Reflectance optimization of second-surface silvered glass mirrors for concentrating solar power and concentrating photovoltaics application

Methods developed to maximize the overall reflectance of the second-surface silvered glass used in concentrating solar power (CSP) and concentrating photovoltaics (CPV) solar systems are reported. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.5% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 3.5% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric enhancement, was measured at National Renewable Energy Laboratory to have 95.4% solar weighted reflectance. For comparison, nonenhanced, wet-silvered reflectors of the same 4-mm thickness show reflectance ranging from 91.6% to 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending. Finally, an experiment capable of determining which junction limits the triple junction cell is demonstrrated.