Just to spell out something that's been implicit in other comments: the entropy level of your energy matters a lot. Electricity > movement > heat. To go down the chain is almost free, to go up the chain you have to spend quite a lot.
You can do a lot of things with electricity: you can heat things, but also move them around and run your TV, all without any loss. With heat you can just... heat things. So you can't call this an "iron battery", because you don't get electricity out of it, just heat. Maybe call it a "heat battery" or "high performance heat pad".
Also note the efficiency numbers: "High-efficiency electrolysis of iron oxide can store as much as 80 percent of your input energy in the iron fuel" is the efficiency of the process itself. "Using this kind of cyclical process to generate electricity could approach a theoretical efficiency around 40 percent" is because you need to climb the ladder to low entropy again (probably by using the equivalent of a steam engine to run a generator).
The most important take away from this is that electrical energy can be easily "multiplied" into thermal energy by using it to move around the almost unlimited thermal energy contained in the environment around us.
For example, a water-water heat pump can easily boost the temperature of your reactor to 80 deg C by using only 1 KWh of electricity for every 4KWh of heat delivered, the rest coming from a nearby large body of water and slowly recovered from the environment throughout the year. So we are looking at on order of magnitude more heat for the same electrical consumption 40% vs 400% efficiency. Instantanous renewable energy prices are low, but rarely that low.
The limit of efficiency is dictated by the boost in temperature needed, so you can't use heat pumps to get the 1000C generated by iron oxidation. There is certainly a niche for industrial processes requiring very high temperatures, but I can't imagine why on earth would a brewery need such temperatures.
Brewery’s need high temperatures simply as a way to quickly heat a large volume of liquid. Most recipes involve boiling the mixture for around an hour, and you want to reach boiling quickly. If your heat pump is only at ~120C you would need a lot of surface area which makes cleaning difficult. On top of this reaching 120C via heat pump is already inefficient and the heat pump ends up being expensive compared to resistive heating.
Breweries need multiple temperatures, and that's an area where heat pumps could still be applied. I think the common mistake is either/or solutions and you're making it here. Water has a high thermal mass. Starting with ground temperature water and heating it up greatly increases the number of BTUs.
And then after you've cooked it, you need to chill it down to around room temperature. Where you dump that heat could make a huge difference.
It takes vastly more energy to start and maintain a rolling boil for an hour than to reach 50c. If we where talking a giant scale industrial process then optimizing this stuff via heat exchangers would be a major consideration. However for a Dutch brewery located in such a cold area it’s much harder to justify such specialized investments with associated maintenance costs.
Remember this inherently allows them to load shift and thus get much lower electricity prices as well as reducing peak demand.
The other way to put it is that your efficiency is proportional to 1/(delta T). In other words if your heat pump is working between two environments with very close temperatures then the heat transfer is very efficient (for example 19 degrees C outside, 21 inside). But if you are trying to heat your house to a livable temperature while it’s -20 outside, your efficiency is terrible.
This is why for example you see heat pumps installed all over the place in homes in the southern parts of the US where you also need little heating all around, while places up north where you do spend quite a bit on heating cannot really take advantage of heat pumps due to winters being too cold.
Though apparently you'd be better off with a ground source heat pump if your average temperature during heating season was below freezing or so.
[Edit: I mention this because I've heard that "you don't see heat pumps in the north" was true in the past but is now outdated -- they've made sense in warmer climates for a while but have only recently crossed over for colder climates, so it will take a while until they're common.]
Note: there are two popular "efficiency" measures, and its important to realize that they're incompatible.
Air conditioners are commonly "energy moved / energy used", which can reach greater than 100%. If you move 150W of heat using only 100W of electricity, you have 150% "efficiency".
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I don't know the term for the other kind of efficiency (I'm not a physicist), but lets call it "inverse engine efficiency". This is "energy out / energy in", which ends up being pretty close to "energy moved / (energy moved+electricity used)"
Under this measurement of efficiency, 150W moved with 100W of electricity is 60% efficient. This follows the more standard physics rule of thermodynamics (you can never go above 100% efficiency: it will always take some number of energy to move heat around).
Note: Car air-conditioners are funny systems. They use the heat from the combustion engine to move heat from inside the cabin to the outside world. So you are literally using heat to move other heat.
> "Note: Car air-conditioners are funny systems. They use the heat from the combustion engine to move heat from inside the cabin to the outside world. So you are literally using heat to move other heat."
Car air-conditioners are not powered by heat from the combustion engine. They are powered mechanically by a belt ("serpentine belt") connected to the engine.
Or, in some cases such as battery electric vehicles, powered electrically with the compressor turned by an integrated electric motor. The extra heat produced is just a byproduct (thermal inefficiency) of producing and transmitting that mechanical/electrical energy.
> Car air-conditioners are not powered by heat from the combustion engine. They are powered mechanically by a belt ("serpentine belt") connected to the engine.
But that belt is powered by the expansion of gas that takes place inside of a piston, due largely to the increase in heat from combusting gasoline. Ultimately, an ICE engine is a heat-engine (like a steam engine or sterling engine, but different).
Yes, but the useful energy in a combustion engine comes from that expansion of gas driving the pistons. The heat is, for the most part, just a wasted byproduct. (It can be used for ancillary purposes like heating the cabin).
With that being said: the octane combustion formula has 25 O2 as input and 16 CO2 + 18 H2O as output (and I assume the H2O is mostly water vapor). So that's 25 molecules of gas input -> 34 molecules of gas output.
So it seems like more "CO2 + vapor" is created than the number of input O2 molecules. But that only accounts for 34% expansion of the volume of the stroke. (25 mols input -> 34 mols output).
The rest of the stroke's power comes from the ideal-gas law: higher temperature means higher pressure and larger volume. Literally the heat generated by the chemical reaction.
> "But if you are trying to heat your house to a livable temperature while it’s -20 outside, your efficiency is terrible."
Heat pumps are designed so that their "efficiency" (performance) is never less than 100% (COP >= 1). In very cold conditions, they may not be more efficient than resistive electric heating, but they will never be worse.
> "places up north where you do spend quite a bit on heating cannot really take advantage of heat pumps due to winters being too cold."
You're talking about air-source heat pumps here. In colder regions, it's better to use ground source/geothermal heat pumps. Ground temperature is consistent year-round even in very cold climates.
True. Geothermal heat pumps will always be better but given that you can get an air source heat pump for 10^3 to 10^4 dollars while a geothermal setup will cost you 10^4 to 10^5, geothermal becomes prohibitive for most residential setups.
Over here in northern Europe, for a typical single-family house, replacing an oil-fired boiler with an air-water heat pump (that is, the heat pump uses outside air to heat water that can be run through the existing radiators) costs around 10k-15kEUR, replacing the boiler with a ground-source heat pump (incl. borehole drilling, the whole shebang) is around 15-25kEUR.
