Developing lean and agile rotor blades

WORLDWIDE: The wind turbine rotors of the future will take advantage of new modelling tools that will allow individual blades independence of movement, greatly improving their performance and extending their working lives.

by Jim Platts 30 April 2015

Long and light… At 75 metres long, the Siemens B75 is the longest one-piece fibreglass blade, weighs only 25 tons and has no seams

Attempting to harness the wind resource offshore requires the costly development of a complete subsea electrical infrastructure, deep seabed foundations and maintenance processes. These all contribute to an energy investment payback ratio for an offshore wind project that is lower than 10:1, rather than over 30:1, as it is for a good onshore site. This is not the road towards sustainability.

Representing only 5% of the wind capacity installed worldwide in 2014, offshore remains eclipsed by the quiet but continuing development of rotor technology. It was this rotor technology that enabled the wind power industry to come into being in the first place, and continues to extend its effectiveness into the low-wind-speed areas close to where electricity is needed, and requiring low levels of infrastructure investment. German turbine manufacturer Enercon provides a good example; its latest developments improve energy capture and extend turbine design life to 30 years.

The watchwords of success in rotor design and manufacture are "leanness" and "agility". But most of all the sector depends on a steady clear-sightedness as to what the real design drivers are. Really good craftsmen design their own resources, because the payback on the development of tools that enable a transformational breakthrough in the technology is almost infinite.

The elephant in the room in any discussion about how the qualities of leanness and agility are imperative for rotor designs, however, is that, as rotors have grown bigger, the turbulence length scale they have to handle has become significantly smaller than the length of a blade. It is now, therefore ridiculous to think of a whole blade, let alone a whole rotor, as a single aerodynamic and structural entity.

At the macro scale, not only is the wind shear from the bottom to the top of the rotor substantial, it also varies continuously. However they are conceived, the individual blades of a rotor are essentially designed as a flat and unified disk, which can never be in the aerodynamically optimum position because the conditions they encounter are continuously changing, and are continuously different for each blade.

The blades need to be able to move independently, but the principle of "leanness" suggests they need to be able to move independently for another reason. One of the fundamental truths of structural design is that, if you make something stiff, it attracts load to itself, and so you have to make it strong as well. Then it becomes heavy and expensive. All the value in a blade is in the aerodynamics in the outer part, but all the weight and cost is in the root end. The golden rule is to let it move, and to use "agility" actively as a design tool. This principle offers many advantages but, until now, it has been avoided by the wind energy industry because it has been hard to model. But at last recent developments have opened the door.

In the 1990s, Peter Jamieson of research and consultancy firm Garrad Hassan thoroughly laid out the potential of what he called "the coning rotor", a downwind rotor with individually hinged blades, and damping and control on the hinging. It can become a larger rotor in light winds and smaller in heavy winds and thus increase the energy capture, especially in the former conditions. But more than that, the aerodynamic load is balanced by the centrifugal load on a blade, so each becomes lighter and less expensive and the fatigue loading drops. At this time, the aerodynamics of such coned blades was hard to model because propeller theory was developed assuming a flat rotor disk. But Curran Crawford of the Unviersity of Victoria in Canada rewrote the theory a decade ago to include the coning effects, adding only two terms to the basic equation so this no longer presents a problem.

Complex dynamics

A different problem was how to model and understand the more complex dynamic behaviour of a rotor with individually hinged blades. But several years ago, aerodynamicist David Delamore developed the process first explored by Bill Leithead at the University of Strathclyde, algebraically developing representational models of wind turbines with a small number of carefully developed variables, so that the full complexity of the behaviour was retained within a set of equations that were still possible to handle using algebra.

If you then apply Cramer's Rule - which solves linear simultaneous equations using determinants - in deriving the frequency domain model from the spatial model, it is possible to retain the algebraic transparency and see what in the spatial model drives the dynamic behaviour in the frequency domain model. You can then design the dynamic behaviour.

At the mid-scale level there is also a requirement for flexibility because the turbulence varies along each blade and on a moment-by-moment basis, driving a lot of fatigue loading into the whole wind turbine. But using hinged blades offers a second freedom. The blades need to be stiff in torsion and on the edge, but they no longer need to be stiff in terms of flexure. When a section of the blade is hit by a gust, if it can flex backwards - which is also inwards because of the coning - then the extra local centrifugal force reacts to the increased local load, and nothing is passed on to the rest of the wind turbine. The whole value of designed flexibility enables you to dodge loads before they bite you.

Softening the load

At the micro level, there is a final and highly desirable flexibility — the same as the flexibility feathers give to the aerodynamics of a bird's wings. If you begin with an airfoil (see diagram, below), which has a very good lift-to-drag ratio and a shape involving a substantially undercut trailing edge, the pressure underneath the trailing edge and its thinness suggest that it can become a very responsive load softener if a little elasticity is incorporated into the underside of the trailing edge.Airfoil dynamics Thinness can enable very responsive load softener if elasticity is incorporated into the underside of the trailing edge

It pays to remember that the reinforced rubber structure of tyres is part of this family of composite materials and has the same good fatigue characteristics. Rubber also bonds well to epoxy, so incorporating elasticity in a composite moulding is, in fact, very easy. Allowing the trailing edge to yield both reduces the camber and the angle of attack, truly letting go of the unwanted gust without getting bitten.

Other features worth remarking on are the use of spline curves - that connect to multiple points - to develop the parametric definition of shape, which simplifies modelling and enables the blade shape to be optimised, and the use of Taguchi quality control methods to accentuate those design parameters in the highly turbulent conditions that wind turbines experience. The parallel-processing computer tools now available also mean that fully turbulent time-step computational fluid dynamics modelling of wind turbines' aero-elastic behaviour is now less costly than any other investment entailed by turbine development. The return on an investment that achieves clear sight is almost infinite.

Future rotors will be lean and agile, and the tools to develop them are at hand. If you also make the tower and foundations lean, as French manufacturer Vergnet does, turbine lifetimes can be as long as 40 years, and energy investment payback ratios of 100:1 are within reach. Then the road to true sustainability opens up.

Dr Jim Platts is a lecturer at the Institute for Manufacturing, Department of Engineering, University of Cambridge. He is speaking at �u�X��ʘ��'s Blade Manufacturing and Composites Forum, 12 May 2015.