Optical Durability Testing of Candidate Solar Mirrors

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 Concentrated Solar Power (CSP) Program. Outdoor exposure testing (OET) at up to eight outdoor, worldwide exposure sites has been underway for several years. This includes collaboration underthe auspices of the International Energy Agency (IEA) Solar Power and Chemical Energy Systems (SolarPACES) agreement. Outdoor sites are fully instrumented in terms of monitoring meteorological conditions and solar irradiance. Candidate materials are optically characterized prior to being subjected to exposure in real and simulated weathering environments. Optical durability is quantified byperiodically re-measuring hemispherical and specular reflectance as a function of exposure time. By closely monitoring the site- and time-dependent environmental stress conditions experienced by the material samples, site-dependent loss of performance may be quantified. In addition, accelerated exposure testing (AET) of these materials in parallel under laboratory-controlled conditions maypermit correlating the outdoor results with AET, and subsequently predicting service lifetimes. Test results to date for a large number of candidate solar reflector materials are presented in this report. Acronyms are defined in Table 1. Based upon OET and AET results to date, conclusions can be drawn about the optical durability of the candidate reflector materials. The optical durability ofthin glass (from Naugatuck, Schlaich, Bergermann und Partner, or Steinm?ller), thick glass (from ATS or Flagsol), and two metallized polymers (SA-85, ECP-305+) can be characterized as excellent. The all-polymeric construction, several of the aluminized reflectors (Alanod's improved product, materials from Metalloxyd), and a metallized polymer (ECP-305) can be characterized as having intermediatedurability and require further improvement, testing and evaluation, or both. A metallized polymer (SS-95), metallized fluoropolymers (until specularity can be sufficiently improved), and constructions in which adhesives are in direct contact with a silver reflective layer can be characterized as poor and do not warrant further consideration for solar applications. Recently, a number of newpromising constructions have been identified including: several front-surface mirrors under an ongoing SunLab subcontract and prepared by SunLab staff; a new all-polymeric construction using improved interlayer resins and incorporating UV screens; a newly available commercial solar reflector material called SolarBrite 95; and a novel commercial laminate construction co-invented by SunLab staffand industry collaborators