The drive towards lighter rotors, which lead to lighter hubs, nacelles and towers, means that GRP is now widely used for the load-bearing structural spars, as well as the aerofoil. Other materials used include aluminium, wood epoxy, and carbon-fibre-reinforced plastic.
As the industry has matured, a better understanding of blade loads has enabled weights to be almost halved and costs to be reduced by increasingly sophisticated manufacturing. The turbines’ increasing size means novel solutions, such as jointed blades, are now being considered.
Many of the large turbines built in the early 1980s had two blades and a steel spar. Both blades of Boeing’s 2.5MW experimental MOD-2 turbines funded by the US Department of Energy weighed 25 tonnes and so a set of blades weighed 50 tonnes. Today, a set of blades for an almost identical machine, such as Gamesa’s 2MW machine, weigh 17.4 tonnes -— and the Gamesa machine has three blades, rather than two.
The weight of each blade of Germany’s 3MW 1980s Growian machine weighed 28 tonnes — 56 tonnes for the set of two. Today, the three blades of the 5MW Repower turbine also total 56 tonnes.
Material choice is vital
The designers of the Growian and MOD machines recognised the importance of blade materials at an early stage. Design studies from Germany suggested that, compared with steel, GRP would reduce weights by about a third, while carbon-fibre-reinforced plastic would reduce them by two thirds. In the US, a joint US Department of Energy/National Aeronautics and Space Administration (NASA) report that looked at aluminium and wood epoxy suggested that both these materials would reduce weights by about 40%.
Although the glass and carbon-fibre alternatives are more expensive than steel, the weight savings make them worthwhile, and cost savings can be made elsewhere in the structure, particularly the tower.
Aluminium has not been used for large turbines to any significant extent but a wood and epoxy construction was popular around 20 years ago, and the material is still used. The majority of today’s large wind turbine blades, however, use GRP, often with some carbon-fibre reinforcement as well. Turbine manufacturers Dewind, Repower, Vestas and Gamesa all use this latter type in their largest machines.
Several other factors have contributed to reductions in weight and cost of blades. Manufacturing processes are continuously improving and an increasing degree of automation helps to guarantee the integrity of the blades. The industry’s understanding of the wind and of the complex loads that are imposed on blades has also advanced considerably. In the early days, uncertainties over aerodynamic loads led to safety factors in blade design that could be seen as overly generous. During the past 30 years, considerable analysis of turbine loads has removed much of the uncertainty and safety factors can be applied with greater confidence.
Long and lean
Blade design processes have evolved rather than jumped forward, and they are still evolving. LM Wind Power, for example, recently announced that its latest 42.1-metre blades would be thinner than the earlier 40.3-metre blades. Manufacturers can now accurately assess the performance and characteristics of blade aerofoil sections using wind tunnel tests.
Further advances are in the pipeline. LM recently announced participation in a project with the Danish National Advanced Technology Foundation that will investigate the use of laser measurements to monitor the oncoming wind with the objective of adjusting the blade settings to maximise energy capture and reduce loads. More new materials may also emerge. Researchers at Cambridge University, for example, are looking into bamboo, as it is light, strong and cheap.