Why measure solar irradiation on site?

When developing a large-scale solar project it is generally necessary to predict the energy production of the system with the highest practical level of accuracy. In any energy production assessment, estimation of the long term resource at the site is a significant source of uncertainty.

Solar data has been recorded at meteorological stations around the world for a long time. However, there are often significant distances between these reference stations and proposed project locations; and therefore, the uncertainty associated with extrapolating this data is often significant.

There are also a number of long term databases available based on satellite imagery. These databases often do not account for the effects of aerosols and may mistake reflective surfaces for clouds, resulting in high levels of uncertainty.

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On site measurements correlated to long term reference data generally provide the best indication of the resource that could be captured by a future project.

What should be measured?

Solar irradiation consists of two components: the direct component from the sun and the diffuse component reflected from the surrounding atmosphere. These two components combine to make up ‘global’ solar irradiation.

Solar power generation systems may convert either the global irradiation or the direct irradiation, depending on their design. It is therefore important to consider the proposed generation technology when selecting the site monitoring equipment. It is also important to record the temperature at the monitoring location as the efficiency of solar power generation equipment is often dependent on this variable.

What equipment is required?

The ISO 9060:1990 Standard and the World Meteorological Organisation’s Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8) define classification levels for measurement instruments based on reliability and accuracy.

Depending on the desired irradiation information, a selection of the following instruments and a suitable data logger would be required to measure the solar resource:

  • Pyranometer – measures global solar irradiance on a planar surface and must be mounted exactly horizontally to minimise the uncertainty in long term resource predictions.
  • Pyrheliometer – Measures direct solar irradiance and must be installed with a solar tracking system to ensure that the instrument is always aimed at the sun.
  • Solar Tracking System – Enables pyrheliometers to always be pointing at the sun. As part of a solar tracking system, shading discs can be installed to shade individual pyranometers from direct solar irradiation. Shaded pyranometers measure the diffuse component of solar irradiation.
  • Thermocouple – Measures the temperature at the monitoring location and must be installed with a radiation shield to ensure that it is not exposed to direct sunlight.

Where, and how, should the equipment be installed?

The location selected for the installation of solar irradiance measurement equipment should represent of the proposed power generation site and readily accessible for maintenance purposes.

The instrumentation should not be in a position where objects are likely to reflect solar energy onto it or cast shadows over it. The ISO 9901:1990 Standard and WMO-No. 8 provide details of appropriate installation and usage of solar radiation measuring equipment.

How can the measured data be used?

After recording one year or more of valid data at the site, it can be correlated with long term reference data to predict the long term solar energy resource at the site.

This can then be used to design an appropriate solar power generation system and predict its long term energy output with a quantifiable level of accuracy, which is often important when seeking finance for a large-scale project.