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Derryadd Wind Farm – EIAR

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10 MATERIAL ASSETS - SHADOW FLICKER

Note: Chapter 10 (Material Assets - Shadow Flicker) and Chapter 11 (Material Assets -

Telecommunications, Aviation and EMF) of this Environmental Impact Assessment Report (EIAR) can

be classified as “Material Assets” and, as such, are often presented together within one chapter of an

EIAR. However, for the purposes of clarity and a detailed assessment of each parameter, it has been

decided to deal with each topic separately within this EIAR.

10.1 INTRODUCTION

10.1.1 Background

This chapter assesses the potential for shadow flicker from the proposed Derryadd Wind Farm

development to impact on sensitive receptors in the surrounding area.

Shadow flicker is a phenomenon that arises when an operational wind turbine is located between an

observer (located indoors) and the sun – most common when the sun is low in the sky. Shadow flicker

effects are only possible if there is an unobstructed path from the turbines to a window. If there is no view

of the turbines from a dwelling, there will be no noticeable effects because the turbine’s shadow will not

pass over the window.

Rotating wind turbine blades can cause brightness levels to vary periodically at locations where they

obstruct the Sun’s rays. This can result in a nuisance when the shadow is cast over the windows of

residential properties. This intermittent shadow is described by the term ‘shadow flicker’ and it can be a

cause of annoyance at residences near onshore wind turbines if it occurs for a significant period of time

during the year. Shadow flicker is an indoor phenomenon and can be noticeable inside a room if the

rotating blades obstruct the direct sunlight that is illuminating the room’s window. This is largely dictated

by the relative position of the turbine(s) and the window, in combination with the time of day (position of

the Sun). The frequency of the flicker effect is related to the frequency of the rotating blades. It can also

be dependent on the number of turbine rotors that are casting shadows on the window.

Shadow flicker could only occur if one or more turbine rotors was located between an observer within a

dwelling and the sun. Shadow flicker would not occur under various real-world conditions, for example if

the sky is overcast, the rotor was not spinning for any reason or blinds/curtains were drawn at the receptor

location.


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10.1.2 Wind Development Details

The proposed development comprises 24 turbines and is described in detail in Chapter 2 – Description

of the Proposed Development. The shadow flicker assessment is based on the turbine locations

described in Chapter 2 and depicted in Figure 10.1 and Appendix 10.1. All coordinates and elevations

within this chapter are in Irish National Grid (TM65).

10.1.3 Statement of Authority

The technical assessment was prepared by Pager Power. Kai Frolic (Pager Power), was the lead author

of the technical report on which this chapter is based. His qualifications include a Masters degree in

Physics (MPhys, first class honours) from the University of Surrey (2008). He is a member of the Institute

of Physics (MIsntP) and he has 10 years of experience undertaking assessments for wind farm

developments, including shadow flicker assessments, on behalf of Pager Power. Siobhán Tinnelly

(Associate Director, TOBIN Consulting Engineers) prepared and formatted this chapter using the

technical report provided by Pager Power. Siobhán has over eighteen years of experience in

environmental assessment and project management and her qualifications include a degree in Natural

Sciences (Environmental Science), a Masters in Applied Hydrogeology and a Higher Diploma

(postgraduate) in both Environmental Engineering and Business Management. The content of the Pager

Power technical report, including the assessment of potential impact, has been directly reflected in this

chapter.

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DERRYADD WIND FARM

Proposed Wind Turbine Locations

F. Healy S. Tinnelly January 2019D. Grehan

Figure 10.1 A

Scale @ A3:

Issue Date Description By Chkd.

Client:

Project:

Title:

Prepared by: Checked: Date:

Project Director:

Consulting, Civil and Structural Engineers,Block 10-4, Blanchardstown Corporate Park, Dublin 15, Ireland.tel: +353-(0)1-8030406fax:+353-(0)1-8030409e-mail: [email protected]

Issue:No part of this document may be reproduced or transmitted in any form or stored in any retrieval system of any nature without the written permission of Patrick J. Tobin & Co. Ltd. as copyright holder except as agreed for use on the project for which the document was originally issued.

