MONITORING STATION INSTRUMENTATION AND MEASUREMENTS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 47

4.2.6 Global Solar Radiation
The solar energy resource can be measured as part of a wind monitoring program.
Solar radiation, when used in conjunction with wind speed and time of day, can also
be an indicator of atmospheric stability.
A pyranometer is used to measure global horizontal (total) solar radiation, which is
the combination of direct sunlight and diffuse sky radiation striking a horizontal plane.
One common type of pyranometer uses a photodiode, which generates a very small
current proportional to the amount of incident sunlight (called insolation ). Another
type uses a thermopile, a group of thermal sensors, which produces a very small
voltage. The data logger (or a supplementary interface device) applies a predetermined
multiplier and offset to calculate the global solar radiation reading. Since the output
signal from the sensor is usually very small (microamps or microvolts), it may have
to be amplified to be read by the logger.
The pyranometer must be leveled to measure global horizontal solar radiation accu-
rately. When installed on a tower in the northern hemisphere, it is best to locate the
sensor on a boom extending southward, above or beyond any obstructions to minimize
shading from other instruments and the tower; the reverse applies, of course, in the
southern hemisphere. The recommended measurement height is 3-4 m above ground.
Pyranometers may require frequent maintenance visits for cleaning and releveling.
4.3 RECORDED PARAMETERS AND SAMPLING INTERVALS
It is highly recommended that the parameters to be measured and their sampling
and recording intervals conform with typical wind industry practice. Adherence to
these standards will facilitate the wind resource analysis and any subsequent external
review. The industry standard recording interval is 10 min, although occasionally
other (usually shorter) intervals are used. The parameters are generally sampled once
every 1 or 2 s (depending on the logger model) within each 10-min interval (to avoid
errors, the sampling frequency should not be greater than the pulse frequency from the
anemometer). Depending on the parameter, the data logger records interval averages,
standard deviations, and maximum and minimum values. Data recording should be
serial in time, with all records marked by a time and date stamp. These requirements
are all standard functions of data loggers designed for wind energy applications. It
should be noted that the time stamp for some data loggers refers to the preceding
interval, while for others it refers to the following interval.
The recorded values are the basis for the data validation procedures described in
Chapter 9. Each is presented below and summarized in Table 4-5.
4.3.1 Average
The average or mean value in each 10-min interval is recorded for all parameters
except wind direction. For the direction, the average is defined as a vector resultant
value , which is the direction implied by the means of the northerly and easterly