Whole-Ocean Gravity Battery

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This topic contains 26 replies, has 6 voices, and was last updated by  cbthiess 7 years, 5 months ago.

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• #505

cbthiess
Participant
• 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?
#2274

thebastidge
Participant

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.

#2288

Jesrad
Participant

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.

#2300

cbthiess
Participant
• 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.
#2306

thebastidge
Participant
• 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.
#2310

cbthiess
Participant
• The distance should cause very little loss if the pipe is wide enough for your power usage. And 1500 psi (pounds-per-square-inch) is not hard to do, whatever the energy source. The engineering challenge is to get back out the energy you put in, and there’s historically been a lot of work put into that (steam engines, air compressors, actual compressed air energy storage systems). They got excellent efficiency from steam engines in the 1800’s, don’t see why we can’t get good efficiency out of a different source of high-pressure gas now.
• The temperature’s lower than at the surface, but still 5C at 1000m – http://www.windows.ucar.edu/tour/link=/earth/Water/temp.html&edu=high . At the surface it would be warmer, and that’s where you’d put your heat exchanger to re-warm the expanded air. The cooling due to re-expansion would be far greater than that due to bottom temperature.
• The pipe doesn’t need to withstand compression, because the external/internal pressure is balanced at the bottom and is higher internally than externally at the surface.
• How to get the shell level? Pick a flat spot. The ocean’s bottom is probably less uneven than dry land.
• What about currents? A rounded shell wouldn’t have much drag and could have a fair amount of extra mass to keep it in place. I’m just guessing here, but I’d think deep currents would be much slower than surface currents.
• I’m assuming industrial electric motors and generators last a lot longer than diesel engines. I don’t believe they generally use motors with brushes, and they have a lot fewer moving parts. In fact, just one major moving part and some bearings.
• Seems like it could be practical to me. Cheap, reliable, and takes advantage of a resource unique to seasteads.
#2313

Anonymous

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.

#2317

cbthiess
Participant

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.

#2343

thebastidge
Participant

“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).
#2400

polyparadigm
Participant

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

#2501

Sundiver
Participant
• 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.”

#2500

Sundiver
Participant
• 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.”

#2502

Sundiver
Participant
• 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.”

#2511

cbthiess
Participant
• 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.
#2516

cbthiess
Participant
• 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.

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