A New Instrument for Measuring the Reflectance Distribution Function of Solar Reflector Materials

AbstractDuring the last years, the importance of correctly measuring the reflectance properties of solar reflector materials has increased due to the fact that more and more different kinds of materials compete on the market. Customers and manufacturers likewise need reliable tools to ensure the quality of the reflector and evaluate its performance in the desired concentrated solar power (CSP) application. The SolarPACES Reflectance Measurement Guideline [1] defines the parameters that are necessary for a thorough reflector qualification. The work on this guideline made it evident, that the state of the art of measurement technology is not sufficient for measuring all of these parameters in the ideal way. For some reflector materials, this can lead to improper estimations of their reflectance characteristics and their performance. Until now it was not possible to measure the most important parameter, the specular reflectance, ρs(λ,θ,φ), in the ideal format as a function of wavelength, λ, acceptance angle, φ, and incidence angle, θ. The currently most common method is to measure ρs(λ,θ,φ) at few selected φ and λ and to estimate the solar weighted specular reflectance, ρs(SW,θ,φ). One solution is the measurement of the bidirectional reflectance distribution function (BRDF) and use it to calculate ρs(λ,θ,φ) as a function of φ and θ. At the OPAC laboratory a new instrument MIRA (Mirror Reflectance Function Analyzer) was developed that offers the possibility to measure the bidirectional reflectance distribution of the light reflected by a mirror into the hemisphere at variable incidence angles [2]. In advantage to traditional goniometers or gonioreflectometers of which only few reach the necessary angular measurement resolution, MIRA uses a time saving technique and occupies comparably little space that is available in any laboratory. The instrument allows measurements at different selected wavelengths for the approximation of the solar weighted specular reflectance. This paper gives a detailed description of the instrument prototype and first preliminary measurement results are compared with a reference measurement. The instrument design and data evaluation method has a German patent [3]