The ability of a wind turbine to generate electricity relies fundamentally on its blades "capturing" the wind. However, wind conditions are inconsistent-varying in strength and availability-so blade angles must be adjusted in real-time to maximize power generation efficiency. This adjustment is performed by the pitch control system.
Where does the power for the pitch system come from? Hydraulic fluid. This fluid must be transported from the stationary nacelle to the rotating hub without leaking or interrupting the flow, and the component handling this transfer must withstand tens of thousands of rotations without wearing out. This is the role of the rotary union.
Installed between the nacelle and the hub, the rotary union allows hydraulic fluid to enter one end and exit the other while enabling continuous 360-degree rotation without breaking the fluid circuit. Essentially, it acts as a "dynamic pipe" within the turbine: one end remains stationary with the nacelle while the other rotates with the hub, using seals to securely contain the fluid.
The logic behind the pitch system is straightforward: the main control system calculates the optimal blade angle based on wind speed and direction, then transmits commands through the rotary union to the hydraulic cylinders on the hub, which push the blades to the required angle. The correct angle ensures high wind energy conversion efficiency, whereas an incorrect angle leads to a drop in power output. Consequently, a malfunction in the rotary union can range from reduced efficiency to a loss of control over the blades, potentially forcing an emergency shutdown of the entire turbine.
Offshore wind power imposes even more rigorous demands on rotary unions. Offshore environments feature high salinity, high humidity, and strong corrosive elements, compounded by constant vibrations from waves and wind-conditions that standard seals cannot withstand for long. Modern hydraulic rotary unions for wind turbines often employ multi-stage sealing structures and materials such as silicon carbide sealing rings and fluororubber seals; they can handle operating pressures up to 35 MPa, withstand temperatures ranging from -20°C to 200°C, and offer a design lifespan of up to 20 years. The goal is to ensure that once installed, the unit requires minimal maintenance.
Market data indicates that global sales of hydraulic rotary unions for wind turbines reached $877 million in 2024, and the market continues to grow. These products come in single-channel, dual-channel, and multi-channel configurations; large-scale units typically utilize multi-channel versions, which allow for the simultaneous transmission of hydraulic fluid, lubricating oil, and electrical signals.
Another noteworthy trend is the shift away from traditional rotary joints-which rely on physical contact for signal transmission and are prone to wear and debris generation that can compromise signal stability-toward non-contact transmission technologies. For instance, fiber-optic rotary joints eliminate mechanical friction and debris generation, offering distinct advantages in environments like offshore sites where maintenance costs are exceptionally high.
A wind turbine is designed to operate for 20 years, and the rotary joint must last just as long. Though often overlooked, it is indispensable: without it, the blades cannot adjust their pitch, rendering the turbine unable to generate electricity regardless of wind strength.