Readers will remember the ding-dong between Gordon Hughes and DECC's Chief Scientist David Mackay over the rate of decline of wind turbine performance over the years. Hughes had written a paper that suggested that this was significant, with Mackay arguing that Hughes' approach was flawed.
At the time Mackay pointed to a forthcoming paper by Staffell and Green that he said would support his case. This has now appeared in the journal Renewable Energy and it indeed supports Mackay's case.
The Renewable Energy Foundation, who published the original report, have issued a statement that suggests the story is far from over:
This study uses different methods, and though it confirms the finding that performance declines significantly over time, it reports a lower rate, of about 1.6% per year, with “average load factors declining from 28.5% when new to 21% at age 19.” Staffell & Green note that this has the effect of increasing the levelised cost of electricity from wind by 9%.
REF believes that the debate has moved on from whether wind farm performance declines with age, as that is clearly confirmed, to the technical issues involved in measuring the rate and profile of the decline. The Staffell and Green paper proposes an approach that compares actual output with ideal output based on interpolated wind data. The potential difficulties of this specification are outlined in the Appendix to Professor Hughes’s paper and are not resolved in the new paper. Hence, we are not convinced that the Staffell and Green’s approach is superior to the statistical methods employed by Professor Hughes.
In one respect, we believe that the Staffell and Green analysis is clearly inferior to the Hughes specification. Namely, their model does not allow for the effect of technical progress in the wind industry through improvements in operating and maintenance practices, which can be treated separately from changes in turbine design. This was built into the Hughes specification and few would dispute that such technical progress has occurred in the past.
Work carried out by Professor Hughes – and made available to Dr Staffell and Professor Green – shows that the rate of such technical progress has fallen markedly since 2008. The reason this matters is that Staffell & Green and Hughes have both confirmed that specifications that allow for such technical progress yield higher estimates of the rate of decline in performance with age than those, including the new paper, which do not. In statistics this is known as omitted variable bias.
In summary, REF’s view is that while the new paper makes a useful contribution to public discussion of the performance of wind farms, Staffell and Green’s methods are not statistically superior to those used in the Hughes study. Indeed, Staffell and Green’s specification contains an assumption that results in a significant downward bias in the estimate of the rate at which performance declines with age. We will provide fuller details of these points in due course.