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The use of leached gradient index anti-reflection surfaces on borosilicate glass solar collector cover tubes
Solar Energy Materials 11 (1984) 231-238 North-Holland, Amsterdam
231
THE USE OF LEACHED GRADIENT INDEX ANTI-REFLECTION SURFACES ON BOROSILICATE GLASS SOLAR COLLECTOR COVER TUBES H e r b e r t A. M I S K A Supervisor, Product Engineering, Advanced Products Department, Coming Glass Works, Main Plant, Building 8-5, Coming N Y 14831, USA
When alkali-borosilicate glasses are heat treated, they can be caused to phase separate. The silica and non-silica constituents are separated on a sub-microscopic scale. This enables small amounts of the non-silica phase to be chemically leached from the surface, which results in a graded reduction in the index of refraction near the surface. The index gradient can be controlled so that it leads to the most effective broad band, antireflection (A.R.) films known. At normal incidence, average reflection per surface can be kept well below 1% in the spectral region from 0.35 to 2.5 p~m. Further, films produced by this method maintain their effectiveness even at incident angles up to 70 o. All these aspects; i.e. manufacturing method, spectral response and angular response, combine to make the leached gradient index A.R. treatment ideally suited to enhance the efficiency of linear trough solar collector systems. Such systems typically use borosilicate glass cover tubes of the absorber tubes to cut heat losses, but in the absence of A.R. treatment, reflective losses from the cover tubes can cut system output by about 7-8%. The leached A.R. surface eliminates reflective losses almost entirely. Exposure to the elements in an industrial environment for five (5) years has resulted in no degradation of the effectiveness of the leached A.R. surfaces. While the surface is somewhat sensitive to contamination by body oils because of its porous nature, once installed, the inherent chemical durability of the nearly pure silica leads to excellent long-term performance.
I. Introduction I n 1976, M i n o t [1] d e m o n s t r a t e d t h a t h i g h l y e f f e c t i v e a n t i - r e f l e c t i o n ( A . R . ) films c a n b e p r o d u c e d o n the s u r f a c e s o f p h a s e - s e p a r a t e d b o r o s i l i c a t e glass b y v a r i o u s e t c h i n g a n d l e a c h i n g t e c h n i q u e s . E t c h t e c h n i q u e s has b e e n e m p l o y e d to p r o d u c e A . R . films as e a r l y as 1887, b u t the b e h a v i o r o f t h e s e e a r l y films was e s s e n t i a l l y t h a t o f single-layer, q u a r t e r - w a v e c o a t i n g s a n d , as such, t h e y e x h i b i t e d n o f u n d a m e n t a l a d v a n t a g e o v e r d e p o s i t e d l o w i n d e x films. O t h e r e t c h e d a n t i - r e f l e c t i o n s u r f a c e s o n l y d i f f u s e the r e f l e c t e d light a n d d o n o t e n h a n c e t r a n s m i t t a n c e . T h e t e c h n i q u e s d e v e l o p e d b y M i n o r , o n the o t h e r h a n d , h a v e r e s u l t e d in e x t r e m e l y e f f e c t i v e b r o a d b a n d anti-reflective properties, good performance over a wide range of incident angles and d e m o n s t r a t e d d u r a b i l i t y . T h e b r o a d b a n d a n d a n g u l a r r e f l e c t a n c e b e h a v i o r is the r e s u l t o f an i n d e x g r a d i e n t at the glass s u r f a c e w h i c h has m a n y of the c h a r a c t e r i s t i c s o f v e r y s o p h i s t i c a t e d m u l t i - l a y e r films w i t h o u t the h i g h cost n o r m a l l y a s s o c i a t e d w i t h s u c h surfaces. T h e c o m b i n e d a s p e c t s o f p e r f o r m a n c e , d u r a b i l i t y a n d cost m a k e g r a d i e n t a n t i - r e 0 1 6 5 - 1 6 3 3 / 8 4 / $ 0 3 . 0 0 © E l s e v i e r S c i e n c e P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)
232
H.A, Miska / Leached gradient index anti- reflection surfaces
flection (G.I.A.R.) films logical candidates for application to glass employed in solar energy systems. One of the most common designs used on solar energy collector systems for the generation of industrial process heat or steam for power generation consists of parabolic trough reflectors which feature a collector tube at the focus. In order to minimize heat losses, the absorptive coated steel pipe is typically covered by a glass tube. When the annular space between the tubes is also evacuated, conductive as well as convective heat losses are minimized. While preventing heat losses on the one hand, the glass cover tubes, if their surfaces are not specially treated, will cause a 7-8% loss in energy input to the collector pipe because of surface reflection. A broad band A.R. surface treatment which is insensitive to incident angle and which features demonstrated durability is therefore needed to maximize the system output. Gradient index anti-reflection surfaces produced by surface leaching of phase-separated glass have been demonstrated to meet this need.