So the price difference is not huge. In southern parts of the country, and for older houses with less expected life left in them, air-source pumps have proved more popular, whereas further up north and for newer buildings ground-source pumps are more often the choice.
Depends on your timeframe of investment. Your air source heat pump will need to be replaced in 7-10 years. The really expensive parts of a geothermo system will last basically forever, the other parts may need to be replaced every 7-10 years, but the costs is about the same as replacing a air source heat pump, and geothemo is more efficient in general. Thus if you live in the same house for 30 years geothermo may pay off.
Your upper limit for geothermal seems a bit high, typical installation is 20-30k. But commercial viability depends on typical outside temperature and time frame for payback
Water heater heat pumps (other than the complexity) do an excellent job of maintaining 40C temperature differentials (eg Vermont or Maine) with better than 130% wall plug efficiency. I think ideal efficiency is proportional to Tcold/dT (T in Kelvin) so it could go as high as 400% if there weren’t fluid, pump, and insulation losses.
The real challenge is reliability and cost of electricity. If your power goes out and you have heating oil you can still live for a week.
Brewing water temperatures peak at ~100C (pressurized steam is dangerous) and even with 20C water that’s an 80K difference so you’re down to 350% ideal, which probably puts you close to breakeven with pump losses. It’s the complexity that kills it.
Either. I am talking about heat pumps. The efficiency formula is exactly the same. The difference is that ground water stays at a consistent 12-15 degrees C IIRC. That is close to what we as humans like to live in, so the efficiency is relatively high heat round. In the meantime air temperature fluctuates by quite a bit. The main drawback of using ground water as your heat source/sink is that it is expensive and costly to maintain relative to a resistive coil with electricity running through it.
Yes but in a renewables world Electricity will not necessarily be ready on demand. In that case the multiplier is irrelevant. Nor may it be available locally.
With iron burning you could theoretically create iron using solar power in Sahara and then ship that to the Netherlands for use in breweries.
Thus you can decouple where and when power is generated from usage.
The energy cost of sea transport is like 50 less per distance traveled than on land and it has being like that for the last 3000 years. So the cost of shipment to Netherlands is just a cost of transporting to the nearest sea port from Sahara.
Iron is heavy, dense as a material, but the cost-efficient of moving it around as an energy store depends on it's energy density (and it's initial cost), not it's basic density as a material.
I think an iron-powered ship would have to be a steamship.
There are still a small number of steam-turbine-powered merchant ships being built- specifically LNG carriers, where gas that boils off from the cargo can be used as fuel.
I can see that heat pumps can only move a certain amount of energy and to a similar extent electricity.
I remember looking at on-demand hot water heating. It's easier to do with natural gas.
But for electricity it would require something like a 100+amp electrical circuit and would have limits on flow rate and/or temperature increase.
for a heat pump, it just wouldn't work.
So for hot water the most common solution is to trickle charge a thermal battery - a hot water heater - and release it fairly quickly during a hot shower.
> So we are looking at on order of magnitude more heat for the same electrical consumption 40% vs 400% efficiency. Instantaneous renewable energy prices are low, but rarely that low.
Wouldn't this imply that when heat is the goal we should always use these pumps? What is the limit of usefulness of these pumps? (eg, what if a whole city did it for all its heat needs?)
For one thing, heat pumps are more expensive than heaters. A heat pump is basically an AC unit that runs in reverse - way more expensive and complicated than a fire or a long thin wire.
The big limit is temperature differential matters. The smaller the better!. 40C differential is about the max modern equipment can do with enough efficiency to be worth bothering with. Your home freezer might do more than 40C in extreme cases, but it isn't efficient (We also don't have many other options for this mode, for heating we have other options that work)
Some industrial processes require very high temperatures, for which electrical generation is ill suited. Glass furnaces are one such case. Having an efficient local energy delivery mechanism that may be "recharged" with electricity is quite interesting.
I don't imagine beer production requiring such high temperatures, but honestly I don't know anything about industrial beer production. Anyhow the tech presented in the article is very cool.
An asynchronous high intensity heat source, asynchronous in that it can be "charged" from electricity ahead of utilization time, whenever intermittent energy sources are high. According to the article at 80% efficiency, that's pretty good for a storage method that is cheap in capacity (think seasonal storage or beyond).
It's not the universal storage solution to end all search for other storage solutions because it's only really applicable in a niche (heat consumption), but that niche might not be as small as people think. I think that besides applications that require high temperatures it could also be a very good match where a particularly high maximum temperature isn't required (so heat pumps and the like could be applicable) but peak power demand is of a rarely occurring, intermittent nature. It might even be applicable to once-in-a-lifetime heat applications if they can be designed for easy oxide recovery.
What remains open is how cheap "charging" throughput can be built, as this is the weak spot of all other power-to-fuel concepts: the conversion facilities tend to be too expensive to remain idle outside of electricity oversupply times. And who knows, theoretically conversion throughput capacity might even turn out to be so exceptionally cheap (I certainly wouldn't expect that, but I'd be glad to be surprised) that even the inefficient electricity->fuel->heat->electricity might become competitive, once conversion outgrows direct heat applications. A market for inefficient but cheap long-term storage would definitely exist if there was any supply.
On the other hand, electricity can create almost "infinite temperature" with relative ease. Temperature is, roughly speaking, the average kinetic energy of a material's molecules, and you can accelerate particles to extremely kigh kinetic energy with electromagnetic fields. So if you need that kind of "temperature", electricity is very suitable, even though it's expensive per unit of energy.
Another example for how electricity is a very valuable type of energy.
For industrial-scale methane/coal burning, sequestering 90% or maybe even more of the CO2 is still a lot cheaper than using electricity. Don't get me wrong: it costs around 10% of the thermal load in electricity (technically, driveshaft power for turbomachinery).
This is a much more plausible use for the energy produced by this method than on-site glass bottle manufacturing, alluded to elsewhere among the comments.
It is true that it is a less efficient way of storing electric power than a battery as you need to created steam to drive turbines.
Yet this has some huge advantages over batteries:
1. We already know how to create iron at massive scale. Various iron smelters are going green already.
2. This has much higher energy density than lithium-ion batteries. More similar to gasoline.
3. The cost of storage is very low. Capital expenses to build batteries is much higher.
4. You can save a lot more of money by using existing industrial infrastructure. We can convert existing coal plants and any industrial process needing heath such as concrete production to use this. This makes it possible to get a solution in quickly . Scaling up global battery production in contrast is an enormous undertaking and we must prioritize EVs over grid storage at least initially.
My counterpoint would be that efficiency isn't always the most important variable in an energy supply chain, especially when your energy supply is renewable, and thus non-polluting, and as the marginal cost for generation of renewable energy continues to drop.
