This is a pretty elegant idea. It takes 826 kJ to split a mole of iron oxide (Fe2O3) and it takes 855 kJ to split 3 moles of water (H2O). So if you take H2 and blow over one mole of Fe2O3 you can strip the O3 for the cost of 826 kJ but then by burning the hydrogen in oxygen you get 855 kJ, for a net exothermic effect of 29 kJ, which is a rounding error. The opposite reaction requires 29 kJ, again negligible, there are probably bigger energy losses bringing the reactant mass at the required temperature (400 degrees C).
Unfortunately, I don't see this making any sense for large scale energy storage. Storage tanks for compressed hydrogen enjoy the square-cube law. The larger they are the less expensive they are proportional to the mass of hydrogen they hold.
With this iron oxide method, you need 27 tons of iron oxide for one ton of hydrogen. You can procure right now tanks that can hold 2.7 tons of hydrogen and weigh 77 tons empty [1], the ratio is 28 to 1. But the round-trip efficiency of the tank is virtually 100%. The efficiency of the iron-based storage is only 50%. The tanks are not very expensive.
I can't see the niche that this idea can apply to.
> Storage tanks for compressed hydrogen enjoy the square-cube law.
Not really. Wall thickness is roughly proportional to diameter, and surface area to the square, so you don't gain anything in terms of storage mass ratio by building bigger tanks.
> But the round-trip efficiency of the tank is virtually 100%
This is oversimplifying quite a bit. Compressing hydrogen, the lightest gas, is very energy intensive per unit of mass, and this energy is not fully recoverable upon decompression (due to general pump efficiency and thermal losses in the intercooler).
27 tons of iron oxide have a volume of 5m^3 and can be stored in pretty much a hole in the ground.
2.7 tons of hydrogen have a volume of almost exactly 30000 m^3, requiring storing it under high pressure in specialized containers. Hydrogen is famous for being hard to store without losses.
For long-term storage storage and losses are a problem.
> But the round-trip efficiency of the tank is virtually 100%. The efficiency of the iron-based storage is only 50%
Maybe I'm missing something, but why? As you mentioned it takes 29kj to restore 3 moles of H2 out of (3 moles of H20 + 1 mole of Fe2O3). Where does 50% comes from?
i.e. the paper[0] states that first "discharging" produced 7.09kg of H2 out of 8.71 theoretically possible
the efficiency is super low, but again, according to the paper, "most of the energy input was due to thermal losses at the reactor surface (83.9%)", which also benefits from square/cube law.
Iron oxide is completely inert. You can store it in piles, under the elements. Iron powder less so, you need to keep it dry, but again you can just pile it up. The only tricky part is moving dense dry solids around at the huge scales required.
Edit: wait I forgot that direct reduced iron powder exothermically reacts with oxygen and water in the air, that's how single-use instant hand warmers work. So yeah you gotta isolate the iron powder a bit more than stick it under a tarp.
I've been playing too much Factorio lately so of course my mind goes towards rail systems (could repurpose coal plants) in combination with pneumatics.
I'm picturing in my head a freight train car parked at a small desolate compound, standardized iron reactant cartridges dripping tar-like preservation liquid robotically unloaded, and local FCEVs and BEVs gathering to charge there. That might make an interesting Sci-Fi cutscene.
There are alternatives to iron that have higher efficiency and lower prices. For instance https://hydrogenious.net/ does exactly that but with benzene like structures. The advantage of this is that you can reuse existing infrastructure for transport and you have higher transport efficiency: while the square cube law exist, the same thing holds for the forces on the chamber walls which have to increase in thickness. Hydrogen tanks are also very expensive as they have to be manufactured to tight tolerances (and they need to be replaced rate often due to hydrogen creep weakening chamber walls)
> I don't see this making any sense for large scale energy storage. Storage tanks for compressed hydrogen enjoy the square-cube law.
This system doesn't store hydrogen. It stores elemental iron (produced from iron oxide, i.e., iron ore, and hydrogen from solar power splitting water into hydrogen and oxygen), and uses steam to get the hydrogen out (and convert the iron to iron oxide) only when the hydrogen is needed.
<< I can't see the niche that this idea can apply to.
Tbh, I am not sure either. I think the main benefit of this is FeO is inert under temps/conditions humans consider normal so maybe it long term storage is not that far fetched. I like the idea. I am just unsure about its practical applications.
