Manufactured from great sheets of steel that are curled into cylinders up to eight metres
in diameter and seam welded at the factory, tower sections arrive onside in sections to be stacked
up. The tower is too big to be moved in one piece, either for ships at sea or conventional trucks,
so modular on-site assembly of prefabbed elements is clearly the way to go.
Craned into position and stacked one by one, the tower elevates the turbine fan
up above the aerodynamic sweet spot of 30 metres above ground,
where winds are usually less turbulent and more desirable from an energy-harvesting
standpoint. The tricky part here is aligning the different tower sections as they rise,
with huge components in windy locations up against engineering tolerances of just a few millimetres.
Atop the shaft sits the brains of the operation, also known as the nacelle. Crammed with upwards of
1,500 bespoke components, the nacelle houses the generator and drivetrain, inverters, sometimes
a gearbox, as well as assorted monitoring, communications and environmental maintenance gear.
It’s affixed to the tower via a sophisticated rotational ‘yaw’ system, which allows the nacelle,
and by extension the turbine blades, to turn and face the wind whenever it changes.
The nacelle itself can be incredibly heavy,
with one recent IEA offshore 15 MW model weighing in at over 800 tonnes.
How on earth is that installed? With great difficulty. Mighty cranes,
lift wires and supporting tugger lines are part of the process. At sea, the impact of wind on the
dangling nacelle is surprisingly less of an issue than the oscillations of the tower itself, as it’s
battered by waves and currents below. Careful mathematical modelling is carried out before
every installation, supported by clever civil engineering gadgets called ‘tuned mass dampers’,
which go some way to help align vibrations between these two massive components before mating.
Of course much of the nacelle is prefabbed before being shipped to
site. Construction bosses need to make tough choices over what’s efficient to make onsite,
against weight considerations, and even simple limits on available deck space.
Once in place, and covered up with a weather-proof fibreglass gondola,
the blades can now be attached to the hub.
Engineers have come to favour a three-blade turbine format. Even numbers of blades
cause problems with resonance, which leads to unseemly wobbles that in turn cause expensive
and frustrating wear and tear. In theory one blade works just fine, or 9 blades,
although they suffer from exciting-sounding ‘vortex issues’. Fundamentally, engineers
are pragmatists, and want an economical, scalable solution. For now at least, that’s three blades.
These blades – which nowadays can stretch over 100metres long,
and be four metres around at the root – were once made of aluminium,
but at current scales that’s far too heavy so fibreglass is now in vogue.
And in case you were thinking the job of winching up a 100m long,
55 ton fibreglass blade – a blade that’s literally designed to catch
the wind – 150 metres in the air, at sea, is easy. Well, it’s not easy.
Each blade root is encircled by guide pins that marry up with flange holes
in the hub. How do engineers compensate for the wobbly tower, and motion-prone
blade? Those passive tuned mass dampers we mentioned earlier play something of a role.
But in recent years engineers have dreamed up two new techniques,
both of which sound a bit like cheesy anime villains.
‘Blade Dragon’ is a lifting yoke that attaches to the crane
and holds the blade more firmly than a regular hook. The advantage here
is it can be affixed to the hub at an assortment of inclined angles.
‘Boom Lock’ is a smart system that sets out to eliminate, or at least compensate for,
any lateral movement on the crane hook, so the blade stays steadier. These techniques make great
economic sense, as they work at relatively high ambient windspeeds using fewer human operators,
ultimately getting the installation done quicker and therefore cheaper.
Indeed, modern wind farms can erect the tower,
install the nacelle and the blades in a day, with just a little extra time for
calibration of the blades pitch angle and optimal yaw before connecting to the grid.
As turbines get taller and more ambitious,
construction obviously gets trickier. There’s a limit to how tall a crane can usefully be,
especially at sea, but some companies in the field – like Mammoet – have developed bold new
solutions; spin-off technologies, if you will. Take this WTA, or ‘Wind Turbine Assembly’ crane.
Instead of being rooted to the ground, or on a jacked-up boat, the WTA 250 uses temporary guide
rails attached to the turbine tower itself. Capable of lifting a hefty 250 metric tonnes,
it’s superior because it has no footprint, and can theoretically rise as high as the tower itself.
Minimising the role of humans in what is often a perilous working environment
is another priority as the turbine construction field matures.
Already, ultra-sophistical mathematical models guide design and development of turbines and
wind farms, and there’s an increasing role for AI and software engineers to
play in conceptualising the next generation of taller, cheaper, more efficient turbines.
So the rewards are massive for any young nerds out there who wish to study the blade.
What do you think? Are modern wind turbines ugly blots on the natural landscape,
or inspiring monuments to human ingenuity and progress? Let us know in the comments,
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