Underwater Tanks Turn Energy Storage Upside-Down

Pumped hydro storage is one of the oldest grid storage technologies, and one of the most widely deployed, too. The concept is simple – use excess energy to pump a lot of water up high, then run it back through a turbine when you want to get the energy back later.

With the rise in renewable energy deployments around the world, there is much interest in finding ways to store energy from these often-intermittent sources. Traditional pumped hydro can help, but there is only so much suitable land to work with.

However, there could be a solution, and it lurks deep under the waves. Yes, we’re talking about underwater pumped hydro storage!

It’s All Down Below

Most concepts for underwater pumped hydro storage rely on concrete spheres as pressure vessels, for their simple construction and good pressure-bearing properties. Credit: Fraunhofer IEE

The basic concept of an underwater pumped hydro storage system is not dissimilar from that of its land-based cousin. The difference is all in the details of how you make electricity by pumping water around when you’re already under the sea.

The general idea is to have a closed vessel sitting on the seafloor. Surplus energy is then used to pump water out of this vessel, leaving the inside at a near-vacuum. When it’s desired to recover energy from the system, water can be allowed to flow back into the vessel under the pressure generated by the seawater above. As the vessel is filled, the water flowing in turns a turbine, generating electricity in just the same way as a traditional pumped hydro system.

The utility of such a design may not be obvious at first. However, there are several benefits to such a system. Primary among them is that such systems can easily be colocated with off-shore wind farms, prized for their power generation, but with sporadic output. Running underwater also allows the system to take advantage of the great pressure exerted by the sea above. For each 10 meters of depth, pressure increases by roughly one atmosphere (1 bar), and with a system designed to operate with vessels at near-vacuum when fully “charged”, there’s a huge differential to take advantage of. Some designs proposing to operate at pressures in excess of 75 bar. Efficiency of such systems is expected to lie around 70-80%, around the same as traditional pumped hydro storage.

The StEnSea project‘s 3-meter diameter test sphere.

The underwater design also eliminates the issue of evaporation, which saps water, and thus energy, from pumped hydro reservoirs. Installation is readily scalable, too. Each underwater reservoir only needs an electrical connection to the grid, and nothing more. Simply installing more reservoirs underwater with the appropriate electrical infrastructure will easily scale up the capacity of such an installation.

There’s also the simple advantage that there’s no need to find big mountains or valleys in which to build reservoirs, and no risk of those reservoirs bursting and destroying local cities in the surrounding area. Instead, seldom-used areas of seafloor are readily available, with very few housing developments or existing businesses down there to frustrate the building approvals process.

Early Days Yet

The most notable effort in this area is the Stored Energy at Sea project, also known as StEnSea for short. The brainchild of Dr. Horst Schmidt-Böcking and Dr. Gerhard Luther back in 2011, the basic idea lead to a grand concept of 30-meter diameter spheres on the ocean floor. These would be complete with integrated turbine pumps to empty them of water, while also generating electricity as it flows back in.

A 1:10th scale test of the full-scale concept went ahead in 2016. This involved the construction of a 3 m diameter concrete sphere, which would serve as the primary storage vessel. Sunk down to a depth of 100 meters in Lake Constance, Germany, the vessel was tested extensively for four weeks to determine the viability of underwater pumped hydro storage. The test was successful overall, with the engineering team able to operate the sphere, storing energy and recovering it later.

The results of the study, combined with other research, indicated to the team that the idea was feasible at depths of around 700 meters. Pressures at this depth are on the order of 70 bar, and serve to help the system generate large amounts of energy while still remaining in a safe zone regarding material strength concerns and the practicality of installation. It’s expected at this depth, a single sphere could store a full 20 MWh of electricity, paried with a turbine capable of generating 5 MW for a discharge time of four hours.

With multiple spheres ganged up in an off-shore installation, estimated storage costs when up and running would come down as low as a few cents per kWh, likely cheaper than comparable compressed air solutions, with construction costs coming in around $1,300 to $1,600 per kW of power output. The actual financial viability of such an operation, however, depends on the arbitrage price of energy in the market; one study suggests that a system of 80 such giant spheres, operating with a combined output of 400 MW, would be viable in ranges from 4 to 20 Euro cents per kWh.

Other efforts exist, too. Both MIT and a startup known as Subhydro have also explored the idea, similarly based around hollow concrete spheres on the ocean floor. The numbers arrived at by these teams, regarding depths, efficiencies, and power outputs are within the ballpark of those quoted by StEnSea, suggesting the basic engineering behind the concept is sound.

The Ocean Grazer concept uses a bladder paired with a buried concrete vessel in order to run a closed system. Credit: Ocean Grazer

Meanwhile, a Dutch start-up by the name of Ocean Grazer is exploring a twist on the StEnSea concept. Instead of giant spheres, a concrete tube buried in the seabed is to be used as the pressure vessel. Additionally, rather than pumping water from the vessel out into the open ocean, it will instead pump its water into a sealed bladder. This still allows the system to take advantage of the pressure differential at the seafloor, but negates potential issues with a pump being fouled by marine flora and fauna, as it operates as a sealed system. Ocean Grazer has pivoted to the design having explored other renewable energy technologies such as wave power generation in the past. The company expects that one reservoir, with a capacity of 20 million liters of water, could store up to 10 MWh of energy.

The Ocean Grazer project, which won an award at CES 2022,  is perhaps receiving the greatest press for underwater pumped hydro at the moment. Despite this, and the other projects that have bubbled under for the last decade, the technology still largely lives on paper and a large-scale installation seems to be a long way away. Regardless, the fundamentals are there, so if energy storage does suddenly become more important, or, let’s be honest – much more profitable – much of the required basic engineering has already been done. Implementing a major installation may just require the right economic conditions to happen in only a few short years!

126 thoughts on “Underwater Tanks Turn Energy Storage Upside-Down

    1. So Hydroelectric dams are right out then?

      I guess what I’m saying is that just because you had a simple pump fail and didn’t enjoy replacing it that shouldn’t mean that professional engineers should shy away from developing high-tech engineering devices that can allow the storage of energy and allow intermittent sources of energy to be available around0the-clock.

      I personally think this is a worthwhile investigation and I hope the development goes well in the next phase.

