Currently, there are over 65,000 active wind turbines in the United States [1]. With a capacity of 125 GW, wind power is now the third largest source of electricity in the country (8.7%), producing enough to power 39 million Americans’ homes. The Wind turbine technician is the second fastest growing job in the country with the wind industry in general employing 120,000 Americans; In 2020 alone, $25 billion was invested into new wind turbine projects.
How Long Do Wind Turbines Last?
There is very little data on modern turbines reaching their life expectancy so it is largely unknown how long they will be operable. Modern wind turbines have over 8,000 parts (broken down into three major components) and blades as long as 262 feet, the same length as the wingspan of an Airbus [2]. With higher efficiency modern turbines due to additional electronic components and a more powerful and massive design, there is a higher chance of something going wrong with more potential points of failure and overall added stress and load on the structure.
"We don't know with certainty the life spans of current turbines," said Lisa Linowes, executive director of WindAction Group, a nonprofit [3]. With most wind turbines being installed in the last decade, it is largely unknown if they will make it to the designed 20-25 year life.
At 10 years of life, blades and gearboxes are needing to be replaced already so it is unlikely they will make it another 10 years. The cost to teardown a single turbine is $200,000, not including any payback from selling or recycling valuable materials, which is heavily labor intensive and not always cost effective. Instead of decommissioning, more often the site will be ‘repowered’ which means replacing the turbines with newer technology.
The world’s oldest turbine is Tvindkraft in Denmark, which has been in operation since 1978 for a total of 43 years [5]. What makes this turbine special is that it was designed for 2 MW but has mostly operated at half the capacity, 1 MW, allowing it to have a much longer life. Another interesting fact is that the Tvindkraft was organized and built by teachers and over 400 volunteers to prove that wind was a viable alternative to nuclear and oil.
What Factors Determine a Wind Turbine’s Life?
Modern wind turbines are designed to last 20 years and with proper monitoring and preventative maintenance two to three times per year (increasing with frequency as the turbine ages) their lifetime can be extended to 25 years [6].
Wind turbine’s lifespan is determined by the amount of load and stress the structure is put under by the wind, especially since the structure is only fixed at one end. As the wind speed increases, so does the force the turbine is put under. Sometimes wind speeds can reach 100 times the design load so that is why many turbines have a shutdown feature during high wind speeds.
The main factor that determines the life of the wind turbine is the environmental conditions such as average wind speeds, turbulence intensities, and for offshore facilities the cyclical load of the waves against the foundations, jacket structures, and monopiles as well as the inevitable corrosion and erosion. Other components are a concern for fatigue failure such as the wind turbine’s blades and hydraulic systems. The blades are especially prone to damage from lightning strikes, birds, high impact wind, and generally higher levels of loading and fatigue.
Top Causes for Wind Turbine Failure
Why do most wind turbines fail? Externally, the reasons may be birds, lightning, rainfall, blade detachment, delamination, blade cracks [7]. Internally, electrical and mechanical failures are to blame such as a short circuit or if the gearbox stops working.
Electrical failures are the most common reason and very expensive to fix. These failures are mostly due to high humidity levels over 60% in the nacelle and tower where the electrical components are located. The cabinets are not completely airtight, so the outside air puts the electrical system at risk.
Likewise, the moisture in the air will weaken metal components such as the gearbox, bearings, and yaw gear. Better humidity control and filtering any salt from the air is essential to the turbine’s life.
About half of all wind turbines fail due to electrical components and the control system; however, these failures have a short downtime [9]. Generator and gear boxes fail less often but have a longer downtime. 25% of wind turbine failures caused 95% of downtime. On average wind turbines fail at least once a year and have a reliability of 98%. Wind turbine blades failing are still rare with about 0.54% (or 3,800) of all blades in the United States failing every year [10].
The top three types of wind turbine failure are due to the blades, generator, and gearbox.
- Larger blades produce more power yet also put additional strain on the structure and components [11]. Common causes for the blade to fail include de-bonding, joint failure, erosion, splitting along fibers, gel coat cracks, power regulator failure as well as damage from foreign objects.
- Generators fail due to wind loading, extreme weather, and thermal cycling.
- Mechanical or electrical failure in the bearings, vibration, voltage irregularities, and cooling system failures can lead to excessive heat or fire.
- Gearboxes are designed for harsh conditions yet 25% need replacing within ten years [12]. Every year in the USA about 1,200 gear boxes fail, 96% of which are due to bearings and gears.
