Paul Schissel

Silvered-PMMA reflectors

Abstract Metallized, flexible polymeric reflector materials are much lighter and potentially less expensive than the conventional glass/metal mirrors often used in solar thermal concentrators such as heliostats and parabolic dishes and troughs. Unweathered silvered-PMMA reflectors have a solar reflectance at least as high as glass reflectors, but their environmental durability needs to be demonstrated. ECP-305, a silvered-PMMA film available commercially from the 3M Company and developed in collaboration with the National Renewable Energy Laboratory, is the current state of the art. Important progress has been made in overcoming the three primary mechanisms causing ECP-305 to lose reflectance. These mechanisms are: (1) photon-induced silver corrosion, (2) surface soiling, and (3) a form of delamination called tunneling. Given the progress in resolving these performance lifetime issues, silvered-PMMA films should meet the reflectance and durability goals.

Role of inorganic oxide interlayers in improving the adhesion of sputtered silver film on PMMA

Large mirrors are used to concentrate sunlight for renewable power generation. The reflector materials that are used must be inexpensive and maintain high specular reflectance for extended lifetimes in severe outdoor environments. Polymer reflectors are lighter than glass mirrors, offer greater system-design flexibility, and have the potential for lower cost. The state-of-the-art commercial reflector is a silvered polymethylmethacrylate (PMMA) polymer. Interlayer adhesion failure can limit the performance of the optical components used to convert solar energy into thermal or electric energy. Optically transparent, inorganic interlayers can improve adhesion in multilayer structures. We have evaluated interlayer adhesion in sandwich structures that are representative of solar mirrors and other devices. Experimental structures were fabricated by vacuum-sputtering thin layers onto substrates. The structures were evaluated for optical performance and for adhesion. Calculations show that the Lifshitz-van der Wa...

Specular reflectance properties of silvered polymer materials

The specular reflection properties of transparent cast polymer sheets and extruded polymer films, silvered and unsilvered, have been characterized with a newly designed specular reflectometer. The results obtained with this instrument are either absolute reflectances or a measure for the Fourier transform of the reflection function of the specimen in one dimension. Cast polymer sheets are investigated before and after silvering, and silvered polymer films are evaluated by mounting them with an adhesive onto aluminum or glass substrates, or by suspending the thin, silvered polymer as a taut membrane. Silvered polymers have attained a specularity such that over 90% of the incident beam is contained in a 1-2 mrad full-cone angle when mounted on a good substrate or suspended as a membrane. This value is well within the current goals for solar concentrators but silvered polymer mirrors are currently less specular than glass mirrors. The image quality of these mirrors does not change significantly over the wavelength range 400 to 1000 nm and angles of incidence between 20 and 60 degrees. A principal limiting factor to the initial specularity of the polymer mirrors was waviness and/or curvature of the surface, hence, the material being used as a substrate plays an important role in the optical performance of the mirror.

Solar concentrator development in the United States

Abstract Sandia National Laboratories leads the U.S. Department of Energys solar concentrator development program in a joint effort with the Solar Energy Research Institute. The goal of DOEs program is to develop, build and test solar concentrators that are low in cost, have high performance, and long lifetimes. Efforts are currently focused on three areas: low-cost heliostats, point-focus parabolic dishes, and durable reflective films. The status and future plans of DOEs program in each area are reviewed.

Optical Performance and Durability of Silvered Polymer Mirrors

Metallized polymeric reflector materials for solar energy applications offer significant system weight reduction and cost savings. Such materials must maintain high optical performance over extended service lifetimes. Durability studies of candidate silvered polymer mirrors have been carried out as a function of real-time outdoor weathering and accelerated exposure. Major progress has been made toward achieving a long-term goal of demonstrating a specular reflectance of 90% that is maintained for at least five years.

Specularity and stability of silvered polymers

Abstract Silvered polymers have been studied for possible mirror applications in concentrators for solar thermal systems. The polymeric mirrors were prepared by vacuum evaporating or dc-sputtering silver onto several candidate polymers for use as second surface mirrors. Specularities were measured at 7 and 15 mrad acceptance angles at 660 nm. The specularity of silvered polymers depends strongly on the topography and bulk properties of the polymer used. It is independent both of the deposition rate from 0.1 to 7 nm/s and from thermal evaporation and dc-sputtering processes. Silvered glass specularities are also independent of deposition rates from 0.1 to 45 nm/s and are typically 93% at 7 mrad. The specularity does not depend on the substrate temperatures during depositions for the range of 5 to about 100°C. In some cases, a metallic film was deposited onto the silver before mounting onto a Corning 7809 glass substrate. Inconel backing of films from 35 to 150 nm thick does not reduce the specularity of sivered polymers at 15 mrad, but does accelerate the degradation of the multilayer combination during accelerated or real-time weathering. After measuring the solar-weighted hemispherical reflectance and the second surface specularity at 660 nm for acceptance angles of 15 and 7 mrad, the silvered polymeric specimens were subjected to accelerated testing in a Weather-Ometer at 60°C, 80% relative humidity, and UV irradiation, or in a QUV at 40°C, UV irradiation and condensation cycles. Real-time testing has also been carried out in the suburban Denver, CO, atmosphere. With exposure, all mirrors prepared show evidence for some or extensive optical and/or visual degradation. The relative durabilities of the various combinations studied are discussed in terms of the silvering method, polymer used, metallic backing for the silver, and adhesive used for mounting the silvered polymer onto a test substrate.