> Being a highly reduced material, DRI has a tendency to re-oxidise, an exothermic reaction. Thus, without appropriate precautions being taken in its handling, transport and storage, there is a risk of self-heating and fires. The International Maritime Organisation's International Maritime Solid Bulk Cargoes Code classifies DRI - Direct Reduced Iron (B) - as Group B (cargo with chemical hazard) and class MHB (material hazardous only in bulk) and requires that DRI be shipped under an inert atmosphere, usually nitrogen.
It would be nice if the iron could be in an alloy that, in addition to being oxidized/reduced, could further absorb hydrogen when in the reduced state. FeTi absorbs hydrogen, but I don't think the titanium would withstand repeated oxidation/reduction cycles. The Ti would go to the +4 oxidation state and stay there.
We have a cheap, stable, infrastructure-friendly, high-density storage formula for hydrogen. Or better, since the application here isn't hydrogen-specific but is simply looking to find a fuel-storage solution: energy storage.
It's hydrocarbons.
In this case, synfuel hydrocarbons as direct analogues of fossil-fuel based compounds of chain-lengths 1 (methane) to around a dozen or so (kerosene / aviation fuel, at a stretch, diesel fuel).
It stores forever (proved to 300 million years), it is drop-in compatible with extant infrastructure and equipment, it's infinitely miscable with present fuels, it doesn't leak out of storage, it doesn't embrittle metals (and in fact generally lubricates and protects them).
Yes, the round-trip storage efficiencies are low (as low as ~15--20% recovery based on thermal electrical generation, roughly the same as the solution named here), but that's in exchange for something that can readily provide weeks to months of storage capacity in a stable, low-risk form. Where you need storage that's long-term stable, dense, safe, and instantly dispatchable, your options are few.
The technology has been demonstrated in numerous experimental trials, and is similar to processes run at national scale for decades in Germany and South Africa. US-based research has been conducted at Brookhaven National Laboratory, M.I.T., and the US Naval Research Lab, amongst others. The stumbling block to date has been that fossil fuel prices are sufficiently low[1] that synfuels simply are not competitive presuming market-based mechanisms which fail to account for externalities and other market failures.
I've be aware of this for about a decade and have written about the technology, Fischer-Tropsch fuel synthesis, multiple times on HN:
1. A market failure of staggering proportions, as the under-pricing is on the order of a million-fold. See: Jeffrey S. Dukes, "Burning Buried Sunshine", <https://core.ac.uk/download/pdf/5212176.pdf> (PDF)
agreed. there are a fairly wide range of possible hydrogen storage forms like this (ammonia, carbohydrate, hydrazine, metal hydrides) but paraffin has many practical advantages. possibly aluminum (without any actual hydrogen, similar to the iron in the article) is superior, due to lower weight and higher round-trip efficiency, but possibly not. i think we can take widespread fischer-tropsch fuels for granted as part of the transition to renewable energy
>> How much incoming solar power ends up in the methane?
As in how many power to gas plants are operating currently? I would guess not many due to the price vs fossil methane, but as the GP notes this is a major market failure due pricing the externalities of fossil methane.
>> And how do you get the energy back out?
Easy, burn it. For heat, or with a turbine, electricity. Zero carbon impact because it started out as atmospheric CO2.
You really want to store excess energy as natural gas or jet fuel because of all the existing infrastructure. Especially since excess power is available at so many solar sites, we’re so good at transporting these fuels, and the cost of photovoltaic will keep going down
The vast, cheap power that photovoltaic will provide is a giant opportunity. Please review the links below
I thought the same thing, but no, the chemical process is different. Iron-air batteries are traditional flowcell batteries (with some extra complications).
This paper uses hydrogen as an intermediary, which has advantages but also adds some questionable margins in efficiency. But I don't know the efficiency of the suggested iron-air batteries either.
This may be nicer if you want hydrogen rather than an electric battery. But if you turn that hydrogen into a fuel cell... the efficiencies of producing and consuming that hydrogen add up.
I don't think I understand the idea here properly.
When storing energy, the idea is to split water into hydrogen and oxygen, and then let the hydrogen recombine with the oxygen from iron oxide... to make water again. Meanwhile the oxygen from the original water is just released (since it's everywhere anyway)? That doesn't really seem to me like "storing hydrogen", since you just get the water back that you already had. Rather, it's using the energy to deoxidize rust.
