I TECH R.I.C.

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NBSIR 81-2232

Solar Energy Systems - Standards

for Absorber Materials

Larry W. Masters

James F. Seiler

Edward J. Embree

Willard E. Roberts

Structures and Materials Division

Center for Building Technology

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

January 1981

Prepared for Department of Energy

Dffice of Solar Applications for Building

100 /Vashington, DC 20585

.U56

81-2232

1981

c. 2

NBSIR 81-2232

SOLAR ENERGY SYSTEMS - STANDARDS

FOR ABSORBER MATERIALS

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Larry W. Masters

James F. Seiler

Edward J. Embree

Willard E. Roberts

Structures and Materials Division

Center for Building Technology

National Engineering Laboratory

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

January 1981

Prepared for Department of Energy

Office of Solar Applications for Buildings

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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TABLE OF CONTENTS

Page

LIST OF TABLES v LIST OF FIGURES vl

ABSTRACT viii

ACKNOWLEDGMENT lx

1. INTRODUCTION 1

1.1 Background 1

1.2 Objectives 1

1.3 Scope 1

2. PROBLEM ASSESSMENT 2

2.1 Performance Requlranents 2 2.2 Key Properties 2 2.3 Key Degradation Factors 2

3. LABORATORY AND FIELD STUDIES 3

3.1 Materials 3

3.2 Test Methods 3

3.2.1 Property Measurement Tests 3

3.2.2 Accelerated Laboratory Exposure Tests 3

3.2.2. 1 Elevated Temperature (Oven Aging)

3. 2. 2. 2 Thermal Cycling

3. 2. 2. 3 Ultraviolet (UV) Radiation

3. 2. 2. 4 Moisture

3. 2. 2. 5 Cumulative Exposure 4

3.2.3 Outdoor Exposure Tests 4

3.2.4 Outgasslng 5

4. RESULTS AND DISCUSSION 5

4.1 Accelerated Laboratory Exposure Tests 5

4.1.1

Elevated Temperature (Oven Aging) 5

4. 1.1.1 150°C Exposure 5

4. 1.1.2 200°C Exposure 6

4. 1.1.3 250°C Exposure 6

111

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TABLE OF CONTENTS

Page

4.1.2 Thermal Cycling 7 4.1.3 Ultraviolet (UV) Radiation 7 4.1.4 Moisture 7 4.1.5 Cumulative Exposure 8

4.2 Outdoor Exposure 9

4.3 Outgasslng 10

5. CONCLUSIONS 10

6. PROPOSED STANDARDS 12

6.1 Standard Practice for Evaluating Solar Absorptive Materials

for Thermal Applications 12

6.2 Standard Practice for Determining the Effect of Outgasslng on the Transmittance of Glass Test Plates Which Simulate

Solar Collector Covers 12

7. BIBLIOGRAPHY 13

APPENDICES

APPENDIX A - Standard Practice for Evaluating Solar Absorptive

Materials for Thermal Applications

APPENDIX B - Standard Practice for Determining the Effect of

Outgasslng on the Transmittance of Glass Test Plates

Which Simulate Solar Collector Covers

Iv
LIST OF TABLES

Page

1. Absorber Materials and Substrates used in the Study 14

2. Absorptance and Infrared Emittance of Absorbers Prior

to Exposure 15

3. Results of 130°C Oven Aging Exposure 16

4. Results of 200^*0 Oven Aging Exposure 17

5. Results of 250°C Oven Aging Exposure 18

6. Results of Thermal Cycling 19

7. Results of Ultraviolet (UV) Radiation Exposure 20

8. Results of Exposure at 92°C and 97 Percent Relative Humidity ... 21

9. Results of Exposure Using a Condensation Cycle 22

10. Results of Outdoor Exposure at Gaithersburg, Maryland 23

11. Results of Outdoor Exposure at Phoenix, Arizona 24

12. Results of Outdoor Exposure at Miami, Florida 25

13. Results of Outgasslng Tests at 250*^C 26

v
LIST OF FIGURES

Page

1. Test Box Used for Outdoor Exposures 27

2. Reflectance of Exposed and Unexposed Copper Oxide 28

3. Reflectance of Black Chrome on Copper Before and After Oven

Aging 29

4. Blistering of Nickel Foil During Oven Aging 30

5. Appearance of Copper Oxide Specimens Before and After 250®C

Exposure 31

6. Appearance of Polyvinylidene Fluoride (G9) on Galvanized Steel

Before and After 250°C Exposure 31

7. Appearance of Urethane on Copper Before and After 250“C

Exposure 32

8. Appearance of Urethane on Aluminum Before and After 250“C

Exposure 32

9. Appearance of Alkyd on Copper Before and After 250“C

Exposure 33

10. Appearance of Alkyd on Aluminum Before and After 250®C

Exposure 33

11. Reflectance of Exposed and Unexposed Alkyd on Steel 34

12. Loss of Adhesion of Modified Polyester Coating During Thermal

Cycling Exposure 35

13. Appearance of Alkyd on Steel Before and After Exposure at

92 °C and 97 Percent R. H 35

14. Appearance of Alkyd on Copper Before and After Exposure at

92 °C and 97 Percent R. H 36

15. Appearance of Alkyd on Aluminum Before and After Exposure

at 92 °C and 97 Percent R. H 36

16. Appearance of Epoxide on Steel Before and After Exposure at

92 °C and 97 Percent R. H 37

17. Appearance of Epoxide on Aluminum Before and After Exposure

at 92 °C and 97 Percent R. H 37

Vi
LIST OF FIGURES

Page

18. Crosa-Sectlon of Unexposed Epoxide Coating with Phenolic

Butyrate Primer on Aluminum (300 x) 38

19. Cross-Section of Epoxide Coating and Primer on Aluminum

Following 21 Weeks Exposure at 92 *C and 97 Percent R, H, 38

20. Appearance of Polyvinyl Idene Fluoride on Galvanized Steel

(G9) Before and After Exposure at 92®C and 97 Percent R, H. .. 39

21. Cross-Section of Unexposed Polyvinyl Idene Fluoride Coating

with Epoxide Primer on Galvlnlzed Steel (G9) (1000 x) 40

22. Cross-Section of Polyvinyl Idene Fluoride with Primer on Galvanized Steel (G9) Following 21 Weeks Exposure at 92°C and

97 Percent R. H 40

23. Appearance of Alkyd on Aluminum Before and After Cumul.atlve

Exposure (12 Weeks at 200®C Followed by 28 Temperature Cycles

from 200®C to -10®C) 41

24. Appearance of Urethane on Copper Before and After Cumulative

Exposure (12 Weeks at 200®C Followed by 28 Temperature Cycles

from 200®C to -10®C) 41

25. Reflectance Curves of Black Chrome on Steel Before and After

Outdoor Exposure at Gaithersburg, Maryland 42

26. Reflectance Curves of Black Chrome on Copper Before and After

Outdoor Exposure at Gaithersburg, Maryland 43

27. Reflectance Curves of Black Chrome on Copper Before and After

Outdoor Exposure at Miami, Florida 44

vii
ABSTEIACT

Absorber materials used in solar heating and cooling systems absorb energy

from the sun and convert it to thermal energy. These materials must perform

their intended functions both when first installed and after extended use. The environment in which absorber materials are exposed can cause degradation

in their performance. Numerous problems with absorber materials in solar

energy systems have demonstrated the need for standards to assess their

performance and durability.

This study was performed to aid in the development of accelerated test

methods needed for the evaluation of absorber materials and to incorporate

the methods into draft standards for consideration as consensus standards

by the American Society for Testing and Materials (ASTM).

Performance requirements for absorber materials were identified and

laboratory and field studies were performed to measure performance

according to the requirements. The data obtained, using twelve absorber

materials, were used as the technical basis for two draft standards.

This report presents the results of the research and includes the proposed

draft standards.

viii
ACKNOWLEDGMENT

The authors gratefully acknowledge 1) the many suggestions provided by

members of ASTM E44.04 on Materials Performance in planning and carrying out this study, 2) the interest demonstrated by the Department of

Energy in supporting the research, 3) the cooperation of manufacturers

who provided absorber materials for inclusion in the study and commercial exposures test sites who performed outdoor exposures, 4) the technical assistance of Ms. Mildred Post and Mr. David Ballard of NBS in

performing SEM analyses, 5) the technical assistance of Mr. Elmer Streed

in preparing the report and 6) the support and cooperation of

Mr. Robert D. Dikkers and Dr. David Waksman of NBS throughout the

study

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1.1.1 INTRODUCTION

BACKGROUND

The purposes of absorptive coatings used in solar energy collectors for

heating or cooling applications are to absorb the sun's energy and convert

it to thermal energy. Of prime importance are the optical characteristics

of the absorptive coatings, i.e, absorptance (a) and anittance (e), and

the maintenance of these characteristics during the lifetime of the

collector system.

A signficant number of problems with absorber materials and other collector

materials have occurred in the residential and commercial demonstration

programs being conducted by the Department of Housing and Urban Development

(DHUD) and the Department of Energy (DoE). Field inspections of operational

ber of materials related coriecror prooxems iij. nany or rnese prooxems

have been traced to the inability of solar collector materials to withstand

prolonged exposure to stagnation conditions. Such conditions can be expected during the normal lifetime of a solar collector, either initially,

when the collector is installed before the system is put into operation or subsequently when the system is shut down either for maintenance or because

of seasonal considerations, e.g., with heating only systems.

Efforts in the development of reliability/durability tests for the

materials ccwaprising collectors have been hampered by the lack of data

on real time and accelerated degradation. These difficulties point out

the need to develop accelerated test methods for evaluating the reliability

and durability of materials and to incorporate the methods into standards

for use by the solar industry.

1.2 OBJECTIVES

This study, sponsored by the Department of Energy, was performed to aid in

the development of accelerated test methods needed for evaluation of absorber

materials and to incorporate the methods into draft standards for consideration as consensus standards by the American Society for Testing and

Materials (ASTM).

1.3 SCOPE

In order to meet the stated objectives, tasks were performed to 1) identify

performance requirements, 2) develop test methods to evaluate performance

and durability according to the requirements, 3) perform laboratory and

field evaluations to ensure that mechanisms of failure in accelerated tests

are the same as those to be expected in-service, and 4) develop, based upon

the research findings, draft standards for consideration as consensus

standards

.

^ Numbers in brackets indicate references included in the Bibliography.

systems by the National

«

1
The laboratory and field evaluations included four selective and eight nons elective absorber materials used primarily for building heating

and cooling and domestic hot water applications. The optical properties

of the absorbers were measured in conjunction with aging tests intended

to induce degradation in the materials. The aging tests included accelerated laboratory exposure as well as outdoor exposure in simulated solar

collectors. Measurement of properties before and after aging allowed the

amount of degradation caused by the aging process to be determined. The

accelerated laboratory aging tests emphasized elevated temperature, thermal

cycling, ultraviolet radiation and moisture.

The test data contained in this report should not be used for design

purposes. The properties of absorber materials are likely to vary from

manufacturer to manufacturer and from one production run to another.

Therefore data to aid design of collectors must be obtained for the

specific absorber material which is to be used in the collector. In

addition, the emlttance data reported are total normal emittance rather

than total honispherical emittance.

2. PROBLEM ASSESSMENT

2.1 PERFORMANCE REQUIREMENTS

In order to perform their primary function, which is to absorb energy from

the sun and convert it to heat energy, absorber materials for heating and

cooling collector systems should possess, and maintain under in-use conditions, a high solar absorptance in the wavelength range of 0.3 to 3.5 ym.

The absorber material, if a coating, should adhere to the substrate. In

general, the absorber material should be capable of performing its intended

functions when initieilly installed and after long-term exposure to ser- vice conditions (i.e., it should be durable). In particular, the material

should resist degradation resulting from exposure to elevated temperatures,

temperature cycles, ultraviolet (UV) radiation, moisture, and atmospheric

constituents. The material should be chemically and physically compatible

with adjoining materials and degradation products emanating from the

material should not affect the performance of other collector components.

2.2 KEY PROPERTIES

Key properties of absorber materials are solar absorptance (oi) and

Infrared emittance (e). Other relevant properties are adhesion to the

substrate and visual appearance.

2.3 KEY DEGRADATION FACTORS

Key degradation factors which could affect the properties of absorber

materials and thereby reduce their ability to perform their intended

functions were identified as elevated temperature, temperature cycles, UV

radiation, and moisture.

2
Secondary degradation factors Include freezing, oxygen, ozone and airborne

contaminants. Secondary factors were not addressed in this study.

3. LABORATORY AND FIELD STUDIES

3.1 MATERIALS

Absorber materials included in the test program were nickel coated foil,

anodized aluminum, copper oxide, black chrome, polyvinyl idene fluoride

(two), urethane, alkyd, silicone, modified polyester, epoxide, and porcelain

enamel. These were selected to provide a cross-section of generic types

of absorber materials which have been or could be used in solar collectors. Substrates included steel, copper, and aluminum although not all

absorbers were tested on all substrates. Table 1 includes a list of the

materials and the substrates on which they were applied.

A single test specimen was used at each measurement point in the

laboratory tests and triplicate specimens were used in the outdoor exposures.

3.2 TEST METHODS

3.2.1 Property Measurement Tests

Absorptance was calculated from reflectance as measured using ASTM E424-71,

Method A, [2] which utilizes an integrating sphere spectrophotometer, and

total normal infrared emittance was measured using ASTM E408-71 [3].

Absorptance is reported as an integrated value from 0.35 to 2.1 ym for an Air Mass 2 solar spectrum. In addition to optical measurements, the specimens were visually Inspected before and after aging.

3.2.2 Accelerated Laboratory Exposure Tests

The accelerated laboratory exposure tests were selected based upon the

assumption that the primary degradation factors for solar absorber materials are elevated temperature, thermal cycling, ultraviolet (UV)

radiation, and moisture. Temperatures for heat aging of 150, 200 and

250‘*C were selected, based upon absorber temperatures recorded in stagnating collectors and upon calculations of maximum expected temperatures

[4], to provide data over the range of temperatures that can occur under

these conditions.

3.2.2. 1 Elevated Temperature (Oven Aging)

Test specimens were exposed in an oven to elevated temperatures of 150,

200 and 250®C and removed at various time increments for measurement of

properties. While few absorbers in heating and cooling or domestic hot

water applications would be expected to reach temperatures greater than

about 175“C, higher temperatures were used in the laboratory tests to

aid in acceleration of degradation. The 150°C specimens were evaluated

at 1, 3, 11 and 14 weeks, 200“C specimens were evaluated at 1, 2, 4, 6

and 12 weeks and 250°C specimens evaluated at 1, 2, 4 and 12 weeks.

3
3. 2, 2.

2

Thermal Cycling

Test specimens were exposed to thermal cycling with each cycle consisting of:

1. Heating at 200°C for 7.5 hours

2. Cooling to room temperature over a 30 minute period.

3. Cooling to -10°C for 15.5 hours

4. Warming to room temperature over a 30 minute period.

Properties of specimens were measured after 7, 15, 28, 42, and 84 cycles.

3. 2. 2. 3 Ultraviolet (UV) Radiation

Test specimens were exposed to UV radiation using ASTM G26-77 [5], using a Xenon arc accelerated weathering machine of measured solar intensity. A

quartz window was used around the light source and low iron glass was mounted

between the light source and the test specimen to simulate the effect of a cover plate in a solar collector. The air temperature Inside the weathering

machine was approximately 60°C during the exposures. Specimens were removed

at time increments of 125, 625, 1100 and 2100 hours for measurement of

properties.

3. 2. 2. 4 Moisture

Test specimens were exposed in a humidity cabinet at 92®C and 97 percent

relative humidity and removed at time increments of 1, 2, 6 and 21 weeks

for evaluation. Specimens were also tested according to ASTM D2247-68 (1973)

[6], which provides for the formation of condensation on the absorber surface

Specimens were evaluated at time increments of 1, 3, 6 and 12 weeks of con- densation cycle exposure.

3.2. 2. 5 Cumulative Exposure

Tests were performed to expose materials sequentially to elevated temperature

(200®C), thermal cycling, as in 3. 2. 2. 2, UV radiation, as in 3.2. 2. 3 and mois

ture (92°C and 97% r.h. ). Properties of the specimens were measured after

each exposure condition.

This exposure was performed to evaluate the effect of cumulative exposure

relative to the effect of single aging tests described in 3.2.2. 1 through

3.2. 2.4. Exposures were not performed to study the synergistic effects of

combined degradation factors.

3.2.3 Outdoor Exposure Tests

Specimens were exposed outdoors in test boxes which simulate a solar

collector (see figure 1). The exposure boxes included a single, low iron

glass cover plate, a coated metallic mounting plate, and glass fiber insulation behind the mounting plate. For selective coatings, the boxes contained

a black chrome coated mounting plate, while the boxes for nonselective coatings contained a flat black coated mounting plate. Exposures were performed

4
at three sites having distinctly different climates: Gaithersburg, Maryland;

Phoenix, Arizona; and Miami, Florida. The procedure used was in general

accordance with a draft standard being developed by ASTM Committee E44 on Solar Energy Conversion [7], Test specimens (triplicate) were retrieved at

various time increments (approximately every four months) for measurement of

properties.

3.2.4 Outgassing

Several specimens were evaluated at 250"C for outgassing, a phenonemon in

which degradation products condense on the underside of cover plates. The

deposits formed can result in reduced ability of the cover plates to transmit

sunlight. The outgassing test method consisted of collecting condensible

materials on a glass plate of known transmittance. As part of the outgassing

test, the transmittance was remeasured to determine the effect of the condensible materials. The method is further described in an NBS report [8].

Appendix B, Standard Practice for Determining the Effect of Outgassing on

the Transmittance of Glass Test Plates which Simulate Solar Collector Covers,

contains the details of the test procedure used.

4. RESULTS AND DISCUSSION

4.1

ACCELERATED LABORATORY EXPOSURE TESTS

Absorptance (a) and infrared emittance (e) of the absorber materials were determined prior to laboratory exposure using the methods described in 3.2.1.

The optical properties of ten randomly selected specimens of each absorber

were measured to assess the uniformity of the test specimens. With the

exception of the emittance values for absorber C5 (copper oxide), the properties of each absorber material were reproducible within 0.01 unit from specimen to specimen. The average initial absorptance and emittance values of the

ten specimens of each absorber are shown in table 2. The values reported in

table 2 are used as a basis for comparison of absorptance and emittance

before and after accelerated laboratory exposure tests and outdoor exposure

tests. During the study, a set of test specimens was stored as controls in

the laboratory at 20 + 3°C and 50+5 percent r.h. Measurement of the absorptance and emittance after completion of all accelerated laboratory tests

demonstrated that no changes in the optical properties resulted in the control specimens.

4.1.1 Elevated Temperature (Oven Aging)

4 . 1 . 1 . 1 150°C Exposure

The results of oven aging at 150°C for up to 14 weeks are presented in table

3. The absorptance of copper oxide specimens decreased from 0.95 to 0.87

after eleven weeks and was 0.88 after 14 weeks. Black chrome on copper

decreased in absorptance from 0.96 to 0.93 after 14 weeks. The absorptance

of the modified polyester specimens on all three substrates decreased as shown in the table. The 150°C exposure had little, if any, effect on the

absorptance of other absorber materials.

5
The observed change in the infrared emittance of copper oxide specimens must

be interpreted with caution because, as previously noted, the emittance of

unexposed specimens varied considerably from specimen to specimen. Emittance

of other absorber materials was not significantly changed by the exposure.

4. 1.1. 2 200°C Exposure

The results of the 200°C oven aging exposure are summarized in table 4.

Decreases in absorptance of more than 0.01 unit were observed for test specimens of copper oxide, black chrome on copper and modified polyester on steel,

copper and aluminum.

Figure 2 shows the reflectance curves for unaged and aged (12 weeks at 200°C)

copper oxide specimens. Figure 3 shows the shift observed from 0.8-2. lum

in the reflectance curves of black chrome on copper after oven aging at

150°C, 200°C and 250°C.

After 12 weeks exposure, emittance changes of more than 0.01 unit were noted

for copper oxide, polyvinylidene fluoride [on galvanized steel(G9), bonderized steel(Hl) and aluminum (H8)], urethane, silicone on copper and aluminum,

and modified polyester on aluminum. The values for copper oxide are of

questionable significance because of the variability of the absorber. The

G9 polyvinylidene fluoride increased in emittance from 0.85 to 0.91 after 12

weeks exposure while the emittance for the H series specimens decreased.

One absorber material, the epoxide, decomposed during the exposure and was

removed from the copper and aluminum substrates. The same absorber on steel was unaffected by the exposure. Although the nickel foil absorber exhibited

no degradation in optical properties during the exposure, the adhesive which

bonded the foil to the substrate was observed to lose adhesion. As a result,

the foil blistered substantially (See figure 4).

4. 1.1.3 250°C Exposure

The results of the 250“C oven aging exposure are presented in table 5.

Absorbers which exhibited little or no change in absorptance at 250°C were

nickel foil, anodized aluminum, black chrome, alkyd on steel, silicone on

steel and aluminum, and porcelain enamel. Polyvinylidene fluoride (HI and

H8) exhibited little change in absorptance but showed a decrease in emittance

from 0.85 and 0.88 to 0.54 and 0.52, respectively, after 12 weeks.

A number of absorbers exhibited degradation or loss of adhesion during or after the 12 week exposure. These included copper oxide (figure 5), polyvinylidene fluoride on galvanized steel (figure 6), urethane on copper and

aluminum (figures 7 and 8), alkyd on copper and aluminum (figures 9 and 10),

epoxide on all three substrates, silicone on copper and modified polyester

on all three substrates.

Figure 3 shows the reflectance curve for black chrome on copper after the

250°C exposure.

6
4.1.2

Thermal Cycling

The results of the thermal cycling exposures are presented in table 6.

Thermal cycling after 84 cycles, led to a change in absorptance for copper

oxide (from .95 to .86) and black chrome on copper (from .96 to .92); these

changes probably resulted from the high temperature portion of the cycle,

since losses in acceptance were observed during the 200°C oven aging (see

Table 4). Figures 2 and 11 illustrate the effect of thermal cycling on the

reflectance curves of copper oxide and alkyd on steel, respectively.

The modified polyester coating exhibited the most dramatic failure in that,

upon putting the specimens in the freezer during the first cycle, the coating

delaminated. Figure 12 illustrates the appearance after failure. Thermal

cycling also resulted in loss of adhesion of urethane on steel, alkyd on steel and copper, epoxide on steel and copper, and silicone on steel.

4.1.3 Ultraviolet (UV) Radiation

Accelerated UV radiation of more than 2000 hours had little, if any, effect

on the optical properties of the absorber materials, as indicated by the

data in Table 7. Visual inspection of the absorbers after exposure indicated no change in appearance, no loss of adhesion and no surface degradation.

Figures 2 and 11 Illustrates the effect of exposure on the reflectance curves

of copper oxide and alkyd on steel, respectively.

4.1.4 Moisture

Two laboratory tests were performed to determine the effect of moisture

exposures. As described in 3. 2. 2.4, one test consisted of continuous exposure at 92°C and 97 percent r.h. The second included a moisture condensation cycle. Tables 8 and 9 summarize the optical property data obtained

following these exposures.

The moisture exposure for 21 weeks at 92°C and 97 percent r.h. resulted in

decreased absorptance of more than 0.01 unit for urethane on steel (0.97 to

0.95), alkyd on steel (0.97 to 0.90), epoxide on steel (0,97 to 0.90), epoxide

on aluminum (0.97 to 0,90) and modified polyester on aluminum (0.90 to 0.88).

Figures 13, 14 and 15 illustrate the visual appearance of alkyd test specimens

on steel, copper and aluminum, respectively. Surface degradation of the

coating was evident in the 21 week test specimens on all three substrates

and steel and aluminum specimens exhibited metallic corrosion.

Despite the presence of corrosion products on the surface of the alkyd on aluminum specimen, the absorptance and emittance values remained nearly the same as those for unexposed specimens. Figure 11 illustrates the shift in

the reflectance observed for the alkyd on steel following the 21 week exposure at 92°C and 97 percent r.h.

Figures 16 and 17 depict the appearance of epoxide test specimens on steel

and aluminum. Figures 18 and 19 show cross-sections of the unexposed and

exposed epoxide on aluminum, specimens using Scanning Electron Microscopy

(SEM). These latter figures show that the epoxide matrix degraded, cracked

7
and delaminated from the graphite particles as a result of the exposure,

thereby providing a path for moisture penetration to the metallic surface.

Further details of the SEM analysis have been reported by Post and Ballard (9).

