Polymeric Encapsulation Materials for Low-Cost, Terrestrial

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Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 18, 2015 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch022

Polymeric Encapsulation Materials for Low-Cost, Terrestrial, Photovoltaic Modules E. F. CUDDIHY and C. D. COULBERT—California Institute of Technology, Jet Propulsion Laboratory, Pasadena,CA91109 P. WILLIS and B. BAUM—Springborn Laboratories, Enfield,CT06082 A. GARCIA—Spectrolab, Inc., Sylmar,CA91342 C. MINNING—Hughes AircraftCo.,Culver City,CA90230

Solar cell modules must undergo substantial reductions in cost in order to become economically attractive as practical devices for the terrestrial production of electricity. Part of the cost reductions must be realized by the encapsulation materials which are used to package, protect, and support the solar cells, electrical interconnects, and other ancillary components. As many of the encapsulation materials are polymeric, cost reductions necessitate the use of low-cost polymers. This article describes the current status of low-cost polymers being developed or identified for encapsulation application, requirements for polymeric encapsulation materials, and evolving theories and test results of antisoiling technology. The Jet Propulsion Laboratory manages the "Flat-Plate Solar Array (FSA) Project" for the Department of Energy. The project objective is to conduct research on photovoltaic arrays establishing their technical feasibility so that industry could meet a target price for modules of less than T0t per Wpk (in 1980 dollars) and with a minimum service lifetime of 20 years. Assuming a module efficiency of 10 per cent, which is essentially 100 W per m at solar meridian, the capital cost of the modules can be alternately quoted as $70.00 per m . Out of this cost goal, $14.00 per m is allocated for the encapsulation materials which includes both the cost of a structural panel, and edge seals and gaskets. At project inception, approx. 1975, the accumulative cost of encapsulation materials in popular use, such as RTV silicones, aluminum panels, etc., greatly exceeded $14.00 per m . Accordingly, the FSA project established a group called the "Environmental Isolation Task", to identify and/or develop as necessary new materials, and new material technologies in order to achieve the cost and life goal. This article describes the status of this task group relative to the identification and development of an inventory of low-cost polymeric encapsulation materials (1^ - 5), and describes evolving engineering requirements of an encapsulation system which relates to minimum usage of polymeric materials (6). 0097-6156/83/0220-0353$06.00/0 © 1983 American Chemical Society 2

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In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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C o n s t r u c t i o n Elements To perform a survey f o r candidate m a t e r i a l s or m a t e r i a l c l a s s e s which could f i r s t meet the FSA p r o j e c t cost g o a l s , i t was h e l p f u l to examine the design and c o n s t r u c t i o n features of commerc i a l t e r r e s t r i a l p h o t o v o l t a i c modules, i n order to i d e n t i f y the b a s i c b u i l d i n g blocks of encapsulation systems. A basic b u i l d i n g block i s defined as a c o n s t r u c t i o n component f o r which a d i s t i n c t m a t e r i a l i s r e q u i r e d . C u r r e n t l y , commercial p h o t o v o l t a i c modules can be c l a s s i f i e d according to two engineering design o p t i o n s , a substrate system, and a s u p e r s t r a t e system. These design c l a s s i f i c a t i o n s r e f e r to the method by which the encapsulated s o l a r c e l l s are mechanically supported. A substrate design means that the encapsulated c e l l s are supported by a s t r u c t u r a l backside panel, and the s u p e r s t r a t e design means that the encapsulated c e l l s are supported by a transparent, sunside s t r u c t u r a l panel (e.g., g l a s s ) . For these two design options, up to nine m a t e r i a l components of an encapsulation system, c a l l e d c o n s t r u c t i o n e l e ments, can be i d e n t i f i e d . These c o n s t r u c t i o n elements are dep i c t e d i n Figure 1 along w i t h t h e i r designations and encapsulation f u n c t i o n s . Note that not a l l of these c o n s t r u c t i o n elements need be incorporated i n any given encapsulated module, but a l l c u r r e n t day modules have combinations of these elements. Low-cost candidates f o r the substrate panels are mild s t e e l and hardboard, and glass i s the lowest cost candidate f o r the s t r u c t u r a l s u p e r s t r a t e panel. On a s t r u c t u r a l comparison b a s i s , p l a s t i c m a t e r i a l s used s t r u c t u r a l l y as e i t h e r a substrate or as a transparent s u p e r s t r a t e are considerably higher cost ( 1 _ , 2 ) . The low-cost candidate f o r the porous spacer i s a non-woven Eg l a s s mat (_3,4^,5). Low cost candidates f o r a l l of the other cons t r u c t i o n elements are polymeric. Polymeric Encapsulation M a t e r i a l s P o t t a n t . The c e n t r a l core of an encapsulation system i s the p o t t a n t , a transparent, polymeric m a t e r i a l which i s the a c t u a l enc a p s u l a t i o n media i n a module. As there i s a s i g n i f i c a n t d i f f e r ence between the thermal-expansion c o e f f i c i e n t s of polymeric mat e r i a l s and the s i l i c o n c e l l s and m e t a l l i c i n t e r c o n n e c t s ; s t r e s s e s developed from the thousands of d a i l y thermal c y c l e s can r e s u l t i n f r a c t u r e d c e l l s , broken i n t e r c o n n e c t s , or cracks and separations i n the pottant m a t e r i a l . To avoid these problems, the pottant mat e r i a l must not o v e r s t r e s s the c e l l and i n t e r c o n n e c t s , and must i t s e l f be r e s i s t a n t to f r a c t u r e . From the r e s u l t s of a t h e o r e t i c a l a n a l y s i s ( 6 0 , experimental e f f o r t s ( 3 ) , and observations of the m a t e r i a l s of choice used f o r pottants i n commercial modules, the pottant must be a low-modulus, elastomeric m a t e r i a l . A l s o , these m a t e r i a l s must be transparent, p r o c e s s i b l e , comm e r i c a l l y a v a i l a b l e , and d e s i r a b l y of low c o s t . I n many cases, the commercially a v a i l a b l e m a t e r i a l i s not p h y s i c a l l y or chemicall y s u i t a b l e f o r immediate encapsulation use, and therefore must a l s o be amenable to low-cost m o d i f i c a t i o n . The pottant m a t e r i a l s



