Iron Cycle based energy storage and generation
teamsolid.org
teamsolid.org
2. Iron is dense -- ?? Not discussed in the webpage. But something like Hydrogen (despite being very light) takes up a lot of space. Hydrogen is so light that its volume becomes a problem. Iron does seem to solve that issue at least.
3. Iron is renewable -- Iron turns into Rust, and the rust can turn back into Iron easily. This makes Iron more comparable to Redox-flow batteries or Hydrogen.
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Funny thing is: I was just discussing with someone else in my social circles about how Iron/Rust is exactly the same process as battery-chemistries. Just... worse. Iron-electricity has less voltage than Li-ion or Zinc, or other elements.
But the Iron -> Rust -> Iron process is a well known process for "storing energy". We just usually don't care about the energy storage part, and more about the physical properties of Iron, and preventing it from turning into Rust (which is weaker, redder, more stains, etc. etc.).
Experimenting with Iron-based energy storage seems like it'd be destined for failure? Its one of the oldest elements and the chemical process has been known for a while. However, the only way to be sure that its a bad idea is to try it. So we should experiment with it, at least a little bit, to really make sure that we didn't miss anything here. Iron is a hugely abundant element after all... if at all else, it can be used as a "cheap" energy storage mechanism as opposed to a "quality" energy storage mechanism.
After all, a lot of our energy is just wasted (ex: Solar Panels during the peak days just turn off). Instead of turning off solar panels when the grid is overloaded, maybe store the excess electricity in cheap iron? 40% round-trip efficiency is still better than 0%.
I envision iron to be one component of a complicated green energy future if such a future even possible as many sources day it's too late.
Odds-on favorites include hydrogen synthesis, ammonia synthesis, pumped hydro (up hills and up out of underground cavities), liquified air, and various battery technologies. But there will without doubt be some surprise break-outs, and maybe surprise duds.
But round-trip efficiency doesn't matter much, given top-line electrical abundance from epochally cheap solar, wind, and maybe geothermal generation. Geographical placement doesn't matter much, given grid distribution. Battery technologies will yield to better ones. Cost for every kind of storage will only ever decline, and efficiency at every stage will only ever improve.
So, it will come down to cost and value: i.e., what do the batteries or the synthesis and draw-down equipment cost (tankage is always cheap), and what is the industrial value of excess synthetic output after local tankage is full?
Cost for every energy use will be substantially lower than today, as each provider competes to deliver from unlimited energy sources more cheaply than the others. Nukes and fossil fuel have already been priced out of the market.
Whatever green future awaits us, it will be much less convenient then the days of the past.
It is a public fact that renewables cost less than fossil fuels. They cost less than anything in history, no matter how you measure. Clearly you wish this were not so. But reality does not tailor itself to what you wish.
Renewables don't deliver the energy density of fossil fuels. When you factor in storage and density renewables costs more.
A good way to illustrate the extremes is air planes which basically can't run on renewables.
When I say the future is bleak I don't mean that we should continue to burn oil. I think we have to stop burning oil. What I'm saying is that we have to stop, and the result of stopping is a future that is much less convenient then it is now but way better then the future of continuing to burn.
The problem with you and your ass hole comment about Exxon, is that you view it as a team based thing. You're on team environment so anyone else not on your side is team Exxon. Why don't you take a step back and look at is dispassionately. I'm not on ANYONES side so I'm not fucking blind to the fact that oil is a critical part of our civilizations infrastructure and removing it completely will change many things, including your ability to fly to another country. You have to be next level stupid if you can't see the downsides of green energy. Stop playing teams.
Aircraft using LH2 will drive kerosene-driven out of all high-profit routes as quickly as they can be built and fielded, because besides being the cheaper fuel, per joule, LH2's extreme mass-energy density leaves much more of rated lifting capacity for paying cargo. I.e., the LH2 craft will drive prices below the base operating cost of kerosene craft.
