Optical, Mechanical, and Environmental Testing of Solar Collector

6

Downloaded by PENNSYLVANIA STATE UNIV on May 5, 2012 | http://pubs.acs.org Publication Date: June 15, 1983 | doi: 10.1021/bk-1983-0220.ch006

Optical, Mechanical, and Environmental Testing of Solar Collector Plastic Films M . J. B E R R Y

1

and H . W. DURSCH

Boeing Engineering and Construction Company, Solar Systems, Seattle, WA 98124

Optical and mechanical testing of several commercially available plastic films indicated that while desired initial performance was attained, long term weatherability and stability in specularity (specular reflectance or specular transmittance) and strength were observed only in the fluoropolymers studied. The objective of the described testing (characterization - environmental exposure retest) was to identify or stimulate the development of reflective (> 93%) and transmitting (> 92%) films possessing low cost, high specularity, adequate strength and weatherability (moisture, UV, hail). Kynar and Tedlar (PVF and PVF) nearly met the desired specular transmittance and showed little or no optical and mechanical degradation after the equivalent of 16 years exposure. Metalized polyesters and polycarbonates met initial optical and mechanical requirements but showed severe property degradation after relatively short environmental exposure. 3

The Boeing Engineering and Construction Company (BEC) has been involved in the study and use of plastic film for solar collectors since 1975. Several studies resulted in designs that require the use of highly specular plastic films. Solar collector designs for which hardware was fabricated and tested include a thin film heliostat (Figure 1), a parabolic linear trough concentrator (Figure 2), a pressure stabilized point focusing concentrator and a plastic film/steel sheet laminate point focusing concentrator (Figure 3). The designs for the heliostat, trough concentrator and the pressure stabilized point focusing concentrator a l l involve the use of lightweight reflectors and structures which are protected from the 1

Current address: Resources Conservation Co., P.O. Box 3766, Seattle, WA 98124 0097-6156/83/0220-0099$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


POLYMERS IN SOLAR ENERGY UTILIZATION

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


Figure 2 .

P a r a b o l i c L i n e a r Trough.


X

O

c

α d

>

M

w


102


F i g u r e 3, P o i n t Focusing Concentrator ( P l a s t i c F i l m / S t e e l Sheet Laminate).


6.

BERRY AND DURSCH

Testing of Plastic Films

103

environment by p r e s s u r i z e d t h i n f i l m enclosures ( f a b r i c a t e d w i t h panels and seams, or one piece thermoforming process). APPROACH


T h i r t y p l a s t i c f i l m s u p p l i e r s were contacted w i t h 21 companies a c t i v e l y responding. Tables I and I I l i s t those s u p p l i e r s that have been most a c t i v e , along w i t h the m a t e r i a l s supplied. The m a t e r i a l s were screen tested i n Boeing l a b o r a t o r i e s . The screen t e s t i n g e l i m i n a t e d f i l m candidates w i t h poor o p t i c a l and/or mechanical p r o p e r t i e s and provided c o n t r o l values f o r m a t e r i a l s that were exposure t e s t e d . M i c r o t e n s i l e coupons were t e s t e d per ASTM D1708 f o r determination of y i e l d s t r e n g t h , u l t i m a t e strength and u l t i m a t e elongation. Specular r e f l e c t a n c e i s measured using a modified b i - d i r e c t i o n a l r e f l e c t o m e t e r u t i l i z i n g a 633 nm wavelength l a s e r source and a v a r i a b l e aperture system (0.08° t o 1.17° cone angles) (Figure 4 ) . Specular transmittance i s measured using a Beckman DK-2A spectrophotometer and a Gier-Dunkle i n t e g r a t i n g sphere to provide transmittance w i t h i n an acceptance cone angle of 0.5° f o r wavelengths of 250 through 2500 nm (Figure 5) · The r e s u l t s are i n t e g r a t e d over an air-mass 2 s o l a r spectrum. M a t e r i a l s showing promise a f t e r s c r e e n - t e s t i n g were sent t o Desert Sunshine Exposure Testing F a c i l i t y (DSET) l o c a t e d near Phoenix, Arizona. Real time exposure t e s t i n g i s performed on 45° e l e v a t i o n , south f a c i n g racks, p r o v i d i n g d i r e c t (1 sun) exposure. A c c e l e r a t e d t e s t i n g i s performed on EMMA ( e q u a t o r i a l mount w i t h m i r r o r s f o r a c c e l e r a t i o n ) . EMMA a c c e l e r a t i o n f a c t o r s average out to approximately 8 times d i r e c t (8 suns) over a year's period of exposure. The bare transparent f i l m s were placed i n the environment as received while the r e f l e c t i v e m a t e r i a l coupons were placed i n s i d e of transparent (fluorocarbon) f i l m envelopes to simulate BEC s p l a s t i c f i l m h e l i o s t a t design. The same t e s t i n g techniques that were used to screen t e s t the samples were used t o t e s t the samples a f t e r outdoor exposure. ( O p t i c a l t e s t i n g of exposed samples was performed a f t e r c l e a n i n g ) . 1

