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1. If the entire UK national grid annual demand (which is very well defined [[Control of the National Grid (UK)]] and therefore a good example, (but could be any other large grid where the facts are known) were supplied by wind power - typically 140 GW, 35,000 turbines, would be needed in the UK. Then self evidently, when the wind is not blowing, all the existing power stations can be started up in sequence to fill in the gaps. The cost of providing this cover is very roughly the [[Spark spread]] as loss of revenue and is not overwhelmingly significant - about £7.MWh compared to supply prices presently of £70/MWh.
2. It is known that the UK Grid can easily cope with 3 GW swings in a few minutes, (last solar eclipse http://news.bbc.co.uk/1/low/sci/tech/specials/total_eclipse/417650.stm ) and self evidently, 35,000 turbines spread around the coasts of the UK cannot all lose or gain 3 GW in 3 minutes, or the 1800 MW surge in electricity when the half time whistle blew during this evening's World Cup matches. from a change in wind speeds over the entire UK - winds simply do not change their average speed at anything like that rate. Graham Sinden’s paper (http://www.eci.ox.ac.uk/publications/downloads/sinden05-dtiwindreport.pdf ) page 8 shows that the worst rate of change due to wind variation is likely about 20% and is likely to occur about once per year.
So for 60 GW of capacity that is a 12 GW change in 1 hour, or a paltry 0.2GW per minute, far less than the 3 GW change in minutes due to the last eclipse..
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