Other reasons for failure include dirty or water contaminated lubrication, improper bearing settings, significant temperature fluctuations, improper or infrequent maintenance, and transient loads leading to sudden accelerations and load zone reversals. Gearbox repairs are expensive as this component is 13% of the total cost of a wind turbine and may take a few months of downtime for the parts to become available [11].
Decreased Performance With Age
The average age of wind turbines in the United States in 2020 is 7 years with an estimated 11 years by 2025 [12]. Performance of wind turbines greatly decreases with age, about 16% per decade or 1.6% each year [13].
This degradation reduces the wind farm’s output by 12% over a 20-year life and increases the cost of levelized electricity by 9% [8]. When the cost of degradation is too much, it makes sense to replace the turbines with newer models. National fleets have a downtime of 4-7% which results in 11% reduction in energy output as they typically fail in windier conditions.
As wind turbines age, wind farm operators must make a decision: whether to continue operation, repower (use same site and infrastructure with larger wind turbines), or decommission the site completely. Environmental factors and fatigue along with regular maintenance largely determine whether the turbine can continue past its life expectancy. In many instances the wind conditions are less than designed loads so the turbine can continue operation.
Costly Operation and Maintenance
The operation and maintenance (O&M) costs vary depending on the life of the turbine but typically account for about 20-25% of the total levelized cost of electricity production during the turbine’s lifetime [6]. When the turbine is new these costs may only be 10-15% but can increase to up to 35% towards the end of the turbine’s life. Cumulative O&M costs are 65%-95% of the turbine’s investment cost over a 20-year life with unscheduled maintenance resulting in 30-60% of the total O&M cost [9].
O&M costs are expected to rise from $45,000/MW per year for turbines less than 10 years to $60,000/MW per year for those 10-15 years [12]. O&M costs consist of the following: insurance, regular maintenance, repair, replacement parts, and administration. Very few modern turbines have reached their life expectancy, so the data is largely unknown of their end-of-life O&M costs [6].
As wind turbine manufacturing has improved the warranties on parts on the other hand have decreased dramatically from 10 to 5 years [3] as the manufacturers found extended warranties to be an additional revenue stream. This in turn further drives up O&M costs in comparison to wind projects in the past decade or so.
Lifetime Status Report
A lifetime status report will determine if continued operation is possible past the typical 20-year design life. This assessment includes on-site inspections, evaluation of design load data, and theoretical and practical analysis.
The status report will list the maintenance requirements and associated costs of replacement and continual operation to compare against the risk of failure or decommissioning [6]. This report is typically used for applying for insurance policy extensions.
Analytical assessments are conducted to ensure safe operation is maintained. Safety devices, braking systems, and turbine control systems require testing to verify the structural stability as well as comparing the design conditions against the actual loads the turbine has been exposed to using computer simulations of average wind speeds, turbulence, and extreme weather events.
On-site inspections look for signs of corrosion, cracking, fatigue, and weathering on components and equipment such as the turbine blades, cables, supporting structure, and foundation. An audible listen check is also performed searching for any unusual noises coming from the gear and bearing assemblies. If significant damage is found, immediate shutdown is required which leads to costly downtimes until it can be repaired.
Repowering Wind Turbines
By repowering projects, output can be increased by 10% or more allowing for additional electricity generation without applying for new permits or interconnection fees [14]. In 2019, about 2 GW (one fifth of all wind projects built in the United States) were repowering projects.
Lawrence Berkeley National Laboratory did a study and estimated that turbines reengineered in 2019 have an average additional lifespan of 11 years. Turbine owners can receive a federal tax credit as long as they spend enough on new equipment to qualify as a new project. Repowering also allows for increased digitalization and improved monitoring of wind speed and direction and, hence, increased control and efficiency of the turbine.
Turbine technology has greatly advanced over the past few decades with a wind turbines capacity being 22% in the late 1990’s versus 35% today [15]. Also, the cost of wind has dramatically decreased from $0.55/kWh in 1980 to $0.03/kWh today.
Many power purchase agreements (PPAs) last 15-20 years, so a new agreement may need to be negotiated to extend the project. Most wind farms have contract renewal clauses with the landowner up to 60 years, allowing for a timeline of repowering wind power plants [3].
Decommissioning and Recycling
About 85% of all turbine components can be recycled, such as steel, copper wiring, electronics and gearing [2]. The blades, however, are made from fiberglass and more difficult to recycle. They require heavy machinery to be cut and, often times, the blades are taken to landfills or stored elsewhere.
Some civil engineering projects are repurposing the blades for towers, pedestrian walkways, or roofs for affordable housing [2]. In the future, blades may be made from thermoplastic resin, which is easier and cheaper to recycle.