2. Phase separation in borosilicate glasses Alkali borosilicate glasses have long been known to undergo a phase-separation process when reheated and held somewhat below the miscibility temperature. These glasses then separate into a silica-rich phase which is typically rather insoluable and a phase which is depleted in silica and more readily soluable (see fig. 1). For a
• c>
J
r
l,OWm Fig. 1. Spinoidal phase separation.
L
H.A. Miska / Leached gradient index anti-reflection surfaces
233
commercial alkali borosilicate, such as Corning Code 7740 Pyrex brand glass, phase separation can be carried out at temperature ranging from 570 to 660 °C. The scale on which phase separation occurs is a function of treatment time and temperature. Typically, as long as heat treatment is carried out below the miscibility temperature, longer times or higher temperatures both result in more complete separation.
3. The film forming process In its simplest terms, the process of forming gradient index anti-reflection films by the leach technique consists of heat treatment, skin removing etch and surface leaching. The heat treatment establishes the phase-separated structure which permits selective leaching to take place. Spectral response of the films ultimately formed depends on the degree of phase separation and on the scale on which it occurs. Typically, alkali borosilicate compositions such as Corning Code 7740 Pyrex brand glass can be phase separated at temperatures ranging from 570 to 660 °C for times ranging from 3 to 24 h. Glass distortion is most often the factor which governs the choice of temperature. The etching operation is designed to uniformly remove glass which often differs from the bulk material as a result of volatilization during glass forming. One typically finds a silica rich surface layer on glasses which are formed by operations such as tubing or sheet drawing. Acids which uniformly attack the glass structure are used for the skin removal etch. A distilled water rinse is then followed by the actual film forming etch/leach operation. This involves the use of mineral acid solutions which selectively attack the non-silica components in the phase-separated structure. A variety of acids have been found to be suitable. Solution temperatures in the range of 45 to 80°C, and etch/leach times of 30 s to 35 min have been successfully employed. Spectral response of the resulting film is a function of the etch/leach process and is addressed in the next section of this paper. A final rinse in de-ionized water completes the process.
4. Optical characteristics of G.I.A.R. films 4.1. Spectral response The spectral response of glass processed according to parameters which are compatible with production requirements is shown in fig. 2. Transmittance of treated and untreated 2.2 mm thick samples is plotted against wavelength. There is a some evidence of absorption at 1.4 ~tm in both the treated and untreated glass. Furthermore, it is obvious from this plot that over the region from 0.4 to about 1.75 ~m, the improvement over untreated glass in transmittance is very nearly constant at 6.2%. Between 1.75 and 2.1 ~tm the advantage averages 5.5%. The spectral response of G.I.A.R. films as a function of processing parameters has been studied by Minot [1] as well as Elmer and Martin [2]. Process variables studied were: Phase separation
H.A. Miska / Leached gradient index anti-reflection surfaces
234
time and temperature, film forming (leaching) time and solution temperature. Minot found that higher phase-separation temperatures (applied to Corning Code 7740 glass) tended to increase the effective band width of the films so that they showed less than 1% reflectance per surface throughout the visible spectrum and to 2.5 ~m in the infrared. Later experiments at Sandia Corporation and also in a manufacturing setting have shown that longer heat treatment at lower temperatures has essentially the same effect as higher temperatures. When heat treatment time and temperature are held constant and film forming time is varied, a cyclic variation reflectance is noted. The cycle time from maximum to minimum reflectance increases with increasing leach time. Increased leach bath temperature has the effect of shortening the time required for optimum film formation. In general, it can be said that there is considerable flexibility in choosing the process so that manufacturing and product considerations can usually be accommodated. The process chosen by Corning Glass Works for production of solar cover tubes was subject to the following constraints: The cover tubes had to remain round and straight within tight limits. This dictated phase separation at moderately low temperature for a fairly long time. - The process had to be somewhat insensitive to small leach time variations. While nearly optimum film performance can be obtained after very short leach times, this is not always desirable because of the cyclic response which is most prominent for short leach times. The product specification calls for an improvement in solar transmittance of 6%.
