Close up - Blade Dynamics' innovative modular blade technology

Sited next to a Saturn V rocket booster displayed at the historic US government NASA site, Blade Dynamic's New Orleans production facilities, housed in decade-old buildings, feature a state-of-the-art manufacturing environment with stable temperature and fixed humidity control. These are essential conditions for producing a consistently high quality product, explained technical director Paul Hayden.

Traditional blade manufacture involves main composite components like the upper and lower shells and inner reinforcements made to their full length before further final assembly. Blade Dynamics in contrast manufactures its blades split into relatively short easy to handle individual components that can fit into standard 40-foot containers. These smaller parts are easier to make at very high quality, according to the company. A series of relatively uncomplicated moulds are used for the individual inner and outer parts, with assembly done in laser-aligned jigs that are easy to install and dismantle again.

An inner spar technology is core to all Blade Dynamics blade designs. Multiple multi-layer carbon fibre-reinforced epoxy sections are put into a precision mould that can produce repeatable, high quality single spar members. The spar members integrated into structurally stiff box-type structures carry the main structural loads. "The outer shell elements are built predominantly in glass-fibre reinforced composite and their main function is aerodynamic cladding, but they also contribute to the blade's structural integrity," said Hayden.

Thin walls

The patented blade root is another key design. The blade root of conventional blades comprise a thick-wall circular section integrated into the shell's structure. Blade bolts are commonly assembled by the 'IKEA' method, with multiple holes drilled into the root circumference to place the nut, with corresponding blade bolts for attaching blade and pitch bearing drilled vertically in the root's wall section. "Conventional blade roots are inherently heavy and cause high local composite material stresses", Hayden added. "Our root section instead comprises opposing thin-wall inner and outer sections, with a shape-pattern resembling that of corrugated iron. Laminated together, these two sections create multiple tapering round holes in the root circumference for accommodating our patented inner steel thread elements for the blade bolts. Once also laminated into the root, the three parts create a very strong and stiff but lightweight structure."

The blade root wall thickness after the bolt inserts is in fact only around 20–25mm, compared with 80-100mm for a conventional 49-metre blade root. Reinforcement bars incorporated inside the root prevent the blade root buckling under load, and add very little to overall blade mass, said Hayden.

Seams

Despite being constructed from a number of shell sections, the outer prototype blade surface showed no visible seams or other imperfections at the joints, a result of the focus on overall manufacturing quality and accuracy of the smaller parts, according to the company.

Future challenges for the company, says Hayden, will be in maintaining sufficient torsion stiffness control in the next generation of long slender blades. "It is certain that a 20MW turbine blade should not resemble that of a conventional 3MW three-bladed turbine. Such an evolutionary design approach will simply not work. Instead new technologies will be necessary with a design focus on optimized interaction between wind and blades at this scale."