Approximated method for modelling hemispherical reflectance and evaluating near-specular reflectance of CSP mirrors

• Hemispherical-reflectance is measured at normal incidence by a spectrophotometer.
• Off-normal incidence is predicted by a simplified optical model.
• Specular reflectance is measured at several λ and acceptance-angle with simple set-up.
• The knowledge is extended to the whole solar range by modelling the data.
• Specular solar-reflectance is predicted versus acceptance and incidence angle.

Commercial first-surface solar-mirrors, which are alternatives to conventional glass mirrors, reflect solar radiation in a broader manner because of light scattering. However, in concentrated solar power all the radiation reflected in the solid angle of receiver-viewing is useful. This makes a mirror-evaluation methodology which takes into account not only specular, but also near-specular reflectance, mandatory. Two laminated mirrors and one traditional were analysed with a new method based on near-normal incidence reflectance measurements of the hemispherical spectrum, and near-specular at 405.5, 543.5 and 632.8 nm, for several acceptance angles. For a given acceptance angle, the wavelength-behaviour of the ratio near-specular/hemispherical is found well modelled by the well-known Total Integrated Scattering relationship. The angular-behaviour of the hemispherical reflectance is predicted by a simplified optical model of the mirror. As final result, the behaviour of solar reflectance versus the incidence-angle over the investigated acceptance-angle range was obtained.