Wind Resource - Wind Energy: Renewable Energy and the Environment - page 64

propeller's axis. The vane requires a minimum force to initiate movement. The threshold wind
speed for this force, usually, is of the order of 1 m/s. Normally the motion of the vane is damped to
prevent rapid changes of directions. Wind vanes generally produce signals either by contact closures
or by potentiometers. The accuracy obtained from potentiometers is higher than that obtained from
contact closures, but the latter are less expensive.
4.5.3 I NSTRUMENT C HARACTERISTICS
Sensors, transducers, and signal conditioners measure and transform signals for recording.
Resolution is the smallest unit of a variable that is detectable by a sensor. Recorders may limit the
resolution. Reliability is a measure of an instrument's ability to produce useful data over a period of
time. The best indicator of reliability is the past performance of similar instruments.
Accuracy and precision are two separate measures of system performance that are often treated
ambiguously. Accuracy refers to the mean difference between the output of a sensor and the true
value of the measured variable. Precision refers to the dispersion about the mean. For example, an
instrument may produce the same measured value every time but produce a value that is off by 50%.
That system has a high precision but low accuracy.
The accuracy, however, may be a function of time, or dependent on maintenance. Anemometers
are calibrated in wind tunnels, where the airflow is steady. Another calibration of performance,
scale and offset, of anemometers for wind resources assessment uses the controlled velocity method
(boom mounted on truck). Generally, calibrated anemometers produce a signal that is accurate to
within 0.5 to 2% of the true wind speed. Under normal use in the atmosphere, good anemometers
should be accurate to around 2 to 4%.
The distance constant is the length of fluid flow past a sensor required to cause it to respond to
63.2% of a step change in speed. A step change is change from one value to another value, similar in
shape to stair step. The larger and heavier cup anemometers usually have distance constants of 3 to
5 m. For light-weight and smaller cup anemometers, such as those used for turbulence, the distance
constant is typically about 1 m. The time constant is the period that is required for the sensor to
respond to 63.2% of a step change in input signal.
The damping ratio is a constant that describes the performance of a wind vane in response to a step
change in wind direction. The damping ratio is dimensionless and is generally between 0.3 and 0.7.
The sample rate is the frequency (Hz) at which the signal is sampled. This may include the time
for recording the data. Since a large amount of data requires large storage, wind speeds are averaged
over a longer time period, and these are the values stored, along with standard deviations. Typical
values for wind power analysis are sample rates of 1 Hz and averaging time of 10 min. Previously,
1 h averaging times were used for many resource assessment projects.
4.5.4 M EASUREMENT
Anemometers mounted on towers should be mounted away from a lattice tower a distance of two to
three tower diameters to reduce the effect of the tower on the airflow. For solid towers, they should
be mounted six tower diameters away. Met towers have to be located away from the influence of
obstacles: trees, buildings, etc.
The time and money spent for measuring the wind resource depends on whether it is for a wind
farm or a small wind turbine. The difference between finding class 3 and class 4 and above wind
sites will easily determine the economic viability for wind farms. Individuals who install small
wind turbines tend to overestimate the wind resource before their turbine is installed, and then
bemoan the lack of wind afterwards.
Instrumentation for measuring turbulence and the wind inflow for wind turbine response uses
multiple anemometers and a higher sampling rate. A system for characterizing turbulence [28]