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Silvered-PMMA reflectors
Solar Enecw Materials and Solar Cells
ELSEVIER
Solar Energy Materials and Solar Cells 33 (1994) 183-197
Silvered-PMMA reflectors Paul Schissel, G a r y J o r g e n s e n , C h e r y l K e n n e d y *, R i t a G o g g i n National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA
Received 15 September 1993
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 tunnelling. Given the progress in resolving these performance lifetime issues, silvered-PMMA films should meet the reflectance and durability goals.
1. Introduction Mirrors - or reflectors - that concentrate sunlight are critical elements in solar thermal electricity-generating systems. Sustained high performance and long life are two characteristics that reflectors need to exhibit to bring solar electric systems into widespread use. Silvered polymethylmethacrylate (PMMA) films are one of the more promising materials being studied for use in solar reflectors. SilveredP M M A is the most promising metallized polymer film because it exhibits greater optical durability and versatility than other reflecting materials. ECP-305 is a silvered-PMMA film that was developed by the 3M Company in collaboration with the National Renewable Energy Laboratory (NREL). ECP-305 represents the state of the art in polymer reflective materials. Two drawbacks
* Corresponding author. 0927-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0927-0248(93)E0133-X
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- - t h e price of the material ($2.00-$3.00 per square foot) and the loss of reflectance during service-- are currently impeding ECP-305 from being employed on a wide-scale basis in the solar industry. In this article we discuss the progress being made to overcome the three primary mechanisms-corrosion, soiling, and tunnelling that cause ECP-305 to lose reflectance.
2. Background on silvered reflector materials for solar applications
Two silvered materials, back silver-coated low-iron glass and silvered-PMMA films, are among the most promising reflector alternatives for making large-scale solar thermal electricity a reality. The hemispherical reflectance of freshly deposited silver, weighted over the solar spectrum (0.3-3.0 ~m) is greater than 97%. While silvered glass has performed well in solar applications in the United States and other countries, silvered polymer films are much lighter and potentially less expensive than their glass counterparts. They also allow greater flexibility in reflector design, especially in stretched membrane designs for concentrating solar collectors. The US Department of Energy, the agency sponsoring work in advanced optical materials through NREL, has set performance goals for reflector materials: advanced materials should maintain 90% reflectance into a 8 mrad, full-cone acceptance angle averaged over 5 years in outdoor service. Aluminized mirrors are excluded because their initial reflectance before weathering does not meet the 90% goal. The silvered-PMMA films under study at NREL are likely to meet these goals; samples are undergoing accelerated weathering tests in the laboratory and long-term outdoor tests in various sites throughout the country. A schematic cross-section of a silvered-PMMA mirror is shown in Fig. 1. A transparent top layer is required to protect the silver from abrasion, soiling, and corrosion. Typically, acrylic polymer with ultraviolet (UV) absorbers (to inhibit photon-activated degradation) is used for this purpose. The remainder of this paper addresses the degradation that can affect the reflectance of silvered-PMMA films.
3. Silver corrosion
Laboratory studies of silvered-PMMA mirror corrosion have identified several key variables. If no light is present, corrosion is virtually nonexistent even at elevated temperatures and humidity. Light at wavelengths up to about 450 nm triggers the corrosion mechanism. After initiation by light, the corrosion becomes sensitive to temperature, humidity, the ambient atmosphere, and materials used in the mirror construction. It is principally the light that passes through the silver to the rear structure of the mirror that causes corrosion. Silver about 100 nm thick is highly transparent to UV light near 320 nm, and some transmittance occurs at much longer wavelengths. These effects can be mitigated in several ways. UV
P. Schissel et al. / Solar Energy Materials and Solar Cells 33 (1994) 183-197
185
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absorbers are added to the polymer glazing to selectively block the light. Opaque layers behind the silver can block the light completely and, alternatively, a dielectric paint layer that isolates the aluminum substrate is also effective in slowing corrosion. N R E L has investigated improved silvered acrylic materials including high-purity polymers, more effective UV stabilizers, and coated polymers better able to resist scratches and to reject soil. Interlayers in front of and behind the silver and paint layers on aluminum or stainless-steel substrates can retard corrosion. New materials are evaluated in accelerated laboratory tests in a Weather-Ometer (WOM, Atlas Electric Co). In the process, samples are illuminated with a xenon arc lamp, with filters to match the solar spectrum, and are maintained at 60°C in air at 80% relative humidity. Fig. 2 presents the durability of reflectance in the WOM for ECP-305. The experimental mirrors are now so stable that a more accelerated test is also used. The collimated beam from a solar simulator concentrates the near-UV radiation to more than ten suns while the samples are maintained at 80°C in air at 75% relative humidity. Fig. 3 compares three types of mirrors and demonstrates the better performance of the newer materials. In Fig. 3, the corrosion of ECP-300A and ECP-305 has been slowed because the mirrors are mounted on painted aluminum substrates, which are known to provide better durability than unpainted aluminum for these mirror types.
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Fig. 2. Solar-weighted hemispherical reflectance versus months in the Weather-Ometer accelerated weathering chamber for ECP-305 mounted on painted aluminum.
Because of varying meteorological conditions, outdoor performance is site dependent. Denver's climate, for example, is relatively mild, and test samples of the ECP series of mirrors continue to maintain reflectance after years of exposure with only a slight improvement if a dielectric paint layer isolates the metal substrate. Fig. 4 shows data resulting from the m e a s u r e m e n t of solar-weighted hemispherical reflectance versus time for mirror type ECP-300A during up to 6 years of exposure in Colorado. ECP-300A is an earlier version that is now superseded by ECP-305, which is performing even better. However, data are only available for 2.5 years. Because Florida and particularly Arizona are harsher sites, it is necessary to use the dielectric paint layers on the substrates to obtain good mirror life. Even with the dielectric layers, data at 2 years or more show some degradation for ECP-305 (Fig. 4).
P. Schissel et aL / Solar Energy Materials and Solar Cells 33 (1994) 183-197
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Approximately 560 m 2 of ECP-300A were installed 4 years ago by Industrial Solar Technology (IST) on parabolic troughs near Denver. The film has resisted corrosion of the silver and has withstood monthly noncontact spray cleaning that regularly recovers reflectance above 90% (Fig. 5). The data shown are for hemispherical reflectance, which provides a good measure of reflectance loss resulting from corrosion. Silver corrosion is the usual degradation mechanism for ECP-305: rarely have sample mirrors lost only specu-
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Fig. 5. Parabolic troughs, Industrial Solar Technology,Denver.
larity, which would indicate loss caused by scattering. Specular reflectance can be limited by the chance curvature of the mirror substrate. For laboratory tests, substrate curvature effects are avoided either by stretching the mirrors as membranes or by mounting the mirrors on glass, even though silvered-PMMA is less durable when mounted on glass compared to painted aluminum. The specular reflectance at 650 nm wavelength for a full-cone acceptance angle of 8 mrad is plotted in Fig. 6 versus time outdoors at Golden, Colorado, for ECP-305 mounted on glass. Although no instrumentation is available to measure solar-weighted specular reflectance, Pettit [3] has used approximation schemes. If we assume that scattering is essentially independent of wavelength over the solar spectrum and that the beam shape is Gaussian, the ratio of solar-weighted specular reflectance to specular reflectance at 650 nm is the same as the ratio of solar-weighted hemispherical reflectance to hemispherical reflectance at 650 nm. Using this procedure, the initial solar-weighted specular reflectance (8 mrad) is estimated to be about 91% corresponding to the t = 0 point of Fig. 6. At the 36-month point of Fig. 6, the estimated solar-weighted specular reflectance has fallen to about 84%. Visually these mirrors have good specularity. Data on specular reflectance in the WOM are presented in Fig. 7. The specular reflectance at 650 nm wavelength for a full-cone acceptance angle of 8 mrad is plotted versus time in the WOM for ECP-305 samples mounted on glass. Comparison of Figs. 6 and 7 shows that the W O M is accelerated about a factor of seven for changes in specular reflectance compared to performance outdoors in Golden. Similar data are not available at other outdoor sites for mirrors mounted on glass.
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4. Soiling and cleaning Noncontact cleaning using only a medium-pressure spray of deionized water is effective, according to tests at IST [4] (Fig. 5). The tests, conducted in Brighton, Colorado, cleaned 560 m 2 of parabolic troughs that use the silvered-PMMA-mirfor-type ECP-300A. The specular reflectance was measured periodically, including immediately before and after noncontact cleaning, using a Devices and Services Model 14R portable reflectometer. Fig. 8 shows the specular reflectance immediately after washing, at 650 nm wavelength and at a large full-cone acceptance angle (25 mrad, because of the curvature of the troughs). The troughs had been operating for about 4 months when the monthly cleaning test began. Between the monthly washings, reflectance fell at rates between 0.1% and 0.5% per day, depending on weather conditions, with an average rate of 0.25% per day.
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As Fig. 8 shows, the specular reflectance is regularly returned to about 93% after cleaning. Unweathered mirrors have a specular reflectance of about 97% at 650 nm, so that a reflectance loss of about 4% occurs. This loss is caused by tenaciously held soil and is not retrievable by noncontact cleaning. Contact (abrasive) cleaning returns reflectance nearly to initial values, but soiling rates increase so that the average reflectance is not improved much during longer term tests. The study by IST also indicated the following preliminary conclusions regarding soiling and cleaning: (1) Detergents or high-pressure sprays do not improve cleaning. (2) Stowing troughs face down minimizes soil accumulation. Soiling depends on collector position; test samples that are held in a fixed position are of little value for predicting performance of operating systems.
P. Schissel et al. / Solar Energy Materials and Solar Cells 33 (1994) 183-197
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Fig. 8. Average specular reflectance at 650 nm wave length and 25 mrad full-cone, acceptance angle immediately after noncontact spray cleaning of a 560 m2 installation of parabolic troughs successively over a 2-year period (reflective film ECP-300A). (3) The 560 m z field could be washed in 2 hours using 450 liters of water. Laboratory and outdoor studies have identified factors such as hardness, smoothness, hydrophobicity, and low surface energy that may influence soiling [5]. Hardcoats do make silvered-PMMA mirrors more resistant to scratching. Temperature-cured hardcoats also weather well, but temperature limitations of the polymer require low curing temperatures that slow the curing and may increase cost. UV-cured hardcoats are equally scratch-resistant. They cure rapidly, but they have not weathered well. Studies have also indicated that detergents or surface coatings (e.g., fluorinecontaining layers) improve the performance of photovoltaic modules [5]. Fluorinecontaining molecular monolayers can be stable and can alter the performance of mirrors [6]. Other research also finds that surface coatings can improve the performance of mirrors [7], but the effects are lost after continued cleaning cycles. An acceptable coating that resists soiling or allows better maintenance of reflectance during cleaning cycles has not yet been identified.
5. Tunnelling Silvered-PMMA mirrors can experience a sporadic failure mode called tunnelling. During tunnelling the polymer separates from the silver metal in a
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characteristic pattern. Tunnelling usually occurs when the mirror is exposed to high humidity, and it almost always begins at the edges of the polymer. Important exceptions occur when the mirror is damaged away from the edges, but even then internally formed edges play a role. For ECP-305 the tunnels are usually about 2 cm wide and are separated by about 8 cm. Typically, a pattern of tunnels meanders over the complete mirror surface if the tunnels are not repaired as they initiate. Tunnelling is believed to begin when stresses, induced at the silver/polymer interface by differential thermal and hygroscopic expansion, overcome the weak adhesion between the polymer and silver. The adhesion between the polymer and silver is weak initially and is further weakened upon exposure to moisture. It is impossible to predict when tunnelling may occur. If water is allowed to puddle on a mirror, the mirror might fail within a few days. In contrast, a parabolic trough system installed by IST near Denver has not had any tunnelling problems after more than 1 year of operation. N R E L has had a variety of sample mirrors (46 cm by 61 cm) outdoors near Denver and they have not tunnelled in 1 year of exposure. Other installations have experienced sporadic problems. Thus, to solve the tunnelling problem, it was important to develop an accelerated exposure test that replicates failures in a more controlled manner in the laboratory. N R E L uses two laboratory tests to evaluate tunnelling [8]. The first exposes mirrors to moisture by immersing them in a bath of tap water at room temperature. The second procedure is a cyclic test that alternates the mirrors from a tap-water (27°C) bath to a dry oven (2 h at 50°C). Although these tests can induce tunnel failures that qualitatively replicate those observed in the field, they employ considerably harsher conditions than usually found in actual applications. Extrapolation of laboratory results to outdoor survival rates is neither possible nor intended at this time. The benefit of these tests is that they generate data that can be useful for purposes of comparison. The water-bath tests have identified several variables that influence tunnelling. A simple graphical presentation of reliability is obtained by plotting the percent survival of a sample set as a function of time in a particular tunnelling test. A basic plot is shown in Fig. 9. Ten flat samples were placed in the static water test. These particular samples consisted of ECP-305 (46 cm by 61 cm) laminated to aluminum substrates. The samples were cut from a roll 61 cm wide whose edges were slit by the 3M Company. The samples were cut from the roll with a heated knife and the 3M edges were left intact. These samples were edge taped with an aluminized polymer tape (3M ECP-244) and are representative of the current best practice. Fig. 9 graphically represents the current standard against which improvements are to be compared. A dramatic resistance to tunnelling occurs when the mirrors are annealed. Annealing the mirrors increases the force that is required to separate the silver/ polymer bond, perhaps by increasing adhesion or relieving residual stresses at the interface. For example, 31 flat samples (46 cm by 61 cm) were trimmed with a razor to intentionally form poor edges and the edges were not protected with edge tape. The samples were annealed at 80°C for 65 h. All samples were intact at 40 days in the water bath. In contrast, five samples prepared the same way, except
P. Schissel et aL / Solar Energy Materials and Solar Cells 33 (1994) 183-197 100
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with no annealing, had all failed by 28 days in water. The C-rank statistical theorem can be used to evaluate these data to show that mirror reliability in the water soak test is improved from less than 1% to 91% by annealing, with a test confidence of 95% [9]. Annealing has the important advantage in that it improves performance over the entire mirror area and not just at the edges. If damage occurs at the interior of the mirror and forms an internal edge, tunnelling is still resisted. To demonstrate the effectiveness of the thermal treatment at points of the mirror surface interior to the edges, 6 of the above 31 samples with 91% reliability were each drilled, scratched, and hit with a hammer and replaced in the water. These six samples had no tunnel failures after an additional 81 days of exposure to water. Samples that were drilled, scratched, hit, and not annealed fail very quickly ( < 1 day). One concern associated with thermally treating polymeric reflectors is the potential to adversely affect optical performance. Small (5 cm by 5 cm) samples of ECP-305 were mounted on glass substrates, and specular reflectance measurements were made at 4, 8, and 12 mrad full-acceptance angle. The samples were then subjected to 80°C for 65 h, and specular reflectance measurements were repeated. No discernable difference in specular reflectance was found even for the 4 mrad full-cone angle measurements. The temperature (80°C) and time (65 h) for the curing process were chosen based on the known shrinkage characteristics of ECP-305. Preliminary results for annealing temperatures of 60 °, 70 °, and 80°C and times of 4, 22, and 40 h indicated
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that 60°C is ineffective and that at 80°C, times as short as 4 h can improve performance. Edge preparation and protection are also effective because tunnelling virtually always initiates at edges. How the edges are cut affects tunnelling. Microscopic examination of edges reveals cut-induced flaws in the brittle polymer that depend on the cutting method. Razor cuts visually are poor and they perform poorly in the water bath. A heated knife that melts through the polymer yields better edges, probably because local melting anneals the flaws. After cutting, the edges can be protected. The current standard procedure is to tape the edges with an aluminized polymer tape. An alternative Tedlar tape (3M Co.) is easier to apply and is more effective in avoiding tunnelling. Data for samples with Tedlar edge tape are presented in Fig. 10. At 40 days in water there are no failures for 15 samples, which indicates a reliability of 83% at a test confidence of 95% [9]. Of the 15 samples, 5 had all edges trimmed with a razor to intentionally create poor edges. The Tedlar prevented any failures for 44 days and only one failure occurred up to 180 days. Ten of these samples had all four edges trimmed with a heated knife to provide better edges. The first of the ten failed after 149 days. For comparison, the data in Fig. 9 are also replotted in Fig. 10. The improved performance of the Tedlar tape is evident. The positive effect of Tedlar tape is more striking if one compares the five samples that were also trimmed with a razor but had no edge tape (Fig. 10). All five of these samples failed within 28 days, and the first sample failed in 1 day.
