Recently, economist Colm McCarthy noted  that:
Wind generators can be relied on to produce power only about one hour in three over a year, and those productive hours are unpredictable. So conventional capacity has to be kept in reserve for the periods when the wind does not blow. These stations will be utilised less than optimally and this is a hidden cost of wind generation.
In addition to inefficient use of capital, critics have argued that wind generation has a potential cost in terms of CO₂ emissions. When the wind is blowing, priority is given to wind generation over conventional capacity. However an idling thermal plant is like a car crawling along in traffic – not doing very much but still burning fuel. This may cause thermal plant to burn more fuel per unit energy generated than would otherwise be the case.
Is there any direct evidence of reduced CO₂ savings when wind generation is high? Surprisingly, the answer to this question is yes.
The scatterplot below shows the relationship between total instantaneous CO₂ emissions and instantaneous wind generation using data from the Irish grid operator Eirgrid. The data cover the period from 1-Nov-2010 to 30-Aug-2011 at 15 minute intervals (~29,000 data points). The blue line is a local regression (loess) fit. The loess fit has span parameter 1.0. A non-parametric regression curve is shown in grey.
As expected, wind generation does reduce CO₂ emissions. A linear regression fit suggests an emissions saving ~−0.38tCO₂/MWh. However, the real world relationship between wind generation and emissions is clearly non-linear. At wind generation ~600MW, fuel savings begin to slow. Above ~800MW, they cease altogether. Above 1000MW, emissions increase again.
Heat rate curve
Carbon intensity is CO₂ emitted per unit energy generated. To see why emissions savings decrease as wind generation increases, we need to look at the carbon intensity of thermal generation. Thermal generation is extracted from the Eirgrid data as the difference between demand (MW) and wind generation (MW) (assumes no power is dumped). The graph below shows the carbon intensity of thermal generation (tCO₂/MWh) versus thermal generation (MW) for the period Nov 2010 to Aug 2011.
There is an optimum point on the curve around 3000MW. 3000MW is close to the average electricity demand. In the absence of wind generation, thermal generation fluctuates in line with demand around the the optimum point, a design feature which ensures maximum efficiency. Unfortunately, high wind generation forces thermal plant to operate far to the left of the optimal point on the “heat rate curve”.
Another plot …
Wind penetration is defined as instantaneous wind generation as a % of instantaneous demand.
This graph of thermal carbon intensity (tCO₂/MWh) vs wind penetration (%) tells the same story.