Expert Report · Mandatory under BImSchG · LAI Guidelines

Shadow Flicker Report for Wind Turbines

Basis: LAI Guidelines · max 30 h/year astronomical · 30 min/day · 8 h/year real

The shadow flicker report (Schattenwurfgutachten) predicts how many hours per year and how many minutes per day the moving rotor shadow reaches the nearest residential buildings — so-called "periodic shadow effect". It is a mandatory component of every BImSchG permitting procedure.

Shadow flicker zones of a wind turbine: plan view with shadow areas per LAI guidelines -- red above 30 h/year, yellow 8-30 h/year, green below 8 h/year

Shadow flicker zones around a WTG — schematic plan view per LAI guidelines (30/30/8 rule)

What does the report assess?

Astronomically maximum shadow flicker: worst-case calculation assuming the sun is always shining, the wind always blows from the unfavourable direction, and the rotor always rotates. Threshold: ≤ 30 h/year and ≤ 30 min/day.
Real shadow flicker: accounting for average cloud cover, wind conditions, and standstill periods. Threshold: ≤ 8 h/year.
Cartographic representation: iso-lines of shadow flicker as overview + detailed evaluation per immission point.
Mitigation concept: if thresholds are exceeded, shutdown via shadow flicker module with astronomical clock and light sensor.

The LAI guidelines (summary)

The "Guidelines on the determination and assessment of optical immissions from wind turbines" of the LAI (German Federal/State Working Group on Immission Control, last version 2002, currently under revision) are adopted as binding assessment basis in almost all federal states:

Maximum 30 hours per calendar year of astronomically calculated shadow flicker at an immission point
Maximum 30 minutes per day
Real-meteorological maximum 8 hours per year

Practical solution: When thresholds are exceeded, a shadow flicker shutdown module is installed (controlled by astronomical clock, light sensor, optionally wind direction). The module shuts down the turbine preventively once the daily or annual budget at the affected immission point is reached. Typical yield loss < 1%.

What does the report cost?

Guideline value EUR 3,000 – 12,000 for a wind farm with 3–6 turbines. Key factors:

Number of turbines + number of immission points
Topography (complex terrain makes rendering more elaborate)
Shutdown logic design for many immission points

Who prepares the report?

Specialised acoustics and immission engineering firms — typically the same ones that also prepare the sound protection report (combined commissioning often saves 20–30%). Key software: WindPRO (EMD), WindFarmer (DNV), WAsP extensions.

Shadow flicker report + sound protection as a package?

We forward your enquiry to a firm that prepares both reports as a combined commission — this is usually faster and more cost-effective than separate commissions.

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Frequently asked questions

What about light flashes at sunset ("disco effect")?

Rotor blades of modern WTGs have a matte coating that virtually eliminates this effect. Older turbines with high-gloss paint have been decommissioned. In the report, this is generally mentioned only as a note.

Is the shutdown module active from day one of commissioning?

Yes — the shadow flicker module is configured before commissioning with the immission point coordinates and the daily/annual budgets specified in the report. It shuts down the turbine automatically from the first day of operation.

What happens if new residential buildings are built within the impact zone after commissioning?

Protection under the LAI guidelines applies only to residential buildings that existed or were planned at the time of permit approval. New construction in the impact zone after commissioning has no protection claim. Recommendation nonetheless: update the module configuration to avoid complaints.

Can the shadow flicker calculator on this site do this?

The online calculator only shows the astronomically possible shadow flicker of a single turbine — a good initial plausibility check. An official report must additionally aggregate all turbines in the farm, consider topography, and use certified software (WindPRO or similar).