1:40,000

A Jan 2019 FH ST

NE

E

SE

S

SW

W

NWN

DECLAN CUNNINGHAM 2004

Final Issue

LegendPlanning Application Boundary

0 1 2 30.5Kilometres

1. FIGURED DIMENSIONS ONLY TO BE TAKEN FROM THIS DRAWING2. ALL DRAWINGS TO BE CHECKED BY THE CONTRACTOR ON SITE3. ENGINEER TO BE INFORMED OF ANY DISCREPANCIES BEFORE ANY WORK COMMENCES4. ALL LEVELS RELATE TO ORDNANCE SURVEY DATUM AT MALIN HEAD

NOTES

Ordnance Survey Ireland Licence No EN 0016019 ©Ordnance Survey Ireland and Government of Ireland

X Proposed Turbine Locations


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10.2 METHODOLOGY

10.2.1 Guidance

There are various sources of guidance with regard to the assessment and management of shadow flicker

impacts caused by wind turbines. Guidance relevant to the proposed development is summarised below.

Additional shadow flicker information from the UK is also presented to provide technical context.

Wind Energy Development Guidelines (2006)

The guidelines state that:

“Careful site selection, design and planning, and good use of relevant software, can help avoid the

possibility of shadow flicker in the first instance. It is recommended that shadow flicker at neighbouring

offices and dwellings within 500m should not exceed 30 hours per year or 30 minutes per day”.

The guidelines also state that:

“At distances greater than 10 rotor diameters from a turbine, the potential for shadow flicker is very low.

Where shadow flicker could be a problem, developers should provide calculations to quantify the effect

and where appropriate take measures to prevent or ameliorate the potential effect, such as by turning off

a particular turbine at certain times”.

Pager Power’s modelling approach in this assessment is consistent with this recommendation.

Information Note: Review of the Wind Energy Development Guidelines 2006 “Preferred Draft

Approach” (June 2017)

The preferred draft approach as announced by the Department of Housing, Planning, Community and

Local Government (DHPCLG) and the Department of Communications, Climate Action and Environment

(DCCAE) states the following with regard to Shadow Flicker:

“The ‘preferred draft approach’ proposes that technology and appropriate modelling at design stage to

eradicate the occurrence of shadow flicker must be confirmed in all planning applications for wind energy

development. Moreover, there will be clearly specified measures for automatic wind turbine shut down,

where the issue arises as a condition planning permission. In effect, no neighbouring property will

experience the occurrence of shadow flicker.”

This text provides for the prevention of shadow flicker due to automatic shutdown of the turbines. This

means that turbines will be programmed to shut down when shadow flicker effects occur i.e. no amount

of shadow flicker per day/year would be acceptable. The nature of the automatic shutdown process allows


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for a very short period of flicker to occur as the blades are moved into the idle position and the blade

movement comes to a halt.

Parsons Brinckerhoff – Update of UK Shadow Flicker Evidence Base (2011)

Key extracts from this document are:

“This report presents an update of the evidence base which has been produced by carrying out a thorough

review of international guidance on shadow flicker, an academic literature review and by investigating

current assessment methodologies employed by developers and case study evidence. Consultation (by

means of a questionnaire) was carried out with stakeholders in the UK onshore wind farm industry

including developers, consultants and Local Planning Authorities (LPAs). This exercise was used to

gauge their opinion and operational experience with shadow flicker, current guidance and the mitigation

strategies that can and have been implemented.”

“The three key computer models used by the industry are WindPro, WindFarm and Windfarmer. It has

been shown that the outputs of these packages do not have significant differences between them. All

computer model assessment methods use a “worst case scenario” approach and don’t consider “realistic”

factors such as wind speed and cloud cover which can reduce the duration of the shadow flicker impact.”