Retrofitted and new NH3-tanked shipping will similarly drive out the still bunker-oil driven as NH3 price falls, accelerated by dockage for polluting craft becoming increasingly restricted.
Plastics might still be made from the cheapest of pumped oil, but that is not certain.
Except they don't because you can't even get that level of storage without a huge downside.
>Soon, synthetic hydrocarbons will be cheaper than most mined/refined/shipped, where that stuff is still needed at all.
Only because the supply of fossil fuels is dwindling. Using renewables to make this stuff will NEVER be as efficient as the way fossil fuels have been used in the past century. Free fuel out of the ground vs. making it? Free fuel is way more efficient then either biofuels or hydrogen that has to be made.
Ethanol and biodiesel are the two main types of biofuels, derived from organic matter (obtained directly from plants, or indirectly from agricultural, commercial, domestic, and/or industrial wastes). To be a viable alternative for petroleum, a biofuel should provide a net energy gain, offer clear environmental and economic benefits, and not reduce food supplies and/or increase their costs. Biofuels fall short of these requirements and should therefore stay a niche market, used moderately and optionally instead of mandated at wide-scale public use. Environmental groups themselves have found that between 2008 and 2022, biofuels will receive more than $400 billion in subsidies.
>Aircraft using LH2 will drive kerosene-driven out of all high-profit routes as quickly as they can be built and fielded
Unlikely. The large amount of energy required to isolate hydrogen from natural compounds (water, natural gas, biomass), package the light gas by compression or liquefaction, transfer the energy carrier to the user, plus the energy lost when it is converted to useful electricity with fuel cells, leaves around 25% for practical use — an unacceptable value to run an economy in a sustainable future. Only niche applications like submarines and spacecraft might use hydrogen.
If planes use hydrogen getting on one will cost astronomically more than it does today. The business incentive and R&D on such things will be infinitely less. Planes will be running on oil for as long as the industry can.
You also still haven't apologized for being an ass. Please do that.
When you discuss things with people please refrain from offensive statements like "are you Exxon?" It's offensive and akin to like "Are you kkk?" There's really no need for that.
Although I shouldn't have called you an ass. Dang I want to bring to your attention that although the word "ass" is more explicit, this person essentially started the waterfall and is saying things that are much more offensive but sort of more stealthy. The Exxon thing was a deliberate and targeted jab.
The question for any specific technology is: Is there a specific application where it is a better choice than any alternative?
There are some properties here that are interesting (see other comment in this thread) and that may make this into a viable technology for specific applications.
I'm just grabbing a random number. Basically, it doesn't matter how well it performs because the current "solution" is just wasting energy at peak periods on the grid right now.
> A detrimental side of iron fuel that it has a relatively low specific energy density. Therefore, it cannot be used in applications in which weight is of importance such as cars, trucks and planes. Also, it can only be transported in an economic manner by ship or train. However, most of the heavy industries are located on railways or waterways. Therefore, iron fuel is applicable to those industries.
The question to ask is "where could you use this that you couldn't also use electric heaters and/or batteries".
Seems like a similar scam to "gravity batteries". It may work but alternatives are likely cheaper, lighter, and more readily available.
> "The resulting iron oxide is a solid material, so it can be captured after the combustion process. It is then reused by regenerating it with green hydrogen into flammable iron fuel. In this way, iron fuel offers a revolutionary method to store energy in a circular and carbon-free fashion."
It has some real advantages over shipping hydrogen, as hydrogen is tricky to work with, relative to methane. There is an alternative process, developing direct-air-capture of atmospheric CO2 and using the hydrogen to reduce the CO2 to CH4, which has the advantage of being able to use existing natural gas pipelines for transport and distribution. In contrast this iron process has some long-term storage advantages; it could serve as a way to store excess wind/solar power over longer periods i.e. months. The notion it could be used to fuel long-distance shipping is also intriguing: ships could use hydrogen to regenerate their iron fuel at the end of each voyage.
