Donna Guinivan, Trelleborg, Denmark
Wind turbines are dotted all over the green and rolling landscape of Denmark, which has long been associated with wind power. The country now uses this renewable energy source to provide 20 per cent its electricity needs, a higher proportion than any other country in the world.
AVN Energy are a leading supplier of actuators for wind turbines
After travelling from Copenhagen, I arrived at AVN Energy in Silkeborg, where I am met by export sales manager Poul Kristensen. He proudly 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 Kristensen. “At first the turbine producers came to us and told us what they wanted, but over time we have gained a high level of expertise that allows us to recommend the optimum hydraulic system for their application.”
In the last few years wind turbine technology has changed. Until recently, wind turbines were stall machines, and their position would shift only once every ten minutes or so. Such turbines have now been superseded by continuous pitch systems, where the pitch, the position of the nacelle and the angles of the blades change slightly with every rotation on average 15 times per minute.
“While this optimized the production of energy from the turbine, for us, the actuator manufacturer, it presented a real challenge,” says Kristensen. “Instead of hydraulics producing six long strokes per hour, they now had to give perform 900 short strokes in the same period. And it’s not just the pitch that 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 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, but 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, and 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,” adds Kristensen. “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 but losing income. On top of that, operators are often penalized if supply targets are not met. A primary objective for them is to minimize routine downtime, so stoppages due to component failure have to be avoided at all costs.”
AVN puts a great deal of emphasis on R&D: over 20 per cent of the 70 people employed at the Silkeborg site are 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 R&D 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 500 000 km 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 necessary. 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 on the quality of its counterparts,” says Fruekilde. “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, operating 24/7 was constructed specifically to test this.”
The seals within the hydraulics are integral to its performance, and optimizing 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 the sealing of the actuator for wind turbine applications were unique,” says Per Hvidberg, a sales engineer from Trelleborg Sealing Solutions. “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.”
Hvidberg’s relationship with the engineering team at AVN goes back a long way, and when asked to support them in the development of continuous-pitch actuators, he, the engineering team at Trelleborg Sealing Solutions Helsingør 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 Hvidberg. “The unique configuration is specially engineered to enhance lubrication, optimize friction characteristics and maximize service life, while preventing external leakage. Some of the seals are expected to achieve the 20-year target, but it is impossible to guarantee this.”
Fruekilde says: “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 operators carry out for any leak or loss of pressure. We feel that this arrangement is the ideal compromise between minimum required maintenance and guaranteed long-term performance.”
Typical sealing arrangement within a cylinder
“Cleanliness of sub-components is another important factor,” says Kristensen. “Before assembly, the system is flushed through to ensure there is no metal from machining, or other debris, such as dust or sand, in the cylinder. Residual matter such as this has been found to cause wear on the seals, shortening seal life and consequently total system life.
Radial oil seals are commonly used within wind power applications
“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 will eliminate media that could potentially enter the actuator’s hydraulic system before it is enclosed.“
So what does the future hold for AVN?
“Growth and more growth,” says Kristensen. “We see the Silkeborg site expanding even further, but we are also supporting 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.”
The actuator challenge
The wind power actuator and its sealing system must be capable of operating at 250 bars 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 as standard and to -40°C in the Arctic. Below these temperatures the oil within the cylinder cannot function and requires warming with heating elements. The maximum operating temperature is 60°C. Beyond this, the system is cooled, otherwise the oil becomes stressed, its viscosity drops too low and it carbonizes.
The actuators must also withstand high humidity, salt spray and the rigours of wind and rain. Corrosion is prevented with advanced coating technology.
Maintenance is also challenging. It’s hard to imagine when you look at a wind turbine that the nacelle, the structure that houses all the turbine’s generating components for the blades, is large enough for a man to stand up in. It is, and it has to be this big because, to maintain the turbine, the engineer must enter it, either through the side or, more commonly, by climbing to the top of the tower and down into the nacelle. That’s not easy 100 m up on land, and it’s even more daring when the turbines are up to 100 km out to sea.
Wind turbine facts and figures
A wind turbine tower is between 35 m and 120 m high. The blades are between 12 m and 60 m in length. These are attached to a nacelle that is over 2 m high and 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 m to 1.5 m and can be tilted through 90 degrees.
The higher the turbine and larger the blade size the greater the electrical output. The smallest turbines produce 1 MW per hour, while the largest yield up to 5 MW per hour. In Europe, most turbines are between 1.5 MW and 2.6 MW. The biggest used on land is 3.6 MW, while a number installed offshore range between 4.5 MW and 5 MW. In Asia, the trend has been for larger wind farms with smaller 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. In some cases it is beneficial to have the option of shutting off a lower production source than a higher one. There are, however, economies of scale to be gained by running a high-output turbine compared with a smaller one.
On top of a turbine tower are two wind sensors measuring wind direction and speed. One is the primary input and the second is for backup. On installation, the nacelle of the turbine is positioned to face the prevailing wind direction. Using complex arithmetic calculations, the wind turbine’s control system takes the sensors’ inputs and automatically yaws, or turns, the nacelle into the wind, the actuators tilting each blade independently. Positioning is precise, to exacting tolerances, thereby optimizing energy production in a given wind condition. The movement is calculated for every rotation, which could be 15 times per minute, continuously, 24 hours a day, seven days a week.
When choosing a site for a wind farm, analysis must show it to have 2500 hours of wind at 12 m per second over a year for it to be viable to utility companies. Wind turbines will normally operate at between 3 m and 25 m per second. The optimum wind speed is between 12 m and 15 m per second. Although designed to withstand speeds of up of up to 50 m per second, the control system will counter over-rotation for speeds of over 25 m per second for safety reasons.
The utility companies target 98 per cent utilization, with a 2 per cent allowance for maintenance. The turbines can be switched on and off remotely from control rooms anywhere in the world for maintenance or in response to grid changes.
On stall turbines a braking mechanism is employed to stop the windmill. On the new larger turbines this can stress the tower, but tilting a single blade to 90 degrees normally stops them. In an emergency a brake may also be employed, bringing windmills to a halt in less than a minute. The brake will then continue to hold the blades in position.
A 24-hour mains electricity supply finally arrived in February 2008 for residents of the Isle of Eigg, one of the Hebridean Islands off the west coast of Scotland. The island was so remote that it previously had to rely on expensive diesel generators to provide electricity for the island’s homes. A new £1.6 million ($2.4 million) renewable energy system, which incorporates hydro, wind and solar power generation, is now operational and is expected to generate more than 95 per cent of the island’s energy demand.
It has taken a decade for the islanders’ green dream to be realized. 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 forming a high-voltage network. Perhaps renewables really are the future.