Extreme Wind Power
Together with European partners, ECN is developing technology for wind turbines rated at 10 megawatt and more. Stoyan Kanev of ECN Wind Energy finds the prospect exciting: "We believe that our improved control systems can reduce the cost of offshore wind power by 10 to 15 percent."
In 2020 - just eleven years away - wind turbines will have to provide 25 percent of our electrical energy. That is how the Dutch government wants to implement the strategy formulated by the European Union, which calls for rapid replacement of fossil fuels with renewable energy sources.
To meet a quarter of the current demand, an installed capacity of at least 9000 megawatt is required. "That means another 6000 MW offshore," says Kanev. "Which amounts to 1200 wind turbines rated at 5 MW each, with rotor blades measuring 60 meters in length on towers standing 100 meters tall. On land, there is nowhere to put so many of those giants."
But using current technology, offshore wind power is one and a half times more expensive. Maintenance and repair are the main culprits, representing about a third of the total project expenses, "while for wind turbines on land it's just a few percent."
Improved control technology may enable larger wind turbines with reduced cost per kWh (Illustration: EWEA).
Vulnerable gearboxes
Wind turbines suffer most when the load varies. Each time a rotor blade passes in front of the tower it briefly catches less wind, causing a dip in the bending stresses on the blade as well as in the rotor speed. The blades of a large turbine also feel the differences in wind speed and direction between the ground-hugging and slightly higher layers of air. Another factor is the tower's aeroelasticity. Varying forces on the rotor cause bending oscillations, which in turn affect rotor speed.
Gearboxes are especially vulnerable. A series of unexpected gearbox defects occurred during the last decade. So far, the only way to avoid such problems is to use a conservative, read heavy design. So far, the only way to avoid such problems is to use a conservative, read heavy design.
Damping control
A better control system will smooth the load variations. New software developed by ECN Wind Energy already achieves impressive results. Kanev: "A factor ten for the gearbox! Which means that peaks in the load are reduced by 90 percent. And the tower swings about 60 percent less." How it works?
1- An accelerometer inside the nacelle senses the tower's fore-aft motion. The software compensates by altering the blade pitch, thereby adjusting the wind pressure on the tower.
2- A second accelerometer provides information about the sideways tower motions, which are countered by altering the generator torque; briefly reducing or increasing the amount of electric power taken from the wind causes reaction forces which oppose the side-to-side swings.
3- The software also adjusts the generator torque to keep the drive-train loads constant.
4- Strain gauges sense the load on each rotor blade, by measuring the amount of bending. Individual pitch control reduces the stress peaks per blade.
The nacelle of a modern wind turbine (Illustration: WinWinD).
Reliable damping control systems can help today's wind turbines, but their main purpose is to enable much larger machines. "A wind turbine's power increases with the area of the rotor disc, which is the square of the scale factor," says Kanev. "But the cost and whether the turbine can work at all depend mostly on the weight, and there we have to deal with the third power. Making a wind turbine twice as large causes it to be eight times as heavy, unless the load remains constant, allowing a much lighter design."
ECN has joined dozens of European partners in project UpWind, which aims to develop various technologies for wind turbines rated at 10 megawatt and more.
Increasing the size should help to reduce the cost per kWh. Especially offshore, in part because it reduces the number of foundations required for a certain installed capacity. Kanev: "Even so, working offshore will remain relatively expensive, so it's a pretty big advantage if wind turbines last longer and need less maintenance. We think that our improved control system can make offshore wind power 10 to 15 percent less expensive. And we are working on even more advanced technology.
Wind estimator
Kanev describes software able to deal with slowly deteriorating components and impending failures. Sensors, for starters. Control systems need data on wind speed, blade pitch, the amount of bending, rotor speed and so on. Sensors fail sometimes, but to some extent they predict each other's data. This allows software to identify defects and adapt to make do with the remaining information, just as an animal is able to function well enough with one eye instead of two.
A second key element is giving the system knowledge about itself. The software will detect degrading performance by comparing actual data with nominal capabilities. To keep deteriorating mechanical and electrical components going between scheduled maintenance visits, the wind turbine will operate at reduced power levels.
Another way to increase reliability is to improve the way in which extreme gusts are handled. Current control systems try to maintain maximum output, even when operating in quite strong winds, until a gust drives the rotor speed past a limit. Then a separate emergency system takes over and brings the turbine to a full stop by rapidly feathering the blades. The braking action can be aggressive enough to cause damage, and certainly reduces the energy output.
New software will contain a 'wind estimator' module, to allow continued safe operation in high winds. The estimator won't even look at the wind speed indicator on the nacelle. Instead it will closely watch signals like the bending of the blades and variations in the rotor speed. When it reports that extreme gusts are becoming likely, the control system will reduce the rotor speed to create a safe margin.
In practice
"But those are concepts we will be studying and simulating for a while yet," says Kanev. "As to real hardware, in the coming year we are going to test our individual pitch control together with Mitsubishi, in one of their wind turbines. They've already got similar software, but feel that ours might work better."
A paradise for researchers? "Abssolutely," smiles Kanev. "But also important for the future of our energy supply. The wind farms which must provide a quarter of our electric power are going to cost tens of billions. Reducing those expenses by 10 to 15 percent is serious business."
Additional information
Wind Turbine Extreme Gust Control
Exploring the Limits in Individual Pitch Control
ECN Design Tool
Estimating costs of operation & maintenance for offshore wind farms
Contact
Stoyan Kanev
ECN Windenergie
Tel.: +31 22 456 4821
E-mail: Stoyan Kanev
Text: Steven Bolt
This ECN Newsletter article may be reproduced without permission, provided that www.ecn.nl/nl/nieuws/newsletter-en/ is acknowledged as the source of the material.