“Mitigation measures which have been employed to operational wind farms such as turbine shut down

strategies, have proved very successful, to the extent that shadow flicker cannot be considered to be a

major issue in the UK.”

The Companion Guide to PPS22 (PPS22 was a planning policy statement produced by the UK

government in 2004 and referred to in the Parsons Brinckerhoff Report, 2011) makes the following

statements:

• Shadow flicker only occurs inside buildings where the flicker appears through a narrow window

opening;

• Only properties within 130 degrees either side of north of the turbines can be affected at UK

latitudes;

• Shadow flicker has been proven to occur only within ten rotor diameters of a turbine position.

A further extract from the Parsons Brinckerhoff Report refers to the Onshore Wind Energy Planning

Conditions Guidance Note, Renewables Advisory Board and BERR (2007), which states that only

dwellings within 130 degrees either side of north relative to a turbine can be affected and the shadow can

be experienced only within 10 rotor diameters of the wind farm.”


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Pager Power uses WindFarm software for its analysis, which is one of the industry standard models

referenced within the guidance.

Furthermore, cognisance was given to EPA Guidance on EIAR in the preparation of this chapter (as

outlined in Chapter 1, Section 1.8.2).

10.2.2 Discussion – 10 Rotor Diameter Exclusion Zone

It is common to use 10 rotor diameters as a maximum limit within which significant shadow flicker effects

can occur. The validity of this limit is discussed at length within the relevant literature and guidance varies

in different documents and countries, with some stating that effects can only occur within this distance

and others stating that this is a general rule or that the risk beyond this distance is low.

The Parsons Brinckerhoff report referenced above acknowledges that the 10 rotor diameter limit is a ‘one

size fits all’ approach that may not be suitable depending on the latitude of the site. The Onshore Wind

Energy Planning Conditions Guidance Note issued in 2007 by the Renewables Advisory Board and

BERR (United Kingdom) stated that the shadow can only be experienced within 10 rotor diameters.

Planning Advice Note 45 issued by the Scottish Executive in 2002 referred to nearby dwellings (as a

general rule 10 rotor diameters). The Best Practice Guidance to Planning Policy Statement 18 issued in

2009 by the Northern Ireland Department of the Environment stated that ‘the potential for shadow flicker

at distances greater than ten rotor diameters from a turbine position is very low’. The same wording is

used within Ireland’s Wind Energy Development Guidelines (2006), as set out above.

In reality, there is no fixed cut off distance at which effects can occur, because this is sensitive to many

parameters including the exact latitude and the terrain around the development location. This assessment

has considered dwellings within 10 rotor diameters - this is aligned with the current planning guidance in

Ireland and, in practice, effects are most likely to be significant at closer range to the wind farm.

10.2.3 Modelling Methodology

The analysis has been undertaken using WindFarm (Release 4) software which is one of the

recommended software packages for Shadow Flicker assessment. It is a sophisticated model that

incorporates:

The terrain elevation (based on interpolated Shuttle Radar Topography Mission data);

The path of the Sun throughout the year at the development latitude; and

The size, position and orientation of windows at the dwelling location (window orientations were

modelled as facing the wind development to ensure results are conservative).


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Other features of the modelling are highly conservative. For example, there are a number of factors that

could diminish shadow flicker effects namely cloud cover, varying wind direction and low wind speed. In

addition, it has been assumed that all properties have a single window facing the development. The

maximum value per day assumes unobstructed visibility between the window and the turbine rotors,

bright weather conditions and rotor alignment with maximum potential to cast a shadow. The model

therefore considers a ‘worst-case scenario’.

Furthermore, regarding cloud cover, the total annual shadow flicker calculated by the model for each

property assumes 100% sunshine during daytime hours. However, Met Éireann data for this region shows

that the sun shines on average for 29.2% of the daylight hours per year127. Therefore, the total amount

of shadow flicker is likely to be significantly less than the theoretical durations produced by the model.

