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Solarex experience with ethylene vinyl acetate encapsulation
Solar Cells, 30 (1991) 383-387
383
Solarex experience with ethylene vinyl acetate encapsulation J o h n H. W o h l g e m u t h and Raymond C. P e t e r s e n Solarex Corporation, Frederick, MD 21701 (U.S.A.)
(Received October 25, 1990)
Abstract Solarex began using ethylene vinyl acetate (EVA) as an encapsulant for photovoltaic modules during the Jet Propulsion Laboratory sponsored Block IV Program in 1979. Experience was gained in the processing and use of EVA during a number of Department of Energy sponsored projects through the early 1980s. In 1982 Solarex began using EVA in commercial modules and has continued to use it up to the present time. EVA has proven to be a highly reliable encapsulant, with no reported instances of Solarex module field failures being attributed to failure of the EVA encapsulant. The EVA encapsulation system is complex, requiring well controlled manufacture of the film itself and the correct lamination procedure to assure adequate cure and bonding to the glass, cell and backsheet surfaces. The initial Springborn work on EVA included accelerated testing, which indicated that at temperatures considerably higher than experienced during normal module operation, the EVA system will suffer thermally induced degradation. However, no major degradation was experienced under normal operating conditions during either Springborn's testing or Solarex's 10 years of field experience.
1. I n t r o d u c t i o n Solarex b e g a n d e v e l o p i n g the t e c h n o l o g y t o use the S p r i n g b o r n ethylene vinyl a c e t a t e (EVA) f o r m u l a t i o n d u r i n g the Block IV p r o g r a m b e g i n n i n g in 1979 [1 ]. P r e v i o u s to this, all Solarex m o d u l e s h a d utilized silicone r u b b e r e n c a p s u l a n t s . EVA w a s d e v e l o p e d b y S p r i n g b o r n u n d e r a J e t P r o p u l s i o n L a b o r a t o r y (JPL) c o n t r a c t as a l o w e r c o s t alternative to silicone r u b b e r [2]. L a m i n a t e d EVA m o d u l e s were also built b y Solarex for a n u m b e r o f D e p a r t m e n t of E n e r g y s p o n s o r e d p r o j e c t s including b o t h the Solarex a n d M a s s a c h u s e t t s Institute o f T e c h n o l o g y h o u s e s at the N o r t h e a s t Residence, the Solarex h o u s e at the S o u t h w e s t Residence, the Carlisle h o u s e a n d finally B l o c k V. T h e s e p r o j e c t s p r o v i d e d a basis for m o d u l e qualification via Block IV [3] a n d the Block V [4] a c c e l e r a t e d e n v i r o n m e n t a l test s e q u e n c e s a n d p r e l i m i n a r y field t e s t i n g o f the p r o d u c t . B a s e d o n the positive results f r o m t h e s e g o v e r n m e n t projects, Solarex i n t r o d u c e d EVA into the c o m m e r c i a l m o d u l e line in 1982. Today, all o f S o l a r e x ' s terrestrial m o d u l e s are e n c a p s u l a t e d u s i n g EVA lamination. The c r o s s - s e c t i o n a l s t r u c t u r e o f a Solarex EVA m o d u l e is s h o w n in Fig. 1. The c o m p o n e n t s o f the c o n s t r u c t i o n are as follows: low iron t e m p e r e d
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
384
GLASS I
I[
]1
II
IJMATRIX/EVA "BACKSHEET
Fig. 1. Solarex EVA lamination package.