Figure 20, of polyvinylidene fluoride on galvanized steel (G9), shows that

the coating lost adhesion after 21 weeks at 92“C and 97 percent r.h. Although

this coating showed little change in absorptance and emittance (Table 8),

degradation had occurred in its bond to the substrate. Figures 21 and 22

show cross-sections of unexposed and exposed polyvinylidene fluoride on galvanized steel (G9) using SEM. The loss of adhesion between the galvanized

surface and the epoxide primer (Arrow B) and between the steel substrate and

the galvanizing layer (Arrow A) may be noted in Figure 22 (9).

Table 9 illustrates that the 12 week moisture exposure using a moisture

condensation cycle led to a decrease in absorptance of more than 0.01 unit

for nickel foil (0.98 to 0.95), anodized aluminum (B6) (0.92 to 0.90) and

black chrome on copper (0.96 to 0.93). Coatings which exhibited change in

absorptance in the moisture exposure at 92°C and 97 percent r.h. (Table 8),

i.e., urethane on steel, alkyd on steel, epoxide on steel and aluminum and

modified polyester, did not exhibit those same changes in the condensation

cycle.

Analysis of cross-sections of the epoxide on aluminum specimens by SEM,

however, showed that the epoxide matrix degraded in the same manner as observed in the exposure at 92°C and 97 percent r.h. For example, when SEM

cross-sections of the epoxide on aluminum specimens exposed for 6 weeks at

92°C and 97 percent r.h. were compared to cross-sections exposed for 6 weeks

using the condensation cycle, degradation of the epoxide matrix was evident

following both exposures, although the extent of the degradation was less

using the condensation cycle.

Also, visual and SEM analysis of the polyvinylidene flouride on galvanized

steel specimens following condensation cycle exposure indicated signs of loss

of adhesion, as had been observed using the 92°C and 97 percent r.h. exposure.

Microscopic analysis of the alkyd on steel specimens following 12 weeks of

condensation cycle exposure indicated signs of surface cracking and it is

probable that continued exposure would have led to the degradation observed

visually and optically after 21 weeks exposure at 92°C and 97 percent r.h.

It appears from these analysis that both moisture exposures induce the same mechanisms of degradation but that the condensation cycle exposure is less severe than the exposure at 92°C and 97 percent r.h.

4.1.5 Cumulative Exposure

As mentioned in 3. 2. 2. 5, tests were designed to study the effect of sequential exposure of the same test specimens to 1) oven aging, 2) thermal cycling,

3) UV radiation and 4) moisture. However, the tests were terminated after

28 cycles of the thermal cycling because of the large number of the failures

which were observed. At the point at which testing was terminated, specimens

had been exposed to 12 weeks of oven aging at 200°C and 28 cycles from 200°C

to -10°C (as described in the procedure for Thermal Cycling in 3. 2. 2. 2).

8
The test specimens used for this test were those that had been previously

exposed to oven aging at 200°C for 12 weeks. Absorptance and emittance

values for these specimens at the end of the 200°C oven aging are given in

table 4.

Upon commencing the thermal cycling phase of the sequential testing, many of

the absorber materials exhibited rapid degradation leading to delamination

from the substrate. For example, figures 23 and 24 show delamination of the

alkyd on aluminum and the urethane on copper after 28 temperature cycles. In

the 84 thermal cycles completed without prior oven aging (table 6), these

same coatings did not delaminate.

At the end of 28 cycles, the following absorber materials remained intact:

anodized aluminum (B6 and B7), copper oxide, black chrome on steel and copper,

polyvinylidene fluoride on steel and aluminum (HI and H8), silicone on copper

and aluminum and porcelain enamel on steel and aluminum. All other absorber

materials had delaminated.

4.2 OUTDOOR EXPOSURE

Table 10 contains the results of outdoor exposures performed on the grounds of

the National Bureau of Standards, Gaithersburg, Maryland. Specimens were

exposed in test boxes as shown in figure 1 to simulate exposure to stagnation

conditions. Tables 11 and 12 summarize the data from outdoor exposures

performed using identical test boxes at Phoenix, Arizona and Miami, Florida.

The Maryland exposures were started in June 1978. Specimens were first

retrieved for evaluation in October 1978 and re-exposed from November 1978 to

March 1979. They were then exposed from April 1979 to August 1979. The

Arizona exposures were initiated in November 1978. The first evaluation was performed in March 1979. The second exposure period included exposure from

April 1979 to August 1979.

The first four months of Florida exposure covered a period from June 1978 to

October 1978. They were re-exposed from November 1978 to May 1979, prior to

evaluation for the second time. During this exposure increment, the specimens were not exposed for a two month period due to storm damage at the test site.

The third exposure period was from July 1979 to November 1979.

The data in tables 10-12 indicate that little change occurred in the absorptance and emittance of most absorbers as a result of the exposures. Changes

in absorptance and emittance are noted for copper oxide but these data must

be interpreted with caution because of the large variability noted in emittance for unexposed specimens. Also, the limited amount of data available

to date makes it difficult to explain the few changes that have occurred.

For example, the absorptance and emittance of modified polyester on copper

(M4) decreased in Arizona but increased in Maryland and Florida. The relatively small changes observed in the outdoor exposure tests indicate that

the accelerated laboratory exposures, which resulted in degradation of some materials (l.e., oven aging at 200°C and 250°C, thermal cycling and moisture

exposure), provide more severe exposure conditions than outdoor exposure in

9
the test boxes. This is not surprising. Measurements of temperatures in

similar test boxes [10] show that attainment of 200°C is unlikely, at least

for extended periods of time. Also, thermal cycles and moisture conditions

experienced in test boxes outdoors are less severe than the accelerated laboratory conditions. The outdoor exposure will be continued to obtain additional data. These additional data will be used to study rates of degradation so that improved comparisons and mathematical models of laboratory and

field test data can be made.

Figures 25 and 26 show the reflectance curves for black chrome on steel (D2)

and copper (D4), respectively, following outdoor exposure in Maryland.

Figure 27 contains reflectance curves for black chrome on copper following

Florida exposures. Figures 26 and 27 show that the observed shift in

reflectance curves for black chrome on steel occurred during the first four

months of exposure.

4.3 OUTGASSING

The outgassing test data are shown in Table 13. The test at 250°G resulted

in no change in transmittance of the glass collecting plate for nickel foil,

anodized aluminum, copper oxide and black chrome. Polyvinylidene fluoride,

urethane, and alkyd caused a decrease in transmittance of 0.4, 1.8 and 5.5

percent, respectively.

5. CONCLUSIONS

1. The data obtained in this study show that the environment experienced

in solar collectors, e.g. elevated temperature, thermal cycling and moisture, can degrade some materials typically used as absorber materials.

2. Degradation of materials, which can be observed visually or with SEM,

does not always result in a change in absorptance or emittance (or in the

reflectance curves from 0.35-2.1 um). It is important, therefore, to

visually inspect exposed specimens and to note observable changes.

3. Accelerated laboratory tests can be used as an effective means of

determining if absorber materials are likely to be degraded by specific

degradation factors, such as high temperature, thermal cycling and

moisture. These test methods, therefore, can provide a basis for

evaluation standards which aid in screening materials.

4. Exposure to elevated temperature is an important test in evaluating

absorber materials. Oven aging at 150, 200 and 250°C showed that, in

general, the absorbers studied are stable up to 150“C. At temperatures

of 200°C or higher, however, the polymeric materials may be susceptible

to degradation resulting from thermal decomposition.

5. Thermal cycling proved to be an important test to aid in identifying

potential adhesion problems which may be accentuated by expansion and

contraction.

10
6. The accelerated UV radiation exposure of more than 2000 hours had little

effect on the absorptance and emittance of the materials studied. Also,

none of the materials exhibited visually detectable changes such as loss

of adhesion, as a result of the exposures. It is uncertain whether the

test, as performed, was not effective in inducing UV degradation or whether the absorber materials included in the study are not likely to

be degraded by UV exposure.

7. Moisture exposure is an important test in evaluating absorber materials.

Of the two test methods used, the exposure at 92°C and 97 percent r.h.

appears to be a more severe test than the condensation cycle exposure.

8. Cumulative exposure testing appears to be much more severe than exposure

of specimens to single degradation factors. Degradation induced in sev- eral absorber materials during the 12 week oven aging at 200°C decreased

their ability to withstand thermal cycling.

