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Lifting wind power via hydraulics

The latest generation of hydraulics technology is enabling increasing levels of precision in wind turbines, resulting in higher levels of efficiency and lower maintenance requirements.

Andrew Delaney, Hydraulic Cylinder Division, Parker Hannifin, UK

The popularity of wind power as a renewable energy source is greater than ever, with the global capacity of wind generated electricity having increased from less than 5 000 MW in 1995 to a total of just over 59 GW by the end of 2005. Europe, in particular, has embraced this technology, which provides 6 per cent of Germany’s electricity requirements, and as much as 20 per cent in Denmark.

With the operating costs of wind turbines estimated to be as high as 30 per cent of the cost of energy generated, operators of wind farms, particularly those offshore, are continually looking for ways to increase the efficiency of machinery by reducing maintenance and replacement requirements, and optimizing performance. The latest generation of specialized hydraulics technology has been developed in response to these requirements, providing turbines with a reliable power source, precise control of pitch and yaw, and efficient braking mechanisms in even the most hostile environments.

Offshore wind farms

A large number of wind farms are now located offshore, where they benefit from uninterrupted and typically higher air flows, and generally fewer environmental objections than their land based counterparts.

Technically speaking, the way in which offshore installations generate electricity is no different to that of conventional installations. However, their location presents a number of challenges to designers, engineers and managers.

Hydraulic yaw control systems reduce inefficiencies of nacelle rotation
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The remote and inaccessible locations in which they operate make installation, maintenance and replacement difficult, expensive and often dangerous, resulting in the need to keep the turbines operating effectively without attention for as long as possible. In addition, the combination of salt water and high winds makes corrosion of the turbines and component parts of particular concern.

The latest hydraulics technology offers solutions to these problems through its ability to combine precise control with robust construction. Hydraulic cylinders with fully protected integral feedback systems operating under closed loop control allow the speed and direction of wind turbines to be accurately regulated for optimum output. In comparison to electromechanical systems, hydraulics can offer the right balance between cost effectiveness and performance.

Blade turning

Although the way in which wind turbines work is seemingly simple, with the wind turning the rotor, which then turns the generator through a gearbox, both housed in the nacelle at the top of the turbine tower, their operation is actually far more complex. Both the rotor blades and the nacelle have to be constantly controlled and powered in order to optimize performance, and effective braking mechanisms need to be in place for when maintenance is required.

With wind turbines required to produce electrical energy as cheaply as possible, they are generally engineered to yield maximum output at wind speeds of around 15 m/s (30 knots or 33 mph). Designing them to cater for stronger winds is generally not cost effective because such wind speeds are relatively rare, and could also damage the turbine.

Whether in order to optimize performance or protect the turbine from damage, the output can be carefully controlled using hydraulics technology, which alters the pitch of the turbine blades. On a pitch-controlled wind turbine, an electronic controller checks the power output from the generator several times per second. When the wind speed increases and the power output becomes too high the blade pitch mechanism immediately pitches or turns the rotor blades, usually by a fraction of a degree at a time, slightly out of the wind, causing the rotational speed to fall. Likewise, the blades are turned back into the wind whenever the wind drops again, raising the rotational speed until an optimum generating output is obtained.

Specialized hydraulics technology offers many benefits to wind energy
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To ensure that the pitch is accurately controlled, hydraulics technology is used in conjunction with variable speed technology. Typically, three pitch control systems are used, one for each blade, located inside the hub. These consist of a hydraulic cylinder that varies the pitch of each blade through a cam action. Specially designed, these cylinders are part of a closed loop control system, featuring internal transducers that are incorporated into each cylinder’s design, and are completely sealed from the harsh environments encountered by wind turbines, particularly those located offshore.

Hydraulic pitch control systems can be controlled with various degrees of precision by implementing the latest designs of analogue or digital valves. These allow design engineers to choose from standard proportional valves and high response units for more intelligent applications, with integrated blocks containing all the necessary valves being mounted directly onto the units.

Yawing control

As well as controlling the pitch of the blades, the yaw or rotational position of the turbine also needs to be continually adjusted in order to obtain maximum efficiency. Output is optimized when the blades are facing directly into the wind, and this is achieved by the horizontal rotation of the nacelle about the axis of the tower. Unlike electromechanically powered systems, which require a separate transmission, hydraulically powered yaw control systems offer a more compact and simpler direct drive, reducing the frequency of maintenance required or components to be replaced. A hydraulic drive can also incorporate hydro-pneumatic accumulators, which have damping properties that protect the mechanical components in a system from overload by absorbing the effects of external forces.

Secure braking system

Although pitching the blades of a turbine fully with the wind – 90à‚º along the longitudinal axes – will usually be enough to bring the rotors to a halt within a few revolutions, a more secure braking system is required when maintenance work is needed to be carried out. Typically, high speed shaft (HSS) disc braking systems, incorporating compact hydraulic cylinders, are used to provide an effective solution.

This method of braking, i.e. locking the blades in position in relation to the nacelle, offers the level of reliability required with long stand-by times, low leakage operation, and once again the ability to withstand the hostile conditions that many turbines are subjected to.

The power units of these brakes typically include a tank, pump and motor with a single diaphragm accumulator, and a high pressure return filter and tank breather. Although some wind turbines are designed in such a way that hydraulic power for the yaw brake is taken from the main power unit.

Hydraulic power

The latest hydraulic power units, whether designed to power shaft braking systems, yaw braking systems or pitch control cylinders, have been specifically designed so that they can be supplied ready for installation and commissioning, with a range of frame sizes and pumps available to fit most turbines. Compact and reliable, the latest generation of hydraulic power units features low maintenance requirements, even when operated in extreme conditions.

The European Wind Energy Association targets 180,000 MW installed by 2020
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Specialized hydraulics technology offers many benefits in the wind power industry, enabling accurate control, increased efficiency and long low maintenance service life. These benefits together can lower operating costs significantly, essential if the targets set for renewable energy generation are to be met.

With the European Wind Energy Association setting a target of a 180,000 MW installed by 2020, including 70,000 MW of capacity offshore, wind power’s continued growth looks set to continue, with the latest hydraulics technology ensuring that state-of-the-art turbines can be installed and operated cost effectively.