    2. It really shouldn’t be a big issue on this scale though, when there is so much money to be made in the reliable operation these things will end up overengineered and well maintained. The sump pump you bought on the other hand is probably the cheapest fastest to produce chinesium grade pump, designed from the outset to work fine only for so long so you buy a new one again and again, as if it just works for another 50 years they aren’t getting more money out of you…

      There are questions on if such systems are economically viable (especially the short term return on investment that is so loved by those throwing money around no matter that as soon as you get your return its working out worse in the medium and long run), or ecologically viable (the material costs to build such things well enough to survive the conditions may end up making them far less good than they appear – though that comes down a very large amount to how well constructed there were in the first place as that defines their service life and maintenance requirements)

      1. How about a comparison of the engineering and construction costs for this vs nuclear power? I would love to see that. Because the complexity, durability and expense issues that haunt nuclear power, not to mention the intractable danger (accident or terrorism) and waste issues never stopped that industry from expanding all over the world. This power storage combined with wind resource is safer, cheaper, faster and better in every regard than nuclear. Let’s see the stats!!

        1. Not sure they are great comparison really – primary generation vs secondary storage, entirely controllable output vs whatever the wind is doing with a small output smoothing capacitor or perhaps small backup battery if you compare a windfarm with these to nuclear…

          Or in short as Nuclear lets you have a reliable and throttleable output it makes a superb backbone baseline supply. Also just how much electric are you trying to supply – most nuclear reactors blow even the biggest windfarms on the windiest day they can run in away for peak output…

          Where as it stands no single windfarm with these units would either to really make a similarly reliable production, or the same peak output… As time passes and enough of these things are built (so the variability of weather is now not as big a factor for the many distributed windfarms are not stuck in the same weather all at once) along with better demand management to match the supply maybe its a workable comparison, but its still not a 1:1 comparison…

          (Also have to ask what type of Nuclear reactor or reactor chain as that has an impact to its build and running costs)

          1. Nuclear power economics is such that it’s cheapest to run at full power. It comes from the fact that the facility has fixed and recurring infrastructure costs, limited calendar life, and almost negligible fuel costs, so the lowest power prices are attainable by maximum production.

            When you throttle a nuclear power station down, the price of energy goes up, therefore it stands to reason to add storage capacity and use the power later, as long as it’s cheaper than the loss of production.

        2. >safer, cheaper, faster and better in every regard

          Except the infrastructure and materials cost, and the massive environmental damage and land use, and the statistically greater deaths/TWh figures because it involves more worker-hours and accidents naturally happen. Wind power is comparatively more dangerous to human life even including all the nuclear accidents.

          Then there’s the irony of mountains of thorium and uranium waste produced by digging up rare earth metals for the electrical industries.

    3. Not sure why you’re so negative. Sump pumps are so ludicrously cheap, it’s actually kindof amazing when you think about it. I mean, a pump that can shove *three thousand gallons per hour* buried directly in water with no maintenance for around $50?

      Not really the pump that’s the issue, it’s more the lack of easy intelligent monitoring. Those things often cost more than the pump itself!

    4. I hope that you can understand the pump that failed on you was likely something that was designed to be as cheap as possible to satisfy the homeowner market where cost is more important than longevity or reliability.

    5. The small one seems like you could just “lift upp” with a crane from a ship and fix on land if it “crapped out”… The big one looks harder to fix… Unless you build a pressurized facility around the actual tank that people could get to with a submarine…

    6. @Col_Panek – Don’t tell that to the submarine guys… other than burying it is even worse.

      IMHO, EXCELLENT to see these designs being developed. I’ve advocated getting the pumped hydro storage systems underground and/or underwater on more than one sober and drunken occasion. Kind of knew about them all my life having heard lame stories of causation for why not implementing… even underground since seems can have what system is in place now above ground and still have an underground system with minimalist impact where from my perspective with proper prior planning… an increased natural resources system resulting to compensate other volumes losses eslewhere and unrelated. Kind of like with hydro dam systems where for each, you implement life support systems like nurseries and fish hatcheries at the least with budgets to account for those in the life cycle of the plant… and of course remediation plans as well for the general betterment holistically not stupid.

    1. Nah, they build them to never go wrong. Friend of mine worked on paints for offshore wind farms… it’s some serious stuff. Massive mistake asking him what paint to use to prevent rust on my gate…!

    2. Yeh, It seems to be the second dumbest idea a seen for a while.

      Why not just have empty pods that are open to the see water at the bottom and one air line from the top going to shore. Pump air down where it displaces the force of the water then release the air on-shore to allow the force of the water to expel air pressure to generate power.

      Then underwater – no moving parts – no electricity – exceptionally low maintenance underwater where it would be expensive. Perhaps a few plumbing repairs here and there.

      And onshore – no corrosive salt water to deal with further reducing maintenance costs.

      And in case your wondering – the title dumbest goes to a industrial plant – they had to double the scale during design. Simple ^c ^v and they now have two plants but the second was a mirror image of the first. So now there’s a left hand and a right hand “everything”, doubling the maintenance stock they have to keep.

      Now who would do such a thing? Maybe the designer was a “friend” of the person who holds the maintenance contract.

      1. The problem is that air is compressible. You compress it, it loses energy as heat, you decompress it, it gets cold and the pressure drops. The smaller the tank the more difficult it is to keep the temperature of the gas the same between charging and discharging, so you get hysteresis losses.

        It doesn’t work for modular systems because the efficiency goes really low, so you’d have to build on a gigawatt-hour scale to start with, and that presents difficulties with funding and economic risks.

      2. Yeah exactly my thought. You have just explained the basic principle of wave power harnessing which can also use pressure displacement. Why not leverage both sporadic, cheaper power generation storage with higher cost, but constant power generation (wave power)? Solar/wind power storage will reduce wave power generation cost.

      3. I’m not seeing why they would need to keep a left and right handed part for everything. Surely it’s not difficult to design a plant to use the same part but fitted in the left and right position

        1. Not everything is designed by the plant designer. For example, an engine to run a standby generator, The exhaust will be on one specific side of the engine so to rout the exhaust to the side of the building and then out you need two different exhaust systems, one for the left plant and one for the right plant.

    3. If the pump is designed right it shouldn’t have any issues, they operate pumps for long periods of time on ships and submarines that are constantly exposed to the salt.

      1. As a former submariner, I can attest to the constant maintenance needed to keep those vessels operating safely. Pump rebuilds were a regular occurrence, not a rarity. Salt water is aggressive, especially at depths. Not saying the scheme is impossible, but subs have a crew dedicated to keeping things working and that gets expensive for a profit making enterprise. Something to consider.

  1. This really does seem like a really neat solution to the renewable energy storage problems, if you can create a stock design that is good for off shore windfarms with a large range of water depth it could through the more mass production methods work out really good value as an integrated part of future offshore wind turbine towers – every tower needs a foundation structure anyway, so making at least a few of those foundations these energy stores makes sense.

