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Wind capacity and generation capacity in UK

Recent statistics published by the UK's National Grid provide insights into the curtailment of wind power in the country, but also into the amount of dispatchable generation capacity relative to peak power demand. Interestingly, a large share of wind power seems to be counted as a part of this capacity.

In a recent article, I wrote about the low level of wind power curtailment in Germany and asked readers to provide me with statistics for other countries if they had any. One reader provided me with the UK's Winter Outlook published by National Grid (PDF), which contains this chart.

Wind power curtailment UK
NationalGrid UK

The curtailment levels are some 4 to 5 times greater than in Germany, an interesting finding in light of all the talk about alleged grid bottlenecks that the Germans need to fix. And as the figures show, all of the curtailment took place in Scotland,, which has a 100 percent renewable electricity target for 2020.

But some other charts caught my eye as well. Recently, I talked about how power demand fluctuates in France much more than it does in Germany, primarily because of electric heating in France. The following chart shows power demand fluctuations in the UK – seasonally, not daily.

Power curves UK
NationalGrid UK

Here, we see that monthly peak demand fluctuates from around 40 GW to above 50 GW in the UK. The chart is not directly comparable to the data for France and Germany, however, which show absolute lows in power demand and absolute highlights. The UK publication focuses on meeting peak power demand in the winter and does not seem concerned about the range from high to low.

Specifically, the figures are averages. The actual peak winter demand is estimated to be around 55 GW, with a one in 20 chance of demand reaching and surpassing 58 GW.

The Outlook speaks of a margin of eight percent "against forecast demand," quite a narrow margin compared to Germany. But then we see this chart showing "operational generation capacity" forecast for this winter.

UK generation capacity
NationalGrid UK

An eight percent margin above even the maximum estimate of 58 GW still only puts us at around 63 GW, far below the 74.7 estimated in this chart. I have spoken before about how I don't like to write about the UK in particular because I don't understand the country well enough, and I cannot resolve this difference here (if you can, use the comment boxes below).

The UK clearly has a large share of combined-cycle gas turbines (CCGT). Although they have relatively high efficiency levels compared to traditional gas turbines (called OCGT or open cycle gas turbines in the chart above), they are the big losers in Europe at the moment – from Germany to Spain and in France. They do not ramp well, and their efficiency is actually lower overall than cogeneration units fired with gas. But in terms of generation capacity, the UK has clearly locked into gas for the foreseeable future.

The most salient feature for me that requires explaining, however, is the relatively large share of wind power at 6.8 GW. The report says that the estimate for generation capacity "[includes] renewables," but clearly only biomass and wind are included; solar is not. And the report says that figure is an increase year-over-year of 0.8 GW due partly to "an increase in wind capacity."

Yet, the country only had a total installed capacity of 10.8 GW in late 2013, according to official government data published on December 19. None of this wind power is dispatchable; Germans therefore do not count wind power at all when calculating available dispatchable generation capacity. The UK apparently counts a share of wind – but how is that share calculated?

I assumed it was the capacity factor, but that would be lower than 30 percent for onshore wind in the UK and probably not higher than around 40 percent for offshore, even with the fantastic wind conditions in the UK. But 6.8 GW is around 63 percent of the 10.8 GW installed.

On page 60 (Table E6), the report says that the "assumed that generation availability" of wind ranges from 25 to 29 percent. It also says the availability of wind is calculated based on the "equivalent firm capacity" (EFC), which is defined as "how much 100% reliable generation would be required to replace the installed wind generation while maintaining the same level of system security" – that does not sound like a capacity factor at all. In fact, it sounds like something I would subtract from the chart above, not add to it; EFC seems to describe the need for backup, not the provision of capacity. But again, use the comment box below if you know how to clear up my confusion.

Otherwise, the UK turns out to have 1 GW of "non-firm emergency" generation reserve not included in the analysis. Germany has far more – 2.6 GW – and it was hardly used last winter.

Overall, it seems that the UK has sufficient generation capacity for the time being, but also that Germany is slightly better positioned in terms of wind power curtailment and generation capacity margins. (Craig Morris)