Modifying a pyrheliometer to measure direct normal and circumsolar irradiance

Abstract

Direct normal irradiance (DNI) is key to concentrating solar power (CSP) systems energy generation. However, pyrheliometric measurements of DNI also include a circumsolar normal irradiance (CSNI) component because the aperture angle of the instrument is larger than the sun disk. CSNI is due to the scattering of the sun rays by molecules, aerosols and some cloud types such as cirrus clouds [1]. As a result, energy is transferred from the direct solar beam to the circumsolar region, i.e., the region in the vicinity of the sun disk [2]. Typically, CSP systems have a lower aperture angle than that of pyrheliometers, which means that these systems are not able to absorb the same amount of CSNI as the amount that is present in the DNI measurements, leading to an inaccurate forecast of energy generation of CSP systems if DNI measurements are directly used in its sizing and operation. In this way, measuring CSNI for different aperture angles is important to better size, operate and optimize the energy generation of CSP systems. In this work, a conventional pyrheliometer field-of-view is modified through the variation of its collimator length and diameter in order to gather information on the circumsolar irradiance [3]. The modified pyrheliometer is composed by a revolver with four possible combinations of tube lengths and diameters and a stepper motor, which automatically enable varying the field-of-view of the pyrheliometer.