    And if you build them right I can’t see a reason they won’t still be in service long after we are all dead (obviously going to need maintenance and replacement parts in the generator/pump over time, but like most hyrdo dam there is no reason for the core structure to not be there in working order for a very very long time if you get the design right, water is always destructive in geological time scales, but in more human scales not so much)

    1. For daily variations, you need enough storage capacity to catch 1-2 days of full output, on average, so for every 1 MW of turbines you need 24-48 MWh of storage capacity. Off-shore where the winds are a little bit steadier, about half that. If one of these balloons can store 20 MWh you pretty much need one per wind turbine, so you might as well integrate it to the turbine itself.

      For seasonal variations, the problem of storage is still ten times worse.

      1. Along the same lines I was thinking, though you might find it cheaper and more efficient to put in larger one on every nth turbine rather than on every single one – some serious analysis of the life time maintenance and efficiency, along with engineering practically of trying to ship the bigger pressure vessel in the first place. Also have to ask just how much energy you wish to consume/provide – one massive pressure tank holds more energy but if its using the same scale of pump you have limited it to working at that rate.

        I’d also argue that seasonal variation isn’t anywhere near 10 times worse when you look at the grid as a whole for a few reasons, site the renewables in good locations and their seasonal variance in output is predictable and often not very great, when one type of renewable in that location isn’t working well the others are often going to be, and its trivial enough to oversupply at peak output enough you have to dump some (or more likely ramp the steel mill etc up – the highly energy intensive industry will work extra shifts and more machines while the power is cheap as the overtime payments are cheaper (maybe even subsidised by payments from the grid!) than the power bill the rest of the time) while making the more normal output levels sufficient for normal demand. Storage is definitely an issue as grids go greener, and a significant one, but seasonal longer term storage while it will be somewhat needed doesn’t need to be that bad.

          1. But if your average during the lower outputting week/month is even close to sufficient for the baseline required draw then even that 1-2 days of output is enough with a little bit of shaping demand to supply. Its all a question of the degree of over/under supply to demand, and if that demand is at all controllable.

            Longer term storage is only a really big deal if you insist that you will put in only just barely enough generation, where oversupply the renewables a bit more that reduces, more still its gone entirely and the question is what to do with all the excess!

            Also while Wind will usually have some seasonal variance it isn’t nearly as affected by the season as solar (at least when not on/near the equator) – its variance is much more caused by relatively short term weather patterns (at least in most areas of the world).

          2. 10-20 days of output is really the minimum if you wish to have any real impact with storage technologies on a seasonal scale. That’s about 3-5% of your yearly energy demand. Even so, it’s actually a massive amount of energy.

            What you really want is 1-3 months worth of strategic reserves so you’re not living hand-to-mouth waiting for the inevitable system collapse due to something like a natural disaster, or a war.

          3. As for what comes to over-provisioning the generators, that shows up directly in energy prices. You double up the generators, you double the power prices because you’re building twice the infrastructure for the same effect. It’s already difficult enough to keep the power prices palatable.

          4. Oh, and two times zero is still zero.

            50% of the output of a typical wind turbine happens in 15% of the running hours. Wind power is “peaky”. When you have a few turbines, the time series is a sparse bunch of spikes with valleys of pretty much nothing in between over a time scale of 1-2 weeks.

            When you have more turbines in the near geographical area, the spikes just grow taller and you need MORE storage capacity, not less, to make any use of it. Adding more distant turbines makes the forest of spikes denser and it doesn’t really start to smooth out until you count very distant generators around 600 miles out and further, but then you’re dealing with big transmission costs and losses.

            And “demand control” without adding local storage capacity is just another way of saying “energy rationing”. That’s actually just normalizing fault over what the system is supposed to be doing. Same as saying “There doesn’t need to be food in the stores every day if we can control the demand by telling people to tighten their belts.” – true, but not really what anyone wants to do if we can avoid it.

          5. With how cheap all the renewable sources are for the energy they output over their lifetime having more doesn’t cost much really – and even if energy prices went up 20% that largely just means people and more particularly industry will cut back till the prices dip or the demand for their product goes up enough to make it profitable again… And as the renewables are actually now rather cheap forms of power generation double those types of generator and you might well end up with less energy bill – gas imported from the Middle East, Russia coal from Aus, Germany etc all have limits and serious shipping and extraction costs, and that is not to mention likely green taxes and the reliance on the foreign powers for your energy – which means they can choose to turn off the tap or dial up the price as has happened before…

            If you are needing months of stored energy for a war etc you are planning a different game entirely – when there is a war on suddenly all those nice to haves like mobile phone networks drop in power consumption or get turned off entirely, rolling power outages across domestic buildings are to be expected etc, and during a natural disaster your steel industry etc that are huge consumers of power are not going to be working as normal if the power is needed elsewhere. The actual power demands of both situations are going to be massively different and lifestyles changed to suit the situation that you really don’t need that level of reserve – for a war all you need is the ability to keep the production of war material (which includes food etc) and the armed forces going (who will continue to rely on nuclear and fossil fuel for quite some time to come), in a natural disaster all you need is some local power supply capacity largely for the communications around the relief effort – which is often handled when going to the aid of other nations by dropping off the boat or aircraft a solar array and battery these days as its more than good enough and you don’t have to keep dropping in yet more fuel on those disasters that drag on…

            Also transmission losses are a thing but when you are talking on the scale of the grid not a very relevant thing – the losses are so tiny compared to the amount of power going down those lines that adding a few hundred extra miles of transmission hasn’t cost you much, and if its all from ‘infinite’ renewable sources spiking somewhere else whatever it costs in losses is completely meaningless – it was ‘free’ energy anyway. So that windfarm in an entirely different European nation can and has been part of powering the local grid for other nations – there are lots of connections and power trading between the powers of Europe… So while your giant windfarm at x might be barely ticking over or even becalmed entirely if you have twice as many turbines overall you have twice as many at y where its blowing hard and spiking nicely…

          6. Re: Electricity rationing – Demand limiting

            My country did some things differently and we have this problem but here it’s more of a death spiral for distribution and (hopefully) coal fired centralized generation.

            I’m in Australia which has vast open distances. Most of the population is on the coast and the distances between towns inland can be 1000’s of km.

            So we have a vast electricity grid with a relatively small number of generation points.

            Government subsidies for domestic solar over the past 7 or so years has meant that the longer distance distribution system has to carry higher peak loads (general expansion) and a much lower total of distributed power overall.

            This means increasing distribution cost per WattHour has resulted in end consumer costs increasing over about 6 years and it has been a very significant increase. This is pushing more people to go completely off-grid with their own batteries and that in turn leaves less consumers to pay for the distribution costs and an escalation in consumer costs again. I call this the death spiral.

