Actuators must withstand high humidity, salt, wind and rain

Paul Boughton

Wind power now accounts for 20 per cent of Denmark's electricity needs. Donna Guinivan visits AVN's plant, Silkeborg, Denmark, to get the full story on this and the future of wind power in general.

Companies such as AVN Energy, a Danish manufacturer of wind power equipment, are creating the technology which now provides 20 per cent of Denmark's electricity production. AVN co-operated with The Trelleborg Group's Sealing Solutions to develop seals for their wind power actuators.

Dotted all over the green and rolling landscape of Denmark, where the idea for wind power originated, are wind turbines. This renewable energy source now accounts for 20 per cent of the country's electricity needs, more than anywhere else in the world.

After travelling two and half hours from Copenhagen, I arrived at AVN Energy in Silkeborg, where I was met by the Export Sales Manager, Poul Kristensen. He showed me around his company's production site, which has more than doubled in size in the last year.

"We've been involved in wind power since it began back in the 1980s," says Poul. "At first the turbine producers came to us and told us what they wanted, but over time we gained a high level of expertise which allows us to recommend the optimum hydraulic system for their application."

In the past few years wind turbine technology has changed. Previously the wind turbines were stall machines and their position would shift only once every ten minutes. Such turbines have been superseded by continuous pitch systems, where the pitch, the position of the nacelle and angles of the blades, constantly changes in small amounts once every rotation. That could be on average 15 times per minute.

"While this optimised the production of energy from the turbine, for us, the actuator manufacturer, it presented a real challenge," continues Poul. "Instead of hydraulics producing six long strokes per hour, they now had to give nine hundred short strokes in the same period. And it's not just the pitch which is continuous; it is also the turbine's operation, with the actuators needing to initiate those strokes 24 hours a day, seven days a week.

"Customers have high expectations from our products, and the number one requirement of the wind turbine manufacturers is reliability. At first this was not the case. Initially demand for windmills was on a small scale, from farmers with a single turbine powering an individual generator. Then the power distributors became involved. They built relatively small wind farms and quality needs increased. Nowadays wind power is government backed and expansion is on a huge scale; the power suppliers are making the decisions and the demands. These big investors are not prepared to finance installations unless equipment can be guaranteed for 20 years with only the minimum of maintenance.

"Maintenance of turbines is difficult and costly," says Poul. "On land it is hard enough, but offshore it is really tough. And when the windmill is switched off for maintenance, it is not producing energy and losing income. On top of that, operators are often penalised if supply targets are not met. So a primary objective for them is to minimise routine downtime, while stoppages due to component failure have to be avoided at all costs."

AVN put a great deal of emphasis on research and development with over 20 per cent of the 70 people employed at the Silkeborg site involved in R&D. "Here in R&D it's not just about knowing the product, it's about thinking about new solutions to the challenges imposed by turbine design and about finding new ways of doing things," says Johnny Fruekilde from AVN's Research and Development department.

"Meeting the target life of 20 years for an actuator required all our expertise, and initially it seemed almost unfeasible. If you imagine the actuator as a car, it's a bit like saying to its manufacturer that you won't buy his vehicle unless it can travel 500 000kms without replacing the oil filter, brake pads or any other wearing parts. Yet we have strived to accomplish the impossible, and our actuators should provide the 20-year life span stipulated with very little maintenance.

"At the moment though, we are working a little in the dark when it comes to actual performance in application. The continuous pitch systems have only been around for three years so we are basing our expectations on extrapolating performance results from older generation wind turbines. This is combined with virtual modelling and long-term testing on individual elements of the system."

Simulation programs are extensively used by AVN to specify the best hydraulic and actuation system for each design of wind turbine. Following on from this though, automated physical testing is a necessity. The conditions within the wind turbines are very specific to the application. This means that AVN needs to build test rigs to their own designs that can as closely as possible replicate the situation within the nacelle and hub.

"We know that the hydraulic system can only ever be as strong as its weakest link, and early on we realized that the reliability of the sealing configuration was highly dependent upon the quality of its counterparts," continues Johnny. "So one area we have focused on is the interaction between the surface finish of the rods and shafts of the actuators and the sealing components. A special rig was constructed specifically to test this and operates 24/7."

The seals within the hydraulics are integral to its performance, and optimising their life is critical to the long-term effectiveness of the total system. Several other specially built rigs are used to measure sealing characteristics, as the dynamic demands of the application are extreme.

"The requirements for sealing of the actuator for wind turbine applications were unique," says Per Hvidberg, Sales Engineer from Trelleborg Sealing Solutions, Denmark. "Never before had I been faced with a demand for a sealing configuration on a cylinder that produced relatively rapid short strokes continuously. And not only was there linear pressure from the rear, there could be side load too."

Per's relationship with the engineering team at AVN goes back a long way and when asked to support them in development of continuous pitch actuators, he, the engineering team at Trelleborg Sealing Solutions Helsingor and AVN worked together to come up with the best possible design.

"Within the actuators is a complex arrangement of seals ranging from O-Rings to specialist Turcon PTFE based geometries and Slydring in Orkot," says Per. "The unique configuration is specially engineered to enhance lubrication, optimise friction characteristics, and maximise service life, while preventing any external leakage. Some of the seals are expected to achieve the twenty year target, but it is impossible to guarantee this."

