[teruakohatu]

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.

Or would the plan be to slowly heat over fall?

[johndough]

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.

[adrian_b]

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.

[moffkalast]

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.

[DrNosferatu]

I hear peroxide conversion efficiency can be as low as 30%, and concentrated peroxide is quite dangerous.

[moffkalast]

> Efficiency Admittedly, the current, non-optimized, technical trial-level efficiency of the here-built system was very low, with an overall storage efficiency of 11.4%,

So far their is even lower, though they claim a theoretical max of 79%. Storing large amounts of energy that's ready to be used is rarely not dangerous in any case. Except maybe potential energy of a tank of water on a mountain.

[mark-r]

There's an advantage to pure hydrogen vs. synthetic hydrocarbons, the lack of carbon means no greenhouse gas production when you use it.

[jillesvangurp]

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.

[DoctorOetker]

would you mind explicitly listing the 4 conversions?

I see:

1) generation 2) storage efficiency (energy while storing divided by energy upon release)

what are the other 2 you had in mind?

[jillesvangurp]

1) generate hydrogen 2) store hydrogen in iron oxide 3) discharge hydrogen again 4) convert it into something useful (electricity, heat, movement, etc.).

All those steps lose energy. And there's stuff that happens in between involving pipes, leaky valves, tanks, compression, etc.

[DoctorOetker]

this is not compressed hydrogen storage, anyway your 2,3,4 are the same as my round trip storage efficiency 2

[toast0]

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.

[zdragnar]

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.

[MadDemon]

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.

[mark-r]

The difference depends on your latitude. On the equator the seasons won't make a difference, above the arctic circle you'll get diddly-squat from your panels in the winter during the endless night. Switzerland is at 47 degrees, not quite arctic circle but far enough north to see a huge difference.

[jl6]

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.

[londons_explore]

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.

[outop]

Isn't it possible that such a system will over-produce 99% of the year and that therefore, the marginal cost will almost always be $0?

'Take my energy and allow me to stop accelerating my flywheels which regulate production' seems more plausible than 'someone would always like more heat for something' (what?)

Or possibly 'take my energy and I'll cut off some of the people using spare energy to do low priority, low value computation for free'?

[londons_explore]

I think electricity use is far more elastic than you're imagining. Plenty of big users can turn up/down production and already do so based on prices. If wide price swings got more frequent, more stuff would get dynamic.

You can imagine home appliances having an 'eco' setting which runs the appliance like the washing or the dishwasher at the cheapest time in the next 12 hours.

Or the water heating systems which heat more water when prices are cheap.

Or heaters which switch between natural gas and heat pump based on price.

Or electric car chargers which charge during the cheapest hours.

(all of these already exist, but none are yet common).

Over the long term there is also plenty of elasticity. If electric heating is expensive, people will install gas/oil heaters when they renovate. If electricity is cheap, more people buy electric cars. With cheap electricity, maybe fewer people decide to add more insulation to their houses. Businesses don't upgrade energy inefficient equipment to be more energy efficient, etc.

Plenty of demand elasticity in both the short and long term. End result: As long as the market is unconstrained, prices won't hit zero more than say ~5% of the time.

[outop]

I disagree that this argument makes it less likely to have very low prices much of the time. I think it makes it more likely.

If peak to trough is a large gap, say 60% of peak, this tends to make it less likely that peak will be met by overproduction, since that would involve very large capital costs.

The picture you paint above would suggest a very small gap between peak and trough, say 2% of peak. This means that almost certainly there would be enough over capacity to more than meet peak demand. Therefore the total daily demand would be more than met by capacity, leading to some energy being thrown away. So at all times except the peak, the marginal cost would be zero.

You have given an accurate argument for why demand would be elastic at trough. But you haven't given any reason why overall demand would be very elastic.

[zdragnar]

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.

[brazzy]

Energy production from solar is lower in winter, but it's not zero. And other forms (notably wind) are not reduced.

It's a non-problem, really. Especially at scale.