There are many situations where it would be much less expensive and simpler to replace a coal- or oil-fired boiler with one that runs off of iron filings, while still maintaining all of the rest of the equipment as-is. These locations could be off the grid, or in locations where it's not feasible to get a connection capable of supplying the megawatts of electricity it might take to produce enough steam.
You have to compare the efficiency to other similar options. For instance, (hydrogen-based) fuel cells also only have around 40-60% efficiency…
Though, I would assume that iron-powered (steam?) vehicles would not be competitive due to the weight ? (Even despite the extra weight that hydrogen or electric batteries add.) Maybe iron-burning steam boats ? Steam rail ?
Nevertheless, fixed processes (far from hydropower), especially those requiring heat energy, seem to be a great niche for iron burning ! And I say niche, but this niche might be even bigger than transportation, which IIRC is only like 25-30% of total energy consumption ?
Yeah, and concrete making releases a lot of concentrated CO2, so there's already a lot of potential for algae-growing to make liquid hydrocarbon fuels there !
With large amounts of heat in your home, you can do cogeneration (combined heat and power). Heat some water via oxidation, produce electricity with the steam, collect the excess heat to keep the house warm. And you can do that all winter long as long as your basement is full of iron powder. And in the summer, you reduce the iron oxide again with excess solar power. (That's not what they want to do here but that would be the idea)
A really interesting application of this came to mind that seems like an optimal use of this “iron heat battery” from a pure physics perspective.
Each kind of “energy storage application” can be broken down into different properties /requirements that can vary quite a bit.
Real time electricity response for 2-8hr periods so far seems well suited for Li-ion (LFP), given it is electrical energy, speed of response, and that it can be deployed virtually anywhere.
On the opposite end of the spectrum - you have seasonal energy storage (winter in colder climates, for example). When thinking about “energy containers” - Having iron powder doesn’t have the same requirements of alternatives I’ve heard, and seems like it could be the easiest to scale. Physically storing heat requires very large well insulated structures (rocks underground I’ve seen), hydrogen requires compressed gas cylinders, pumped hydro and compressed air needs specific geology (large valleys or large sealed underground caves).
Iron powder? An oxygen free warehouse Amazon sized. Helped that many winter applications ultimately only need heat, this seems like a good candidate that could be rapidly deployed given how on a relative basis - capital light and material accessible this is.
An oxygen free warehouse is overkill. Iron needs both oxygen and water to rust, so you can store it in a warehouse with just some climate control to manage the humidity. You could treat it like any other flammable powder, where you keep dust down as much as possible, and keep flame sources away.
Fine steel wool will burn readily, yet it keeps underneath the sink and only rusts over a period of years.
You are right, of course, but I wonder if you couldn't use "waste heat" as the energy input of this "heat battery"? This would make the economics much better: instead of going, say, nuclear -> steam engine -> electricity -> iron -> heat (-> electricity), you could skip the first conversion losses.
Not sure how doable it would be from a chemistry point of view, though: you probably need at least one extra reagent or catalyst.
Source temperature of waste heat is an important factor. Heat radiating from a 2000C source is a lot more valuable & useful than that from some warm water at 60C.
You can't use that heat to raise the temperature of something to 61C, for example.
But you could use a heat pump and 60C water to raise the temperature of something to 61C. And would take a lot less energy to run that heat-pump than to actually heat that something to 61C.
What I've become interested in, is how easy it is to 'abuse' heat pumps like this. Could you take a heat-pump and this 60C water to heat a boiler to the point you can run a steam-turbine that produces more energy than it takes to run the heat-pump?
If not at 60C water, does this work at any temperature? There remains free energy in the temperature differential between an ambient 21C and the temperature of warmer water. It seems to me this should be harvest able. There are significant limits here from the Carnot theory, but they don't say this is impossible.
No that explicitly violates the second law of thermodynamics.
A heat pump is like a water pump: it's a lot easier to move water to a tank 61 feet off the ground from another tank 60 feet off the ground than from ground level, but you can't power the pump producing 1 foot of pressure head with a turbine being driven by 1 foot of pressure head.
So, say you have a tank at 61 feet, one at 60 feet, and a desire to move the water up.
Just let some of the water out of the 60 foot tank through a turbine. Put a shaft on that turbine, and put a pump on the other end. That pump should be able to pump water from the 60 foot tank up to the 61 foot tank. You would get less water in the 61 foot tank than in the 60 foot tank. Quite a bit less actually, but you could still harness the energy in the 60 ft tank to get some water to the 61 ft tank.
No one is saying it's impossible to lift water up 1 foot (that's incredibly easy), the problem is it always takes more energy to lift water 1 foot than you can ever get from lowering it 1 foot. Likewise a heat pump can easily and efficiently create a heat reservoir, but you need to drain some other, larger heat reservoir to do so.
Yes, we are explicitly losing free energy from this.
Thing is, the free energy in 60C wastewater is hard to use, and the free energy in 200C steam is much easier to use (through steam-turbines). Hence, it might be worth it to sacrifice a large part of the free energy in the 60C wastewater to get it to a useable form.
What are the alternatives to use 60C wastewater to generate electricity? I can think of a sterling engine or a thermoelectric material. These would probably be more efficient, but they are new technologies, with practical limitations and development limitations. Hence, if my idea could generate any form of electricity, it might be more feasible than the other ideas.
From what I know about heat engines, I know it might not be possible to generate electricity this way.
Well in this case you need to pump 10 gallons up to get those 10 gallons to spill down. At some point in the chain, you need to consume energy from the environment to drive the system.
> At some point in the chain, you need to consume energy from the environment to drive the system.
Agreed, that's where spilling water comes into play:
┌───┐
┃ A ┊┃ │
┗━━━━━┛ │
│
│┃┈┈┈┈┈┈┈┈┃
└┨ B ┃
┗━━━━━┳━━┛
┇
┃ C ┇ ┃
┗━━━━━━┛
Here, B->A is driven by B->C spillage. The critical point here is that B is a low quality source, but plentiful. rocqua's point was likely similar, if you replace height with thermal energy. You can't use the energy you put in A to move it from B to A, because it costs you at least as much to move it up.
However, we're not bringing it down to B. We're bringing it down to C (ambient 21°C in their example). There will be losses, but shouldn't it be doable, depending on the B-C potential?
Wouldn't Peltier modules be 100% efficient in theory for rising B to A (as long as thermal losses go to the hot side, that is: tey keep a side cooler), for instance? If so, the generated energy is the one stored between A and C, minus conversion losses, minus A-B. That is, B-C minus conversions losses from A-C. Probably not very efficient at the end, though worth it if you have plenty of B (and a excellent "C" sink).
Draining B into C is consuming energy from the environment.
You most certainly can drain some of the water from B to C so as to move other water from B to A. But you will always do so at a net loss.