Simulated h a i l t e s t i n g was performed on a l i m i t e d number of m a t e r i a l s that had passed i n i t i a l screening; ( f l u o r o c a r b o n , p o l y e s t e r and polycarbonate f i l m s were i n c l u d e d ) . A " s l i n g shot" type launcher c o n s i s t i n g of an i c e b a l l holder, an e l a s t i c p r o p e l l i n g system, and a commercial v e l o c i t y measurement instrument was used (see Figure 6 ) . P o t e n t i a l damage was assessed by measuring the l o s s i n mechanical strength and transmittance a t the point of contact of the i c e b a l l s .



F i g u r e 4»

B i - d i r e c t i o n a l Reflectometer.


Signal to lock-in voltmeter Brower labs Model 131

BERRY

A N D DURSCH

Testing of Plastic

Films


6.


105


Release mechanism -*-E?

(·.

t · • • ' • • • t

"Sling shot'

Calibrated scale

Hailstone

Launcher

Target plane

Material specimen

Normal incidence

Figure 6 H a i l T e s t i n g Apparatus.

Velocity measurement instrument

8.5 ft radius of curvature

45° Incidence


Incidence

60°

Pressurization blower

Drum for specimen support

ο

2

H δ

r

H

d

M δ

M

Ο r

Η-

Ο

OS

6.