Then on the recovery side, why use this steam process? Apparently (because the thermodynamics work out so that this whole thing has efficiency > 0) you get energy out of the process of putting the oxygen back into the iron. So why not just, well, burn (i.e. rust) the iron directly? What exactly is the dissociation and re-combination of the steam accomplishing?
Oxodizing iron gives rather low intensity heat, comparable, iirc, to burning lignite. That might be good for some combined cycle applications (but still a logistics nightmare?), but it's not a drop-in replacement for anything. Hydrogen on the other hand is good for an extremely wide range of applications, from fuel cells to e-fuel production (and all kinds of other chemical processes) to flying Neil Armstrong to the moon.
To succeed in decarbonization we will need an entire "cache hierarchy" to solve the intermittency problem, a single storage solution will never be enough. Batteries to make hydrolyzers able to run around the clock during high availability seasons, hydrogen storage to make converters further down the pipeline run continuously during surplus seasons and not only on the best days.
What will definitely not get us to decarbonization is any of the following three approaches:
(a) building enough production capacity that we don't need storage (most production would be idle most of the time for lack of a buyer)
(b) focusing on one kind of storage (same problem again, now with conversion capacity)
> Rather, it's using the energy to deoxidize rust.
Exactly, really what they are storing is electrons.
Rusting the iron directly releases heat, which limits your efficiency to the delta-T of the process. Reducing steam to hydrogen and then converting the hydrogen in a fuel cell allows higher efficiency to produce electricity.
At scale, what I don’t get is this requires a lot of energy to kickstart the reaction (heating the iron ore to 400 degrees). Where is that energy coming from when energy production is constrained in winter.
Heat losses at the surface of a sphere scale with the square of the radius, while the energy density scales with the cube of the radius, so you can just scale it up until the heat loss is relatively small.
In the paper, the authors mention 11.4% efficiency for this system and a theoretical maximum efficiency of 79% if scaled up, so it might take a lot of scale.
Even most internal-combustion engines require energy stored in a battery to kickstart them, so this is not different.
Obviously the energy efficiency of this process based on iron is modest. It is likely that the energy efficiency is even lower than for the process of storing energy by making synthetic hydrocarbons (e.g. synthetic gasoline), which are much easier to use once energy is stored in them.
The only advantage is the very low cost even for very large storage capacities.
My immediate thought is, why not store it as peroxide? It takes more energy to make too, but at least it's liquid rocket fuel instead of gaseous rocket fuel.
The right question is what the efficiency of this process is. End to end, not just the charging/discharging.
Both charging and discharging seems to require a lot of heat. Waste heat is essentially lost energy that is released in the form of heat. I assume the discharge reaction is exothermic. That would be the energy stored in the summer months. Heating up a lot of tons of iron during charging is also not going to be free. It doesn't matter whether you do it slowly or quickly.
Creating the hydrogen is also not a loss free process. Nor is doing something useful with it like using it in a fuel cell (0.85), burning it (0.45), etc. These inefficiencies multiply.
All that lost energy comes out of the original budget of energy that came out of the solar panels.
Even if you use some wildly optimistic numbers, they multiply to something well below 0.5 pretty quickly even before you consider charging & discharging.
But lets do something silly and unrealistic and just do the math for an average step efficiency at 0.7, 0.8, and 0.9. We're talking four conversions here so that's 0.7^4 =0.24 vs. 0.41 and 0.66. And forget about getting anywhere near average 0.9 efficiencies with all of those steps. I'm assuming 0.7 would already be on the high side. Add more steps to the process and it only gets worse. Pipes aren't perfect. If you need to pressurize the hydrogen before you use it (like in a car), that isn't free either.
Basically, this takes a system that was already quite inefficient end to end and adds two more steps that sound like they involve some pretty significant energy losses to it (i.e. probably well below 0.5 when combined), thus making the system as a whole a lot more inefficient. Hydrogen as a battery already sucked with normal storage. This doesn't improve things.
There's a good reason that most hydrogen produced is used at or close to its site of production: it minimizes the energy losses and producing hydrogen is really expensive so it's not really desirable to lose 80-90% of the energy unless you really need to.
If this is intended to support a grid, rather than be grid forming or isolated, then you'd sequence that somehow.
Somewhere with lots of solar on the grid probably has excess energy, even during winter, during the day, so you'd plan to put in the input energy to start the reaction during the afternoon peak, and if you miss that for some reason, some sort of coordinated startup procedure would likely be used.
