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.
#graphoftheday

This is the graph he is talking about:

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:

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).

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