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must have e i t h e r inherent w e a t h e r a b i l i t y ( r e t e n t i o n of transparen cy and mechanical i n t e g r i t y under weather extremes) or the poten t i a l f o r long l i f e that can be provided by c o s t - e f f e c t i v e protec t i o n incorporated i n t o the m a t e r i a l or the module design. I n a f a b r i c a t e d module, the pottant provides three c r i t i c a l f u n c t i o n s f o r module l i f e and r e l i a b i l i t y : (1) Maximum o p t i c a l transmission i n the s i l i c o n s o l a r c e l l operating wavelength range of 0.4 to 1.1 μπι. (2) Retention of a required l e v e l of e l e c t r i c a l i n s u l a t i o n to p r o t e c t against e l e c t r i c a l breakdown, a r c i n g , e t c . , w i t h the a s s o c i a t e d dangers and haz ards of e l e c t r i c a l f i r e s , and human s a f e t y . (3) The mechanical p r o p e r t i e s to maintain s p a t i a l con tainment of the s o l a r c e l l s and i n t e r c o n n e c t s , and to r e s i s t mechanical creep. The l e v e l of mechani c a l p r o p e r t i e s a l s o must not exceed values that would impose undue mechanical s t r e s s e s on the s o l a r cell. When exposed to outdoor weathering, polymeric m a t e r i a l s can undergo degradation that could a f f e c t t h e i r o p t i c a l , mechanical, and e l e c t r i c a l i n s u l a t i o n p r o p e r t i e s . Outdoors, polymeric mate r i a l s can degrade from one or more of the f o l l o w i n g weathering ac tions: (1) UV photooxidation. (2) UV p h o t o l y s i s . (3) Thermal o x i d a t i o n . (4) H y d r o l y s i s . For expected temperature l e v e l s i n operating modules, ^ 60°C i n a rack-mounted array and p o s s i b l y up to 80°C on a r o o f t o p , three generic c l a s s e s of transparent polymers are g e n e r a l l y r e s i s tant to the above weathering a c t i o n s : s i l i c o n e s , fluorocarbons, and PMMA a c r y l i c s . Of these three, only s i l i c o n e s , which are ex pensive, have been a v a i l a b l e as low-modulus elastomers s u i t a b l e f o r pottant a p p l i c a t i o n . Therefore, a l l other transparent, low-modulus elastomers w i l l i n general be s e n s i t i v e to some degree of weathering degradation. However, l e s s weatherable and lower-cost m a t e r i a l s can be con s i d e r e d f o r pottant a p p l i c a t i o n i f the module design can provide the necessary degree of environmental p r o t e c t i o n . For example, a hermetic design, such as a glass s u p e r s t r a t e w i t h a m e t a l - f o i l back cover and appropriate edge s e a l i n g , w i l l e s s e n t i a l l y i s o l a t e the i n t e r i o r pottant from exposure to oxygen and water vapor, w i t h the glass i t s e l f p r o v i d i n g a l e v e l of UV s h i e l d i n g . The s i t u a t i o n i s d i f f e r e n t f o r a substrate module however, which w i l l employ a weatherable p l a s t i c - f i l m f r o n t cover. Because a l l p l a s t i c f i l m s are permeable to oxygen and water vapor (the only d i f f e r e n c e i s permeation r a t e ) , the pottant i s exposed to oxygen and water vapor, and a l s o to UV i f the p l a s t i c f i l m i s nonUV screening. Because i s o l a t i o n of the pottant from oxygen and water vapor i s not p r a c t i c a l l y p o s s i b l e i n t h i s design o p t i o n , i t becomes a requirement that the pottant be i n t r i n s i c a l l y r e s i s t a n t to h y d r o l y s i s and thermal o x i d a t i o n , but s e n s i t i v i t y to UV i s



