Shadow flicker assessment for wind turbines and wind farms
Shadow flicker assessment for wind turbines: we calculate periodic shadowing, optimise the layout and help prevent conflicts with local residents.
Depending on the site, solar irradiation and terrain conditions, the rotating blades of wind turbines cast a periodic shadow that sweeps across buildings and land at regular intervals. Professional wind farm planning therefore includes calculating the future shadow flicker. Using precise calculations and models, we identify the affected areas, optimise the siting (see micrositing), take measures where necessary and ensure compliance with the requirements relevant in the procedure. In this way, conflicts with local residents are avoided at an early stage. The shadow flicker assessment provides the evidence of at which receptors, and to what extent, shading occurs.
In procedures, a shadow flicker assessment is evaluated against technical guide values depending on the federal state and the procedure; commonly used are no more than 30 hours per year and 30 minutes per day of astronomically maximum possible shading. These are not uniform statutory limits. Where an automatic shutdown system is used that takes meteorological variables such as sunshine intensity into account, a guide value of no more than 8 hours per year for the actual shading duration is often applied in addition; the relevant value is to be set in the respective procedure. The values are considered for all turbines that affect a receptor together, including neighbouring wind farms. According to the LAI guidance on shadow flicker, the daily guide value applies to the astronomically maximum possible duration for planned turbines and to the actual duration for existing ones. If guide values are exceeded, measures are required. This lets developers and authorities see whether a shutdown becomes necessary and how it affects operation.
Relevant receptors are living rooms, bedrooms, classrooms and offices; terraces and balconies count equally during the day. According to common assessment practice, shadow flicker is considered where a rotor blade covers at least 20 per cent of the solar disc; the minimum share is set in the respective assessment concept. The duration decreases with distance, while larger rotors and higher hubs increase the range. Sun elevations below 3 degrees are mostly disregarded, terrain heights are taken into account, but shielding by vegetation and buildings often is not. In the so-called greenhouse mode, a receptor is calculated as if it could be shaded from all sides. The astronomical calculation is therefore a conservative upper limit. Only operation is assessed, not construction or decommissioning, and where necessary a medical expert opinion evaluates the effect on people.
If the guide values are exceeded, shadow flicker can be limited with an automatic shutdown system. It works with light sensors; a direct solar radiation of 120 W/m² is often used as the threshold for shadow-casting sunshine, but the value set in the decision or assessment concept is decisive. Conditions often require proof of the parameterisation before commissioning and operating logs that are kept for at least one year. Matt coatings with a low gloss level on the rotor blades also reduce disturbing light flashes. We feed the findings of the shadow flicker assessment directly into ongoing planning and into the application documents for the authorisation procedure (specialist support for the authorisation procedure). The visual effect on local residents also includes the landscape, which the landscape and townscape assessment evaluates. Further reports in the permitting procedure: operational and construction noise assessment.
