Summary for policymakers
A number of EU Member States are committed to increasing the generation of electricity from renewable resources as part of their bid to cut back on emissions of carbon dioxide. In the UK, this means a focus primarily on wind, both on- and off-shore. Arguments continue to go back and forth on the desirability and effectiveness of this policy, but the governments in both Westminster and Holyrood remain firmly committed at present.
Wind is, by its nature, intermittent and so the extent to which this affects the output of the fleet of wind turbines in a typical year is crucial in determining how much conventional generating capacity is needed by way of backup and thus what the overall system costs are. This study provides a rigorous quantitative assessment of wind variability and intermittency based on nine years of hourly measurements of wind speed on 22 sites across the country. The analysis is based on a model UK wind fleet of 10 GW nominal capacity.
The model reveals that power output has the following pattern over a year:
i - Power exceeds 90 % of available power for only 17 hours
ii - Power exceeds 80 % of available power for 163 hours
iii -Power is below 20 % of available power for 3,448 hours (20 weeks)
iv - Power is below 10 % of available power for 1,519 hours (9 weeks)
Although it is claimed that the wind is always blowing somewhere in the UK, the model reveals this ‘guaranteed’ output is only sufficient to generate something under 2 % of nominal output. The most common power output of this 10 GW model wind fleet is approximately 800 MW. The probability that the wind fleet will produce full output is vanishingly small.
Long gaps in significant wind production occur in all seasons. Each winter of the study shows prolonged spells of low wind generation which will have to be covered by either significant energy storage (equivalent to building at least 15 plants of the size of Dinorwig) or maintaining fossil plant as reserve.
The preceding deficiencies suggest the model wind fleet would require an equal sized fossil fuel generation fleet operating alongside it, especially during winter months.
The study was extended with another 21 sites located in Ireland and across the northern plain of Europe. Performance of the wind fleet in Ireland is slightly better than in the UK, but the northern European fleet (Belgium, Netherlands, Denmark and Germany) is much poorer. Integrating all these with Ireland and European interconnectors will do little to reduce the intermittency levels described above.
The short-term (30 – 90 minutes) variability of wind generation is also studied and reveals swings in output far higher than would be expected from conventional generation. Swings of 10 % of output are normal. This observation contradicts the claim that a widespread wind fleet installation will smooth variability.
Electricity grid management entails balancing generation against demand even within timescales as short as 10 S. The UK has an island grid, with few interconnectors to other European grids and none of these interconnectors are AC links capable of providing grid stabilization and inertia. It was for this reason that the CEGB designed and built (capital cost over £1 B) the Dinorwig pumped storage power station. But the model wind fleet reveals wind energy production is unlike that of all conventional fossil fuelled or pumped storage plants; it does not follow grid demand on diurnal or even seasonal time patterns. Wind generation will therefore make heavy claims on the UK’s response and reserve market. This study has shown that at certain times half of Dinorwig’s units would be needed to mitigate the variability of a 10 GW wind fleet. The entire UK pumped storage capability cannot compensate for the wind power fleet’s intermittency.