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Flat-Plate Collector Glazings* The cover g l a z i n g s protect the inner elements of the c o l l e c t o r from the environment and i n c r e a s e operating e f f i c i e n c y by reducing r e r a d i a t i o n and convection. Collectors with single glazings are l i m i t e d i n t h e i r o p e r a t i n g temperature; however, some recent work suggests that s i n g l y glazed c o l l e c t o r s can work i n the temperature ranges r e q u i r e d by desiccant and a b s o r p t i o n c o o l i n g [ 2 ] . Higher operati n g temperatures a r e obtained by u s i n g two g l a z i n g s . The outer g l a z i n g must withstand the environment while the inner g l a z i n g must be temperature r e s i s t a n t , t y p i c a l l y up to the s t a g n a t i o n temperature of the device. R e s u l t s on c o l l e c t o r g l a z i n g s have been reported [3] and environmental degradation studies of m a t e r i a l s f o r g l a z i n g s a r e i n progress [ 4 ] . None of the m a t e r i a l s are completely s a t i s f a c t o r y e i t h e r as an outer or inner g l a z i n g . The temperature requirement f o r the inner g l a z i n g e l i m i n a t e s most m a t e r i a l s other than fluorocarbon polymers and g l a s s . Glass i s the most common outer g l a z i n g but i t s u f f e r s from weight, c o s t , and impact r e s i s t a n c e l i m i t a t i o n s , w h i l e l a c k of environmental d u r a b i l i t y l i m i t s the a p p l i c a t i o n s of polymers. The transparent honeycomb concept i s an a l t e r n a t i v e t o t h e use of a second g l a z i n g [5] . The honeycomb i s attached t o o r i s an i n t e g r a l p a r t of the outer g l a z i n g f a c i n g the absorber p l a t e . The honeycomb improves c o l l e c t o r performance by suppressing conv e c t i o n and r a d i a t i o n heat l o s s e s w h i l e only s l i g h t l y reducing the incoming s o l a r energy. The e f f e c t i v e n e s s of the honeycomb f o r improving c o l l e c t o r performance i s approximately equivalent t o t h a t of an inner g l a z i n g . I n t e g r a l honeycombs formed from p o l y carbonate have good mechanical p r o p e r t i e s . Other m a t e r i a l s t e s t e d i n c l u d e p o l y e s t e r , fluorocarbon polymers, and polyimide [ 5 ] • Novel approaches t o c o l l e c t o r f a b r i c a t i o n use i n t e g r a l e x t r u s i o n s [6-8] or laminated t h i n f i l m s [ 2 ] . U n l i k e sheet-and-tube designs, some extruded i n t e g r a l u n i t s i n c l u d e the transparent g l a z i n g , h e a t - t r a n s f e r f l u i d pathways, and backing, a l l i n a conf i g u r a t i o n which could be r o l l e d out onto a r o o f t o p . Black f l u i d s can act as absorbers and be drained from the c o l l e c t o r t o prevent excessive s t a g n a t i o n temperatures. The designs vary i n d e t a i l but a common problem has been the i d e n t i f i c a t i o n of a polymer with acceptable environmental d u r a b i l i t y and low c o s t . An extruded polycarbonate c o l l e c t o r w i t h an i n t e g r a l o p t i c a l concentrator has been developed [ 8 ] . Other m a t e r i a l s that have been used i n designs i n c l u d e a c r y l i c and p o l y e t h e r s u l f o n e . Imaginative a p p l i c a t i o n s o f polymers t o f e n e s t r a t i o n can a l s o be used f o r f l a t - p l a t e c o l l e c t o r s . A transparent, coated p o l y meric g l a z i n g which transmits the s o l a r spectrum but r e t u r n s the i n f r a r e d r a d i a t i o n e f f e c t i v e l y increases the i n s u l a t i o n provided by the g l a z i n g , because the i n f r a r e d r a d i a t i o n generated i n s i d e the s t r u c t u r e i s r e t a i n e d . A polymeric f i l m which changes from transparent t o opaque when heated above a t r a n s i t i o n temperature a c t s as an automatic window shade which could help c o n t r o l s t a g n a t i o n temperatures [ 9 ] .
