Wind Turbine Operation Suspended For Modifications
Windflow Technology's prototype windmill at Gebbies Pass near Christchurch has been temporarily shut down while
modifications aimed at further reducing sound levels are carried out.
"The prototype is operating more efficiently than we had predicted," said Windflow Chief Executive, Geoff Henderson. "So
we are confident that it is a commercially competitive source of electricity. However, its sound emissions have also
been higher than predicted. The University of Canterbury has been assisting us with measurement and analysis, and we are
testing several solutions, a process that has been hampered by the lack of appropriate wind conditions."
"We are determined to remain good neighbours", said Mr Henderson. "That is why we originally agreed to the level of 30
dBA (decibels)." In its public consultations before applying for a resource consent, the company identified that a group
of residents in McQueens Valley might be affected by the sound levels from the turbine under certain wind conditions
because of the unique geography of the site. The valley is sheltered in most wind conditions and hence particularly
quiet, especially at night.
Consequently, Windflow undertook to maintain a sound level less than 30 dBA at the head of McQueen's Valley, compared
with the District Council's usual requirement of 40 dBA. The company has monitored the sound level from the windmill and
it is about 35 dBA. (For comparison purposes: a modern household refrigerator has a sound level of 43 to 44 dBA, and a
whispered conversation is about 30 dBA. However these comparisons do not reflect the subjective character of the sound -
the windmill's dominant frequencies around 300 Hz are much lower than a fridge or a conversation.)
"It must be emphasised", said Mr Henderson, "that these are not high sound levels - they simply stand out because of the
low frequency and the very low night-time sound levels. At the same time we have always agreed with the McQueens Valley
residents that we need to meet our own high standards for acoustic performance and we are determined to do so."
The company undertook, as part of its resource consent, not to allow the prototype to operate for more than three months
if the sound levels exceeded the agreed 30 dBA level. That time has been reached, and after discussions with the Banks
Peninsula District Council and affected residents, the company has decided to stop running the windmill, except for
sound level testing purposes, until the remaining modifications have been completed.
"The challenge has been that the sound does not come from a single source", said Mr Henderson. "We have made several
modifications and reduced sound levels in some locations but not sufficiently at the head of McQueen's Valley. Acoustics
is a complex art, it has taken us longer than expected to isolate all the causes of the problem due in part to lack of
appropriate wind conditions. These low frequencies around 300 Hz are notoriously difficult to deal with. The tower
ringing we reported initially, which was caused by a gearbox vibration, was dealt with in September. For the last month
we have been working to reduce other gearbox-related sound paths."
"We are now left with two possibilities which we need to work through. The generator fan has a slightly different
frequency from the gearbox. We are redesigning the generator cooling air duct, which will reduce the sound level of that
frequency. The other possibility is that the blades themselves are picking up the gearbox vibration structurally, ie in
a similar way that the tower was. If so the solution will be to inject foam into the blades to absorb those vibrations.
We are confident that either or both of these solutions, in combination with the other measures we have implemented to
reduce gearbox noise, will bring us within the 30 dBA level."
"We will start up again once testing has proved to our satisfaction that the 30 dBA level has been achieved. Naturally
we are working with our suppliers to ensure the lessons we have learned from this prototype are included in all future
turbines we produce."