However this all depends on massive scale-up of hydrogen-from-water. That hydrogen can be used in many industrial processes, including also fossil-fuel-free steel production. Without that, this iron process can't be run at any scale.
EDIT: My search-engine abilities apparently failed me. I'm seeing 0.44V as the voltage... well... whatever Iron's voltage is... it is pretty small. That's my point.
There's a reason we didn't make Iron batteries. Iron definitely stores electrons, but the voltage dropoff leaves a lot to be desired...
Iron stores 2 electrons at very low voltage. That fundamentally limits its energy density.
Li-ion stores ... some number... of electrons at 3.7V. Looking at the periodic table, I think 1 electron? In any case, the big 3.7V drop really changes its energy / power characteristics.
You'll need additional cells to increase either voltage or current (doesn't matter which: electric-engineers can convert either voltage or current into power). But without fundamentally good voltage or current (ie: stored electron) stats, you're just not going to have much energy storage.
And if each of those cells are heavier because you're using say... a heavier element (ex: Iron) instead of lighter elements (ex: Hydrogen or Li-ion), then its that much worse.
But the status-quo for chemical energy storage (that is burned / re-released) is maybe Hydrogen?
If you don't care about weight / density issues, then a giant steel-tank containing many/many tons of Hydrogen Syn-gas might be a better storage option than iron?
Hard to say really. I'm willing to see people experiment with the technology. Iron is one of the most abundant elements after-all, so any use of Iron will almost certainly have raw cost-benefits.
But will it be cheaper to use the iron as energy storage? Or is it cheaper to turn the iron into steel, and then pump Hydrogen into the Steel like a balloon and pressurize-store Hydrogen at 10,000PSI or whatever?
That's usable but not great and the installations tend to be costly and somewhat fragile due to all kinds of attributes that hydrogen atoms have which they impart on the materials that that installation is made out of. For instance, hydrogen tends to make metals brittle so regular ductwork isn't going to work. Hydrogen installations also have the bad habit of going 'foom' due to the extremely low activation energy.
So there is some pressure to try to find a storage system that is both cheap, doesn't use exotic materials, is chemically stable and reasonably energy dense and has a round-trip efficiency as high as can be achieved.
This is a complex problem to put it mildly and there is a whole raft of technologies that are being examined all of which have different trade offs making them more or less applicable to certain applications.
The 'Iron cycle' is one of those, it isn't a winner on all or even a majority of dimensions but it has some interesting properties, notably: the cycle is uses only commonly available materials, it doesn't require long term storage of the hydrogen (which can be produced and used in the same cycle step), the materials involved are stable in both forms (pure iron vs iron oxyde), it is reasonably (not super) energy efficient and it has an energy density which for stationary applications is not problematic.
The biggest questions at this point in time are: does it scale and does it do so cost effectively. If the answer to both of those are 'yes', which this experiment in scaling up the cycle to 5MW should answer then it will at least be a viable contender, much more so than many other schemes that I've seen come by in the last couple of years.
Most of the bulk hydrogen storage plans involve underground storage at a pressure that would not cause trouble. Where more density is needed, e.g. aircraft fuel, liquid form is favored, again at low pressure.
And for liquid storage you have to count on some H2 boiling off and for underground storage you will need to be extremely careful about possible ignition sources.
You need to worry about material degradation mainly at high pressure where the material has to be very strong, and where the hydrogen is driven into the crystal structure, weakening it. At low pressure, ordinary aluminum is resistant to hydrogen degradation, and doesn't need to be strong.
Whether liquid hydrogen is kept refrigerated below its boiling point, or kept well insulated but allowed to boil off, will depend on detailed engineering analysis. As the energy needed to produce it gets cheaper, the extra complication of keeping it too cold to boil gets less attractive. Maybe to refrigerate the tank, it needs to be above-ground so servicing is possible; but without, it can be wholly underground and thereby better insulated.