glass superstrate; clear EVA above the cell matrix; interconnected matrix of polycrystalline silicon solar cells; EVA with Craneglas ® behind the cell matrix; the Craneglas ® assists in removal of gas from the laminate and eliminates cell movement during lamination; a three part backsheet of blue modified polyethylene, Mylar and white Tedlar. 2. E t h y l e n e vinyl a c e t a t e f o r m u l a t i o n s The original EVA formulation used by Solarex was Springborn A9918. It is based on DuPont Elvax 150 EVA. The formulation also contained 0.2% Uniroyal Naugard P as an antioxidant, 0.1% Ciba-Geigy Tinuvin 770 for UV stabilization, 0.3% American Cyanamid Cyasorb UV-531 for UV stabilization, and 1.5% Lupersol 101 organic peroxide curing agent. The only change that Solarex has ever made in the formulation was a switch in 1987 to Springborn formulation 15295, where the Lupersol 101 is replaced by Lupersol TBEC, to reduce the temperature and time requirements for cure. Solarex has never utilized an EVA formulation with primer. We prefer to apply the primer directly to the glass. In this way, we are assured the presence of sufficient primer at the glass--EVA interface and do not have to worry about how uniformly the primer was dispersed during the extrusion. Finally, potential problems of evaporation of the primer during storage or unwanted chemical reactions with primer left in the bulk of the EVA are eliminated when there is no primer in the film. Over the years, Solarex has evaluated EVA samples from a number of vendors, but Springborn is our only qualified vendor. Because of the cure system in the EVA formulation, mixing of the formulation must be accomplished and extrusion begun quickly or curing will start before the film is extruded. However, the extrusion itself must be performed slowly or the resultant film will shrink excessively during lamination. Springborn is the only vendor that has satisfied Solarex with uniform dispersion of the additives and minimal shrinkage of the films during lamination. Material from other vendors tends to yellow before use, possibly indicating non-uniform dispersion of the additives, and/or shrink excessively during lamination. 3. L a m i n a t i o n
process
Control of the lamination process, especially the assurance of adequate time at the cure temperature, is an important factor in achieving reliable
385 long lifetime EVA modules. Adequate time at the cure temperature is necessary to ensure module integrity. Inadequate cure will result in flow of the encapsulant at operating temperatures and inadequate bonding to the glass and backsheet. Delamination is a major problem for inadequately cured modules, easily observed after the humidity freeze portion of the JPL Block V test [4]. The entire Solarex lamination and cure cycle is performed in the laminator to remove the volatile products of the cure from the laminate rather than their being trapped in the cured matrix, possibly causing future degradation of the encapsulation system. The peroxide cure agents, either Lupersol 101 or Lupersol TBEC, decompose at elevated temperatures yielding free radicals. Some of the free radicals react with EVA to cause cross-linking of the polymer with an attendant increase in the encapsulant's softening temperature. Relatively low boiling point byproducts of the reaction can create gas bubbles in the encapsulant during the lamination process and they may remain dissolved in the encapsulant following lamination, unless the process is designed to remove these gaseous byproducts. Finally, to eliminate these undesirable results, Solarex has never used a metallic barrier layer in the backsheet. If the Lupersol 101 system is cured in the lower part of its cure temperature range, the free radicals are not very effÉcient in effecting a cure, and the ratio of volatile byproducts to cross-links is high. It is tempting to process the material in this temperature range since the release of volatile byproducts is slow, resulting in minimum bubbling problems, but trapping a maximum amount of byproducts in the laminate and giving a very slow cure. Solarex switched to the TBEC system in 1987 because Springborn data indicated a substantially more efficient cross-linking reaction. With TBEC, processing temperatures are lower and you do not have to worry about having a temperature range in which the cure agent breaks down but does not cross-link the film.
4. Accelerated t e s t i n g During the early 1980s Springborn conducted a series of accelerated life tests on EVA. The initial test was a high temperature soak with minimodules exposed to 90, 105 and 130 °C [5, 6]. After 7200 h there was no change in any mechanical or optical properties for the 90 °C samples. At 105 °C there was no change after 1000 h. The samples at 130 °C did experience major failure of both mechanical and optical properties after 7200 hours. Even then, the sample still had 74% optical transmission. This degradation was attributed to a simple thermal degradation (thermolysis) of EVA in which the EVA breaks down with a thermal activation energy of 30 to 40 kcal mo1-1, producing acetic acid. This process was only observed at the 130 °C level and, according to the Springborn reports [5, 6], should not be a problem for normal flat-plate operations of their EVA formulation.
