PD ISO-TR 9901-2021 pdf free download – Solar energy — Pyranometers — Recommended practice for use.
4 Selection of pyranometers and accessories4.1General
A pyranometer performs a hemispherical irradiance measurement in W/m2. lt is important to realizethat in many applications for example when working according to monitoring standards IEC 61724-1[ 21_1,ASTM G183[ 22_l and wMO[ 3 l a measurement are accompanied by a time stamp. Both theirradiance and the time stamp have a measurement uncertainty. See 5.2.5.2 and 5.6 for more details onuncertainty evaluation.
The pyranometer selection is often based on the wish to attain a certain measurement uncertainty.There also may be other considerations:
a) Task-specific criteria, such a maximum response time, or the requirement to comply with astandard.b) Operational criteria,such as dimensions，weight,stability,measures to mitigate dew, frost,
precipitation and soiling, and maintenance requirements of the instrument and accessories.
c)Economic criteria, costs of mechanical and electrical integration in a system depend on the
instrument characteristics. Also costs of recalibration,inspection and maintenance maybe considered.
When selecting an instrument there are two common ways to make a choice,described in thefollowing clauses:
-related to the pyranometer accuracy class;
-related to the specifications of the pyranometer and its accessories.4.2Pyranometer selection based on accuracy class
In some applications the choice of instrument is driven by the pyranometer accuracy class. The classis often, but not necessarily related to the type, i.e. the technology used (e.g. with photodiode orthermopile sensors).
The choice of a certain accuracy class is often driven by the requirements of standards.Table 1summarizes the required pyranometer accuracy class for the most common application of PV systemperformance monitoring according to IEC.
NOTE IEC61724-1 is due for revision in 2021,and requirements will possibly change.
ISO 9060:2018 defines 3 pyranometer classes,A, B and C.These classes are “accuracy classes”, whichare defined by JCGM 200:2012 to meet stated metrological requirements that are intended to keepmeasurement errors or instrumental uncertainties within specified limits under specified operatingconditionsl 241.
The accuracy classification as used in Iso 9060 does not by definition mean that a higher classpyranometer will provide a higher accuracy measurement; this entirely depends on the application.Besides classification as class A,B, and C, ISo 9060 makes a further distinction between 2 main typesand an independent sub-category:
-spectrally flat pyranometers; most thermoelectric pyranometers are in this category;—(non-spectrally flat) pyranometers; photodiode pyranometers may qualify for this category;
Pyranometers classified in ISO 9060as “spectrally flat” have aspectralselectivity of less than 3 %(guardbands 2 %) in the 0,35 um to 1,5 um spectral range. This is the same requirement as in the previousISO 9060:1990 for secondary standard pyranometers. Spectrally flat pyranometers are typicallymore accurate over a wide range of conditions, and applicable not only for horizontal measurementof global horizontal irradiance, GHl, but also for measurements of plane of array irradiance,POA, andreflected irradiance,Rl, as well as for artificial solar sources such as lamps.IEC 61724-1 requires use ofinstruments of a specified accuracy class for its class A and B monitoring systems. There is consensusthat the spectral selectivity specifications of ISO 9060 “spectrally flat” pyranometers have a negligible(zero) spectral error and that they can be used for all the common outdoor measurements in solarenergy studies with the same calibration (typically performed with the clear sky solar spectrum asthe source) without significant loss of accuracy.The clear sky solar spectrum is one of the referenceoperating conditions for pyranometers if it is the source under which an instrument is calibrated or thesource under which a calibration reference standard has been calibrated.
Pyranometers employing photodiodes (otherwise known as silicon-pyranometers), are not classified as”spectrally flat” in ISo 9060.The spectral error of pyranometers is defined for a set of clear sky solarspectra only. This implies that their spectral error for other than clear sky spectra cannot be basedon the classification alone. The spectral error of pyranometers, in particular if they are not spectrallyflat, may be larger for measurements of DHI, POA or Rl than for clear sky GHI. The user may performan individual uncertainty evaluation depending on the manufacturer specification of the instrumentand the spectra of the measured radiation.T’he factory calibration of non spectrally flat instruments istypically valid for a set of clear sky solar spectra.Their sensitivity and uncertainty of their sensitivitymay both change for different conditions.PD ISO-TR 9901 pdf download.