Spectral data of specular reflectance, narrow-angle transmittance and angle-resolved surface scattering of materials for solar concentrators

The spectral specular reflectance of conventional and novel reflective materials for solar concentrators is measured with an acceptance angle of 17.5 mrad over the wavelength range 300−2500 nm at incidence angles 15–60° using a spectroscopic goniometry system. The same experimental setup is used to determine the spectral narrow-angle transmittance of semi-transparent materials for solar collector covers at incidence angles 0–60°. In addition, the angle-resolved surface scattering of reflective materials is recorded by an area-scan CCD detector over the spectral range 350–1050 nm. A comprehensive summary, discussion, and interpretation of the results are included in the associated research article “Spectral reflectance, transmittance, and angular scattering of materials for solar concentrators” in Solar Energy Materials and Solar Cells.


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The spectral specular reflectance of conventional and novel reflective materials for solar concentrators is measured with an acceptance angle of 17.5 mrad over the wavelength range 300 À 2500 nm at incidence angles 15-60°using a spectroscopic goniometry system. The same experimental setup is used to determine the spectral narrow-angle transmittance of semitransparent materials for solar collector covers at incidence angles 0-60°. In addition, the angle-resolved surface scattering of reflective materials is recorded by an area-scan CCD detector over the spectral range 350-1050 nm. A comprehensive summary, discussion, and interpretation of the results are included in the associated research article "Spectral reflectance, transmittance, and angular scattering of materials for solar concentrators" in Solar Energy Materials   The angle-resolved surface scattering data is useful for the accurate simulation of the solar flux distribution on the receiver and the optical design of solar collectors with small acceptance angles such as far-distant heliostats.

Data
Spectral data of specular reflectance of solar reflector materials and narrow-angle transmittance of semi-transparent materials for solar collector covers are experimentally measured at an acceptance half-angle of 17.5 mrad, wavelengths 300-2500 nm, and incidence angles 0-60°. The angle-resolved surface scattering of reflective materials is characterized by the parameters of a superposition of two Gaussian distributions over the spectral range 350-1050 nm and incidence angles 15-60°.

Materials
Three types of specular reflective materials (back-silvered glasses, metallized polymer films and metallized aluminum sheets) and two types of semi-transparent materials (glasses and polymeric films) are characterized. An overview of the materials is given in Table 1. A comprehensive description of materials is included in Chapter 2 of the associated research article [1].

Methods
For each spectral data point, three sequential measurements are performed at the same wavelength: (1) a reference measurement with the sample moved out of the beam and the detector arm positioned at 180°; (2) a sample measurement with the sample placed in the source beam at an incidence angle θ and the detector arm rotated to the corresponding angular position (2θ for reflection; 180°for transmission); and (3) a second reference measurement.

Specular reflectance and narrow-angle transmittance
The spectral specular reflectance and narrow-angle transmittance are calculated as the ratio of the voltage from the sample measurement to the average voltage from the reference measurements. For polarization-dependent optical properties the above procedure is performed twice, with the electric field of incident light oscillating once in the plane parallel to the plane of incidence (parallel or p-polarized), and once in the plane perpendicular to the plane of incidence (perpendicularly or s-polarized), and the optical property for unpolarized sunlight is calculated as the average of p-and spolarized light [16]. Type A measurement uncertainties of spectral specular reflectance and narrowangle transmittance are calculated from the estimated variance of the recorded voltage signals using the Gaussian error propagation formula. The maximum type A uncertainty occurring at 300 nm and the root-mean-square uncertainty over the measured spectral range are below 0.02 and 0.003, respectively. The type B uncertainty is estimated as 0.004 from spectral transmittance and reflectance measurements of an N-BK7HT glass sample with known optical properties [17]. Accordingly, the combined measurement uncertainty calculated with a confidence factor 2 (95% confidence) is usually within 0.01 for spectral specular reflectance and narrow-angle transmittance. The spectra and type A uncertainties of specular reflectance and narrow-angle transmittance are given in the data Tables A.1 and A.2, respectively.

Angle-resolved surface scattering
The angular scattering is quantified by the parameters of statistical surface scattering models [18]. For the scattering function of silvered polymer films and silvered aluminum sheets with the manufacturing marks perpendicular to the plane of incidence, a superposition of two Gaussian distributions is used, where θ s,x is the angular deviation of the reflected ray from the perfectly specular direction, σ 1 and σ 2 the standard deviations of the first and second distributions, and F 1 the fraction of rays following the first distribution. The parameters of f are identified by matching the convolution of the scattering function and the measured reference beam shape to the scattered beam shape of the sample measurement using the least-square technique. The method is described in more detail in Section 4.2 of the associated research article [1]. The identified parameters at different wavelengths and incidence angles are included in Table B1.