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allowed i f the weatherable f r o n t - c o v e r p l a s t i c f i l m can provide UV shielding. Therefore surveys (1_>4_) were done to i d e n t i f y the lowestc o s t , transparent, low-modulus elastomers w i t h expected r e s i s tance to h y d r o l y s i s and thermal o x i d a t i o n at temperatures up to 80°C, but these m a t e r i a l s were allowed to be s e n s i t i v e to UV det e r i o r a t i o n . I t was envisioned that i f such a set of pottant cand i d a t e s were s e l e c t e d on the b a s i s of a l e s s - p r o t e c t i n g substrate-module design, they would a l s o be useable i n a p o t e n t i a l l y more-protecting g l a s s - s u p e r s t r a t e design. I n a d d i t i o n to the foregoing requirement f o r candidate pottant s e l e c t i o n , these mat e r i a l s must a l s o be capable of being f a b r i c a t e d i n t o modules by i n d u s t r i a l f a b r i c a t i o n methods. This requirment becomes important as i t i s d e s i r a b l e to have i n d u s t r i a l e v a l u a t i o n of the m a t e r i a l s being developed, and t h e r e f o r e the m a t e r i a l s must be r e a d i l y useable on commerical equipment. The two i n d u s t r i a l f a b r i c a t i o n techniques i n common use are l a m i n a t i o n and c a s t i n g . With a l l of these requirements, four pottant m a t e r i a l s have emerged as most v i a b l e and are c u r r e n t l y i n various stages of development or i n d u s t r i a l use. The four pottants are based on ethylene v i n y l acetate (EVA), ethylene methyl a c r y l a t e (EMA), p o l y n - b u t y l a c r y l a t e (P-n-BA), and a l i p h a t i c polyether urethane (PU). EVA and EMA are dry f i l m s designed f o r vacuum-bag l a m i n a t i o n at temperatures up to 150°C. Above 120°C during the l a m i n a t i o n process, EVA and EMA undergo peroxide c r o s s l i n k i n g to tough, rubbery thermosets. P-n-BA and PU are l i q u i d c a s t i n g systems. P-n-BA, a polymer/monomer syrup, i s being developed j o i n t l y by JPL and Springborn L a b o r a t o r i e s . P-n-BA i s being formulated to cure w i t h i n 15 minutes a t 60°C. Candidate polyurethane systems are being s u p p l i e d f o r FSA e v a l u a t i o n by various polyurethane manufacturers, from which one promising PU system has been i d e n t i f i e d . A b r i e f d e s c r i p t i o n of each of the four pottants f o l l o w s . a. Ethylene V i n y l Acetate (EVA). EVA i s a copolymer of ethylene and v i n y l acetate t y p i c a l l y s o l d i n p e l l e t form by Du Pont and U.S. I n d u s t r i a l Chemicals, Inc. (USI). The Du Pont name i s Elvax; the USI trade name i s Vynathane. The cost of EVA t y p i c a l l y ranges between $0.55 and $0.65 per l b . A l l commercially a v a i l a b l e grades of EVA were examined and the l i s t reduced to four candidates based on maximum transparency: Elvax 150, Elvax 250, E l v a x 4320, and Elvax 4355 ( 3 ) . Because EVA i s t h e r m o p l a s t i c , processing i n t o a module i s best accomplished by vacuum-bag lamina t i o n w i t h a f i l m of EVA. Therefore, based on f i l m e x t r u d a b i l i t y and transparency, the best choice became Elvax 150. Elvax 250 was an extremely c l o s e second choice. E l v a x 150 softens to a viscous melt above 70°C, and therefore i s not s u i t a b l e f o r temperature s e r v i c e above 70°C when employed i n a f a b r i c a t e d module. A cure system was developed f o r Elvax 150 that r e s u l t s i n a temperature-stable elastomer ( 3 ) . Elvax 150 was a l s o compounded w i t h an a n t i o x i d a n t and UV s t a b i l i z e r s , which improved i t s weather s t a b i l i t y and d i d not a f f e c t i t s transparency. The f o r m u l a t i o n of the encapsulation grade ethylene v i n y l acetate i s given i n Table I . These i n g r e d i e n t s are compounded i n t o Elvax


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CUDDIHY E T A L .

Table I .

Formulation of Ethylene V i n y l Acetate Encapsulation Film

Component


357


EVA (Elvax 150, Du Pont) L u p e r s o l 101 (peroxide) Naugard-P ( a n t i o x i d a n t ) T i n u v i n 770 (UV s t a b i l i z e r ) Cyasorb UV-531 (UV s t a b i l i z e r )

(EVA)

Composition (Parts-By-Weight)