386 The second group of accelerated tests utilized RS/4 lamps to perform UV stress tests. Under the RS/4 lamp at 50 °C, EVA formulations A9918 and 15295 exhibited no change in optical or mechanical properties after 35 000 h exposure. This was equivalent to 27 years of solar UV exposure. The third group of tests p e r f o r m e d at Springborn were the photothermal aging tests, where the modules were externally heated to either 70, 90 or 105 °C, while being e x p o s e d to natural sunlight. The tests were run for 12 000 h [7]. At 70 °C there was no change in optical or mechanical properties. At 90 °C there was no change in mechanical properties but visual inspection indicated slight yellowing of the EVA although not enough to decrease electrical p er f or m ance of the module. At 105 °C there was a m oderat e degree of yellowing of the EVA. The r e por t also indicated that samples cured with the Lupersol TBEC system exhibited less yellowing than samples cured with Lupersol 101 [7].
5. Field experience Solarex has not observed any optical or mechanical degradation of EVA used under normal flat plate operating conditions. Sample modules with up to 9 years of exposure in the U.S. Southwest have been returned to our factory for electrical tests and visual inspection. Electrically the modules produce exactly the same power as when m easured before shipment 9 years ago. Preliminary visual inspection indicates a browning of the module. However, this browning disappears when the t ext ured front surface .of the glass is decoupled from the optical path (covering it with index-of-refraction matched alcohol). The browning is clearly related to having the textured surface of the glass on the outside, a construction practice Solarex st opped by the time commercial modules were made with EVA. Silicone rubber modules of the same era have the same brown coloration from the textured glass. Visual inspection through the alcohol is inconclusive. The metallization pattern (silver) has certainly yellowed. However, it is very difficult to determine if the EVA itself has discolored. Further tests of these modules, that have been subjected to long term o u t d o o r exposure, is underway. The initial r e por t of EVA darkening occurred on the mirror-enhanced portion of the Arco array at Carrisa Plains [8]. The high temperature, approximately 90 °C, and increased light intensity, nearly 2 Suns, appear to be the cause. Solarex has also seen failures of EVA modules that have been utilized u n d e r concentrated sunlight and at high operating temperatures. This result does not mean that under normal flat-plate conditions, where the modules should never see more than 70 °C (and will average much less), there will be any degradation over a 20 or 30 year lifetime. From Springborn's accelerated testing, EVA degradation would be e x p e c t e d under the conditions at Carrisa Plains, but no major degradation would be e x p e c t e d under normal flat-plate operating conditions.
387
6. C o n c l u s i o n s The Springborn EVA system has proven to be a reliable encapsulation system for crystalline silicon photovoltaic modules. Fabrication of the films as well as lamination of the photovoltaic module requires careful engineering and process control. When fabricated correctly, these modules are extremely reliable with an expected lifetime of 20 to 30 years in fiat-plate applications.
References 1 J. E. Hoelscher, Proc. 15th IEEE Photovoltaic Specialists' Cove., IEEE, New York, 1981, p. 745. 2 P. B. Willis and B. Baum, Investigation of test methods, material properties, and processes for solar cell encapsulants, Ann. Rep. JPL Contract 954527, 1979 (Jet Propulsion Laboratory, Pasadena, CA). 3 Block IV solar cell module design and test specification for intermediate load center applications, JPL Document 5101415, 1978 (Jet Propulsion I_xtboratory, Pasadena, CA). 4 Block V solar cell module design and test specification for intermediate load applications, JPL Document 5101-151, 1981 (Jet Propulsion Laboratory, Pasadena, CA). 5 P. B. Willis, investigation of test methods, material properties, and processes for solar cell encapsulants, 7th Ann. Rep., JPL Contract 954527, 1983 (Jet Propulsion Laboratory, Pasadena, CA). 6 P. Willis, Proc. flat.plate solar array project research forum on quantifying degradation, JPL Document 5101-231, 1983 (Jet Propulsion Laboratory, Pasadena, CA), p. 445. 7 P. B. Willis, Investigation of test methods, material properties, and processes for solar cell encapsulants, 9th Ann. Rep., JPL Contract 954527, 1985 (Jet Propulsion Laboratory, Pasadena, CA). 8 D.D. Sumner, C. M. Whitaker and L. E. Schlueter, Proc. 20th IEEE Photovoltaic Specialists' Core., IEEE, New York, 1988, p. 1289.
