Solano Wind Farm

The Sacramento Municipal Utility District's Solano Wind Farm. 

Wind plant performance across the United States strongly correlates with a turbine's age relative to the facility's eligibility for production tax credits, according to Lawrence Berkeley National Laboratory researchers.

The findings are from what the researchers call "the first comprehensive study of how U.S. wind plant performance changes with age."

Based on a survey of 917 plants in the continental U.S., the researchers separated the plants into "new" and "old" using the year 2008 as the dividing line. Facilities that came on line in or after 2008 are defined as "new" by the researchers. The plants also had to have at least five years' worth of data available.

The researchers found that plant performance "declines smoothly with age, until a stepwise drop in performance occurs when plants age out of eligibility for the production tax credit."

Since this pattern is not similar to those in any other countries, the researchers conclude that policy mechanisms, of which the tax credit is a significant one, have a direct effect on wind plant performance decline. Performance decline "is not an immutable function of physical degradation of the wind turbines," they wrote, adding that the decline rate is lower compared with those observed in other countries.

Dev Millstein, one of the researchers, said in a May 20 webinar that the expected generation profiles over the lifetime of a plant can be useful for other important analyses, such as calculating the levelized cost of energy and input into other long-term electric system models. 

There has been previous research into wind turbine component reliability, Millstein said, but this does not provide any insight into plant or fleet decline. Performance is typically not accounted for and should be, he said, adding that the concepts can be applied to other power generation technologies.

The researchers looked at performance changes and plant characteristics to better understand why some wind plants have more and others less variation in performance decline with age. Some adjustments were made for curtailment, for example. The researchers looked at independent system operator-reported curtailments and distributed those across plants in a region based on local nodal pricing, since no specific curtailment data is available, and whether the plant was receiving production tax credits.

What they observed, Millstein said, is that the first year a wind plant is on line, it has lower performance, but tends to reach its full production level in the second year of operation. Among newer plants, throughout the first 10 years of operation, performance degrades at a rate of roughly 0.17 percent per year; among older plants, that rate is roughly 0.53 percent per year.

Then, after 10 years—"at the close of the production tax credit window"—the performance rate drops suddenly, by 3.6 percent. The rate for the next four years is a 1.23-percent drop, after which there is a small rebound followed by a steeper decline. The rebound is thought to be the result of component replacement. The older plants, after roughly 17 years on line, still operate at about 87 percent of their Year 2 performance rate, Millstein said.

This suggests that once a facility is no longer eligible for a production tax credit, maintenance and operations priorities change. Specifically, the "loss of credits reduces profit incentives for aggressive monitoring and maintenance," Millstein said. 

Two other factors considered for performance in newer plants were component reliability and newer technologies, including sensors and controls, as well as changes in turbine design that allow equipment to operate at rated power more often.

A total of 13 different characteristics with the potential to affect performance were examined. These were what the researchers refer to as "a priori hypotheses"—project nameplate capacity, turbine specific power, terrain roughness, average wind speed and others. They found statistically significant correlations for only three of these characteristics or factors: turbine specific power, average wind speed and terrain roughness. Turbines with lower specific power and direct drive technologies experienced less degradation, the researchers found, while facilities located in rougher terrain saw greater degradation, thought to be the result of increased turbulence affecting performance.

As to whether repowering is a factor in performance degradation, Millstein said the researchers do not have "too much data from the field on this." Repowering is an increasingly popular option for California wind facility owners (see related story).

Researcher Mark Bolinger said tax credit availability is the primary driver for those types of decisions. "If they have the opportunity to capture another 10 years [of tax credits], it might make economic sense to repower that project," he said.

Other potential factors attendees asked about that might be associated with operations and maintenance budget allocations that could translate into performance issues were not statistically significant, according to the researchers. These included whether the facility had a power-purchase agreement or whether the equipment was under warranty.

Having this sort of information will be helpful as wind generation becomes an increasingly important power source throughout the U.S. Wind generation supplied 7.3 percent of total power generation in the U.S. in 2019, the researchers said in their paper. That same year, 9,143 MW of new wind capacity came on line.

Wind power has been on the rise since 2008. U.S. Energy Information Administration data shows wind generation at utility-scale facilities increased from 55,363 thousand MWh in 2018 to 272,667 thousand MWh in 2018. In California, there is roughly 5,535 MW of capacity operating, according to the latest data from the California Wind Energy Association. Wind was responsible for supplying 7.2 percent of all power generated within the state and contributed to 29 percent of the renewables portfolio standard as of 2018.

California has the nation's oldest operable turbine—one that has been in service for 44 years, according to a 2019 report by the Texas Renewable Energy Industries Alliance; however, Tennessee has, on average, the oldest operating turbines in the nation, with an average project age of 17 years. The average project age in California is 16 years. The other states in the top five are Wyoming, Minnesota and Arkansas, the latter of which has facilities with an average age of 12 years.

California does have a higher number of turbines relative to wind capacity because it was an early adopter of the technology. It also has the second-largest number of installed wind turbines, but its 6.1 GW of capacity ranks it fourth among states, according to data TREIA derived from the EIA's Electric Power Annual 2018, released in October 2019. The next report is due for release in November.

Time, plus a wider variety of key data would help provide clear answers to future research questions, such as whether the newer wind projects will maintain reduced levels of performance decline into their second decade of life, Millstein said. Improved curtailment and wind speed information would help resolve questions and also aid researchers in further diagnosing the driving factors of performance decline, such as terrain roughness and interplant wake effects. There is also a lack of data on precipitation and icing—conditions that could also affect performance over time. The researchers also want to compare and harmonize their datasets against those of the European Union fleets.

The LBNL team's research has been published in the journal Joule.