Wind Turbine Operation Suspended For Modifications

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."

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