100.0 1.5 0.2 0.1 0.3

150 p e l l e t s , followed by e x t r u s i o n at 85°C to form a continuous f i l m . The thickness of the c l e a r f i l m i s nominally 18 m i l s . The s e l e c t i v e c u r i n g system i s i n a c t i v e below 100°C, so that f i l m extruded at 85°C undergoes no curing r e a c t i o n . The extruded f i l m r e t a i n s the b a s i c t h e r m o p l a s t i c i t y of the Elvax 150. Therefore during vacuum-bag l a m i n a t i o n , the m a t e r i a l w i l l soften and process as a conventional laminating r e s i n . This EVA pottant has undergone extensive i n d u s t r i a l evaluat i o n , and manufacturers of p h o t o v o l t a i c (PV) modules have reported c e r t a i n advantages of EVA when compared to p o l y v i n y l b u t y r a l (PVB), a laminating f i l m m a t e r i a l i n common use w i t h i n the PV module i n d u s t r y . The reported advantages are: (1) Lower c o s t . (2) Better appearance. (3) B e t t e r c l a r i t y . (4) Non-yellowing. (5) E l i m i n a t i o n of c o l d storage. (6) Dimensional s t a b i l i t y . (7) No need to use a pressure autoclave. (8) Good flow p r o p e r t i e s and volumetric f i l l . Although t h i s encapsulation-grade EVA has been favorably r e ceived by the i n d u s t r y , i t s status i s s t i l l considered to be experimental. To advance EVA, s e v e r a l developmental tasks remain to be completed: (1) Faster processing, p r i m a r i l y i n the cure schedule, which i n v o l v e s a r e d u c t i o n i n cure time and temperat u r e ; the minimum cure temperature w i l l be d i c t a t e d by the requirement that the curing system must not become a c t i v e during f i l m e x t r u s i o n . (2) O p t i m i z a t i o n of the U V - s t a b i l i z a t i o n a d d i t i v e s ; the present a d d i t i v e s were s e l e c t e d on the b a s i s of l i t e r a t u r e c i t a t i o n s and i n d u s t r i a l experience w i t h polymers s i m i l a r to EVA. (3) I d e n t i f i c a t i o n of the maximum s e r v i c e temperature allowed f o r EVA i n a module a p p l i c a t i o n , to ensure long l i f e . (4) I n d u s t r i a l e v a l u a t i o n of the d e s i r a b i l i t y of having a s e l f - p r i m i n g EVA, r e c o g n i z i n g the p o s s i b i l i t y of


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Table I I . Formulation of Ethylene Methyl A c r y l a t e (EMA) Encapsulation Film


Component

EMA (TD 938, Gulf O i l Co.) L u p e r s o l 231 (perioxde) Naugard-P ( a n t i o x i d a n t ) T i n u v i n 770 (UV s t a b i l i z e r ) Cyasorb UV-531 (UV s t a b i l i z e r )

Composition (Parts-By-Weight)

100.0 3.0 0.2 0.1 0.3

an a d d i t i o n a l cost component ( c o s t - b e n e f i t - p e r f o r mance t r a d e - o f f ) . b. Ethylene Methyl A c r y l a t e (EMA). This r e c e n t l y i d e n t i f i e d m a t e r i a l (_3), a copolymer of ethylene and methyl a c r y l a t e , has pot e n t i a l as a s o l a r - c e l l l a m i n a t i o n p o t t a n t . There are three supp l i e r s of EMA r e s i n s ; two are domestic, Du Pont and Gulf O i l Chem i c a l s . The Du Pont EMA r e s i n , designated "VAMAC N-123", cannot be used because of i t s lack of transparency. The t h i r d s u p p l i e r i s foreign. Gulf markets three h i g h l y transparent EMA r e s i n s that are designated 2205, 2255, and TD-938. Grade 2255 i s the same base r e s i n as 2205, except that i t contains l u b r i c a n t and a n t i b l o c k i n g a d d i t i v e s . Gulf l i t e r a t u r e f o r these r e s i n s i n d i c a t e the f o l l o w ing features: (1) Low-extrusion temperatures. (2) Good heat s e a l a b i l i t y . (3) Thermal s t a b i l i t y to 315°C (600°F) f o r short periods of time (manufacturer's c l a i m ) . (4) S t r e s s - c r a c k r e s i s t a n c e . (5) Low melt v i s c o s i t i e s . (6) Good adhesion to a v a r i e t y of s u b s t r a t e s . The three Gulf EMA r e s i n s were e x p e r i m e n t a l l y evaluated and TD-938 was selected on the b a s i s of f i l m transparency, e x t r u d a b i l i t y , and ease of module f a b r i c a t i o n by l a m i n a t i o n . The TD938-base r e s i n s e l l s f o r about $0.60/lb ( A p r i l 1981). A t r i a l f o r m u l a t i o n i s shown i n Table I I . Modules have been f a b r i c a t e d w i t h t h i s EMA by the vacuum-bag l a m i n a t i o n process, and have succ e s s f u l l y passed module engineering q u a l i f i c a t i o n t e s t s . Primer formulations f o r bonding EVA and EMA to glass and p o l y e s t e r f i l m have been developed by Dow Corning and the formulations are given i n Table I I I . c. P o l y - n - B u t y l - A c r y l a t e (PnBA). No commercially a v a i l a b l e , a l l - a c r y l i c l i q u i d - c a s t i n g and c u r a b l e - e l a s t o m e r i c system could be found. A c c o r d i n g l y , the Environmental I s o l a t i o n Task undertook a developmental e f f o r t . A requirement of encapsulation-grade pottant s i s r e t e n t i o n of e l a s t o m e r i c p r o p e r t i e s over the temperature range from -40°C to +90°C. This requirement i s met by PnBA, which has a g l a s s - t r a n s i t i o n temperature of -54°C ( 7 ) .


