Burnt Hill Wind Development

Transcription

1 Report OSE3581 Appendix D Noise and Shadow Flicker Assessment

2 Environmental Health Studies: Noise and Shadow Flicker Assessment 1 Summary results 1.1 Wind turbines can create two types of environmental health nuisance; disturbance from noise and the potential for shadow or flicker affects. This study considers the impact that these possible nuisances could have upon neighbours and concludes that these impacts are not adversely significant for the Burnt Hill development. 1.2 Wind turbine noise is normally generated by the turbine blades along with mechanical noise from the gearbox and generator. The Burnt Hill project has been revised to become a single moderate scale turbine, restricting maximum height to 50m. The proposed turbine is now the Vestas 500kW, and due to good aerodynamic design practices, the impact of noise from the wind turbine is not significant and achieves planning guidance. 1.3 A desktop assessment has been carried out, with distances measured from properties around the site, then calculations performed to show the impact of the turbine at neighbours. The Burnt Hill wind energy project has been designed to minimise impact and achieves recommended sound levels; neighbouring properties have noise levels below 35dB(A). 1.4 Shadow flicker can occur under certain combinations of turbine position, time of day and time of year, when the sun may pass behind the rotor and cast a shadow over neighbouring properties, and can cause a nuisance when the shadow appears in narrow window openings. Scottish planning guidance recommends 10 rotor diameter separations between wind turbines and dwellings to avoid this nuisance, fully achieved by the Burnt Hill project; the small cluster of buildings around the landowner s farmhouse are approximately 440m away from the turbine position, a clearance greater than 10 rotor diameters. Surrounding farms, houses, the school and the nearby village of Thrumster are further away, with negligible effects due to significantly longer distances and the relative geometry of the site. 2

3 2 Potential disturbance from noise 2.1 Sound is generated by wind turbines when they operate and generate power, and occurs over the operational windspeed range. Below the cut-in wind speed there is insufficient strength in the wind to turn the blades and in extreme conditions the turbines enters a storm control mode, with the rotor turning only very slowly as a safety measure. The production wind speed range for wind turbines is from 4 to 25 metres per second (ms -1 ) and the principal source of noise is from the blades rotating in the air; these modern turbine models do not have a separate gearbox, reducing any tonal element of the noise characteristics. 2.2 Noise levels are normally expressed in decibels. Noise in the environment is measured using the db(a) scale which includes a correction for the response of the human ear to noises with different frequency content. A 3dB change in noise level is perceptible and a 10dB change in noise level is heard as a doubling or halving of the perceived level. The sound level from the Vestas 500kW wind turbine is 101 db(a) at a wind speed of 10 ms -1, with this value decreasing in lower windspeeds, with distance from the turbine and with atmospheric absorption. Table 1 shows the predicted impact at neighbours around the Burnt Hill project, along with other typical noise levels. Source/Activity Indicative noise level db (A) Threshold of hearing 0 Rural night-time background Burnt Hill wind turbine Quiet bedroom 35 Busy road at 5km Car at 65 km/h at 100m 55 Busy general office 60 Conversation 60 Truck at 50 km/h at 100m 65 City traffic 90 Pneumatic drill at 7m 95 Jet aircraft at 250m 105 Threshold of pain 140 Table 1 Typical noise levels 3

4 2.3 The Planning Advice Note on Renewable Energy Technologies, PAN 45, has now been superseded by PAN 1/2011, Planning and Noise [1], and by a series of online advice sheets on the different renewable technologies. PAN 1/2011 identifies two sources of noise from wind turbines; mechanical noise and aerodynamic noise. It states that good acoustical design and siting of turbines is essential to minimise the potential to generate noise. The report The Assessment and Rating of Noise from Wind Farms, ETSU-R- 97 [2], describes a framework for the measurement of wind farm noise and gives indicative noise levels thought to offer a degree of protection to wind farm neighbours, without placing unreasonable restrictions on wind farm development or adding unduly to the costs and administrative burdens on wind farm developers or planning authorities. 2.4 These planning guidelines suggest that noise limits can be applied to locations where a quiet environment is desirable. Where a wind farm is near to neighbours, noise limits can be set relative to background noise and should reflect the variation in both turbine source noise and background noise with wind speed. Separate noise limits can apply for day-time and for night-time as the protection of the external amenity becomes less important in the evening and the emphasis should be on preventing sleep disturbance 2.5 The recommended day-time noise limits are db(a) or 5dB(A) above the prevailing background, whichever is the greater. For night-time periods the recommended noise limit is 43 db(a) or 5dB(A) above the prevailing background, again whichever is the greater. The 43 db(a) lower limit is based on a sleep disturbance criteria of 35 db(a) with an allowance of 10dB(A) for attenuation through an open window and 2dB(A) subtracted to account for the use of L A90 rather the L Aeq. Quiet day-time periods are defined as evenings from plus Saturday afternoons from and Sundays from Night-time is defined as Where predicted noise levels are low at the nearest residential properties, due to large distances from wind turbines or if a small scale development is planned, a simplified noise limit is permitted by the planning guidelines, such that noise is restricted to a maximum level of 35 db(a) for wind speeds up to 10 ms -1. This recognises that extensive background noise measurements should not be necessary for single turbines or low density schemes. 4

