Output from a typical US wind farm shrinks by about 13% over 17 years, with most of this decline taking place after the project turns ten years old, the Lawrence Berkeley National Laboratory (LBNL) found.
On average, output decreased by only 0.17% per year in the first ten years of operation, the researchers found.
Increased downtime for maintenance, the erosion of blade edges, and increased friction within rotating components all contributed to declines in output, they explained.
The research suggested project operators are incentivised to maintain turbines during the first decade by the tax credit support system. The fall in performance is noticed more acutely after a project is no longer receiving the subsidy.
As the production tax credit (PTC) is paid in line with a turbine’s output, operators maximise the benefit of the support before it is phased out by keeping their turbines in better condition to maintain output levels, the researchers explained.
The acceleration of declining performance after ten years — observed in the data from 917 wind farms across the United States — was not found in prior studies focusing on European wind fleets, in which output declined consistently over time, they added.
This rate of decline is apparently unique to US sites further supporting the hypothesis.
Elsewhere, the researchers found a variety of project specifics afford gentler rates of decline.
They suggested a flatter terrain around projects means turbines encounter less wind turbulence and so reducing stresses put on them.
However, data on turbulence is not systematically available, the researchers noted.
Meanwhile, turbines with lower specific power ratings — which have longer blades relative to their generator size and are increasingly common — also fared better.
The researchers said this might be because these turbines are capable of harvesting a greater portion of the available wind energy, which partially offsets the decreasing aerodynamic efficiency experienced by all turbines.
Direct-drive turbines were also found to perform better than geared turbines, as gearboxes may be more subject to mechanical failure, leading to higher levels of output degradation. Although the data set for this was small.
The Berkeley Lab’s study was based on data from 917 wind farms across the US and was included in the peer-reviewed journal Joule.