Pyranometer selection guide: how to choose the best sensor for your application

Next level instruments in every class at the most attractive price level

Hukseflux offers a wide range of solutions for measurement of solar radiation. This guide offers you general guidelines for selection of the right instrument. The application of pyranometers in PV system performance monitoring according to IEC 61724-1 is highlighted as an example. Sensors specific for diffuse radiation and meteorological networks are also addressed in this selection guide.

The right instrument for the application

Choosing the right instrument for your application is not an easy task. We can offer assistance. But first, you should ask yourself the following questions:

When discussing with Hukseflux, our recommendation for the best suited pyranometer will be based on:

Accuracy improvement by a factor 2

Pyranometers are subject to classification according to ISO 9060:2018. The 3 classes are:

From Class C to Class B and from Class B to Class A, the achievable accuracy improves by a factor 2.

Hukseflux pyranometers

Measurand	hemispherical solar radiation
ISO 9060 clasification	spectrally flat Class A, B and C
Options	analogue and / or digital output; with Recirculating Ventilation and Heating (RVH TM), sapphire outer dome, or use with VU01 ventilation unit; cable length; heaters and internal temperature sensors; various mounting and levelling fixtures

Hukseflux pyranometer benefits

Hukseflux is a leading manufacturer, both in technology and market share, of pyranometers. We offer you the best measurement accuracy in every class. In more detail, superior instrument design allows us to claim:

Whatever your application is: Hukseflux offers the highest accuracy in every class at the most attractive price level.

Compliant with IEC, Class A and B

IEC 61724-1: Photovoltaic System Performance Monitoring – Guidelines for Measurement, Data Exchange and Analysis – requires ventilation and heating for Class A monitoring. Only SR30 offers both, without the need for additional accessories. No other manufacturer offers a pyranometer complying with class A in its standard configuration. Only Hukseflux’s SR30 does. Alternatively, you may consider SR20 pyranometer with an external VU01 ventilation unit. SR15-A1 and –D1, ISO 9060 Class B models equipped with a heater, comply with IEC 61724-1 Class B. Most competing pyranometers do not even comply with Class B, which requires heating. A general summary of the IEC standard can be found in a quick explanation of IEC 61724-1:2017: what’s new? A separate memo offers comments on consequences of the new standard concerning the selection of pyranometers.

Asset management of large scale PV

Asset managers of industrial and utility-scale PV power plants prefer digital secondary standard pyranometers. The reasons why:

Asset managers monitor for various reasons. Apart from monitoring as a tool to assess day-to-day performance, they are interested to have documented proof of performance in case of warranty claims, when negotiating (re-) financing and when selling the asset. For monitoring the plant performance, the irradiance sensor must be more stable than the cells used in the PV system. This is only the case with secondary standard instruments, offering a stability of < 0.5 %/yr compared to a typical > 1% /yr PV cell degradation.

High data availability: use SR30-D1

High data availability is attained by heating of the outer dome using ventilation between the inner and outer dome. RVH™ – Recirculating Ventilation and Heating – technology, developed by Hukseflux, suppresses dew and frost deposition and is as effective as traditional ventilation systems, without the maintenance hassle and large footprint.

low power consumption: SR30 requires only 2 W, compared to 10 W for traditional ventilation systems
low maintenance: SR30 does not require filter cleaning

The dome of SR30 pyranometer is heated by ventilating the area between the inner and outer dome. RVH™ is much more efficient than traditional ventilation, where most of the heat is carried away with the ventilation air. Recirculating ventilation is as effective in suppressing dew and frost deposition at 2 W as traditional ventilation is at 10 W. RVH™ technology also leads to a reduction of zero offsets.

Diffuse radiation: use SR25

Diffuse solar radiation is usually measured using shaded pyranometers. The dominant measurement error is the zero offset a. SR25, equipped with a high thermal conductivity sapphire dome, has very low offsets. SR25 outperforms the quartz dome instruments, traditionally used for this purpose, at a much lower cost level. SR25 has been tested at NREL National Renewable Energy Laboratories of Golden, Colorado, USA, and has been adopted by NREL as one of its diffuse radiation reference sensors.

National networks

In WMO-No. 8, Guide to Meteorological Instruments and Methods of Observation, WMO recommends use of first class or “good quality” pyranometers such as our model SR12 for network operation. Modern networks often use one level higher: secondary standard; such as our model SR30.

Sensors made by Hukseflux passed validation and acceptance testing for a large number of National Meteorological Networks:

India: Centre for Wind Energy Technology (CWET), solar resource assessment network
USA: National Ecological Observatory Network (NEON), meteorological observation network
UK: Centre for Ecology & Hydrology (CEH), measurement / monitoring network
India: India Meteorological Department (IMD), national measurement network
Japan: Japan Meteorological Agency (JMA), national measurement network
China: China Meteorological Administration (CMA), national measurement network, sensors supplied through a technology transfer project.
Ecuador: National Meteorological and Hydrological Institute (INAMHI), national measurement network

NOTE: the fact that a sensor is tested or used in a network does not constitute a formal endorsement by the test institute or network owner.

Need for recalibration

In case you require a first class pyranometer, but wish to avoid a 2-yearly recalibration, use a higher class instrument: secondary standard. Our SR30 secondary standard pyranometer offers better stability and therefore can work within first class limits at a lower recalibration interval.

Our calibration services include traceable calibration of pyranometers of the most common brands, pyrheliometers and heat flux sensors.

Influence of instrument cleaning

The performance of high class instruments strongly depends on cleaning. At a low maintenance level, the achievable accuracy will not be reliably attained. You may then consider using multiple instruments. The use of redundant instruments allows remote checks of one instrument using the other as a reference, which leads to a higher measurement reliability.

For lower class instruments, the relative loss of accuracy at a low maintenance level is less significant. At low maintenance intervals, although this is not formally complying with the IEC, ASTM and ISO standards, use of multiple low class instruments is a good alternative to using a single high class instrument.

Table 1 gives an overview of pyranometers and the most common considerations for choosing a particular one.