Because they are bulky and cause a lot of troubles, wind turbine gearboxes are to be shunned out of the wind power industry slowly but surely, being replaced with direct drive systems. These are indeed bigger, but the nacelle’s overall weight is lower, and the chances of breaking up are also reduced, because of the reduced complexity.
One big player in the wind turbine industry is Siemens, planning to come up with a new design that reduces the weight of the generator. In conventional wind turbines, the gearbox increases the speed of the wind-driven rotor several hundred fold, which radically reduces the size of the generator required. Direct-drive generators operate at the same speed as the turbine’s blades and must therefore be much bigger–over four meters in diameter for Siemens’s three-megawatt turbine. Yet Siemens claims that the turbine’s entire nacelle weighs just 73 metric tons–12 tons less than that on its less powerful, gear-driven 2.3-megawatt turbines.
They plan to use permanent magnets in the generators’ rotor, instead of electromagnets fed with electricity by the generator’s own force. Henk Polinder, an expert in parmanent-magnet generators, from the Delft University of Technology in the Netherlands, estimates that a 15 millimeter-thick segment of permanent magnets can generate the same magnetic field as 10- to 15-centimeter section of copper coils.
Henrik Stiesdal, chief technology officer of Siemens’s wind power unit says Siemens reduced weight further by inverting its generator’s design. Rather than a steel rotor covered with permanent magnets spinning inside a stationary doughnut-shaped stator (the design GE is using in its four-megawatt direct-drive turbine) Siemens’s rotor is a steel cylinder with permanent magnets on the inside, and this rotor spins around a column-like stator.
Siemens has already built a prototype of a direct-drive system in Brande, Denkmark, in December, with plans to install another 10 during 2010. Mass production is to begin in 2011, and a 4 MW machine is to be released in 2012.