9. The substrate to which a coating is applied affects its resistance to

the various expsoure tests. Therefore, it is essential to evaluate

absorbers on the substrates to which they will be applied in solar

collectors.

10. Little change was observed in the absorptance, emittance and appearance

of most specimens exposed outdoors to stagnation conditions at three

locations, Maryland, Arizona and Florida. This indicates that the outdoor exposure test is less severe for some absorber materials than the

accelerated laboratory exposures. Despite the fact that little change

was observed in the properties of most test specimens during outdoor

exposure, the test method is a valuable tool for evaluating absorbers

because the exposure conditions are more closely related to real-life

than the accelerated laboratory exposures. Since correlations have not

yet been established between laboratory tests and in-service performance,

stagnation test data are particularly useful in selecting absorbers for

specific applications.

11. The test procedure developed during this study to evaluate the effects of

outgassing on the transmittance of a glass plate appears to provide an effective tool for identifying, in the laboratory, potential outgassing

problems. Future work is needed to compare results of the test with

these obtained in operational solar collectors.

11
6. PROPOSED STANDARDS

6.1 STANDARD PRACTICE FOR EVALUATING SOLAR ABSORPTIVE MATERIALS FOR

THERMAL APPLICATIONS

Appendix A is a proposed practice for evaluating solar absorptive materials

for thermal applications.

The practice provides two methods for evaluating absorber materials. Method 1 (Laboratory Exposure) includes exposure of separate test specimens to elevated

temperature, thermal cycling, moisture and UV radiation. Test temperatures

are linked to in-use or stagnation temperatures for the materials to be

evaluated. Moisture exposure utilizes an environment at 90+ 5®C and 95+ 5

percent r.h. This exposure is the more severe of the two methods used in

this study. Steps 1 (heating) and 3 (cooling) of the thermal cycling procedure were shortened from 7.5 and 15.5 hours, respectively, as used in

the laboratory study, to one hour each. This permits the accumulation of an increased number of cycles per unit time.

Method 2 provides for outdoor exposure for approximately 12 months and

references the practice being developed by ASTM E44 (7) and utilized in this

study for outdoor exposure.

Before and after exposure, absorber materials are evaluated for reflectance,

emittance and visual appearance.

The practice, as outlined, excludes its use for 1) concentrating collectors

with concentrating ratios exceeding 5, 2) absorbers used in direct contact

with, or suspended in, a heat transfer fluid and 3) absorbers used in

evacuated collectors.

6.2 STANDARD PRACTICE FOR DETERMINING THE EFFECT OF OUTGASSING ON THE

TRANSMITTANCE OF GLASS TEST PLATES WHICH SIMULATE SOLAR COLLECTOR COVERS

Appendix B is a proposed test method for determining the effect of outgasslng

on the transmittance of glass test plates which simulate cover plates used in

solar collectors.

The method consists of heating a test specimen and collecting the condensible

outgasslng products on a glass plate. A filtered air stream serves to cool

the glass plate. The transmittance of the glass plate is measured, using

ASTM E424, Method A, before and after the test.

Key components in the test apparatus include a hot plate, a specimen holder,

a Teflon gasket and sleeve, a glass test plate, a filtered air stream and an Instrument for measuring the temperature of the test specimen.

12
7 BIBLIOGRAPHY

1. Skoda, L. F. and Masters, L. W., "Solar Energy Systems - Survey of Materials Performance", NBSIR 77-1314, National Bureau of Standards,

Washington, D.C., October 1977.

2. E424-71, Test for Solar Energy Transmittance and Reflectance (Terrestrial)

of Sheet Materials, Annual Book of Standards, American Society of Testing

and Materials, 1916 Race Street, Philadelphia, Pa.

3. E408-71, Test for Total Normal Emlttance of Surfaces Using Inspection

Meter Techniques, Annual Book of Standards, American Society for Testing

and Materials, 1916 Race Street, Philadelphia, Pa.

4. Clark, Elizabeth J., Roberts, Willard E., Grimes, John W. and Embree,

Edward, J. , "Solar Energy Systems - Standards for Cover Plates for Flat

Plate Solar Collectors," Technical Note 1132, National Bureau of Standards,

Washington, D.C., December 1980.

5. G26-77, Standard Practice for Operating Light-Exposure Apparatus (Xenon-Arc

Type) With and Without Water for Exposure of Nonmetallic Materials,

Annual Book of Standards, American Society for Testing and Materials,

1916 Race Street, Philadelphia.

6. D2247-68 (1973), Testing Coated Metal Specimens at 100 Percent Relative

Humidity, Annual Book of Standards, American Society for Testing and

Materials, 1916 Race Street, Philadelphia, Pa.

7. Draft Standard Practice for Evaluating Absorptive Solar Receiver Materials

When Exposed to Conditions Simulating Stagnation in Collectors with Cover

Plates, Under development by ASTM E44 on Solar Energy Conversion, available

from ASTM, 1916 Race Street, Philadelphia, PA 19103.

8. Stiehler, Robert D., Hockman, Arthur, Embree, Edward J., and Masters,

Larry W. , "Solar Energy Systems - Standards for Rubber Seals", NBSIR

77-1437, National Bureau of Standards, Washington, D.C. , March 1978.

9. Post, M. and Ballard D. , "Degradation of Solar Coatings Observed by

Scanning Election Microscopy", Journal of Coatings Technology , 52 (664),

pp. 55-62, May 1980.

10. Waksman, David, National Bureau of Standards, Washington, D.C., Private

Communication.

13
TABLE 1. ABSORBER MATERIALS AND SUBSTRATES USED IN THE STUDY

Code Number 1 / Absorber Material

A8 Nickel coated :foil

B6 Anodized aluminum

B7 Anodized aluminum

C5 Copper oxide

D2 Black chrome

D4 Black chrome

G9 Polyvinylidene fluoride

HI Polyvinylidene fluoride

H8 Polyvinylidene fluoride

11 Urethane

14 Urethane

18 Urethane

J1 Alkyd

J4 Alkyd

J8 Alkyd

K1 Epoxide

K4 Epoxide

K8 Epoxide

LI Silicone

L4 Silicone

L8 Silicone

Ml Modified polyester

M4 Modified polyester

M8 Modified polyester

N2 Porcelain enamel

N8 Porcelain enamel

The letter refers to the absorber material and the

Includes 0.5 mil Nickel flash.

' Includes 0.1 - 0.2 mil Nickel flash.

Substrate

Aluminum, alodlne

Aluminum, 6061, anodized

Aluminum, 1100, anodized

Copper

Steel, cold-rolled^'

Copper, DHP 122 ' 20 gage, G90, hot dipped

galvanized steel

Steel, bonderized

Aluminum, aIodine

Steel, bonderized

Copper, DHP 122

Aluminum, alodine

Steel, bonderized

Copper, DHP 122

Aluminum, alodine

Steel, bonderized

Copper, DHP 122

Aluminum, alodine

Steel, bonderized

Copper, DHP 122

Aluminum, alodine

Steel, bonderized

Copper, DHP 122

Aluminum, alodine

Steel, cold-rolled

Aluminum, alodine

number to the substrate.

14
TABLE 2. ABSORPTANCE AND INFRARED EMITTANCE OF

ABSORBERS PRIOR TO EXPOSURE

Code Number Absorber Material Absorptance (a) Emlttance (e)

A8 Nickel foil 0.98 0.09

B6 Anodized aluminum 0.92 0.81

B7 Anodized aluminum 0.90 0.82

C5 Copper oxide 0.95 0.74

D2 Black chrome 0.95 0.07

D4 Black chrome 0.96 0.06

G9 Polyvinylidene fluoride 0.95 0.85

HI Polyvinylidene fluoride 0.96 0.85

H8 Polyvinylidene fluoride 0.96 0.88

11 Urethane 0.97 0.92

14 Urethane 0.97 0.92

18 Urethane 0.97 0.92

J1 Alkyd 0.97 0.92

J4 Alkyd 0.97 0.91

J8 Alkyd 0.97 0.92

K1 Epoxide 0.97 0.91

K4 Epoxide 0.97 0.91

K8 Epoxide 0.97 0.91

LI Silicone 0.97 0.88

L4 Silicone 0.97 0.87

L8 Silicone 0.97 0.88

Ml Modified polyester 0.93 0.86

M4 Modified polyester 0.89 0.84

M8 Modified polyester 0.90 0.81

N2 Porcelain 0.93 0.85

N8 Porcelain 0.93 0.84

Emittance for 10 specimens of this sample varied from 0.65 to 0.80.

