But as turbines grow larger and blade lengths correspondingly longer, there are signs of greater interest in segmented blades — for transport and manufacturing reasons.
Two pioneering turbine suppliers have already developed a segmented blade, while another is considering using one for its wind turbine.
In the late 1990s, the largest blade transportable by road in one piece was around 34 metres.
Only a few years later this had risen to 40 metres and today, in many industrialised countries, cost-effective road transport is feasible for single blades between 52 and 57 metres.
Developments in transportation equipment has largely kept pace with the lengthening blades, with innovations such as a high-tech trailer that allows a blade to rotate around its longitude axis, and a feature that can vary the distance between truck and trailer wheels to deal with tight and narrow road bends.
But research has continued into segmented blades, in the face of challenges over cost and production time as well as renewed transport issues as blades continue to get longer.
Areas such as the Mediterranean, with its curving roads and dense population, will possibly experience greater problems with longer blades.
Current segmented designs consist of an inner and outer section joined by mechanical fasteners; an alternative to the steel fasteners could be chemical bonding methods.
The design of the joint itself is a major challenge and a major development risk that may have delayed market introduction.
Steel and plastic composite
In 2007 Enercon of Germany introduced the first segmented rotor blade, a 127-metre rotor diameter model with in-house-developed 59-metre segmented blades, for its large 6MW turbine.
These E-126 blades consist of a 24-metre conical, fabricated steel inner section and a 35-metre glass-fibre-reinforced-epoxy composite outer section, with an aluminium blade tip.
Attached to the steel structure is a sub-frame covered by pre-shaped aluminium cladding that gives the inner segment Enercon’s familiar airfoil shape.
Both blade sections are jointed by 20 huge M52 bolts, which are fastened from inside the steel blade structure.
Steel blades themselves are considered relatively heavy and susceptible to material fatigue if not properly designed, but strength and fatigue calculations are claimed to be more straightforward than calculations for composites.
In April, 25 E-126 turbines with up to 7.5MW power ratings were turning.
Around a year ago, Gamesa of Spain installed an in-house-developed segmented blade onto its 4.5MW G128 onshore prototype with a 128-metre rotor diameter.
The Innoblades consist of carbon-fibre and glass-fibre reinforced epoxy composite inner and outer sections connected by a single bolted joint.
The maximum blade section length is less than 35 metres.
Reinforced plastics composites are known for combining low material density (kilogram per metre cubed) with high stiffness, high strength and good fatigue performance.
Gamesa says a major challenge for designers was eliminating the risk of differences in the blade’s bending abilities occurring in the joining surface cross-section.
But by mid March, the company had completed construction of a second G128 prototype.
This April it introduced a G136-4.5MW sister model with segmented blades and an enlarged 136-metre rotor diameter for low-wind sites.
Nordex of Germany is also considering segmented blades for a new 4MW onshore turbine in development. The key reason is easier road transportation.
Progress has been made and, with the ongoing trend for onshore turbines with large rotors, the market for segmented blades could be growing.