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CUDDIHY E T AL.


Table I I I .

1)


Primer Formulations

Primer f o r Bonding EVA and EMA to Glass

Component

Z-6030 S i l a n e (Dow Corning) Benzyl Dimethyl Amine L u p e r s o l 101 Methanol

2)

359

Composition

9.0 wt. 1.0 wt. 0.1 wt. 89.9 wt.

% % % %

Primer f o r Bonding EVA and EMA to P o l y e s t e r Films

Component

Z-6030 S i l a n e (Dow Corning) Cymel 303 (Am. Cyanamid) Methanol

Composition

2.5 wt. % 22.5 wt. % 75.0 wt. %

PnBA i s not commercially a v a i l a b l e i n a form s u i t a b l e f o r use as an encapsulation p o t t a n t , but the η-butyl a c r y l a t e monomer i s r e a d i l y a v a i l a b l e at a bulk cost of about $0.45/lb. As a r e s u l t of the developmental program, a 100%-pure PnBA l i q u i d was d e v e l oped that could be cast as a c o n v e n t i o n a l l i q u i d - c a s t i n g r e s i n , and that subsequently cures to a tough, temperature-stable e l a s t o mer. Modules f a b r i c a t e d w i t h the PnBA elastomer have s u c c e s s f u l l y passed module engineering t e s t s . In g e n e r a l , the process f o r producing the prototype l i q u i d PnBA c o n s i s t s of f i r s t p o l y m e r i z i n g a batch of η-butyl a c r y l a t e to achieve a high-molecular-weight elastomer, then d i s s o l v i n g the elastomer i n an η-butyl a c r y l a t e monomer to o b t a i n a s o l u t i o n of acceptable v i s c o s i t y . F o l l o w i n g t h i s , a c r o s s l i n k e r , c u r i n g agent, UV s t a b i l i z e r s , and an a n t i - o x i d a n t are then added. The c u r r e n t f o r m u l a t i o n i s given i n Table IV. This f o r m u l a t i o n w i l l cure i n 20 minutes at 60°C. The p r o j e c t e d high-volume cost f o r t h i s m a t e r i a l i s estimated at about $0.85 to $0.90/lb. compared w i t h the commercial s e l l i n g p r i c e of $9 to $11/lb f o r RTV s i l i cones which are used i n commercial modules as a c a s t i n g p o t t a n t . d. A l i p h a t i c P o l y e t h e r Urethane. Almost a l l commercially a v a i l a b l e polyurethanes are of the aromatic, p o l y e s t e r type, which are not favorable because of t h e i r tendency toward h y d r o l y s i s of the ester groups, and UV degradation due to UV a b s o r p t i o n by the aromatic s t r u c t u r e . Only a few a l i p h a t i c , polyether urethanes have been i d e n t i f i e d , and one of the more promising f o r photovol t a i c module a p p l i c a t i o n i s a urethane designated Z-2591, marketed by Development A s s o c i a t e s , North Kingston, Rhode I s l a n d . This ma-


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Table IV.

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UTILIZATION

Formulation of P o l y - n - B u t y l A c r y l a t e (P-n-BA) Casting L i q u i d

Component


IN

η-Butyl A c r y l a t e (monomer) P o l y - n - B u t y l A c r y l a t e (polymer) 1,6 Hexanediol D i a c r y l a t e (crosslinker) Alperox-F ( c u r i n g agent) T i n u v i n Ρ (UV s t a b i l i z e r ) T i n u v i n 770 (UV s t a b i l i z e r and antioxidant)

Composition

(Parts-By-Weight)