The unit of water moved from B=60 to A=61 gains A-B=1 unit of potential energy, in addition to the B-C=40 units of potential energy it already had for a total of A-C=41 units. However, without a 100% efficient (read impossible) pump and turbine combo, it takes more than 1 unit of energy to raise up that unit of mass. This means you need to spill more than 1/40 units of water from B to C to get that 1 unit from B to A. Now dropping that water from A to C would get you 41 units of energy, 1 more than if you had spilled it straight from B to C, but it cost you more than 1 unit of energy to get into this situation.
A is indeed a higher quality source than B, but you are using that low quality source of B to generate your energy, and the efficiency losses there will always be greater than the gains you make on the higher one. To break even the initial height/temperature differential would have to be infinitely high.
Now if for some reason you could not harvest energy directly from B-C, yes you could use water going from A to C to power the pump from B to A, it would just be less efficient. This would be the equivalent of a siphon.
I think we agree on everything, thank you for expanding your reasoning.
> Now if for some reason you could not harvest energy directly from B-C, yes you could use water going from A to C to power the pump from B to A, it would just be less efficient.
I also think that was the initial point: some higher-efficiency generators ("steam turbine") are only available for high potential differentials. Whether raising the potential offsets the gains likely depends on the specific setup and energy source.
> Not if the total free energy in the system goes down right?
The heat pump requires power to run, the question is whether the power to run the heap pump is smaller than the power required to just generate the same heat differential directly.
No, the question is whether the power to run the heat pump is smaller than the additional power you'd gain by increasing the differential, which is always no.
You would always get more energy out of draining the whole 60 foot water tank directly to ground than you would by lifting the water to 61 feet and then draining it to ground.
For short-term storage at a nuclear plant, you could use molten salt thermal storage. In terms of a 'battery,' decomposing ammonia to H2+N2 is one way to directly convert heat to chemical stored energy: https://eeme.anu.edu.au/research/research-projects/ammonia-b...
Interesting - essentially the iron is oxidised to release energy, then deoxidised using renewable energy. So it's another way of buffering renewable energy, which doesn't require a large body of water to store it. I'd be interested to know the overall efficiency of the process, but it sounds great - on the surface.
Not bad; of course there is also the energy cost related to moving the material around. Iron is kind of heavy. If you are doing the electrolysis on site that's probably not a big factor. But if we're talking about converting coal plants (which the article does), presumably the electrolysis would happen elsewhere; which means tons of iron needs to be moved back and forth.
Moving fuel around is also a problem with coal and oil. We're basically burning fossil fuels to move fossil fuels around. In the case of oil, we than also have to run it through energy intensive processes to turn it into more palatable fuels than bunker oil, which is what is typically used for shipping things around.
A typical oil tanker consumes a couple of hundreds of tons of bunker fuel per day to move a couple of thousands of tons of fuel around. So, that's a sizable percentage of its load that is lost just to get the raw product from A to B. I'm sure the math is similar for producing and moving coal around.
It only makes sense if you get to pollute at will without financial consequences; which happens to be the case with shipping, which mostly takes place in international waters without any constraints or regulations on the amount of crap put in our atmosphere.
Ships actually switch to cleaner fuels when they get close to populated areas because otherwise they'd end up killing some of the locals with their fumes. Bunker fuel is nasty. Even more so than coal.
So, if we are debating efficiencies, we'd do well to apply the same scrutiny to existing solutions as well. The efficiency of coal should also include digging it up using diesel burning heavy duty equipment, moving it around, etc. I'd bet that knocks some percentage points of even the most efficient plants or ICE cars.
It is easy to get too hung up on efficiency though. Overall cost matters more. If a solar plus metal powder solution gives the same reliable power as a solar plus battery solution at lower cost then that will often be a superior solution, even if it requires more solar panels.
Efficiency matters when you run out of space or resources. But you got vast areas like Sahara where solar power production could be combined with metal production. Wasting space in Sahara is a non-issue if it means you get the overall cost of system down.
This article suggests at top speed Panamax tankers carry 1.5-2mn gallons of fuel while burning 63,000 gallons per day (i.e. around 3-4% of capacity) at top speed.
That is considerably less than the 10% (200 tons to move 2,000 tons) that you suggest.
Also the article notes that reducing speed by 10% can reduce fuel use by a third. The tankers travel at 19mph, significantly less than their top speed of 23-28mph so fuel consumption is likely closer to 1%.
> A typical oil tanker consumes a couple of hundreds of tons of bunker fuel per day to move a couple of thousands of tons of fuel around.
While it certainly makes sense to calculate for the entire journey, if you were talking about the "whole journey" why did you specify "per day"? It's hard to blame 'ximeng' for relying on your written comment rather than what you intended to say. :)
Rust is heavier than the equivalent amount of pure iron, so perhaps it would be possible to put the furnace at the top of a hill and the refiner stage at the bottom and let the weight difference do the heavy lifting.
Interesting idea. I wonder how much height difference you would need so that gravity gives you enough energy to power the regenerator at the bottom? The actual energy would come from ... the gravity field of the Earth?
The energy comes from the mass of oxygen that is released at the bottom and captured at the top.
But second law of thermo dynamics applies. If this works at all it is because you are capturing a little more of the energy that was put in at the top and would otherwise be lost. You stil need to put in external energy to keep this system running.
Shouldn't the needed energy increase the more the height difference? Seems it must be made up for in some other place too, or less waste heat at some stage.
The energy out increases by the height level as well.
If the process was 100% reversible, and friction free (probably a couple other loss factors I forgot about) it would be energy positive as the oxygen is making its way back up in the air is an external energy input. Of course in the real world both assumptions are wrong by more than enough that you can never get this to work.
> But if we're talking about converting coal plants (which the article does), presumably the electrolysis would happen elsewhere; which means tons of iron needs to be moved back and forth.
You could build the infrastructure to do the electrolysis immediately next to the coal plant to solve this problem. If this technology takes off that's most likely what will happen anyway.
Generating electricity with iron as fuel would be suboptimal, that's like using electricity to turn co2 back into coal, and then using said coal to generate electricity again. Using iron in coal power plants would only male sense if transporting it from somewhere where IRS more efficient to produce iron is economically viable - say use solar in Spain to regenerate iron and then use it further up north where solar isn't as good.
The problem being solved is the bursty nature of green energy, we are trying to store solar energy generated during the day for usage at night or store excess wind energy for still days.
We are not trying to transfer green energy to less windy/less sunny places, the grid can take care of that.
Right, but in that case you'll get way better efficiency out of pretty much any other proposed solution: gravity, momentum, battery solutions are all more efficient than this for storing energy from renewables.
Yeah, or in the article’s case (where you’re brewing beer), you might as well use a solar furnace to boil water directly with sunlight. You’ll have a hard time beating that efficiency with other methods. If you have huge water storage tanks anyway then keeping the water hot overnight is not a problem either.
