Spectral reflectance, transmittance, and angular scattering of materials for solar concentrators

The narrow-angle spectral specular reflectance and angular scattering of conventional and novel reflective materials for solar concentrators are measured over the wavelength range 300-2500 nm at incidence angles ranging from 15° to 60° using a spectroscopic goniometry system. The solar-weighted specular reflectance at near normal incidence and an acceptance half-angle of 17.5 mrad is 0.941 for back-silvered glass, 0.908-0.926 for silvered polymer films, 0.895 for aluminized polyester film, 0.939-0.954 for silvered aluminum sheets, and 0.860 for aluminized aluminum sheet. The angular scattering, quantified in terms of the standard deviation of a Gaussian distribution, is found to be negligible for aluminized polyester (<0.05 mrad) and back-silvered glass (<0.07 mrad), and noticeable for silvered polymer films (0.27-1.12 mrad) and silvered aluminum sheets (0.12-1.66 mrad). In addition, the spectral transmittance of semi-transparent materials suitable for protective covers is measured, yielding solar-weighted normal transmittance values of 0.913 and 0.946 for 100 μm thin films of ETFE (ethylene-tetrafluoroethylene) and FEP (fluorinated ethylene propylene), respectively. The measured optical properties are incorporated in a Monte Carlo ray-tracing program and applied to analyze the optical performance of solar concentrators.