            Our country installed “the biggest battery in the world” and is in the process of installing an even larger one but these centralized resources aren’t fixing the overall problem.

            I think advances in battery technology are just going to destroy all value in the longer distribution networks.

            The real problem here in this country has mostly been political with network providers making up absolutely stupid “stories” to preserve their cut of the pie which is now mostly being subsidized by the government. There are obvious ways to provide some relief to the problems but the networks won’t get onboard as it isn’t as profitable for them while they can argue for increasing subsidies. Here it will end with the wire being ripped off the poles and out of the ground by the most part.

            Anyway, the real elephant in the room is that the real problem is consumption.

            We talk of carbon emissions from fuels for cars, trucks and industry but we never talk of the carbon emissions from manufacturing all these things in the first place. Worst are metals and concretes (as far as I know).

            I traveled off grid in the bush for about 5 years. My solar was peak 880 Watts. When I came back about a year ago, the idea of paying for electricity had become totally foreign to me. I had become accustomed to electricity being “free”.

            The real problem with carbon emission is the energy used to create all the short life consumer goods that quickly end up in land fill.

            I had a domestic electronics repair center in the era of the cassette, VCR, CTR TV etc.

            A good VCR, Reel to Reel, TV or high end stereo could easily last 10 or 15 years. They were made to be serviceable instead of the opposite we see today. We used reusable glass drink containers at home. We had a different mindset.

            Future generations will see our era as if we wasted hundreds if not thousands of years of resources in mere decades like drunken sailors (no offense to sailors).

          7. RÖB I hope you are wrong on distrubution in Aus – as while being so sparsely populated does mean much longer and pricier than normal runs between the developed area it also means you have a huge amount of land area you can put solar/wind on providing a high degree of energy security and allowing for a big expansion in the high energy industrial sectors the rest of the world will start to find harder to achieve – but none of that works if the grid is gone so you are all little tiny islands often with some excess power you can’t use…

            Also metal is arguable the cleanest building material we use – as its generally very easy to recycle, while also being very enduring – everything has cost and the energy required to work with metal seems stupendous, but then it might well last 100+ years… As with all things humans do its not what the material is its how you use it, which these days often means how fast it gets thrown in the landfill…

          8. @[Foldi-One]

            We already have Too Much solar. The big thing now seems to be Wind but this just exasperates the problem.

            We have so much roof top solar that the network needs to turn some of it off at times for two reasons: 1) It exceeds demand, 2) It effects network phase and the network can’t correct with reactive regeneration.

            Phase misalignment is where different generators on the network drift away from the same phase and this causes excess loading on network infrastructure. The misaligned power is dissipated as heat in the network and reduces the total “normal” load the distribution network can handle.

            Reactive regeneration can be thought of as a big flywheel that aligns phase by taking power at one phase point and returning the power at another phase point, it doesn’t really generate its own power.

            The simple solution to phase correction is to synchronize the solar inverters but it is not considered because the networks are in the governments hip pocket and devise different solutions that are both stupid more profitable to them.

            Anyway the real problem is that the daily generation cycle doesn’t match the load cycle due to the excess solar so storage is the only practical solution in the longer term. And in this country (for consumers) it will end up being consumer provided storage due to the unreasonable cost demands of the monopoly network. I see this as completely inevitable for consumer power with the advances and reduction in cost of battery technology.

            What (What’s left of the network after that) can do is industrial. Large remote solar generation (not necessarily PV) can be used close to large solar generators for metal processing etc. I recently saw an article on HaD where a re-melting stage of metal ore processing was eliminated (ore to billet) and will dramatically reduce carbon emissions. Solar is very useful for this as a lot of the power demand is daytime BUT and it’s a big “but”, still high amounts of power are used overnight to keep things hot as it’s too much power waste to let it cool and then reheat it. Plants like this can take a week to shut down and a week to restart.

            And the issue with concrete is the cement. It accounts for 8% of carbon emissions.

          9. >If you are needing months of stored energy for a war etc you are planning a different game entirely – when there is a war on suddenly all those nice to haves like mobile phone networks drop in power consumption or get turned off entirely

            That’s exactly what you’d like to avoid, because the point of the attacker is to disrupt the energy system to disrupt the society first. Strategic energy reserves are needed to keep the lights on and the society functioning as normally as possible to sustain effective counter-measures.

            But the same situation can happen if you have a major system failure, like a storm that sweeps a significant portion of your wind turbines to the ground, because it takes many months to fix it. If you lose 10% of your generators for 6 months, you need 20 days worth of alternative supply to make up the difference.

          10. Of course you could argue for backup generators, but then you have a Texas situation where there’s no economic incentive to keep strategic offline generators that are needed once in 20 years. It costs money just the same as having active online storage that actually benefits the energy market by stabilizing prices over the long term.

          11. @ RÖB
            If you all end up running your own personal grid you are duplicating so much effort, and wasting so much of the potential energy – the right solution is partly storage to smooth the daily cycle but you also have such good generation potential that your heavy industrial should be able to work nearly for free quite often – which just ends up being good for everyone in Aus, and useful for the rest of the world having decent products with low carbon costs, something its harder for much of the rest of the world to provide.

            @Dude the cost per KW and cost for service lifetime are now better than any other generation method every time its looked at (Nuclear decommisioning costs hurt it, fossil fuels are expensive and only getting more so), the only caveat that brings that back some in the real world is you will need some energy smoothing and/or greater oversupply potential (realistically some of both or more sensibly the nuke for the backbone).

            Also in a real war you don’t care about having all 60million TV channels, 5G connectivity everywhere, AC, dishwashers – all the energy consuming fluff of normal life becomes rather irrelevant when you/your family members and friends are likely to wind up dead, and even more of them WILL wind up dead if you don’t produce enough war material because you are wasting all that energy on garbage – if your priority is living the full on wasteful life of today while your nation is in a real war (not just squashing some smaller nations troops a bit in the little proxy war) you deserve to be dead, as you for damn sure are making it certain more other folks, including those willing to risk their lives to protect your bloody liberties will wind up dead…

      1. I think your numbers are a bit wonky. A 30 metre diameter sphere has a volume just over 14,000 cubic meters. At 1000kg/cubic metre, that’s 14,000,000 kg. Build the sphere of out concrete 1 metre thick and it’s going to weigh 7,000,000 kg.

  2. I worked on something like this, sort of. Basic compressed air storage underwater using a caisson, but open to flooding. Everybody looked at the power stored in the compressed air.
    I noticed that a water pumping air compresser/expander was more efficient ant that the water pumped out of the volume also added to the power storage. This plan is just the latter, without the pressurized air.