In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by PENNSYLVANIA STATE UNIV on May 5, 2012 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch001
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Polymeric Glazings - Mirrors* The i n s t a l l e d p r i c e of h e l i o s t a t s i s estimated t o account f o r about h a l f of the t o t a l c a p i t a l cost o f a c e n t r a l - r e c e i v e r s o l a r thermal e l e c t r i c plant and a l a r g e r f r a c t i o n of the cost of systems f o r process heat product i o n [10]. M e t a l l i z e d t h i n polymeric f i l m s a r e one means to make l i g h t w e i g h t m i r r o r s that are l e s s expensive than current design. F l e x i b l e , l i g h t w e i g h t mirrors a l s o a l l o w l e s s expensive designs o f a u x i l i a r y equipment. Thin, f l e x i b l e f i l m s can be attached w i t h adhesives t o substrates w i t h s i n g l e o r compound curvature. E a r l i e r s t u d i e s of alurainized o r s i l v e r e d polymers have included a c r y l i c s , f l u o r i n a t e d polymers, polycarbonate, s i l i c o n e s , and p o l y e s t e r [11]. Tests a t Phoenix, A r i z o n a , showed n e g l i g i b l e degradation o f aluminum and s i l v e r m i r r o r s protected by a c r y l i c , T e f l o n , and glass during exposures exceeding two years, w h i l e s i m i l a r t e s t s a t other s i t e s r e s u l t e d i n severe degradation i n about one year [12]. I t was decided that the r e l i a b i l i t y of polymer-coated mirrors was i n s u f f i c i e n t f o r t h e i r use as h e l i o s t a t s a t the Barstow demonstration f a c i l i t y [13]. More systematic environmental degradation s t u d i e s o f some o f these m a t e r i a l s a r e i n progress [4] and s e v e r a l m i r r o r c o n f i g u r a t i o n s , i n c l u d i n g aluminized a c r y l i c s , are being t e s t e d c u r r e n t l y a t s e l e c t e d l o c a t i o n s around the U. S. I n recent t e s t s conducted i n dry, r e l a t i v e l y benign c l i m a t e s , aluminized a c r y l i c s have performed w e l l f o r up t o f i v e years, polymeric g l a z i n g s that p r o t e c t s i l v e r surfaces f o r comparable time periods have not been i d e n t i f i e d . L o c a l i r r e g u l a r i t i e s ( s l o p e - e r r o r ) i n the shape of r e f l e c t o r s present a problem w i t h polymer-glazed m i r r o r s . A slope-error t o l e r a n c e as low as one m i l l i r a d i a n i s needed f o r some point-focus concentrators [14]. This tolerance has been met w i t h g l a s s m i r r o r s ; however, m e t a l l i z e d polymeric f i l m s have a poorer t o l e r ance. Dome Enclosures. An e n c l o s e d - h e l i o s t a t (dome) design e n v i sions l a r g e ( 3 0 - f t diameter) bubbles made of t h i n , a i r - s u p p o r t e d , transparent polymeric f i l m s as p r o t e c t i v e covers f o r m e t a l l i z e d polymeric m i r r o r s . Studies [10] i n d i c a t e that use of domeenclosed s o l a r concentrators may r e s u l t i n s i g n i f i c a n t cost reductions. The air-supported dome c o n f i g u r a t i o n i s capable of w i t h standing wind loads and can p r o t e c t l i g h t gauge p l a s t i c membrane h e l i o s t a t s and d r i v e mechanisms which lower c o s t s . The o r i g i n a l concept from the Boeing Engineering and Construction Company used i n t e g r a l domes w h i l e l a t e r designs by the General E l e c t r i c Company used segments assembled w i t h adhesives. A number of transparent polymers have been examined and t e s t e d f o r t h i s purpose; prototype domes have been f a b r i c a t e d from p o l y v i n y l f l u o r i d e which was l a t e r determined t o be too expensive and not s u f f i c i e n t l y s t a b l e . The dominant requirement f o r t h i s a p p l i c a t i o n i s good specular t r a n s m i s s i o n . Several p o l y e s t e r s
In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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were t e s t e d and had e x c e l l e n t i n i t i a l o p t i c a l transmittance. However, t h e i r environmental d u r a b i l i t y was too l i m i t e d . Energy l o s s e s due to absorption or s c a t t e r i n g decrease the system e f f i c i e n c y , reducing the cost advantages. In the case of enclosures f o r h e l i o s t a t s i n c e n t r a l r e c e i v e r systems, l o s s e s are m u l t i p l i e d because the s o l a r beam must pass through the dome t w i c e . The m a t e r i a l must be compatible w i t h c o s t - e f f e c t i v e dome f a b r i c a t i o n methods and must have s u i t a b l e mechanical p r o p e r t i e s and durab i l i t y to maintain operation under the combined e f f e c t s of wind, h a i l , temperature, s u n l i g h t , e t c . , f o r s e v e r a l years. Other m a t e r i a l s t e s t e d i n c l u d e p o l y v i n y l i d e n e f l u o r i d e , polycarbonate, and polypropylene. B i a x i a l l y oriented polyvinylidene fluoride i s p r a c t i c a l to manufacture commercially and i s s a i d to have good o p t i c a l p r o p e r t i e s and e x c e l l e n t w e a t h e r a b i l i t y [10]. Flat-Plate Photovoltaic (PV) Encapsulation.