The article says they're currently powered from a grid connection but hope to be fully solar powered soon.
What I'm not getting is how this process produces more energy than the solar input to power the process.
Unless they're getting solar collectors to try to generate 400 degree temperatures rather than PV solar to electricity, but that seems like a sketchy proposition at best in winter.
It does not, but they are storing the energy for winter. Solar produces a lot more energy in summer, which is especially true in Switzerland or Europe in general.
They mention using waste heat from the reaction to minimize the energy cost of discharge. As long as they still get some power out of it, it could be a win even if it’s quite inefficient. When the input hydrogen is “free” in the summer (due to excess production), inefficiency can be tolerable.
I do wonder if “free” will actually pan out, or whether someone will find a way to demand-shift from winter to summer and use it all up.
I suspect in the next 50 years electricity will end up globally transportable via undersea cables, like the internet does for data today.
At that point, it's always summertime somewhere and it's always daylight somewhere, and if prices were to fall to zero there is always someone who would like more heat for something.
Therefore I suspect zero-priced energy will stop existing.
They're storing the hydrogen as water, though. The only reason to produce it in the summer from excess production via electrolysis is to be able to reduce iron oxide to pure iron rather than just buying the pure iron to start with.
Isn’t a more viable economic “battery” to just turn on your LLM training massive data center, or Aluminum smelter (or any other high-consumption, low-urgency industrial process) when there’s excess electric production?
The issue is that it's a self-defeating mechanism. PV doesn't produce zero energy during wintertime, they just produce less. You're going to be building additional PV to charge the Season Battery, but those additional panels will also be providing power during the winter.
If your battery's efficiency gets bad enough the added winter power from those extra panels is going to be enough to cover the winter shortage - so you don't even need the battery at all. You'd essentially just be turning a huge amount of power into heat for nothing.
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Unfortunately, I don't see this making any sense for large scale energy storage. Storage tanks for compressed hydrogen enjoy the square-cube law. The larger they are the less expensive they are proportional to the mass of hydrogen they hold.
With this iron oxide method, you need 27 tons of iron oxide for one ton of hydrogen. You can procure right now tanks that can hold 2.7 tons of hydrogen and weigh 77 tons empty [1], the ratio is 28 to 1. But the round-trip efficiency of the tank is virtually 100%. The efficiency of the iron-based storage is only 50%. The tanks are not very expensive.
I can't see the niche that this idea can apply to.
[1] https://www.iberdrola.com/press-room/news/detail/storage-tan...
Not really. Wall thickness is roughly proportional to diameter, and surface area to the square, so you don't gain anything in terms of storage mass ratio by building bigger tanks.
> But the round-trip efficiency of the tank is virtually 100%
This is oversimplifying quite a bit. Compressing hydrogen, the lightest gas, is very energy intensive per unit of mass, and this energy is not fully recoverable upon decompression (due to general pump efficiency and thermal losses in the intercooler).
2.7 tons of hydrogen have a volume of almost exactly 30000 m^3, requiring storing it under high pressure in specialized containers. Hydrogen is famous for being hard to store without losses.
For long-term storage storage and losses are a problem.
> But the round-trip efficiency of the tank is virtually 100%. The efficiency of the iron-based storage is only 50%
Maybe I'm missing something, but why? As you mentioned it takes 29kj to restore 3 moles of H2 out of (3 moles of H20 + 1 mole of Fe2O3). Where does 50% comes from?
the efficiency is super low, but again, according to the paper, "most of the energy input was due to thermal losses at the reactor surface (83.9%)", which also benefits from square/cube law.
[0] https://pubs.rsc.org/en/content/articlelanding/2024/se/d3se0...
Edit: wait I forgot that direct reduced iron powder exothermically reacts with oxygen and water in the air, that's how single-use instant hand warmers work. So yeah you gotta isolate the iron powder a bit more than stick it under a tarp.
I've been playing too much Factorio lately so of course my mind goes towards rail systems (could repurpose coal plants) in combination with pneumatics.
This system doesn't store hydrogen. It stores elemental iron (produced from iron oxide, i.e., iron ore, and hydrogen from solar power splitting water into hydrogen and oxygen), and uses steam to get the hydrogen out (and convert the iron to iron oxide) only when the hydrogen is needed.