15
Table

3.

RESULTS

OF

150°C

OVEN

AGING

EXPOSURE

LO

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—I OOOOOvOOOOO o^a^o^o^o^a^o^o^o^oooooooooooooooo

00 ^ < 0> vD vO CO O 00 00 ^ O O 00 m

00

00

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0)o

C

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4-1 w

-H --I m r>. v£) 'O' O 00 00 vO O O 00

CM

00

00

00 iCMcMogcM'—i--(--4-H'^OOvOOOiO'0''0'tO'0' ON(Ti(T\ONCT\cy\(T><y\ONOOOOOOOOOOOOOOOO

w

w

o

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H

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(U CU (U M M

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z

16
TABLE

4.

RESULTS

OF

200“C

OVEN

AGING

EXPOSURE

00

00

H o^o^Or^'0L^lLrl<J

ONaN<y'ONONaNO> + + ooooc^oooooooooo

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O'O'O'OOO'O'O' O' O' O'lO'O'O'O'O'O' + O'O'O'O'O'OOOOO'O'

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(U (U

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<! 00 CO u Q Q o 33 33 M M M T-) •“3 lic! hJ 2 2 2 z z CJ> 2

17

+

designates

removal

of

absorber

material

during

exposure.
TABLE

5.

RESULTS

OF

250°C

OVEN

AGING

EXPOSURE

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O CM O 00 vO lO o^ + ^-o^^-^- + ^- + o^ + oo + + + oooo

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(U 01 0) M u

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t-H o rH tH iH >N d d

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CO 4J T3 73 a o o o >N o >N o 4J 4J U t>N >N >N X X X •H •H *H • • • O 3

JO ccJ o O O a- ccJ cd rH 3 iH D iH 3 (U (U (U JlJ M J<i o o O rH rH H T3 x» T3 M d

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z < <! CJ PO PQ CM Oi Cu CM [H 03 00 < < c M t=d w CO CO CO a a CL4 O.

M

0) (U 03-0 OOOr^LOCM<tCT\ —4 00 ^<f0O-J'^0O-H-^0O^'0'0O'—t-CfCJOCMCX) OS <dC5cquqQCJ U3 PO 3

z

18

+

designates

removal

of

absorber

material

during

exposure
TABLE

6.

RESULTS

OF

THERMAL

CYCLING

CM CM nO m pH m <r pH pH CJN CJN LO 00 00 nO o o ON 00 00 + CJN CJN + + CJN + CJN + CO CO + + + 00 00 00 • « • • • • • • • • • • • • 9 • •

On •H CM m NO NO NO CM 'S- pH CM pH CO CO CO <r O 00 00 NO o o 00 00 00 CJN CJN CJN ON CJN + + CJN CO CO CO + + + CO CO

• • • • e • • • • • • • • • • • 9 9 9 •

00 <y> pH *H <r nO NO o CJN fO m ro o pH o CO CO 00 NO in

CM o 00 oo NO O o 00 00 00 ON CJN CJN CJN + CJN CJN + CjN CO CO CO CO + 00 CO CO

U)

0>oc o f-H 00 nO NO <r 00 CM CO ro iH pH fH ^H pH CO CO in m pH 00 00 vO o o 00 00 00 CJN CJN CJN CJN CjN CJN CJN CJN ON CO CO 00 CO + 00 CO CO

4J

•H0

u

a\ o CJN NO <r CO CJN CM CM CO pH o o o NO m 00 o 00 00 o o 00 00 00 CJN ON CJN CJN CJN CJN CJN CJN CJN CO CO CO 00 + CO CO 00

CO

b] ON CM NO m lO 00 CM CM CM CM iH CM pH f“H iH CO CO vO pH m <r O o o 00 00 o o 00 00 00 CJN CJN CJN CJN ON ON ON CJN CJN CO CO 00 CO CO CO CO CO

><u

o

<r 00 CM o nO CM m NO NO 1^ cn

aS 00 C7N ON CJN 00 CJN CJN CJN CJN CJN + CJN CJN + + CJN + + CJN + CJN ON + + + CJN CJN bl • • • • • • • • • • • • • • • • 9 Z CM ON pH o CM m NO NO r>. CO cn

C3N C3N CJN 00 CJN CJN CJN CJN CJN CJN CJN CJN CJN + CJN + + CJN CJN CJN ON + + + CJN CJN

• • • • • • • • • • • « • • • • • • • •

O

00 CM o m NO <r m nO NO CO CO CO CO CM CO cn

00 C3N ON CJN 00 CJN CJN CJN CJN CJN CJN CJN CJN ON + ON CJN + CJN CJN CJN CJN CJN + CO ON CJN OJ CM o

c

cC 4ba

w in 00 pH ON 00 NO m to NO NO r>. O CJN o C3N (3N 00 00 CJN CJN ON CJN CJN CJN ON CJN CJN CJN ON CJN CJN CJN CJN CJN CJN CJN + CO ON CJN CO JQ<

00 »H o o m m m m 00 cn CM

CJN C3N ON CJN CJN ON CJN CJN CJN CJN CJN CJN CJN CJN ON CJN CJN CJN CJN CJN ON CJN + CO CJN CJN

o 00 CM o lO U-) nO m NO NO r>. CO CJN o CO cn

<J\ C3N CON C3N CJN ON ON CJN CJN CJN CJN CJN ON CJN CJN CJN CJN CJN CJN CJN ON CJN z CJN ON CJN

cu (U cu 3 3 3

a c 3 3 3 3

cu cu 9i cu cu cu 4J 4J 4J 0 0 T) "TJ TJ CO W 3

•H •H o o •H •H •H 3 3 3

)b rH o X >-i fH r-H iH 0^ >. >N 3 3

<u <c Eh 'O TO o Xi cu CU >N cu cu CU 3 3 3 3 Z *H iH •H •H CU CU CD CD c T3 c 3 z 3 3 3 3 3 3 3 3 3 O o o CO 3

U4 rH N N >-l •H •H •H •iH •H •H 3 3 3 Z Z z O O O a a a rH rH O (U (U •H •H cu M > V4 > M > V4 Z Z Z z Z Z •rj •H •H CJ O O 3 3

CO 4-) M T3 o a O O >N o >N O >N o 4-> 4b 4J >N X X X •H •H •H • • • O O

eO O O o a CCJ rH 3 rH 3 iH 3 CU 3 3 M Z M o o o rH rH rH z z z 3 3

C Z •H C c o rH rH o rH o rH o rH V4 U U iH iH iH a a a •H •H •rH o o o O O Z <; c o PQ PCJ a Eh a Eh CM Eh Z z Z < <3 <3 w z w CO CO CO g g g a a

cu CU t3 J3 00 vO m CM CJN pH 00 pH <r 00 *H <r CO fH 00 fH 00 pH CO CM 00 O 0 <3 CO PQ o Q Q o Z z M M M z z Z z z z g g g z z

Z

19

+

designates

removal

of

absorber

material

during

exposure.
TABLE

7.

RESULTS

OF

ULTRAVIOLET

(UV)

RADIATION

EXPOSURE

<uo

c

cO

4-1 4-1

c/1

Pli O

X

bJXX

c/1 O

X

X

w

[jLi o

<uo

c

CO

4-1 a.

)-i o

CO X

<

o

o

cN

O

cso

m

CN

o

o

o

o

cn

CN

CN

>-i (U CU T3 X) O S

CJ 3

z

o <31 CO CO m

00 c» sO O O 00 m

00 oi

00 CNCNCN^^CN-M^—toooooor^CT>ONCTNCTiCTiaiONO\ON30 00 00 00

01 1—I •—I ^ vO v£> -O’ O 00 00 O O oo 00 Ol

00 cNcNCNr-4-—

0^0^0'CT><3'O^CJ^a^O^OOOOOOOO

-O’ -O’ 00 00

m -O’ 00 00

00 -o o oo 00 o o 00

-O’ 00

00

00 fNcNCN^-^-H^^.—tOOr^OOX) o^e3^a^cJ^cT\a^o^<J^o^oooooooo

Q Q

W W

CZ) crt o o m -O’ 04 CM 00 00 X X! • • w w

01 »“H •—i f-H \0 vO o 00 00 r-. o o 00 m

00

00

00 CNCNCN^^—4r-(--4.—ir-.cOOOiO

CT><T>CT><TiCTiCT>0101CTv<30000000

H

O

z

m -i’ 00 00

o^ CN lO O 00 00 r-^ O o 00 m

00

0000 CNCNCNCN'^CN'-H'-^^OOI’^OOvO-d’^m'd’

CJS<T>CT>CT\CTiOlCy>CT>CT>00C0 00 00 00 00 00 00

00 CN o X) cn -d’ cn O' CT> 0^ 01 Cl 01 CT^ m

O' mO' r'-.oooooor-.oooooooot'^r-^r'.cnO' O' O' O' O' O' O' O' O' O' O' O' O'

00 (N O r-. iD CN lO

O' O' COi O' O' O' O' mCTi lO

O' r'-r'-r^oor’-r-'-r— r^cxit-'-i'^i-'-ro O'O'O'O'O'O'O'O'O'O'O'O'O'

O' CJi

-cf -Cf cTi cy.