* Polymers can serve s e v e r a l f u n c t i o n s i n PV encapsulation systems [15]. The s i n g l e polymer a p p l i c a t i o n common to a l l c o n f i g u r a t i o n s and the core of the encapsulation package I s the p o t t a n t , which embeds the s o l a r c e l l s and r e l a t e d e l e c t r i c a l conductors. The key r e q u i r e ments f o r a pottant are high transparency i n the range of s o l a r c e l l response, mechanical cushioning of the f r a g i l e s o l a r c e l l s from thermal and mechanical s t r e s s e s , e l e c t r i c a l i n s u l a t i o n to i s o l a t e module voltage, and c o s t - e f f e c t i v e m a t e r i a l and module f a b r i c a t i o n processes. Other encapsulation a p p l i c a t i o n s of polymers f o r s p e c i f i c designs i n c l u d e s o i l , u l t r a v i o l e t , and a b r a s i o n - r e s i s t a n t f r o n t covers. The cover can serve as a transparent s t r u c t u r a l superstate. Substrate support designs r e q u i r e a hard, durable f r o n t cover f i l m to p r o t e c t the r e l a t i v e l y s o f t pottant from mechanical damages and excess s o i l accumulation. A polymeric f r o n t cover must be low i n c o s t , h i g h l y transparent, and weather r e s i s t a n t to compete w i t h g l a s s . For a p p l i c a t i o n s out of the o p t i c a l path between the sun and the s o l a r c e l l s (adhesives, i n s u l a t i o n , edge s e a l s , gaskets) requirements f o r polymeric use i n encapsulation are the same as f o r other a p p l i c a t i o n s . Luminescent Solar Concentrators (LSCs). The LSC uses the p r i n c i p l e of l i g h t pipe t r a p p i n g , t r a n s m i s s i o n , and c o u p l i n g i n t o a p h o t o v o l t a i c c e l l (PV) to concentrate s o l a r r a d i a t i o n [16,17]. This use of a low-cost concentrator can reduce the area r e q u i r e ments of the more expensive PV c e l l s . The LSC has s e v e r a l important advantages. I t can be made from inexpensive m a t e r i a l s , can be nontracking and i t can concentrate the l i g h t input from e i t h e r d i r e c t or d i f f u s e i n s o l a t i o n . The LSC can act as a wavelength
*Encapsulation of p h o t o v o l t a i c s f o r concentrator systems depends on c o n c e n t r a t i o n r a t i o and other system s p e c i f i c parameter t e s t s , i s s u e s that are not discussed i n t h i s paper.
In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by PENNSYLVANIA STATE UNIV on May 5, 2012 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch001
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matcher between the s o l a r r a d i a t i o n and the s p e c t r a l response o f the PV c e l l . A l s o , the s o l a r i n f r a r e d r a d i a t i o n and the r e s u l t a n t heat load are prevented from reaching the s o l a r c e l l . One planar c o n f i g u r a t i o n , shown i n Figure 1, uses a polymeric host (polymethylmethacrylate) i n t o which dye molecules are d i s persed randomly. Photons w i t h wavelengths i n the adsorption band of the dye enter the host, a r e absorbed by the dye, and ate reradiated i s o t r o p i c a l l y . Reradiated photons w i t h i n a c e r t a i n s o l i d angle are trapped by t o t a l i n t e r n a l r e f l e c t i o n and guided t o the edge of the p l a t e where s o l a r c e l l s are attached. The concent r a t i o n f a c t o r i s determined by the r a t i o of the areas of the face to an edge, by the f r a c t i o n of r e r a d i a t e d photons which i s trapped (75%), and other f a c t o r s r e l a t i n g to the dye. The trapping e f f i ciency i s p a r t l y determined by the r e f r a c t i v e index o f the polymer. A second planar c o n f i g u r a t i o n uses t h i n (25 urn) polymeric f i l m host ( c e l l u l o s e acetate butyrate) coated onto a support (PMMA, g l a s s ) . When the support i s p o s i t i o n e d as a s u p e r s t r a t e i t acts as a g l a z i n g t o protect the t h i n f i l m . Some designs use s e v e r a l t h i n l a y e r s , each c o n t a i n i n g a dye matched t o d i f f e r e n t s o l a r wavelengths. The p h y s i c a l separation of the dyes can improve t h e i r d u r a b i l i t y . The t h i n f i l m approach means that more expensive m a t e r i a l s may be acceptable. F l u o r i n a t e d or deuterated dyes can improve o p t i c a l e f f i c i e n c y and h i g h l y concentrated dyes can a l t e r the mechanism and e f f i c i e n c y of energy t r a n s p o r t . System l i f e t i m e i s an important unknown f a c t o r p r i n c i p a l l y i n f l u enced by dye l i f e t i m e . Questions r e l a t i n g t o dye-host i n t e r a c t i o n s and the i n f l u e n c e of the host on system l i f e t i m e s a r e unanswered. Fresnel Lenses. F r e s n e l lens concentrators have been s t u d i e d f o r both thermal and p h o t o v o l t a i c systems. The economic v i a b i l i t y of t h e i r use depends on a l a r g e number of system-related f a c t o r s , i n c l u d i n g the performance, c o s t , and d u r a b i l i t y of the lenses. Performance requirements i n c l u d e minimum a b s o r p t i o n , s c a t t e r i n g , and surface r e f l e c t i o n . T o t a l cost depends on costs of m a t e r i a l s and f a b r i c a t i o n , or minimum thickness defined by