BERRY AND DURSCH


107

PROGRAM RESULTS


Test r e s u l t s are based upon s i n g l e coupon o p t i c a l measurements and t e n s i l e measurements using 4-8 coupons/sample. (Successive measurements on same m a t e r i a l , not same sample). This l i m i t e d number of coupons, coupled w i t h apparent v a r i a t i o n s i n the p l a s t i c f i l m process v a r i a b l e s such as non-uniformity of o r i e n t a t i o n , c o a t i n g t h i c k n e s s e s , r e s u l t s i n property data s c a t t e r . When a small number of t r u l y promising m a t e r i a l s have been i s o l a t e d , l a r g e numbers of coupons should be t e s t e d , p r o v i d i n g s u f f i c i e n t data f o r s t a t i s t i c a l treatment. Shown i n Table I a r e the enclosure (transparent) f i l m m a t e r i a l s c u r r e n t l y being exposure t e s t e d at DSET. A f t e r 24 months of exposure, the two fluoropolymers, Kynar and Tedlar, have proved t o be the most promising enclosure f i l m s . Figures 7 and 8 give the u l t i m a t e e l o n g a t i o n and specular transmittance of these and other m a t e r i a l s . Experience has shown u l t i m a t e e l o n g a t i o n to be the most s e n s i t i v e i n d i c a t o r of degradation. A f t e r an e q u i v a l e n t of 16 years of s o l a r exposure, Kynar has shown a 2% decrease i n specular transmittance and 33% increase i n e l o n g a t i o n . During the same time, Tedlar decreased 5% i n s p e c u l a r transmittance and 11% i n e l o n g a t i o n . Data f o r coupons cut from a Tedlar enclosure, exposed f o r 2 years i n northeastern Oregon, i s shown i n Figure 7. Also shown i n Figures 7 and 8 are the r e s u l t s from exposure t e s t i n g of a t y p i c a l p o l y e s t e r and a t y p i c a l polycarbonate. Both m a t e r i a l s l o s t considerable mechanical s t r e n g t h i n EMMA a f t e r 6 months (4 years e q u i v a l e n t ) . In both cases, e l o n g a t i o n was reduced to near zero. The l o s s e s i n transmittance were 29% f o r 3M p o l y e s t e r and 60% f o r the Cryovac polycarbonate. I t was decided to d i s c o n t i n u e a c c e l e r a t e d t e s t i n g of the p o l y e s t e r and polycarbonate a f t e r 6 months since the damage had been so severe. Real time t e s t i n g was continued to determine i f the a c c e l e r a t i o n e f f e c t s of EMMA are causing damage at a r a t e that i s i n excess of 8 times d i r e c t . The 3M and Cryovac samples were uv s t a b i l i z e d , but as r e s u l t s i n d i c a t e , the s t a b i l i z a t i o n was inadequate. L i s t e d i n Table I I a r e the r e f l e c t i v e f i l m m a t e r i a l s undergoing outdoor exposure t e s t i n g a t DSET. T y p i c a l r e s u l t s f o r coupon t e s t i n g of these m a t e r i a l s are shown i n Figures 9 and 10. A l s o shown are data from coupons cut from an uncoated aluminized p o l y e s t e r r e f l e c t o r supported i n a Tedlar enclosure that was exposed i n northeastern Oregon. Of the r e f l e c t o r m a t e r i a l candidates s t u d i e d , only the coated, s i l v e r i z e d , s t a b i l i z e d p o l y e s t e r (OCLI) maintained i t s specular r e f l e c t i v i t y f o r 18 months on EMMA ( e q u i v a l e n t of 12 y e a r s ) . A l l r e f l e c t i v e f i l m s showed severe r e d u c t i o n i n mechanical p r o p e r t i e s . None of the 4 f i l m s i n v e s t i g a t e d f o r h a i l r e s i s t a n c e f a i l e d when impacted w i t h 25mm (1 i n ) i c e b a l l s t r a v e l l i n g at t e r m i n a l


108



Table I

Enclosure Materials Undergoing Exposure

Supplier 3M* Dupont* Pennwalt* Allied Chemical* Mobay 3M 3M XCEL Pennwalt Allied Chemical W.R. Grace (Cryovac)* National Metalizing* HOECHST 3M Martin Processing* Celenese* ICI ICI* Morton Chemical* Martin Processing* W.R. Grace (Cryovac)*

Material Anti-reflective coated/internally stabi 1 ized/polyester Fluorocarbon (Tedlar) Fluorocarbon (Kynar, lab material) Polyester (Petra A) UV stabilized polycarbonate PMMA (Acrylar) Coated/internally stabilized/ polyester Acrylic (Korad) Fluorocarbon (Kynar, production) Fluorocarbon (H-2) UV stabilized polycarbonate UV stabilized polyester Fluorocarbon (Hostaflon) Acrylic/polyester lamin. (Flexigard) UV stabilized polyester (Llumar) UV stabilized polyester (Celenar) Internally UV stabilized polyester Internally UV stabilized polyester (Melinex OW) AR coated fluorocarbon (Tedlar) UV stabilized polyester (Llumar) UV stabililized polycarbonate

Testing at

DSET

Solar specular transmittance (%) at 0.59° cone angle (control value) 93 90 89 89 89 89 89 88 86 86 86 86 86 86 85 85 85 83 83 82 73

* Materials with exposure test results



6.

BERRY AND DURSCH


F i g u r e 7. T y p i c a l Enclosure M a t e r i a l O p t i c a l Data.

F i g u r e 8. T y p i c a l Enclosure M a t e r i a l Mechanical Data.