383
Solarex experience with ethylene vinyl acetate encapsulation J o h n H. W o h l g e m u t h and Raymond C. P e t e r s e n Solarex Corporation, Frederick, MD 21701 (U.S.A.)
(Received October 25, 1990)
Abstract Solarex began using ethylene vinyl acetate (EVA) as an encapsulant for photovoltaic modules during the Jet Propulsion Laboratory sponsored Block IV Program in 1979. Experience was gained in the processing and use of EVA during a number of Department of Energy sponsored projects through the early 1980s. In 1982 Solarex began using EVA in commercial modules and has continued to use it up to the present time. EVA has proven to be a highly reliable encapsulant, with no reported instances of Solarex module field failures being attributed to failure of the EVA encapsulant. The EVA encapsulation system is complex, requiring well controlled manufacture of the film itself and the correct lamination procedure to assure adequate cure and bonding to the glass, cell and backsheet surfaces. The initial Springborn work on EVA included accelerated testing, which indicated that at temperatures considerably higher than experienced during normal module operation, the EVA system will suffer thermally induced degradation. However, no major degradation was experienced under normal operating conditions during either Springborn's testing or Solarex's 10 years of field experience.
1. I n t r o d u c t i o n Solarex b e g a n d e v e l o p i n g the t e c h n o l o g y t o use the S p r i n g b o r n ethylene vinyl a c e t a t e (EVA) f o r m u l a t i o n d u r i n g the Block IV p r o g r a m b e g i n n i n g in 1979 [1 ]. P r e v i o u s to this, all Solarex m o d u l e s h a d utilized silicone r u b b e r e n c a p s u l a n t s . EVA w a s d e v e l o p e d b y S p r i n g b o r n u n d e r a J e t P r o p u l s i o n L a b o r a t o r y (JPL) c o n t r a c t as a l o w e r c o s t alternative to silicone r u b b e r [2]. L a m i n a t e d EVA m o d u l e s were also built b y Solarex for a n u m b e r o f D e p a r t m e n t of E n e r g y s p o n s o r e d p r o j e c t s including b o t h the Solarex a n d M a s s a c h u s e t t s Institute o f T e c h n o l o g y h o u s e s at the N o r t h e a s t Residence, the Solarex h o u s e at the S o u t h w e s t Residence, the Carlisle h o u s e a n d finally B l o c k V. T h e s e p r o j e c t s p r o v i d e d a basis for m o d u l e qualification via Block IV [3] a n d the Block V [4] a c c e l e r a t e d e n v i r o n m e n t a l test s e q u e n c e s a n d p r e l i m i n a r y field t e s t i n g o f the p r o d u c t . B a s e d o n the positive results f r o m t h e s e g o v e r n m e n t projects, Solarex i n t r o d u c e d EVA into the c o m m e r c i a l m o d u l e line in 1982. Today, all o f S o l a r e x ' s terrestrial m o d u l e s are e n c a p s u l a t e d u s i n g EVA lamination. The c r o s s - s e c t i o n a l s t r u c t u r e o f a Solarex EVA m o d u l e is s h o w n in Fig. 1. The c o m p o n e n t s o f the c o n s t r u c t i o n are as follows: low iron t e m p e r e d
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
384
GLASS I
I[
]1
II
IJMATRIX/EVA "BACKSHEET
Fig. 1. Solarex EVA lamination package.