"As this was the case," says Johnny, "the hydraulics were designed for easy exchange of the seal set. This is mounted in a module that can be quickly bolted on and off. The minimum life expectancy of the sealing configuration, allowing for the seal that has the shortest predicted life, is seven years, but replacement is recommended after five. Other than this, and routine rod replacement, the actuators should run without maintenance except for the systematic checking that the operators do for any leakage or loss of pressure. We feel that this arrangement gives the ideal compromise between minimum required maintenance and guaranteed long-term performance."

"Cleanliness of subcomponents is another important factor," comments Poul. "Before assembly the system is flushed through to ensure there is no metal from machining or other debris such as dust or sand within the cylinder. Any residual matter such as this has been found to cause wear on the seals, shortening seal life and consequently total system life.

"The expanded factory has allowed us to construct a cleanroom. It's not quite like the cleanrooms used in semiconductor or chemical processing, but it's advanced in our type of manufacture. The cleanroom will be completely enclosed with barriers between it and the outside world and an extraction system to eliminate media that could potentially enter the actuator's hydraulic system before it is enclosed."

And what does the future hold for AVN?

"Growth and more growth," says Poul. "We see the Silkeborg site expanding even further, but we are also supporting the turbine manufacturers as they enter booming wind power markets globally. We already have production facilities in India and are planning expansion in China and the US."

Challenging requirements

The wind power actuator and its sealing system must be capable of operating at 250 bars/3625 psi with constant pressure on the rod from behind and differential side loads that control positioning. Seals must give minimal wear and facilitate dynamic movement that is continuous in short strokes, on average 900 times per hour.

Temperature resistance is needed down to -30°C/-22°F as standard and to -40°C/-40°F in the Artic. Below these temperatures the oil within the cylinder cannot function and requires warming with heating elements. Maximum temperature is 60°C/140°F. Beyond this the system is cooled, otherwise the oil becomes stressed, its viscosity is too low, and it carbonises.

In addition, the actuators must withstand high humidity, salt spray and the rigors of wind and rain. Corrosion is prevented with advanced coating technology.


It's hard to imagine when you look at a wind turbine that the nacelle, or the structure that houses all the turbine's generating components for the blades, is large enough for a man to stand up in. It has to be, because for maintenance the engineer has to enter this either through the side, but more commonly by climbing to the top of the tower, and down into the nacelle from there. That's not easy 100metres/330feet high on land and even more daring when the turbines are up to 100kilometres/60miles out at sea.

The wind turbine tower is between 35 and 120metres/115 to 395feet high with blades of 12 to 60metres/40 to 195feet in length. These are attached to a nacelle which is over two metere/7 feet high and that can rotate 360 degrees on top of the tower. Each of the three curved blades of the turbine is positioned by an independently operated actuator with a stroke of 1.2 to 1.5metres/4 to 5feet and can be tilted through 90 degrees.

The higher the turbine and larger the blade size, the greater the megawatts of electricity produced each hour. The smallest turbines are producing one megawatt per hour while the largest yield up to five megawatts. In Europe most turbines are between 1.5 and 2.6megawatts. The biggest used on land is 3.6 megawatts, with a number installed offshore between 4.5 and 5megawatts. In Asia the trend has been for larger wind farms with smaller wattage turbines.

Bigger turbines are not always better; it depends on the size of the wind farm, the stability of the electricity grid it supplies and the promised output. So in some cases it is beneficial to have the option of shutting off a lower production source than a higher one, even though there are economies of scale in running a high output turbine compared to a smaller one.

On top of a turbine tower are two wind sensors checking wind direction and speed. One is the primary input and the second for backup. On installation the nacelle of the turbine is positioned inline with the predominant wind direction. Based on complex arithmetic calculations the wind turbine's control system take the sensors input, and automatically yaws, or turns the nacelle to the wind, the actuators tilting each blade independently. Positioning is precise, to exacting tolerances, thereby optimising energy production in the wind condition. The movement is calculated for every rotation, which may be 15 times per minute, continuously for 24 hours, seven days per week.

When choosing a site for a wind farm, analysis must prove it to have 2500 hours of wind at 12 metres/39 feet per second over a year to make them viable to the utility companies. Wind turbines will normally operate from three metres/10 feet per second to 25 meters/80 feet per second, with the optimum wind speed being between 12 to 15 metres/40 to 50 feet per second. Though designed to withstand speeds up to 50 metres/165 feet per second, the control system will counter over rotation for speeds of over 25 metres/80 feet per second due to safety concerns.

The utility companies target 98 per cent utilisation with two percent allowance for maintenance. The turbines can be switched on and off remotely from control rooms anywhere in the world. This is done for maintenance or in response to grid changes.

On the stall turbines a braking mechanism is employed to stop the windmill. On the new larger turbines this can stress the tower, so tilting a single blade to 90 degrees normally stops them. In an emergency situation this method plus a brake will be employed. In these circumstances the windmills are stationary in well under a minute. The brake, in all cases, then holds the blades in position.

A 24-hour mains electricity supply finally arrived in February 2008 for residents of the Isle of Eigg, which lies in the Small Isles archipelago off Scotland's west coast. So remote, it previously had to rely on expensive diesel generators to run homes. Now operational, a £1.6 million renewable energy system, which includes hydro, wind and solar power, is expected to generate more than 95 per cent of its annual energy demand.

It has taken a decade for the islanders' green dream to be realised. The idea was first raised after the community of less than 100 people bought the island from its previous owner in 1997. Now, a total of 45 households, 20 businesses and six community buildings are linked together by six miles of buried cable that form a high voltage network. This proves the future really can be renewable.

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