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195
The above tests were on flat 46 cm by 61 cm panels. Curved substrates were expected to introduce additional mechanical stresses on silvered polymer reflectors. These stresses could increase the likelihood of tunnel initiation and accelerate propagation. To explore this concern, tunnel experiments were carried out on curved samples in the static-soak screening baths. Recent success with heat treating samples of ECP-305 mounted on aluminum substrates prior to water immersion was incorporated into these tests. One set (10 replications) was laminated fiat, curved, (36 cm radius of curvature) and then heat treated. The other set (9 replications) was laminated onto curved substrates and then heat treated. All samples were trimmed with a razor blade; no edge protection (tape, etc.) was used. After 5 weeks of testing, none of the samples whose final step was thermal treatment had tunnelled. These encouraging results suggest that significant resistance to tunnel failure can be obtained by application of thermal treatment to ECP-305 laminated to curved surfaces. All the above tests involved placing the samples in water and leaving them in place. NREL has also cycled samples between the water bath (27°C) and a drying chamber (2 h at 50°C) and back to the water bath three times per week. A series of cyclic exposures confirm the improved performance for samples mounted on flat substrates that had been annealed prior to cycling or had their edges protected with Tedlar tape. Cyclic delamination tests have also been carried out to investigate concerns with ECP-305 laminated to curved substrates. Five curved (178-cm radius of curvature) samples each of the following constructions (of ECP-305 laminated to bare aluminum substrates, 46 cm x 61 cm in size, and cut with a razor blade) were prepared: (1) No heat treatment, no edge tape (2) 80°C heat treatment (65 h) after bending, no edge tape (3) 80°C heat treatment (65 h) before bending, no edge tape (4) 80°C heat treatment (65 h) before bending, ECP-244 edge tape (5) 80°C heat treatment (65 h) before bending, Tedlar edge tape. After 24 cycles in 59 days, sample sets 2-5 were replicated and introduced into test. In addition, five samples having Tedlar edge tape but no thermal treatment were prepared. Results are presented in Fig. 11. All five samples that had been neither heat treated nor edge taped tunnelled catastrophically after only 1 day in the water bath (1/2 cycle). The use of Tedlar tape without thermal treatment improves the delamination resistance of curved sample constructions but does not appear to provide complete protection. In contrast, none of the samples that were heat treated and edge taped (independent of the type of edge tape) have evidenced any delamination. Results also suggest that thermal treatment before curving provides protection against the mechanical stresses introduced into the reflector film during the bending process. When delamination damage of the thermally heated samples has been observed, it generally does not exhibit the characteristic tunnel patterns associated with such failure. Rather, the polymer film tends to pull away from the silver along the curved edges of the substrate.
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The water-bath tests are very harsh. Samples (46 cm by 61 cm) have also been tested outdoors near Denver. Some samples were ECP-305 mounted on stainless steel without edge tape, or ECP-305 mounted on aluminum with ECP-244 or Tedlar edge tape. Another set of mirrors consists of ECP-305 mounted on aluminum without edge tape and annealed at 80°C for 65 h. Parts of each sample set were drilled, scratched, and hit with a hammer to simulate damage in the field. By only 3 months of outdoor exposure seven of eight mirrors that were drilled but not annealed had tunnelled. In contrast, the two drilled samples that had been annealed remain intact at 1 year of outdoor exposure. All other samples, except one, remain intact at 1 year independent of how their edges were formed or whether or not they were edge taped.
6. C o n c l u s i o n s
Based on outdoor tests in Denver, silvered-PMMA films seem likely to meet the reflectance and durability requirements established by the Department of Energy. More long-term weathering data on corrosion are needed, particularly for harsher sites such as Arizona. Additional tests also need to be conducted to determine why weathering has a greater effect on specular reflectance than hemispherical reflectance. Based on accelerated testing of experimental silvered-PMMA samples, further improvement in optical durability may be expected. For the problem of silver corrosion, the test results show that this phenomenon is initiated by near-UV
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light and occurs uniformly over areas of test coupons instead of initiating from the edges. Soiling appears not to be a major issue affecting the long-term performance of silvered-PMMA reflectors. Non-contact cleaning could regularly return reflectance to more than 90% over a two-year period as demonstrated with a solar system for providing industrial heat. The delamination-or tunnelling-problem can be eliminated or substantially reduced by properly treating the mirrors. Annealing the reflector materials and protecting the mirror edges dramatically reduce tunnelling in test samples. The ability of these measures to reduce delamination in full-scale reflectors is currently being investigated in Albuquerque, New Mexico. The performance lifetime issues associated with silvered-PMMA reflectors have largely been resolved. As the costs of these materials are reduced, they should play an important role in future solar applications.
Acknowledgements The authors thank B.A. Benson of the 3M Company for supplying test materials and weathering data. IST kindly supplied results of its cleaning studies. This work was supported by the U.S. Department of Energy ( D O E ) under Contract No. DE-AC02-CH10093.
References [1] Five Year Research and Development Plan 1986-1990, US Department of Energy, DOE/CE-0160, September 1986. [2] I. Susemihl and P. Schissel, Sol. Energy Mater. 16 (1987) 403. [3] R.B. Pettit, Characterizing Solar Mirror Materials using Portable Reflectometers, SAND 82-1714, Sept. 1982. [4] Industrial Solar Technology, Soiling Rates and Cleaning Techniques for ECP 300, Final Report, Prepared for the National Renewable Energy Laboratory, Contract No. HL-9-19017-1, 1989. [5] E.F. Cuddihy and P.B. Willis, Antisoiling Technology:Theories of Surface Soiling and Performance of Antisoiling Surface Coatings, Jet Propulsion Laboratory, Pasadena, California, DOE/JPL-1012102 (DE85006658), 1984. [6] J.D. Schutt et al., Antisoiling Solar Reflector Research, Georgia Institute of Technology,Atlanta, Georgia, Prepared for the National Renewable Energy Laboratory, Contract No. XX-7-07029-1, 1989. [7] B. Baum et al., Protective Treatments for Membrane Heliostat Mirrors, Springborn Laboratories, Enfield, Connecticut. Prepared for the National Renewable Energy Laboratory, Contract No. HX-0-19028-1, 1990. 18] G. Jorgensen et al., Improved Tunnel Resistance of Silvered-PMMA Mirrors, NREL/TP-257-4419, Oct. 1991. [9] C. Lipson and N.J. Sheth, Statistical Design and Analysis of Engineering Experiments (McGraw Hill, NY, 1973) pp. 178, 251,252.
ELSEVIER
Solar Energy Materials and Solar Cells 33 (1994) 183-197
Silvered-PMMA reflectors Paul Schissel, G a r y J o r g e n s e n , C h e r y l K e n n e d y *, R i t a G o g g i n National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA
Received 15 September 1993
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 tunnelling. Given the progress in resolving these performance lifetime issues, silvered-PMMA films should meet the reflectance and durability goals.