The modelled results, therefore, overestimate the likely effects. This is an appropriate conservative

approach because sunshine data is statistical and will vary throughout the year, however it is likely that

the level of cloud cover will reduce the effects.

The assessment has considered a ‘view height’ of 1.8 metres (nominal view height for an adult) above

ground level and a minimum Sun elevation of 2 degrees (typical value to accommodate terrain obstruction

at the horizon for low solar elevation angles). The dimensions of the turbine have been set in accordance

with the turbine details included in Chapter 2 of this report.

The model has considered windows with a size of 1 metre by 1 metre with a centre that is 1.5 metres

above ground directed towards the centre of the wind farm. These dimensions are considered typical for

dwelling windows. The model has also assumed that each dwelling has a window facing the nearest

group of wind turbines i.e. those with the most potential to cause shadow flicker effects. Where

appropriate, dwellings have been modelled with two windows, each facing a different group of turbines.

This has been done in cases where turbines on two separate bearings have a reasonable prospect of

causing an issue.

In addition, the model has assumed that the rotor is turning at all times. In reality, low wind speeds and

maintenance requirements will reduce the operational time throughout the year. The model has assumed

a maximum aspect to observers, which will not be the case in all instances.

This approach represents a worst-case scenario because it maximises the potential for shadow flicker

effects to occur and, therefore, predicts an over estimated potential impact.

127 This percentage is based on Met Éireann data recorded at Mullingar over the 30-year period from 1971 to 2000 (www.met.ie).


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10.2.4 Acceptable Limits

According to the Wind Energy Development Guidelines (2006) the acceptable limit for shadow flicker in

Ireland is 30 hours per year with a maximum of 30 minutes per day.

Within this assessment, reference has been made to the current limit as outlined in the 2006 guidance.

However, the shut-down times in Appendix 10.2 have been defined for a ‘minimal effects’ scenario in

response to the requirements of the ‘preferred draft approach’ and as the worst-case impact on the

operation of the windfarm.

10.3 EXISTING ENVIRONMENT

10.3.1 Sensitive Receptors

The sensitive receptors included in the assessment are depicted in pink in Figure 10.2 below. The turbine

locations are shown in blue and the 10-rotor diameter buffer (orange area) is included for reference

purposes.

Figure 10.2: Assessed Receptors (Map Source: © 2018 Google and DigitalGlobe)


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Receptor locations were identified by the author of the technical shadow flicker assessment (Kai Frolic,

Pager Power) based on inspection of mapping including aerial and street-level imagery. A total of 151

receptor locations (comprising habitable residences) were identified within the 10-rotor diameter buffer

from the turbine locations.

The dwelling data, including coordinates for all habitable and derelict potential receptors within the 10-

rotor diameter buffer zone and also in the vicinity of the buffer zone, was collated by Pager Power (as

presented in Appendix 10.1) and totalled 187 receptors. Based on a 10-rotor diameter buffer zone, the

locations of the 187 potential receptors were reviewed and the number of receptors was reduced to 151

receptors i.e. the receptors identified within the buffer zone and were not deemed to be derelict. The table

of data within Appendix 10.1 titled “Summary of merged shadow times on each house from all turbines”

includes the 151 receptors that were identified (out of the original total of 187, also detailed in Appendix

10.1) within this 10-rotor diameter buffer zone (as shown in Figure 10.2 above).

In addition, Appendix 10.1 includes more localised images than Figure 10.2 above, for information and

illustrative purposes (Map Source: © 2018 Google and DigitalGlobe). The main image in this appendix

includes an overview of all relevant sensitive receptors (shown as pink icons) with numbers in groups

relative to the turbines (shown in blue) and the 10 rotor diameter zone (shown in orange). The supporting

images include more localised images of each group of receptors.