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Fig. 2. S p e c t r a l t r a n s m i t t a n c e - 2.2 m m b o r o s i l i c a t e glass G . I . A . R . t r e a t e d (7744) a n d u n t r e a t e d (7740).
H.,4. Miska / Leached gradient index anti-reflection surfaces
235
4. 2. Angular response In addition to being largely independent of wavelength, G.I.A.R. films have been shown to provide low reflectance over a wide range of incident angles [3]. Measurements were made in the visible regime (from 0.35 to 0.7 ~m) at incident angles of 20 ° , 30 ° , 40 ° , 50 ° , 60 ° and 70 ° . Reflectance (at ~ = 0 . 5 ~tm) is plotted as a function of incident angle in fig. 3. Nearly identical performance is exhibited at all wavelengths from 0.35 to 0.7 ~tm. Theoretical analysis of the behavior leads to the conclusion that good anti-reflection behavior at high-incident angles can be expected to extend well into the infrared [3].
5. Weathering tests Power and Elmer [4] as well as Waiters and A d a m s [5] have carried on long term durability studies of gradient index anti-reflection films which have led to the conclusion that a significant transmittance advantage over the untreated glass will be retained for at least 10 years. This is based on the results of 68 months of weathering-exposure testing on the roof of the Sullivan Park Research L a b o r a t o r y near C o m i n g , NY. According to Powers, the tests were begun in June 1976 and are still going on. Transmittance measurements were carried out at the end of 3, 6, 9, 12, 24, 37 and 68 months.
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Fig. 3. R e f l e c t a n c e @ A = 0.5 p,m G . I . A . R . t r e a t e d (7744) borosilicate glass c o m p a r e d to u n t r e a t e d (7740) borosilicate glass, 98.
H.A. Miska / Leached gradient index anti-reflection surfaces
236
Table 1 Average solar energy transmittance of uncleaned samples before and after weathering Sample type
Transmittance (%) for a Weather Exposure Period of 6 months
A B C D
Before 96.69 96.82 91.05 82.52
After 95.77 96.04 89.98 82.03
12 months
24 months
37 months
68 months
Before 97.03 96.63 91.04 82.65
Before 96.69 96.82 91.05 82.52
Before 96.69 96.82 91.05 82.52
Before 96.79 96.06 90.90 82.33
After 96.07 96.34 89.80 82.86
After 95.81 96.15 90.21 82.80
After 95.72 96.21 90.84 83.08
After 97.25 97.12 90.72 83.80
Sample types A B C D
7740 glass w / t y p e A gradient index anti-reflection film. 7740 glass w / t y p e B gradient indes anti-reflection film. 7740 glass w / n o film. L-O-F soda lime glass.
The test specimen configuration is a 25 x 19 mm 2 section cut from 100 mm diameter x 3 mm wall thickness tubing *. Two different processes for producing the anti-reflection surfaces were employed. These are designated types A and B. Untreated control specimens are designated type C. Finally, plates of commercial soda lime glass, also 25 x 19 x 3 mm 3 were included in the test and designated type D. The test specimens were inclined at a 45 ° angle, facing south on the roof of the Corning Glass Works Research Laboratory. The elevation at this location is 1261 feet above sea level. Rainfall, as monitored by the National Oceanographic and Atmospheric Administration, averages about 36 in per year. Temperatures range from a low of - 2 5 ° C to a high of 34°C. In addition to normal atmospheric influences, the glass is subject to what can be considered a "mild industrial environment". Transmittance measurements were made with a Varian Cary 17D spectrophotometer at wavelengths from 350 to 2100 nm. Solar energy transmittance of the glass is calculated by integrating the spectral transmittance from 350 to 2100 nm according to the following: X=2100
E
(1)
~350
where Tse = total solar energy transmittance, Tx = transmittance at wavelength )~, E x = relative energy at 50 nm intervals based on weighted coordinates as reported by Moon [6,7]. Four specimens of each type were included in the tests. Each time the samples were evaluated, one of each of the four types was ultrasonically cleaned in 40 ml of detergent water for one minute, followed by a rinse in distilled water and blow drying with a heat gun. Table 1 shows that in the total of 68 months exposure, essentially no degradation * C o r n i n g Code 7740 untreated; Code 7744 treated.
