Recent advances in long-lived mirrors for terrestrial and space applications

Solar Energy Materials 16 (1987) 423-433 North-Holland, Amsterdam

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R E C E N T A D V A N C E S IN L O N G - L I V E D M I R R O R S F O R T E R R E S T R I A L AND S P A C E A P P L I C A T I O N S Frank L. B O U Q U E T , Richard G. H E L M S and Carl R. M A A G Applied Technologies Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Philip M. H E G G E N Energy General, 158 Laurel Avenue, Menlo Park, CA 94025, USA

Experience from solar terrestrial mirrors can be applied to the design for use in space. In this paper, a brief review of terrestrial experience, space flight tests of mirrors, and future design considerations for long-lived mirrors are treated.

1. Introduction A considerable amount of terrestrial data on fabrication, deployment, environmental exposure, and evaluation of solar mirrors has been accumulated over the past decade [1-4]. In this paper, a brief review of terrestrial experience, space flight tests of glass mirrors, and future design considerations for long-lived mirrors will be treated.

2. Terrestrial mirrors 2.1. Glass mirrors: mirror fabrication

Solar research has shown that the techniques used in mirror fabrication and subsequent handling can be important in the achievement of long mirror life. Some fabrication aspects are shown in table 1. This table shows some of the mirror defects that are believed to be fife-limiting for commercially produced mirrors. Only a few of these m a y occur on one production line. For example, a low-sulfur adhesive will not exhibit mirror degradation, and adhesives without amines should not be a problem. The technology for making "commercial mirrors" is mostly proprietary and does not meet solar mirror requirements. Recently, Czanderna et al. [5] have reviewed the state-of-the-art of the design, manufacturing, testing and performance for first and second surface mirrors for solar thermal applications. An example of a typical manufacturing process is shown in fig. 1. However, certain modifications are shown, namely, the heat for the I R drying stages is applied below the mirrors so that the 0165-1633/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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t"]L. Bouquet et al. / Recent adt~ances in long-lwed mirrors

'Fable 1 Possible mirror defects from mirror production. Process or fabrication step

Possible defect source

Glass fabrication Glass transportation Glass cleaning Silver and copper deposits

Sn (metal) contamination on float surface Powder rack residue and reaction with glass Incomplete scrub; incomplete removal of cleaning agents Non-uniform Ag reduction and deposition; reactive impurities in reagents: incomplete rinsing of ionic chlorine, atmospheric impurities Overheating; Ag aggregation, inadequate driving off of solvents Trapped H20, other reagents, coating component reactions with Ag or Cu Incompatible adhesives, ionic sulphur, amines Physical damage of penetration; accumulation of H 2 0 reactants

Drying Coating Substrate adhesives Mounting

solvents and chemical residues can evolve upwards. It is claimed that the complete removal of these products wilt contribute to the long life of the mirrors. Other important points are the following: - For solar mirrors, special composition and processing is needed in the production of float glass to change the iron in the glass from ferrous to ferric state (the so-called low-iron glass). Otherwise, the absorption in the visible region may be unacceptable. - Back-sealing of the mirror is needed by special paint, metal, or preferably, a glass barrier. In addition, edge sealing is usually desirable to protect the edges of the reflective surface or its immediate backing material, such as copper or nickel, from degrading. - The end product achieved is a compromise between performance, durability and cost.

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Fig. 2. The three soiling layers. The top layer is the easiest to clean with the lower layers increasingly more difficult [6]. 3. Surface contamination during deployment on earth After fabrication and deployment, surface contamination is an important factor for achieving high performance and it is related to the cleaning strategy. Surface contamination, in general, can be divided up into three layers that are categorized by their tenacity (see fig. 2). The differences in construction of the underlying layers for first and second surface mirrors are shown in fig. 3 [6]. Terrestrial effects on mirrors due to surface contamination buildup can be significant. Data from the top and bottom of the same building at Albuquerque (fig. 4) have been compared to soiling at the Solar Thermal Test Facility (STTF) which is located approximately 6 miles south [7]. N o cleaning was done on the mirrors, except by the natural environment (i.e., rain, snow or dew). This shows that contamination buildup varies drastically with location. Morris [8] has treated both washed and unwashed mirrors with similar results. Similar data have been compiled at JPL [4]. As a result, a model (fig. 5) of the corrosion and degradation processes was formulated of the form: relative specular reflectance = fa (ec*,) + f2 ( ecot ) + constant,