1. Introduction Mirrors - or reflectors - that concentrate sunlight are critical elements in solar thermal electricity-generating systems. Sustained high performance and long life are two characteristics that reflectors need to exhibit to bring solar electric systems into widespread use. Silvered polymethylmethacrylate (PMMA) films are one of the more promising materials being studied for use in solar reflectors. SilveredP M M A is the most promising metallized polymer film because it exhibits greater optical durability and versatility than other reflecting materials. ECP-305 is a silvered-PMMA film that was developed by the 3M Company in collaboration with the National Renewable Energy Laboratory (NREL). ECP-305 represents the state of the art in polymer reflective materials. Two drawbacks
* Corresponding author. 0927-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0927-0248(93)E0133-X
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- - t h e price of the material ($2.00-$3.00 per square foot) and the loss of reflectance during service-- are currently impeding ECP-305 from being employed on a wide-scale basis in the solar industry. In this article we discuss the progress being made to overcome the three primary mechanisms-corrosion, soiling, and tunnelling that cause ECP-305 to lose reflectance.
2. Background on silvered reflector materials for solar applications
Two silvered materials, back silver-coated low-iron glass and silvered-PMMA films, are among the most promising reflector alternatives for making large-scale solar thermal electricity a reality. The hemispherical reflectance of freshly deposited silver, weighted over the solar spectrum (0.3-3.0 ~m) is greater than 97%. While silvered glass has performed well in solar applications in the United States and other countries, silvered polymer films are much lighter and potentially less expensive than their glass counterparts. They also allow greater flexibility in reflector design, especially in stretched membrane designs for concentrating solar collectors. The US Department of Energy, the agency sponsoring work in advanced optical materials through NREL, has set performance goals for reflector materials: advanced materials should maintain 90% reflectance into a 8 mrad, full-cone acceptance angle averaged over 5 years in outdoor service. Aluminized mirrors are excluded because their initial reflectance before weathering does not meet the 90% goal. The silvered-PMMA films under study at NREL are likely to meet these goals; samples are undergoing accelerated weathering tests in the laboratory and long-term outdoor tests in various sites throughout the country. A schematic cross-section of a silvered-PMMA mirror is shown in Fig. 1. A transparent top layer is required to protect the silver from abrasion, soiling, and corrosion. Typically, acrylic polymer with ultraviolet (UV) absorbers (to inhibit photon-activated degradation) is used for this purpose. The remainder of this paper addresses the degradation that can affect the reflectance of silvered-PMMA films.
3. Silver corrosion
Laboratory studies of silvered-PMMA mirror corrosion have identified several key variables. If no light is present, corrosion is virtually nonexistent even at elevated temperatures and humidity. Light at wavelengths up to about 450 nm triggers the corrosion mechanism. After initiation by light, the corrosion becomes sensitive to temperature, humidity, the ambient atmosphere, and materials used in the mirror construction. It is principally the light that passes through the silver to the rear structure of the mirror that causes corrosion. Silver about 100 nm thick is highly transparent to UV light near 320 nm, and some transmittance occurs at much longer wavelengths. These effects can be mitigated in several ways. UV
P. Schissel et al. / Solar Energy Materials and Solar Cells 33 (1994) 183-197
185
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absorbers are added to the polymer glazing to selectively block the light. Opaque layers behind the silver can block the light completely and, alternatively, a dielectric paint layer that isolates the aluminum substrate is also effective in slowing corrosion. N R E L has investigated improved silvered acrylic materials including high-purity polymers, more effective UV stabilizers, and coated polymers better able to resist scratches and to reject soil. Interlayers in front of and behind the silver and paint layers on aluminum or stainless-steel substrates can retard corrosion. New materials are evaluated in accelerated laboratory tests in a Weather-Ometer (WOM, Atlas Electric Co). In the process, samples are illuminated with a xenon arc lamp, with filters to match the solar spectrum, and are maintained at 60°C in air at 80% relative humidity. Fig. 2 presents the durability of reflectance in the WOM for ECP-305. The experimental mirrors are now so stable that a more accelerated test is also used. The collimated beam from a solar simulator concentrates the near-UV radiation to more than ten suns while the samples are maintained at 80°C in air at 75% relative humidity. Fig. 3 compares three types of mirrors and demonstrates the better performance of the newer materials. In Fig. 3, the corrosion of ECP-300A and ECP-305 has been slowed because the mirrors are mounted on painted aluminum substrates, which are known to provide better durability than unpainted aluminum for these mirror types.
186
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Because of varying meteorological conditions, outdoor performance is site dependent. Denver's climate, for example, is relatively mild, and test samples of the ECP series of mirrors continue to maintain reflectance after years of exposure with only a slight improvement if a dielectric paint layer isolates the metal substrate. Fig. 4 shows data resulting from the m e a s u r e m e n t of solar-weighted hemispherical reflectance versus time for mirror type ECP-300A during up to 6 years of exposure in Colorado. ECP-300A is an earlier version that is now superseded by ECP-305, which is performing even better. However, data are only available for 2.5 years. Because Florida and particularly Arizona are harsher sites, it is necessary to use the dielectric paint layers on the substrates to obtain good mirror life. Even with the dielectric layers, data at 2 years or more show some degradation for ECP-305 (Fig. 4).
