They boost a turbine's energy yield at the same time limiting rotor and turbine loads, which in turn contributes towards reducing lifecycle-based costs of energy. By enabling turbines to generate more in lower wind speeds, they can also generate more power throughout year.
Long blades can even improve wind's ability to supply the network with base-load power and improve wind's utilisation of power cables and transformers.
Suppliers that already offer long, slender blades include Vestas, Siemens, Nordex, LM Wind Power and Enercon. Siemens' 75-metre B75 blade currently holds the length record, beating LM's 2011 record of 73.5 metres.
A key characteristic of slender blades is a narrow airfoil width in relation to total blade length. But, as blades get longer it becomes increasingly challenging to build in sufficient strength and stiffness to protect against blade deflection (bending) under load and prevent blade tips hitting the tower.
Other than the use of costly carbon fibres, makers have looked at stiffening blades by extending The relatively thick rounded root section. However, a cylinder moving through the air creates resistance rather than aerodynamic lift. A counter measure is to fit a spoiler to create a lift force similar to that created by airfoils. Vortex generators comprising multiple small fins and mounted at the lowest part of the airfoil can also boost blade performance.
Gerard van Bussel, a professor in wind energy at Delft University of Technology in the Netherlands, has been researching the future use of thermoplastic materials for very long blades. "Compared to thermoset composites, thermoplastic materials offer several benefits, including the fact that modular serial production will become possible and individual sections can be welded," says Van Bussel. "Unfortunately imperfect adhesive bonding between the thermoplastic resins and fibres in the composite matrix hampers adequate loads transfer. As these issues still have to be addressed, large-scale commercial application will be something for the longer term."
Delft researchers and international partners are also working on so-called smart solutions for power output control. Ideas being researched include flaps for the rotor-blade trailing edge controlled by memory materials, and piezoelectric elements.
Memory material would allow parts of the blade to temporarily take on a different shape during certain load conditions, such as high winds. Piezoelectricity is a way to produce electricity from continuously changing mechanical pressure during motion. By applying electric current to into piezoelectric elements connected to a blade part, control movements could be introduced on demand.
However, both solutions are still at the laboratory stage. The researchers are not aiming to eliminate state-of-the-art output control through blade pitching, but support it to deal with small variations in conditions.
Most rotor blades, including the longest units of up to 75 metres, are still manufactured in a single piece.
Only Enercon and Gamesa use segmented blades for their most powerful machines. California-based Modular Wind Energy has developed a 45-metre blade that can be transported in 15-metre segments.
US-based Blade Dynamics, part-owned by AMSC, is also developing segmented blades.
The blades are made in small individual components, while the patent-pending blade technology enables shipping in two sections. This modular technology greatly improves quality and performance, says company spokesperson Theo Botha. Design innovations include a lightweight root attachment and modular central spar-joining technology. The blades are very light and generate low machine loads compared with similar products: six tonnes for the D49 versus eight tonnes for a competitor's 49-metre blade.
"For a 2MW AMSC turbine these combined load and mass-reducing features enable a rotor diameter increase from the standard 93 metres to 100 metres without any turbine modifications," says Botha.
Most importantly, they boost annual energy yield by 6-12%, which significantly improves financial performance, he adds.