Measuring Solar Irradiance – Differences in Tilted and Horizontal Pyranometers

An ISO 9060 compliant pyranometer is used to very accurately measure solar irradiance. Most of the meteorological data is obtained using horizontally mounted pyranometers. However, with photovoltaic (PV) panels, knowing the tilted global solar irradiance is important.

This article describes the measurement of solar irradiance, and the difference between tilted and horizontal pyranometers.

Measurement of Solar Irradiance

Radiometers provide ground-based solar radiation measurements of superior quality and uniform response across a wide range of spectral bandwidth. These measurements are obtained using a ‘thermopile’ detector with a black coating. The coating absorbs the incoming energy, heats it and converts the rise in temperature to an electric signal. Further, the detector is covered by a window, or a glass or quartz dome, whose quality and precision influence the radiometer performance.

Solar radiation is ultraviolet, visible and near infrared energy, with a wavelength of between approximately 300 to 3000 nm. It consists of three components. The global horizontal irradiance (GHI) that hits the earth’s surface includes the direct normal irradiance (DNI) from the sun and the diffuse horizontal irradiance (DHI) from the sky.

   GHI = DHI + DNI * cos(θ)

where θ is the solar zenith angle.

GHI is measured using a horizontally mounted pyranometer with a hemispherical view. Figure 1 shows the schematic of horizontally a mounted pyranometer. DHI measurements are carried out using a pyranometer shaded from the direct sun beam, while DNI measurements are carried out using a pyrheliometer with a narrow view that allows measurement of the direct beam coming from the sun. However, a high precision automatic sun tracker is required for measuring DNI. The output signals are converted into irradiance in W/m2 using radiometer sensitivities.

Schematic of horizontally mounted pyranometer

Figure 1. Schematic of horizontally mounted pyranometer

The International Standards Organisation (ISO) has defined the pyranometers grades, their performance, and calibration methods in ISO 9060:1990. ISO compliant instruments provide precise measurements of solar radiation in all weather conditions. Figure 2 shows the schematic of an ISO Secondary Standard thermopile pyranometer.

ISO Secondary Standard thermopile pyranometer

Figure 2. ISO Secondary Standard thermopile pyranometer

Types of Pyranometers

These instruments have been used in the scientific community and meteorological world for several years. The results of the horizontal pyranometer measurements are compared with past and present historical data from weather and climate station networks and satellites through various locations irrespective of the type of solar energy system being used.

The wide field of view of PV panels enables them to be placed at a fixed angle to receive a large amount of solar radiation throughout the year. In such cases, another pyranometer should be installed at a suitable angle for measuring the ‘tilted global irradiance’ with the same view as a fixed panel. Figure 3 shows the rugged adjustable tilt mounting for a pyranometer.

Rugged adjustable tilt mounting for a pyranometer

Figure 3. Rugged adjustable tilt mounting for a pyranometer

The tilt angle can be precisely set when the pyranometer features a stable, rugged mounting. Year-round data on the solar energy resource is usually provided by placing a pair of horizontal and tilted pyranometers at various locations. This allows for the selection of optimal locations for solar energy plants.

This kind of setup can often be observed in generating sites with fixed angle panels to carry out reference measurements of the available energy. In these cases, the data obtained from the pyranometer was transmitted to the control room to monitor the overall plant efficiency.

Pyranometer domes remain cleaner when compared to the flat PV panels. They can easily monitor the drop-off in efficiency. However, the integrated digital interfaces in the recent ‘smart’ pyranometer models ensure direct connection to plant data acquisition and control systems.

Conclusion

In general, instruments that exceed or meet the ISO 9060 requirements for secondary standard pyranometers are usually recommended for use. As far as maintenance is concerned, the domes need to be kept clean and periodically checked. Similarly, the desiccant that keeps the internal portion of the instruments dry should be replaced periodically. In order to maintain accuracy, recalibration is required every two years.

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