RES to vary Den Brook condition? 

Wind farm developer RES is consulting on a variation of a tight condition on amplitude modulation (AM) at the proposed Den Brook wind farm in Devon.

Den Brook was the scene of a high profile battle between near-neighbour Mike Hulme and the developer. Hulme was concerned that amplitude modulation would cause disturbance, and the case went in and out of the High Court (Noise Bulletin June 2011 p1).

The outcome was that RES won permission – but with a tight amplitude modulation requirement (Condition 20). RES has since argued that the condition, put together by consultant Mike Stigwood, is unworkable and would be triggered by background noise (NB December 2011 p1).

Its application states: “Condition 20 risks providing considerably less protection to the interests of local residents than all parties envisaged at the time it was drawn up. If the condition were under consideration today, it would be likely to fail the “enforceability” advisory test of the Secretary of State.

“A Section 73 application to vary the existing Condition 20 of the planning permission is therefore to be submitted to ensure that the Den Brook wind farm, and in particular the inhabitants of nearby properties, benefit from a workable and enforceable approach to defining an acceptable AM threshold. The proposed revised condition does not use the methodology of the existing condition and instead draws on recent research into the measurement of, and subjective response to, AM.”

RES then outlines its version of the complicated equations it would like to see in the condition. A key objection to the original Stigwood condition was that background noises could trigger failure, making the condition meaningless. Stigwood said non-turbine sounds could be excluded.

For the proposed new condition, RES is agreeing: “External sources of noise, for example crows and other birds, can have a noise signature which ‘looks’ like AM, but does not in fact derive from wind turbines. If an EAM penalty, as define in revised Condition 20, results from application of the above procedure, the appointed consultant shall listen to the one minute of recorded noise data from which the EAM penalty was derived. If it is clear that the AM is generated by an external noise source other than the wind turbines, then the EAM penalty shall not be valid and shall be discounted.”

Etsu and AM condition scrutiny

A methodology intended to patch up gaps in Etsu wind turbine assessment – dubbed the IoA Bulletin Agreement – may worsen noise.

The agreement has already been criticised by consultant Mike Stigwood (Noise Bulletin November 2011 p5). The IoA agreement focuses on the handling of wind shear – when wind is faster higher up than at ground level – which is blamed for causing amplitude modulation (thumping).

Now the Renewable Energy Foundation, which has been critical of wind turbines in the past, has released its thinking on the matter.

REF says: “We used actual wind speed data to model the impact of the revision on noise conditions and likelihood of noise complaints from neighbours. The IoA revision is designed to correct for site- specific wind shear that was erroneously assumed to be constant between two heights in the Etsu guidance. The impact of this assumption increases the uncertainty of the background noise curves and reduces confidence in the reliability of noise conditions based on them.

“We show that the revised methodology can produce different noise conditions depending on the dates chosen for the baseline background noise survey, thus yielding differences in permitted wind farm noise levels of as much as 5dB for the same site.

“Consequently, use of the methodology can lead to the situation where predicted noise levels from turbines at a given proximity to dwellings are deemed acceptable, whereas measurements taken two weeks later would give the opposite result.

“Use of the new methodology can even result in site layouts where the wind farm noise would breach the old-style Etsu conditions, which are sometimes mistakenly believed to be the less benign of the two methodologies.”

Independent noise consultant and wind specialist Dick Bowdler has looked at REF’s criticisms: “Yes there is more scatter in the standardised graphs in high wind shear areas. But in hilly areas, where wind shear is nearer to the standard value and where most wind farms are built in the UK, the degree of scatter is much greater than flat areas anyway. This is because the wind speed in valleys, where most of the houses are, bears hardly any relation to the wind speed up on the hill.

“So it is more accurate to say that, using measured rather than standardised wind speed in flat areas, artificially reduces the scatter in flat areas because it measures the wind speed closer to that generating the background noise. Measuring wind speed closer to the noise generating objects is bound to reduce scatter but it is no good for the assessment because the turbine noise you compare it with depends on the hub height wind speed on the development site.”

Bowdler continued: “As to the variation in background noise levels from week to week or from season to season, this happens anyway whichever method of analysis is used.

“So this is all a diversion from the real debate which is whether the average background noise should be used to assess turbine noise. If average less one standard deviation, for example, were used, the scatter would be irrelevant.”

www.ref.org.uk/publications/255-ioa-critique