Seasonality of solar is huge in northern latitudes. Using iron to store energy for months is an interesting prospect, although it needs to be kept dry so you can't just leave big heaps of it outdoors.
At first glance it seems reasonably transportable, but I'm worried about how you prevent it rusting in the air. Maybe it would be useful to blend it with biodiesel for that purpose.
The advantages I see of this process are: Ease of storage, re-use of the coal plants, ease of fuel generation.
Methane seems nice for the re-use of gas fired plants. I do think its gaseous nature has more downsides than upsides on the other two factors.
There's also a gas-leak hazard with methane that doesn't exist here.
If the capital requirements are low enough, I could see a 40% efficient energy battery for smoothing out peaks being nicer than more complicated storage schemes.
Chemical batteries will likely eat the smoothing market.
For fuel that can be transported, methane is a winner. In addition to the distribution infrastructure, there's an awful lot of residential use, with lots of appliances that don't need replacement.
This is still going to be less efficient than an actual battery though, right? I'm all for alternative renewable energy storage solutions, but this one seems pretty poor.
Not even close. Second law of thermodynamics puts a limit on heat engine. Gravity storage is not subject to such limit.
But it is a great idea to burn any fuel whose oxidized product is not a gas. The only reason why burning hydrocarbons is problematic is because carbon dioxide is a gas, otherwise it could be captured and recycled like this.
Right, conversion of iron to rust yielding heat is 100% efficient, less only the work needed to move the rust out of the way.
If you wanted to convert that heat back to electricity, there would be losses, but nowhere near as much as some people are saying. 1800 degrees minus ambient is a very big delta T. You are not bound to a closed cycle, so Carnot does not apply. So it is likely 80+% would be achievable, maybe over 65% round trip.
Gravity storage IS subject to the same law, why do you believe it isn’t?
Burning hydrocarbons is problematic for a host of reasons besides CO2 release: fracking, oil spills, release of other pollutants, political issues with the supplier countries, pipelines, ...
Gravity storage converts work to potential energy, and back, efficiencies can be 90-almost 100%... heat engines at room temperature generally top out in the 35%ish range. Using the "waste" heat improves that, but nowhere close to 90%.
Because the Carnot limit is particularly pessimistic.
70% vs 40% is a big difference, especially when you consider it from the losses column instead of the wins column. If I gave you a system that was losing 30% of a resource to replace a system that lost 60%, you've cut your losses in half.
If I told you a mechanical system could get that to 15%, you'd replace that again. Anyone who tells you they can get a heat engine down to 15% losses is a charlatan and should be reported for fraud.
I've seen designs for renewable energy storage that use gravity. These are often with water, but I've also seen designs using cranes and regenerative braking. I've seen designs like this that use load shedding for generating some useful feedstock material.
What I haven't seen is many people doing both at the same time. I might have seen a desal unit powered by green energy, which tops up the water towers when power is cheap, but not much beyond that.
Remarkably good science journalism - I appreciated the caution at the end about the economics of the process and the understanding that there are many different factors that can make a process attractive.
Using more ... aggressive metals might help energy density here, though it would require more energy losses during rebuilding the oxides.
Now before one points out that burning lithium just makes this a battery; kinda but also no. If you burn lithium to turn a generator, I'd argue it's not much more a battery than burning oil to turn a generator. If you wanted a battery, you'd need a non-generator variant. That is where I'd differentiate.
Could also use Flourine and burn CO2, might be a viable carbon sink. Flouroalkanes from burning CO2 would be organically inert, don't deplete ozone if released and don't bioaccumulate. Only downside is they're very good greenhouse gases if you don't burn them down to the alkanes that are solid or liquid are normal temperatures. Those you could easily bury deep below the earth.
The only issue is obtaining a shitton of flourine to burn your carbon with and then not blowing yourself up in the process.
(Also yes, Flourine will burn CO2 and act as the oxidizer)
The trouble with more exciting metals is they want to self-oxidise, whereas iron powder in dry air isn't particularly enthusiastic about oxidising. Aluminium produces its surface oxide and then stays relatively inert, although aluminium powder is prone to powder explosions (as is coal dust and even grain/flour).
Everything on the left side of the table gets lively with air and especially water; everything from sodium downwards explodes in water.
(I wonder if anyone is doing the 21st century version of Ignition!'s approach of setting random things on fire to see if they're good rocket fuels with potential electrolysis-cycle storable fuels?)
>Using more ... aggressive metals might help energy density here
I think breweries are one of the types of business that could more easily use low density fuels. Right now a lot of the beer is moved by road, and the trucks that transport beer are often empty on their way to a brewery.
> the trucks that transport beer are often empty on their way to a brewery.
You have to transport the iron oxide back to where it can be recycled. That's the "advantage" of burning fossil fuels: most of the combustion products are volatile, so you can release them into the atmosphere.
That is also the problem with fossil fuel. No simple way of capturing and reusing waste. Anyway transport is a small part of cost. For fossil fuel drilling and refining adds much more cost.
Not that much, but I imagine a lot of bars would spring up in the vicinity of iron electrolysis plants, on account of the steady supply of cheaply transported beer.
There is a scatter plot on exactly this in the background story linked in TFA [0]. B and Be come out on top, both with around 10 times the mass density and 3 times the volume density of Fe.
A lithium-air battery running an inductive heater strikes me as likely more efficient, but Lithium is much more expensive than Iron.
I can't remember if it was Fluorine or Chlorine Trifluoride referenced in the book "Ignition" as being capable of burning as fuels water, sand, and rocket test engineers.
Flourine is electronegative enough to be able to burn with asbestos, but not willingly, it needs a lot of encouragement to do so. It'll happily burn spent coal or other oxygen burn byproducts.
For asbestos you'll need some chlorine to make that flourine very excited about the idea of tearing apart some strong molecular bonds. Chlorinetriflouride is the candidate here, though Dioxygendiflouride also likes doing it, if in a more explosive fashion (it'll probably detonate in a hypergolic fashion with asbestos or sand).
I have seen this story in several outlets, but haven't heard about NOx emissions from this process. Could someone more knowledgeable shine a light on this?
After this downvoted I've read around and found that coal power plants also contribute to NOx emissions.
But higher temperatures seem to be much more dangerous. I've heard that from critiques of hydrogen burning, they said burning pure hydrogen is unacceptable due to high temperature leading to NOx emission.
Found this:
> It is believed that an increase in the maximum temperature in the combustion zone above 1850 K leads to unacceptably high NOx emissions , and one of the main ways to reduce emissions by the thermal mechanism is to prevent the formation of hot spots in the flame front.
And this brewery previously was just poisoning people with alcohol, now they in addition pollute atmosphere with highly toxic oxigen-nitrogen compounds. And proud of it.