As detailed in Chapter 5 (Population and Human Health) of this EIAR, the closest dwelling is located in

excess of 750m from the nearest turbine. The study area was also the subject of a planning history search

(as described in Chapter 5, Population and Human Health), to identify properties that may have been

granted planning permission but that have not yet been constructed. All such properties have been

included in the assessment.

10.3.2 Potential Impacts Assessment Results

Table 10.1 below summarises the key findings from the assessment of the receptors. This phase of the

analysis assesses the level of predicted effect, based on conservative assumptions, in the absence of

any mitigation. It categorises the number of dwellings within 10-rotor diameters that could experience

effects under these conditions, with reference to the acceptable limits within the guidance. Detailed data

can be found in Appendix 10.1.


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Table 10.1: Results – Effects at Receptors

Number of

unaffected

receptors

Number of receptors

predicted to experience

less than 30 minutes

per day and less than 30

hours per year

Number of receptors

predicted to experience

more than 30 minutes

per day or more than 30

hours per year

Maximum

hours per

day

predicted

at any

receptor

Maximum

hours per

year

predicted

at any

receptor

32

(out of 151)

68

(out of 151, including

dwellings with zero

effects)

83

(out of 151)

0.82

(Dwelling 5)

100.6

(Dwelling 7)

Note: This table includes all habitable receptors identified within 10 rotor diameters of the proposed turbines

(151 receptors).

Table 10.2 below quantifies the worst-case shadow flicker effects by turbine (within 10-rotor diameters)

and Appendix 10.1 details the potential shadow flicker impact at each identified receptor.

This phase of the analysis identifies how much shadow flicker could be caused by each individual turbine,

based on conservative assumptions, in the absence of any mitigation.

Table 10.2: Quantification of the predicted Shadow Flicker per Turbine

Turbine

Number

Days per year

of shadow

flicker

Maximum hours

per day

Mean hours per

day

Total hours per

year

1 257 0.94 0.6 155.1

2 245 1.02 0.58 142.8

3 193 0.96 0.62 119.4

4 97 0.82 0.62 59.9

5 113 0.81 0.51 57.1

6 163 0.8 0.61 99.3

7 209 1.09 0.63 130.8

8 221 1.19 0.62 136.7

9 201 0.96 0.51 102.4

10 362 1.09 0.68 244.8

11 128 0.48 0.35 44.3

12 247 1.2 0.82 202.6


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13 146 0.69 0.45 65.9

14 86 0.46 0.35 30.1

15 221 1.42 0.95 209.6

16 229 1.24 0.82 187.9

17 191 0.97 0.54 103.3

18 116 1.15 0.72 83.2

19 64 0.57 0.44 28.3

20 0 0 0 0

21 105 0.69 0.56 58.8

22 139 0.9 0.66 91.6

23 167 0.8 0.56 93.9

24 269 1.26 0.69 186

Note: The minimum shut-down per year is zero hours for T20 and the maximum is 244.8 hours for T10.

Figure 10.3 illustrates the combined shadow flicker times on all dwellings from all turbines (red hatched

areas), within 10-rotor diameters. The red lines illustrate the sunrise and sunset times. It can be seen that

effects generally occur when the Sun is low in the sky, which is to be expected since this equates to the

longest shadows.

Figure 10.3: Shadow Times on all houses from all Turbine


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Further detailed breakdowns are presented in the appendices, including:

• Summary of worst-case effects for each individual receptor (Appendix 10.1).

• Receptor Map (Appendix 10.1).

• Worst-case turbine shut-down scheme (Appendix 10.2).

10.3.3 Do Nothing Effects

The shadow flicker effect examined in this chapter is solely related to the proposed development of a

windfarm. Therefore, should the development not proceed the effects described and examined in this

chapter would not occur.

10.3.4 Potential Effects

Shadow flicker effects are only possible if there is an unobstructed path from the turbines to the window.

