Ever since Gordon Hughes' report noted that wind power was more likely to produce more carbon dioxide emissions than gas, I have been looking for the figures behind the claim. In the comments, someone has now posted some details that seem to meet the bill. Although these are not Hughes' own numbers -they were submitted in evidence to Parliament by an engineer - I assume they are similar.
[A]s wind rarely produces more than 25% of its faceplate capacity it needs 75% backup - which due to the necessity of fast response times needs OCGT generation (CCGT can respond quickly but the heat-exchanger systems upon which their increased efficiency relies, cannot - so CCGT behaves like OCGT under these circumstances). CCGT produces 0.4 tonnes of CO2 per MWh, OCGT produces 0.6 tonnes. Thus 0.6 tonnes x 75% = 0.45 tonnes. Conclusion: Wind + OCGT backup produces more 0.05 tonnes of CO2 per MWh than continuous CCGT.
Now, where does the alternative view - the one proclaimed by Grantham Institute man Robert Gross - come from?
Tim Worstall has picked up on this post. There is an interesting exchange in the comments.