P. Schissel et aL / Solar Energy Materials and Solar Cells 33 (1994) 183-197
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Approximately 560 m 2 of ECP-300A were installed 4 years ago by Industrial Solar Technology (IST) on parabolic troughs near Denver. The film has resisted corrosion of the silver and has withstood monthly noncontact spray cleaning that regularly recovers reflectance above 90% (Fig. 5). The data shown are for hemispherical reflectance, which provides a good measure of reflectance loss resulting from corrosion. Silver corrosion is the usual degradation mechanism for ECP-305: rarely have sample mirrors lost only specu-
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188
P. Schissel et al. /,Solar Energy Materials and Solar Cells 33 (1994) 183-197
Fig. 5. Parabolic troughs, Industrial Solar Technology,Denver.
larity, which would indicate loss caused by scattering. Specular reflectance can be limited by the chance curvature of the mirror substrate. For laboratory tests, substrate curvature effects are avoided either by stretching the mirrors as membranes or by mounting the mirrors on glass, even though silvered-PMMA is less durable when mounted on glass compared to painted aluminum. The specular reflectance at 650 nm wavelength for a full-cone acceptance angle of 8 mrad is plotted in Fig. 6 versus time outdoors at Golden, Colorado, for ECP-305 mounted on glass. Although no instrumentation is available to measure solar-weighted specular reflectance, Pettit [3] has used approximation schemes. If we assume that scattering is essentially independent of wavelength over the solar spectrum and that the beam shape is Gaussian, the ratio of solar-weighted specular reflectance to specular reflectance at 650 nm is the same as the ratio of solar-weighted hemispherical reflectance to hemispherical reflectance at 650 nm. Using this procedure, the initial solar-weighted specular reflectance (8 mrad) is estimated to be about 91% corresponding to the t = 0 point of Fig. 6. At the 36-month point of Fig. 6, the estimated solar-weighted specular reflectance has fallen to about 84%. Visually these mirrors have good specularity. Data on specular reflectance in the WOM are presented in Fig. 7. The specular reflectance at 650 nm wavelength for a full-cone acceptance angle of 8 mrad is plotted versus time in the WOM for ECP-305 samples mounted on glass. Comparison of Figs. 6 and 7 shows that the W O M is accelerated about a factor of seven for changes in specular reflectance compared to performance outdoors in Golden. Similar data are not available at other outdoor sites for mirrors mounted on glass.
P. Schissel et al. / Solar Energy Materials and Solar Cells 33 (1994) 183-197
189
4. Soiling and cleaning Noncontact cleaning using only a medium-pressure spray of deionized water is effective, according to tests at IST [4] (Fig. 5). The tests, conducted in Brighton, Colorado, cleaned 560 m 2 of parabolic troughs that use the silvered-PMMA-mirfor-type ECP-300A. The specular reflectance was measured periodically, including immediately before and after noncontact cleaning, using a Devices and Services Model 14R portable reflectometer. Fig. 8 shows the specular reflectance immediately after washing, at 650 nm wavelength and at a large full-cone acceptance angle (25 mrad, because of the curvature of the troughs). The troughs had been operating for about 4 months when the monthly cleaning test began. Between the monthly washings, reflectance fell at rates between 0.1% and 0.5% per day, depending on weather conditions, with an average rate of 0.25% per day.
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190
P. Schissel et aL / Solar Energy Materials and Solar Cells 33 (1994) 183-197
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As Fig. 8 shows, the specular reflectance is regularly returned to about 93% after cleaning. Unweathered mirrors have a specular reflectance of about 97% at 650 nm, so that a reflectance loss of about 4% occurs. This loss is caused by tenaciously held soil and is not retrievable by noncontact cleaning. Contact (abrasive) cleaning returns reflectance nearly to initial values, but soiling rates increase so that the average reflectance is not improved much during longer term tests. The study by IST also indicated the following preliminary conclusions regarding soiling and cleaning: (1) Detergents or high-pressure sprays do not improve cleaning. (2) Stowing troughs face down minimizes soil accumulation. Soiling depends on collector position; test samples that are held in a fixed position are of little value for predicting performance of operating systems.
P. Schissel et al. / Solar Energy Materials and Solar Cells 33 (1994) 183-197
191
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Fig. 8. Average specular reflectance at 650 nm wave length and 25 mrad full-cone, acceptance angle immediately after noncontact spray cleaning of a 560 m2 installation of parabolic troughs successively over a 2-year period (reflective film ECP-300A). (3) The 560 m z field could be washed in 2 hours using 450 liters of water. Laboratory and outdoor studies have identified factors such as hardness, smoothness, hydrophobicity, and low surface energy that may influence soiling [5]. Hardcoats do make silvered-PMMA mirrors more resistant to scratching. Temperature-cured hardcoats also weather well, but temperature limitations of the polymer require low curing temperatures that slow the curing and may increase cost. UV-cured hardcoats are equally scratch-resistant. They cure rapidly, but they have not weathered well. Studies have also indicated that detergents or surface coatings (e.g., fluorinecontaining layers) improve the performance of photovoltaic modules [5]. Fluorinecontaining molecular monolayers can be stable and can alter the performance of mirrors [6]. Other research also finds that surface coatings can improve the performance of mirrors [7], but the effects are lost after continued cleaning cycles. An acceptable coating that resists soiling or allows better maintenance of reflectance during cleaning cycles has not yet been identified.
