In previous post, I showed graphs presenting a comparison between the average daily production of intermittent energy sources and the average daily import/export balance over a one year period. Although this way of presenting is perfectly suitable to find the average relationship between the import/export balance and the production of solar and/or wind, it is of course a highly idealized situation that will hide the influence of intermittent electricity sources on the grid. Production of electricity by solar and wind doesn’t only vary during the day, but there is also seasonal variability involved.

This becomes clear when I repeat that exercise on a monthly basis instead of an annual basis. This is the monthly solar performance in 2023:

This graph shows that there is hardly any solar electricity production in the beginning of the year, it then increases in the first half of the year to decrease again in the second half to end at hardly any solar electricity production in December.

However, looking closer at this graph, the values of the months July and August don’t make much sense. Solar electricity production should be the highest in those two months, but that doesn’t seem to be the case here.

Generally, I expect January production the lowest of the year, then gradually increase until its peak in July to finally slowly tapering down back towards the minimum at the end of the year. However, the production in July is considerably lower than in June and August is roughly as low as July, followed by September that is higher than August. This is not as expected because at our latitude, July and August have the longest daytime of the year and that doesn’t match what is shown in that graph.

I initially assumed that this July-August dip might be the result of curtailment in the summer months, but then that didn’t seem right. It is mostly wind that is curtailed, not solar. That made me curious whether there was thát much more sunshine in July and August that it necessitated curtailment or whether there was just too much intermittent production relative to the demand.

I quickly found the sunshine hours of 2023 as compared to the average over the 1981 – 2010 and 1991 – 2020 reference periods:

This is how it looks as a graph.

That explains a lot! Although the number of sunshine hours over the year seems slightly higher compared to the two reference periods, the distribution was rather different. The number of sunshine hours in May was quite higher than the average, but it was June that stuck out head and shoulders above the rest. July and August were both below average, followed by September that again was higher than average. May and June had the highest number of sunshine hours in 2023, instead of July and August as I would expect.

So, this aberration in the first graph in this post apparently is a combination of the well above average sunshine hours of May, June and September contrasting with the slightly below average number of hours of July and August, resulting in that production dip during those two summer months.

I also re-created the graph for wind:

Wind is much more stable over the year and there is slightly more wind in winter than in summer, therefore somewhat complementing solar (which is on average less present in winter and strongly present in summer). There are however exceptions. February is a winter month, so one would expect the production of electricity to be among the highest, yet this is not the case.

Wind is more stable because, give it enough time, surpluses and shortages will cancel each other out over the day. On the other hand, the sun will always be absent during the night and only present during the day, having its peak just after noon. That is what gives average solar electricity production that bell shape and average wind electricity production its rather stable trajectory.

Even in this highly averaged environment, it shows that solar and wind are intermittent and even general claims like there is more solar in summer months or there is more wind in winter months, don’t necessarily fly.

In this case, a lower production of solar electricity in July and August combined with low wind conditions might have been a good thing, at least from a balancing perspective (because of the traditionally low demand in this period). That is however not a given and the opposite (high demand combined with low production) could also be true.

Post Scriptum
The same exercise for solar and wind combined:

via Trust, yet verify

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September 29, 2024 at 03:50PM