1. Solar plant monitoring:
The Role and Importance of Weather Data
in Solar Plant Performance Evaluation

2. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 2
Meteo Sensors used in PV Plant Monitoring
Pyranometers - quantify the solar irradiance incident on their surface with sensitivity over the entire
spectrum for which modules may be active.
Reference cells - incorporate one or more photovoltaic cells and can measure the irradiance and the cell
temperature together.
Back-of-module temperature sensors - are mounted in direct contact with the back of a PV module.
Ambient air temperature sensors - quantify the temperature of the air around the PV plant under
shade and isolated from wind.
Hygrometers, barometric pressure sensors, wind speed sensors (anemometers), wind direction
sensors and rain gauges - measure factors that may be of interest but are not commonly used in
performance estimates.

3. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 3
The importance of the correct use of meteo sensors (1/4)
Since accurate meteo data are
critical to the evaluation of plant
performance, recording
erroneous values is not an
option.
The data acquisition system must:
support outlier detection
identify the root cause of the
failure leading to the injection of
outlier values.
Outliers can occur in many places along the data acquisition path. However, they usually
occur at the sensor and the cabling infrastructure connecting it to the data acquisition
device.

4. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 4
The importance of the correct use of meteo sensors (2/4)
Another method to ensure
correct meteo-data
measurements is to install
identical redundant sensors at
the plant.
If a sensor fails, known and
adequate backup sources are
already online.
If readings from multiple sensors –
which are expected to be very
similar - can be compared for each
sampling period, any unreasonable
values can be excluded from
subsequent processing as well.

5. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 5
The importance of the correct use of meteo sensors (3/4)
Data acquisition period
It is industry practice to monitor the
operational parameters of solar
power plants using 10 or 15-minute
averaged values.
Meteo conditions, especially
irradiance, are known to exhibit a
highly time-variant behavior.
Consequently, taking a snapshot
every 15 minutes is not an
appropriate option.
Data acquisition devices must sample
considerably faster (down to 1 second
as far as irradiance measurements are
concerned) and average the acquired
time series across 15 minutes.

6. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 6
The importance of the correct use of meteo sensors (4/4)
Meteorological conditions, apart
from being time-variant, are also
expected to differ from one
place to another within a large
PV plant.
For a plant of sufficient size, KPIs for
specific parts of the plant must be
collected using only the most locally
relevant meteo data.
In the case of a PV plant with
geographical variations - namely PV
modules on multiple slopes - this
averaging function needs to take into
consideration the tilt and orientation
of the slopes.
Certain KPIs can only be formulated for
the entire site therefore PV plant
monitoring system shall have to support
spatial averaging of measurements
coming from different parts of the site.

7. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 7
The use of weather stations
In order to:
The location of a meteo sensor
set must be clearly defined in
the PV plant monitoring system
Exploit a meteo
sensor’s redundancy
Apply specific meteo
sensor measurements
to the calculation of
location-aware KPIs
All meteorological services around the world use the notion of weather station as the
basis for meteorological condition monitoring.
The weather station is a set of meteo sensors, including any redundant ones, along with
any supporting structure, power supply and possible local digitization, processing and
storage of measurement values.

8. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 8
The case of Irradiance Measurements (1/4)
A comparison study of different types of irradiance measurement instruments based on real data.
The data are from a 1.7 MWp solar plant.
Installed irradiation sensors:
2 WMO secondary standard pyranometers
7 WMO second-class standard pyranometers
2 reference cells.
The observation period selected was a fortnight of October.
The solar plant had been fully operational for more than one year before this observation period.
Only irradiance values above 200 W/m2 are taken into consideration in order to rule
out any shading effects during the early-morning or late-afternoon values or very cloudy intervals.
The same threshold of 200 W/m2 is also used for the calculation of the Performance Ratio.

9. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 9
The case of Irradiance Measurements (2/4)
The figures below show the percent error of measurements regarding each of the
second-class pyranometers and reference cells from the average value of the
two reference pyranometers:

10. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 10
The case of Irradiance Measurements (3/4)
The importance of correct irradiance
values is clearly visible when it comes
to the calculation of performance
indices, for example,
the Performance Ratio.
The following graph shows the PR
calculated having as irradiance input
data:
a) the average of the secondary
standard pyranometers
b) the average of the second-class
pyranometers
c) the average of the reference cells
The variation of the differently calculated PRs is significant. In addition, the erroneous irradiance
measurements taken from the reference cells lead to unacceptable values (PR > 100%).

11. The Role and Importance of Weather Data in Solar Plant Performance Evaluation 11
The case of Irradiance Measurements (4/4)
Instrument placement is another very
significant one that may have a major
effect on the data’s credibility.
A pyranometer with a slightly different azimuth
(e.g. 3° east) compared to the PV panels’ azimuth
cannot be a reliable source of irradiance data.
The phase difference between the power and the irradiance curve reveals that the azimuth of
panels and of the pyranometer is not identical.

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