5. Tunnelling Silvered-PMMA mirrors can experience a sporadic failure mode called tunnelling. During tunnelling the polymer separates from the silver metal in a
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P. Schissel et al. / Solar Energy Materials and Solar Cells 33 (1994) 183-197
characteristic pattern. Tunnelling usually occurs when the mirror is exposed to high humidity, and it almost always begins at the edges of the polymer. Important exceptions occur when the mirror is damaged away from the edges, but even then internally formed edges play a role. For ECP-305 the tunnels are usually about 2 cm wide and are separated by about 8 cm. Typically, a pattern of tunnels meanders over the complete mirror surface if the tunnels are not repaired as they initiate. Tunnelling is believed to begin when stresses, induced at the silver/polymer interface by differential thermal and hygroscopic expansion, overcome the weak adhesion between the polymer and silver. The adhesion between the polymer and silver is weak initially and is further weakened upon exposure to moisture. It is impossible to predict when tunnelling may occur. If water is allowed to puddle on a mirror, the mirror might fail within a few days. In contrast, a parabolic trough system installed by IST near Denver has not had any tunnelling problems after more than 1 year of operation. N R E L has had a variety of sample mirrors (46 cm by 61 cm) outdoors near Denver and they have not tunnelled in 1 year of exposure. Other installations have experienced sporadic problems. Thus, to solve the tunnelling problem, it was important to develop an accelerated exposure test that replicates failures in a more controlled manner in the laboratory. N R E L uses two laboratory tests to evaluate tunnelling [8]. The first exposes mirrors to moisture by immersing them in a bath of tap water at room temperature. The second procedure is a cyclic test that alternates the mirrors from a tap-water (27°C) bath to a dry oven (2 h at 50°C). Although these tests can induce tunnel failures that qualitatively replicate those observed in the field, they employ considerably harsher conditions than usually found in actual applications. Extrapolation of laboratory results to outdoor survival rates is neither possible nor intended at this time. The benefit of these tests is that they generate data that can be useful for purposes of comparison. The water-bath tests have identified several variables that influence tunnelling. A simple graphical presentation of reliability is obtained by plotting the percent survival of a sample set as a function of time in a particular tunnelling test. A basic plot is shown in Fig. 9. Ten flat samples were placed in the static water test. These particular samples consisted of ECP-305 (46 cm by 61 cm) laminated to aluminum substrates. The samples were cut from a roll 61 cm wide whose edges were slit by the 3M Company. The samples were cut from the roll with a heated knife and the 3M edges were left intact. These samples were edge taped with an aluminized polymer tape (3M ECP-244) and are representative of the current best practice. Fig. 9 graphically represents the current standard against which improvements are to be compared. A dramatic resistance to tunnelling occurs when the mirrors are annealed. Annealing the mirrors increases the force that is required to separate the silver/ polymer bond, perhaps by increasing adhesion or relieving residual stresses at the interface. For example, 31 flat samples (46 cm by 61 cm) were trimmed with a razor to intentionally form poor edges and the edges were not protected with edge tape. The samples were annealed at 80°C for 65 h. All samples were intact at 40 days in the water bath. In contrast, five samples prepared the same way, except
P. Schissel et aL / Solar Energy Materials and Solar Cells 33 (1994) 183-197 100
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with no annealing, had all failed by 28 days in water. The C-rank statistical theorem can be used to evaluate these data to show that mirror reliability in the water soak test is improved from less than 1% to 91% by annealing, with a test confidence of 95% [9]. Annealing has the important advantage in that it improves performance over the entire mirror area and not just at the edges. If damage occurs at the interior of the mirror and forms an internal edge, tunnelling is still resisted. To demonstrate the effectiveness of the thermal treatment at points of the mirror surface interior to the edges, 6 of the above 31 samples with 91% reliability were each drilled, scratched, and hit with a hammer and replaced in the water. These six samples had no tunnel failures after an additional 81 days of exposure to water. Samples that were drilled, scratched, hit, and not annealed fail very quickly ( < 1 day). One concern associated with thermally treating polymeric reflectors is the potential to adversely affect optical performance. Small (5 cm by 5 cm) samples of ECP-305 were mounted on glass substrates, and specular reflectance measurements were made at 4, 8, and 12 mrad full-acceptance angle. The samples were then subjected to 80°C for 65 h, and specular reflectance measurements were repeated. No discernable difference in specular reflectance was found even for the 4 mrad full-cone angle measurements. The temperature (80°C) and time (65 h) for the curing process were chosen based on the known shrinkage characteristics of ECP-305. Preliminary results for annealing temperatures of 60 °, 70 °, and 80°C and times of 4, 22, and 40 h indicated
194
P. Schissel et aL / Solar Energy Materials and Solar Cells 33 (1994) 183-197
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that 60°C is ineffective and that at 80°C, times as short as 4 h can improve performance. Edge preparation and protection are also effective because tunnelling virtually always initiates at edges. How the edges are cut affects tunnelling. Microscopic examination of edges reveals cut-induced flaws in the brittle polymer that depend on the cutting method. Razor cuts visually are poor and they perform poorly in the water bath. A heated knife that melts through the polymer yields better edges, probably because local melting anneals the flaws. After cutting, the edges can be protected. The current standard procedure is to tape the edges with an aluminized polymer tape. An alternative Tedlar tape (3M Co.) is easier to apply and is more effective in avoiding tunnelling. Data for samples with Tedlar edge tape are presented in Fig. 10. At 40 days in water there are no failures for 15 samples, which indicates a reliability of 83% at a test confidence of 95% [9]. Of the 15 samples, 5 had all edges trimmed with a razor to intentionally create poor edges. The Tedlar prevented any failures for 44 days and only one failure occurred up to 180 days. Ten of these samples had all four edges trimmed with a heated knife to provide better edges. The first of the ten failed after 149 days. For comparison, the data in Fig. 9 are also replotted in Fig. 10. The improved performance of the Tedlar tape is evident. The positive effect of Tedlar tape is more striking if one compares the five samples that were also trimmed with a razor but had no edge tape (Fig. 10). All five of these samples failed within 28 days, and the first sample failed in 1 day.
