Performance decline in newer US wind farms is less acute compared with older plants, a new study by Berkeley Lab has found.
Researchers measured output from a typical wind plant declines by about 13% over 17 years. Wind farms under 10 years of age showed smaller performance decline overall, at -0.17 % per year.
The researchers based their study on data from 917 wind farms in the US and said it is the “first comprehensive evaluation of its kind for the United States”.
The study stated: “This performance decline rate is an important input into estimates of a plant’s lifetime generation output and estimates of a plant’s financial viability.
“This rate is also important in research contexts, for example, as an input into long-term energy sector models and as input to estimates of the levelised cost of wind energy. Despite its importance, little information is publicly available as to how US wind plant performance changes with age.
“Due in part to the lack of information, the performance decline rate is often not accounted for by investors, energy modelers, and policy makers.”
The study said a “major influence” on the performance of older plants, built before 2008, is the federal production tax credit, which provides incentives for energy output during the first 10 years of plant life.
Plant performance declined abruptly after the 10-year tax credit ran out. The authors said this suggests that the tax credit gives plant operators strong incentives to maintain turbines.
This drop in performance after 10 years was not found in prior studies focused on European wind fleets, in which performance decline was constant over time.
The study also found that turbines sited on flatter terrain, with lower specific power and direct drives, instead of gear boxes, have lower rates of performance decline.
Flatter terrain may be associated with less wind turbulence, and thus less stress on the turbines, stated the study.
Direct drive turbines are not common in the US projects, with less than a dozen in the projects sample.
Factors that had no discernible effect on performance decline included overall project capacity, amount of nearby capacity at other wind plants, size of plant owner, and turbine make.
The researchers said performance of wind plants may decline over time due to various reasons, such as increasing downtime for maintenance, erosion of blade edges, or increased friction within rotating components.
They also said performance can be improved if software, or hardware, updates are installed. However, in most cases plant performance tends to decline slightly with age.
The findings have been published in the peer-reviewed journal Joule.