mechanical requirements, and on a d d i t i o n a l m a t e r i a l required f o r o p t i c a l design. L i k e other s o l a r a p p l i c a t i o n s o f o p t i c a l polymers, durab i l i t y f o r extended periods i s r e q u i r e d . Other Applications. Polymers can a l s o be used as edge s e a l s i n g l a s s m i r r o r s , f i l m s f o r m i r r o r backings, adhesives, s t r u c t u r a l members, s o l a r pond l i n e r s , and energy storage systems. Glass m i r r o r s are more s t a b l e than m i r r o r s w i t h polymeric g l a z i n g s , but they are expensive, heavy, and probably not s t a b l e enough. The s t a t e of the a r t i s e x e m p l i f i e d by the developments of m i r r o r s f o r h e l i o s t a t a p p l i c a t i o n s [18]. The s t r u c t u r e c o n s i s t s o f s i l v e r e d g l a s s backed w i t h a p o l y i s o b u t y l e n e f i l m and mounted on an aluminum sheet-paper honeycomb s t r u c t u r e . The m i r r o r edges are sealed
In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by PENNSYLVANIA STATE UNIV on May 5, 2012 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch001
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w i t h p o l y i s o b u t y l e n e and s i l i c o n e , and the m i r r o r i s h e l d t o the supporting s t r u c t u r e using a neoprene phenolic adhesive. Other low-cost h e l i o s t a t m i r r o r modules have been designed and developed which use p l a s t i c to reduce weight and to accommodate high-volume production of complex forms by molding. Molded r i b , extruded panel, or sandwiched honeycomb s t r u c t u r e s a r e combined w i t h sprayed s i l v e r m e t a l l i z a t i o n , sprayed polymeric overcoats, o r laminated f i l m s [19]. Molded r e i n f o r c e d p l a s t i c s are a l s o used i n p a r a b o l i c trough module designs [20]• A survey of thermal energy storage p r o j e c t s i s a v a i l a b l e [21]•
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Research Opportunities Optical Elements. Problems which are common to many s o l a r r e l a t e d o p t i c a l elements i n c l u d e d i r t r e t e n t i o n , c l e a n i n g , surface a b r a s i o n , and photodegradation. A common f e a t u r e of some of these problems i s that the d e l e t e r i o u s e f f e c t s occur a t an i n t e r f a c e . U l t r a v i o l e t r a d i a t i o n , atmospheric components, mechanical s t r e s s , etc., can have a profound e f f e c t on performance by changing surface c h a r a c t e r i s t i c s . The l i f e t i m e s o f UV s t a b i l i z e r s can be l i m i t e d by exudation; p e r m e a b i l i t y can cause harmful r e a c t i o n s a t i n t e r f a c e s ; and mechanical p r o p e r t i e s can be i n f l u e n c e d by surface crazing. I n other a p p l i c a t i o n s mechanical behavior of the bulk polymer i s c r i t i c a l and v i r t u a l l y a l l a p p l i c a t i o n s r e q u i r e that the polymer system withstand m u l t i p l e environmental s t r e s s e s simultaneously. Surface/Interface Properties of Polymers Surface phenomena play a s i g n i f i c a n t r o l e i n the major problem areas associated w i t h polymers and, t h e r e f o r e , are b a s i c to most o f the s t u d i e s . For example, the l i f e t i m e s of UV s t a b i l i z e r s can be l i m i t e d by exudation and accumulation a t the s u r f a c e , p e r m e a b i l i t y can cause harmful r e a c t i o n s a t i n t e r f a c e s , adhesion Is an i n t e r f a c e phenomenon, and mechanical p r o p e r t i e s can be i n f l u e n c e d by surface c r a z i n g . Examples o f surface problems a f f e c t i n g m i r r o r s i n c l u d e abrasion, dust adhesion, and c l e a n i n g procedures. Surface i n t e r a c t i o n s a l s o occur during the production of polymers; the subsequent behavior o f a polymer can be c r i t i c a l l y dependent upon the m a t e r i a l against which i t i s formed. Surface measurement techniques w i l l form a general experimental b a s i s f o r work on s p e c i f i c a p p l i c a t i o n s . Experimental and a n a l y t i c a l studies are needed t o improve understanding of the chemistry, p h y s i c s , and morphology of s u r f a c e s . Study o f i n t e r faces between polymers and other m a t e r i a l s i s a l s o needed both f o r model i n t e r f a c e s and f o r candidate engineering m a t e r i a l i n t e r faces. Such s t u d i e s should c h a r a c t e r i z e the i n t e r f a c e s as o r i g i n a l l y f a b r i c a t e d and a f t e r changes caused by t y p i c a l e n v i r o n mental exposures. The accumulation of a i r b o r n e p a r t i c u l a t e s and aerosols on the
In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by PENNSYLVANIA STATE UNIV on May 5, 2012 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch001
