Evaluation of a reflective polymer film for heliostats

Commercially available Solar Mirror Film (SMF) 1100 from 3M was evaluated for application in concentrating solar power tower applications, where large arrays of heliostats are used to reflect and concentrate sunlight toward a central receiver at potentially large distances. The reflectance and soiling rate of SMF1100 was compared to silvered glass mirrors during outdoor exposure for over a year. In addition, the reflected beam quality and peak flux resulting from solar reflections from SMF1100 and silvered glass facets at distances up to ∼1700 m were compared. Results showed that the impacts of soiling and outdoor exposure on the solar-weighted reflectance of coupons of SMF1100 did not differ significantly from that of silvered glass over a year of testing. However, the initial (clean) specular reflectance (at 660 nm) of SMF1100 was found to be ∼2-4% lower than that of silvered glass for acceptance angles ranging from 25 mrad to 15 mrad, which contributed to a lower overall heliostat beam power projected onto the tower ∼200 m away when compared to an adjacent heliostat with silvered glass. The peak flux was measured from individual facets with SMF1100 and silvered glass at distances up to ∼1700 m. Slight differences existed in the focal length, specular reflectance, and time of testing of the individual facets, but results showed that the mean of the measured peak fluxes (normalized to the direct normal irradiance at the time of testing) of the SMF100 and silvered glass facets were statistically similar.