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Noise and meteorological monitoring of a wind farm
Massimiliano Masullo, Francesco Sorrentino and Giuseppe Ciaburro
Second University of Naples, 81031 Aversa, Italy, Email: massimiliano.masullo@unina2.it
sorrentino_francesco@ymail.com giuseppe.ciaburro@unina2.it
Introduction
Wind energy is one of the cleaner and more environmentally
friendly of renewable energy, nevertheless the installation
sites of Wind Farms (WF) must comply with different
requirements (e.g. wind speed and direction, absence of
obstacles, distances to dwelling). Mountain ridges and hills
sites far from urban centre fulfil these requirements and were
the firsts to be occupied by WF. However, the continuous
increase of wind energy systems has implied that even more
portions of territory were involved by the WF installation.
Moreover rather than their high level of environmental
compatibility to others energy plants, a public resistance
toward WF is frequently encountered. The motivations
ascribed to the opposition are mainly connected to physical
(noise and visual impact) and individual aspects (green
energy or wildlife preservation attitude, degree of
involvement in the project, economic benefits, NIMBY).
The methodologies for noise assessment of WF at the
receiver points are rather standardized and widely described
by national legislations. Two methods are generally used.
These methods assess the WF noise contribution from the
measurements of the Combined Noise Level and of the
Background Noise Level. In the first method, the
measurements must be carried out in two different temporal
stages: when the wind turbines (WTs) are working and then,
immediately after the time that the WTs are parked [1]. In
this way the background noise may be assessed, assuming
that the measurements are recorded under the same weather
conditions (remaining unchanged between these short time
intervals). Subtracting the Background Noise Level from the
Combined Noise Level it is possible to obtain the
contribution of the WTs noise. Moreover some legislation
recommend to measure the Background Noise Level “prior
to site construction and operation” with a long-term
measurement survey [2].
For the second method two simultaneous measurements
must be carried out: "on-site", as measurement of the
Combined Noise Level, and "off-site", as the likely
Background Noise Level on-site. The requirements to select
off-site point for measurements are: 1) far enough, where
WTs noise is negligible; 2) close enough to be representative
of the area [3]. As in precedence the WTs noise contribute is
the difference between Combined Noise Level and
Background Noise Level.
In this paper are suggested an analysis to detect the impact
of different functioning conditions by long term
measurements. A cluster analysis was applied to the noise
and meteorological long term measurements at an outdoor
receiver point (Fig. 1) situated in the yard of a dwelling, at
the distance of 200 from the closest REpower MM92 wind
turbine of a WF.
Figure 1: Measurement activities at receiver point
Measurements
A sound level meter (SLM) supplied with batteries in a IP65
box, was used for the long term noise monitoring. The SLM
equipped with a ½” microphone (50 mV/Pa, class 1) and a
double wind screen was positioned at 2 meters height far
from reflecting surfaces. Every 10 minutes the overall sound
equivalent (Leq) and the statistical (L
50
, L
90
, L
95
) levels, as
well as theirs 1/3
rd
octave band spectrum, were logged.
Additionally a Vantage Pro2 weather station was positioned
at the same height, close to the microphone position. The
average and the maximum wind speed, wind direction,
temperature, humidity, barometric pressure and rainfall were
logged at the same temporal interval. The measurement
activities lasted about 1 month.
Monitoring results
In order to observe the worst condition at the receiver the
wind direction data were used to identify and extract the
downwind conditions. Moreover, exclusively the nocturne
measurements (22-06) were taken into account to reduce as
far as possible the contributions of others noise sources
(lower background noise). According to the literature [4] the
level L
95
was used as major descriptor of the phenomenon.
At the same time, the wind speed at the height of 10m were
estimated by using a power law (1):
α
⋅=
rif
rif
z
z
zUzU )()(
[m/s] (1)
where: