Thin Glass CSP Mirrors: “From Reflection to Concentration”

Abstract

The goal for Concentrated Solar Power (CSP) mirror is not just reflection, but the complete capture and utilization of the entire solar spectrum. Solar radiation is emitted over a range of wavelengths, analytically measured between 250–2500 nm, to ensure maximum captivation. An effective solar mirror, utilized for concentrating this energy must be capable of maintaining a high level of reflectance, under adverse environmental conditions for a prolonged duration in a CSP system. Thin (1-mm) flat low-iron, silvered glass mirrors have been utilized for CSP applications for many years, but obstacles with respect to quality and durability have had to be overcome. Developments have improved the reflectance from averages in the low 90% range to averages between ∼96%–97%. The reflectance durability standard for utilization of mirror for solar applications requires a minimal reflective loss of less than 5% over a 15 year period in the field. The ultimate goal is to expand the solar mirror’s field life to 20–30 years, the life of a CSP system. Overcoming harsh accelerated testing parameters continues to be the focus, as these tests attempt to correlate the lifetime to actual field applications. Test chambers with elevated temperatures and humidity conditions continue to be the most severe, and results continually show dramatic improvement. Focus was drawn on the loss of spectral reflectance, as degradation was occurring at a rapid rate specifically with the lower wave spectra. Drawing on the expertise and direction of the National Renewable Energy Laboratory (NREL), CSP thin-glass mirrors are emerging to be a viable choice for solar concentration. Thinglass mirrors offer a low-weight, highly reflective option, that resists harsh weather conditions, including water and humidity variances along with surface contamination. Mirror coating advancements have exceeded the physical and chemical resistance properties of standard “off the shelf” mirror coating products to precise, industry specific components. This study will review the obstacles and highlight the progress that has led to the success of the thin-glass mirror CSP market. A compilation of test results from NREL and other analytical, laboratories along with the collaboration of mirror manufacturing expertise from a vast knowledge base in the chemically plated mirror industry. It is the primary focus of the industry to continue to strive for a superior quality concentrating mirror while making it economically viable to the solar industry.