    1. I was thinking the same thing. I use vacuum tankers regularly for agriculture and the pumps are quite reliable mechanically, so long as the oiler is metered correctly. there are balls to prevent any slurry fluid from entering the pump. the only issue i’ve seen is that the steel tanks tend to collapse under vacuum, but that shouldn’t be an issue with concrete tanks.

      1. That’s been done at least since the 1990s (in Germany), filling mines with water, putting a huge concrete plate on top and use the weight of that plate (500m radius) as a pressure source. Works.

          1. There was a proposition to cut a kilometer-wide plug out of bedrock and pump water under it to use as a hydraulic piston, but it needs so much water that it couldn’t be built anywhere. It would empty most regular lakes to lift the cylinder up, so you can only build like five of them anywhere in Europe, except right next to the sea.

            And the seismic effects of pumping it up… I won’t even speculate on that. All I know is that when you’re engineering on that scale, the ground is basically jello and pushing down on it makes other places go up…

  3. Great article, but I find it ironic (and maybe even a little misleading) that you spent all this effort to detail this interesting system without mentioning on the the biggest, fundamental challenges of doing anything in a marine environment: biological fouling. Anything you put in the sea will become colonized, even at significant depths. The closer you are to land, the more likely you are to have significant growth of marine life. Factoring in maintenance/regular cleaning may totally alter the cost-benefit analysis and ROI.

    1. Yeah, Like the sump pump comment right at the start and how they’re so simple and reliable.

      Well perhaps a three stage pump would be reliable.

      Stage 1) Crush and liquefy barnacles
      Stage 2) Chemically prevent ferric oxide from solidifying in the pump
      Stage 3) Pump liquid

  4. I can’t quite wrap my head around how some of the numbers net out.

    First, it seems that this has to use a lot of material in the very deep water where it’s planned. At 700 feet the 100 foot wide pressure sphere has to resist about 300PSI, which, integrated over the area, is a lot.

    I suppose there’s no real weight penalty, and it has to be heavy enough to stay submerged when it’s empty anyway, so you can use a material that’s very strong in compression and relatively cheap, like concrete, but still, a 100′ concrete ball that can resist 300 psi is going to use a lot of material.

    A compressed air system, on the other hand, uses almost no structure, you could do it with a fabric bag as long as you can hold it down (on the other hand, you’ll loose the heat energy of the compressed gas. And you have to hold it down. Thermodynamics giveth, thermodynamics taketh away…)

    Second, I wonder how much energy is lost in pumping out the water and leaving a void. They talk a bout a near vacuum, but that has issues, You can’t really have a very low pressure or the water will boil into (sea temperature) steam. That’s not really a technical issue, I suppose, but that phase change does use a *lot* of energy, and I’m not sure how you get that back when you flood the sphere again. The water will get a bit warmer, but unless the sphere is insulated it’ll loose the heat to the sea, so you’ll loose that energy forever.

    I suppose you can pump in just enough air to keep the water under it’s boiling point, but that is wasting some fraction of the available pressure difference.

    It’s kind of like the guys who want to store energy by stacking blocks, it’s an interesting idea, but at the end of the day how much do you really net?

    I guess against all this is the fact that the power is essentially free. There are times of low load where you would just literally dump anything you couldn’t use, so you might as well store it in a wasteful battery because that’s still better than tkhrowing it away.

    It’s an interesting engineering problem

    1. A “near vacuum” is most likely not 0 Pa, but likely still up in the few kPA region. So boiling water at 4-8 C isn’t really going to happen. Wasting a few kPA when one has a few MPa to deal with isn’t much, like 0.1% and this is just a loss in capacity, so not that major of a worry.

      Though, personally I think compressed air is a more trivial solution and more widely applicable.
      Also, losing heat for compressed air isn’t an issue, and honestly a rapid exchange of heat during compression is preferred in an isothermal system.

      Though, isothermal compression/decompression tends to be “slow” and not that power dense compared to the adiabatic approach (but isothermal is still plenty power dense), but the adiabatic process also needs to thermally isolate the storage vessel, adding a lot of extra complexities to the puzzle. (So I think isothermal is the better solution, especially if one stores power over the course of days.)

      But a downside with compressed air balloons under the sea is creasing of the fabric, this tends to lead to a lot of wear and tears can form. Having such a balloon rupture might not be particularly safe. (One reason I think using a TBM drilled tunnel lined with steel bellow ground is a better storage vessel. The ground preferably “doesn’t move” (don’t built it in a fault zone) and holds the pressure while the steel makes it gas tight.)

    2. I’m with you in questioning the use of concrete. For its mass / psi ratio, I don’t think it’s a competitive vessel material. Also, the two guarantees with concrete are that it gets hard and it cracks. So they ultimately are depending on a liner, which is prone to degredation over time. I’d like to see this redesigned as a giant array of piping that looks like a heat exchanger, but is tens of thousands linear feet of 2′ diameter pipes looping back on itself. The volume will add up to whatever they are capturing in these concrete vaults, yet the vessel material will be readily sourceable from industry so it can scale however large is necessary. Schedule 40 24″ and 36″ steel pipe is rated at well over 1000psi ( https://www.steeltubesindia.net/schedule-40-steel-pipe.html ). I wonder if projects like this are avoiding industry-standard materials simply to appear to have proprietary, patentable designs in the eyes of investors.

      1. One of the differences between steel and concrete, steel is really strong under tension (and bad under compression), and concrete is really strong under compression (and bad under tension).

        This these storage vessels are deep (for some value of deep) under water with their inner void at 1 ATM pressure, they will be under compression. Steel doesn’t perform well under compression, and to be able to accurately calculated it’s failure depth it would need reinforcement rings installed (or to be incredibly thick). Concrete on the other hand is great under compression, and so much cheaper than steel, construction would also be simpler.

        In fact, construction could be as simple and building a mold/form and (slip) casting the concrete sphere, then reusing the form/mold to make more. Compare that to the amount of forming and welding (and inspections) required to build the same sphere out of steel.

        Given the above, concrete makes a lot of sense for this application.

        1. Steel isn’t bad under compression. It is very strong in every way. It is much more expensive than concrete, and constructing a giant steel tank would be less practical. If you made the same thickness steel tank as the concrete plan, it would work just fine.

      1. That’s a side effect of subsidies. The producers pay you to take the power because they get paid more by the state. You’re still paying the full price for the power – just indirectly.

