What is Dunkelflaute?
Interview with Lars Schernikau
“Dunkelflaute” is a German term (dark and windless) that is used to describe a period when there is both little wind and little irradiance, making solar and wind installations sit idle. Needless to say, during periods of a dunkelflaute, which can last days and sometimes weeks, power generation relies on conventional forms of power generators such as coal, gas, nuclear, hydro or biomass.
The term is often used in discussions about the challenges associated with intermittent renewable energy sources and the need for reliable energy storage solutions, or alternative power generation methods to ensure a consistent and stable energy supply during periods of low wind and solar activity.
Lars Schernikau is an energy economist, entrepreneur, commodity trader, and book author. He is one of the German Energiewende’s biggest critiques. His well articulated arguments are all published in his book The Unpopular Truth. Him and I had a discussion last week to talk about his concerns, and in particular what occured between the end of November and the beginning of December 2023, when there was a Dunkeflaute all across Europe.
That was when the Germans were caught smoking coal again!
Mr. Schernikau asks the basic question: “how big should your battery be if there is absolutely no wind or solar in the country?”
If Germany’s demand is approximately 60 GW, and a Dunkelflaute lasts for example, for only 10 hours, then how big should that battery be?
60 GW x 10 hours = 600 GWh or 600 000 000 kWh!
But what is the current price of batteries? Currently the various studies put them on a range of $600/kwh to $200/kwh with an average of 5000 recharge cycles.
If batteries are priced at $200/kwh, then it would cost Germany only a “modest” sum of $120 billion for 10 hour in upfront cost, and $24.0 million per 10 hour Dunkelflaute!
The battery storage problem isn’t so much that we don’t know how to make a battery, it’s that the cost of that battery is rather expensive, which is why, even in Germany, reliable “dirty” king coal remains supreme.
As I showed above, if you multiply out that money, then maintaining a safety of supply during dunkelflaute isn’t exactly a cheap exercise, and this is why Germany is still keeping its coal fire stations on backup. Yes, Germany might not be burning more coal, but they are still stockpiling coal, and effectively running their plants on an economically inefficient model, because they only operate them for a few days per year.
As Lars and I discussed, there currently isn’t a technological solution for this problem, and it in turn makes the operation of emergency fuel very expensive, because a coal plant that operates for only a few days per year is a highly inefficient usage of the asset!
Could Dunkelflaute be a contributing factor to why German household electricity prices, despite the apparent decrease in the cost of renewable energy, are among the highest in the world?
We both concur that electricity is a service, and the pricing of this service is driven by the cost of emergency fuel, particularly from economically inefficient coal plants, rather than the operational case, such as that of a wind turbine during strong winds. As I posit, a revision in the pricing model would entail incorporating the cost of batteries into the fixed tariff.
This adjustment would provide customers with a transparent depiction of the Full Cost of Electricity, enabling them to precisely assess the financial impact of these policies on their bottom line, and hopefully, only then will they vote the politicians out who advocate for them!
Below is a link for our conversation.
Revised Note: A reader pointed out that I neglected to mention that renewables are generally overbuilt, and I also overestimated the installed capacity. I appreciate the correction. However, it's crucial to consider that I may have underestimated the duration of a "dunkelflaute," which can often persist for as long as 50 to 150 hours, and sometimes even longer. Please take this into account when evaluating the overall analysis. Here is a better article with a more detailed calculation that he referred me to.
If you find any error on my assumptions or calculations, please let me know, I appreciate the corrections.
I clicked on this article because I saw the eye-popping pull quote about the cost of battery storage, but the math doesn’t check out.
The first big problem is that it’s wrong to look at replacing the total installed renewable capacity with battery storage. Power producers know renewables are intermittent and never produce at 100%, so they overbuild renewable capacity. During a Dunkelflaute, you only have to replace the actual deficit, and not the extra overbuilt margin. The graph included in the article conveniently shows the worst case deficit is closer to 60 GW.
The second problem is your figure for how much battery storage costs appear to be way off reality. My best guess is that the report gives a projection of the upfront cost of building battery storage in the future, and not the cost per incident of using it, however I have to admit I am very confused by the cited report. Nonetheless, we can sanity check this by looking at the cost to build battery storage plants. A random one I picked off the top of my head that was built recently is the Australian Hornsdale Power Reserve plant, which Wikipedia says cost $120 million USD for 200 MWh of installed capacity. So, to scale up a plant that can handle a 600 GWh 10 hour Dunkelflaute would cost an upfront cost of $360 billion if built today. The ongoing operational costs are very small compared to this, so I’ll ignore them.
I can’t find for sure how many uses the Australian plant is designed for, but Wikipedia says the battery technology it’s based on, the Powerwall, can deliver 5,000 cycles, so we’ll use that as an estimate. This would bring the per Dunkelflaute, per German cost down to $0.87, rather than the $3,557.69 which caught my attention.