Efficient cycles to store heat/cold will be very valuable in the next few years. There are a lot of industrial/logistics processes where by far the biggest energy usage is in heating/cooling. In a recent refrigerated warehouse 40% of the electricity is coming from solar panels on the roof. Going higher than that is not economical because energy will be sold too cheap to the grid and batteries are still too expensive. But there are solutions starting to appear to store cold while the sun is out and then release it over the night as needed. If we can get a lot of these types of loads working like that switching the grid to solar can go even faster because of not having to wait for cheaper grid-scale storage.
I'm most interested in the the rust->iron electrolysis process - neither the article nor the video describes how that's done, except to mention that the process used clean electricity. Certainly it's possible, but I don't think there's much electrolytically produced iron today. I wonder if they're using a process that requires the oxide to be melted (very simple but needs really high temps) or a lower-temp, more chemically involved process. If they have a good-enough way to produce elemental iron, it seems like replacing an existing coal/gas fired iron smelter with a renewable electrolytic one would be a cool experiment too.
So it's more of a battery rather than a fuel. I wonder what the energy density and longevity is compared to other industrial batteries. On a side note, what's the point of a brewery (single business) using it? Do they have some unique energy requirements?
As I understood the article, the brewery is not using the process to generate electricity, but to heat fluids (water and wort).
In essence brewing process is a hell of a lot about moving heat around: You heat water to mash the grain to make wort, you boil the wort, then you remove the heat (via heat exchanger) as quickly as possible to get the wort down to where the yeast is happy. What are you going to do with all that energy you've pumped in to the fluid in the first place and have just pumped out again? If you're clever (energy conscious) you find a way to cycle that back into the process. There are lots of opportunities to optimise and conserve the energy that gets shunted back and forth in a brewery and it's a whole art/science in itself.
> On a side note, what's the point of a brewery (single business) using it? Do they have some unique energy requirements?
It's a collaboration with a local university to test the technique on industrial scale. According to the researchers there the goal is to grow to grid-scale and convert coal-fired power plans in the coming decade.
The brewery is not using the process to generate electricity, but to heat fluids (water and wort). The research team behind it hope to scale it up to electricity generation in the future.
Iron has a specific energy of 4.9 MJ/kg (for comparison: wood 18, coal 26-33, natural gas 53.6, jet fuel 43, diesel 45.6, gasoline 46.4).
So having ships, planes or even cars powered by burning iron seems to be unfeasible (at least from energy production standpoint). Albeit iron has much higher density, so if weight is not a problem then it may work. Iron has energy density of about 40 MJ/liter which is comparable with coal, diesel, petrol, which have ~34-40 MJ/liter.
That's still 5-10 times higher than lithium based batteries (0.3-1MJ/kg) and those are being used in cars and almost good enough for some airplane use cases. Heat engine efficiency isn't as good as electric motors, but it isn't bellow 20%. Downside is that recharging by converting back to iron probably can't be done within device. While pouring in metal powder in for fast refilling might be doable getting it out is probably messier.
Yes, but even best combustion engines have at very best 50% efficiency using direct transformation of thermal energy through gas expansion into mechanical energy. This involves numerous technologies, which better mix fuel and air, inject fuel directly, adjust timing of injection and exhaust following the RPMs, etc. Cars don't generate steam or use any other indirect method of transformation of thermal energy into mechanical energy, due to losses and impracticality (otherwise we would see other types of combustion engines). Burning of powder iron would not work with normal ICEs since powders are not liquids, they are abrasive and burn residuals are not gaseous (rust dust, which is even more abrasive). At very least this is non trivial engineering challenge. So 5-10 times lower specific energy of lithium batteries wins because of simplicity (and lower weight) of whole power-train (inverters, engines) and their high overall efficiency (90-95%). But indeed, on big ships (e.g. tankers), which can have a steam turbines, iron might be a feasible fuel (especially if using onboard solar power while cruising it is possible to recover part of it as a fuel again).
I this is is largely on point - this should work for larger ships that have steam turbines. Also, said turbines have better efficiency than an ICE engine.
They don't, actually. Steam turbines are less, not more efficient than marine diesel engines. In modern marine propulsion, turbines are used pretty much only on gas and coal carriers.
You would need a steam engine to convert heat into work, which is clearly out of the question for planes, and might make cars and trucks impractical. Maybe rail and boats though ?
>So having ships, planes or even cars powered by burning iron seems to be unfeasible
aluminum of course. It is cheap too and has higher specific energy than iron. Using aluminum-air fuel cell instead of burning makes the efficiency about 2 times higher. The actual cars powered by aluminum have range up to 2000km per refueling.
One nice thing is that in brewing it can be used to generate heat for boiling during brewing rather than converting to electricity first. They mention a furnace but not a generator. 81% of Dutch power is fossil fuel based, if you have an environmental concern about that and it's good press why not.
The brewery is like lots of other industrial processes. They need a lot of heath which typically coal produce today. Metal powder can replace coal in all these industrial settings.
How much iron is wasted in the process? I’m pretty sure they are not able to recycle 100% of the oxide and there must be some material losses.
Also what temperature do they need ? I assume they are making their own bottles so I wonder if a much more simple process to heat up using electricity directly would not be more efficient?
I really doubt the claim about 80% efficiency of converting iron oxide back to iron. Industrial processes widely used today are based on fossil fuel (natural gas or coal). There is some research on electrolysis, but this process is not an easy one either: you have to heat oxides up to 1600 C, apply electricity, extract resulting iron, cool it down, powder it. Each step takes energy, so I guess the 80% are only for the electricity part, without accounting for other required steps.
And how do they get the 40% round-trip efficiency? Even if we assume the 80%, modern gas turbines have efficiency of up to 38%. In complicated combined cycle mode plants efficiency can be boosted up to 60%. And it is natural gas, a very convenient fuel to work with.
for those wondering why more applications do not source metal as a fuel, burning metal is frighteningly difficult to extinguish.
metal fires often burn at more than 5000 degrees F. That’s hot enough to disassemble water into its component parts, and one of those parts is hydrogen gas, which is not only flammable but explosive. any uncontrolled release of liquid into the fire would be catastrophic. Metal fires cannot generally be quickly extinguished in an emergency or uncontrolled accident.
metal fires also release toxic gasses and byproducts that often require more consideration than electric or gas.
as an update to a few questions: NEVER add water to a metal fire. it will cause an explosion.
depriving the fire of oxygen works, but only insofar as it remains deprived until the fuel source cools from 5000 degrees, or it risks spontaneous reignition. it generally has to be monitored similar to a crucible as it cools.
most accidental metal fires do not have a cogent or quick option to deprive the fuel source of air.