H.A. Miska / Leached gradient index anti-reflection surfaces
237
of performance has occurred. It should be noted that the samples on test represent early efforts which were not truly optimized for solar transmittance. Neither the type A nor the Type B, which differ primarily in the nature of the phase-separation heat treatment, exhibited the level of performance routinely achieved in the manufacturing plant where the heat treatment and chemical processing are optimized. Waiters and Adams carried out a separate study of the effect of various types of atmospheric and environmental pollution on the performance of a number of anti-reflection films. Their main findings for the gradient index film produced by methods desribed here are as follows: 1. A significant transmittance advantage over untreated glass will be retained for at least 10 years. 2. Heavy soil, including bird droppings and tree sap, was removed by rain and snow.
3. Sand abrasion resistance was superior to the other film types tested * and significantly exceeded nonfilmed soda lime window glass. 4. Resistance to detergent scrubbing was excellent. Compared to the other surfaces tested, the porous gradient index anti-reflection films on Corning Code 7744 glass retained their overall transmittance advantage after one year in the pigeon roost, tree sap, road dust, high humidity, weatherometer (6 months only), mild industrial, Florida marine, SO 2 and HC1 environments. Only in a salt spray and N a O H environment did the G.I.A.R. surfaces suffer degradation which left them inferior to untreated borosilicate glass. Based on the above, Walters and Adams have estimated that G.I.A.R. surfaces on borosilicate glasses will retain between 93 and 94% transmittance after 20 years in a chemically-polluted environment.
6. Cleaning of G.I.A.R. surfaces Power and Elmer [4] found that the action of rain and snow alone appears to be sufficient to keep G.I.A.R. surfaces clean in an ordinary outdoor environment. They report that after 68 months ultrasonic cleaning in distilled water with a surface active agent only improved the transmittance by 0.6 to 0.9% over the level measured without cleaning. At that time, samples from both the A and B treatment each exhibited 97% total solar transmittance before ultrasonic cleaning. Walters and Adams confirm that G.I.A.R. surfaces which are periodically " w a s h e d " by the action of rain will be less prone to weathering than unwashed specimens. It should also be noted that in spite of the porous nature of the surface, there is no evidence of water absorption as a result of exposure to rain. Freezing appears to have no adverse effect on the surface which seems to confirm the absence of water absorption. * Soda lime window glass with anti-reflection surface produced by SiO2 redeposition according to Nicoll method; Code 7740 flat glass coated with MgF2; Code 7740 flat glass coated with Teflon; Code 7740 flat glass coated with SiO2/TiO 2.
238
H.A. Miska / Leached gradient index anti-reflection surfaces
7. Conclusion The c o m b i n a t i o n of c o n v e n i e n t a n d readily-controlled m a n u f a c t u r i n g process, excellent spectral a n d angular response a n d d e m o n s t r a t e d durability has made G.I.A.R. films the logical choice for solar collector cover tubes. System efficiency increases of 7% are realized. This allows fewer collector modules to be employed for a given a m o u n t of energy output. The economic benefit of using cover tubes with G.I.A.R. surfaces is thus very readily calculated a n d it has been shown that the savings achieved are substantial compared to the added processing costs. At a time when fossil fuels appear, at least temporarily, to be a b u n d a n t l y available, solar energy systems must strive for m a x i m u m efficiency, a n d this can only be achieved when losses are minimized. G.I.A.R. films allow the designer to do this.
References [1] [2] [3] [4] [5]
M.J. Minot, J. Opt. Soc. Am. 66 (6) (1976). T.H. Elmer and F.W. Martin, Am. Ceram. Soc. Bul. 58 (11) (1979). M.J. Minot, J. Opt. Soc. Am. 67 (8) (1977). J.M. Power and T.H. Elmer, Am. Ceram. Soc. Bul. 59 (11) (1980). H.V. Waiters and P.B. Adams, Performance Evaluation of Anti-reflective Films for Solar Collectors, Internal Communications, Corning Glass Works (November 1982). [6] P. Moon, J. Franklin Inst. 230 (1940) 583. [7] 1978 ASTM Annual Book of ASTM Standards, Part 46 (Am. Soc. for Testing and Materials Philadelphia, PA, 1978) p. 281.