5 2.7 The Burnt Hill project has a single moderate scale turbine, located over 400m from the farmhouse and any neighbour, and it is felt that a simple desktop assessment is appropriate. When considering the impact from a windfarm, the noise levels from individual wind turbines are normally combined to give an overall impact, with neighbours downwind of all turbines at the same instant, which in practice cannot happen. Cumulative assessment in this instance is not necessary as only a single turbine is proposed and there are no other grid-connected wind turbines of this scale or larger within the immediate surroundings of the community. 2.8 Current guidance on noise impact of wind energy projects is based upon a short article published in The Acoustics Bulletin Volume 34 Number 2 [3], and although this assessment has been conducted to follow the guidance provided in the Bulletin, it should be noted that the leaflet has no legal standing; the guidance is appended to this report. A good practice guide to wind turbine noise assessment is being produced however this has not yet been published. 2.9 Noise predictions have been carried out using International Standard ISO , Acoustics Attenuation of Sound During Propagation Outdoors [4]. The propagation model described in Part 2 of this standard provides for the prediction of sound pressure levels based on either short-term downwind (ie. worst case) conditions or long term overall averages. Only the down-wind condition has been considered in this assessment, with the wind blowing from the proposed turbine location towards the nearby houses. When the wind is blowing in the opposite direction noise levels will be significantly lower, especially where there is any shielding between the site and the houses The ISO propagation model calculates the predicted sound pressure level by taking the source sound power level for the turbine in separate octave bands and subtracting a number of attenuation factors according to the following: Predicted Octave Band Noise Level = Lw + D A geo - A atm A gr - A bar - A misc Where: LW = Source Sound Power Level D = Directivity Factor (down-wind is worst-case) A geo = Geometrical Divergence A gr = Ground Effects (G = 0.5 used) A atm = Atmospheric Absorption A bar = Barrier Attenuation A misc = Miscellaneous (propagation through vegetation etc) 5

6 Geometrical divergence has the largest impact on noise reduction, with lesser effects from the other attenuation factors. Noise predictions have been based on source sound power levels for the Vestas 500 kw turbine as warranted by the manufacturer and shown in Table 2. The noise spectra have been taken from measurements carried out on a sample V47 turbine, Table 3, according to IEC , normalised to the warranted sound power level at each integer wind speed. Wind speed at hub height (m/s) V Sound Power Level (dba) Table 2 Predicted sound power levels from the Vestas 500kW Octave Band Centre Frequency (Hz) k 2k 4k 8k V Table 3 Standard octave band spectra from the Vestas 500kW 2.11 Geometric Divergence calculation: Ageo = 20 x log(d) + 11 where d = distance from the turbine Atmospheric Absorption attenuation calculation: Aatm = d x α where d = distance from turbine and α = atmospheric absorption coefficient in db/m Published values of α have been taken from ISO9613 Part 1 corresponding to a temperature of 10ºC and a relative humidity of 70% which give relatively low levels of atmospheric attenuation, and subsequently worst-case noise predictions, Table 4. These atmospheric conditions are in line with current good practice Octave Band Centre Frequency (Hz) Atmospheric Absorption Coefficient (db/m) k 2k 4k 8k Table 4 Atmospheric absorption coefficients 6

7 2.12 The level of attenuation attributed to Atmospheric Absorption was calculated using the formula Aatm = d x α where d = distance from the turbine and α = atmospheric absorption coefficient in db/m. Published values of α have been taken from ISO9613 Part 1 corresponding to a temperature of 10ºC and a relative humidity of 70% which give relatively low levels of atmospheric attenuation, and subsequently worst-case noise predictions. These atmospheric conditions are in line with current good practice Ground effect is the interference of sound reflected by the ground interfering with the sound propagating directly from source to receiver. The prediction of ground effects depends on the source height, receiver height, propagation height between the source and receiver and the ground conditions. The ground conditions are described according to a variable G that varies between 0 for hard ground (includes paving, water, ice, concrete & any sites with low porosity) and 1 for soft ground (includes ground covered by grass, trees or other vegetation). The predictions have been carried out using a source height corresponding to the proposed height of the turbine nacelle, 31m, a receiver height of 4m and an assumed ground factor G = 0.5. This ground factor corresponds to a mixture between hard and soft ground and represents current good practice detailed in ISO There has not been any attenuation of the sound levels for Barrier Effects (Abar) or Miscellaneous Effects (Amisc) The process used to determine the sound levels was thus to identify a selection of neighbours, measure distances from the facing façade of these properties to the turbine, and then to subtract the various attenuation factors. The calculation has been completed using the relevant module in Windfarm Designer, the industry standard programme produced by RESoft. Table 5 shows the range of theoretical sound levels, calculated from the above formulae, with the predicted noise levels taking into account the grassy moorland ground conditions on the site, octave band spectra, and a source sound power level of 101 db(a). 7

