Monster turbines

Deok-je Bang next to the 10 kW prototype of his direct-drive generator (Photo: Bong-jun Kim)

Scaling up wind turbines from the current 3 megawatts to 5, 10 or even 20 megawatts demands daring and creative concepts. Dr Deok-je Bang presented some in his thesis.

To harvest more power from the wind, you need to build monstrously large devices. A 10 megawatt (MW) wind turbine would need a blade diameter as large as the Euromast. Mounted on its support mast, it would actually dwarf that Rotterdam monument.

Not only would its size be impressive, but so too would its weight. The turbine would weigh anywhere between 200 and 800 tonnes, depending on the design. The 800 tonnes is the calculated weight for the Concept-1, a turbine design by Enercon, a German company, while the 200 tonnes design is the result of a concept by Dr Deok-je Bang, who defended his thesis on the design of a generator for direct-drive wind turbines at the faculty of Electrical Engineering, Mathematics and Computer Sciences.

Direct-drive is the industry’s buzz-word. Dr Henk Polinder, Bang’s co-supervisor, explains that four or five years ago major wind turbine producers, such as Siemens, General Electric, Enercon and Goldwind, started conducting research into direct-drive turbines. The main reason for this interest is that manufacturers are keen to rid their designs of gearboxes, as gearboxes have proven to be major sources of failures, especially offshore. The current paradigm is to fit the generator directly to the rotor, thus simplifying the design and reducing the chance of failure. Polinder points out however that, statistically, direct-drive turbines have not yet performed more reliably than geared ones. An additional advantage is said to be the reduction in weight that comes from eliminating the gearbox. But Polinder doesn’t buy that, either: “The direct-drive generator becomes huge. Instead of 100 tonnes of gearbox, you have a 200 tonne generator.”

Dr Bang didn’t shy away from this challenge. A mechanical engineer by education, he gained experience in innovative electromagnetic conveyor belts in Korea. He became obsessed with devising an alternative alignment of the magnetic fields. Normally, the magnetic flux is parallel to the direction of movement; however, in transverse flux, the magnetic flux stands perpendicular to the movement. Dr Bang used this concept on his conveyor belts, because transverse flux increased the force density (force per area). But especially the curves proved to be challenging. Dr Bang’s desire to learn more about electromagnetic design ultimately led him to pursue his PhD project at TU Delft.

At the Delft lab he designed a direct drive generator based on the transverse flux concept. He also built a scaled-down prototype at WinTech in Korea – a 10 MW version would sport a diameter of 8.7 metres instead of 1.2 metres. Being a mechanical engineer, Bang was also able to design a unique floating bearing intended to keep the weight down.

“Generally, people don’t like water in their turbines,” Polinder remarks, while also admitting that “the design does show a lot of creativity.”

Deok-je Bang, Design of Transverse Flux Permanent Magnet Machines for Large Direct-Drive Wind Turbines, 20 October 2010, PhD supervisor: Professor Bram Ferreira.