One interesting recent development is the replacement of the cobalt needed in lithium batteries with iron. Cobalt wars in Africa's Congo are a current issue:
https://nextmoneyng.com/2021/12/22/the-cobalt-war-the-sino-a...
Now it seems that cobalt can be replaced with iron with no efficieny/storage capacity losses:
https://crm.org/articles/a-clean-industry-revolution-the-lit...
Lithium itself doesn't really seem replaceable for where it's used, in applications that demand relatively high energy density and voltage.
Evaporating naturally occurring brine in poverty stricken shitholes is just a lazier way of extracting the tiny amount that we've used historically rather than capital intensive extraction from spodumene, clay, and geothermal or desalination wastewater.
Come to think of it, I guess Nevada is a poverty stricken shithole, but it's my poverty stricken shithole and I'm proud of it.
It's being sold as a complementary solution to more efficient LFP cells (for evening peak shifting), and as an alternative to pumped hydro and peaking gas turbines.
It would be better to waste the energy than to let them have it for free.
I really want them to succeed for the sake of the planet, but it's another "revolutionary new battery just five years away" story.
As solar gets cheaper and cheaper, the ability to mitigate efficiency loss by just increasing the size of the solar installation gets easier and easier.
The Future of Energy Storage - Professor Yet-Ming Chiang, MIT
https://www.youtube.com/watch?v=E76q-9q7ZDg
Professor Yet-Ming Chiang, one of the world’s most prominent researchers in energy storage, will give a personal perspective on some of the challenges and opportunities for better and cheaper energy storage. From a historical perspective, battery performance has improved steadily, but now the divergent needs for batteries in high energy density applications such as EVs (and, coming soon, electric aviation), and those for grid storage, which emphasizes ultra-low cost and earth-abundant materials, are becoming clear.
He will give examples of emerging innovations that may address these different needs, including solid-state and other batteries that use alkali metal electrodes, and approaches that make use of the most widely available electroactive elements.
Battery technologies are notoriously difficult to scale given high technology risk and high development costs. Yet-Ming will share some of the unique lessons learned at startups he has co-founded, including A123, 24M, and most recently Form Energy.
https://www.spglobal.com/marketintelligence/en/news-insights...
Whether with iron or some other energy-consumptive process, imagine if a municipal truck rolls up to your house with a bucket of rust and picks up your bucket of iron. You roll the bucket of rust into a receptacle in your furnace and through the winter, slowly process rust back to iron as you heat your home with the waste heat.
You're happy, because your heating costs are offset by the sale of processed iron. We're happy, because someone somewhere isn't consuming even more energy just to create processed iron and you're probably able to process iron at a lower cost, reducing the price of processed iron.
Depending on the process, the thermodynamics may or may not pencil out, but it's a dream I keep contemplating. The key requirement is that the process be dead-simple, hard to game, and very energy-intensive.
Stirling engines are quite tricky to get right, as I understand it. And that sounds like you'd need a good sized one, not a toy.
It's quite magical to see sunlight turned into motion without any intermediary.
A system more similar to what we have in New England would be the opposite of what you describe: a truck drops off a load of fresh (elemental) iron, and it's oxidized over the winter to heat the house. Then, in the spring, someone comes back and removes all the rust for recycling. This plan would allow iron to replace a lot of propane and heating oil if the energy densities work out.
There's no need for this to be done at a home, since the home is on the grid.
I have severe doubts that iron has as much energy density as methane or propane.
I also doubt that Iron is as easy to carry around as methane pipelines.
The heat of reaction of elemental iron oxidizing to rust is about 1648kJ per 4 moles Iron. Propane burning is about 2220kJ per mole. Also, Iron atoms are heavier than molecules of propane (55.8g/mol vs 44.1g/mol). This means that propane has an energy density of about 50 joules/gram while Iron has about 7 Joules/gram. So no, not very energy dense on a per kilogram basis.