Optical properties of high-temperature materials for direct absorption receivers

Abstract Recent solar central receiver designs emphasize direct absorption receiver (DAR) concepts primarily because of their ability to absorb high flux densities. An attractive design utilizes molten salt as the transport/storage fluid, which is pumped to the receiver and allowed to flow over a high-temperature absorber surface. As the salt runs down the absorber surface in a thin film, concentrated solar flux heats the salt to 900°C, either directly (blackened fluid) or by convective heat transfer with the irradiated absorber (clear fluid). The feasibility of such a design depends on the optical efficiency of the absorber/fluid combination. The optical properties of candidate absorber materials and transport fluids are therefore required at appropriate elevated temperatures. Because such salts can be extremely reactive at high temperatures, it is important to measure optical properties as a function of exposure history of the salt/absorber. Optical characterization of a clear molten carbonate salt (Li 2 CO 3 Na 2 CO 3 K 2 CO 3 ternary eutectic) and a high-temperature metal alloy (Inconel 600) has been carried out at elevated temperatures using a recently developed integrating sphere solar spectrometer. Reflectance measurements of oxidized Inconel 600 alone and covered with several thicknesses of molten salt have been made. Measurements were also made of the reflectance of Inconel 600 samples that had been exposed to molten salt under high cyclic temperatures.

Polymer degradation on reflecting metal films: Fourier transform infrared (FTIR) reflection-absorbance studies

Abstract The technique of Fourier transform infrared reflection-absorption (FTIR-RA) spectroscopy has been successfully adapted to the study of bulk and interfacially activated photodegradation of several types of polymers on various metallic substrates. The technique enables qualitative and quantitative study of photochemical reaction mechanisms and rates. A controlled Environmental Exposure Chamber (CEEC), which permits collection of IR-RA spectra of the polymer/metal samples during their exposure to controlled spectral distributions of UV, temperatures, and gas mixtures, was built into the sample compartment of a Nicolet 7199 FTIR spectrophotometer. Surface analysis, gel permeation chromatography (GPC), UV spectroscopy, and UV spectroradiometry were used to complement the FTIR-RA results.

Effective antireflection coatings of transparent polymeric materials by gas-phase surface fluorination

Abstract There is a dramatic need in solar energy collection systems for lightweight, inexpensive polymeric materials,that exhibit improved performance and durability. One approach to altering the properties of polymeric materials, surface fluorination, is appealing because of its potential for low cost. The literature indicates that such properties as permeability, wettability, bondability, thermal stability, weatherability and optical transmittance can be improved by treating the surface with gaseous fluorine. A gas phase fluorination reactor system (GPFRS) was designed, built and used. The initial emphasis was on improving optical transmittance by having an effective antireflection coating form on the surface of a wide variety of commercially available transparent polymeric films. These included such materials as polypropylene, acrylic, polyacrylonitrile, highly cross-linked polyethylene, polyester, polycarbonate and polymethylpentene. Two techniques were used to quantify the effect of exposing the surface of the polymers to gaseous fluorine. Transparent films were characterized before and after fluorine exposure by specular transmittance measurements. Surface analysis of selected treated and untreated samples was accomplished by X-ray photoelectron spectroscopy and depth profiling. Surface analysis confirmed the deposit of fluorine at the surface and into the bulk of all specimens examined after treatment in the GPFRS. Optical measurements revealed substantial improvement in specular transmittance following surface fluorination of almost all materials considered. Increases in solar weighted specular transmittance as high as 4.6% were measured.

Polymer glazings for silver mirrors

Abstract This paper reports on our evaluation and modification of polymeric glazings to protect silver mirrors. The mirrors were made using Corning 7809 glass as a substrate onto which a thin silver film is deposited. The modified polymeric films are then cast from solution onto the silver. The mirrors were characterized by measuring the hemispherical reflectance and the specular reflectance at 660 nm and selected acceptance angles (7.5 or 3.5 mrad). The mirrors were exposed to environmental degradation using accelerated weathering devices and outdoor exposure. Empirical evidence has demonstrated that polymethylmethacrylate is a stable polymer in a terrestrial environment, but the polymer does not provide adequate protection for the silver reflector. The crucial role in degradation played by ultraviolet (UV) light is shown by several experimental results. We have demonstrated that UV stabilizers added to the polymer improve the weatherability of mirrors. The relative effectiveness of different stabilizers will be discussed in terms of the weathering modes, retention of optical properties, and effectiveness of the additives. The process for silver deposition influences the reflectance of silver mirrors, and the optical properties depend on subtle relationships between the metallization and the dielectric (polymeric) films that are in conctact with the silver.