ReflecTech® Mirror Film Attributes and Durability for CSP Applications

Abstract

Reflectors are an essential part of parabolic trough solar electric and other concentrating solar power (CSP) systems. Reflectors in CSP systems require a high reflectance over the solar wavelength spectrum and they must be durable to outdoor exposure and resist all forms of degradation over time. All utility-scale CSP systems installed to date use glass reflectors. Glass mirrors have maintained their reflectance very well in CSP environments, but they are susceptible to wind-related breakage and are expensive to transport and install. Alternative lower-cost reflectors are needed to reduce the cost of CSP systems [1]. ReflecTech® Mirror Film is a highly reflective polymer-based film co-developed with the National Renewable Energy Laboratory (NREL) for concentrating solar energy applications. The attributes of ReflecTech® Mirror Film and test results for weatherability are described herein. This paper discusses field and lab test results and properties of ReflecTech® Mirror Film, specifically: 1. Stability under ultraviolet (UV) light through accelerated testing and outdoor real-time testing. 2. Mechanical stability and resistance to moisture through water immersion tests for delamination and “tunneling”. 3. Mechanical resistance to high wind events common in utility-scale concentrating solar power applications. 4. Lighter weight and resistance to breakage that reduces transportation and installation costs, and allows greater design flexibility of concentrator geometries. 5. Lower initial cost compared with curved glass mirrors. To test for weatherability, reflector samples were subjected to controlled conditions more extreme than actual outdoor environments. NREL maintains a world-class testing capability for solar reflectors that includes a Solar Simulator (SS), QUV (an accelerated exposure chamber manufactured by Q-Lab Corp. that subjects materials to alternating cycles of light and condensation at elevated temperatures), and several WeatherOmeter® (WOM) exposure chambers that allow accelerated testing of reflector samples. In addition, samples of ReflecTech® Mirror Film have been subjected to ACUVEX® accelerated outdoor weathering tests (natural sunlight in Phoenix, AZ, concentrated 7 to 8 times with a Fresnel-reflector while the samples are cooled with a fan to near ambient conditions and sprayed with de-ionized water 8 min per natural sun hour). Immersion tests were also performed to test the resistance of ReflecTech® Mirror Film to extreme moisture. Test results compared ReflecTech® film performance to past film products like 3M’s ECP-300 and ECP-305+ which suffered from “tunneling” [2], a problem whereby the silver reflective layer delaminates from the polymer film in the presence of moisture.