Taking turbines to breaking point

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Spain's national wind turbine test laboratory says it has become a fully-fledged one-stop shop for the industry, offering a range of facilities that include full-scale drive train analysis and fatigue testing of rotor blades with lengths of up to 50 metres. More testing facilities are planned

With the recent completion of a EUR 20 million drive train laboratory, Spain is now home to "one of the top public wind turbine and blade test centres for big turbines" in the world, says Jesús Busturia, operations director of Spain's National Renewable Energy Centre, Centro Nacional de Energías Renovables (CENER). Funded with public money and officially inaugurated by King Juan Carlos I in September, the national wind turbine test laboratory, Laboratorio de Ensayos de Aerogeneradores (LEA), is located in Sangüesa, near Pamplona, the capital city of the far northern region of Navarra.

CENER developed, owns and now operates the laboratory alongside other renewable energy test centres in Pamplona. Spanning 30,000 square metres, LEA is dedicated to testing drive trains, generators and blades for large wind turbines ranging in capacity from 2 MW to 5 MW. A blade test facility for certifying complete blades for turbines with rotor diameters of up to 75-100 metres has been operating since 2006, together with a composite materials and manufacturing processes laboratory and a division for testing turbine performance in the field. With the addition of the drive train laboratory, LEA is now a comprehensive resource for turbine testing, says centre manager Gurutz Urzelai. LEA is testing drive trains, generators, nacelles and blades "all under one roof and we are doing it for any customer," he says.

Turbine test site

Facilities for testing even more components under research and development (R&D) are planned, which will take total investment in LEA to EUR 50 million. "Next year there will be six turbines turning up there in our experimental wind plant, each up to 5 MW and all undergoing vigorous testing in complex terrain," says Urzelai, pointing a finger at the ridge of the Alaiz mountain outside his office window. Swapping to a thumb over his shoulder he points in the other direction. "Round the back, we've reserved a site for carrying out test runs mounting the complete nacelle on top of a tower; a unique practice ground, saving on expensive learning curves in the field," he says. In 2010, CENER hopes to add a wind tunnel. When fully complete, the entire LEA will cost around EUR 6 million a year to run.

Clients are already lining up to use the new drive train laboratory. These are mainly Spanish turbine manufacturers, although a few component companies and foreign turbine makers are also showing interest, says Urzelai. "It looks like we have got enough to keep us in business over the next six years, with some gaps for new customers."

CENER director general Juan Ormazábal reveals that a prototype drive train from wind turbine supplier Gamesa will be the first to undergo trial on the LEA test bed. "We have a launch agreement, with special conditions," he says. "LEA plays an important role in Gamesa R&D and, in particular, in the design and development of our new high power turbine in the 5 MW range," adds Gamesa, although the company declines to reveal further details.

Rigorous tests

The Gamesa 5 MW prototype will certainly be taken through its paces. "The bashing we give to the drive train over six months is equivalent to what it will receive in the field over a 20-year lifespan," shouts Urzelai above the noise in the new laboratory during a tour of the facility. "That helps manufacturers avoid producing prototypes in series."

While he talks, an eight megawatt capacity motor, simulating a rotor's revolutions, gets to work on a 5 MW drive train. The turbine's actual generator is substituted by a higher load, as the 8 MW motor would burn it out otherwise. Meanwhile, other rotor forces are also simulated using hydraulic mechanisms, buffeting the drive train across the axis at different angles. There are also hefty jolts both ways along its axis. To monitor all this, the customer's equipment is riddled with sensors, feeding a data logger.

In another part of the same laboratory, the real generator is tested using a 6 MW motor. Tests for overheating, overspeed and overloading are conducted, together with sudden breaking and accelerations, with steps of up to 300 revolutions per minute. In yet another area, the entire generating unit is run to test its short circuit response and capability for riding through sporadic voltage dips on the grid network without tripping offline, now an obligation for all new turbines under Spain's grid code.

On the other side of the building, blades are put to the test at the certified blade test centre. They are subjected to forces equivalent to 20 years of operation in the field in a three-month period. The customer can then use the resulting data when applying for type certification for the blade and turbine in question.

One area houses the dynamic blade test unit, testing for stress and fatigue. Here, a blade up to 50 metres long has five motorised clamps along its length, set to oscillate the blade at its resonant frequency.

At the other end of the floor, another blade is mounted for static testing. Motors will bend it to just beyond nominal maximum strength. "That will take the blade to a single extreme it is unlikely to see in the field throughout its lifetime," says Urzelai. "We can take the blade to break point if the client wants." This, he says, can show if a manufacturer is spending too little or too much on blade strength.

Seeking guarantees

"Throw-away finance is suddenly a thing of the past and banks demand solid guarantees," adds Ormazábal, noting the impact of the global credit crisis. He says that, unlike for blades, certification bodies do not require testing for complete nacelles and drive trains. Instead, these are certified on design-only at present. Ormazábal thinks this might change or that banks at least will demand some form of testing before agreeing to finance.

"Even if the banks do not start requiring drive train testing, the manufacturers themselves will, to reduce risk and improve output and operational hours before hitting series production," he adds, suggesting it is a price worth paying. It costs around EUR 1 million to put a turbine through the whole drive train test procedure. It is "a drop in the ocean" compared to gains through efficiency and reduced risk, says Ormazábal.

Meanwhile, CENER is likely to share its experience with the US government's National Renewable Energy Laboratory (NREL) in America. NREL currently tests blades up to 50 metres in length and gearboxes for machines up to 2.5 MW. It "wants to upgrade and we are in talks to help with that," Ormazábal says.

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