60.00 35.00 5.00 0.50 0.25 0.05

t e r i a l i s c u r r e n t l y undergoing extensive e v a l u a t i o n f o r t h i s ap plication. UV Screening P l a s t i c F i l m s . The module f r o n t cover i s i n d i r e c t contact w i t h a l l of the weathering elements: UV, humidity, dew, r a i n , oxygen, e t c . ; t h e r e f o r e , the s e l e c t e d m a t e r i a l s must be weatherable. Only four c l a s s e s of transparent m a t e r i a l s are known to be weatherable, g l a s s , f l u o r o c a r b o n s , s i l i c o n e s and polymethyl methacrylate. In a d d i t i o n to w e a t h e r a b i l i t y , the f r o n t cover must a l s o f u n c t i o n as a UV screen, to p r o t e c t underlying pottants that are s e n s i t i v e to degradation by UV photooxidation or UV p h o t o l y s i s . The outer surface of the f r o n t cover should a l s o be e a s i l y c l e a n able and r e s i s t a n t to atmospheric s o i l i n g , a b r a s i o n - r e s i s t a n t , and a n t i r e f l e c t i v e to increase module l i g h t t r a n s m i s s i o n . I f some or a l l of these outer-surface c h a r a c t e r i s t i c s are absent i n the f r o n t - c o v e r m a t e r i a l , a d d i t i o n a l , s u r f a c i n g m a t e r i a l s may have to be a p p l i e d . E x c l u d i n g g l a s s , the only commercially a v a i l a b l e , t r a n s parent, UV screening p l a s t i c f i l m s which have been i d e n t i f i e d are f l u o r o c a r b o n f i l m s , Tedlar (Du Pont), and PMMA f i l m s , A c r y l a r (3M Co.). a. T e d l a r . Du Pont markets three 1 - m i l - t h i c k , c l e a r , Tedlar fluorocarbon UV-screening f i l m s . The d e s i g n a t i o n of these three f i l m s are: (1) Tedlar 100 AG 30 UT (2) Tedlar 100 BG 15 UT (3) Tedlar 100 BG 30 UT An i n i t i a l d i f f i c u l t y w i t h Tedlar had been poor adhesion to EVA and EMA. This has been corrected by the use of an a l l - a c r y l i c contact adhesive that can be coated d i r e c t l y onto one surface of Tedlar f i l m s . The coated adhesive, a Du Pont product designated 68040 i s dry and non-tacky a t ambient c o n d i t i o n s ; thus coated Tedlar can be r e a d i l y unwound from supply r o l l s . Experimental t e s t i n g i n d i c a t e s that when the adhesive i s heated during the EVA


22.

CUDDIHY E T AL.


361

Table V. Back Covers (White-Pigmented P l a s t i c F i l m s )


1. 2. 3. 4. 5.

Tedlar 150 BL 30 WH, 1.5 m i l s (Du Pont) Tedlar 400 BS 20 WH, 4.0 m i l s (Du Pont) Scotchpar 10 CP White, 1.0 m i l s (3M Co.) Scotchpar 20 CP White, 2.0 m i l s (3M Co.) Korad 63000 White, 3.0 m i l s ( X c e l Corp.)

and EMA l a m i n a t i o n c y c l e , strong adhesive bonding develops between the pottants and the Tedlar f i l m s . The thickness of the adhesive c o a t i n g ranged between 0.3 and 0.4 m i l . b. A c r y l a r . 3M markets UV screening, b i a x i a l l y o r i e n t e d PMMA f i l m s under the tradename " A c r y l a r " . The f i l m s are a v a i l a b l e i n two t h i c k n e s s e s , a 2-mil v e r s i o n designated X-22416, and a 3m i l v e r s i o n designated X-22417. An i n i t i a l concern w i t h these f i l m s i s t h e i r tendency f o r thermal shrinkage when heated above 105°C, the g l a s s t r a n s i t i o n temperature of PMMA. Although true f o r a free standing f i l m , t h i s has not been a problem when u n i formly pressed at 150°C i n a module assembly by one atmosphere of l a m i n a t i o n pressure. Adhesion strength between these f i l m s and EVA and EMA a f t e r module f a b r i c a t i o n i s f a i r , but not e x c e l l e n t . Chemical coupling primer systems f o r these f i l m s are being developed. Back Covers. Back covers are back surface m a t e r i a l l a y e r s which should be weatherable, hard, and mechanically durable and tough. Engineering a n a l y s i s i n d i c a t e s that the c o l o r of the back surface m a t e r i a l l a y e r should be white, to a i d module c o o l i n g . Back covers f u n c t i o n to provide necessary back side p r o t e c t i o n f o r s u b s t r a t e s , such as f o r example c o r r o s i o n p r o t e c t i o n f o r low-cost m i l d s t e e l panels, or humidity b a r r i e r s f o r moisture s e n s i t i v e panels. For s u p e r s t r a t e designs, the back covers provides a tough o v e r l a y on the back surface of the s o f t , elastomeric pottant. I f the back cover f o r a s u p e r s t r a t e design i s s e l e c t e d to be a metal f o i l , an a d d i t i o n a l i n s u l a t i n g d i e l e c t r i c f i l m should be i n s e r t e d i n the module assembly between the c e l l s and the metal f o i l , as shown i n Figure 1. Candidate back cover f i l m s are l i s t e d i n Table V. Edge Seals and Gaskets. Trends based on t e c h n i c a l and econom i c a l a n a l y s i s (3) suggest that b u t y l s should be considered f o r edge s e a l s , and EPDM elastomers should be considered f o r gaskets. S e v e r a l m a t e r i a l s f o r each a p p l i c a t i o n are under i n v e s t i g a t i o n . At t h i s time, one of the more promising edge s e a l m a t e r i a l s i s a b u t y l edge s e a l i n g tape designated "5354" (3M Co.), and one of the more promising EPDM gasket m a t e r i a l i s designated "E-633" ( P a u l i n g Rubber Co.).



