Silver/Glass Mirrorsfor Solar Thermal Systems t- L! Notice This publication was prepared under a contract to the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontrac- tors, or their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness or usefulness of any’,information, product or process disclosed, or represents that its use would not infringe privately owned rights. Printed in the United States of America Available in print from: Superintendent of Documents U.S. Government Printing Office Washington, DC 20402 Available in microfiche from: National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 Stock Number: SERUSP-271-2293 Mirrors for Solar Thermal -terns DE85000537 SERI/SP-271-2293 UC Categories: 62, 62a, 62b, 62c June 1985 Preface The research and development (R&D)described in this document was conducted within the US. Department of Energy’s (DOE) Solar Thermal Technology Program. The goal of the Solar Thermal Technology Program is to advance the engineering and scientific understanding of solar thermal technology and to establish the technology base from which private industry can develop solar thermal power production options for introduction into the competitive energy market. In solar thermal technology, tracking mirrors or lenses concentrate sunlight onto a receiver. The heat absorbed by the receiver is converted into electricity or used as process heat. The two primary solar thermal technologies, central receivers and distributed receivers, use various point and line-focus optics to concentrate sunlight. Central receiver systems use fields of heliostats (two-axis tracking mirrors) to focus the sun’s rays onto a single tower-mounted receiver. Parabolic dishes up to 17 meters in diameter track the sun in two axes and use mirrors or Fresnel lenses to focus radiant energy onto a receiver. Troughs and bowls are line-focus tracking reflectors that concentrate sunlight onto receiver tubes along their focal lines. Concentrating collector modules can be used alone or in a multi-module system. The concentrated radiant energy absorbed by the solar thermal receiver is transported to the conversion process by a circulating working fluid. Receiver temperatures range from loo°C in low-temperature troughs to over 1500’C in dish and central receiver systems. The Solar Thermal Technology Program is directing efforts to advance and improve each system concept through research and development of solar thermal materials, components, A Product of the Solar Technical tnformation Program Produced by the Technical Information Branch A Division of Midwest Research Institute Solar Energy Research Institute Operated for the 1617 Cole Boulevard, Golden, Colorado 80401 U.S. Department of Energy i and subsystems, and the testing and performance evaluation of subsystems and systems. Work is done under the technical direction of DOE and its network of national laboratories who work with universities and private industry. Together they are pursuing a comprehen- sive, goal-directed program to improve Performance and provide technically proven options for eventual incorporation into the nation’s energy supply. To contribute to the national energy supply, solar thermal energy must eventually be econom- ically competitive with other energy sources. Components and system-level performance targets have been developed as quantitative program goals. The performance targets are used in planning research and development activities, measuring progress, assessing alternative technology options, and optimizing components. These targets will be pursued vigorously to ensure a successful program. This document reviews the state of the art of the design, manufacture, testing, and perfor- mance of silver/glass mirrors for solar thermal systems applications. Acknowledgments This document was prepared by A1 Czanderna, Keith Masterson, and Terence M. Thomas of the Materials Research Branch of the Solar Energy Research Institute. Contents Page Introduction .......................................................... iii Chapter 1 Overview of Silver/Glass Mirrors ............................ 1 Chapter 2 Properties of SiIver/Glass Mirrors ........................... 5 Chapter 3 Physics and Chemistry of SiIver/Glass Mirrors ............... 19 Chapter 4 Mirror Product Testing Procedures ......................... 45 Chapter 5 Solar Mirror Manufacturing Methods ........................ 55 ii Introduction 8ackg rou nd over the economic life span of the facility in which they are used, during which time the mirror may be A significant aspect of research and development degraded by storms and stressed by temperature (R&D)of solar thermal energy systems concerns the cycling, water, humidity, vapor, ultraviolet (UV) design, manufacture, testing and performance of radiation, dust, and physical abuse. mirrors for solar energy applications. Mirrors have Most mirrors have been manufactured by bonding a an essential role in various solar thermal technology thin layer of silver to glass - the glass being the applications. For these applications, mirrored sur- protective barrier, light transmitter, and, in part, a faces are used to redirect and/or to concentrate the structural support. A copper film to protect the rays of the sun. The most extensive application of silver’s back surface and layers of paint to protect the mirrors is expected to be in solar thermal systems. copper also are necessary. Most industrial and utility applications of solar thermal energy systems are to produce process heat The advantages of using glass as a superstrate are its fluids or develop other forms of energy, such as elec- clarity (high solar transmittance), low cost, smooth tricity for mechanical power, requiring concentrat- surface, physical strength, abrasion resistance, im- ing mirrors. Mirrors are needed to concentrate the pertneabilit y, resistance to soiling, ease of cleaning, naturally available solar flux in order to attain and inertness. Compared with other mirrors, those higher temperatures. made of glass and silver are preferred for their high reflectance, good specularity, durability, and resist- Mirrors are used mostly in such solar thermal sys- ance to distortion from loads. However, glass is heavy temsisubsystems as parabolic troughs, parabolic and brittle, requiring massive structural support. dishes, spherical bowls, and heliostats. Each has particular design requirements. However, the prin- Mirrors of silvered glass are fairly tolerant of mirror cipal attributes sought in the design specifications of compositional variations and/or impurities. They the “ideal” solar thermal reflector should include the can be expected to provide high reflectance as long as following: the silver remains intact and in contact with the glass. High optical performance: Retaining this integrity depends on chemical and - reflectanceitransinit tance, physical processes that take place in and between the layers of materials composing the mirrors. - specularity, - geometrical configuration, Although glass has many advantages as a super- Low maintenance (dust free), strate for silver mirrors, certain factors can mitigate its use. Glass is a relatively heavy (high density) and Low initial cost, and brittle material; therefore, it requires either stronger Long life. and stiffer mechanical support structures or thinner, more fragile layers than lighter materials. Most mirrors presently in use employ either silver (domestic and decorative applications) or aluminum Mirror manufacturing techniques can play an impor- films (automotive applications] for their reflecting tant role in the performance of glass as a mirror surfaces. Mirrors made of silver must be protected superstrate. Poorly made glass may contain bubbles, from chemical and physical deterioration of the which can cause dispersion of the light beam, dimin- silver; whereas,aluminum mirrors are more resistant ishing mirror specularity. The glass manufacturing to degradation, technique also influences mirror performance in that Since both silver and aluminum reflecting surfaces it controls surface smoothness and thickness uni- in solar applications must retain their highest reflec- formity. Float glasses have problems for solar appli- tivity for many years, the reflecting surfaces are pro- cations; their facings tend to be wavy and lack paral- tected with transparent covering materials referred lelism. On the other hand, the float glass process to as superstrates. Also, this adds to the physical allows the production of very thin glass desired for integrity of the reflector since the superstrate sup- heliostats. Polished sheet glass has the best unifor- ports the kilver. mity but is thicker than float glass. Thick glass tends to be less wavy than thin glass. The material or materials attached to the silver (1) The age of the glass prior to its being silvered can should not detract from the silver’s reflective qual- affect the longevity of a mirror’s high optical perfor- ity, (2) should not contribute to degradation of the mance. Apparently, the surface of many glasses silver, and (3) should bond well. Moreover, all of “foam up” during storage which leads to a destabili- these properties should remain substantially intact zation of the subsequent silvering process. iii The attributes sought in the design specifications