Experimental Comparison between Direct and Indirect Methods Adopted for Evaluating CPV System Optical Performance

In concentrating photovoltaic (CPV) systems, energy conversion efficiency depends on solar cell characteristics and optical parameters such as the optical efficiency (ηopt) that affects solar radiation concentrated on TJ cells. Hence, it is important to standardize ηopt definition, measuring methods and instruments to compare different CPV technologies. ηopt is linked to optical concentration factor (Copt) representing how many times solar flux is increased by optics. The CPV systems optical characterization must be practical; so, two methodologies of concentrated solar flux are compared. Indirect method is based on generated photocurrent, while direct method on thermal sensors. The aim is to evaluate experimentally methodologies able to estimate Copt and ηopt in CPV systems. An increase of Copt from 16 to 310 until lens height of 24 cm with ηopt of 14%, is obtained by indirect methodology. As for direct methodology, an increase of cell-lens distance from 10.5 to 21 cm leads to variation of Copt from 22 to 315 with ηopt of 16%. The methodologies results are comparable; indirect method underestimates the optical performances. The novelty of this paper is the comparison under same operation conditions (TJ cell size, Copt range, ηopt and focal distance) of two methodologies able to determine optical parameters and standardisable for similar point-focus CPV configurations.