231
THE USE OF LEACHED GRADIENT INDEX ANTI-REFLECTION SURFACES ON BOROSILICATE GLASS SOLAR COLLECTOR COVER TUBES H e r b e r t A. M I S K A Supervisor, Product Engineering, Advanced Products Department, Coming Glass Works, Main Plant, Building 8-5, Coming N Y 14831, USA
When alkali-borosilicate glasses are heat treated, they can be caused to phase separate. The silica and non-silica constituents are separated on a sub-microscopic scale. This enables small amounts of the non-silica phase to be chemically leached from the surface, which results in a graded reduction in the index of refraction near the surface. The index gradient can be controlled so that it leads to the most effective broad band, antireflection (A.R.) films known. At normal incidence, average reflection per surface can be kept well below 1% in the spectral region from 0.35 to 2.5 p~m. Further, films produced by this method maintain their effectiveness even at incident angles up to 70 o. All these aspects; i.e. manufacturing method, spectral response and angular response, combine to make the leached gradient index A.R. treatment ideally suited to enhance the efficiency of linear trough solar collector systems. Such systems typically use borosilicate glass cover tubes of the absorber tubes to cut heat losses, but in the absence of A.R. treatment, reflective losses from the cover tubes can cut system output by about 7-8%. The leached A.R. surface eliminates reflective losses almost entirely. Exposure to the elements in an industrial environment for five (5) years has resulted in no degradation of the effectiveness of the leached A.R. surfaces. While the surface is somewhat sensitive to contamination by body oils because of its porous nature, once installed, the inherent chemical durability of the nearly pure silica leads to excellent long-term performance.
I. Introduction I n 1976, M i n o t [1] d e m o n s t r a t e d t h a t h i g h l y e f f e c t i v e a n t i - r e f l e c t i o n ( A . R . ) films c a n b e p r o d u c e d o n the s u r f a c e s o f p h a s e - s e p a r a t e d b o r o s i l i c a t e glass b y v a r i o u s e t c h i n g a n d l e a c h i n g t e c h n i q u e s . E t c h t e c h n i q u e s has b e e n e m p l o y e d to p r o d u c e A . R . films as e a r l y as 1887, b u t the b e h a v i o r o f t h e s e e a r l y films was e s s e n t i a l l y t h a t o f single-layer, q u a r t e r - w a v e c o a t i n g s a n d , as such, t h e y e x h i b i t e d n o f u n d a m e n t a l a d v a n t a g e o v e r d e p o s i t e d l o w i n d e x films. O t h e r e t c h e d a n t i - r e f l e c t i o n s u r f a c e s o n l y d i f f u s e the r e f l e c t e d light a n d d o n o t e n h a n c e t r a n s m i t t a n c e . T h e t e c h n i q u e s d e v e l o p e d b y M i n o r , o n the o t h e r h a n d , h a v e r e s u l t e d in e x t r e m e l y e f f e c t i v e b r o a d b a n d anti-reflective properties, good performance over a wide range of incident angles and d e m o n s t r a t e d d u r a b i l i t y . T h e b r o a d b a n d a n d a n g u l a r r e f l e c t a n c e b e h a v i o r is the r e s u l t o f an i n d e x g r a d i e n t at the glass s u r f a c e w h i c h has m a n y of the c h a r a c t e r i s t i c s o f v e r y s o p h i s t i c a t e d m u l t i - l a y e r films w i t h o u t the h i g h cost n o r m a l l y a s s o c i a t e d w i t h s u c h surfaces. T h e c o m b i n e d a s p e c t s o f p e r f o r m a n c e , d u r a b i l i t y a n d cost m a k e g r a d i e n t a n t i - r e 0 1 6 5 - 1 6 3 3 / 8 4 / $ 0 3 . 0 0 © E l s e v i e r S c i e n c e P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)
232
H.A, Miska / Leached gradient index anti- reflection surfaces
flection (G.I.A.R.) films logical candidates for application to glass employed in solar energy systems. One of the most common designs used on solar energy collector systems for the generation of industrial process heat or steam for power generation consists of parabolic trough reflectors which feature a collector tube at the focus. In order to minimize heat losses, the absorptive coated steel pipe is typically covered by a glass tube. When the annular space between the tubes is also evacuated, conductive as well as convective heat losses are minimized. While preventing heat losses on the one hand, the glass cover tubes, if their surfaces are not specially treated, will cause a 7-8% loss in energy input to the collector pipe because of surface reflection. A broad band A.R. surface treatment which is insensitive to incident angle and which features demonstrated durability is therefore needed to maximize the system output. Gradient index anti-reflection surfaces produced by surface leaching of phase-separated glass have been demonstrated to meet this need.
