Metal proves preferable for wind power

July 2015 - With many industries seeking lighter weight materials, metal isn’t always the go-to choice where composites are available. However, researchers have found metals perform better than fiber-reinforced polymers for wind turbine blades. 

In fact, “it’s a much cheaper solution because of material prices and primarily because” it can be mass produced, says Marco Pröhl researcher at the department of hydroforming and die concepts at the Fraunhofer Institute for Machine Tools and Forming Technology IWU, Chemnitz, Germany. 

The team at Fraunhofer collaborated with the University of Brussels. The researchers produced prototype blades using HC340LA because “micro-alloyed high-strength steels are good for such an application,” Pröhl says.

The turbines are not yet commercialized, “but we changed the rotor of an existing turbine to test our steel blades,” he says. The team’s findings are eliciting notice: the HyBlade project won the Steel Innovation Award 2015 for the “Climate protection with steel” category from the German Steel Trade Association.

Possibilities unfold

The team used airfoil geometry (shapes of wings or blades) to sheet metal specifications. For example, they used bending radii at the trailing edge and simulated the aerodynamic and structural behavior of the rotor under operational conditions in a laboratory environment. According to the team at Fraunhofer, blades were manufactured under near-series conditions using CNC bending, laser welding and hydro forming. Researchers manufactured the entire metal rotor and field-tested the prototype on a 1.5 kW wind turbine along the Belgian coast of the North Sea.

After extensive testing, Fraunhofer engineers found manufacturing blades from sheet metal provided a cost savings of 90 percent compared to fiber-reinforced blades. They also found using metal significantly reduced energy use for blade manufacturing (parts made from fiber reinforced plastics need 15 times more energy than blades made of steel). The blade/rotor can be recycled without degradation, multiple times. 

Weight wasn’t the only consideration when creating more efficient wind turbine blades, but it played a significant role when considering the rotor startup. Pröhl says the researchers also considered titanium, aluminum and magnesium—metals that might be tested in follow-up projects.

According to Pröhl, the weight of the blade is just as critical as the design criteria for larger blades (greater than 10 m). “Since fiber-reinforced plastics are not ideal regarding their ecobalance, metals are a good alternative,” he says. “Especially when you consider building large amounts of turbines that will produce thousands of tons of hazardous waste after their lifespan.”

“Corrosion protection of course plays a role when using metals,” adds Pröhl. “We have protected our steel rotor with a matte, clear paint (base coat and finish). For series applications we would do it the same way as the automobile industry protects their steel car bodies.” 

While plastics may not face the same corrosion issues it also has to be painted in matte colors as well to avoid a “disco effect” discoloration, Pröhl says. 

Maintenance for the metal blade would involve routine inspection. “Metal has a much better predictable behavior compared to fiber reinforced plastics—this is where we clearly see the benefits. MM