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3.1. Large terrestrial arrays Advances have occurred and are occurring in solar terrestrial mirror technology. In this section, a few of the advances in development of large solar arrays made with segmented mirrors are presented. (1) A 40 ft diameter concentrator has been developed by Solar Steam, Inc. [10]. Triangular second-surface glass mirror sections are mechanically bent and held in place by the patented structure. Fig. 6 shows the detail of the triangular mirrors. The mirrors are designed for easy replacement. (2) Texas Tech University is planning a 200 ft diameter "fixed bowl" concentrator. Laminated glass mirror panels (48 in. x 48 in.) designed and built by one of the authors (P.H.) are undergoing tests for this project (see fig. 7). (3) The large Keck Telescope being developed by Caltech for deployment on a 13 600 ft mountainshop in Hawaii uses 36 active mirror segments (fig. 8). The mirror segments are continually adjusted in position 300 times per second. Each first

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Fig. 6. Detailed view of triangular curved reflective surfaces of the dish concentrator by Solar Steam, Inc.

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Fig. 7. Solar concentrator being planned by Texas Tech University. It contains 2000 mirror panels, each 48 in 2.

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surface silvered mirror is continually adjustable for improved optics. The segments can be resilvered when contamination/corrosion effects become unacceptable.

4. Metallic mirrors 4.1. Thin films

A number of thin films have been developed that exhibit relatively high reflectance. An example is the FEK-244, an aluminzed acrylic film with an adhesive backing manufactured by the 3M Company. On the other hand, GE's Llumar Y-91 uses a polyester film. For some time, concentrators have been made using first-surface metallic mirrors, overcoated with a protective polymeric layer in terrestrial applications. Small samples were tested at JPL, while large parabolic concentrators were built at Sandia National Laboratory, Albuquerque, NM, USA. The metallization is either silver or aluminum, while the protective layer is either polyester or acrylic. The film can be cut to conform to the underlying substrate. The primary advantage is the relatively rapid replacement of damage sections because the film is held on by adhesive to the substrate. By pulling, this tape can be pulled off, and replaced with a new film. Silvered polymers have attained a specularity such that over 90% of the beam is contained within a 1-2 mrad full-cone angle when mounted on a good substrate. Although these films do not have the specularity of the glass mirrors, they may find application in space because they can accept the g-loadings at launch better. Further, research is needed to explore their applications in space. 4.2. Bulk metallic mirrors

Another class of mirrors are bulk metallic materials, such as polished copper, silver or aluminum, overcoated with a protective coating. The space telescope is an example of the latter. Like the polymeric films, the bulk metal surfaces are capable of withstanding the launch forces. Although these mirrors exhibited poorer specularity and durability on earth, they have considerable potential for space. As with the polymeric mirrors, research is needed to explore their potentialities. In concluding the section on terrestrial mirror systems, much progress has been made. Much or all of the experience is directly applicable to long-lived space applications. The trend toward the use of modular, replaceable mirrors is evident.

5. Space application of mirrors 5.1. General

A possible space application of a solar dynamic system using two 25 kW(th) units is shown in fig. 9 [11]. For the mirrors to be long-lived, a number of new considerations are involved. Six points are important:

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Fig. 9. Space station showing two solar concentrators.

(1) The fabrication and handling techniques developed for terrestrial solar mirrors are applicable to space, including the segmented mirrors presented above. (2) The types of contamination found in space are different from those for terrestrial mirrors and should be investigated. The primary sources of space contamination will be condensations from rocket plumes. Surface coatings should be low-energy surfaces and, of course, be compatible with cleaning agents to be used. (3) Space debris can impact mirrors in space resulting in loss of a section [9]. Therefore, small mirrors are preferred because their segments can be replaced (if accessible by the Shuttle or other means). Recently, JPL has flown mirrors on the Space Shuttle STS-8 at approximately 128 nm altitude; one mirror exhibited two small craters from space debris. A photo is shown in fig. t0. (4) At low altitudes, less than 700 km, atomic oxygen may cause effects on the reflective surface or its overcoating. These effects could result in erosion of the outer surface with a subsequent decrease in reflectance. Careful selection of the outer coating is needed for successful orbiting of mirrors at low altitudes. (5) If the outer surface of the mirror is a dielectric, such as glass, the space radiation can cause a charge buildup in the dielectric which can lead to subsequent discharging. These discharging events can produce visible optical discharge paths in

F.L. Bouquet et al. / Recent advances in long-lived mirrors

Fig. 10. Exposed mirrors on STS-8 showing on-orbit particle impacts,

Table 2 Some design considerations for long-lived space mirrors. Mirror

Layer

Design considerations (preliminary)

First surface (AI203 or Ag)

Coating

Contamination resistant UV resistant Atomic oxygen resistant Radiation resistant Proton, electron and cosmic ray resistant UV resistant Special edge sealant Have similar coefficient of thermal expansion to substrate Structure integrity Segmented design