Tbh, I am not sure either. I think the main benefit of this is FeO is inert under temps/conditions humans consider normal so maybe it long term storage is not that far fetched. I like the idea. I am just unsure about its practical applications.
https://www.metallics.org/dri.html
> Being a highly reduced material, DRI has a tendency to re-oxidise, an exothermic reaction. Thus, without appropriate precautions being taken in its handling, transport and storage, there is a risk of self-heating and fires. The International Maritime Organisation's International Maritime Solid Bulk Cargoes Code classifies DRI - Direct Reduced Iron (B) - as Group B (cargo with chemical hazard) and class MHB (material hazardous only in bulk) and requires that DRI be shipped under an inert atmosphere, usually nitrogen.
It would be nice if the iron could be in an alloy that, in addition to being oxidized/reduced, could further absorb hydrogen when in the reduced state. FeTi absorbs hydrogen, but I don't think the titanium would withstand repeated oxidation/reduction cycles. The Ti would go to the +4 oxidation state and stay there.
It's hydrocarbons.
In this case, synfuel hydrocarbons as direct analogues of fossil-fuel based compounds of chain-lengths 1 (methane) to around a dozen or so (kerosene / aviation fuel, at a stretch, diesel fuel).
It stores forever (proved to 300 million years), it is drop-in compatible with extant infrastructure and equipment, it's infinitely miscable with present fuels, it doesn't leak out of storage, it doesn't embrittle metals (and in fact generally lubricates and protects them).
Yes, the round-trip storage efficiencies are low (as low as ~15--20% recovery based on thermal electrical generation, roughly the same as the solution named here), but that's in exchange for something that can readily provide weeks to months of storage capacity in a stable, low-risk form. Where you need storage that's long-term stable, dense, safe, and instantly dispatchable, your options are few.
The technology has been demonstrated in numerous experimental trials, and is similar to processes run at national scale for decades in Germany and South Africa. US-based research has been conducted at Brookhaven National Laboratory, M.I.T., and the US Naval Research Lab, amongst others. The stumbling block to date has been that fossil fuel prices are sufficiently low[1] that synfuels simply are not competitive presuming market-based mechanisms which fail to account for externalities and other market failures.
I've be aware of this for about a decade and have written about the technology, Fischer-Tropsch fuel synthesis, multiple times on HN:
<https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...>
________________________________
Notes:
1. A market failure of staggering proportions, as the under-pricing is on the order of a million-fold. See: Jeffrey S. Dukes, "Burning Buried Sunshine", <https://core.ac.uk/download/pdf/5212176.pdf> (PDF)
As in how many power to gas plants are operating currently? I would guess not many due to the price vs fossil methane, but as the GP notes this is a major market failure due pricing the externalities of fossil methane.
>> And how do you get the energy back out?
Easy, burn it. For heat, or with a turbine, electricity. Zero carbon impact because it started out as atmospheric CO2.
Deleted Comment
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https://www.scientificamerican.com/article/rusty-batteries-c...
This paper uses hydrogen as an intermediary, which has advantages but also adds some questionable margins in efficiency. But I don't know the efficiency of the suggested iron-air batteries either.
This may be nicer if you want hydrogen rather than an electric battery. But if you turn that hydrogen into a fuel cell... the efficiencies of producing and consuming that hydrogen add up.
When storing energy, the idea is to split water into hydrogen and oxygen, and then let the hydrogen recombine with the oxygen from iron oxide... to make water again. Meanwhile the oxygen from the original water is just released (since it's everywhere anyway)? That doesn't really seem to me like "storing hydrogen", since you just get the water back that you already had. Rather, it's using the energy to deoxidize rust.
Then on the recovery side, why use this steam process? Apparently (because the thermodynamics work out so that this whole thing has efficiency > 0) you get energy out of the process of putting the oxygen back into the iron. So why not just, well, burn (i.e. rust) the iron directly? What exactly is the dissociation and re-combination of the steam accomplishing?
Direct use of reduced iron as an energy carrier has been discussed here:
https://news.ycombinator.com/item?id=24996153
Oxodizing iron gives rather low intensity heat, comparable, iirc, to burning lignite. That might be good for some combined cycle applications (but still a logistics nightmare?), but it's not a drop-in replacement for anything. Hydrogen on the other hand is good for an extremely wide range of applications, from fuel cells to e-fuel production (and all kinds of other chemical processes) to flying Neil Armstrong to the moon.