However, energy density is only relevant to cost of transportation. If elemental iron is stupid cheap compared to methane due to taxation of external costs associated with emitting carbon dioxide, then the additional cost to transport it would be less relevant. In addition, since it is renewable, the cost could be reduced further. But yeah, we are a long way from that since methane and propane are very cheap right now.
I mean, if weight is the only issue, then maybe don't transport it ever?
Its not like we try to "transport" the Bath County Pumped hydro station around (aka: the largest battery in the USA). We just let it sit in one location and convert from electricity (usually nuclear at night) into energy-storage (pump water uphill), and then convert it back later (release the water during daytime peak-electricity usage).
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If the energy "needs to be transported", then convert the energy into another form (Ethanol fuel? Syngas Kerosene? ). Each conversion loses efficiency of course, but if Iron is cheap enough to use as "energy storage", then it can be our "energy storage of last resort", since any storage is better than waste. (IE: Never turn off your solar panels. We always have "something" to dump our excess electricity into)
Anytime you hear 'use waste heat for X', the thermodynamically more efficient version uses a heat pump. And with time, the thermodynamically more efficient version will also be the economically more efficient version.
Cities are the solution there. Waste-heat from power-plants is ~200F (just under boiling), because the water needs to be steam to efficiently move a turbine. The 200F waste-water is no longer useful at turning the generators, but its still useful to pump around the city and warm up homes.
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IBM was playing around with waste-heat systems to increase the efficiency of solar panels. Solar Panels lose efficiency above 140F, which is pretty common because the sun heats up the solar panels. By running water through the panels and dropping the temperature down a bit, you end up with say, 130F water and 130F panels (increasing panel efficiency, but now you gotta deal with 130F water).
The 130F water is waste heat, but unlike New York City steamworks, its in a hot location that won't be able to use the waste-heat in a useful manner. Unfortunately, I don't think this is high enough temperature to be used for this chemical-iron process...
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That being said: Iron Fuel itself is something I haven't considered personally. But the chemical process sounds quite similar to Hydrogen Fuel / Syngas. Iron being a solid sounds like its easier to move around and store, but Hydrogen has pumping benefits (ex: hydrogen pipelines).
Chemical energy storage methodologies seem useful. There's all sorts of low-grade waste heat (130F to 200F), that's really difficult to figure out how to use. Anything, even if its like 30% or 40% efficient, will be better than the 0% efficient "Waste" that we do right now.
Somebody is selling plastic frames you attach your panels to, to blow air through to cool the panels and claim the excess heat. Air ducting makes less trouble than plumbing does. The air can go through a heat exchanger attached to a heat pump to concentrate it.
Keeping the panels cool increases their efficiency enough to pay for the fan and heat pump, and makes them degrade much more slowly; and, you get the heat if you want it.
I've seen systems where heat from A/C is used to heat swimming pools. Mostly just needs an extra exchanger on the A/C coolant line that transfers the heat to the pool water. You generally run both system at the same time when it's hot out. (assuming you are fortunate enough to have a pool)
Might be a nice alternative for storage of energy surpluses, because storage is dense in terms of volume, and storage is not complicated by storage tanks and massive pressures.
So I'd expect the closed cycle efficiency of this to be at best 42% - assuming a 100% efficient regeneration phase (highly unlikely). It's probably more like 20% or less.
Poor efficiency for energy storage isn't necessarily a dealbreaker - if the capital cost per kWh stored is low then it may still have a place in the mix - but it's unlikely to be a silver bullet.
'CO2 free' - H2 cycle systems are also CO2 free
'Compact' - the energy density of iron is already pretty low and the proposed machinery also doesn't seem very compact. Maybe storing uncompressed H2 would be somewhat inconvenient depending on your location, but you can always compress it. And if you need longer term storage, you can actually make hydrocarbons from it, although that wouldn't technically be CO2 free (only CO2 neutral if you use captured carbon)
'Cost-effective' - this system seems way more expensive, per kWh and in absolute terms, than gaseous cycle concepts.
