32 — cbthiess

It's pretty neat, actually.

The air is just in a bubble under an open-bottom concrete dome resting on the ocean floor. It's kept compressed by the enormous water pressure at that depth. At 1000m it's about 100 atmospheres of pressure, or 1500 psi / 10 megapascals. When air is pumped under the dome, the bubble would grow and displace more water. That's what essentially pushes the ocean up. Thinking of it as ocean being pushed up rather than compressed air being stored just makes it easier to calculate the amount of energy being stored - either way the answer should come out the same.

Wed, 05/28/2008 - 11:56 — vtoldude

A dome, ok gotcha.

A dome, ok gotcha.
What would you need in terms of dimensions of the dome, depth etc to equal the storage of for instance X numbers of lead acid (car) batteries for instance?
What do you do when you need to move the seastead? Leave the dome?
I´m not grasping the heat loss discussion but it seems to me like there would be massive losses of heat to the ocean. Ambient air gets compressed - heats up - gets sent down where it directly touches very cold water.
edit: Nevermind the last question. I guess you just suck the heat out of the air after you compress it.

Sat, 05/24/2008 - 07:13 — thebastidge

Steam engines aren't really that efficient

"Pushing up the ocean" is gimmicky way of marketing the idea.

Steam engines in the 1800s were not very efficient. They were effective. Efficiency depends on how much of the heat you can recapture. What you're proposing is not a heat engine, you actually have to add heat to keep it from freezing up.
5C, under extreme pressure, and then you're going to greatly release that pressure at the top of your system. It's going to be COLD. A can of compressed air at room temperature forms frost on it when I'm spraying dust out of electronics if I don't use short bursts.
If the internal pressure is needed to withstand crushing pressure on the pipe, there will always be a certain reservoir of pressure below which you cannot go or your system fails. This leaves no room for maintenance of the valvles systemm at the top, and means there is a certain amount of energy cost just to set the system up which you can never recover without damaging the system.
If you have an open-bottomed dome (or whatever kind of open vessel), you can't exceed the water pressure at that depth in your storage facility or air starts escaping from under your dome. So there fore you have both an upper and lower limit on how much energy you can store, both of which limits are greatly expanded by simply using a high pressure tank at the surface.
You also won't really know for sure if your pressure is dropping slightly because of leaks of if you're exceeding your pressure limit (except for an approximate guess) and if you keep pumping energy into this system, you're diverting it from other uses.
A quick google search found high pressure tanks rated to 60,000psi (60k).

Mon, 05/26/2008 - 04:07 — polyparadigm

Phase change

As I posted on the "defense" topic, it might be worthwhile to store a mix of methane and distilled water deep in the ocean, as a bank of energy. (The right mix will freeze solid at the T & P available.)

I imagine the methane would come from anaerobic digestion of waste.

With an open bottom, the system you proposed might have a problem with gas solubility: the nitrogen and oxygen would all just soak in to the water, as happens to CO2 in a soda machine.

-Joel

Tue, 05/27/2008 - 21:33 — cbthiess

Can't seem to find the post

Can't seem to find the post you're referring to. Is there a search box hidden somewhere around here?
You mean clathrates? http://en.wikipedia.org/wiki/Methane_clathrate How would you propose to deposit and extract clathrates? One way that comes to mind is as a pumpable slurry of some sort.
Good point on the solubility. You could probably avoid the problem by storing the air in a plastic bag under the dome.

Thu, 05/29/2008 - 09:22 — thebastidge

Testing

Well, this idea doesn't need fullly developed Seateads to test. Just a buoy big enough to hold the equipment and an anchor heavy enough to hold the bag, and a means to put it in place. Maybe space could be rented or borrowed from an existing platform.

Anyone got grant-writing experience and/or a university or oil company contact that might under-write such a study?

Fri, 06/06/2008 - 00:08 — Sundiver

Not really needed

because there is ongoing research into using subterrain vaults for the same thing. The physics are the same, the bags, hoses, anchors are all normal industry stuff.

It could be done but would be mostly academic in nature.

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