A “skewed” distribution: the volatility of solar and wind

This tweet sums up the biggest problem with intermittent power sources:

Yesterday, a fairly dramatic low in terms of solar & wind output. Fortunately, there were also great days this year.
Although sun and wind often complement each other, the total forms a rather ‘skewed’ distribution. We will have to learn to deal with that soon.

Tweet WM_visser 2021-11-17

This is the graph he is talking about:

Sorted contribution of solar wind (The Netherlands)

This graph depicts the output of solar and wind power of the Netherlands from January 1 until November 16 (the day before the tweet), sorted from big to small, showing the “skewed” distribution of intermittent output

Output varies between 177 GWh on July 29 (summer) and a mere 6.3 GWh on November 16 (late autumn). That is quite a range and still 1.5 months to go in 2021.

I don’t have much data about energy in the Netherlands, but I do have a lot of data about energy in Belgium. That made me wonder how this plays out in Belgium. That is rather simple to figure out. Solar and wind data can be found on the Elia website. After downloading the data from January until November and sorting the data, this is the result:

Chart15a: sorted contribution solar and wind (Belgium)

The two graphs are rather similar. They have roughly the same shape and when total output is the highest, then solar output is pretty low. The highest total output in the Belgian graph is 110.6 GWh and the lowest total output is 4.7 GWh. The smallest total output is therefor about 4% of the largest total output, which is in the same ballpark as the graph of the Netherlands (3.6%).

The statement that solar and wind often complement each other is correct, but only if you look at it ON AVERAGE. In the months with less sun, there if often more wind and in the months with more sun, there is often less wind.

Although average intermittent output is similar over the months, electricity demand isn’t. Electricity demand is highest in winter and lowest in summer. More, if we look at demand throughout the day, then most power is needed during the morning peak and the evening peak. During those peaks, solar energy is absent in winter at our latitude and it therefor will totally depend on wind energy to fill in peak demand. Although there is on average more wind in winter, it is highly variable and it is surely possible that there is no(t much) wind during peak demand.

Electricity supply and demand have to be matched simultaneously. If there is a high demand and low production of intermittent power, then a dispatchable power source needs to jump in to boost the frequency. If there is a low demand and a high production of intermittent power, then the excess needs to be redirected away from the grid before frequency goes way up and causes damage.

Even when solar and wind power “complement” each other on average, that doesn’t necessarily mean that production matches demand at each moment. If solar and wind become our primary power sources, then there is a need for seasonal storage or adequate dispatchable power sources that can be halted during summer.

The graph shows us that there is a huge difference between the minimum and the maximum intermittent output. This difference has no relation with demand, it is dependent on external factors like the length of the day, the intensity of the sun and the strength of the wind. The “there were also great days this year” statement is pretty meaningless in that light. I do however agree that the manager of the Dutch grid will have to learn to deal with that volatility soon (if not already).


2 thoughts on “A “skewed” distribution: the volatility of solar and wind

  1. Linley

    Very interesting post. I was under the impression that off shore wind was supposed to be more reliable than on shore. The graphs don’t seem to support that impression.


    1. trustyetverify Post author

      Thanks for bringing this to my attention. You are right, something doesn’t add up. I checked the numbers from Belgium and those seem to be correct.
      Looking a bit deeper, I am not really sure about the numbers from the wikipedia article on wind power in the Netherlands. They state at the top of their article that “as of October 2020, wind power in the Netherlands had an installed capacity of 4,990 MW, 19% of which is based offshore” and that is the share that I mentioned in my post. However, with this capacity it is very unlikely to produce ± 120 GWh (that would be very close to 100%), so a lot of capacity needed to be installed between October 2020 and July 2021 in order to reach a maximum production of 120 GWh.
      Later in the article, the cumulative wind capacity is given as 2,460 MW offshore plus 4,159 MW onshore and this means 37% offshore.Those numbers are much more plausible.
      I am a bit surprised by the industriousness of the Dutch lately. They were (and still are) behind their schedule, so they might have brought even more capacity online between January 1 and July 29. I am not going to guess how much the share was in 2021 (but it is surely way above 19%), so I will remove that part of the comparison from my post.



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