'Easy to store and transport' - the iron is certainly not easy to transport, nor is the machinery using or recycling it. It is easy to store safely, just not efficiently.
'Scalable' - while iron is abundant on Earth, most of our iron is actually at inaccessible depths, so we're stuck with traditional mining. If you think iron is cheap, you haven't tried to use it as a building material recently. Cheap and abundant metals would be calcium or potassium. Even cheaper, again: H2.
'Safe' / 'Commodity' - true, but how relevant are these, really?
> It is quite energy intensive to produce it
We're talking about a cycle that is supposed to store and release energy. "Quite energy intensive to produce" would be a plus in this context. It would matter though if it was inefficient to produce, which is not the case. On top of all that, H2 is actually a part of the proposed iron cycle system.
> and currently is produced from oil & gas products
It would obviously not be produced from oil and gas products in the case we're discussing here. If you're going to have an H2 cycle, it makes zero sense not to start with water.
> but if you're going to do that, it turns out that just using the electricity is almost always more efficient.
As far as I'm aware, the iron cycle system also does not claim to produce energy in order to compete with solar or wind. It claims to be a good and portable storage solution. Thermodynamics absolutely assures that any such system will always be less efficient than using the electricity right away. It is absolutely ridiculous to use an argument of inefficiency against an H2 cycle system while speaking in favor of an even more complex other cycle system!
I think we need a number of different solutions. Solar takes up vast real estate and isn't very efficient, wind - we bury how many thousands of 747 size blades in the ground each year that won't break down? Surely, that's not sustainable. Nuclear needs to be on the table, too.
This is worth worrying about, however landfill is one of the most plentiful non-renewable resources we have. These blades are completely non-toxic and can be buried safely so it is one of the least worrying problems about wind. The most damage that wind causes is destruction of ecosystems and excessive land use. Even that is pretty minor compared to other power generation methods. Wind is pretty awesome. (Also, yay nuclear!)
https://www.siemensgamesa.com/newsroom/2021/09/launch-world-...
Just grind them down and put them in landfill or whatever, it's fine.
https://www.theguardian.com/environment/2020/aug/06/nautical...
As mentionned by other the fact that we talk about "waste" from wind turbine is likely due to fossil companies funding media campaigns about it.
Example: ICE cars vs. Electric: electric cars need more resources to create and thus use more CO2 in reality!
Simple truth: initial energy cost might be high but battery-electric cars catch ICE cars in resource friendliness after only few years.
They just had to find one for wind turbines that was better than ”they are an eyesore”.
In practice storing labor is a very difficult or maybe even impossible thing, so the ability to store additional labor in the form of physical capital is very useful. Lots of people want to retire early. They want to work a lot in their youth and then relax instead of working until they die. If you get an ICE car right before your retirement, you are dependent on a lot of young people running the infrastructure necessary for that car. Meanwhile with EVs that infrastructure is a lot less labor intensive so a demographic decline isn't that bad. All the work was done before demographics got worse.
Now try doing the same with food. You can drive a car for 20 years off a wind turbine that lasts 20 years but you can't store food for 20 years. You are reliant on young workers who will take care of you.
In other words, I wonder if iron is so heavy that you pretty much have to do this burn/reduce cycle in close proximity of eachother. Useful for a battery maybe but not as much for a transportable fuel, except maybe for boats and trains
https://news.ycombinator.com/item?id=24996153
What I do wonder is how well the storage part really scales: what would be the most economical way to keep large energy stockpiles in that shape safe from, well, getting a little rusty over time?
https://www.nu.nl/economie/6194949/nutsbedrijf-veolia-gaat-e... (dutch)
Conventional blast furnaces use a reducing gas or coal for the reduction.
The website has no link to anything concrete here (but mentions conventional furnaces).