109


110


Table I I Reflector Materials Undergoing Exposure Testing at

Supplier OCLI* Dunmore* National Metalizing* Sheldahl* 3M Dunmore* Morton Chemical* Dunmore* Morton Chemical* Sheldahl* Mobay* Dunmore* XCEL

Material Silverized/UV stabilized/polyester Aluminized/internally UV stabilized/ polyester Coated/aluminized/polyester UV stabilized polycarbonate/ silverized/polyester Acrylic coated/aluminized/polyester Coated/aluminized/polyester Coated/aluminized/polyester Aluminized polyester Coated/aluminized/polyester UV stabilized polycarbonate/ aluminized/polyester Aluminized polycarbonate Coated/aluminized/polyester Aluminized acrylic

DSET

Reflectance (%) at 0.14° cone angle (control value) 94 88 86 85 85 84 83 76 73 66 58 43 27

* Materials wit h exposure test results


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ΑΙ/Polyester reflector

Ag/Polyester (OCLI)


Coated ΑΙ/Polyester No. 2 (Morton) • Δ Coated ΑΙ/Polyester No. 1 (Natl. Metal.)

1 - Real time, years

6

8

_L 10

12

14

16

Accelerated time, years —

F i g u r e 9. T y p i c a l R e f l e c t o r M a t e r i a l O p t i c a l Data.

F i g u r e 10. T y p i c a l R e f l e c t o r M a t e r i a l Mechanical Data.



POLY CARBONATE

7.8 7.0

7.5 9.0

8.4 7.3 9.1

8.0 8.1 7.9

7.9 8.8 8.6

long

0° 45 60

0° 45* 60

0° 45 60

0° 45 60

0° 45 60

Hailstone Angle Weight From average Normal (Grams) Incidence

Fine Scratch Lines, h inch

3.5

5

PETRA A ( P o l y e s t e r )

4

2

On H e l i o s t a t In F i e l d

4

MELINEX 0 ( P o l y e s t e r )

•

TELAR ( P o l y v i n y l F l o u r i d e ) • In Laboratory

MATERIAL

Film Thickness (Mils)

73.4 73.4

74.8 74.8

74.8 74.8 73.4

76.2 77.8 77.8

76.3 74.8 74.8

Velocity (FPS)

.46 NONE

.23 NONE

.37 NONE NONE

NO^E

NONE

U>°

Indentation Diameter (Inches)

TABLE I I I TEST RESULTS

24

71

77

—

56

Specular Transmittance (%)


0

0

2.9

0.7

Loss In Yield Strength (%)

DAMAGE

9.5

0

0

0.9

>117

104

113

106

122

Velocity At Failure (FPS)

. .

Loss I n Ultimate Strength (%)

6.

BERRY AND DURSCH


113

v e l o c i t y (see Table I I I ) . The maximum mechanical damage was 9.5% l o s s i n u l t i m a t e strength f o r the polycarbonate; others showed l e s s than 3% l o s s of u l t i m a t e or y i e l d strength. The specular transmittance was reduced w i t h i n the a f f e c t e d areas. An a n a l y s i s , which considered c o l l e c t o r geometry, h a i l s t o n e i n c i d e n c e angle, and storm s e v e r i t y and frequency f o r the southwestern U.S. was used to p r e d i c t the u s e f u l area that would be l o s t through transmittance r e d u c t i o n i n a 15 year period. Losses of 1 t o 3% were estimated.


CONCLUSIONS Test r e s u l t s to date i n d i c a t e that the fluoropolymers, Kynar and T e d l a r , e x h i b i t the best w e a t h e r a b i l i t y of the enclosure m a t e r i a l s s t u d i e d . L i t t l e or no o p t i c a l or mechanical degradation was observed a f t e r the equivalent of 16 years. The m e t a l i z e d p o l y e s t e r s and polycarbonates, although s t a b i l i z e d , were severely degraded i n mechanical p r o p e r t i e s i n both r e a l time and a c c e l e r a t e d exposures. There was some evidence that r e f l e c t i v i t y can be s t a b i l i z e d through the use of p r o t e c t i v e c o a t i n g s . M e t a l i z i n g of fluorocarbons i s under i n v e s t i g a t i o n . Several f i l m s e x h i b i t e d a good r e s i s t a n c e to damage from 25mm h a i l f a l l i n g at terminal v e l o c i t y . RECEIVED November 22, 1982


Optical, Mechanical, and Environmental Testing of Solar Collector

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