362

POLYMERS

MODULE SUNSIDE

LAYER DESIGNATION SURFACE 1) MATERIAL 2) MODIFICATION

I

I

IN S O L A R

ENERGY

UTILIZATION

FUNCTION • t • •

LOW SOILING EASY CLEANABILITY ABRASION RESISTANT ΑΝΤΙ REFLECTIVE

FRONT COVER

• UV SCREENING • STRUCTURAL SUPER STRATE

POTTANT

• SOLAR CELL ENCAPSULATION

POROUS SPACER

• AIR RELEASE • MECHANICAL SEPARATION

DIELECTRIC

• ELECTRICAL ISOLATION

SUBSTRATE

t STRUCTURAL SUPPORT

BACK COVER

• MECHANICAL PROTECTION t WEATHERING BARRIER • INFRA-RED EMITTER

PLUS NECESSARY PR IMERS/ADHESIVES

Figure 1.

C o n s t r u c t i o n elements of p h o t o v o l t a i c encapsulation systems.


22.

CUDDIHY ET AL.


Encapsulation


363

Engineering

An engineering a n a l y s i s of encapsulation systems (6) i s being c a r r i e d out to achieve a r e l i a b l e and p r a c t i c a l engineering design. This a n a l y s i s i n v o l v e s four necessary features of a module: 1) S t r u c t u r a l adequacy. 2) E l e c t r i c a l isolation (safety). 3) Maximum o p t i c a l transmission, and 4) Minimum module temperature. One of the goals of t h i s a n a l y s i s i s a generation of g u i d e l i n e s f o r minimum m a t e r i a l usage f o r each of the c o n s t r u c t i o n elements. The analyses f o r s t r u c t u r a l adequacy i d e n t i f i e d that the thermal expansion or wind d e f l e c t i o n of p h o t o v o l t a i c modules can r e s u l t i n the development of mechanical s t r e s s e s i n the encapsulated s o l a r c e l l s s u f f i c i e n t to cause c e l l breakage. The thermal s t r e s s e s are developed from d i f f e r e n c e s i n the thermal expansion p r o p e r t i e s of the load c a r r y i n g panel, and the s o l a r cells. However, the a n a l y s i s i n t e r e s t i n g l y i d e n t i f e d that the s o l a r c e l l s t r e s s e s from e i t h e r thermal expansion d i f f e r e n c e s or wind d e f l e c t i o n can be reduced by i n c r e a s i n g the thickness t of the pottant, or by using pottants with lower Young's Modulus E. I n other words, the a n a l y s i s i n d i c a t e s that the load c a r r y i n g panel can be considered to be the generator of s t r e s s , and that the pottant acts to dampen the transmission of the s t r e s s to the cells. The pottants a b i l i t y to dampen transmitted s t r e s s i s d i r e c t l y r e l a t e d to the r a t i o of i t s thickness to modulus, t/E. For example, the a n a l y s i s f i n d s f o r a four foot square glass s u p e r s t r a t e module undergoing a 50 mph wind d e f l e c t i o n , that the pottant t/E r a t i o should be equal to or greater than 4, where t i s i n m i l s , and Ε i s i n u n i t s of KSI. At a r a t i o of 4, the s o l a r c e l l stresses are j u s t at t h e i r allowable l i m i t . I f the pottant were EVA having a Young's module Ε of 0.9 KSI, then the minimum thickness of EVA would be between 4 to 5 m i l s . The use of a pot tant having a higher Young's modulus would n e c e s s i t a t e that the thickness of that pottant be correspondingly increased. I t should be mentioned that the t/E requirement of a glass superstrate mod u l e undergoing thermal expansion i s only 2. Thus s o l a r c e l l s t r e s s e s generated by the wind d e f l e c t i o n of a glass superstrate module, rather than thermal expansion e f f e c t s , d i c t a t e the minimum usage requirements of p o t t a n t s . This kind of output from the engineering a n a l y s i s begins to enable a cost-comparison basis f o r candidate m a t e r i a l s . For ex ample, compared to EVA, a higher c o s t i n g pottant having a higher Young's modulus would be much more c o s t l y to use both f o r reasons of higher m a t e r i a l s c o s t , and the need f o r more t h i c k n e s s . On the other hand, a higher c o s t i n g pottant having a lower Young's modulus may be j u s t as c o s t - e f f e c t i v e due to an allowed thinner usage.