Depends on the type of fire extinguisher really. Most extinguishers work to suffocate the fire (e.g. ABC or CO2 extinguishers), but depending on the type of burning metal the chemical(s) in the extinguisher might react with the metal and make the fire worse. They make special "class D" extinguishers specifically for metal fires, which spray a non-reactive powder (usually graphite dust I think) over the metal to suffocate it.
Tangent: this again reminds me of the surprising number of breweries or beers called "Bavaria" which are not in fact located in Bavaria. But I guess Bavaria should take that as a compliment...
Look at the URL [1] and indeed, the original title and text was wrong ('Germany') [2]. They left the URL intact. Such blasphemy... (kidding aside, it actually is at least from a journalism standpoint)
And it would be difficult to 'aerosolize' the powder injection into the combustion chamber and all sorts of problems. It would take a fundamentally different design. But it's still a fun thought :)
I don't see how an iron powdered internal combustion engine – where the fuel is usually a liquid and the waste is a gas – could possibly work. We do have a lot of experience with steam engines though, but AFAIK they're much harder to miniaturize to be able to fit in a car ?
This technology is based on the research from McGill’s Alternative Fuels Laboratory [1]. There are three stages involved: 1. excess electricity is used to make the initial powdered metal, 2. powdered metal burners replace coal or gas burners in existing or new power plants, and 3. excess electricity is used to recycle the powdered metal oxide output from stage 2 back into combustible metal powder.
The research is focused on the efficiency and CO2 footprint of all three stages.
"That rust can be regenerated straight back into iron powder with the application of electricity, and if you do this using solar, wind or other zero-carbon power generation systems, you end up with a totally carbon-free cycle."
Making solar panels is not carbon free. Making batteries instead of emitting gases is not carbon free. We still have to recycle those panels and those batteries and take in account the impact of it.
I feel like we are changing the place where gases are emitted or residues stored instead of making less cars, consuming less in general, etc.
One thing to keep in mind is that while commercial and consumer users of electricity generally require steady, high-quality, alternating current, electrolysis can be done with low-quality, intermittent direct current of the sort that can be generated with low-tech electrostatic generators.
A whole new parallel infrastructure of low-quality electricity generation could be built without too much effort for electrolysis of iron oxide, powered by low-grade heat and motion sources that currently can't be effectively utilized.
Same applies to production of ammonia, which has numerous uses other than fuel--right at the point of production, if that happens to be under a wind turbine placed at the edge of a farmer's field.
Waste oxygen is a byproduct of ammonia production from water and air. You really want to use pure oxygen to burn your iron, to avoid producing NOx.
Consuming less in general is very good, but it's not a strategy that can take you to net zero emissions. Building large amounts of renewables and using excess energy to sequester some carbon on the other hand can become a net zero system.
This is the perfect solution for an off grid heat source that wouldn't fatigue electrochemical batteries with limited lifetimes I'd rather reserve for smaller, precise loads (not heating element voltage).
I've been saving 1lb propane containers because I thought I would experiment with low pressure (for safety) hydrogen storage as flame source. Then I see videos where people are putting on spark arrestors and using more involving methods to totally remove oxygen (electrically interactive element in a steel container - an amount as low as 4% mixed with hydrogens low ignition point could be hazardous). Combine this with all the hoses and couplings I'd have to put in and it could add up and get complicated (although Alex Lab is a neat channel for hydrogen experimentation).
Could I just put a grinder wheel to some pig iron and create a powder stock (high surface area)? Since the powder flows it could be delivered like a wood pellet stove with auto-feed and hopper storage, and maybe for cooking I could spoon feed powder into a bowl with air flow rate control for temperature adjustment? Then another batch would "charge" as a short between two electrodes of a voltage source. Would the constant voltage of a charge controller be necessary for this redox? Could it just be a container of oxidized iron that reacts as voltage is available?
Iron powder burned cleanly producing iron oxide, then reformed into iron powder with electricity. Effectively a battery with combustion as the output.
> "the idea certainly seems to have some advantages over hydrogen, pumped hydro, batteries or kinetic energy storage"
What advantages though? If the process needs combustion then it's interesting but if the combustion is just used generate electricity then how is this better than the other methods?
Hydrogen embrittles metal, combusts too readily, and generally must be stored at pressure for transport; pumped hydro requires the right geology/topology; batteries wear out; kinetic energy storage is low-density.
> but if the combustion is just used generate electricity
This is where the brewery angle is interesting, since (as others have mentioned as well) what brewing needs is mostly heat for boiling. Combustion gives you heat directly.
Thermodynamically, the higher concentration of CO2 in industrial flue gas means you can capture CO2 there more efficiently than you can from the ambient air. However, industrial carbon capture typically is a loser because processing the huge volumes of gas produced by industrial equipment is neither cheap nor easy, requiring very large & energy intensive facilities. The products produced by such purification aren't valuable enough to offset those costs, and industry will not shoulder that burden out of the goodness of its cold mechanical heart.
Chemical looping can reduce the cost of these processing facilities (or eliminate them completely) by dramatically reducing the number of impurities in the CO2 that is produced from combustion, especially nitrogen. I.e. it makes a more pure CO2 product from the get-go, which means the total volume of gas to process is lower, and purification is even easier.
Right, "carbon capture" either from a hydrocarbon stream (carbon neutral) or biomass (carbon negative) competes with many energy sources (solar, wind, nuclear, hydro, conventional fossil fuel combustion) so you'd have to pay operators for the service of stashing CO₂ underground.
Most gas streams from combustion contain a mixture of N₂ and CO₂. CO₂ is disposed of by compressing it 1500 psi and injecting it underground. Even small amounts of N₂ or H₂O will cause the gas to misbehave in the pipeline.
but they have a liquid oxygen plant at the head end of the thing and they are separating acid gases from a stream of hydrogen and carbon monoxide about to be built up into methane.
There are other methods of combustion with oxygen but they are tricky: temperatures would be high (melt your turbine) if you really used pure oxygen, but if you recycle some of the output gas back into the turbine you might make it work.
Of course there is the cost of the oxygen separator so it is hard to be competitive. The hope with CLC is that you might be able to bolt it onto a fluidized bed combustion system and not raise the cost as much as the alternatives.
Another article mentions the fuel gets heaver as it burns. So if it was used in a ship it'll get lower in the voyage as you travel.
Personally I'm waiting for thunderf00t on this one. (But it's also cool to do strange stuff to brew beer, the story is an important part of the drinking, allegedly)
> High-efficiency electrolysis of iron oxide can store as much as 80 percent of your input energy in the iron fuel
Looks promising for home heating during the winter; especially promising for areas of the world where the winter day is so short that using solar + battery for heating is impractical.
> Using this kind of cyclical process to generate electricity could approach a theoretical efficiency around 40 percent
My heat pump (air based) has a COE of 2.7. If the electricity was stored at 40% efficiency, that means I'm getting back 108% of the energy if combusting iron is used to store the energy!