8 House Status Distance to turbine (m) Table 5 Predicted sound and noise levels Ground conditions Total of attenuations (db) Total Sound db(aeq)) Theoretical noise level db(la90 10min) H1 Landowner T1: 440 Rough pasture T1: H1A Neighbour T1: 440 Rough pasture T1: H2 Neighbour T1: 538 Rough pasture T1: H3 Neighbour T1: 459 Moorland pasture + road T1: H4 Neighbour T1: 444 Rough pasture T1: The properties have been chosen to represent the impact upon different parts of the community, and include the nearest neighbours; five property sites have been assessed, Figure 1. The open nature of the countryside, the flat moorland pasture setting and good distances between the turbine and neighbours are important considerations. Note that the Ordnance Survey map does not show the location of the neighbouring dwelling house at Burnt Hill, with H1 marking the position of both the farmhouse and the neighbour to the north The assessment reveals that the project achieves planning guidelines, with predicted sound levels for neighbours below the recommended level of 35 db(a), Figure 2. The project has sound levels much less than the 43 db(a) night-time disturbance figure, and is below the 45dB(A) limit recommended for project owners. The estimated noise levels predicted to occur at Thrumster are a maximum of 26 db(a), and the local school can expect an impact of around 27dB(A), again well within planning guidance Measurements of low frequency and infra-sonic noise around modern wind farms have shown that levels of such noise are below accepted thresholds of perception even on the wind farm itself [5]. There is no evidence that there are any effects of low frequency sounds at levels below perceptibility. The World Health Organisation has also stated that there is no reliable evidence that infrasound below the thresholds of hearing produce physiological or psychological effects [6]. 8

9 Figure 1 Neighbours to Burnt Hill Development 9

10 Figure 2 Sound dispersal, showing the 35 db LA90 noise level 35 db LA90 10

11 3 Potential disturbance from shadow flicker 3.1 Under certain combinations of geographical position, time of day and time of year, the sun may pass behind the rotor of a wind turbine and cast a shadow over neighbouring properties. The Scottish Government has provided online planning advice regarding onshore wind turbines at Planning-Policy/themes/renewables/Onshore. This guidance indicates that shadow flicker occurs within buildings where a blade flicker appears within a narrow window opening, and that the seasonal duration of this effect can be calculated from the geometry of the machine and the latitude of the potential site. The online planning advice indicates that provided a good separation is provided between the turbines and nearby dwellings (as a general rule 10 rotor diameters), shadow flicker should not be a problem. For the Burnt Hill development 10 rotor diameters is 390m. 3.2 The Burnt Hill project has been designed in conjunction with the software programme WindFarm. This programme uses Ordnance Survey Panorama gridded topographic data at a scale of 1:50,000 to create a virtual landscape for assistance in designing layouts. The programme calculates the risk of shadow flicker impact, based on the relative geometry of the turbine, the time and season, and the location of any nearby properties. The nearest properties are the farmhouse of Burnt Hill, and a neighbouring property in the process of being renovated; these buildings are approximately 440m from the turbine, and accordingly there little risk of shadow flicker effects. 3.3 It has been suggested that shadow flicker from wind turbines can have an influence upon individuals susceptible to photosensitive epilepsy. Epilepsy Scotland indicate that television is the most commonly reported trigger of seizures in photosensitive people, and that flickering sunlight through trees or strobe lighting are also triggers. The most common flash rates which are reported to produce seizures are between 12 and 24 Hz [7]. The flash rate which triggers epilepsy in photosensitive people is at a much higher frequency that the shadows produced by rotating wind turbines; a three bladed wind turbine would require a rotational speed of 240 to 480 rpm, approaching the rotational speed of helicopter blades. The shadow flicker from wind turbine of the scale proposed for Burnt Hill has a maximum frequency 1.5 Hz, with little risk of impact. 11

12 4 References and bibliography 1 Planning Advice Note PAN 1/2011. Planning and Noise. The Scottish Government, March ETSU-R-97. The Assessment and Rating of Noise from Windfarms. DTI Noise Working Group 3 The Acoustics Bulletin Volume 34 Number 2. Institute of Acoustics March ISO Acoustics Attenuation of Sound During Propagation Outdoors. 5 ETSU W/13/00385/REP. A critical appraisal of wind farm noise propagation. ETSU for the DTI. 6 ETSU W/13/00392/REP. Low frequency noise and vibrations at a modern windfarm. ETSU for the DTI. 7 Community Noise. Archives of the Centre of Sensory Research Vol.2(1), eds. Bergland and Lindvall. World Health Organisation. 8 Epilepsy Scotland. Factsheet 12

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