A Report by Stuart Young, supported by the John Muir Trust
Principal findings in respect of analysis of electricity generation from all the U.K. windfarms which are metered by National Grid, November 2008 to December 2010. The following five statements are common assertions made by both the wind industry and Government representatives and agencies. This Report examines those assertions.
- “Wind turbines will generate on average 30% of their rated capacity over a year.”
- “The wind is always blowing somewhere.”
- “Periods of widespread low wind are infrequent.”
- “The probability of very low wind output coinciding with peak electricity demand is slight.”
- “Pumped storage hydro can fill the generation gap during prolonged low wind periods.”
This analysis uses publicly available data for a 26 month period between November 2008 and December 2010 and the facts in respect of the above assertions are:
- Average output from wind was 27.18% of metered capacity in 2009, 21.14% in 2010, and 24.08% between November 2008 and December 2010 inclusive.
- There were 124 separate occasions from November 2008 till December 2010 when total generation from the windfarms metered by National Grid was less than 20MW. (Average capacity over the period was in excess of 1600MW).
- The average frequency and duration of a low wind event of 20MW or less between November 2008 and December 2010 was once every 6.38 days for a period of 4.93 hours.
- At each of the four highest peak demands of 2010 wind output was low being respectively 4.72%, 5.51%, 2.59% and 2.51% of capacity at peak demand.
- The entire pumped storage hydro capacity in the UK can provide up to 2788MW for only 5 hours then it drops to 1060MW, and finally runs out of water after 22 hours.
Other findings have emerged in the course of this analysis in addition to the Principal Findings which related to the testing of five common assertions. These Other Findings are listed below.
1. During the study period, wind generation was:
- below 20% of capacity more than half the time;
- below 10% of capacity over one third of the time;
- below 2.5% capacity for the equivalent of one day in twelve;
- below 1.25% capacity for the equivalent of just under one day a month.
The discovery that for one third of the time wind output was less than 10% of capacity, and often significantly less than 10%, was an unexpected result of the analysis.
2. Among the 124 days on which generation fell below 20MW were 51 days when generation was 10MW or less. In some ways this is an unimportant statistic because with 20MW or less output the contribution from wind is effectively zero, and a few MW less is neither here nor there. But the very existence of these events and their frequency – on average almost once every 15 days for a period of 4.35 hours – indicates that a major reassessment of the capacity credit of wind power is required.
3. Very low wind events are not confined to periods of high pressure in winter. They can occur at any time of the year.
4. The incidence of high wind and low demand can occur at any time of year. As connected wind capacity increases there will come a point when no more thermal plant can be constrained off to accommodate wind power. In the illustrated 30GW connected wind capacity model with “must-run” thermal generation assumed to be 10GW, this scenario occurs 78 times, or 3 times a month on average. This indicates the requirement for a major reassessment of how much wind capacity can be tolerated by the Grid.
5. The frequency of changes in output of 100MW or more over a five minute period was surprising. There is more work to be done to determine a pattern, but during March 2011, immediately prior to publication of this report, there were six instances of a five minute rise in output in excess of 100MW, the highest being 166MW, and five instances of a five minute drop in output in excess of 100MW, the highest being 148MW. This indicates the requirement for a re-assessment of the potential for increased wind capacity to simulate the instantaneous loss (or gain) of a large thermal plant.
6. The volatility of wind was underlined in the closing days of March 2011 as this Report was being finalised.
- At 3.00am on Monday 28th March, the entire output from 3226MW capacity was 9MW
- At 11.40am on Thursday 31st March, wind output was 2618MW, the highest recorded to date
- The average output from wind in March 2011 was 22.04%
- Output from wind in March 2011 was 10% of capacity or less for 30.78% of the time.
The nature of wind output has been obscured by reliance on “average output” figures. Analysis of hard data from National Grid shows that wind behaves in a quite different manner from that suggested by study of average output derived from the Renewable Obligation Certificates (ROCs) record, or from wind speed records which in themselves are averaged. It is clear from this analysis that wind cannot be relied upon to provide any significant level of generation at any defined time in the future. There is an urgent need to re-evaluate the implications of reliance on wind for any significant proportion of our energy requirement.
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