And also, how on earth do you make iron dust from the iron after reducing it back from its rust state?
As for the second: you don't recast it as pig iron, you keep it in its fragment state.
"fragment state" is something that does not exists.
You got a large vessel of molten iron, you pour that into a form, take the slack off and that is it.
Maybe one could use a fall tower to make small iron pellets like the ones used for making lead shot.
That would need to be done in an oxygen free environment using some inert gas.
The whole process seems quite impractical.
And something else: Pure Iron rusts quite easily so how do you store it? You can not store a large bucket full of iron pellets for your oven in a cellar.
So storage would need an enclosed container.
The whole idea is just weird.
Storing and using hydrogen to turn it back to electricity at scale is not a simple problem (see my other comment in this thread).
> "fragment state" is something that does not exists.
In the article they are pretty clear about using very small particulate, both for the 'pure' state as well as for the 'burned' state.
> You got a large vessel of molten iron, you pour that into a form, take the slack off and that is it.
This has absolutely nothing to do with the process as described in the article, you are discussing the casting of pig iron, here we are talking about an oxidization and a reduction process based on powder sized grains of iron.
> Maybe one could use a fall tower to make small iron pellets like the ones used for making lead shot. > That would need to be done in an oxygen free environment using some inert gas.
This also has absolutely nothing to do with the process as described in the article.
> The whole process seems quite impractical.
That's the problem with strawmen.
> And something else: Pure Iron rusts quite easily so how do you store it? You can not store a large bucket full of iron pellets for your oven in a cellar. > So storage would need an enclosed container.
Yes, like the storage for any other kind of fuel.
> The whole idea is just weird.
It's weird because you don't appear to understand the first principles of the process described in the article, which is already operating at a smaller scale and which they will now scale up to 5 MW to gather more data about costs and scaling efficiency.
>
Fuel has other valuable properties, though. I hope both of these can help soften our landing.
Using deep underground cavities to drain water into, and pump back out of, extends pumped hydro to a lot more places than are usually assumed suitable. And, it multiplies the energy stored up a hill if you can drain the water to somewhere deep.
The most surprising method I have enountered is deep-ocean pumped storage. This doesn't need a pipe, just a tank anchored down deep with a pump/turbine at the bottom. You charge it by pumping the water out, leaving water vapor, and get the energy back by letting the water back in. The only connection to shore is the wire. The amount of energy you can store in a 30-meter diameter tank under 300m of water is amazingly large. The tank does need to be very strong to hold back the pressure.
The same principle applied here: heavier fluid and the lighter one are separated by a membrane, you pump the lighter one in under pressure to force the heavier one out and the heavier one will descend by itself and force the lighter one out due to gravity. Efficiency will be impacted by the ratio between the lighter one and the heavier one. Ideally the lighter one would be a gas but at those pressures that may no longer work.
Or, deep under the water table, and no tank or bladder needed because there is a big cavern down there. I suppose this could work with a lightweight, immiscible liquid, instead...
A pipe down to a closed bladder, and pumping water in, would be simpler.
In practice, I'd have reservations about widespread manufacture of methane. Existing methane infrastructure is often leaky and methane itself is a far more powerful greenhouse gas than carbon dioxide. That isn't to say that manufacturing methane shouldn't be considered, but it's extremely shortsighted to say that methane manufacture is the only storage technology that should be considered.
The cost of equipment will be coming down fast.
It needs, anyway, a supply of hydrogen and carbon. As hydrogen gets more valuable for other processes, its price might even go up. Extracting carbon from the atmosphere costs a few hundred dollars a ton; that cost might go down. The oxygen has to be stripped off, which costs energy you will hope to get back.
Do you have a source for that? Also, where does the carbon for the methane come from? With iron/rust there is no carbon in the cycle.
Generally, anhydrous ammonia is a more practical medium where pure hydrogen won't do. It just needs hydrogen and air as input.