This company profile on Dutch blade manufacturer Aerpac reports on the company's view of the future. Aerpac is opening a new 6500 square metre production facility and the old production works will be closed. There is a growing desire among wind turbine customers for blades to be produced locally. Aerpac is making blades in Spain, the US, Scotland and India. The company says the technical problems of large machines, especially with regard to rotor loads, is not fully understood and more time should go on testing prototypes, though pitch controlled, variable speed rotors have encountered few problems.

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The market for wind turbine rotor blades will grow at a respectable rate of 25% a year, predicts Rob Roelofs, commercial director and one of the shareholders of Aerpac BV in Almelo, the Netherlands. With 150 workers, Aerpac is one of the largest suppliers of blades for wind turbines, with an expected turnover this year of NLG 30 million.

This month the company is opening a new 6500 square metre blade production facility at Almelo, next to an existing Aerpac works used for testing and developing prototypes. The old production works in Hengelo will be closed.

Aerpac started in 1989 as a management buy-out of the wind turbine division of large Dutch engineers, Ventilatoren Stork Howden. The purchase was backed by the county council, the OOM. Of the four founding fathers, one will withdraw this year as a shareholder, but remain in the company. The participation of the OOM is not infinite and Aerpac will shortly be looking for fresh capital. With rapid growth predicted, finding a new investor should be far less difficult than it was six years ago when the outlook for wind power was decidedly gloomy.

The expected growth will mainly be overseas, according to Roelofs. "In northwest Europe the siting of wind turbines is becoming an ever bigger problem," he says. "Growth here will be small, at least in comparison with area's like India, China and Mongolia. Still there is room for growth in western Europe. In Spain there are large areas available for wind farms and the potential in France and Italy has hardly been explored."

Production in Spain, Scotland and the US

There is a growing desire among wind turbine customers for blades to be produced locally using local labour, says Roelofs. The cost of transport for ever bigger blades is becoming prohibitive. The job is also labour intensive and using local workers will usually pay off, even if the higher productivity of countries like Holland and Denmark is taken into account. In response to this localisation of production, Aerpac has set up a joint venture in Spain, Aertusa (Windpower Monthly, April 1995). For the US market Aerpac blades are being produced by Merrifield Roberts in Rhode Island. Another production line in Scotland, near Edinburgh, should start turning out blades for the UK market in 1996, while Aerpac is now researching the possibilities of blade production in India.

As a supplier Aerpac works in close co-operation with wind turbine manufacturers. Its main customers are WindWorld and Micon from Denmark and Enercon and Hanseatische AG in Germany. Until now most wind turbine manufacturers have either required suppliers to make blades to their own specifications, or made their own blades. But Roelofs says this will change towards a car manufacturing model, where wind turbines will be assembled from parts made by certified suppliers. Certification is not enough though; there also has to be a long standing working relationship if a blade is to be optimal.

Size and the unknown

As wind turbines get bigger, says Roelofs, the fine tuning of the whole structure is becoming increasingly critical. He speaks from bitter experience. Earlier this year two Micon 600 turbines were halted after cracks were discovered in their Aerpac 19 m blades, the APX 40/500-T. Extensive research has shown the cracks were caused by aero-elastic instabilities, says Roelofs. That looks like another way of saying "we don't know," though Roelofs prefers to add "yet" to the end of that statement.

"What we do know is that the cracks have nothing to do with the quality of the blade. Other turbine manufacturers haven't got any problems with our 19 metre blade, while on the other hand some of our competitors have run into similar problems," he comments, pointing to problems reported with similar sized blades from Danish companies LM Glasfiber, the world's largest wind turbine blade company, and turbine manufacturer, Vestas (Windpower Monthly, April 1995). Roelofs says these kind of problems arise because of the behaviour of large turbines (over 450 kW) which is different from that of smaller turbines. "You just cannot scale up from 300 to 600 and then to 1 MW," says Roelofs. "This is partly because the frequencies of the rotor and the tower, combined with instant changes in wind direction, cause effects that cannot be predicted with existing models."

He says wind turbine manufacturers must take more time to test their prototypes under different conditions before starting commercial production. "I'm worried that this type of problem will occur more often because there are some companies that have no experience with turbines, but want to start from scratch with the production of 750 kW units. I'm afraid they are doomed to fail, because turbines, especially large turbines require a long learning curve."

Problems with blades are not limited, though, to 500 kW turbines and beyond. In 1994 defects were reported on Aerpac blades on 300 kW units in Greece, supplied by Dutch-Belgian company, HMZ WindMaster (Windpower Monthly, November 1994). The Greek problem is still being investigated, but according to Roelofs it has been confirmed that the failure was not due to faulty blades. Again the root of the problem seems to lie in a combination of causes, says Roelofs. The WindMaster units in northern Greece should begin producing electricity again next year, he says.

Safety in variable speed

Recent problems with blades on large turbines have mainly occurred on either stall controlled machines, or pitch-controlled turbines running at fixed speed, says Roelofs. Pitch controlled, variable speed rotors have encountered very few problems so far. For Roelofs that confirms the necessity of proceeding much faster with the Flexhat concept, long under development in the Netherlands by the wind division of the national laboratory, ECN, with input from Stork Product Engineering, Aerpac and NedWind. Development costs of the Flexhat project are met with subsidies from Dutch agency Novem and from the European Commission's Joule programme. In essence Flexhat consists of a flexible, variable speed rotor with passive pitch control. That means the blade angle varies automatically with rotor speed. The Flexhat concept has met with renewed interest because it opens the possibility for weight reduction on the bigger rotors needed to drive turbines of larger installed capacity. "It is a very interesting concept for either enlarging capacity on existing towers or designing turbines with an installed capacity of 2 to 3 MW," says Roelofs.

Within the Flexhat framework Aerpac is working on an integrated passive pitch control system. Most of today's turbines are equipped with active pitch control where the blade angle is adjusted with hydraulically operated steel pull bars. The only exemption is the Dutch Lagerwey turbine, which has passive pitch control.

A lightning standard

Aerpac is also looking at ways of integrating lightning protection on its blades. Today the company's blades are fitted with a "lightning spot" -- a metal cable moulded into the blade which conducts electricity to earth.

More important than lightning devices themselves, though, is the establishment of standards for lightning protection and test procedures to confirm these standards. "Insurance companies are increasingly asking for guarantees against lightning damage," explains Roelofs. "We are now involved in developing a standard and the accompanying test procedures. One of these is the actual bolt of lightning. At the Kema institute's high voltage laboratory in Arnhem we have run 22 bolts of lightning through our blade. Of these, 21 caused no damage, giving a reliability of 95%. These are figures that insurance companies can live with."

Looking to the future, a confident Roelofs comments: "As the wind industry matures, price and guaranteed quality are becoming more important. An independent blade supplier can play a key role in lowering prices and enhancing performance."

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