2. Phase separation in borosilicate glasses Alkali borosilicate glasses have long been known to undergo a phase-separation process when reheated and held somewhat below the miscibility temperature. These glasses then separate into a silica-rich phase which is typically rather insoluable and a phase which is depleted in silica and more readily soluable (see fig. 1). For a
• c>
J
r
l,OWm Fig. 1. Spinoidal phase separation.
L
H.A. Miska / Leached gradient index anti-reflection surfaces
233
commercial alkali borosilicate, such as Corning Code 7740 Pyrex brand glass, phase separation can be carried out at temperature ranging from 570 to 660 °C. The scale on which phase separation occurs is a function of treatment time and temperature. Typically, as long as heat treatment is carried out below the miscibility temperature, longer times or higher temperatures both result in more complete separation.
3. The film forming process In its simplest terms, the process of forming gradient index anti-reflection films by the leach technique consists of heat treatment, skin removing etch and surface leaching. The heat treatment establishes the phase-separated structure which permits selective leaching to take place. Spectral response of the films ultimately formed depends on the degree of phase separation and on the scale on which it occurs. Typically, alkali borosilicate compositions such as Corning Code 7740 Pyrex brand glass can be phase separated at temperatures ranging from 570 to 660 °C for times ranging from 3 to 24 h. Glass distortion is most often the factor which governs the choice of temperature. The etching operation is designed to uniformly remove glass which often differs from the bulk material as a result of volatilization during glass forming. One typically finds a silica rich surface layer on glasses which are formed by operations such as tubing or sheet drawing. Acids which uniformly attack the glass structure are used for the skin removal etch. A distilled water rinse is then followed by the actual film forming etch/leach operation. This involves the use of mineral acid solutions which selectively attack the non-silica components in the phase-separated structure. A variety of acids have been found to be suitable. Solution temperatures in the range of 45 to 80°C, and etch/leach times of 30 s to 35 min have been successfully employed. Spectral response of the resulting film is a function of the etch/leach process and is addressed in the next section of this paper. A final rinse in de-ionized water completes the process.
4. Optical characteristics of G.I.A.R. films 4.1. Spectral response The spectral response of glass processed according to parameters which are compatible with production requirements is shown in fig. 2. Transmittance of treated and untreated 2.2 mm thick samples is plotted against wavelength. There is a some evidence of absorption at 1.4 ~tm in both the treated and untreated glass. Furthermore, it is obvious from this plot that over the region from 0.4 to about 1.75 ~m, the improvement over untreated glass in transmittance is very nearly constant at 6.2%. Between 1.75 and 2.1 ~tm the advantage averages 5.5%. The spectral response of G.I.A.R. films as a function of processing parameters has been studied by Minot [1] as well as Elmer and Martin [2]. Process variables studied were: Phase separation
H.A. Miska / Leached gradient index anti-reflection surfaces
234
time and temperature, film forming (leaching) time and solution temperature. Minot found that higher phase-separation temperatures (applied to Corning Code 7740 glass) tended to increase the effective band width of the films so that they showed less than 1% reflectance per surface throughout the visible spectrum and to 2.5 ~m in the infrared. Later experiments at Sandia Corporation and also in a manufacturing setting have shown that longer heat treatment at lower temperatures has essentially the same effect as higher temperatures. When heat treatment time and temperature are held constant and film forming time is varied, a cyclic variation reflectance is noted. The cycle time from maximum to minimum reflectance increases with increasing leach time. Increased leach bath temperature has the effect of shortening the time required for optimum film formation. In general, it can be said that there is considerable flexibility in choosing the process so that manufacturing and product considerations can usually be accommodated. The process chosen by Corning Glass Works for production of solar cover tubes was subject to the following constraints: The cover tubes had to remain round and straight within tight limits. This dictated phase separation at moderately low temperature for a fairly long time. - The process had to be somewhat insensitive to small leach time variations. While nearly optimum film performance can be obtained after very short leach times, this is not always desirable because of the cyclic response which is most prominent for short leach times. The product specification calls for an improvement in solar transmittance of 6%.
