How Can Shadow Flicker Impacts Be Reduced?

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. 

Unmitigated shadow flicker can lead to planning objections, project delays, and reputational risk. 

Shadow flicker assessments determine whether a development could cause significant impacts on surrounding dwellings by modelling expected shadow patterns throughout the year. The assessment identifies which properties may be affected, estimates the duration of any flicker events, and compares results against relevant national or international planning guidelines.

UK Guidance and Thresholds

There is no formal limit on the amount of shadow flicker that is considered acceptable within the UK. 

Other European countries do have limits, and these vary from one country to another. A typical limit, which has been utilised in Northern Ireland, Germany and Belgium, is 30 hours per year with a maximum of 30 minutes per day. Since there is no formal guidance on this subject in the UK, potentially significant effects are generally categorised based on threshold limits of 30 minutes per day and/or 30 hours per year.

If shadow flicker effects are predicted beyond this limit, mitigation may be required to eradicate the occurrence of shadow flicker. 

Mitigation Strategies and Shutdown Schemes

Where issues are found, mitigation strategies are limited beyond wind turbine layout adjustments or turbine shutdown schedules to remove or reduce impacts. There are two approaches to mitigation:

• Complete removal of effects;
• Reduction of any effects to the acceptable limit specified above.  

A shutdown scheme defines the times between which a turbine should be shut down to eliminate shadow flicker effects on each dwelling, assuming clear sunny skies. The term ‘shutdown’ means that the rotating blade is completely still and does not move for the period of time specified. The shutdown scheme should either reduce shadow flicker effects to below the recommended threshold limits or eliminate all shadow flicker effects completely throughout the year. Pager Power can recommend shutdown times as part of a shadow flicker assessment that can fulfil either of these functions.  

Shadow flicker effects can only occur under specific conditions so, in reality, turbine shutdown may not be required to eliminate effects i.e. shadow flicker cannot occur if the weather at the time of predicted effects is not clear and sunny or if the rotor is not face-on to the receptor. A shutdown scheme can therefore be enhanced by the use of a light intensity detection system, which would ensure that turbines are only shutdown when the prevailing weather conditions are such that shadow flicker is possible.

Mitigation options can be explored post-consent if required, on the basis of further detailed consideration (this could include the consideration of weather data and ground surveys). If it can be confirmed that the shadow flicker effects received in reality would be less than the threshold limits of 30 minutes per day and/or 30 hours per year, then mitigation would not be necessary.

About Pager Power

Pager Power’s industry-standard modelling and almost two decades of wind project experience ensure accurate assessments that give stakeholders confidence and help keep projects on track.

For more information about what we do, please get in touch.

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