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.
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.
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?
All those steps lose energy. And there's stuff that happens in between involving pipes, leaky valves, tanks, compression, etc.
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.
'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'?
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.
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.
It's a non-problem, really. Especially at scale.