Which is why it is having a go at making steel ones, of which 90% can be recycled.

“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,” it said.

The one in the photo is formed from a 1.0mm steel sheet and features integrated folded reinforcement. Final shape comes from hydro-forming with an oil-water mixture.

Production is: fold a shaped flat sheet in bending die to make a typical blade shape, laser weld the edges to seal the structure, put the proto-blade into a tool internally-profiled to the final blade shape, then inflate the blade inside the tool using several thousand bars of pressure.

“The geometry ends up perfect, with the blades matching the flow profile milled into the tool to within 0.1mm,”said researcher Marco Pröhl.

A blade 150mm wide and 300mm long has been made. The next step is an entire rotor for a vertical axis turbine with 2.8m-long blades and a diameter of 2m – which wil then be installed at a turbine test site on the Belgian coast.

Fraunhofer IWU is working with the Free University Brussels.