Monitoring Solar Irradiance

A number of issues are common to stakeholders, providers, existing and prospective customers in the solar energy industry. These are typically highlighted in exhibitions, workshops or via support requests.

Monitoring Solar Irradiance

Image Credit: OTT HydroMet - Solar Energy

To help address these common issues, Kipp & Zonen have compiled a white paper that summarizes the most common requests for information and clarification. Many of these questions stem from confusion resulting from recent changes in key international standards – a common issue across the industry. 

The white paper explains what can be understood as solar radiation, its specific properties and how the different components arriving at the Earth’s surface are not only related but how they are influenced by the atmosphere and sky conditions.

The ISO 9060 Standard

ISO 9060 is entitled “Solar energy – Specification and classification of instruments for measuring hemispherical solar and direct solar radiation”. This standard defines the use of a pyranometer in terms of measuring global horizontal or tilted irradiance (GHI and GTI) and - when shaded - diffuse horizontal irradiance (DHI).

The standard also defines and explores the use of a pyrheliometer for measuring direct normal irradiance (DNI). Plane of array (POA) irradiance is the most important parameter for the majority of PV applications.

This standard was first introduced in 1990, with many individuals working in solar energy being familiar with its defined performance classifications of Second Class, First Class and Secondary Standard pyranometers – these classifications are listed here in order of improving measurement performance.

Monitoring Solar Irradiance

Image Credit: OTT HydroMet - Solar Energy

Kipp & Zonen’s white paper highlights the reasons why it is only possible to adhere to the specifications by using thermoelectric detectors - generally, a thermopile fitted with a black absorptive coating and a glass dome.

Measuring Solar Irradiance

These definitions changed in November 2018 with the updated ISO 9060 standard. New ‘pyranometer’ classifications are defined - C, B and A - allowing photoelectric detectors with an uneven and limited spectral response.

In the guidance, a well-designed thermopile pyranometer would now be described as ‘spectrally flat’ class C, B or A.

Another sub-category has been introduced for ‘fast response’ instruments. Class A units must also be individually tested to ensure that these sufficiently meet temperature and direction (cosine) response limits.

The IEC 61274-1 Standard

The IEC 61274-1 standard was first published in March 2017 - Photovoltaic system performance – Part 1: Monitoring. This standard defines three distinct classes of monitoring for every type of PV solar power plant.

These range from basic (C), to medium (B) to high accuracy (A). The guidance still refers to the original ISO 9060:1990 pyranometer classifications, however.

It should be noted that ISO 9060:2018 Class A pyranometer would not automatically meet the Class A monitoring requirements of IEC 61724. A range of other implications should also be considered, such as maintenance costs.

The white paper from Kipp & Zonen provides guidance on these complexities, also highlighting other commonly misunderstood topics - pyranometer measurement uncertainty, soiling monitoring and approaches to keeping optical domes and windows clean.

The white paper also contains a section that focuses on the importance of accurately monitoring solar irradiance in each phase of utility-scale solar power projects, beginning with site prospecting and plant design, through to installation and commissioning and operation and maintenance.

The white paper provides a robust explanation of the benefits of high quality irradiance data when looking to quantify performance ratios before concluding with several examples of international solar energy projects that rely upon Kipp & Zonen’s solar radiation monitoring equipment.

This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet - Solar Energy.

For more information on this source, please visit OTT HydroMet - Solar Energy.


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