Note: Where I live we have an old oil plant that only runs during cold snaps; and a newer gas plant next to it that runs when renewables are scarce.
The nice thing in this process of the oxidation of the metal, it is done with hydrogen and does not involve any emission of CO2. Effectively what they do is:
Electricity from renewables -> Hydrogen (electrolysis) -> (iron oxide to iron, in loop) -> heat -> iron oxide.
I would say this is a nice solution to the problem of storing hydrogen.
This iron originally took considerable energy to turn it from rust into iron, when it's burned it becomes rust again.
Presumably, this will be useful on a small scale to absorb certain types of scrap iron and steel that is contaminated with other elements that would make it unsuitable for normal recycling.
High quality iron would be better off being recycled, as the huge amount of energy originally expended in its reduction from oxide to iron doesn't have to be expended on the production of new iron.
I think the point of this is one can use intermittently available renewable sources of power (wind, solar, tidal) to electrolyse the rust to produce the iron powder for on-demand burning. The rust produced by burning can then be re-electrolysed to make more pure iron powder.
Let me get this straight. The theory is you use some other means to heat up the iron to its burning point (probably by burning fuel, unless electricity can be used to bring material >1000 C?). Then you can turn off the energy input because the fuel is burning, at which point it will oxidize on its own...? I'm not really sure I'm grasping how this is an efficient system
It seems like they've made a battery that only ballasts heat, with a material that is tricky to move around and has a complex mechanical cycle. It will be interesting to see what the eroi is on this vs. lithium ion, pumped storage, or even old school boiler heat ballast.
Can someone explain the process behind burning iron and it being carbon free? How does that work? Don't you need something combustible to keep the process going? I mean, I have rusty iron at home, but holding a match to it doesn't set it alight.
Brewing generally consists of: Adding your grain to water to form the mash. Cooking the mash to extract the sugars et al. Separating out the mash into the wort and the solids. Boiling the wort. Cooling the wort, then pitching in yeast. Then storing for a while while it ferments. Heating the wort and mash could be done electrically, but generally (especially at brewery scale) is combustion.
Why not heat molten salt directly? Insulation can be effectively 100% (silvered vacuum bottle) and molten salt through a heat exchanger will boil water pretty fast and heat can be pumped into the salt with at least 100% efficiency.
Wonder if this can be used for heating homes via small rechargeable cells that one recharges during summer with nothing more than a magnifying glass (well, or a concentrated solar plant).
I don't know how this would scale. Like, you can't deliver iron filings/powder efficiently via pipes, can you? Certainly not via existing infrastructure. Clogs would probably be certain.
Without water, rust is Fe2O3. It's commonly used as red die for cements and cheapest iron oxide. It's also useful with aluminum to make thermite (which generates VERY high temperatures). Long ago I bought some Fe2O3, it was very fine powder and apparently waste from some chemical process.
"Our ambition is to convert the first coal-fired power plants into sustainable iron fuel plants by 2030.”
They make it sound so clean with the ability to recover and reuse the iron. But if you replace coal with iron you'll need more than one coal fired plant to produce the energy to recover the burned iron. You can use wind or solar to power the electrolysis instead, but then theres no need to bother with the iron at all.
I bet they're hoping to just sequester the rust in a landfill or something.
If solar prices keep trending the way they're trending, then we may have way more solar than we have anything useful to do with during peak sunlight hours in the relatively near future.
Using iron as a big chemical battery seems at least plausible. More so if you get to reuse existing infrastructure to turn it back into energy.
Renewable energy sources like wind and solar are intermittent (one can't create electricity using solar at night, or using wind when the air is still, after all), whereas furnaces can, effectively, run whenever there is demand.
Electrolysing rust into iron can be done when there is excess renewable energy available, and it can be burned when there is a deficit.
I have to think that if this form of energy was practical, humanity would have gravitated towards using it in the 3200 years since the start of the Iron Age.
Iron needs to be mined, transported, ground and then the rust recycled somehow. Is the value of the energy released substantially more than the aggregate costs of releasing it?
The article discusses using electrolysis with clean energy to recycle the rusted iron dust back to combustible state. This isn't something we've been able (or willing) to do in the past as we didn't have an excess of clean energy.
First we must convert to Kelvin by adding 273.15 to the Celsius temperature. Here is a table with Carnot efficiencies calculated, assuming that the cool end of the cycle is something like a car radiator at just below the boiling point of water at 373 K (100 C or 212 F):
Material Temperature(Kelvin) Efficiency:
---
Coal 2250 83%
Iron 2073 82%
Propane 1533 76%
Wood 893 58%
Im having a hard time finding efficiencies for iron oxide electrolysis because all of the papers are behind paywalls. A big portion of the energy required is in heating the iron oxide in the first place, which could be done easily by solar collectors for free:
I think a 95% efficiency might be reasonable for iron oxide if the temperature is raised by free solar thermal energy. So round trip efficiency is:
efficiency = 0.95 * 0.82 = 78%
This could be raised by a few percent by using a colder radiator (closer to room temperature at 300 K) and recapturing some of the waste heat with a Stirling engine. So I think that the article is accurate.
If someone has a table of electrolysis efficiencies for various compounds, that would be helpful.
Edit: after thinking about this for a moment, I realized that the Carnot efficiency should be calculated against room temperature if only the heat is being used and we aren't generating electricity. It only increases the efficiencies in the table above by about 3-8% from hottest to coldest, respectively.
Edit 2: for anyone curious, capturing heat and converting it to electricity is usually about 70% efficient at a turbine, and 95% efficient at a generator, for about 65% total. That's why a jet engine is limited to about 0.80 * 0.70 * 0.95 = 55% efficiency (40% in practice). Stirling engines are much closer to their ideal Carnot efficiency because their losses to turbulence (friction/entropy) are much lower. If my numbers are a little off here, please correct me.
Which middleman? The suggested method of obtaining pure iron powder involves electrolysis of oxidized iron powder. I don't even see where you could insert coke into this process.
The process for actually making iron, rather than the one they've imagined, involves smelting with coke. I'm pretty sure that electrolysing iron oxide is not a thing that's done on an industrial scale.
You can do a lot of things with electricity: you can heat things, but also move them around and run your TV, all without any loss. With heat you can just... heat things. So you can't call this an "iron battery", because you don't get electricity out of it, just heat. Maybe call it a "heat battery" or "high performance heat pad".
Also note the efficiency numbers: "High-efficiency electrolysis of iron oxide can store as much as 80 percent of your input energy in the iron fuel" is the efficiency of the process itself. "Using this kind of cyclical process to generate electricity could approach a theoretical efficiency around 40 percent" is because you need to climb the ladder to low entropy again (probably by using the equivalent of a steam engine to run a generator).