It is often said that larger wind turbines will perform better than smaller ones, thus bringing electricity at lower cost. It is undoubtedly true that larger turbines produce a higher percentage of their nameplate capacity (the capacity factor), because wind speeds are higher further from the ground. However, the effect is relatively small, as the figure below, taken from my 2021 paper on the cost of offshore wind suggests. The yellow dots are the actual figures for UK windfarms in their first few years of operation, so larger turbines appear to be delivering better capacity factors than smaller ones, although in fact some of the improvement will be due to the fact that larger wind turbines are being installed further from shore as well. The other data points are capacity factors predicted by various other commentators, in each case with a high figure, representing the early years’ performance, and a lower one, representing the lifetime average. (The ludicrous nature of BEIS’s figures is obvious, but that’s another story).
That distinction is important. Windfarms deteriorate over their lifetimes, and capacity factors fall.
It’s therefore interesting to look at the analysis below. This divides the offshore fleet into cohorts by turbine size and then looks at how the capacity factor for each cohort changes over time. The pattern is striking. Small turbines – the longer traces – start low, but deteriorate slowly, if at all. Then, for each step up of turbine size, you get a higher starting point, but a faster rate of deterioration. (Ignore the drops at the end of the grey and blue series, which are anomalies of one kind or another).
I get a similar pattern for onshore wind turbines.
This is important in light of recent news that the Scottish Government intends to allow much larger wind turbines to be deployed onshore. There may in fact be very little benefit.