[jhgb]

> This is a bit of a nitpick, but this is a physical impossibility. On a AC electric network, if the power input is higher than the power output, the frequency of the current goes up quite quickly, until the grid collapses (because there are security to avoid frequency deviation). You cannot “not consume all the power you produce”, all you can do is not producing as much as you could.

I probably should have said "all the power you could produce", since for example with photovoltaics you can produce at any moment any amount of power from zero up to the MPPT point on the I/V curve, depending on how much charge you remove from the panel. I hope this clears it up.

> Btw, I'm interested by the sources of your “1.6 GWh of storage for your 1000 MW nuclear” because it sounds really low to me

I used the figures in the 2018 Zerrahn et al. article: https://www.sciencedirect.com/science/article/pii/S001429211...

> I did a simulation[1] a while ago based on French data, for a 100% RE scenario and my calculation arrived at around 250GWh per GW of installed capacity.

Maybe you've just taken Sinn's approach instead of Zerrahn's? That number would seem to fit it.

[littlestymaar]

> I used the figures in the 2018 Zerrahn et al. article: https://www.sciencedirect.com/science/article/pii/S001429211...

Thanks for the link !

> Maybe you've just taken Sinn's approach instead of Zerrahn's? That number would seem to fit it.

I wasn't aware of that paper (thanks again!), but from skimming Sinn's paper, our methodology seems to be pretty similar. I'm even more excited to read Zerrahn's paper now!

[littlestymaar]

Ok so I read Zerrahn's paper, and I have to admit I didn't expect to be that shocked by the methodology. Of course I'm a bit biased because my own work is really close to Sinn's which is criticized in this paper, but this paper really makes me think the author didn't even try to understand what Sinn studied in their paper: Zerrahn seems to believe that Sinn's scenario stores all the energy because they don't want to waste electricity, but it doesn't occur to them that Sinn's stores all that electricity because at some point the grid will needs it!. In fact, I think the culprit is just using RLDCs (residual load duration curves) instead of time series, because this way you just erase the temporal dimension of the problem, which is − unfortunately − the most important one.

For instance, in my own data (France, year 2017, for the record the scenario was 100% RE) from the first of January 12am, to the 3rd at 3pm the wind capacity factor barely exceed 10%, three days in a row. For this period only you'd need 3TWh of storage[1]! No reasonable level[2] of curtailment is gonna help here.

[1]: of course it doesn't have to be storage, you just need 50GW of controllable power and any fossil fuel would work (and that's what the Danish do for instance) but this is outside of the scope of this discussion, which is about how storage allows you to avoid pairing RE with fossil sources.

[2]: I assume that nobody would consider something above 90% curtailment to be reasonable.

[jhgb]

> but it doesn't occur to them that Sinn's stores all that electricity because at some point the grid will needs it!

I'm pretty sure that they understand that. What they don't understand (and what I don't understand) is why is Sinn making the amount artificially high by ignoring the economics. I immediately understood what Zerrahn was getting at, and even before I knew how different authors approached this problem in literature, I would have myself intuitively gone for an approach like Zerrahn's. MRTS is surely not a difficult concept to grasp.

> For instance, in my own data (France, year 2017, for the record the scenario was 100% RE) from the first of January 12am, to the 3rd at 3pm the wind capacity factor barely exceed 10%, three days in a row. For this period only you'd need 3TWh of storage! No reasonable level[1] of curtailment is gonna help here.

I can't tell you what Zerrahn's approach would tell you for the French grid. You can't really extrapolate that from German results. You'd have to pretty much re-do the whole work, including getting equivalent data for the French grid.

[littlestymaar]

> I'm pretty sure that they understand that. What they don't understand (and what I don't understand) is why is Sinn making the amount artificially high by ignoring the economics. I immediately understood what Zerrahn was getting at, and even before I knew how different authors approached this problem in literature, I would have myself intuitively gone for an approach like Zerrahn's. MRTS is surely not a difficult concept to grasp.

