Wind turbine evolution can win on reliability

EUROPE: The next decade should see both global onshore and offshore wind markets continuing to expand fast. Today's state-of-the-art wind technology applied in both areas can be described as both conventional and evolutionary, with power ratings having reached 7.5MW and rotor diameters 128 metres.

Last year about 85% of the new global wind capacity comprised conventional fast-speed geared wind turbines, and they have been used for all offshore operations today.

Last year conventional fast-speed geared wind turbines made up 85% of the new global wind capacity, and all of the new offshore capacity, with a number of manufacturers involved. Enercon of Germany and China's Goldwind together take about 15% of the world market with their non-geared direct-drive wind turbines.

Larger share

While this is still a modest market share, it is expected to increase substantially, both onshore and offshore. Proponents of direct drive expect that this technology will make wind turbines more reliable and curb the costs of operations and maintenance. Already, Siemens has launched a 3MW direct-drive model and GE a 4MW offshore model, while Vestas is rumoured to be working on a 6MW offshore turbine.

The industry focus is on 1.5-2.5MW turbines with a gradual shift towards 2.5-3MW and beyond. Modular design is popular with the larger turbines, as it limits individual hoist loads to 70 tonnes and, therefore, reduces the cost of crane hire during erection and repair.
Another main trend is that the rotor-swept area for new turbines and model upgrades alike increases faster than power rating. So, while 2MW turbines that were erected ten years ago were typically fitted with a 70-80 metre rotor, today they are likely to have rotors of 90-93 metres, producing much more energy at low- and medium-wind-speed sites. The largest-size rotor blades for new 3MW plus models are now about 55-57 metres long. Two companies are developing segmented rotor blades to aid transportation.
Offshore wind turbines vary between 2-6MW, with Siemens dominating the market with a geared 3.6MW workhorse model.  For offshore application, the demands on wind turbine reliability are much more stringent than for onshore equivalents, largely because access for personnel can be problematic during the windy winter season.

Siemens, therefore, has great expectations for its new direct-drive technology and says that this SWT-3MW-101 turbine offers 25% more power compared to the smaller geared 2.3MW model, but contains only half the parts. GE speaks equally positively about the prospects of its new direct-drive technology, stating in its brochure that "the 4MW wind series platform removes the single most costly failure in offshore - gearboxes - and replaces it with reliable slow-speed components specifically designed for the offshore environment".

Grid compliance

Siemens, GE and most other new direct-drive turbine market entrants have also selected a permanent magnet generator (PMG), which offers improved partial-load efficiency. PMG is also said to provide easier compliance with future stringent grid codes than the more popular double-fed induction generators.

However, the iron elements in the magnet do make the system prone to corrosion, necessitating proper long-lasting insulation for the magnets and a total seal against the aggressive marine environment. Additionally, the operating temperatures inside the generator rotor will need to be limited to 80oC in order to retain the magnetic properties. PMG sceptics also like to add that operational experience in the wind industry is only about seven years.

Several suppliers continue with geared turbines, for reasons of cost cutting amongst others. Clipper Windpower is developing its 10MW Britannia offshore turbine with a potential 150 metre rotor diameter. Repower of Germany, part of the established offshore wind market, initially developed a dedicated 5MW wind turbine that has been scaled up to 6MW. Areva Multibrid offers its 5MW medium-speed M5000 offshore turbine, and Bard Engineering's 5MW turbine is now being up scaled up to 6.5MW. This clearly indicates that gradual scaling up of proven concepts is the preferred strategy for most suppliers.
While it is unlikely that scaling up of existing models can go on endlessly, it removes the risk of introducing a radical switch in concept. And banks, utilities and other investors, who play an important part in the future of wind turbines, are naturally focussed on proven concepts with a sound track record. Each of these individual onshore and offshore turbine models demands a dedicated strategy for operations and maintenance. Only time will tell which concepts will finally turn out to be the real winners in terms of lowest costs of energy.   
Eize de Vries is a technical writer and adviser for Windpower Monthly

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