How hydraulics are used in wind turbines?

Wind turbines depend on hydraulic systems to control pitch (adjusting the angle of the blades) as well as the control of yaw (rotating the nacelle so that it faces the wind) as well as brakes. They can handle large forces in a compact size and are an ideal fit for the harsh environment of the turbine tower.

Why do wind turbines require hydraulics?

Wind turbines appear to be at a distance, sluggish-spinning blades, and a tower that is still. However, inside the nacelle and hub, the wind turbine is continuously making mechanical decisions about how much to eat into the wind, what direction to look in, or when the turbine should stop. The majority of these decisions are made via hydraulic mechanisms.

The reason why hydraulics remain in wind energy even though electromechanical alternatives have increased is due to the force density and reliability. Hydraulic actuators can generate immense force in a small, compact, lightweight package. A nacelle can be mounted anywhere from between 80 and 120 meters high; every kilogram of equipment is important. Hydraulics are also able to withstand temperatures, shock loads, and continuous duty cycles with ease—exactly what the wind turbine is exposed to over its 20-plus years of service.

Controlling pitch is the most vital hydraulic function

Controlling the pitch of the blade is the most significant application of hydraulics in the wind turbine. Each blade in the modern three-blade turbine is able to be rotated around its own axis of longitudinal rotation by changing its angle where it encounters the wind—a pitch angle.

When winds are low, blades tilt to maximize lift and to capture the most energy possible. If wind speed increases above the maximum threshold, the blades move to the side of the wind to restrict the speed of the rotor and to protect your generator against overload. When an emergency situation occurs, blades quickly pitch until they are feathered, which brings the rotor down to a complete stop within a few moments.

Hydraulic pitch systems utilize the central motor (HPU) installed within the nacelle. This distributes pressurized fluid to actuators located at the base of every blade. Double-acting cylinders pull and push the blade across its pitch range, usually from 0° to 90°. Control valves proportional to the flow regulate speed and direction, which allows precise blade placement regardless of aerodynamic load.

The most important safety feature is a fail-safe operation. When hydraulic pressure is cut off because of a pump malfunction or a ruptured hose -- the blades should remain able to feather. This is the reason why hydraulic pitch systems have accumulators, which are pre-charged vessels that hold pressurized fluid. Even if the pump is not in operation, the energy stored in the accumulator will propel all three blades into the position of feathering, which can prevent an uncontrolled rotor.

Yaw control: keep the turbine facing towards the wind

The direction of the wind changes constantly. In order for a turbine to function efficiently, its rotor needs to be directly facing the wind. This is what is handled by the yaw system that rotates the entire nacelle that sits on top of the tower.

In larger turbines the yaw drive is typically motor-driven pinion gears that are acting on the ring gear of a huge size. However, hydraulic yaw brakes are virtually universal. After the nacelle has been positioned in the yaw ring, hydraulic caliper brakes are able to attach to the yaw rings to protect it from wind-induced yaw-related moments. They are spring-applied and released hydraulically -- which means they will automatically engage in the event of pressure loss, which is a further inherent fail-safe.

Certain designs of turbines also utilize hydraulic yaw motors directly. Low-speed, high-torque hydraulic motors are well-suited to yaw drive due to the fact that the required rotation is slow, but the torque requirement is excessive, particularly in the correction to yaw with high winds. A hydraulic motor could remain in a stall for a long time without harm, something that electric motors are unable to overcome.

Rotor brake

Beyond pitch feathering, the majority of turbines come with a mechanical rotor as a second braking system and also to lock out maintenance. It is usually a hydraulic disc brake installed in the primary shaft or high-speed gearbox shaft.

The spring-applied, hydraulically released disc brakes are the norm. The hydraulic pressure keeps the brake open throughout normal operation. The release of pressure permits spring force to force the brake pads to be firmly held. This means that the brake is automatically applied in a power loss situation—exactly the same way as using the brake yaw.

They aren't typically employed for dynamic stopping when the load is full (the pitch system can handle this) However, they keep the rotor steady for maintenance, preventing the creep of the rotor in low winds, and act as an emergency backup in case the pitch control is not working.

