White Paper: The Role and Importance of Weather Data in Solar Plant Performance Evaluation

Solar plant monitoring:The Role and Importance of Weather Data

in Solar Plant Performance Evaluation

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.

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.

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.

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.

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.

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.

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 ruleout 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.

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:

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 pyranometersb) the average of the second-class pyranometersc) the average of the reference cells

The variation of the differently calculated PRs is significant. In addition, the erroneous irradiancemeasurements taken from the reference cells lead to unacceptable values (PR > 100%).

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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