Optical Durability of Candidate Solar Reflectors

Concentrating solar power (CSP) technologies use large mirrors to collect sunlight to convert thermal energy to electricity. The viability of CSP systems requires the development of advanced reflector materials that are low in cost and maintain high specular reflectance for extended lifetimes under severe outdoor environments. The long-standing goals for a solar reflector are specular reflectance above 90% into a 4-mrad half-cone angle for at least 10 years outdoors with a cost of less than $13.8/m² (the 1992 $10.8/m² goal corrected for inflation to 2002 dollars) when manufactured in large volumes. Durability testing of a variety of candidate solar reflector materials at outdoor test sites and in laboratory accelerated weathering chambers is the main activity within the Advanced Materials task of the CSP Program at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. Test results to date for several candidate solar reflector materials will be presented. These include the optical durability of thin glass, thick glass, aluminized reflectors, frontsurface mirrors, and silvered polymer mirrors. The development, performance, and durability of these materials will be discussed. Based on accelerated exposure testing the glass, silvered polymer, and front-surface mirrors may meet the 10-year lifetime goals, but at this time because of significant process changes none of the commercially available solar reflectors and advanced solar reflectors have demonstrated the 10-year or more aggressive 20-year lifetime goal.