Expert: Impressive and rare
An 18-year-old nacelle with original gearbox intact is a rarity. That is the assessment from mechanical expert Strange Skriver from the Danish Wind Turbine Owners Association.
- The size of the wind turbine was ahead of its time. Turbines above 500 KW were not widespread until 1993-94. And among those, we’ve seen a lot of failures, says Strange Skriver. He has inspected 3000 turbines in over 25 countries as a technical consultant.
- And when you look at 600 KW wind turbines and above, everyone will have their gearbox replaced within 10 years, says Strange Skriver. His experience stretches from 1 KW turbines to 3 MW. - It’s a question of dimensions and standards. The simulations today are one thing – and the reality for the turbine is completely different. Margins are too small in design, and the gearboxes are constructed to be lightweight rather than long lasting, says the consultant.
By Karsten Prinds
Mentioning the word ‘gearbox’ will probably make any wind industry engineer break out into a cold sweat and make any maintenance operator furious. Gears crack and bearings crumble as the wind grinds the rotor without remorse.
But in the early 1990s, a team of engineers in northern Denmark designed what was to become a very durable wind turbine, perhaps one of the world’s most stable in relation to its size. In 1992, they raised the DANmark wind turbine, at its time very large in production and until today very low in maintenance.
-It’s a myth that you can not build wind turbines with lasting gearboxes, says Preben Maegaard, one of designers on the DANmark project, a wind turbine of record-breaking 525 KW power.
- We looked to the ship industry. There, you build the gearbox into the bottom of the hull, and you never remove it. It’s basically the same philosophy behind our turbine.
Preben Maegaard and the rest of the team at the Nordic Folkecenter for Renewable Energy put their money where their mouth was. The design was to be integrated so that if the gearbox cracked, they would have to take down the whole nacelle.
As a result, construction was cautious:
- We used a safety margin of 1.8 up to 2.0 in our calculations. In normal wind turbine designs you see margins as low as 1.4, says Preben Maegaard.
And there are several examples of this method. The yaw bearing should have been 50 millimeters thick according to the size and standards at the time, but the designers chose 100 millimeter thickness. Being a 525 KW generator, they had to select between gearboxes of either 600 KW or 1000 KW rated power. Size matters, they thought, and they selected the 1000 KW gearbox.
- We made expensive choices. At a total price of DKK 2,500,000, we selected a gearbox that cost DKK 600,000 rather than a small one costing DKK 350,000. But in the long run, who knows whether it would have lasted? Preben Maegaard asks.
Installing the turbine back in 1992: Every component is still the same.
The price per KW was 15% higher than other commercial wind turbines, but the additional costs have long been paid back, Preben Maegaard believes.
- We have had one repair. Shortly after completion, lightning struck the grid next to the turbine. The electronic control exploded, but other than that, it’s just been routine maintenance all these years.