To succeed in decarbonization we will need an entire "cache hierarchy" to solve the intermittency problem, a single storage solution will never be enough. Batteries to make hydrolyzers able to run around the clock during high availability seasons, hydrogen storage to make converters further down the pipeline run continuously during surplus seasons and not only on the best days.
What will definitely not get us to decarbonization is any of the following three approaches:
(a) building enough production capacity that we don't need storage (most production would be idle most of the time for lack of a buyer)
(b) focusing on one kind of storage (same problem again, now with conversion capacity)
Exactly, really what they are storing is electrons.
Rusting the iron directly releases heat, which limits your efficiency to the delta-T of the process. Reducing steam to hydrogen and then converting the hydrogen in a fuel cell allows higher efficiency to produce electricity.
Or would the plan be to slowly heat over fall?
In the paper, the authors mention 11.4% efficiency for this system and a theoretical maximum efficiency of 79% if scaled up, so it might take a lot of scale.
Obviously the energy efficiency of this process based on iron is modest. It is likely that the energy efficiency is even lower than for the process of storing energy by making synthetic hydrocarbons (e.g. synthetic gasoline), which are much easier to use once energy is stored in them.
The only advantage is the very low cost even for very large storage capacities.
Both charging and discharging seems to require a lot of heat. Waste heat is essentially lost energy that is released in the form of heat. I assume the discharge reaction is exothermic. That would be the energy stored in the summer months. Heating up a lot of tons of iron during charging is also not going to be free. It doesn't matter whether you do it slowly or quickly.
Creating the hydrogen is also not a loss free process. Nor is doing something useful with it like using it in a fuel cell (0.85), burning it (0.45), etc. These inefficiencies multiply.
All that lost energy comes out of the original budget of energy that came out of the solar panels.
Even if you use some wildly optimistic numbers, they multiply to something well below 0.5 pretty quickly even before you consider charging & discharging.
But lets do something silly and unrealistic and just do the math for an average step efficiency at 0.7, 0.8, and 0.9. We're talking four conversions here so that's 0.7^4 =0.24 vs. 0.41 and 0.66. And forget about getting anywhere near average 0.9 efficiencies with all of those steps. I'm assuming 0.7 would already be on the high side. Add more steps to the process and it only gets worse. Pipes aren't perfect. If you need to pressurize the hydrogen before you use it (like in a car), that isn't free either.
Basically, this takes a system that was already quite inefficient end to end and adds two more steps that sound like they involve some pretty significant energy losses to it (i.e. probably well below 0.5 when combined), thus making the system as a whole a lot more inefficient. Hydrogen as a battery already sucked with normal storage. This doesn't improve things.
There's a good reason that most hydrogen produced is used at or close to its site of production: it minimizes the energy losses and producing hydrogen is really expensive so it's not really desirable to lose 80-90% of the energy unless you really need to.
I see:
1) generation 2) storage efficiency (energy while storing divided by energy upon release)
what are the other 2 you had in mind?
Somewhere with lots of solar on the grid probably has excess energy, even during winter, during the day, so you'd plan to put in the input energy to start the reaction during the afternoon peak, and if you miss that for some reason, some sort of coordinated startup procedure would likely be used.
What I'm not getting is how this process produces more energy than the solar input to power the process.
Unless they're getting solar collectors to try to generate 400 degree temperatures rather than PV solar to electricity, but that seems like a sketchy proposition at best in winter.
I do wonder if “free” will actually pan out, or whether someone will find a way to demand-shift from winter to summer and use it all up.
At that point, it's always summertime somewhere and it's always daylight somewhere, and if prices were to fall to zero there is always someone who would like more heat for something.
Therefore I suspect zero-priced energy will stop existing.
It's a non-problem, really. Especially at scale.
I would say it's only worth it if the marginal cost of producing the hydrogen is close to 0.
Unless you also have a nuclear reactor to perform the high-temperature electrolysis process. https://en.wikipedia.org/wiki/High-temperature_electrolysis
Which only Japan seems to be working on building at the moment. https://www.hydrogeninsight.com/production/japan-plans-hydro...
It's cool tech, and I like that it's researched. But I think it's not a breakthrough that could realistically help the grid at the moment.
If your battery's efficiency gets bad enough the added winter power from those extra panels is going to be enough to cover the winter shortage - so you don't even need the battery at all. You'd essentially just be turning a huge amount of power into heat for nothing.
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