Generating eco-friendly power with metal rotor blades

May 7, 2015, Fraunhofer-Gesellschaft
Technology demonstrator: formed from a 1.0 mm steel sheet and featuring integrated, folded reinforcement, the rotor blade was given its final shape with the help of an oil-water mixture. Credit: Fraunhofer IWU

Wind turbines deliver environmentally friendly electricity. Yet the fiber-reinforced plastics often used in very large rotor blades are almost impossible to recycle. Not so with steel blades: since these are composed of steel, their recyclability exceeds 90 percent. Plus they cost significantly less than comparable plastic blades.

Wind turbines feed eco-friendly power into the grid. To keep their weight down, the majority of larger rotor blades are made from fiber-reinforced plastics. These materials are rarely recycled at present, in part because it is very complicated to do so. Researchers at the Fraunhofer Institute for Machine Tools and Forming Technology IWU in Chemnitz are therefore focusing on metal, and especially steel, as a blade material. In smaller installations, the greater weight of the steel blades is inconsequential; as installations get larger, light alloys can be used to keep blade weight down. Collaborating with colleagues from the Free University Brussels (VUB) in the HyBlade project, Fraunhofer IWU is developing the required aerodynamics as well as the necessary manufacturing process chains. Manufacturing steel blades offers numerous advantages. "First, it makes turbines significantly more ecological, since more than 90 percent of the steel can be recycled – so using metal makes wind power truly environmentally friendly," explains Marco Pröhl, a researcher at the IWU. "What's more, compared to similar blades made of fiber-reinforced plastic, the cost of rotor blade mass production drops by as much as 90 percent – and the blades can be manufactured more accurately."

Metal blades can also be produced more quickly. Provided that processes are run in parallel – for instance, that a new metal sheet is fed into the production line as soon as the first blade has completed the first process step – then a completed rotor blade rolls off the conveyor belt roughly every 30 seconds. With fiber-reinforced plastics, the same process usually takes several hours.

Suitable for large-scale and automated manufacturing processes

The primary cause of these differences lies in the manufacturing process. Fiber-reinforced plastic blades often require significant manual processing: first, a suitable mold has to be made for the blades. Depending on the production variant, workers layer fiber mats in this mold, inject resin, and leave the component to harden for several hours in an oven. This produces two half shells; once their edges have been trimmed, the halves can be glued together. These steps can be performed simultaneously, as in sheet metal blade manufacturing – but that doesn't make them any quicker. It would take dozens of installations running in parallel to produce plastic blades at the same rate as metal ones.

In contrast, it is easy to automate the manufacturing metal rotors: the processes are similar to those in the auto industry, which means they are suitable for series production. The researchers start with a flat sheet of metal, which they fold using a bending die to give it a typical blade shape. Next, they laser weld the edges to form a closed profile. After placing the preformed piece in a tool with the desired final shape, the researchers then pump a reusable water-oil mixture into the interior of the blade and put it under several thousand bars of pressure. This is equivalent to the pressure experienced underwater at a depth of many thousands of meters. This effectively inflates the blade, giving it its final form. "The fact that we're shaping the blade from the inside out lets us compensate for any inaccuracies in previous steps," explains Pröhl. "The geometry ends up perfect after the first production step, with the blades matching the flow profile milled into the tool to within 0.1 millimeters."

The researchers have already produced a blade 15 centimeters wide and 30 centimeters long, using it to optimize the individual processing steps. Their next step will be to produce an entire rotor for a vertical axis turbine with 2.8-meter-long and a diameter of two meters. Once it is installed at a test site for small on the Belgian coast, it will be put through its paces.

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4 / 5 (1) May 07, 2015
I'm suspicious of this advert. Metal blades would be an obvious automatic first option for wind turbines.

Many smaller windmills at farms for water pumping DO have metal blades.

I have a very strong suspicion that metal blades aren't used on large turbines for very good reasons. Otherwise, they wouldn't have passed over that option so quickly in the first place...

This manufacturing method they're suggesting isn't exactly innovative. It is essentially right in line with what you would expect them to do if they wanted to make metal turbine blades on an industrial scale.
1.5 / 5 (4) May 07, 2015
"Many smaller windmills at farms for water pumping DO have metal blades."

Those have flat planes, not aerodynamic surfaces.
2.6 / 5 (5) May 07, 2015
Metal blades seem more eco-friendly by quickly slaughtering birds and bats, preventing these unfortunate animals from suffering too much.

2.4 / 5 (7) May 08, 2015
Willie, give it up. You already lost.
5 / 5 (4) May 08, 2015
I have a very strong suspicion that metal blades aren't used on large turbines for very good reasons

While they say that for larger installations some alloys can "keep the weight down" I doubt they can reach the same stability as fibreglass blades for the same weight.
(OK, you could use titanium blades. But those would be WAY more expensive and titanium is also very hard to work with so the advantage of "one blade every 30 seconds" would drop by the wayside)

Weight determines at what minimum wind speed your installation works. And that is a very crucial number.
1.8 / 5 (5) May 08, 2015
Willie, give it up. You already lost.
I do agree.
Lucky birds, now with titanium blades instead of fiberglass, they will suffer much less when they are slaughtered. It is really a true Eco-friendly technology.
1.7 / 5 (3) May 09, 2015
I like the sheet metal design of the fan blade.

Very impressive lateral thinking.

Its a wonder someone never thought of this sooner.

The central spar formed in the blade could be laser or plasma machined as a Warren Truss or equivalent, to reduce weight and improve structural efficiency.

The trailing edge is not as sharp as a classical wing profile but should be sufficient if its thickness is less than 1% of chord.

The steel design also allows welding of structural attachments as required - something a plastic blade can't do simply.

Finally, the folding sheet metal design allows for a 'conically' cambered blade - one with taper.

Its like a slender paper funnel being pressed flat.
5 / 5 (3) May 10, 2015
I have a very strong suspicion that metal blades aren't used on large turbines for very good reasons.

Metal fatique and fracture growth leads to sudden and violent failures.

When you got something that weighs tons and measures in the hundreds of feet rotating around, made of thin sheets of steel, you risk the whole thing tearing apart like the De Havilland Comet. It goes happily through some largely unpredictable number of stress cycles and then just snaps.

Fiberglass at least has some resistance to failure, and more importantly, to corrosion. If you want to keep steel blades for 30 years, you got to keep cleaning and painting them.

5 / 5 (1) May 11, 2015
Its a wonder someone never thought of this sooner.

It isn't, because they have.

The Chinese are cranking out aluminium extrusion blades for wind turbines, and you can buy them online from Alibaba. Minimum order 1000 pieces, $2-6 a piece. Aluminium too, of course, has the problem of cracking under repeating stress - even more so than steel.

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