Saw teeth make wind turbines quieter
03 September 2009
Halve the sound wind turbines make without any energy loss. Placing saw teeth on the blades of wind turbines is now possible. The saw teeth are designed by Stefan Oerlemans of the University of Twente and the National Aerospace Laboratory (NLR). Oerlemans first studied the sounds wind turbines made and then used this knowledge to design the saw teeth and halve the noise. Oerlemans graduates with a PhD from the Faculty of Engineering Technology on 4 September.
Rotating wind turbine blades make a characteristic whooshing
sound. This noise often annoys those living nearby, so wind
turbines are not always operating at full capacity and plans for
new wind turbines are frequently rejected. Stefan Oerlemans, PhD
candidate at the Twente of University and employee of the National
Aerospace Laboratory (NLR), researched the sounds caused by wind
turbines and created a solution to halve the noise without loss of
energy. To achieve noise reduction, saw teeth were placed on the
back edge of the outer quarter of the blades (see figure).
Saw teeth on the rear edge of the outer quarter of the
blades.
Sound detection
To reduce the sound of wind turbines, Oerlemans first
investigated what caused the noise and how it was produced. For
this he utilized an acoustic antenna system, which uses a large
number of microphones mounted on flat surfaces. By measuring and
comparing the separate times that it takes a sound to reach the
different microphones, the precise origin of a sound can be
calculated. These measurements showed that most of the wind
turbine's noise is created by airflow through the blades, and the
actual sounds caused by mechanical components of wind
turbines are minimal. Specifically it also revealed that the
majority of the noise came from the outer parts of the blades while
the blades moved downward (see figure). This noise is due to
turbulent airflow eddies around the blades. However when saw teeth
were placed on the blades' rear edges, the sound was halved.
A wind turbine showing the noise sources. The red area has the
loudest sound.
Owls Brushes
This new method and results are part of extensive research into
aircraft and wind turbine noise. Oerlemans plans to continue this
research and dreams of developing even quieter wind turbines. His
inspiration is that owls fly almost silently. It is assumed that
the soft down feathers found on the back edge of owl wings allow
this silent flight. Therefore, a row of bristles on the back edge
of a wind turbine's blades would reduces any sound produced. Wind
tunnels trials have already demonstrated that this is effective,
but further testing is required.
Press note:
On September 4th, Stefan Oerlemans graduates with a PhD from the
Faculty of Engineering Technology from the Research Group
Engineering Fluid Dynamics and the Institute of Mechanics,
Processes and Control Twente (IMPACT). Oerlemans was supervised by
Professor A. Hirschberg and Dr P. Sijtsma. This research was
completed at the National Aerospace Laboratory in cooperation with
the project partners Gamesa, General Electric, Universität
Stuttgart, and the Energy research Centre of the Netherlands
(ECN).
The thesis Detection of aeroacoustic sound sources on aircraft
and wind turbines is available digitally on request.
Press contacts:
Rianne Wanders
053-4892721 or Joost
Bruysters 053-4892773