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o p t i c a l surfaces of s o l a r conversion equipment causes unwanted absorption and s c a t t e r i n g which has lowered operating e f f i c i e n c i e s more than 30%. Accumulation i s most serious f o r s o f t polymers (e.g., s i l i c o n e rubber) and l e a s t s e r i o u s f o r g l a s s . Data i n d i cate that hard polymers l i k e polymethylmethacrylate are n e a r l y as r e s i s t a n t as g l a s s . An understanding of adhesion mechanisms i s r e q u i r e d to develop c o s t - e f f e c t i v e c l e a n i n g methods and s o i l r e s i s t a n t polymeric m a t e r i a l s [22-24]. M o d i f i c a t i o n s of polymeric m a t e r i a l s , e i t h e r i n bulk or by surface treatment or c o a t i n g , may r e s u l t i n m a t e r i a l s that are " s e l f - c l e a n i n g " or that do not tend to hold s o i l , a l l o w i n g i t t o be removed e a s i l y by n a t u r a l forces such as wind and r a i n . Regular c l e a n i n g probably w i l l be r e q u i r e d to maintain high o p t i c a l performance of s o l a r systems, and c l e a n i n g w i l l be a major o p e r a t i o n a l cost f a c t o r . Automatic c l e a n i n g can be made c o s t e f f e c t i v e i f i t i s based on an understanding of s o i l adhesion mechanisms. P o s s i b l e problems i n c l u d e mechanical damage to the surface by c l e a n i n g and p o t e n t i a l c o n t r i b u t i o n s of r e s i d u a l c l e a n i n g agents to m a t e r i a l aging. The o p t i c a l f u n c t i o n of p o l y meric components can be s e r i o u s l y degraded by abrasion due to the c l e a n i n g process or by n a t u r a l causes. Considerations such as c o s t , mechanical c o m p a t i b i l i t y with supporting m a t e r i a l , or UV r e s i s t a n c e may preclude the use of i n h e r e n t l y a b r a s i o n - r e s i s t a n t materials. Since only a shallow l a y e r of r e s i s t a n t m a t e r i a l i s r e q u i r e d , adding a c o a t i n g o r using surface processes that produce a r e s i s t a n t " s k i n " can y i e l d the necessary r e s i s t a n c e . A c o a t i n g might have s e v e r a l uses, p r o v i d i n g abrasion r e s i s t a n c e , improving a n t i r e f l e c t i v e performance, screening UV r a d i a t i o n , or combining several functions. Adhesive f a i l u r e i s a problem i n s o l a r systems. In the past, polymers have been used to p r o t e c t the mechanical i n t e g r i t y of wood and metal s t r u c t u r e s i n severe outdoor environments and to p r o t e c t s e n s i t i v e e l e c t r o n i c components In r e l a t i v e l y benign enclosed environments. Polymers used i n s o l a r equipment w i l l have to p r o t e c t the o p t i c a l p r o p e r t i e s of r e f l e c t o r s , t h i n - f i l m e l e c t r i c a l conductors, and t h i n - f i l m p h o t o v o l t a i c s from the e f f e c t s of moisture and atmospheric p o l l u t a n t s i n severe outdoor environments w h i l e simultaneously maintaining o p t i c a l , mechanical, and chemical integrity. I n some systems, the prevention of mechanical f a i l u r e i s important; f r e q u e n t l y , adhesive f a i l u r e a t the metal/polymer i n t e r f a c e i s of p a r t i c u l a r concern because the ensuing c o r r o s i o n causes o p t i c a l f a i l u r e . Loss of adhesion may be caused by permeation problems. However, i n t e r n a l formation of v o l a t i l e species (outgassing) and primary bond f a i l u r e can a l s o c o n t r i b u t e t o l o s s of adhesion. A l l polymers are i n h e r e n t l y permeable, but t o widely varying degrees. Oxygen, moisture, a i r p o l l u t a n t s , e t c . , can penetrate polymer f i l m s and a t t a c k underlying r e f l e c t o r m e t a l i z a t i o n , cond u c t o r s , or other f u n c t i o n a l elements. Furthermore, these gases
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can modify the mechanical and o p t i c a l p r o p e r t i e s o f the polymer and a l t e r mechanical i n t e r f a c e s between l a y e r s . The p e r m e a b i l i t y of a polymer can be s e n s i t i v e t o how the m a t e r i a l i s processed and used and i s sometimes enhanced as the m a t e r i a l degrades. Although a fundamental understanding o f polymer permeability e x i s t s and experimental data are a v a i l a b l e , current information i s inadequate to model o r c o n t r o l the e f f e c t s of permeation of v a r i o u s species i n s o l a r equipment. Experimental and a n a l y t i c a l studies are needed t o consider modern t h e o r e t i c a l approaches (e.g., nonequilibrium thermodynamics, non-Fickian d i f f u s i o n ) with the g o a l of developing models f o r the transport o f H2O, O2, S0 , and other molecules i n polymers, and to develop and compile q u a n t i t a t i v e engineering data on transport through bulk m a t e r i a l and across and along i n t e r f a c e s s p e c i f i c a l l y f o r s o l a r a p p l i c a t i o n s [25, 26]. A l t e r n a t i v e l y , delamination may not be r e l a t e d d i r e c t l y t o permeation, but may be due instead t o thermal and/or UV e f f e c t s that are followed by the c o r r o s i v e f a i l u r e . Some studies and models i n d i c a t e that the polymer/metal i n t e r f a c e morphology, and the changes i n the morphology with exposure t o the environment, play a key r o l e i n c o r r o s i o n r a t e s . These c h a r a c t e r i s t i c s may be even more important i n c o r r o s i o n c o n t r o l than e i t h e r the d i f f u s i o n of vapors through the polymer or the inherent c o r r o s i o n r e s i s t a n c e of the metal.