Hydraulic power unit (HPU)

The above tasks are usually performed by the centralized HPU situated in the nacelle. A typical HPU for wind turbines comprises the following:

• A variable- or fixed-displacement hydraulic pump powered by an electric motor.
• An aquifer (typically 500-200 litres, for turbines that are utility-scale)
• Pressure relief valves for the System
• Proportional control valves and directional valves for every function
• Accumulators to be used in emergency pitch operations
• Filtration -- usually an amalgamation of pressure-line and return-line filters
• Controlling temperature through heaters or oil coolers (critical for colder climates, where the viscosity of fluid is required to be controlled)

The HPU operates at the speed of need. If no pitch adjustment or yaw braking adjustment is required The system will idle at a low pressure. This decreases the amount of heat generated and increases the duration of component life.

Selection of the fluid in the wind turbine hydraulics

The choice of fluid in wind turbines is more limited than the majority of industries that use hydraulics in their applications. There are a variety of reasons for this:

Fire risk. A leaky hydraulic system inside an enclosed nacelle coupled with electrical equipment and a warm working environment can create a serious risk of fire. Numerous turbine OEMs have mandated the use of fire-resistant hydraulic fluids (water-glycol chemicals or synthetic esters) in systems that are mounted on nacelles.

Biodegradability. Offshore and sensitive onshore installations often require biodegradable liquids. Synthetic ester fluids satisfy the requirements for biodegradability and fire resistance, and that's why they are the most common offshore wind hydraulic specifications.

Low-temperature performance. When it is cold, the fluid should flow efficiently at startup temperatures that can be as low as 20 degrees Celsius or less. Fluids with a high viscosity index or specifically blended cold-climate hydraulic oil are referred to as such.

Seal compatibility. The choice of fluids must be made to seal materials across the system. Moving fluid types within an existing turbine—like changing between mineral oil and ester—requires a full system flush and inspection of the seal.

Considerations for maintenance

Maintenance of the hydraulics in the wind turbine is made more difficult due to access. Technicians need to climb or be lifted up to the nacelle—usually 100 meters or more—making regular fluid testing and filter replacement more significant than those in a plant that is located at ground level.

Maintenance based on condition is the most preferred method. The oil sampling process at specific intervals examines particle counts against ISO cleanliness standards (typically ISO 16/14/11 or better for valves operated by pilots) as well as water content and indicators for degradation of fluid. Differential pressure indicators for filters flag elements that are near bypass, prompting targeted replacements instead of fixed-interval adjustments.

Accumulators require periodic pre-charge pressure checks. A nitrogen charge that is depleted in a pitch accumulator can be an important safety indicator that means that the ability to pitch in emergency situations is diminished or is not present.

The condition of the hose is an additional important area of focus. Flexible hoses in the hub are subjected to rotational movements with every blade pitch cycle, which is millions of cycles throughout the course of the turbine's lifespan. They are usually on a regular replacement schedule irrespective of the apparent conditions.

Hydraulics and electromechanical: What do things are

Electric pitch systems—which use separate servo drives as well as backup battery packs for each blade—have been gaining significant market share, particularly in more modern offshore turbine models. They remove hydraulic fluid completely from the hub, which decreases the risk of leaks and makes maintenance easier.

But hydraulic pitch systems are the dominant offshore turbines and in a number of established onshore platforms. The force capability along with the compactness and safety of hydraulic accumulators are real advantages. Hydraulic yaw and the rotor brakes are not facing major competition from electromechanical alternatives; the spring-applied disc brake is the ideal tool for those tasks.

Hydraulics are incorporated into the design of wind turbines, not because of inertia, but rather because they can solve engineering problems efficiently. Pitch control, yaw brake, and rotor braking all require high force as well as compact packaging and failure-safe behavior in the event of power loss—an ideal system that can provide consistently over years of service. As wind power grows and the hydraulic system becomes more reliable, along with fluid selection and maintenance based on condition, it will remain crucial to turbine performance and security.