If there is no view of the turbines from the location of a receptor, there will be no noticeable effects

because the turbine shadow will not pass over the window.

Survey data regarding the level of visibility may inform the results of this assessment further. However,

based on the scale of the wind farm it is reasonable to assume that a significant level of visibility will be

available from the surrounding areas. It is reasonable and conservative to model effects based on

assumed visibility as this captures a worst-case scenario.

The technical assessment has shown that the majority of the 151 assessed receptors would experience

some effects, based on a conservative approach to the assessment, in the absence of mitigation

measures. Less than half of the receptors would experience less than 30 minutes per day and less than

30 hours per year – which is acceptable based on the current limits. Mitigation is to be applied that will

ensure that all effects are within acceptable limits, should the wind farm be consented.

Factors including cloud cover, variable wind speeds/direction and likely maintenance requirements will

act to reduce the potential effects in real terms. The modelling is based on a comprehensive and

conservative approach whereby statistical and variable mitigating factors are assumed to be worst-case.

There will be no potential effects relating to shadow flicker during the construction phase of the proposed

development. For the duration of the operational life of the proposed development, unmitigated, the worst-

case potential impact from shadow flicker at specific localised receptors will be likely, significant and long-

term but have a momentary effect with respect to the duration of impact on a daily basis.


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10.4 MITIGATION MEASURES

Screening Assessment

The shadow flicker modelling predicts a worst case ‘bare earth’ impact. In reality, existing screening in

the form of buildings, vegetation and local topographic variations will have a significant impact on the

level of shadow flicker that is predicted to be experienced by the sensitive receptors. When these

additional screening features are taken into account, the actual impact in terms of incidence and duration

may be significantly reduced or even eliminated. If existing screening reduces the impact below

acceptable levels then no further mitigation will be required.

Screening Measures

If existing screening is not sufficient to reduce shadow flicker to acceptable levels (either the existing

levels outlined in the Wind Energy Development Guidelines (2006) or Guidelines finalised during the

consenting process) then additional screening measures will be proposed. Through interaction with the

individual sensitive receptors, the incidence and level of shadow flicker at the specific location will be

verified. Once verified, a number of measures will be proposed to the property owner such as installation

of blinds/curtains in the affected room(s), planting of new screening at identified locations within the

curtilage of the property and any other site-specific measures that might be agreeable with the affected

party. Once the agreed measures are implemented, the effectiveness of the measures will be monitored

over a period of months to establish the reduction in impact. The costs of the agreed mitigation measures

will be borne by the developer. If the proposed measures are not agreeable, or the implemented

measures are not effective in reducing the incidence and duration of shadow flicker to acceptable levels,

then a turbine(s) shutdown scheme will be developed and implemented.

Turbine Shutdown Scheme

A worst case turbine shutdown scheme is presented in Appendix 10.2. In principle, the required times

and dates for the proposed shutdown scheme (if implemented) will be programmed into the wind farm

control system to automatically stop the rotor of the specific turbine(s) from turning at times when potential

effects would result from the operation of the turbine. The worst-case turbine shutdown scheme

presented in Appendix 10.2 will be updated (should the wind farm be consented) to reflect the impact of

both the screening assessment, applied screening measures and the acceptable limits.

The implementation of the proposed mitigation measures, including a shutdown scheme to minimise any

potential significant effects from the proposed development, will reduce the overall effect of the

development to slight and long term.


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10.5 RESIDUAL IMPACTS

There will be no residual shadow flicker impacts associated with the proposed development following the

implementation of the mitigation measures outlined in Section 10.4. In summary, the potential effect of

the proposed development will be slight and long term.

In conclusion, the potential for shadow flicker effects has been technically assessed considering the

appropriate technical parameters and conservative assumptions. The results have been interpreted with

reference to the appropriate guidance. The applicant is committed to mitigation that will ensure that any

residual effects are within the acceptable limits.

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