Progress toward Roll Processing of Solar Reflective Material

The goal of this work is to demonstrate that high performance solar reflective material can be produced in a roll format using vacuum deposition techniques. The material consists of a multilayer thin film stack on a substrate. The essential feature of the film stack is an alumina film several microns thick deposited over a silver film. The alumina film is deposited under high vacuum using a physical vapor deposition technique called ion beam assisted deposition (IBAD). The alumina film is optically transparent, scratch resistant, and durable. Its purpose is to protect the silver film and maintain high optical reflectance. Work on this reflective material began five years ago. Samples were produced on polyester substrates by batch coating. Those samples showed excellent durability in accelerated testing at the National Renewable Energy Laboratory (NREL) in Golden Colorado. The principal limitation to commercialization of the process was a low alumina deposition rate. Over the past three years the alumina deposition rate has been increased from 1 nm/s to 20 nm/s. The next step is to produce material in a roll format. A roll coater has been built to process 30.48-cm wide rolls of solar reflective material. A steel strip with a highly specular surface finish will be used as the web material. The advantage of this material compared to polyester is that it withstands a higher process temperature and lowers final product installation costs. A major technical challenge will be to reproduce high reflector durability in a continuous, high rate process. INTRODUCTION Concentrating solar power systems use large solar reflectors to concentrate sunlight to generate electricity. The widespread application of concentrating solar power generation depends in part on developing a durable, lowcost reflector. NREL working jointly with Sandia National Laboratories as Sun♦Lab, is developing reflectors for the U.S. Department of Energy Concentrating Solar Power Program. The goals are specular reflectance above 90% for at least 10 years under outdoor service conditions and a large-volume manufacturing cost of less than $10.8/m (Short, 1988). Currently, the best candidate materials for solar reflectors are silver-coated thin-glass, and silveredpolymer films. Polymer reflectors are lighter in weight, offer greater system design flexibility, and have the potential for lower cost than glass reflectors. However, silvered polymer has several limitations, including— relatively high cost, a lifetime less than 30 years, and poor adhesion between the silver and the polymer on exposure to water (Schissel, 1995). Silvered thin (1.0 mm thick) glass is durable but has relatively high cost, fragility in shipping and handling, and the availability of only a single U.S. manufacturer. These materials may meet the current reflector goal, but it is uncertain whether they can meet a longer lifetime goal of 30 years of outdoor service. Over the past five years Sun♦Lab has funded SAIC to develop a promising low-cost “ultra-thin glass” reflector combining the best of both thin-glass and silvered-polymer reflectors. The reflector consists of a polymer substrate coated with a copper layer, followed by a layer of silver, and finally by a protective optically transparent alumina top