Vestas blade plays the long game

A crew of Vestas engineers in white clean-suits inch their way along the mould for what will be the world’s longest wind turbine blade, guided by a grid of razor-thin green laser beams, hunting for minuscule flaws in the laminate.

This high-tech prototype 80-metre blade — being fabricated at Vestas’ West Medina Mills R&D centre on the Isle of Wight, off southern England — has been under development for more than two years, soaking up a large amount of the company’s annual €300m-400m ($400m-533m) spend on R&D.

The new blade is destined for Vestas’ 8MW V164 turbine, the machine specially designed for the coming round of wind-power mega-developments in Northern Europe’s hostile waters.

The road to first fabrication has not been smooth. Vestas launched the V164 to great fanfare in March 2011, but then decided to slow its development to concentrate on returning the company to profitability after a series of disappointing financial results.

The slower pace of commercialisation has had its advantages, however, says managing director of UK operations Rob Sauven. “The prototype will be as near to a production blade as you can get, because all the thinking has been done up front,” he explains.

Senior project manager Alistair Tillman adds: “Offshore, we are looking for reliability, reliability, reliability — this applies to the blade like every other aspect of the turbine. “Robust design does not happen by chance.”

The slimline blade, derived from the design used for its V112-3.0 workhorse, departs from Vestas’ trusty box-shell-and-spar design, instead employing “structural shell” technology, in which the wind loads are borne by the outer surface of the blade, rather than inner supports.

This concept — which will make the blade 10% lighter and cheaper to manufacture in large numbers — was first used in a wood-carbon blade devised in 1996 by NEG Micon, which was then taken into mass production by Vestas for the V82 turbine after the two companies merged in 2004.

The structural shell design is also being used for the blades for Vestas’ new low-wind V126-3.0 machine. “One of the main drivers for this technology is that we have a very low number of structural interfaces, that is, bonded surfaces, because where there is a bond, there is a greater opportunity for failure,” says Tillman.

The construction process involves “laying up” layers of laminates inside gigantic carbon-skin moulds, with a team of three engineers smoothing each one by hand. The next step is for the 150-tonne moulds to be “bagged up” — sheathed in airtight thermal covers — to begin the vacuum-infusion process, in which epoxy resin and a hardening agent are drawn deep inside the shell halves.

The blade uses a combination of dry glass-fibre and pre-cured pultruded carbon (in which carbon fibres have been pulled through resin as they are cured) laid up around a PET (polyethylene terephthalate) foam core supported by three infused-glass sheer webs.

“We have plenty of experience of doing this with our old wood-carbon [structural shell] blades, but we ran a series of mock-ups to determine the right pressures to do this with the 80-metre model,” says Tillman.

“If the infusion is not deep enough, you won’t have the right fibre fraction and the blade will be weak; too much and it will be overweight,” adds Sauven.

The blade also features a lightning-protection package that includes a copper “solid metal tip”.

Testing of the 80-metre model will give new meaning to the word “exhaustive”. After being fitted into the West Medina Mills test rig — a massive steel structure with a 35-metre-deep concrete foundation — and then wired up to 250 strain gauges, each blade will be put through a battery of tests.

These include extreme loads “flap” testing, where each prototype will be flexed almost 18 metres in either direction for one million cycles over three months to simulate 20 years of operation in the field.

At the same time, a range of cutting-edge “non-destructive” testing techniques — shearography (a “CAT-scan for blades”, says Sauven), ultrasonics and x-rays — will be used to ferret out the near-invisible faults that the sensors might have missed.

And the testing doesn’t end when the blade is taken down from the rig. Vestas will apply a Calt (calibrated accelerated lifetime test) programme to check the “reserve life” of a component once it has been through the rigours of the simulated decades of operation, and then a “coupon” will be cut out of a tested blade to further examine the integrity of its material.

“For a blade to have 20-year life, statistically, when you get down to the coupon, you’ve got to show the materials has way more than 20 years’ life left in it,” says Sauven.

“In theory, if a blade is built perfectly, then it will have an infinite life.

“The test rigs run so many years ahead of the prototypes that the real de-risking is all done in advance.

“The risks involved in going from a prototype to an offshore wind farm [of V164s] are something that everyone is very, very aware of. Everything we are doing is to prove to ourselves and our customers that these risks are and will be completely minimised and managed.”

Long gone are the days, notes Sauven, when “we’d buy materials off the guy next door, build a blade, test it and carry out some very basis 2D analysis”.

The first set of 80-metre blades are slated to be ready for in-house trials by the second quarter of this year, with Vestas on track to start commissioning a prototype V164 a year later, in which it will go through 12 months of testing and certification of the machine’s power curve.

Manufacture of “zero series” versions of the turbine — the blades for which could be manufactured at the Isle of Wight facility — could start as early as 2015, so long as “a satisfactory firm order pipeline is in place”, according to Vestas.

“We will have tested every component, every module and every system before the prototype is flying,” says Sauven.

“The prototype is not about seeing if it works: we will have already proven the concept to ourselves on the test rigs. It’s to show our customers and their banks that it works.”