      2. Pass that through to the consumer and they’ll find creative ways to solve the problem. Probably the lowest cost way would be to fit every electric water heater and every dedicated freezer with a smart relay to run them more when there’s a surplus of energy. Adding a similar feature to smart thermostats will allow HVAC to participate as well, then addition of thermal storage will help even more.

        1. If you get energy for negative price, despite it having a real cost, you tend to use it for things that aren’t worth it. Someone heats their swimming pool with wind power and gets paid, while everyone else pays taxes for the subsidies that the producer gets for making the energy. This is a problem.

          1. You are assuming the producer gets anything extra, or paid any more in subsidies for the excess they pay you to take away – with how many different company and governments are involved that seems rather unlikely to be true in many cases (and quite possible true in others)…

            Also oversupply from wind doesn’t really have a ‘real’ cost – you needed all those turbines to provide enough when its not that windy – it really doesn’t cost anything extra to use the existing infrastructure for more production than required and any subsidies paid to build the infrastructure that you can have green cheap renewable power on the calmer days – but its quite possible to work the system without subsidies despite the grids paying folk to use the energy, and without any extra payments or subsidies to the turbine owners – its just definitions or market trading on how things should be done between the companies/governments/consumers in that area and universal hard and fast rules on how it should be handled fiscally just plain won’t exist as the social and political landscape of the world isn’t the same…

          2. >You are assuming the producer gets anything extra, or paid any more in subsidies for the excess they pay you to take away

            They do. That’s how renewable feed-in tariffs work. Usually they’re arranged so that the government pays the difference between the sales price and a fixed subsidy rate. Whether negative rates are considered differently depends on the country/law, but the point is that the producer gets guaranteed profits regardless of the selling price.

          3. >oversupply from wind doesn’t really have a ‘real’ cost

            Only if it’s “accidental”, if you don’t plan and build deliberately to over-provision the capacity. If you build FOR oversupply then the additional capacity has a cost, and the over-supply has a cost, and the fact that you have to give it away just to utilize it means you’re doing it wrong because your other customers have to pay the difference.

          4. >you needed all those turbines to provide enough when its not that windy

            That’s a really poor design approach. Studies from the UK show that wind power can be relied on for only 3% of the nominal capacity on the “idle hours” which is most of the time. When you have a system that produces 50% of the total energy output in 15% of the running hours, the ratio between the peak hours and the rest of the hours is about 5.7.

            In other words, when your system is sized to utilize the peak output, you get about 17% of the peak at other times. When you want to double that to 34% by increasing supply, you’re doubling your peak and wasting about a third of the total production into over-supply.

          5. You can rely on wind power entirely very easily Dude if you want to build it that way – you just need a sufficiently distributed network, which nowhere really have yet at least, the UK for instance really only has a handful of sizeable windfarms (many of those right next to each other so you could just call them one) and a few odd turbines elsewhere – so you run into issues when any of the bigger farms are becalmed, but if you have more or even the same damn number over a wider area the odds of meeting your target output become much more certain.

            When they are all distributed enough you can say with some certainty that x% are spiking right up to the redline, y% are producing 10%,20% etc of the peak, and z% are not functional right now. There is always wind, lots of it, all over the place – its just not always where turbine x happens to be, but if its not there it 100% has to exist somewhere else not all that far away as the sun still exists to drive the weather… Statistically its a certainly over a wide enough network to produce a fairly stable output – obviously its not controllable stable output the way fossil fuel and nukes are, but still pretty damn stable.

          6. > you just need a sufficiently distributed network

            Yes, and a “sufficiently distributed network” is something that spans entire continents over thousands of kilometers. It’s not a practical proposition.

          7. >”the interconnected regions have the greatest amount of firm power, with 17% of installed wind capacity available 79% of the time and 12% of capacity available 92% of the time.”

            https://cedmcenter.org/wp-content/uploads/2017/10/The-effect-of-long-distance-interconnection-on-wind-power-variability.pdf

            Here we’re talking about entire states and regions, CAISO and ERCOT etc. which span entire US states or multiple states. Still the variability and peak-to-average ratio is huge. These networks span hundreds of miles. In Europe as well, networking all the northern states and Germany would only reduce the overall variability by half (Hence why the DESERTEC plan).

          8. The power grids are already continent spanning Dude, that really isn’t all the much of a hurdle.

            The only real hurdle to trying to seriously use only wind is the level of co-operation required over larger distances across political divides and building them all in the first place…

            I do agree full wind power isn’t all that practical as things stand, but the point stands the variability and spike nature of wind power effectively drops the larger the distributed network, yes it never entirely goes away but it doesn’t have to either, add in just a day or two of average output stored at the turbines smooths it out further (a day or two of peak output even more so of course) – you quickly start getting to the point that you can rely on it enough, and without your several months of energy store even when working only within the bounds of a single small nation (particularly a nice windy island like the UK that has many many good spots turbines can go onshore and off)

          9. Also Dude measuring ‘by capacity’ really doesn’t make sense – as clearly they have to be talking redline max output peak capacity, but that is something almost no powerstation will ever be running at no matter what type – you have 8 big powerstations all running at say 70% during the highest load so you can then accommodate the maintenance downtime – if you needed to run all 8 flat out when one goes down so does the grid, if you can only just barely cover one of them being down you haven’t really got a comfortable level of redundancy – so by that same metric normal powerstations will be supplying x% (x<100%) y% of the time too..

            Really it only makes sense to consider the required/average output you expect from such things not their maximum – so if you need to always have 25MW of wind you end up building a few extras, and perhaps adding energy stores so with your calculations you can hit that target 100% of the time – which is exactly what is done with fossil fuel, you overbuild somewhat and stockpile energy too, it can't run without fuel and JIT is not satisfactory for that so you have nice big bunkers. The calculations are the same, the viability of either is pretty much the same too, except that one has been built around for 100+ years the other is just breaking in so isn't built around in the same fashion.

  5. Nice idea. But few nice ideas are truly compatible with sea water and ocean conditions. In this case the mechanisms will likely be the weak link, and placing them at depths requiring trimix saturation diving will be very *very* (did I mention very?) expensive. Time will tell, though I wish them well.

    1. Well, best case they have a few lifting points attached so that one can just send down a robot with a hook and lift it up for maintenance.

      Though, a 30 meter diameter concrete ball likely weighs a “tiny” bit. So perhaps one seeks help from a few floatation devices to keep it surfaced for repairs.

      But I see little reason to dive down to an otherwise moveable object that has few things attaching it to the ocean floor. If sparsely enough placed, putting it down on the cable for another shouldn’t be an issue either. Not that placing things with a degree of precision on the ocean floor is a particularly unique challenge, so getting them inside their allotted square/hexagon within +/- 10-20 meters should be doable unless one places them somewhere were a lot of currents are present, then one might need a larger buffer.