It's not about being difficult to grasp, it's about whether they are the right tool for the job. Which they aren't, because the temporality of the phenomenon disappear, while it is the single most crucial factor when talking about storage: 24 hours without wind in a row have a dramatically different impact from 24 days each without wind for one hour. In the first case you need enough storage for an entire day, while in the second case all you need is one hour of storage! (And that's where the two orders of magnitude come from: «several days» being ~100 times as long as «1 hour». The storage you need is strictly superior the sum of consecutive hours with a positive residual load (minus what can be produced by you non-renewable plants), to calculate this value you must keep the time (and also factor in the availability and economics of your back-up non-renewable power supply if you want to go one step further, which neither I nor Sinn did).

Sinn doesn't take economics in account, because it's not relevant to the discussion here, it's all about physics here. (And Sinn being an economist, he really deserves credit for focusing on the physics aspect).

> I can't tell you what Zerrahn's approach would tell you for the French grid. You can't really extrapolate that from German results. You'd have to pretty much re-do the whole work, including getting equivalent data for the French grid.

It would be easier to just grab the German data used by Zerrahn to reproduce Sinn's findings (because they claim them to be easily accessible). Maybe I'll have some time later in the week to do so.

[jhgb]

> It's not about being difficult to grasp, it's about whether they are the right tool for the job. Which they aren't, because the temporality of the phenomenon disappear, while it is the single most crucial factor when talking about storage: 24 hours without wind in a row have a dramatically different impact from 24 days each without wind for one hour.

I don't see how this changes anything. The difference between the two approaches is not the difference between assuming multi-day troughs in wind power vs. not assuming them (both Zerrahn and Sinn assume their existence) -- it's a difference between blindly modeling storage for all generated power so that it never goes to waste vs. modeling a grid with minimum total cost of all components included that still satisfies expected electricity production demands in all parts of a year (= that does not exceed the capabilities of any component of the system in any part of the year).

The latter approach (the feasible set of which is a superset of the feasible set of the former approach) will converge to the former ONLY IF storage costs are disproportionately low. If storage costs are substantial, the optimum will likely lie in the part of the expanded feasible set that lies outside of the original feasible set, with the consequence that the old optimum was very much local, and formed a huge red herring.

> In the first case you need enough storage for an entire day, while in the second case all you need is one hour of storage! (And that's where the two orders of magnitude come from: «several days» being ~100 times as long as «1 hour».

No, that's NOT where the difference is, and I'm dismayed that this is your takeaway from all this even after reading TFA by Zerrahn.

The difference is that Sinn assumes that if there's 1 GWh to be fulfilled in the middle of January and there's a matching 1 GWh of PV overgeneration in the middle of July, then it's perfectly reasonable to say "fine, let's store that 1 GWh for half a year until we need it in the middle of January, regardless of how expensive it is" -- because THAT is what you necessarily end up with if you're going for 0% curtailment like Sinn did.

And it turns out that economically, this is terrible idea, and once you realize it and include economics in your models, they will steer you away from the idea of zero curtailment.

> The storage you need is strictly superior the sum of consecutive hours with a positive residual load

...and Sinn makes that positive residual load artificially high compared to the economic optimum because of striving for 0% curtailment for no good reason.

> Sinn doesn't take economics in account, because it's not relevant to the discussion here, it's all about physics here. (And Sinn being an economist, he really deserves credit for focusing on the physics aspect).

Which makes it all the sadder if he first constructs a straw man and then sets fire to it, especially if it's a straw man from his own department.

> It would be easier to just grab the German data used by Zerrahn to reproduce Sinn's findings (because they claim them to be easily accessible).

But...that's what Zerrahn did? It's mentioned in the paper that they replicated Sinn's findings with their own data as a validation that they're calculating with comparable data.

[littlestymaar]

> it's a difference between blindly modeling storage for all generated power so that it never goes to waste

No, that's Zerrahn's take on Sinn's paper, but you should not take it for granted. And the cheap shot about the «Non-robustness» of Sinn's paper should serve as a warning that Zerrahn is not really giving Sinn's paper a fair treatment.

> But...that's what Zerrahn did? It's mentioned in the paper that they replicated Sinn's findings with their own data as a validation that they're calculating with comparable data.

Yes, and now I want to re-use the same dataset, but with a proper time-based methodology so I can find a specific time period for which Zerrahn's-level of storage would lead to a network collapse (Like I did for the French data above).