Wind of change

A fundamental rethink over the way wind turbines generate electricity could lead to improved efficiency and lower maintenance costs. Jon Excell reports.

With politicians, celebrities, David Bellamy and anyone else with a passing interest weighing in with an argument about the impact and aesthetics of wind farms it’s easy to forget about the technology. But while the public debate rages on, wind industry engineers are keeping their heads down and working on improving the core science.

In one of the latest advances Norwegian wind turbine manufacturer ScanWind has turned to a design that dispenses with gears and employs the kind of direct-drive technology more readily associated with the factory floor.

Most turbines currently use gearboxes to connect the low-speed shaft turned by the rotor to a high-speed shaft, increasing rotational speed of 10-60rpm to the speed required to generate electricity (around 1,200-1,800rpm). But the gearbox is far from ideal. As well as the inherent energy losses due to friction and heat, a gearbox requires significant maintenance and oil.

Enter ScanWind’s system. Equipped with a permanent magnet generator designed by engineers at Siemens, the 80m tower and 43.5m rotor blades make it the world’s largest gearless wind turbine.

For the past year, a one-off prototype of the 3MW system has been generating power at Norway’s Hundhammerfjellet wind farm, a remote utility on a wind-lashed tongue of land 200km north of Trondheim.

With a power output of 3MW, the unit is said to be able to generate enough power for wind farm owner NTE to supply around 3,000 Norwegian households with electricity each year. On the back of this success 13 more units are likely to be ordered.

ScanWind is now establishing an assembly line for the turbines at its factory in Verdal harbour, 60km north of Trondheim. From there, they will be directly shipped to a newly built quay at the Hundhammerfjellet site.

Stefan Rausch, Siemens’ head of marketing for the turbine’s generator, explained that the unit, which has an extremely high efficiency rating of 98 per cent, uses permanent magnets to convert wind energy from the rotor into electricity. The system reaches its rated power of 3MW at a wind speed of 14m/s.

One of the chief advantages of the direct-drive system, explained Rausch, is the low level of maintenance required. With fewer mechanical parts subject to wear and damage and no need for gear oil, the system is ideally suited for use in remote and hard-to-access locations.

In addition, direct-driven generator systems are said to be better at exploiting the existing wind potential. They have a significantly lower breakaway torque than geared systems, so the rotors on a gearless turbine start to spin even at low wind speeds or during brief gusts where a traditional turbine would remain motionless. Direct-drive turbines are also much quieter than gearbox machines as they don’t produce mechanical noise, thus helping to limit their environmental impact.

The synchronous generator at the heart of the system operates in much the same way as a synchronous motor. In a synchronous motor the rotor (a magnet at the centre of the motor) exactly follows the rotation of a magnetic field around a series of stationary electromagnets (the stator). However, by forcing the magnet (rotor) around, the device works like a generator, sending AC back into the grid. The more torque that’s applied the more electricity you generate.

In practice, permanent magnet synchronous generators are not used very much. There are several reasons for this. One is that permanent magnets tend to become demagnetised by working in the powerful magnetic fields inside a generator. Another is that powerful magnets (made of rare-earth metals, such as neodymium) are expensive, even though prices have recently dropped.

However, Rausch explained that while it’s possible to generate the rotor’s magnetic field either electrically or using a permanent magnet, the design team opted for a permanent magnet largely because it takes up around a quarter of the space of an electrically excited system and requires fewer parts, thus substantially reducing transportation and installation costs. He added that in recent years high-quality permanent magnets have become more cost effective. Siemens has also developed a special coating for the magnets which ensures that the extreme humidity and salt-laden air around the Hundhammerfjellet wind farm do not result in any corrosion.

ScanWind is not the only company moving into gearless turbines, however. Mitsubishi Heavy Industries and ABB have also demonstrated smaller versions of the technology in recent months, and a 1.5MW turbine developed by energy company Ecotricity has been providing electricity in Norfolk for the past couple of years. This turbine also uses a synchronous generator and begins operating at a wind speed of just 2.5m/s, with rated power output available from wind speeds of 13m/s upwards. The turbine currently produces enough electricity for around 3,000 people – over a third of the population of Swaffham.

Although ScanWind’s system appears to be the most advanced at the moment, gearless turbines look set to become one of the keytechnology trends that will lead to a further reduction in the cost of wind energy.

It is estimated that 22,000MW of wind energy capacity, in the form of 40,000 wind turbines, will be installed around the world in the next 10 years. And with six European companies currently supplyingover half the world’s turbines, Europe clearly stands to benefit greatly from this move towards sustainability.