ISO/TR 9901:2021

TECHNICAL ISO/TR
REPORT 9901
Second edition
2021-08
Solar energy — Pyranometers —
Recommended practice for use
Énergie solaire — Pyranomètres — Pratique recommandée pour
l'emploi
Reference number
©
ISO 2021
© ISO 2021
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ii © ISO 2021 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Selection of pyranometers and accessories . 4
4.1 General . 4
4.2 Pyranometer selection based on accuracy class . 4
4.3 Pyranometer and accessory selection based on other considerations . 6
4.4 Measuring system redundancy and spatial resolution . 6
4.5 Common pyranometer accessories . 7
4.5.1 Electronics, data acquisition and power supply . 7
4.5.2 Heating and ventilation systems . 8
4.5.3 Mounting stands and supports . 9
4.6 Personal safety . 9
5 Recommended practice for use .10
5.1 General .10
5.2 Pyranometers measuring plane of array and global horizontal irradiance .10
5.2.1 General.10
5.2.2 Installation .10
5.2.3 Heating and ventilation .13
5.2.4 Inspection and maintenance .13
5.2.5 Data acquisition and storage .16
5.2.6 Data quality control and correction .19
5.3 Pyranometers measuring diffuse radiation .20
5.3.1 General.20
5.3.2 Installation .22
5.3.3 Heating and ventilation .23
5.3.4 Inspection and maintenance .23
5.3.5 Data acquisition and storage .23
5.3.6 Data quality control and correction .23
5.4 Pyranometers measuring reflected radiation .24
5.4.1 General.24
5.4.2 Installation .24
5.4.3 Inspection and maintenance .25
5.4.4 Data acquisition and storage .25
5.4.5 Data quality control and correction .25
5.5 Pyranometer calibration and performance verification .26
5.5.1 Calibration .26
5.5.2 On-site performance verification/check .28
5.5.3 Introduction of a new pyranometer sensitivity .28
5.6 Uncertainty evaluation of the measurement .29
5.7 Indoor use of pyranometers .32
Annex A (informative) Heating and ventilation systems .34
Annex B (informative) Shading losses in reflected radiation measurement .36
Bibliography .38
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
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on the ISO list of patent declarations received (see www .iso .org/ patents).
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expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 180, Solar energy, Subcommittee SC 1,
Climate — Measurement and data.
This second edition cancels and replaces the first edition (ISO/TR 9901:1990), which has been
technically revised.
The main changes compared to the previous edition are as follows:
— adaptation of the terminology to the revised ISO 9060:2018 including reference to new “non-
spectrally flat” and “fast response” instruments;
— added recommended practices for use of modern pyranometers with a digital output, including
internal diagnostics;
— added recommended practices for use of pyranometers to measure “plane of array” and reflected
radiation;
— added references to the main standards used in solar energy application of pyranometers:
IEC 61724-1:2017, ASTM G213-17 and ASTM G183-15.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2021 – All rights reserved

Introduction
This document contains recommendations for use of pyranometers in solar energy applications. It
summarises the state of the art and updates the first edition of 1990. In recent years the application
of solar radiation measurement, using pyranometers, has risen sharply. The main application of
pyranometers now is no longer scientific research, but assessment of the performance of PV solar
power plants, that is power plants employing photovoltaic solar modules. The reflected irradiance
measurement also has become more relevant with the increasing application of bifacial modules.
Between 1990 and now the use of pyranometers has been further standardized. Two examples are
the 2017 revision of IEC 61724, the group of standards governing use of PV system performance
monitoring, and the 2018 revision of ISO 9060 covering pyranometer and pyrheliometer specification
and classification. The IEC standard implicitly recognises that solar irradiance is a critical and often
the least accurately known parameter in solar energy performance assessment. For those users that
choose to work according to this standard, IEC 61724-1 now defines 3 monitoring system classes and
offers detailed guidelines for use of pyranometers including requirements (not recommendations) for
the pyranometer classes that must be used, for instrument heating and for inspection-, cleaning and
recalibration intervals.
The solar community also has come to realize that a measurement without an uncertainty evaluation
is meaningless. IEC 61724-1 requires this evaluation when measurement results are reported, usually
as PV performance ratio and performance index. ASTM has issued the G213 standard in 2017 for
uncertainty evaluation of the measurement with pyranometers.
The 1990 version of ISO TR 9901 included reference only to “spectrally flat” pyranometers. Now
that ISO 9060 in its latest version also defines and classifies “non-spectrally flat” pyranometers, this
document also refers to the use of these instruments.
As in all above documents, uncertainties mentioned in this document are expanded uncertainties with
a coverage factor k = 2.
TECHNICAL REPORT ISO/TR 9901:2021(E)
Solar energy — Pyranometers — Recommended practice
for use
1 Scope
This document gives recommended practice for the use of pyranometers in solar energy applications
(e.g. testing of solar photovoltaic panels, solar thermal collectors or other devices, and performance
monitoring of solar energy systems). It is applicable for both outdoor and indoor use of pyranometers,
when measuring plane of array, global horizontal and refl
...