Close up - Mitsubishi's 81-metre SeaAngel blade

GERMANY: Last week Mitusibishi took a big step towards completing its SeaAngel offshore turbine when blade manufacturer Euros despatched the first 81-metre blades for testing. Windpower Monthly technology writer Eize de Vries spoke to Euros about the blade and its evolution.

The 81-metre SeaAngel blade
The 81-metre SeaAngel blade

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Euros delivered a first in-house developed offshore blade from its prototype production facility on the German Baltic Sea island Rügen to Mitsubishi of Japan. The 81.6-metre blade weighs 32.5 tonnes and is currently the world's second longest — the largest is an 83.5-metre blade made by SSP Technologies for a Samsung turbine. Euros designed the blade to last 25 years for Mitsubishi's 7MW SeaAngel offshore turbine featuring a 167-metre rotor.

The first test blade has been sent to Bremerhaven-based German wind technology institute Fraunhofer IWES. The institute will conduct extensive static and dynamic load testing on its test rig, effectively compressing the blade's lifespan into five to eight months. Speaking about the testing, Euros chief technical officer Hinrich Graue said: "The main aim is to ensure that the structural design model and manufacturing process fully comply with blade calculations and verification modelling results, and key certification requirements."

Long-term experience

In 2009, Euros launched its own offshore rotor-blade programme. But its experience dates back as far as 2003 when it manufactured 56.5-metre Aerodyn-designed blades for the 5MW Multibrid (now Areva) M5000-116 prototype. Graue said: "Aerodyn Energiesysteme designed this rotor blade, but we jointly developed the production technology. The fact that this blade contains carbon fibres in highly stressed sections substantially helped us understand the physics and underlying mechanisms, especially in keeping dynamic loads under control."

Graue stressed that these combined design and manufacturing experiences have proven useful when moving to the next levels in rotor blade design. "However, scaling up from state-of-the-art 55–60-metre long blades to our latest 81.6-metre rotor blade represents a step of a completely different magnitude. As it involves an significantly increased innovation ratio, each step in the development process has to be considered very carefully," he said.

Joint programme

All aspects regarding design, manufacturing, transport/logistics and long-term upkeep were carefully evaluated in a joint research and development programme with Mitsubishi. Extensive wind-tunnel tests had to ensure the highest aerodynamic performance, and resulted in a "thick" yield-enhancing trailing edge for the rounded inner blade section called "flat-back".

Structurally, Euros engineers incorporated four (2x2) load-carrying reinforcements in between the shells. Called spar caps, they are made of carbon-reinforced epoxy laminate and a new core filler material originating from the aerospace industry. Graue said: "This core material is a new kind of foam, characterised by favourable specific mass [kg/m3] and reduced resin demand. Innovative materials usage, tailored production processes and a focus on the highest possible manufacturing quality all contributed to optimal strength, stiffness and durability plus a favourable blade mass. Our first blade was manufactured within less than 1% mass deviation from the target design value."

In parallel to the test blade delivery, Euros built two SeaAngel blade sets. The first is destined for an onshore prototype in Scotland for later this year, the second set for a floating offshore turbine envisaged near Fukushima, Japan, during 2014.

Euros' headquarters and design office are both in Berlin, while rotor-blade production is concentrated in southern Poland. A massive expansion programme will be finished this month, increasing total production capacity five-fold to 600MW annually.

"Our facility at Rügen is only temporary and proved convenient for the Mitsubishi prototype blade project, including the initial sets of pre-series blades. In the future, Euros plans to establish a new offshore wind facility in the port of Rostock for both pre-series and serial production, serving international clients from the blade design phase to serial production", Graue concluded.

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