00 CN o m cn -d’ O' O' O' O' CT> O' O' m

COl mCON

r-« r-. 00

O' O' O' O' O'

Q Q

W W

r-.r^r^r-sr^r^r^cNc/lc/i O'O'O'O'O'O'O'O'OO

04&4

X X

Cd W

CJl O'

00 CN o CO -d’ m

O' O' O' O' O' O' O'

H H

o o lO vo r^r’^r^oooor^t-^cof^r^r-r'-.cozZ'd’CN

O' O' O' O' O' O' O' O' O' O' O' O' O' O' O' O' O'

00 rg O cO lO vO lO O' O' O' O' O' O' O'

vOO'vO

O'

I— r^f'»r-.r^r---r’-.r^r''.r''.r’^r^coo'Ocnco

O'O'O'O'O'O'O'O'O'O'O'O'O'COO'O'O'

CUaCUaCUc (U CU

CU (U CU CU (U CU 4J 4-1 4J rH X) a a XI X) TO CO CO CO •H •H o o -H *H •H y y y

Ml rH O X Ml Ml iH iH rH X X X 3 3

CU cd X) XJ o X! X X CU X CU X (U CU CU CU (U 3 y rH rH iH •H •H X •H CU (U O C-) c X) c Xl c xl 3 3 3 CU 3 CU C3 3 3 O O O cd Cd Ml U rH N N Ml •H •iM •H •H *H •H cd 3 3 X) t3 •Xi o o o om 04 CM iH rH O <D 0) *H •H (U X X > Ml > Ml > Ml X X X X3 TO xl -i-j •H •i y o y y

CO 4J X» TO CM O o X O X O X O 4-) 4-1 4-> X X X X X X -H •H •H • • • y y

JO cd a O O CM cej cd iH 3 tH 3 rH 3 CU <U 3 X X

a

o O iH 1—

1

fH XJ 'O TO Ml Ml < iz: •H c c O rH iH O iH o iH o rH Ml Ml Ml iH rH CM CM -H •H •H o o o o o

'Z < < CJ P5 cd CM fci CM (H CM, Cm CO CO CO <! <! <: w W W C/1 CO CO s z s CM CM

00 cO CO CN -ct O'

<; CQ OQ CJ Q Q O

00

re

4 -d’ 00 •—

I

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20
—< ocM'croor^r^^ m on POcofOr^'-H-HON-^rgoor^oovooo‘J^LO

cNi ooooo>^oooo 00 00 onononooosonooononoooooooooooooooo

nO ON 00 v£> 00 m 00 CN on CM F^ 00 00 vO o A A

NO o 00 00 m o O 00 00 00 ON ON CJN ON CON CON CJN ON CON 00 00 00 00 00 00 00 00

/T«\ w

'w'

(Uo

q 00 PO 00 m UO CM ON ONl CM CM r—

o O'. f'» f-H cn F-^ A <}

(0 fNj o 00 00 NO o o 00 00 00 ON <0N CON CJN CON (ON CON CON ON 00 00 00 00 00 00 00 00

4J >< 4J HM

Q b3

l-H Si 00 00 CO r-H nO m ONj ON CN CM CM cn «-H 1—

f-H o r>- (ON CM ^H A 'd- /-> o 00 00 nO o O 00 00 00 CJN (ON ON (ON (ON CON CON CON ON 00 00 00 00 00 00 00

3d CO

3^

Cd Cd >M s

H

<c ON <N 'd‘ nO m m 00 CM CM CM CM r~4 CM 1-H I-H 00 00 v£> -d- I-H A

Id o o 00 00 o o 00 00 00 ON (ON CON CJN CON ON CJN ON CON 00 00 00 00 00 00 00 00 W oS

as 30

CO H OOi w ><S o u 00 CNl CO -d- m m NO lA o oo 00 o 00 o vO CO CM CO vO

DS CN ON ON ON ON ON ON ON ON ON ON (ON CON ON CON CJN CON CON (ON (ON (ON (ON ON ON 00 ON CJN w bt4 0( O

ON s

Q H 00 oo 00 m NO to m m m p^ 00 00 i-H 00 00 vO CO F—H CJN -d- A

nO ON ON ON ON ON ON C3N ON ON ON CON ON CON (ON CON ON (ON CJN CJN CON (ON ON (ON 00 (ON ON o /—S O 8

CMON

0) 00 OM 'd- nO <! m m 00 00 <>0 o ON -d- mT H u es| C3N ON C3N ON ON ON ON ON ON CJN CON CJN CON CON CON CON CJN ON (ON CJN (ON (ON (ON 00 CJN (ON <3 0

0

w 4J aS a

30 u

c/3 o o CO ON m •<0 UO <r m m r*^ 00 00 CO 00 00 '-O 00 o -d- a. ON ON ON ON C3N ON ON ON ON ON CON CON CON CON CJN ON CON CON CJN (ON (ON ON 00 CJN ON ON

X! C W

[jc-i o

CO 00 CN o m to NO m NO NO CO ON o CO CO H o ON ON ON C3N ON ON ON ON ON (ON (ON CJN ON ON (ON CON CON CON CON (ON (ON ON 00 (ON CJN (ON kJ3

c/5 w

0(S 0 0 0 X X X

0 0 0 0 0 0

• CU (U (U 0 0 0 X X X

00 TO a a TJ TO TO w m M

•H •H o o •H •H •H 0 0 0

Cd l-l iH o d iH T—

H >% >N 0 0

J <u cfl Id TJ TO O X X 0 0 0 0 0 0 0 0 0 iH H tH •H •rH cd XI •H (U cu u o 0 TO 0 TO 0 X 0 0 0 0 0 0 0 0 0 O O O 0 0

< X N N d •H •H •H •H •H H 0 0 0 X X X O o O Ph CM (d rH t—

H o (U (U •H •H 0 X X > 0 > U > 0 X X X X X X H •H *rl 0 CJ U 0 0

CO 4J a<! TJ •CJ O. o CJ >N o !X o O X X X >, >% >> X X X •H •H • • • O o X cd o o O a 0 0 rH 3 tH 3 iH 3 0 0 0 X X M o o o tH H rH X X X X

<; s: •H 0 0 o rH rH o iH O I—

O rH u X X tH tH iH cx a a •H •H •H o o o O o Z <3 <3 o « M p-( d Oh d a, pC4 X X X <3 <3 <3 w w w C/3 CO CO 23 23 23 Oh Oh

(U (U TJ X X X A CM <t (ON 1—

CO fH -cf CO i“H CO F“H 'd' CO iX <!• 00 i-H 'd- CO CM CO o a < M CJ Q Q o X X M M M X X X i4 3»S 3.d X X 23 23 23 Z Z

CJ 3

2:

21
TABLE

9.