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Photochemistry of Polymers V i r t u a l l y a l l polymers d e t e r i o r a t e under exposure to outdoor weathering and s o l a r r a d i a t i o n , but a t g r e a t l y varying r a t e s . Polymers i n s o l a r equipment must maintain o p t i c a l , mechanical, and chemical i n t e g r i t y despite prolonged exposure to s o l a r u l t r a v i o l e t radiation. F o r most outdoor a p p l i c a t i o n s of polymers, s o l a r r a d i a t i o n exposure i s i n c i d e n t a l , but f o r many s o l a r a p p l i c a t i o n s , exposure t o s o l a r r a d i a t i o n i s d e l i b e r a t e l y maximized i n the equipment design. Transparency i s e s s e n t i a l f o r many o f t h e p o t e n t i a l l y most c o s t - e f f e c t i v e a p p l i c a t i o n s , and conventional approaches t o u l t r a v i o l e t p r o t e c t i o n such as opaque coatings and f i l l e r s are unacceptable. Photodegradation i n polymers begins w i t h the primary e x c i t e d s t a t e s produced by absorption of u l t r a v i o l e t photon energy by the polymer. These e x c i t e d s t a t e s undergo f a s t - r e a c t i o n sequences t o form chain r a d i c a l s which, i n t u r n , decay through chemical r e a c t i o n s w i t h i n the polymer o r with O2, R^O, e t c . These r e a c t i o n s can produce changes i n chemistry or molecular s i z e . The r e a c t i o n products can absorb a d d i t i o n a l photons, r e s u l t i n g i n f u r t h e r degradation by analogous processes. The cumulative changes may r e s u l t i n y e l l o w i n g ( l o s s o f transparency), change i n r e f r a c t i v e index, and d e t e r i o r a t i o n o f surface p r o p e r t i e s (e.g., crazing)• Changes i n the mechanical p r o p e r t i e s cause increased creep or c r a c k i n g , w h i l e changes i n the chemical p r o p e r t i e s r e s u l t i n increased permeability to R^O, S0 , e t c . , and subsequent corX
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r o s i v e i n t e r a c t i o n w i t h m e t a l l i c components i n contact w i t h the polymer. Polymers that are s e n s i t i v e to u l t r a v i o l e t r a d i a t i o n but that otherwise have d e s i r a b l e p r o p e r t i e s can be s t a b l i z e d by u l t r a v i o l e t screens, absorbers, quenchers, r a d i c a l scavengers, and antioxidants. U l t r a v i o l e t s t a b l i z e r s can be incorporated by simple a d d i t i o n or by chemical combination w i t h the polymer molecule. Long l i f e t i m e can best be a t t a i n e d by i m m o b i l i z i n g the a d d i t i v e as part of the molecular s t r u c t u r e . Using e x i s t i n g photochemistry, a n a l y t i c a l and experimental s t u d i e s are needed t o develop models of photochemical processes i n s o l a r - r e l a t e d polymers. Recent screening s t u d i e s u s i n g commercial a d d i t i v e s have been d i r e c t e d toward s o l a r a p p l i c a t i o n s . They can provide some input to the system s e l e c t i o n s [28-31]. Thermomechanical Behavior. Requirements f o r o p t i c a l performance impose unprecedented requirements f o r dimensional s t a b i l i t y of polymers used i n h i g h - c o n c e n t r a t i o n r e f l e c t o r s . Requirements f o r mechanical c o m p a t i b i l i t y are a l s o s t r i c t f o r p h o t o v o l t a i c systems subjected to moisture and thermal s t r e s s e s . Moisture, temperature, and UV, s e p a r a t e l y and i n combination, can change the volume and thus the s t r e s s s t a t e of polymers. For example, temperature and humidity c y c l e s alone do not cause surface microcracks i n polycarbonate. However, i n the presence of UV r a d i a tion, such c y c l e s cause m i c r o c r a c k s , w h i l e UV alone does not [32]. An understanding of these r e l a t i o n s h i p s i s e s s e n t i a l t o permit r e l i a b l e design of equipment that uses polymers. These f a c t o r s , coupled w i t h need f o r r e l i a b l e design and low c o s t , n e c e s s i t a t e both a fundamental understanding of mechanical behavior and r e l i a b l e mechanical design d a t a . The r e l a t i o n s h i p s between process and environmental e f f e c t s to mechanical behavior have been developed f o r e l a s t o m e r i c polymers to the degree t h a t these m a t e r i a l s can be s e l e c t e d , and t h e i r long-term performance r e l i a b l y p r e d i c t e d , by a knowledge of some fundamental parameters determined from a few