glass superstrate; clear EVA above the cell matrix; interconnected matrix of polycrystalline silicon solar cells; EVA with Craneglas ® behind the cell matrix; the Craneglas ® assists in removal of gas from the laminate and eliminates cell movement during lamination; a three part backsheet of blue modified polyethylene, Mylar and white Tedlar. 2. E t h y l e n e vinyl a c e t a t e f o r m u l a t i o n s The original EVA formulation used by Solarex was Springborn A9918. It is based on DuPont Elvax 150 EVA. The formulation also contained 0.2% Uniroyal Naugard P as an antioxidant, 0.1% Ciba-Geigy Tinuvin 770 for UV stabilization, 0.3% American Cyanamid Cyasorb UV-531 for UV stabilization, and 1.5% Lupersol 101 organic peroxide curing agent. The only change that Solarex has ever made in the formulation was a switch in 1987 to Springborn formulation 15295, where the Lupersol 101 is replaced by Lupersol TBEC, to reduce the temperature and time requirements for cure. Solarex has never utilized an EVA formulation with primer. We prefer to apply the primer directly to the glass. In this way, we are assured the presence of sufficient primer at the glass--EVA interface and do not have to worry about how uniformly the primer was dispersed during the extrusion. Finally, potential problems of evaporation of the primer during storage or unwanted chemical reactions with primer left in the bulk of the EVA are eliminated when there is no primer in the film. Over the years, Solarex has evaluated EVA samples from a number of vendors, but Springborn is our only qualified vendor. Because of the cure system in the EVA formulation, mixing of the formulation must be accomplished and extrusion begun quickly or curing will start before the film is extruded. However, the extrusion itself must be performed slowly or the resultant film will shrink excessively during lamination. Springborn is the only vendor that has satisfied Solarex with uniform dispersion of the additives and minimal shrinkage of the films during lamination. Material from other vendors tends to yellow before use, possibly indicating non-uniform dispersion of the additives, and/or shrink excessively during lamination. 3. L a m i n a t i o n
process
Control of the lamination process, especially the assurance of adequate time at the cure temperature, is an important factor in achieving reliable
385 long lifetime EVA modules. Adequate time at the cure temperature is necessary to ensure module integrity. Inadequate cure will result in flow of the encapsulant at operating temperatures and inadequate bonding to the glass and backsheet. Delamination is a major problem for inadequately cured modules, easily observed after the humidity freeze portion of the JPL Block V test [4]. The entire Solarex lamination and cure cycle is performed in the laminator to remove the volatile products of the cure from the laminate rather than their being trapped in the cured matrix, possibly causing future degradation of the encapsulation system. The peroxide cure agents, either Lupersol 101 or Lupersol TBEC, decompose at elevated temperatures yielding free radicals. Some of the free radicals react with EVA to cause cross-linking of the polymer with an attendant increase in the encapsulant's softening temperature. Relatively low boiling point byproducts of the reaction can create gas bubbles in the encapsulant during the lamination process and they may remain dissolved in the encapsulant following lamination, unless the process is designed to remove these gaseous byproducts. Finally, to eliminate these undesirable results, Solarex has never used a metallic barrier layer in the backsheet. If the Lupersol 101 system is cured in the lower part of its cure temperature range, the free radicals are not very effÉcient in effecting a cure, and the ratio of volatile byproducts to cross-links is high. It is tempting to process the material in this temperature range since the release of volatile byproducts is slow, resulting in minimum bubbling problems, but trapping a maximum amount of byproducts in the laminate and giving a very slow cure. Solarex switched to the TBEC system in 1987 because Springborn data indicated a substantially more efficient cross-linking reaction. With TBEC, processing temperatures are lower and you do not have to worry about having a temperature range in which the cure agent breaks down but does not cross-link the film.
4. Accelerated t e s t i n g During the early 1980s Springborn conducted a series of accelerated life tests on EVA. The initial test was a high temperature soak with minimodules exposed to 90, 105 and 130 °C [5, 6]. After 7200 h there was no change in any mechanical or optical properties for the 90 °C samples. At 105 °C there was no change after 1000 h. The samples at 130 °C did experience major failure of both mechanical and optical properties after 7200 hours. Even then, the sample still had 74% optical transmission. This degradation was attributed to a simple thermal degradation (thermolysis) of EVA in which the EVA breaks down with a thermal activation energy of 30 to 40 kcal mo1-1, producing acetic acid. This process was only observed at the 130 °C level and, according to the Springborn reports [5, 6], should not be a problem for normal flat-plate operations of their EVA formulation.
