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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.

https://www.hellenicshippingnews.com/how-many-gallons-of-fue...

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%.

I was talking about the whole journey. You don't get anywhere in this world in less than a few days/weeks with a big oil tanker.

> 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.

Haha, love that solution!

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.

Go ahead and stick a generator in between to capture some of the energy of the rust "flow" moving downhill for extra efficiency.

> 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.

Batteries are expensive. Iron is abundant and cheap, and these seem much less difficult/costly to build than to create a dam, for example.

It also seems like it can make use of existing infrastructure and save money there.

I don’t think the expense of other solutions justifies the abject inefficiency of this one though.

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.

Limit the air exposure.

I imagine methane synthesis will beat some thinglike this out. Pipeline infrastructure exists, is convenient, etc.

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.

Or maybe add some oils ?

Well, firstly we do store grain and sand and many other things in massive quantities.

But secondly, wind power makes a lot more sence in northern latitudes.

Iron is here used as an energy storage system. That is the whole point. You store excess energy from wind and solar power e.g. in iron to use later.

Yes for short term batteries are better but for say seasonal storage or when power is generated far away from where it is used then metal is better.

Yes, exactly as with coal. What is the problem with that? Railroad and ships are really efficient at moving large chunks of cargo cheaply.

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.

The principle can be effective, I've not seen anyone seriously consider iron however.