Measuring the Difference Between Global Horizontal and Tilted Solar Irradiance

A pyranometer conforming to ISO 9060 provides the highly accurate measurement of solar irradiance. Although most meteorological data is derived from horizontally mounted pyranometers, knowing the tilted global solar irradiance is crucial for photovoltaic panels. This article covers solar irradiance measurement and the difference between horizontal and tilted pyranometers.

Solar Radiation Measurement

Radiometers that have a uniform response over a broad spectral bandwidth are typically used for making high-quality ground-based measurements of solar radiation. The measurement involves the use of a ‘thermopile’ detector with a black coating, which absorbs the incident radiation, heats up, and transforms the temperature rise into a small electrical signal. Since a glass or quartz dome or window covers the detector for protection, their precision and quality will influence the radiometer performance.

Solar radiation is ultraviolet, visible and near infrared energy in the wavelength range of roughly 300-3000nm, with three components. The global horizontal irradiance (GHI) falling onto the surface of the Earth comprises the direct normal irradiance (DNI) from the sun and the diffuse horizontal irradiance (DHI) from the sky.

i.e., GHI = DHI + DNI*cos (θ)


θ is the solar zenith angle (vertically above the location is 0°, horizontal is 90°).

A pyranometer with a hemispherical view that is mounted horizontally is used for GHI measurement, while a pyrheliometer with a narrow view that can only measure the beam directly from the sun is used for DNI measurement and a pyranometer shaded from the direct sun beam for DHI measurement. It is essential to have a high accuracy automatic sun tracker for DNI measurement. The conversion of output signals into irradiance in W/m2 is performed using the radiometer sensitivities (calibration factors).

Pyranometers and Pyrheliometers

ISO 9060:1990 describes the grades of pyranometers and pyrheliometers, and their performance and calibration techniques. Instruments complying with ISO specifications deliver solar radiation measurements accurately under all weather conditions. It is possible to compare the horizontal pyranometer measurements with current and historical data from satellite, weather and climate station networks across different locations irrespective of the solar energy system type that might be utilized.

Photovoltaic panels have a broad field of view and need to be positioned to collect the maximum amount of solar radiation all over the year. They are often mounted at a fixed angle rather than following the movement of the sun in the sky for cost/benefit reasons. Hence, it is essential to mount a second pyranometer at the ideal angle for that location for measuring the ‘tilted global irradiance’ with the same view as a fixed panel. It is necessary for the pyranometer to have a stable, rugged mounting that allows precise setting of the tilt angle

A pair of horizontal and tilted pyranometers are typically installed at several locations to collect data on the solar energy resource throughout the year, thereby enabling the selection of the optimal locations for solar energy plants. A similar set-up is often installed at generating sites with fixed angle panels to collect reference measurements of the available energy. By transmitting the pyranometer data to the control room, the overall plant efficiency can be monitored. ISO Secondary Standard thermopile pyranometer is depicted in Figure 1.

ISO Secondary Standard thermopile pyranometer.

Figure 1. ISO Secondary Standard thermopile pyranometer.

The latest ‘smart’ pyranometers contain integrated digital interfaces that directly connect to plant data acquisition and control systems. It is sensible to employ devices that satisfy, or exceed, the ISO 9060 specifications for Secondary Standard pyranometers. Rugged adjustable tilt mounting for a pyranometer is depicted in Figure 2.

Rugged adjustable tilt mounting for a pyranometer.

Figure 2. Rugged adjustable tilt mounting for a pyranometer.

Well-maintained instruments can limit the overall uncertainties in the solar irradiance measurement to roughly 2%. Cleaning the domes and periodically checking and replacing the desiccant that maintains the dryness of the devices internally are the only maintenance work. To maintain accuracy, it is recommended to carry out recalibration every two years.

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