s t r a i g h t f o r w a r d experimental measurements. If the current l e v e l of understanding of v i s c o - e l a s t i c i t y of elastomers can be extended i n t o the range of g l a s s y polymers, then i t w i l l be p o s s i b l e to make comparable p r e d i c t i o n s of mechanical s t r e s s / t i m e / t e m p e r a t u r e / s t r a i n response and failure relationships. U l t i m a t e l y , e q u i v a l e n t understanding of g l a s s y polymers w i l l g r e a t l y reduce the need f o r c o s t l y e m p i r i c a l t e s t i n g each time a new a p p l i c a t i o n i s contemplated. Combined Environmental Effect. Any l i s t of s i g n i f i c a n t e f f e c t s of the environment on polymers i n s o l a r a p p l i c a t i o n s w i l l i n c l u d e UV degradation, weathering, p e r m e a b i l i t y , high-temperature performance, d e l a m i n a t i o n / f a t i g u e , dimensional s t a b i l i t y , and soiling/cleaning. These effects are not necessarily independent: polymers are expected to s u f f e r more s e r i o u s degradation during exposure to combined environmental s t r e s s e s
In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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E f f e c t i v e methods f o r p r e d i c t i n g and v e r i f y i n g the performance o f polymers under i n t e r a c t i v e e f f e c t s are r e q u i r e d , p a r t i c u l a r l y f o r those a p p l i c a t i o n s unique to s o l a r energy systems. The simultaneous and s e q u e n t i a l combinations of environmental s t r e s s e s that a l t e r the p r o p e r t i e s and a f f e c t the performance o f polymers should be i d e n t i f i e d e x p e r i m e n t a l l y . Assessment of the s t a b i l i t y o f polymers from fundamental r a t e data o r from e x p e r i mental engineering data r e q u i r e s an understanding of the i n t e r a c t i v e e f f e c t s o f environmental s t r e s s [11, 32]. Testing o f a l l combinations of the s t r e s s e s would r e q u i r e an unacceptable number of experiments. The number of combinations s t u d i e d can be l i m i t e d by r e c o g n i z i n g the unique p o s i t i o n o f o p t i c a l elements which may use transparent polymers and o f s t r u c t u r a l members; by f i r s t ranking the importance of i n d i v i d u a l s t r e s s e s ; and by using screening t e s t s to i d e n t i f y promising m a t e r i a l s [28-30]• Improved a n a l y t i c a l and t e s t methods are needed. Accelerated t e s t s s t r e s s some parameters to decrease the time before f a i l u r e occurs, abbreviated t e s t s use a n a l y t i c a l techniques t o estimate f a i l u r e r a t e s from i n c i p i e n t degradation during short-term exposures at u s u a l s t r e s s l e v e l s . Methods are needed t o demonstrate c o r r e l a t i o n s between these t e s t r e s u l t s and r e a l - t i m e behavior. E a r l i e r s t u d i e s [33, 34] provide some b a s i s f o r t h i s work. Performance p r e d i c t i o n modeling (PPM) i s one method f o r e v a l u a t i n g m a t e r i a l s performance that has been defined and i s being a p p l i e d a t the Jet P r o p u l s i o n Laboratory. I n i t s simplest form, PPM can v e r i f y the s a t i s f a c t o r y performance of a p a r t i c u l a r m a t e r i a l used i n a s p e c i f i c design and subject to a defined set o f s t r e s s e s (e.g., temperature, thermal c y c l i n g , u l t r a v i o l e t r a d i a t i o n , mechanical l o a d s ) . Conversely, I t can d e f i n e l i m i t s o f s t r e s s e s f o r the m a t e r i a l s i n an a v a i l a b l e piece of hardware o r design. This approach has been used s u c c e s s f u l l y as part o f the demonstration of the f e a s i b i l i t y of using an u l t r a - t h i n polymer f i l m on a s o l a r s a i l f o r space p r o p u l s i o n [35], f o r a n a l y t i c a l assessment of an experimental f a c i l i t y t o study space r a d i a t i o n e f f e c t s [36], and f o r a n a l y t i c a l assessment and i d e n t i f i c a t i o n o f c r i t i c a l technologies f o r ceramic r e c e i v e r s [37]. The method i s c u r r e n t l y being a p p l i e d to p h o t o v o l t a i c encapsulation [38]. A second procedure, using the methods of thermodynamics a p p l i e d to i r r e v e r s i b l e processes, o f f e r s another new approach f o r understanding the f a i l u r e of m a t e r i a l s . For example, the e q u i l i b r i u m thermodynamics o f closed systems p r e d i c t s that a system w i l l evolve i n a manner that minimizes i t s energy (or maximizes i t s entropy). The thermodynamics of i r r e v e r s i b l e processes i n open systems p r e d i c t s that the system w i l l evolve i n a manner that minimizes the d i s s i p a t i o n o f energy under the c o n s t r a i n t that a balance of power i s maintained between the system and i t s environment. A p p l i c a t i o n o f these p r i n c i p l e s o f nonlinear i r r e v e r s i b l e thermodynamics has made p o s s i b l e a formal r e l a t i o n s h i p between thermodynamics, molecular and morphological s t r u c t u r a l parameters,
