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It’s about 12 weeks in Germany:

https://iopscience.iop.org/article/10.1088/1748-9326/ac4dc8

It also depends how much one overbuilds the supply, since the batteries need to be fully charged at the beginning of that 12 week drought.

Based on a quick reading it seems they are assuming the average supply is 130% of the average load over the year.

That 12 weeks almost certainly doesn't refer to what you think it refers to.

It appears to be a somewhat arbitrary notion of how long would it take the full storage to be completely depleted, if it was being partly offset by continuing renewable generation over that time.

This accounts for the most initially bizarre claim of the paper, that introducing bioenergy into the system (i.e. storage of natural gas from non-fossil sources) would increase this 12 week period to a full year:

> Interestingly, the decrease in renewable overcapacity in parallel to the increase in overall storage volume means that the period when storage is fully used, that is, the period that defines storage requirements, is prolonged to more than 1 year (10 October 1995 to 3 February 1997).

But obviously a longer period is actually better by this weird metric.

They give some more reasonable numbers of 12 days of energy storage elsewhere, which corresponds with figures given in models like this one, which suggest 13 days of power-to-X fuel would be a low cost optimum for Germany:

https://www.wartsila.com/energy/towards-100-renewable-energy...

i.e. the stored gas would if burned and used exclusively for electricity production would last 13 days as it equals 4% of the total electricity production. Of course, it wouldn't be used in that manner, but in concert with other energy sources, leading to the inflated number you quote from the paper.

And of course, an electricity system that burned 4% fossil gas would hardly be the end of the world. I personally would rather see nations do that and pay a carbon fee to let poorer nations achieve their low hanging goals than obsess about the last 4% in an unhealthy and (often seemingly intentionally) conuterproductive manner.

> Do you have any links to those studies? Because the ones I've seen indicate the exact opposite. You only need 2-3 days of storage or so at most.

It depends very much on where you live. Famously, California can get to 100% renewable production with 3 hours of storage, because production is very stable, load peaks match production well and there is sufficient natural hydropower resources available.

In contrast, Finland would need about 3 months worth to hit 100% renewable. Because worst load peaks happen when production from both wind and solar can be zero for a prolonged period, and natural hydro output is limited at the same time. 3 months is absolutely not actually feasible, so there will always need to be some baseload from nuclear or fossil sources.

But 2-3 days of storage is still quite a lot. The recently started OL3 power plant had a total construction cost of ~11B€, making it one of the most expensive construction projects ever. It has a nameplate capacity of 1600MWe, assuming 95% capacity factor (it goes up when it's cold and down when it's warm), if you spent it's construction cost building grid-scale batteries, assuming the lowest cost of a completed battery project anywhere in the world, you'd get something like 27 hours of storage. So even if the primary production was free, if you need more than that, you'd be better off building the world's largest and most expensive nuclear power plant instead of batteries + renewables.

> Marc Z Jacobsen has some fairly detailed studies for going 100% renewables. He doesn't generally assume any improvements in technology, so his estimates are conservative. I don't remember seeing anything about seasonal storage.

He was a coauthor on a recent review article on 100% RE energy systems. One conclusion of the review article is that e-fuels are very useful, and that with e-fuels costs are similar to those of energy systems based on fossil fuels.

E-fuels (like hydrogen) inherently provide very long term storage.

https://ieeexplore.ieee.org/document/9837910

This report, which is often quoted,

https://www.eia.gov/analysis/studies/powerplants/capitalcost...

gives a crazy low cost for a solar + battery plant that assumes storage for an hour and a half which is certainly too little. When I split out their generation and storage numbers and put in the assumption that 12 hours of storage gets you through the night the price is getting in the same range as gas turbine power plants.

There's the seasonal problem too, the answer to that is some combination of building more solar capacity or adding huge amounts of storage. I'd estimate that the daily insolation varies by a factor of 2 or so in NY

https://www.solarenergylocal.com/states/new-york/new-york/

so you could build maybe twice the solar capacity and have enough generation in the winter. Judged that way the system cost is creeping in the direction of what nuclear energy costs, though you've got a lot of "free" electricity in the summer although that could be "free as in puppy". Hypothetically you could do something like desalinate seawater and pump it uphill into reservoirs but operating any kind of industrial factory intermittently is going to be murder for capital and operating costs. There is this idea

https://www.moderndescartes.com/essays/factobattery/

where you could smooth out diurnal variation in a "hydrogen economy" factory by overbuilding electrolyzers, but to take advantage of "free" summer electricity you might have to lay off all your workers half the year not to mention building surplus transmission infrastructure.

Of course it takes detailed modeling of supply and demand to get good cost estimates for renewable plus storage systems and one thing I find irksome about that EIA report is that it quotes one number for a solar energy plant which is just wrong because the exact same solar plant will product a lot more power in Nevada and it will in Wisconsin. Many people are quoting these numbers and not really aware that they are discrediting themselves and the renewable energy cause because quoting a number that doesn't depend on time and place just violates common sense.

>to take advantage of "free" summer electricity you might have to lay off all your workers half the year not to mention building surplus transmission infrastructure.

Great comment.

Whichever industry you choose as a Factobattery, you should expect some added costs due to seasonal intermittency. The question is: which industry has the lowest added cost per kWh?

Has there ever been a study to rank order which industrial processes make the best Factobatteries?

Here is a study for two areas, Germany and California:

http://euanmearns.com/the-cost-of-wind-solar-power-batteries...

The "trick" here (sadly common in this debate) is the paper assumes you're never allowed to overbuild the solar/wind generation capacity. You can only time-shift, even when oversupply would actually be cheaper.

The most economical solution uses a mix of both, but they quietly discard the best approach to reach the (preordained?) conclusion that batteries are "ruinously expensive." Bad form.

To stabilize the grid you don't buy batteries that cycle just once per year. There's a better way.

https://pubs.aip.org/aip/jrse/article/13/6/066301/285194

It's not a "trick" just an acknowledgment that overbuilding wind/solar generation capacity does nothing but waste money and fail to significantly improve outcomes.

You're utterly wrong there. The optimum, cost minimizing solution can involved overprovisioning of renewables.

Indeed, we already see this internally in PV installations. It's best to overprovision the modules beyond what the inverters can handle and just clip some of their output at times of peak insolation. That's because inverters that could handle the peak would hardly ever operate at top power and could be downsized without losing much overall output.

I'm willing to accept I'm missing something, but overbuilding is not a sufficient approach because you still need to have generation available at night when there is no wind. Doesn't matter how much you overbuild solar and wind you can't overcome the problem of no sun and no wind.

That study isn't hiding anything, it is an attempt to estimate how much storage is required. If you adjust the solar/wind capacity (i.e. overbuild), you'll reduce the storage requirements but there are diminishing returns resulting in very expensive systems long before your solved the storage problem.

If we had grid-scale storage that was economical, it should be very easy to build a production system to demonstrate that capability. I've not seen any examples. And it certainly seems wise to actually build a system that demonstrates the viability of grid-scale storage before decommissioning base load generating capacity.

>overbuilding is not a sufficient approach because you still need generation at night

The energy storage provides generation at night, of course.

I said an optimal mix of oversupply and energy storage. You need both.

Your linked paper tries to use only 100% storage and 0% oversupply, which results in very suboptimal economics. The correct approach is to find the cost-optimal mix.