Assessment of the optical efficiency in solar collectors: Experimental method for a concentrating solar power

The amount of incoming solar radiation captured by an absorber in a solar collector is known as optical efficiency. However, applications at medium–high temperatures necessitate thermal insulation of the absorber, which lowers the collector's capacity to absorb solar energy. This fact justifies the requirement for experimental investigation of optical efficiency for all solar devices. This assignment typically entails evaluating the instantaneous efficiency of a solar collector when the absorber and surrounding temperatures are equal. However, this strategy's fundamental disadvantage is that a lack of adequate mass flow rate value might impact heat transmission and lead to misleading results. The significance of this study is to design a more robust and repeatable methodology to assess the optical efficiency of a solar collector. Consequently, this aim proposes the transient heating of the empty collector as an indicator to deduce its optical efficiency, thus avoiding measuring the fluid temperature increase. As a case study, the research methodology experimentally investigated an existing CPC and evacuated pipe to compare the proposed technique with the other based on fluid heating. These methods provided comparable optical efficiency values for airflow rates higher than 5 kg/h with corresponding uncertainties between 7.8 and 9.5% and 5.2–8.6%, respectively.