Hemispherical Reflectance Results of the SolarPACES Reflectance Round Robin

Abstract

Mirrors are the first link in the energy-conversion chain from Sun to electricity-delivery in the grid. Shape and solar reflectance are the key-parameters of mirrors, respectively affecting how solar radiation is concentrated around the focus, and how much of the impinging solar power is reflected. In SolarPACES Task III, an expert group is drafting the solar reflectance guidelines; in order to speed up the discussion the SRRR round robin was launched at the beginning of 2013. Identical kits, each one consisting of ten specimens collected from eight cooperating producers, were distributed and measured at six research institutes, acting as evaluators. The kit includes both traditional (glass based) and innovative (first-surface) solar mirrors. The paper only reports on the simplest task among those of SRRR: the solar hemispherical reflectance measurement. Near-specular solar reflectance was also measured and compared but the results are still under investigation and are not part of this paper. The measurements were accomplished according to the guidelines. The differences among the achieved results are within the typical accuracy of spectrophotometers, demonstrating the reliability of the reflectance guidelines. The statistic of the deviations from the true value is analysed separately for each evaluator, and allows us to infer information abut the gauging-status of the adopted reference mirror, as well as the measurement reproducibility.

Simplified analysis of solar-weighted specular reflectance for mirrors with high specularity

“…This test campaign consisted on evaluating 10 different reflector materials (including both conventional and innovative mirrors) by 6 independent laboratories (CEA, CENER, CIEMAT, DLR, ENEA and ISE Fraunhofer), named "evaluators". According to the results obtained, solar-weighted hemispherical reflectance values, ρ h (SW,θ,h), measured by the evaluators with commercial instruments are in good agreement because the differences among the achieved values are within the typical measurement uncertainty of spectrophotometers [3]. This result demonstrated that the protocol included in the SolarPACES reflectance guideline to measure this parameter is appropriate.…”

“…Their main characteristics are mentioned in Table 1. The first ten samples correspond to the specimens already used in the SRRR [3]. Except the University of Zaragoza, all institutions participating in this research work were already involved as evaluators in the SRRR.…”

“…In this case, a lack of commercial or validated instrumentation to adequately characterize it, according to the SolarPACES reflectance guideline, was noticed during the SRRR [3]. Some new instruments to properly measure the spectral near-specular reflectance, ρ s (λ,θ,φ), of all type of reflector materials are under development or have been recently developed by some of the institutions, such as the SMQ by ENEA [6,7], the MIRA by DLR [8,9] and the VLABS by ISE Fraunhofer [10].…”

Reflectometer comparison for assessment of back-silvered glass solar mirrors

“…If we assume that the Pettit approximation is true (Pettit, 1977), which is the case for highly specular mirrors such as the facet used in this work, then the ratio of specular reflectance to hemispherical reflectance is wavelength independent. As there is no significant difference between the hemispherical reflectance of solar glass mirrors in the range 650-660 nm (Montecchi et al, 2015), it may be assumed that there is no significant impact on specular reflectance in this range and the reflectance values from both devices are perfectly comparable.…”

Advanced Analysis of Corroded Solar Reflectors

“…Several agreements reached by this group are collected in a document in which current version is titled "Parameters and method to evaluate reflectance properties of reflector materials for concentrating solar power technology" (hereinafter "SolarPACES Reflectance Guideline") [15]. This document has been used as a reference by the CST community for the last years and it has even been mentioned by the first standard published about solar reflectors' durability testing, the UNE 206016:2018 [16], as the measurement method to assess durability experiments.The measurement protocol described in this guideline, in order to measure new and clean solar reflectors, has proven to be very accurate and easy-to-use for any laboratory or company equipped with conventional commercial instruments [17,18]. However, the proposed conventional method used to characterize aged and/or soiled reflector samples is insufficient and inaccurate since only some specific spots of the samples are assessed and it does not take into consideration θ i and ϕ dependence.…”