P. Schissel et aL / Solar Energy Materials and Solar Cells 33 (1994) 183-197
195
The above tests were on flat 46 cm by 61 cm panels. Curved substrates were expected to introduce additional mechanical stresses on silvered polymer reflectors. These stresses could increase the likelihood of tunnel initiation and accelerate propagation. To explore this concern, tunnel experiments were carried out on curved samples in the static-soak screening baths. Recent success with heat treating samples of ECP-305 mounted on aluminum substrates prior to water immersion was incorporated into these tests. One set (10 replications) was laminated fiat, curved, (36 cm radius of curvature) and then heat treated. The other set (9 replications) was laminated onto curved substrates and then heat treated. All samples were trimmed with a razor blade; no edge protection (tape, etc.) was used. After 5 weeks of testing, none of the samples whose final step was thermal treatment had tunnelled. These encouraging results suggest that significant resistance to tunnel failure can be obtained by application of thermal treatment to ECP-305 laminated to curved surfaces. All the above tests involved placing the samples in water and leaving them in place. NREL has also cycled samples between the water bath (27°C) and a drying chamber (2 h at 50°C) and back to the water bath three times per week. A series of cyclic exposures confirm the improved performance for samples mounted on flat substrates that had been annealed prior to cycling or had their edges protected with Tedlar tape. Cyclic delamination tests have also been carried out to investigate concerns with ECP-305 laminated to curved substrates. Five curved (178-cm radius of curvature) samples each of the following constructions (of ECP-305 laminated to bare aluminum substrates, 46 cm x 61 cm in size, and cut with a razor blade) were prepared: (1) No heat treatment, no edge tape (2) 80°C heat treatment (65 h) after bending, no edge tape (3) 80°C heat treatment (65 h) before bending, no edge tape (4) 80°C heat treatment (65 h) before bending, ECP-244 edge tape (5) 80°C heat treatment (65 h) before bending, Tedlar edge tape. After 24 cycles in 59 days, sample sets 2-5 were replicated and introduced into test. In addition, five samples having Tedlar edge tape but no thermal treatment were prepared. Results are presented in Fig. 11. All five samples that had been neither heat treated nor edge taped tunnelled catastrophically after only 1 day in the water bath (1/2 cycle). The use of Tedlar tape without thermal treatment improves the delamination resistance of curved sample constructions but does not appear to provide complete protection. In contrast, none of the samples that were heat treated and edge taped (independent of the type of edge tape) have evidenced any delamination. Results also suggest that thermal treatment before curving provides protection against the mechanical stresses introduced into the reflector film during the bending process. When delamination damage of the thermally heated samples has been observed, it generally does not exhibit the characteristic tunnel patterns associated with such failure. Rather, the polymer film tends to pull away from the silver along the curved edges of the substrate.
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P. Schissel et aL / Solar Energy Materials and Solar Cells 33 (1994) 183-197
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The water-bath tests are very harsh. Samples (46 cm by 61 cm) have also been tested outdoors near Denver. Some samples were ECP-305 mounted on stainless steel without edge tape, or ECP-305 mounted on aluminum with ECP-244 or Tedlar edge tape. Another set of mirrors consists of ECP-305 mounted on aluminum without edge tape and annealed at 80°C for 65 h. Parts of each sample set were drilled, scratched, and hit with a hammer to simulate damage in the field. By only 3 months of outdoor exposure seven of eight mirrors that were drilled but not annealed had tunnelled. In contrast, the two drilled samples that had been annealed remain intact at 1 year of outdoor exposure. All other samples, except one, remain intact at 1 year independent of how their edges were formed or whether or not they were edge taped.
6. C o n c l u s i o n s
Based on outdoor tests in Denver, silvered-PMMA films seem likely to meet the reflectance and durability requirements established by the Department of Energy. More long-term weathering data on corrosion are needed, particularly for harsher sites such as Arizona. Additional tests also need to be conducted to determine why weathering has a greater effect on specular reflectance than hemispherical reflectance. Based on accelerated testing of experimental silvered-PMMA samples, further improvement in optical durability may be expected. For the problem of silver corrosion, the test results show that this phenomenon is initiated by near-UV
P. Schissel et al. / Solar Energy Materials and Solar Cells 33 (1994) 183-197
197
light and occurs uniformly over areas of test coupons instead of initiating from the edges. Soiling appears not to be a major issue affecting the long-term performance of silvered-PMMA reflectors. Non-contact cleaning could regularly return reflectance to more than 90% over a two-year period as demonstrated with a solar system for providing industrial heat. The delamination-or tunnelling-problem can be eliminated or substantially reduced by properly treating the mirrors. Annealing the reflector materials and protecting the mirror edges dramatically reduce tunnelling in test samples. The ability of these measures to reduce delamination in full-scale reflectors is currently being investigated in Albuquerque, New Mexico. The performance lifetime issues associated with silvered-PMMA reflectors have largely been resolved. As the costs of these materials are reduced, they should play an important role in future solar applications.
Acknowledgements The authors thank B.A. Benson of the 3M Company for supplying test materials and weathering data. IST kindly supplied results of its cleaning studies. This work was supported by the U.S. Department of Energy ( D O E ) under Contract No. DE-AC02-CH10093.
References [1] Five Year Research and Development Plan 1986-1990, US Department of Energy, DOE/CE-0160, September 1986. [2] I. Susemihl and P. Schissel, Sol. Energy Mater. 16 (1987) 403. [3] R.B. Pettit, Characterizing Solar Mirror Materials using Portable Reflectometers, SAND 82-1714, Sept. 1982. [4] Industrial Solar Technology, Soiling Rates and Cleaning Techniques for ECP 300, Final Report, Prepared for the National Renewable Energy Laboratory, Contract No. HL-9-19017-1, 1989. [5] E.F. Cuddihy and P.B. Willis, Antisoiling Technology:Theories of Surface Soiling and Performance of Antisoiling Surface Coatings, Jet Propulsion Laboratory, Pasadena, California, DOE/JPL-1012102 (DE85006658), 1984. [6] J.D. Schutt et al., Antisoiling Solar Reflector Research, Georgia Institute of Technology,Atlanta, Georgia, Prepared for the National Renewable Energy Laboratory, Contract No. XX-7-07029-1, 1989. [7] B. Baum et al., Protective Treatments for Membrane Heliostat Mirrors, Springborn Laboratories, Enfield, Connecticut. Prepared for the National Renewable Energy Laboratory, Contract No. HX-0-19028-1, 1990. 18] G. Jorgensen et al., Improved Tunnel Resistance of Silvered-PMMA Mirrors, NREL/TP-257-4419, Oct. 1991. [9] C. Lipson and N.J. Sheth, Statistical Design and Analysis of Engineering Experiments (McGraw Hill, NY, 1973) pp. 178, 251,252.