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than from a simple sum of e f f e c t s of i n d i v i d u a l s t r e s s e s , and t h e i r r a t e of change. E x p e r i m e n t a l l y , these p r i n c i p l e s emphasize dynamic measurements t h a t make p o s s i b l e the s e p a r a t i o n o f the d i s s i p a t i v e and the conservative components of energy i n c i d e n t upon the system. Dynamic mechanical a n a l y s i s has been an important area o f research f o r over 40 years. Computer-controlled experimentation now makes i t p o s s i b l e t o apply analogous techniques t o the measurement o f many other thermodynamic s t r e s s e s . One example c u r r e n t l y under i n v e s t i g a t i o n , dynamic photothermal spectroscopy, i s expected t o provide a new approach t o p r e d i c t i n g the long-term e f f e c t s of u l t r a v i o l e t r a d i a t i o n on m a t e r i a l s [39]. Acknowledgments
The authors wish t o express t h e i r g r a t i t u d e f o r the support of the M a t e r i a l s Research Branch and, i n p a r t i c u l a r , Barry B u t l e r , A.W. Czanderna, and R.F. R e i n i s c h f o r t h e i r c o n t r i b u t i o n s . Thanks a l s o a r e due t o members of the Energy and M a t e r i a l s Research S e c t i o n of the J e t P r o p u l s i o n Laboratory f o r t h e i r help i n preparing t h i s document. This document was prepared f o r the U.S. Department of Energy under Contract No. EG-77-C-01-4024. The J e t P r o p u l s i o n Laboratory i s a 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 f a c i l i t y , and the S o l a r Energy Research I n s t i t u t e i s a Department of Energy f a c i l i t y .
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Arrays"; DOE/JPL-954328-78/2; Batelle Columbus Laboratories: Columbus, OH, 1978. Roberts, F. R.; Schonhorn, H. "Polymer Preprints"; Amer. Chem. Soc., Div. Polym. Chem. 1975, 16, 146. Baum, B.; Binnette, Mark. "Solar Collectors--Technical Status Report No. 14, 5 Oct.-5 Nov. 1979"; Contract EM-78-C04-5359; Springborn Laboratories, Inc.: Enfield, CT, 1979. Baum, B.; Cambron, R.; White, R. "Technical Report No. 2, 19 Oct.-19 Nov. 1979"; Contract DE-AC01-79ET21106; Springborn Laboratories, Inc.: Enfield, CT, 1979. Willis, P. B.; Baum, B. "Investigation of Test Methods, Material Properties, and Processes for Solar Cell Encapsulants"; DOE/JPL-954527-79-10; Springborn Laboratories, Inc.: Enfield, CT, 1979. Willis, P. B. et al. "Investigation of Test Methods, Material Properties, and Processes for Solar Cell Encapsulants: Annual Report"; ERDA/JPL-954527-77/2; Springborn Laboratories, Inc.: Enfield, CT. Blaga, A; Yamasaki, R. S. Journal of Material Science. 1976, 11, 1513. Kolyer, J.M.; Mann, N.R. "Accelerated/Abbreviated Test Methods: Interim Report, Apr. 1-Oct. 24, 1977; ERDA-JPL954458-77/2; Rockwell International: Anaheim, CA, 1977. Thomas, R.E.: Carmichael, D.C. "Terrestrial Service Environments for Selected Geographic Locations." ERDA/JPL-95432876/5; Battelle Columbus Laboratories: Columbus, OH, 1976. Jet Propulsion Laboratory. "Sail Film Materials and Supporting Structures for a Solar Sail"; JPL Report 720-9; Pasadena, CA:, 1979. Jet Propulsion Laboratory. "Effects of Space Environment on Composites: An Analytical Study of Critical Experimental Parameters"; JPL Report 79-47; Pasadena, CA, 1979. Jet Propulsion Laboratory. "Performance Prediction Evaluation of Ceramic Materials in Point Focusing Solar Receivers"; DOE/JPL-1060-23; Pasadena, CA. 1079. Jet Propulsion Laboratory. "Physical/Chemical Modeling for Photovoltaic Module Life Prediction"; presented at the International Photovoltaics Conference, Berlin. Jet Propulsion Laboratory: Pasadena, CA, 1979. Lindenmeyer, P.H. "Principles of Nonlinear Irreversible Thermodynamics Applied to the Testing of Materials"; D18025583-1; Boeing Co.: Seattle, WA.
RECEIVED February
9,1983
In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
