Three Blades or Two Blades?

Just when we all thought that the three-bladed wind turbine was the industry standard,  several major wind-power companies are testing a departure by dropping one of the three blades and spinning the rotor 180 degrees to face downwind.  The attraction is that if a two-bladed turbine can produce as much energy as a three-bladed one, the cost of manufacture will be around 20% cheaper and of course there’s less maintenance.  So who are the people looking at lopping off a blade?

China’s Ming Yang Wind Power is the world’s ninth-largest wind-turbine manufacturer and they recently announced plans for the largest test of the two-rotor design to date. The company plans to build a six-megawatt, two-bladed turbine in China this year that will generate as much power as the largest commercial offshore turbine. Ming Yang also plans to build another in Norwegian waters next year.

Ming Yang prototype tw bladed wind turbine 300x2001 Three Blades or Two Blades?

Hitachi and Fuji Heavy Industries collaborated in 2013 to build two two-megawatt wind turbines that use the two-blade downwind design.

In March of this year Dutch wind-tech developer 2-B Energy raised £26.5 million to fund the installation of two downwind, two-bladed, six-megawatt turbines in Scottish waters.

In most environments, offshore wind blows steadier than most places on land. But installing and maintaining offshore wind turbines is expensive, costing twice as much as onshore wind turbines on average. The simple truth is that two-bladed turbines cost less than conventional three-blade turbines because they use fewer materials. The removal of one blade makes the rotor lighter, which in turn makes it possible to place the rotor on the downwind side of the tower.  Conventional wind turbine rotors face the wind and must resist bending back into the turbine’s tower, but downwind rotors can use lighter (and even hinged) blades that bend away from heavy gusts.

It’s not just the cost of two blades as compared to the cost of three, there are other economies: The lighter more flexible rotors yield savings in the turbine’s gearbox, tower and foundation.  As an example the 140-meter diameter rotor, gearbox and generator for Ming Yang’s six-megawatt prototype weighs just 308 tons—about 40 tons less than those of offshore market leader Siemens’s conventional six-megawatt offshore turbines.

That’s not all; two-bladed wind turbines are easier to install. Whereas the three-bladed rotors spinning in today’s offshore farms must be assembled on site, two-bladed rotors can be pre-installed on the turbine’s machinery onshore.  The assembled package then fits more conveniently on ships and is light enough to lift onto the tower.

Broken two bladed wind turbine 300x2251 Three Blades or Two Blades?

But it’s not all positive- the two-bladed downwind design is noisier as it spins faster- although this is hardly a problem for off-shore wind farms. Of greater impact is that in some wind conditions, the flexible blades can spring back and hit the turbine tower. In 2002 this problem caused the abandoning of a major attempt to revive two-blade turbines!