      1. Of course the assumption is that things will be remotely repaired whenever possible, and the structure would require buoyancy control to place properly to begin with. But the ocean gets to choose what curve balls it throws at folks, and strangely there seems to always be that One Thing that forces a choice between abandonment or an expensive human fix. Just ask any saturation diver.

        1. Yes, there will likely occasionally need to be a human touch.
          But if one can avoid it 90% of the time, then that is still a good step forward.

          Though, with a few solid loops for hooks, getting it up shouldn’t be impossible.

          But personally I honestly think other solutions for energy storage might be a better choice.

      2. The balls can be constructed to be only slightly positively weighted under water; a few tons of weight added to there mass/waterdisplacement ratio will keep them down, without much force needed to lift them once they are in their ’empty’ state. Maybe put them in a seabed-attached cradle. The heavy lifting of the sphere will have to be done in the last 30 meters, but one also can imagine dock-like solutions to that.
        And with the pump and a valve in a separate (external, detachable) compartment (cradle?), servicing would even become simpler.

        1. Maybe have the spherical concrete shell and a core that goes through the top and attaches on somehow, so for maintenance you just detach the core with all the pumps etc on it and lift the core out. It would be a lot less weight to lift out and you could just send down a robot to detach it and then use a robot to attach it again and precisely get it back in the hole. Not sure how you would attach it with a watertight seal though.

  6. I love the concept. I wonder how much could be done with existing sunk marine architecture rather than building new pressure vessels, could you use the bladders noted, shore up existing large undersea volumes in sunk craft near tidal areas and use them for power generation? Thus greatly reducing the needed infrastructure cost.

    1. Probably not. The idea seems to be premised on high pressure at depth to increase efficiency. Hence tidal areas are out. And at the deep ocean pressures it’s unlikely that any abandoned article will be able to survive the pressure differences. Plus the considerable development costs likely need to be amortized over a large number of installations in order for the whole exercise to make any kind of financial sense, hence standardization on a single design is important.

    2. Creating one design to use is much cheaper than needing a unique design for every individual one they want to build, most sunk ships, etc aren’t designed to hold the pressure at those depths anyway so there isn’t really any point trying to turn them into a pressure vessel. It would be very expensive from a design perspective and very expensive to implement as well considering most of the construction would have to be done underwater not in a factory and then dropped in.

  7. I was experimenting a while back with an atmospheric version of this, a bit like a barometer, but on a much larger scale. Not so much for earth, but for space, where the temperature swings can be far more extreme. Imagine a rotating satellite that heats and cools each side periodically and uses the pressure differential to produce power for the other parts of the cycle.
    This sort of thing could also work on Mars, all be it requiring a bit more scaling up.

    1. Interesting way to combine solar thermal to electric energy and storage.

      Moving masses of the thermal working fluid aught to create a center of balance and station keeping and maneuvering issues to work out.

      1. Maybe have multiple smaller ones offset at an angle from each other so the forces are more uniform throughout the craft rather than just at one point, also since the craft would be spinning with a constant force in one direction maybe you could just angle that in a useful direction so it doesn’t create any issues or compensate for it for longer flights. It might not work as well in orbit though than travelling out of orbit.

  8. I did not read every link. Does it discuss building these into the bases of the wind mills and using direct mechanical linkage to the pumps? Is that more efficient? I’m sure it cuts the parts count and electronics. But which is more serviceable? Or hydraulic pumps in the mill and hydraulic motors in the pumps? Efficiency versus longevity? Consider repairability if exotic materials become unavailable (rare-earth magnets or motor control chips for example)?

      1. Add some glass fiber to the concrete and it’s not going to crack. The building across the street from where I live has a concrete parking apron that’s around 30 years old. Zero cracks in it because it had strands of glass fiber mixed in.

        1. It’s also not going to get recycled and it will be a huge pain to remove once the structure has to be torn down. Concrete “recycling” is a joke anyhow – it takes huge amounts of energy, makes emissions, and the end result is low value rubble.

    1. Also soak it in salt water and see how long it holds up. I am not sure if there is some way to protect concrete long term from salt, however I know that using salt on concrete for melting ice is not recommended.

  9. One day when there are floating cities out at sea, I can see solar and wind energy stored in tanks attached to upside down skyscrapers under water. And having to deal with changes in buoyancy as the tanks fill and empty.

  10. Obviously these things can be linked to the same grid as the offshore windfarms we have in the UK, which keeps the storage co-located with where the excess power is being generated too.

    What I’m not sure about is the need for extreme depth. Water is fairly incompressible so you don’t have to suck much out of the vessel to go from 10ATM to 1ATM. So that’s not much energy, in my book. It’s the full emptying with a vacuum left behind that is where you are moving a lot of water in or out, is where the energy is. So they don’t have to be very deep.

    I was also thinking that instead of making them out of solid concrete, why not burrow into the bedrock and line the cavity? You could then make the structure much bigger, and the anchoring problem is solved too.

    1. I did eventually realise that there is extra work done at increasing depth because you are pushing against the 10ATM (or whatever) of the surrounding sea. Interestingly this means that only the inlet turbine needs to be at depth, the pump to evacuate the chamber can even be at the surface.

  11. I do not see the advantage here
    Why not just place the storage tanks,(they would not have to be tanks just pools) on shore?
    Easier to build, easier to maintain, simply empty and fill like other pumped hydro
    No need for a sealed concrete container subject to daily stress from trying to create a pressure difference. The tank will expand and contract on a daily basis, cracks will quickly form, catastrophic failure is certain, repair will be impossible
    The deeper this system is placed the more difficult it will be to build and maintain, divers will die
    Any pump system that works under water will work just as well on shore but as the pressure differential increases, the pumps will become less efficient, using more energy to pump less water, so the energy return when the system runs in reverse, will be far less.
    Another project that is overengineered, simply for the sake of it

    1. You would have to build the concrete containers on 1000feet tall stilts/towers, in order to have the same advantage as the same volume at 1000feet depth (ignoring salt water density vs freshwater,etc.). If you want to put the energy storage system near a major coastal city (say San Francisco), you’ll be hard pressed to find a site with in 20 miles that doesn’t cost a fortune and doesn’t cause an eye sore when build these towers.

      While I’m not sure this is really a viable solution, there is an elegant solution here that is more than “overengineered, simply for the sake of it”.