Thin film reflector Substrate

Second surface (silver, aluminum or indium)

Coating Superstrate Thin film reflector Backing metalliTation Backing paint Metal/glass environment barrier

Contamination resistant UV resistant Atomic oxygen resistant Radiation resistant Medium conductivity to reduce charge effects Dopant to prevent radiation-induced color or opacity Good adhesion control Good contamination Special edge sealant control Good production control Adequate thickness Special edge sealant of Cu and/or Ni Good drying techniques Special edge sealant Match coefficient of expansion Structural integrity Segmented design

431

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the dielectric resulting in loss of transmittance. One method of preventing this is the use of a conductive outer surface coating, such as indium-tin-oxide. (6) Although applications for terrestrial and space reflectors can be very similar (solar thermal, optical and non-optical imaging), performance requirements are typically more stringent in the space environment. Typical requirements are the following: (a) Long-term dimensional stability; (b) Extreme thermal gradients; (c) Unique dynamic loadings. The panels should be stiff to withstand microgravity vibrational modes and structural damping requirements; (d) Space mirrors will have longer maintenance intervals; (e) Remote pointing controls will be needed. JPL has been involved in a preliminary assessment of large deployable reflectors (LDR) using simulation techniques [12]. Van Allen [13] has treated other problems of mirrors in space, such as deployment, alignment, pointing and tracking. Design considerations for mirrors in space are given in table 2. These are preliminary and by no means all-inclusive. The life of a mirror, once in orbit, will depend upon the harshness of the particular orbit, i.e., the intensity and kinds of phenomena intercepted.

6. Summary The technology for the use of reflective surfaces in space is in its infancy, but much has been learned from terrestrial efforts. The mirrors must be designed to be resistant to the unique space environmental components if long life and high reflectance are to be achieved. Some aspects have been treated herein based upon the more mature terrestrial experience. Briefly, the more important considerations for space mirrors are summarized as follows: (1) Proper fabrication and sealing of the reflective surface; (2) Use of low-energy outer surfaces that resist contamination buildup; (3) Use of small sections that restrict local damage and permit rapid replacement; (4) Use of surface coatings that reduce or minimize the effects of atomic oxygen and/or charge buildup [9,14]; (5) Design for the special structural requirements of space. These conclusions, although preliminary, should be useful to designers of space mirror systems primarily but also for terrestrial systems.

7. Recommendations From this review, it is concluded that the technology concerning successful design of long-lived reflective surfaces is complex. Research to define further the basic phenomena, engineering parameters, and useful areas and constraints of the items listed in table 2 is needed. The solar dynamic power system on the Space Station (fig. 9) may utilize two large solar thermal concentrators. Prior shuttle experiments indicate power loss could occur from back scattered contamination. Research on contamination buildup is needed for that application.

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Acknowledgement The research described in this paper was carried out by the Applied Technologies Section (354), Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by DOD and USAF through an agreement with the National Aeronautics and Space Administration.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

L.E. Murr, ed. Solar Material Science (Academic Press, New York, 1980). J.F. Kreider and F. Kreith, Solar Energy Handbook (McGraw-Hill, 1981). R.B. Pettit, Solar Energy 19 (1977) 733. F.L. Bouquet, SPIE Paper 428-10, San Diego, CA (August 1983). A. Czanderna, K. Masterson and T.M. Thomas, Solar/Glass Mirrors for Solar Thermal Systems, SERI/SP0271-2293 (June 1985). C.R. Maag and A.R. Hoffman, Photovoltalc Module Soiling Studies, May 1978-October 1980, DOE/JPL-1012-49 (November 1, 1980). Unpublished data from J. Freeze, Sandia National Laboratory, Albuquerque, NM, USA. V.L. Morris, Environmental Degradation of Solar Optical Materials, Vol. I, Final Report, SAND827068/1 (August 1982). F. Bouquet, Terrestrial and Space Effects on Reflective Surfaces and Dielectrics, IASTED Conf. Proc., Santa Barbara, CA (May 30, 1985) (ISBNO-88986-076-9). Solar Steam, Inc., Suite 400, Old City Hall, 625 Commerce St., Tacoma, WA 98402, USA. Space Power: Photovoltalc or Solar Thermal, Aerospace America, Vol. 9, (September 1985) p. 60. R.G. Helms et al., Analysis of Composite Panel for Applications in Space Station Solar Dynamic Power Systems, a JPL Internal Report, to be published. Robert L. Van Allen, Aerospace America (April 1986) pp. 43-44. F.L. Bouquet, E.F. Cuddihy and C.R. Maag, IASTED J. Energy Systems 7 (1986) 44.