Why Two Blades?
Two Blades or Three?
While on the surface it may seem that adding more blades to a wind turbine would enable it to extract more energy from the wind, this is actually not the case. In fact, trying to extract too much energy from the wind causes it to flow around the wind turbine rather than through the blades, leading to a drop in power output. The theory governing this is called Betz's law and all wind turbines, regardless of the number of blades, are subject to this natural limit.
Albert Betz based his theory on a wind turbine with an infinite number of blades. Compared to that, it's clear that two or three blades is simply a design decision. While the international wind industry is dominated by three-bladed turbines, there are many advantages to two-bladed rotors.
Two-Bladed Rotor Design
Two-blade wind turbine rotors offer fundamental advantages in light weight and low cost—if they teeter stably. The rotor assembly of a mainstream three-blade wind turbine moves with one mechanical degree of freedom: it rotates. As blades turn and bend unequally due to turbulence, wind shear, and tower shadow, the resulting bending moments are reacted by the bearings and structure supporting the main shaft.
The rotor assembly of a teetering two-blade wind turbine moves with two mechanical degrees of freedom: as it rotates, it teeters on a hinge between its hub and main shaft. Within engineered limits, this teetering can be mechanically free, eliminating bending fatigue loads through the main shaft due to turbulence, wind shear, and tower shadow. Eliminating these fatigue loads enables reduced weight throughout the main bearings, gear case, mainframe, yaw bearing and tower. See the brief video below showing how a wind turbine rotor teeters.
But the caveat above still applies: the rotor must teeter stably.
Two-bladed Teeter Control System™:
The teetering motion of a Windflow wind turbine rotor is linked to a passive, asymmetric pitching of the turbine blades which stabilizes the rotor. For every one degree of teeter, the blades pitch asymmetrically about one and a half degrees. This helps to stabilize the teeter motion without reducing the advantages of teetering. Windflow's package of hardware and controls that does this is called the Teeter Control System, and it is an area of expertise for the Windflow team and a major part of our internal know-how.
To learn more, read our article Teetering Toward Two-Blade Turbines (published in Windpower Engineering & Development).
Click here to learn why Windflow specializes in mid-size turbines.