364

POLYMERS


Low-Soiling Surface

IN

SOLAR E N E R G Y

UTILIZATION

Coatings

E v o l v i n g s o i l i n g t h e o r i e s (8) and p h y s i c a l examinations of s o i l e d surfaces (_5) suggests that s o i l i n g accumulates i n three l a y e r s . The f i r s t l a y e r i n v o l v e s strong chemical attachment, or strong chemisorption of s o i l matter on the primary surface. The second l a y e r i s p h y s i c a l c o n s i s t i n g of a h i g h l y organized arrange ment of s o i l matter e f f e c t i n g a gradation i n surface energy, from a high associated with the energetic f i r s t l a y e r , to the lowest p o s s i b l e state on the outer surface of the second l a y e r . The low est p o s s i b l e surface energy s t a t e i s d i c t a t e d by the chemical and p h y s i c a l nature of the r e g i o n a l atmospheric s o i l i n g m a t e r i a l s . These f i r s t two l a y e r s are r e s i s t a n t to removal by r a i n and wind. A f t e r the f i r s t two l a y e r s are formed, the t h i r d l a y e r t h e r e a f t e r c o n s t i t u t e s a s e t t l i n g of loose s o i l matter, accumulating i n dry periods and being removed during r a i n y periods. The aerodynamic l i f t i n g a c t i o n of wind can remove p a r t i c l e s greater than about 50 μ from t h i s l a y e r , but i s i n e f f e c t i v e f o r smaller p a r t i c l e s . Thus, the p a r t i c l e s i z e of s o i l matter i n the t h i r d l a y e r i s gen e r a l l y found to be l e s s than 50 μ. Theories and evidence to date suggests that surfaces which should be n a t u r a l l y r e s i s t a n t to the formation of the f i r s t two r a i n - r e s i s t a n t l a y e r s should be hard, smooth, hydrophobic, free of f i r s t period elements ( f o r example, sodium), and have the lowest p o s s i b l e surface energy. These e v o l v i n g requirements f o r low s o i l i n g surfaces suggest that surfaces, or surface coatings should be of fluorocarbon chemistry. Two fluorocarbon coating m a t e r i a l s , a fluoronated s i l a n e ( L 1668, 3M Co.), and perfluorodecanoic a c i d are under t e s t . The perfluorodecanoic a c i d i s chemically attached to the surfaces with a Dow Corning chemical primer, E-3820. The coatings on g l a s s , and on the 3M " A c r y l a r " f i l m , are being exposed outdoors i n E n f i e l d , Conn., and the l o s s of o p t i c a l transmission by n a t u r a l s o i l accu mulation i s being monitored by the performance of standard s o l a r c e l l s p o s i t i o n e d behind the glass and f i l m test specimens. These t e s t specimens are not washed. F i v e months of t e s t r e s u l t s to date are shown i n Figure 2 f o r glass and A c r y l a r . A f t e r 5-months outdoors, s o i l accumulation on the uncoated g l a s s c o n t r o l has r e s u l t e d i n about a 3% l o s s of c e l l performance, whereas the glass coated w i t h L-1668 has r e a l i z e d only about a 0.5% l o s s . The glass sample coated w i t h perfluorodecanoic a c i d has r e a l i z e d about a 1.5% l o s s . The uncoated A c r y l a r c o n t r o l has r e a l i z e d about a 5% l o s s , whereas the l o s s on the sample coated w i t h perfluorodecanoic a c i d i s only about 2.5%, and the l o s s on the A c r y l a r sample coated w i t h L-1668 i s about 3.5%. The t e s t r e s u l t s to date i n d i c a t e that compared to untreated c o n t r o l s , s o i l accumulation i s being reduced on those t e s t s samples treated with the candidate fluorocarbon surface c o a t i n g s .



22.

CUDDIHY ET AL.


MONTHS

365

MONTHS L-1668. FLUOR INA TED SILANE PERFLUORODECANOIC ACID CONTROL

Figure 2.

Experimental materials·

e v a l u a t i o n of l o w - s o i l i n g fluorocarbon coating


366

POLYMERS IN SOLAR ENERGY UTILIZATION

Acknowledgment s The research described i n t h i s paper was c a r r i e d out by the J e t P r o p u l s i o n Laboratory, C a l i f o r n i a I n s t i t u t e of Technology, and was sponsored by the U.S. Department of Energy through an agree ment with the N a t i o n a l Aeronautics and Space A d m i n i s t r a t i o n .


Literature Cited 1. 2. 3. 4. 5.

6. 7. 8.

Cuddihy, E . F . , "Encapsulation Material Trends Relative to 1986 Cost Goals", LSA Project Report 5101-61, JPL, Pasadena, California, April 13, 1978. Cuddihy, E . F . , Baum, Β., and Willis, P., "Low-Cost Encapsulation Materials for Terrestrial Solar Cell Modules", Solar Energy, Vol. 22, p. 389 (1979). Springborn Laboratories, Third, Fourth, and Fifth Annual Reports for JPL's LSA Project, Contract No. 954527, June 1979, 1980 and 1981, respectively. Cuddihy, E . F . , "Encapsulation Materials Status to December 1979", LSA Project Report 5101-144, JPL, Pasadena, California, January 15, 1980. Photovoltaic Module Encapsulation Design and Materials Selection, prepared and edited by the FSA Environmental Isolation Task, FSA Project Report 5101-177, JPL, Pasadena, California, August 15, 1981. Spectrolab, Inc., Phase I Technical Report for FSA Contract 955567, November 1981. Brendlay, W.H., J r . , "Fundamentals of Acrylic Polymers", Paint and Varnish Production, Vol. 63, No. 7, pp. 19-27, July 1973. Cuddihy, E . F . , "Theoretical Considerations of Soil Retention", Solar Energy Materials, Vol. 3, pp. 21-33, 1980.

RECEIVED

November 22, 1982


Polymeric Encapsulation Materials for Low-Cost, Terrestrial

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