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Fig. 2. S p e c t r a l t r a n s m i t t a n c e - 2.2 m m b o r o s i l i c a t e glass G . I . A . R . t r e a t e d (7744) a n d u n t r e a t e d (7740).
H.,4. Miska / Leached gradient index anti-reflection surfaces
235
4. 2. Angular response In addition to being largely independent of wavelength, G.I.A.R. films have been shown to provide low reflectance over a wide range of incident angles [3]. Measurements were made in the visible regime (from 0.35 to 0.7 ~m) at incident angles of 20 ° , 30 ° , 40 ° , 50 ° , 60 ° and 70 ° . Reflectance (at ~ = 0 . 5 ~tm) is plotted as a function of incident angle in fig. 3. Nearly identical performance is exhibited at all wavelengths from 0.35 to 0.7 ~tm. Theoretical analysis of the behavior leads to the conclusion that good anti-reflection behavior at high-incident angles can be expected to extend well into the infrared [3].
5. Weathering tests Power and Elmer [4] as well as Waiters and A d a m s [5] have carried on long term durability studies of gradient index anti-reflection films which have led to the conclusion that a significant transmittance advantage over the untreated glass will be retained for at least 10 years. This is based on the results of 68 months of weathering-exposure testing on the roof of the Sullivan Park Research L a b o r a t o r y near C o m i n g , NY. According to Powers, the tests were begun in June 1976 and are still going on. Transmittance measurements were carried out at the end of 3, 6, 9, 12, 24, 37 and 68 months.
17 16 15 14 13 ,_, w I....u "'
12 11 10
,., ,Y ~ z
9
8
~ ~
7 8 4
_
~
. 25
. 30
3 2
0
T 20
.
. 35
. 40
INCIDENT []
CORNING
CODE
7740
. 45
. 50
.
. 55
ANGLE DEGREES o CORNING
60
65
70
CODE 7744
Fig. 3. R e f l e c t a n c e @ A = 0.5 p,m G . I . A . R . t r e a t e d (7744) borosilicate glass c o m p a r e d to u n t r e a t e d (7740) borosilicate glass, 98.
H.A. Miska / Leached gradient index anti-reflection surfaces
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Table 1 Average solar energy transmittance of uncleaned samples before and after weathering Sample type
Transmittance (%) for a Weather Exposure Period of 6 months
A B C D
Before 96.69 96.82 91.05 82.52
After 95.77 96.04 89.98 82.03
12 months
24 months
37 months
68 months
Before 97.03 96.63 91.04 82.65
Before 96.69 96.82 91.05 82.52
Before 96.69 96.82 91.05 82.52
Before 96.79 96.06 90.90 82.33
After 96.07 96.34 89.80 82.86
After 95.81 96.15 90.21 82.80
After 95.72 96.21 90.84 83.08
After 97.25 97.12 90.72 83.80
Sample types A B C D
7740 glass w / t y p e A gradient index anti-reflection film. 7740 glass w / t y p e B gradient indes anti-reflection film. 7740 glass w / n o film. L-O-F soda lime glass.
The test specimen configuration is a 25 x 19 mm 2 section cut from 100 mm diameter x 3 mm wall thickness tubing *. Two different processes for producing the anti-reflection surfaces were employed. These are designated types A and B. Untreated control specimens are designated type C. Finally, plates of commercial soda lime glass, also 25 x 19 x 3 mm 3 were included in the test and designated type D. The test specimens were inclined at a 45 ° angle, facing south on the roof of the Corning Glass Works Research Laboratory. The elevation at this location is 1261 feet above sea level. Rainfall, as monitored by the National Oceanographic and Atmospheric Administration, averages about 36 in per year. Temperatures range from a low of - 2 5 ° C to a high of 34°C. In addition to normal atmospheric influences, the glass is subject to what can be considered a "mild industrial environment". Transmittance measurements were made with a Varian Cary 17D spectrophotometer at wavelengths from 350 to 2100 nm. Solar energy transmittance of the glass is calculated by integrating the spectral transmittance from 350 to 2100 nm according to the following: X=2100
E
(1)
~350
where Tse = total solar energy transmittance, Tx = transmittance at wavelength )~, E x = relative energy at 50 nm intervals based on weighted coordinates as reported by Moon [6,7]. Four specimens of each type were included in the tests. Each time the samples were evaluated, one of each of the four types was ultrasonically cleaned in 40 ml of detergent water for one minute, followed by a rinse in distilled water and blow drying with a heat gun. Table 1 shows that in the total of 68 months exposure, essentially no degradation * C o r n i n g Code 7740 untreated; Code 7744 treated.
