Wind Power Generation - Renewable Energy: Technology, Economics and Environment - page 344

sounds. According to current knowledge, there is also very little impact on ani-
mals.
Disco effect. In times of high direct solar radiation, luminous reflectance may oc-
cur at rotor blades, if solar radiation is reflected by the mirror-like surface of the
rotor blades. However, luminous reflectance occurs accidentally and is only per-
ceived temporarily at certain sun positions. Due to the concave rotor surfaces
disco effects are very low. A constant impact over several consecutive hours can
be almost ruled out. The described effects may be further reduced by low reflect-
ing rotor blade design, which has become common practice for rotor blade manu-
facturing.
Shadow impact. The term shadow impact refers to the moving shadow cast by the
rotor blades at times of sunshine /7-13/. Shadow impact depends among other fac-
tors on the weather and the solar altitude as well as on the converter size and the
form of operation. For e.g. a 1.5 MW converter maximum shadow impact is per-
ceivable up to a distance of about 1,000 m. Shadow impact is measured by the
theoretical maximum impact duration (for any case sunshine, rotating rotors and
unfavourable wind direction, impacting the rotor position towards the sun, are as-
sumed) and real impact duration (the shadow is calculated for the actual weather
conditions). Actual shadow impact time amounts to approximately 20 % of the
theoretical absolute shadow impact duration.
Nevertheless, shadows may impact the people living in the neighbourhood of
wind energy converters or wind farms. For this reason a survey has been con-
ducted on the impact of periodic shadows in terms of perceiving and managing
the problem and behavioural aspects. The test persons exposed to a mean shadow
duration of 5 to 10 h/a showed individual effects with regard to the above three
criteria; for instance for over 15 h/a of weighted shadow duration, strong effects
have been observed for all three areas of investigation. Further investigations have
been conducted to determine whether periodic shadow impacts with duration of
30 minutes and single occurrence may cause stress. Although none of the test per-
sons experienced considerable strain, the proven increased strain on psychological
and physical resources are indicative that cumulative long-term effects may fulfil
the criteria of being an added stress factor /7-14/.
However, adequate planning in the preliminary stages of wind converter pro-
jects and appropriate micro-siting allows to considerably reduce the duration of
shadow impact /7-15/, /7-25/ and thus minimises all related problems for resi-
dents.
Ice throw. Under certain meteorological conditions icing may occur at the blades
of a wind turbine that may become loose and fall off during start-up. The risk of
icing mainly depends on the respective meteorological framework conditions and
thus on the particular site (e.g. low or high mountain ranges). However, the mortal