By Dr David McHale

David McHale moved to Galloway to enjoy the beautiful, peaceful countryside.  He is a scientist and statistician, and supports the aims and efforts of TW312. David has been looking at a report recently published by the John Muir Trust and a subsequent response by Scottish Renewables. He has published a summary of his analysis below:-


In  April of this year the John Muir Trust released a report prepared by Stuart Young Consulting which analysed wind power data .


The statistics quoted are derived from data publicly available through the Elexon exchange site   That site, run on behalf of the National Grid,  records in five minute intervals the electricity generated by each different form of fuel type:- Coal, Nuclear, Hydro,  Pumped Storage, Wind etc.  for generators connected to the National Grid Transmission Network.  The outputs are available here:-


Scottish Renewables released a statement which criticised the report and its author.


They quoted a short report on average capacity factors based on the same data, compiled on their behalf, which gave slightly different results from the John Muir Trust Report.   I downloaded and analysed the data in order to understand whether Scottish Renewables’  comments on the report were fair.  More importantly I also wanted to understand the distribution of wind power and to highlight the limitations of using average capacity factors in discussions about wind power energy.


The data show that wind does not blow somewhere all the time, and that for a significant amount of time wind output is low.   


An analysis of the data for the generation of electricity from wind shows:-


The average output from Wind over the two year period 2009-2010 was 24.1% of Total Available Metered Capacity.  In 2009 this capacity factor was 27.2% and in 2010 it was 21.1%.


In 2010, output was less than 10% of Total Available Metered Capacity for nearly 40% of the time( 38.6% to be exact)  
In 2009, output was less than 10% of Total Available Metered Capacity for about 30% of the time ( 28.9% to be exact).


This analysis is in line with the results from the John Muir Trust Report.  The report quoted by Scottish Renewables described difficulties with the data source, missing days, and some percentage output over 100%.  The data I analysed showed no such characteristics.


Operating the Grid in 2020


National Grid have consulted on “Operating the Grid in 2020”, including a range of issues raised by the use of intermittent wind power.  The response from E.On UK  is very interesting and emphasises the many difficulties faced by National Grid over the coming years.  The significant issues of forecasting wind speed ( Question 14), the impact on Short Term Operating Reserve Requirement (Question 15),  and low speed wind events ( Questions 17 and 19) are all highlighted.


The UK distribution of measured wind power is in line with data presented by E.On in their 2005 Wind Energy report,  a link to which is available here,  ,  and consequently the issues raised by E.On are relevant to the UK as the proportion of our energy derived from wind increases.  The report describes clearly the difficulties that the intermittent nature of wind power produces for secure and stable supply.  E.On have expended much effort on estimating the capacity credit for wind power.  This is a measure of how much an energy operator can rely on the wind power available in the system to be able to supply power at any given point in time, to cope with peak demand etc.   E.On estimate that the capacity credit for wind power is around 8%.   It is more or less certain that no conventional power stations would be closed due to wind power – conventional power station output, which can be turned on and off as required, is required to ensure security and stability of supply at peak periods.




The estimated capacity factors of 27% and 21% for 2009 and 2010 are below the oft quoted 30-35% level from wind farm developers.  This does raise legitimate questions over the economic effectiveness of wind power.  In my view however the more important question is linked to the wind power distribution, its intermittency, and its implications for a secure and stable supply.


Click here to view or download David's report in full.


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