DNI measurements with a pyrheliometer
Solar Direct Normal Irradiance (DNI) represents the direct solar component and contains about 80% of the total solar energy budget on earth. Outside the atmosphere (AM0) the solar irradiance is considdered as a constant (1367 W/m2) that slightly fluctuates with a 11 year cycle. The DNI at the earth surface is highly variable due to the atmospheric conditions (clouds, aerosols, water vapor and molecules).
The DNI can be measured with a pyrheliometer (MS-56 or MS-57) that will, installed on the sun tracker and follows the course of the sun during the day. The sun tracker is fully autonomous and can be connected to a data logger to monitor it instant position when following the sun.
HOW-TO Application Guide
Measuring the DNI
Defined by the ISO 9060, DNI is supposed to be measured with a thermopile radiometer, which is sensitive to measure the spectrum in the range from 300nm - 3000nm. The field of view (acceptance angle) must be 5 degrees, to capture the sun (0.5 degree) and part of the circumsolar.
The MS-56 or MS-57 pyrheliometer complies to the ISO standard and is perfectly suited to measure the DNI with high precision when used in combination with the automated sun trackers STR-21G/22G and 32G.
Other parameters (GHI, DHI) can be measured with one or two additional pyranometers and shading assembly.