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237
of performance has occurred. It should be noted that the samples on test represent early efforts which were not truly optimized for solar transmittance. Neither the type A nor the Type B, which differ primarily in the nature of the phase-separation heat treatment, exhibited the level of performance routinely achieved in the manufacturing plant where the heat treatment and chemical processing are optimized. Waiters and Adams carried out a separate study of the effect of various types of atmospheric and environmental pollution on the performance of a number of anti-reflection films. Their main findings for the gradient index film produced by methods desribed here are as follows: 1. A significant transmittance advantage over untreated glass will be retained for at least 10 years. 2. Heavy soil, including bird droppings and tree sap, was removed by rain and snow.
3. Sand abrasion resistance was superior to the other film types tested * and significantly exceeded nonfilmed soda lime window glass. 4. Resistance to detergent scrubbing was excellent. Compared to the other surfaces tested, the porous gradient index anti-reflection films on Corning Code 7744 glass retained their overall transmittance advantage after one year in the pigeon roost, tree sap, road dust, high humidity, weatherometer (6 months only), mild industrial, Florida marine, SO 2 and HC1 environments. Only in a salt spray and N a O H environment did the G.I.A.R. surfaces suffer degradation which left them inferior to untreated borosilicate glass. Based on the above, Walters and Adams have estimated that G.I.A.R. surfaces on borosilicate glasses will retain between 93 and 94% transmittance after 20 years in a chemically-polluted environment.
6. Cleaning of G.I.A.R. surfaces Power and Elmer [4] found that the action of rain and snow alone appears to be sufficient to keep G.I.A.R. surfaces clean in an ordinary outdoor environment. They report that after 68 months ultrasonic cleaning in distilled water with a surface active agent only improved the transmittance by 0.6 to 0.9% over the level measured without cleaning. At that time, samples from both the A and B treatment each exhibited 97% total solar transmittance before ultrasonic cleaning. Walters and Adams confirm that G.I.A.R. surfaces which are periodically " w a s h e d " by the action of rain will be less prone to weathering than unwashed specimens. It should also be noted that in spite of the porous nature of the surface, there is no evidence of water absorption as a result of exposure to rain. Freezing appears to have no adverse effect on the surface which seems to confirm the absence of water absorption. * Soda lime window glass with anti-reflection surface produced by SiO2 redeposition according to Nicoll method; Code 7740 flat glass coated with MgF2; Code 7740 flat glass coated with Teflon; Code 7740 flat glass coated with SiO2/TiO 2.
238
H.A. Miska / Leached gradient index anti-reflection surfaces
7. Conclusion The c o m b i n a t i o n of c o n v e n i e n t a n d readily-controlled m a n u f a c t u r i n g process, excellent spectral a n d angular response a n d d e m o n s t r a t e d durability has made G.I.A.R. films the logical choice for solar collector cover tubes. System efficiency increases of 7% are realized. This allows fewer collector modules to be employed for a given a m o u n t of energy output. The economic benefit of using cover tubes with G.I.A.R. surfaces is thus very readily calculated a n d it has been shown that the savings achieved are substantial compared to the added processing costs. At a time when fossil fuels appear, at least temporarily, to be a b u n d a n t l y available, solar energy systems must strive for m a x i m u m efficiency, a n d this can only be achieved when losses are minimized. G.I.A.R. films allow the designer to do this.
References [1] [2] [3] [4] [5]
M.J. Minot, J. Opt. Soc. Am. 66 (6) (1976). T.H. Elmer and F.W. Martin, Am. Ceram. Soc. Bul. 58 (11) (1979). M.J. Minot, J. Opt. Soc. Am. 67 (8) (1977). J.M. Power and T.H. Elmer, Am. Ceram. Soc. Bul. 59 (11) (1980). H.V. Waiters and P.B. Adams, Performance Evaluation of Anti-reflective Films for Solar Collectors, Internal Communications, Corning Glass Works (November 1982). [6] P. Moon, J. Franklin Inst. 230 (1940) 583. [7] 1978 ASTM Annual Book of ASTM Standards, Part 46 (Am. Soc. for Testing and Materials Philadelphia, PA, 1978) p. 281.