    2. You can’t just put a pool on shore to do the same thing, flow is proportional to the pressure difference and the turbines generate power based on flow. There has to be some kind of pressure difference, whether that is from traditional pumped hydro just by pumping it up high or from putting it deep under water. Even if you had a pipe going into it from deep underwater the pressure would decrease the further up to sea level you got until it is useless. Pumped hydro from the sea does not work at sea level.

  12. I love all the engineering based comments…

    Do we not think though, that filling a bucket with pebbles raises the water level?

    Then somebody mentioned about the heat exchange and basically this will warm the water.

    Then it’s made of concrete which gram for gram releases CO2 into the atmosphere during creation.

    So anybody that’s pointing to how green this method is needs to look at the wider picture.

    I don’t know what happens when any tiny creatures / alge, etc. Is sucked in. The closed systems seem better for that.

    1. All good points.

      I would think on creatures algae etc it won’t be a huge issue – either the thing is cycling so much they are flushed in and out rapidly so can’t end up as large clumps that might cause issue or you keep the thing at low pressure long enough anything left behind as the water was evacuated dies…

      This should not raise the water level a meaningful amount – the scale of the sea and even most of the worlds larger lakes are just so vast this structure displaces an irrelevantly tiny fraction of the total.

      Concrete use is certainly a ecological concern, but again I don’t see this as being all that bad there if we make a few assumptions – reasonably long lifespan, that it reduces the number of turbines/solar/tidal powerstations that need to be built (which while acting as foundations for off shore wind means its a nearly free value add there – you needed a foundation that was probably concrete anyway) and that it removes burning fuels so reduces the carbon output of the grid overall. Its like any big hydro dam you flood a large area that was once something else, and used heaps of concrete is the construction, but that structure will keep doing its job practically forever, at least in human timescales…

      Certainly points that need some real world study, and some simultaneous equations from those tests to decide what works out best between greater oversupply of renewable on average and these stores on environmental and cost basis etc.

  13. I have thought about a buoyancy system for years that’s similar. You’d have a tank attached to a chain and gearing system and use the solar energy to drag the ball down under water then once locked in place at the bottom pump the water out. When you need energy allow it to float back up pulling the chain and with gearing should be capable of a bit more generation then the other gear ratio that’s used to just pull a water filled tank down. Of course at the top it would open a hatch on bottom and top to make dragging it down slowly effortless. I guess their way sounds better as the energy would go way up further down you go.

  14. The inside of the empty sphere is not “nearly vaccum”, that will be silly as the pump will run in cavitation mode most of the time, destroying itself fast.
    In fact the sphere is probably filled with air at a atmospheric pressure, then you let the water flow. Once emptied, you still have a good pressure differential (for example 99bar instead of 100 at 1000m depth) and you won’t run into cavitation issues.

  15. With compressed air pushed into the spheres to blow out the water, there wouldn’t need to be anything electrical under the water. Control valves could be air operated via a second, much smaller air line. The pressure of the water would reduce the thickness requirement of the concrete.

    To tap energy from the spheres, let the high pressure sea water flow in and run the air through turbines. If you want more power generation, water turbines could be put into the spheres at the inlets, but that would require electric cables, generators etc to withstand the cold, pressure, and salt water of the deep ocean.

  16. The technology is amazingly feasible. Actually I patented an invention using the principle of underwater pressure to produce electricity. It is already being used in one of the schools in Pasay City Philippines. The Ocean Grazer concept is great although there are parts in the system that can be improved for it to function perfectly.

  17. Interesting idea!
    I started thinking about if it would be possible to do it with a minimum of concrete. Maybe instead of a concrete sphere, a bladder with some structure inside to withstand the pressure. Inspired by the mechanics of an umbrella… maybe it could be folded at manufacturing, transport, and submerging. And when it’s secured to the ocean floor you could pump water in and it would unfold and snap into a locked unfolded state.
    The bladder, if it’s even possible I don’t know, would of course have to be very tough. Possibly several layers of Kevlar-reinforced plastic.
    Also, maybe it would be possible to connect several bladders (or spheres) via a manifold to one generator instead of having one for each.
    Regardless of the solution shouldn’t it be possible to anchor the devices by drilling deep into the ocean floor, and thus avoid concrete structures?
    S little hard to explain my ideas in text, so I did a little napkin sketches. https://i.postimg.cc/yxq3TZHx/bladder.png

  18. Can’t the same degree of excess energy storage be achieved by winching solid concrete blocks up from the seafloor? And then retrieving the energy by spinning a generator while allowing the blocks to sink again? That would put all the electrical stuff at the surface where it would be easier to maintain.

    1. All that means is you have to build it to last a long time between services – something that is very possible, though more expensive than normal in the throwaway societies of today or develop the methods to make servicing easy, which is again very possible.

      I’m not sold this is the best possible solution but the fact its so easy to deploy nearly anywhere – there is a huge pile of ocean just off many major cities and industrial areas, and it can dovetail really nicely into the offshore wind developments makes it seem to me like at the very worst it can end up with a mediocre outcome, successful enough but not as easy/cheap/effective as x…

  19. Seems the intent is to use pumps / turbines that remove all but a couple percent of the water from a large tank at the bottom of the ocean. Whatever remains of the water in the tank will vaporize giving the tank a pressure equal to the vaporization pressure of water at the temperature of the ocean water. Somewhere under one psi. I would worry about water infusing through porous concrete. So I would cast the concrete container inside a stainless steel shell, cylinder or sphere. A liner would do no good protecting against water infusing through a concrete shell without a surrounding water barrier. An empty tank would be more buoyant when it contains only a volume consisting of water vapor then it would be with a lot of compressed air. That would make it easy to bring to the surface for maintenance. The trick would be keeping it on the bottom of the ocean when it is very buoyant and ocean currents and storm currents might dislodge it. Of course these are the same problems the wind turbine would experience in same ocean region. Fascinating ideas, fun to play with

  20. Seems neat. But putting it only a little to the plausible side of spin launch.

    Even if the headline numbers look goodish, it all seems small and to need rather a lot of modules. Much prefer big machines.

    1. While there is some element of truth to that once you get far enough offshore so the water is deep the seafloors are often lifeless or nearly so – varies by climate, depth etc but there are big stretches of ocean floor that won’t find the intrusion of a funny concrete orb a problem at all – it might even be a good anchor point for expanding onto, and others where it will just be another bolder on the bottom.

  21. It’s an interesting take on a gravity battery, as you could store greater amounts of energy over ever shortening distances. Like a spring storing mechanical energy though, you have to find a way to get output that is not reliant upon the distance traveled. Maybe something like a pulley block, or a series of floating air chambers moving between different depths would work.

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