The IEC 61724-1:2021 standard for PV

The IEC 61724-1:2021 standard for PV monitoring systems: a quick explanation

Selection of pyranometers for compliance with the new standard

The 61724-1 standard for PV system performance monitoring has been revised. The latest version, released July 2021, defines 2 “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 standard concerning the selection of pyranometers. It shows requirements for solar radiation measurements and which pyranometers comply.

Introduction

The first edition of IEC 61724-1: Photovoltaic system performance monitoring – Guidelines for measurement, data exchange and analysis –, dates from 2008. It has been updated twice. The latest 2021 version of the standard is fundamentally different from the 2008 version and has slightly changed relative to the 2017 version.

The new standard includes:

2 accuracy classes, A and B, for monitoring systems, to be used in conformity declarations. The 2017 Class C is now Class B
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
new in 2021: requirements for reflected radiation and albedo measurement
requirements for tilt sensors included

Consequences

The 2021 version of the standard recognises that the solar irradiance measurement is one of the weakest links in the measurement chain. For Class A systems, it specifies the Class of pyranometer that may be used, including requirements for dew and frost mitigation, azimuth and tilt angle accuracy. It also defines cleaning and calibration intervals for pyranometers. Furthermore, the standard defines requirements for measurement of module- and air temperature, wind speed and direction, soiling ratio, and (AC and DC) current and voltage.

Why heating?

IEC 61724-1:2021 requires pyranometer dew and frost mitigation for class A monitoring systems. 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 keep the glass temperature above dewpoint and free from dew and frost deposition. This significantly increases the reliability of the measured data. There is an exception for locations where dew and frost is expected for less than 2 % of annual GHI hours.

Figure 1 Frost and dew deposition: clear difference between a non-heated pyranometer (back) and SR30 with heating.

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.

**Table 1** *The main elements of the IEC 61724-1:2021 PV monitoring system classification system.*
	CLASS A	CLASS B	CLASS C
accuracy	high	medium	low
target application	utility scale PV systems and large commercial	rooftop or small to medium size commercial New in 2021: Old (2017) Class C is now Class B	New in 2021: No longer used