This article, a Q&A with Georg Bohmeke, comes from the International Quality & Productivity Center in Germany, www.iqpc.de. Böhmeke is the General Technical Advisor, wind turbine division at Samsung Heavy Industries offers a personal outlook on drivetrain development. He also discusses in detail the “gearbox vs. direct drive” controversy.
IQPC: Reducing the cost of energy remains a strong driver for innovation in drivetrain technology. Which strategy or technology is most promising for further improvements?
Georg Böhmeke: 1. Reducing loads by intelligent operation control
2. Reducing cost on substructure and foundations
3. Reducing cost of the electric system by especially tailored inverter/generator units
IQPC: In a dynamic and competitive market like wind energy, how do you set yourself apart from other companies?
Böhmeke: Samsung Heavy Industries
1. Large, able to make investments
2. Synergy to the existing ship business
3. Active in Asia
IQPC: Where do you see the main challenges for drivetrain development? How do they touch on your work?
Böhmeke: 1. Reliability versus cost. Cost pressure causes quality problems.
2. Detail problems of noise and vibration, it needs special effort to get the gearbox silent
3. Hitting the manufacturing limits of bearings
IQPC: Which of today’s developments will shape future drivetrains?
Böhmeke: 1. Compact multiplanet gearboxes with superposition stages
2. The mechanical superposition should not be overlooked it might be slightly more economic than full power conversion
3. In my private opinion we will need to use segmented main bearings for everything over 7MW, just to take the rotor loads, then a pure-torque gearbox which has its own small play-free bearings. Else we cannot match the requirements of the main bearings for taking the rotor loads and at the same time have small deformations and play to ensure a good gear contact.
IQPC: Last but not least – gearbox or direct drive?
Böhmeke: Gear will win, because the DD needs either permanent magnets (like Siemens/Bonus) or a huge lot of copper for the excitation coils (Enercon). Both solutions are raw-material intensive. Although these precious materials can be recycled, the investment costs are very high and contra-productive to the aim of reducing the COE of wind power. Also, the magnets are subject to political pricing (China). As the raw material costs dominate, the DD drive has not much cost reduction in series.
A gearbox reacts to series manufacture with reduced price because the machining costs go down and they are comparatively large. The argument for DD against GD is usually that the gearbox will cause too much service and risk. But in myprivate opinion this risk can be handled by preventive maintenance. As soon as the condition monitoring system indicates a defect, the whole gear can be exchanged.
Simplified: if you deliver a wind farm, deliver 105 GD’s instead of 100DD’s and use the 5 additional machines to compensate possible defects or repair of the gears.
2. Damage propagation
What speaks for the DD is, that a cleverly designed DD generator can suffer a certain damage without damage propagation. For example, a winding segment burns up and is replaced. But everything else remains ok. If a gearbox really fails, metal particles will spread all around in a few seconds and destroy nearly everything. This difference in failure behavior speaks for a DD, or for a special design of the gearbox with whatsoever separation walls or seals inside to stop failure propagation. For example we replace exactly the third stage and we are sure that the first and second stage is still perfectly ok.-
Maybe we come to a more modular design, possibly we will even split up the gear into a non-suspended first and second stage and then a separate casing for the third stage which is integrated with the generator, which is suspended on rubber elements against structure-born noise. So we limit the damage propagation and at the same time keep the high-speed stage structure-born noise off the supporting mainframe.
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