Minerva Díaz Heras

Accurate quantification of solar absorptivity $(\alpha_s)$ is essential for optimizing solid particle materials in Concentrated Solar Power (CSP) plants. Traditional spectrophotometric methods are costly and restricted to specialized laboratories. To address this limitation, this work evaluates a low-cost, compact fiber-optic spectrometer methodology across sixteen representative granular materials under different probe geometries ($90^{\circ}$ and $45^{\circ}$). The results demonstrate that the $90^{\circ}$ perpendicular configuration significantly outperforms the oblique setup due to superior backscattering capture. Among the evaluated metrics, visible-range absorptivity ($\alpha_{VIS,90}$) provides the most accurate uncalibrated estimation with a mean absolute error of $5.2\%$, while extended-range absorptivity ($\alpha_{c,90}$) yields the strongest linear correlation ($R^2 = 0.987$) for calibration models. Overall, compact fiber-optic spectrometry is validated as a fast, accessible, and reliable alternative for characterizing solar-thermal granular media.