What is Shadow Flicker?
The intermittent shadow cast over the window of a property caused by rotating wind turbine blades obstructing the Sun’s rays is described by the term ‘shadow flicker’. This can be a cause of annoyance at residences near onshore wind turbines if it occurs for a significant period of time during the year.
What Does the Guidance Say?
There is no formal process regarding the assessment of shadow flicker the UK. However, there are various sources of guidance that have influenced the thresholds and methodology used within Pager Power impact assessments; these include:
- Parsens Brinckerhoff, 2011 – Update of UK Shadow Flicker Evidence Base;
- Department for Communities & Local Government (July 2013): Planning practice guidance for renewable and low carbon energy.
These guidance documents provide context for the assessment area and the thresholds applied to determine the level of impact towards a dwelling.
How are the Receptors Identified?
Typically, the assessment area is defined by considering the turbine rotor diameter and the location of the dwelling relative to the turbine.
It is usually only residential dwellings that are considered within the assessment due to the potential impact on amenity; however, commercial properties and educational establishments may also be considered in specific instances.
What are the Effects?
Shadow flicker effects can only occur if there is a clear, unobstructed path between the turbine and the affected window(s). If the turbine is not visible from a receptor i.e. a dwelling, school etc., no shadow flicker effects will be experienced, as the turbine’s shadow will not pass over any window.
If the view is unobstructed, the duration of the shadow flicker is considered in line with the relevant guidance documents and industry best practice to determine whether mitigation is a requirement. The limits commonly applied within the UK, and in other countries such as Germany, Belgium and Northern Ireland, is a maximum of 30 hours per year with a maximum of 30 minutes per day.
How can Shadow Flicker be Mitigated?
The two most common methods of mitigation are:
- A shutdown scheme – the wind turbine will be shut down at the dates and times at which shadow flicker occurs. This may only need to be for the time it occurs over and above the relevant shadow flicker thresholds.
- Screening – physical screening at the location of the receptor can be implemented to remove views of the turbine from the affected windows.
How can Pager Power Help You?
Pager Power offer comprehensive services relating to shadow flicker and can assist with the following:
- Initial modelling;
- Mitigation analysis (e.g. shutdown schemes);
- Assessment of additional complex parameters such as localised weather and prevalent wind conditions;
- EIA input.
About Pager Power
Pager Power undertakes technical assessments for developers of renewable energy projects and tall buildings worldwide. For more information regarding shadow flicker and what we do, please get in touch.
Image accreditation: Reference: Pixabay, White Windmill.Last accessed on 16th May 2025. Available at: https://www.pexels.com/photo/white-windmill-414807/
