How Wind Turbine Pitch-Control and Sealing Systems Work

With over 350,000 wind turbines currently in use globally, wind continues to be one of the fastest growing forms of renewable energy. In 2021, 93.6 GW of new installations brought global cumulative wind power capacity to 837 GW, showing year-over-year growth of 12%. The Global Wind Report 2022 notes that 557 GW of new capacity will be added in the next five years—more than 110 GW of new installations each year until 2026.

A Q&A with Eric Bucci, global technical manager for Energy with Trelleborg Sealing Solutions.

Question: We know that wind turbines are a source of renewable energy generation, just how much energy is produced from a single turbine?

Bucci: Today’s wind turbines are highly technical precision machines, utilizing the latest digital and computer technology to maximize the wattage from every wind gust. An average size 2-MW wind turbine can generate enough electricity to supply about 1,000 households. Running 24/7 with an operational expectancy of 98% over 20 years, wind turbines function in extremely dynamic environments where reliability is paramount.

Q: Will wind turbines continue to complement the global zero emissions goal in years to come?

Bucci: Wind turbines are key in working toward the International Energy Agency’s goal to reach net-zero emissions by the year 2050. This means all electricity produced will have zero carbon emissions and will help limit the global temperature rise to 1.5C. And as wind turbines have increased in size so has their output capability with pitch-controlled systems becoming the standard choice for efficiency gains.

Q: Can you tell us more about pitch-controlled systems?

Bucci: With shaped blades that adjust and rotate to maximize productivity, active pitch control enables configuration of a wind turbine to operate efficiently at any wind speed. By actively managing the blade angle, operators can optimize the rotational energy from the wind, no matter the wind speed.

Housed within the wind turbine pitch actuator, a hydraulic pitch cylinder controls movement, slightly rotating the turbine’s rotor blades and adjusting the pitch for changing wind speeds. If wind speeds are slow, the pitch cylinder adjusts the blade angle to increase the blade surface area to capture the wind energy. As wind speeds increase, blades are angled to reduce the amount of surface area in contact with the wind, preventing excessive rotary speeds to protect system integrity. Drag at higher wind speeds also adversely affects system efficiency and changing the blade pitch can dramatically improve performance.

Q: What kind of sealing configurations are required for the pitch cylinder?

Bucci: The sealing system must be capable of operating at pressures of 3,625 psi (250 bar) with constant linear pressure on the rod and piston seals, and differential side loads that control positioning. While holding a turbine blade at an angle, on extension and retraction of the pitch cylinder rod, the sealing system must not exhibit any drift or slippage. This can result in the blade pitch deteriorating and drastically reducing efficiency. Seals must also easily release their holding position with minimum vibration and friction, or stick slip, and move in a smooth linear fashion. This requires low-friction sealing materials.

In addition, seals must demonstrate minimal wear characteristics and facilitate dynamic movement that is continuous in short strokes. These, which occur on average 900 times per hour, can cause particularly aggressive wear as the seal cannot fully flex and relieve stress as it would in long stroke applications.

Q: What kinds of materials are used for wind turbine sealing?

Bucci: Standard hydraulic cylinder sealing materials may be insufficient and low-wear materials, such as polytetrafluoroethylene (PTFE)-based compounds filled with anti-wear agents and a hard durometer, stable urethane, are needed to extend service life.

Standard polymer-based wear rings may also be incapable of reaching the desired extended lifecycles. As an alternative, wear rings made of composite materials are engineered to withstand high loads and resist wear.

Q: How do varying climates affect the sealing conditions in wind turbines?

Bucci: Seals used in wind turbines must be able to withstand cold temperatures as low as –22F (–30C) as a standard, and even lower, down to –40F (–40C) in very cold climates. When temperatures drop below these levels, the oil inside the cylinder can no longer work properly and requires warming with heating elements.

Maximum temperature resistance for seals in wind turbine hydraulic cylinders is 140F (60C). Above this, cooling systems are necessary so the oil does not become stressed, decreasing viscosity and carbonizing. In addition, actuator seals must withstand demanding conditions including high humidity, dirt particles, salt spray, and the rigors of wind and rain.

Q: How can the performance of the pitch cylinder be optimized?

Bucci: The seals within the hydraulic systems are integral to the overall performance of the turbine and optimizing their life is critical to the long-term effectiveness of the total system. An example of a sealing arrangement for a wind turbine is pairing Trelleborg’s Turcon Stepseal V LM, a single-acting O-ring energized seal made from PTFE-based materials engineered for dynamic applications, with Trelleborg’s composite Orkot Slydring, which prevents metal-to-metal contact between the static and dynamic metallic components. Specially engineered configurations enhance lubrication, optimize friction characteristics, and maximize service life, while preventing any external oil leakage.

Q: How else might operators extend the life of the seals used in wind turbines?

Bucci: To further extend seal life and performance, operators should incorporate an advanced corrosion-resistant coating to all dynamic seal running surfaces. Anti-corrosion coatings reduce the possibility of rust and pitting associated with ferrous metal surfaces. Polished to a seal supplier’s surface finish recommendation, these will provide smooth hardware running surfaces to lower friction, reduce wear, and extend overall system reliability.

—This article was contributed to POWER by Trelleborg Sealing Solutions Americas.