The 61724-1 standard for PV system performance monitoring has been revised. The new version, released February 2017, defines “accuracy classes”. In conformity declarations, providers must state the accuracy class of the measurement. The class is not only determined by the hardware that is used, but also by quality checks and measurement procedures. The standard contains detailed specifications at monitoring system component level. This memo offers comments on consequences of the new standard concerning the selection of pyranometers. It shows requirements for solar radiation measurements and which pyranometers comply.
The first edition of IEC 61724-1: Photovoltaic system performance monitoring – Guidelines for measurement, data exchange and analysis –, dates from 2008. It now has been updated. The new 2017 version of the standard is fundamentally different from the 2008 version. The new scope not only defines the measuring system components and procedures (as in the 2008 version), but also aims to keep measurement errors within specified limits. It does so by establishing accuracy classes for monitoring systems.
The new standard includes:
- 3 accuracy classes, A, B and C, for monitoring systems, to be used in conformity declarations
- accuracy requirements for monitoring equipment per class
- required quality checks (i.e. calibration and maintenance) per class
- recommended minimum number of instruments used as a function of the PV system scale
The 2017 version of the standard recognises that the solar irradiance measurement is one of the weakest links in the measurement chain. It specifies for each class of monitoring system the pyranometer class that must be used, including required instrument heating, azimuth and tilt angle accuracy. It also defines cleaning and calibration intervals for pyranometers. The standard also defines requirements for measurement of module- and air temperature, wind speed and direction, soiling ratio, and (AC and DC) current and voltage.
Why heating and ventilation?
IEC 61724-1:2017 requires pyranometer heating and ventilation for class A, and heating only for class B. Why? Pyranometer domes are made of glass. When facing the sky on a clear night, glass temperature tends to go below dewpoint, so that water condenses on the dome. Heating and ventilation of solar radiation sensors keeps the glass temperature above dewpoint and free from dew and frost deposition. This significantly increases the reliability of the measured data.
The following tables offer an overview of the main elements of the IEC 61724-1 monitoring classification system, its requirements for solar radiation measurement and which pyranometers comply in which accuracy class.
|CLASS A||CLASS B||CLASS C|
|purpose||utility scale PV systems||
large commercial PV systems
|small PV systems|
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