386 The second group of accelerated tests utilized RS/4 lamps to perform UV stress tests. Under the RS/4 lamp at 50 °C, EVA formulations A9918 and 15295 exhibited no change in optical or mechanical properties after 35 000 h exposure. This was equivalent to 27 years of solar UV exposure. The third group of tests p e r f o r m e d at Springborn were the photothermal aging tests, where the modules were externally heated to either 70, 90 or 105 °C, while being e x p o s e d to natural sunlight. The tests were run for 12 000 h [7]. At 70 °C there was no change in optical or mechanical properties. At 90 °C there was no change in mechanical properties but visual inspection indicated slight yellowing of the EVA although not enough to decrease electrical p er f or m ance of the module. At 105 °C there was a m oderat e degree of yellowing of the EVA. The r e por t also indicated that samples cured with the Lupersol TBEC system exhibited less yellowing than samples cured with Lupersol 101 [7].
5. Field experience Solarex has not observed any optical or mechanical degradation of EVA used under normal flat plate operating conditions. Sample modules with up to 9 years of exposure in the U.S. Southwest have been returned to our factory for electrical tests and visual inspection. Electrically the modules produce exactly the same power as when m easured before shipment 9 years ago. Preliminary visual inspection indicates a browning of the module. However, this browning disappears when the t ext ured front surface .of the glass is decoupled from the optical path (covering it with index-of-refraction matched alcohol). The browning is clearly related to having the textured surface of the glass on the outside, a construction practice Solarex st opped by the time commercial modules were made with EVA. Silicone rubber modules of the same era have the same brown coloration from the textured glass. Visual inspection through the alcohol is inconclusive. The metallization pattern (silver) has certainly yellowed. However, it is very difficult to determine if the EVA itself has discolored. Further tests of these modules, that have been subjected to long term o u t d o o r exposure, is underway. The initial r e por t of EVA darkening occurred on the mirror-enhanced portion of the Arco array at Carrisa Plains [8]. The high temperature, approximately 90 °C, and increased light intensity, nearly 2 Suns, appear to be the cause. Solarex has also seen failures of EVA modules that have been utilized u n d e r concentrated sunlight and at high operating temperatures. This result does not mean that under normal flat-plate conditions, where the modules should never see more than 70 °C (and will average much less), there will be any degradation over a 20 or 30 year lifetime. From Springborn's accelerated testing, EVA degradation would be e x p e c t e d under the conditions at Carrisa Plains, but no major degradation would be e x p e c t e d under normal flat-plate operating conditions.
387
6. C o n c l u s i o n s The Springborn EVA system has proven to be a reliable encapsulation system for crystalline silicon photovoltaic modules. Fabrication of the films as well as lamination of the photovoltaic module requires careful engineering and process control. When fabricated correctly, these modules are extremely reliable with an expected lifetime of 20 to 30 years in fiat-plate applications.
References 1 J. E. Hoelscher, Proc. 15th IEEE Photovoltaic Specialists' Cove., IEEE, New York, 1981, p. 745. 2 P. B. Willis and B. Baum, Investigation of test methods, material properties, and processes for solar cell encapsulants, Ann. Rep. JPL Contract 954527, 1979 (Jet Propulsion Laboratory, Pasadena, CA). 3 Block IV solar cell module design and test specification for intermediate load center applications, JPL Document 5101415, 1978 (Jet Propulsion I_xtboratory, Pasadena, CA). 4 Block V solar cell module design and test specification for intermediate load applications, JPL Document 5101-151, 1981 (Jet Propulsion Laboratory, Pasadena, CA). 5 P. B. Willis, investigation of test methods, material properties, and processes for solar cell encapsulants, 7th Ann. Rep., JPL Contract 954527, 1983 (Jet Propulsion Laboratory, Pasadena, CA). 6 P. Willis, Proc. flat.plate solar array project research forum on quantifying degradation, JPL Document 5101-231, 1983 (Jet Propulsion Laboratory, Pasadena, CA), p. 445. 7 P. B. Willis, Investigation of test methods, material properties, and processes for solar cell encapsulants, 9th Ann. Rep., JPL Contract 954527, 1985 (Jet Propulsion Laboratory, Pasadena, CA). 8 D.D. Sumner, C. M. Whitaker and L. E. Schlueter, Proc. 20th IEEE Photovoltaic Specialists' Core., IEEE, New York, 1988, p. 1289.