RESULTS

OF

EXPOSURE

USING

A

CONDENSATION

CYCLE

CM C» O vO vO O <Ti r>. O O 0000

00 cMCNCNj^-H^^T^r-nr^r^r^^omfOLnun

o^o^o^o^a'0^o^o^o^oooooooooooooooo

LT) CM —I 00 lO O 00 00 1^ O O 00 00

o^

00 CMCMCM^—<^^00\£>^OOOr^lO.-HLn-4'

a^CJ^C3^0^C3^CT^CJ^a^CJ^OOOOOOOOOOCX5oOOO

(Uo

c

to (T) cr>-^cMinoor^m vO oo cMcMCM.-Hr^^ooOvOvOr^inrO'^Ln'^ ooooo^oooo 00 00 ONO\cTicr>ONcri<y>cT>aNoooooooooooooooo

•Hsw

COW

c7'cMco-<foooOLr» 'vO O' cMCMCM»-H--^^.-<-Hf-iv^vot^r^cMinLn'd’ O 00 00 O O 00 oo 00 O'O'O'CJ'O'O'CJ'O'O'OOOOOOOOr^OOOOOO

w

od

COo

a,XW

o

o

cTi^cM-^tr'.'sOLn m oo cmcmcmcM'-hcM'—<^»-H oor^oO'0 'i-'-<LO'd' OOOOOr^OOOO 00 00 O'O'O'O'CJ'O'O'CJ'O'OOOOOOOOOOOOOOOO

CM moOr^r^fOLTi m \0 r^r^r^oooooor^oooor^r^r^'d’cocMMi-cM

I—I O' O' O' O' O' O' O' O' O' O' O' O' O' C7' O' O' O' O' O' O' O' O' O' O' ^ O'

H >.0 0'^-^t^OL^^L^ ld lo r^r^r^r^oor^r^f^oO'O'O'OMt’CMOO'^co

O'O'O'O'O'O'O' O' O' O'O'O'O'O'O'O'O'O'O'O'O'O'O'OOO'O'

a

(U as <N 00 lO in sO sD 00 00 00 00 00 00 v£) viJ sO CO o CO o

c

ro as O' O' O' as (S' O' as Os CT» O' as Os as Os O' O' O' O' as as Os as O' as O'

cd

4-) a

o

CO as m m sO m m m 00 00 00 00 sO sO <f

JO OS as O' O' O' as as O' CTi CTn as cyi O' O' O' as O' CTl O' as Os O' 00 as cyi O'

<d

o

cocMOmcnvOLn o sO r^r^r^r^r^p^r^f^r^r^i— r>»cno'Oc»'ro

O'O'O'O'O'O'O' O' O' O'O'O'O'O'O'O'O'O'O'O'O'O'OOO'O'O'

cu CU (U G G G

c c G 3 3 3

cu cu CU (U cu cu G G G

a a 'O '3 3 3 3

•H •H o o •H •H •H 3 3 3

)-l 1—

1

0 X jj iH iH rH X En E>^ G G

d) CCJ U-i t3 TO o JG X! X CU >s cu X CU 3 3 3 3 3 3 iH rH rH •H •H JO •H 0) (U u O G TJ G TO G -3 G G G 3 3 3 G G G O O O 3 3

>-l u rH N N Jj •H *H "rJ •H H •H 3 3 3 TO TO TO O O O CG o. CL, rH rH o cu (U •H •i-t (U J«S > Vj > VJ > J2 JG JG TO TO •H •H *H O O O 3 3

CO XJ -o T3 a O o X o X O En O 4-) 4-) 4J tn En X X X •H •H •H • • • o O

JO CCJ O O O a cd cd -H 3 iH 3 rH a 3 3 3 o o o rH rH rH TO TO TO G G

< s •H CJ C o rM tH O rH O rH o 1—

i

H G G ^ rH rH a. G. 04 •H •H •H o o o O O

:z; <1 c o 00 CQ Oj Ct( CU (H CL, CO CO CO <3 <3 < u w w CO CO C/3 S a a O, CG

>-i (U (U T3O B U 0

z

oO'Or^LOCM'd'O' —

t

Cpqdqcjqqo pd 00 ^-d-OOr-H-^tOO-—•'CfOO'-H'd'QO^'i'OO CM 00

22
TABLE

10.

RESULTS

OF

OUTDOOR

EXPOSURE

AT

GAITHERSBURG,

MARYLAND

cn

00

r>. 00 CMCNJCS-HO—<— <OOLO'do^o^o^o^o^a^0'0^a^ooooooo'o^o^oooo <n

cs o -H ^ iT> m

^ 00 00 m o o 00

(Uo

C

ttj 4-) u

w

COw

!3w

CO0

01d

pX4 o

•

CM00 ooooomoooo 00

CM CM CM -H <-H .-4 —

CO ON ON o^ o^ -HOooi^ooooooom<- o^o^ooooooo^oooooooo

CM ooooomoooo 00 m CM CM CM ^ ^ CM 00 0'0^0^0^0'0^0^ -^-Hr>»r^r^oom'^tn'<r O^O^OOOOOOOOOOCOOOQO

CT\ >—I CM vo cn m

ooooor^oooo 00

00 CMcMCMCM>-ICMO-H-HCOr^CXDv£)-^-^in<l00 O^O^O^O^O^O^O^O^O^OOOOOOOOOOQOoOOO

00-hO—ivocncn m

o^o^o^o^o^o^o^ o^ m r^r^r^oooooooo

o> o^o^o^Q^o^a^o^ oooor^t^r^<J’^o<rcn0^0^0^0^c3^0^0^0^0^0^

CMoo

<T> ^ O

0\ ON o>

.H r>. cn lO

ON ON ON ON m

C3N nOON r>.r*.r^c»oOr^oOoooor^r^r^-<r.-H'—

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(UCJc

CO 4Ja

V4o

CO Xi<

ON CM o CM cn m

ON ON ON ON ON On ON mCJN NO

ON r>.r>.r>.oooor^oooooor^t^r^'d'00-cr'dON ON ON ON ON ON ON ON ON OnONONONONONONON

00 CM o m cn no ON ON O' ON ON CJN ON

NO

CJN NO

CJN r>.r^r^r^i^r^r^r^r'.rN.r^i^cnoNOcncn

OnOnOnonOnonOnonOnonOnonOnoOOnonOn

(U <U (U >-l C3 c 3 3 01 3

(U <u 0) (U 0) (U 4J 4-1 4-1 iH TJ a a -o T3 *3 (0 CO CO •H •H o o •H •H •H 3 3 3

tH O X u V4 rH f—r—

>N >N >N 3 3

(U cd (X4 TJ -o o X X ^ <U 0) >N 3 3 3 3 <U 3 3 rH iH iH *H •H XI •H 0) (U CJ CJ fl T3 13 T3 3 'O 3 8 3 3 3 <U 3 3 3 O O O 3 3

fH N N u •H *H •H •H •H •H 2 3 3 13 13 13 O O O Ol, Oh CL, rH tH O <U (U •H •H <v X X > J-i > > X X X *3 T3 13 •H •H •i-t o O CJ 3 3

CO 4-1 T3 'O cx o o >N O >N O tn O 4J u 4J >N >N >N X X X •H •H •H • • • O 3

XI cd O O O a cd cd ^ 3 iH 3 rH 3 3 <u lU X X o o o rH iH rH T3 X X )-i Vh c s •H a C3 o iH rH O .H O r—

o iH M t-4 tH rH a o. 3- •H •H •H O o o O O

IS <j <J CJ oq PQ (^4 pI4 CL4 [X4 04 Cl4 30 33 33 •< < <3 Ci3 Cd fd C/3 Cd C/3 S s CL, CL,

01 TJo

CJ

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CU Xie

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CM CO z z —« CM cn

23

Exposed

from

6/78

to

10/78.

Exposed

from

11/78

to

3/79.

Exposed

from

4/79

to

8/79.
TABLE

11.

RESULTS

OF

OUTDOOR

EXPOSURE

AT

PHOENIX,

ARIZONA

(Uy

c

(0 4J

4J 1

03

CM00

<uo

c

«u

a

h

o

(0

<!

CM

oo

01 0) T3 JO o a O 3

z

0\ ^ -H vO vO 00 4‘ o 00 00 m o o 00 o00 00 OOO^O^O^O^O^O^CT^O^oOQOoOOO^>.OOoOOO

CT» I—I .-H -<f ON o 00 00 m o o 00 CO

00

0000

CM CM CM *-H 1—4 '—4 t—4

I—4 OQ —4 lO o^o^o^a^o^cJ^o^a^o^Qooocooor^oocooo

cri-^cM-4-r^vom m oo cmcmcmcm-—tcM—'^^oof^oo^<i-^LO-<f O 00 00 O O 00 00 00 CTiCTNa\CTi<Tia\C^<TiON0000Q0000000Q000

00 -H o^ ON ao m m

0^ ON 00 00 ON o> O' m

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CJN r^r^r^oooooooooooo

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OnONONOncOOOONON

ON CM O —I 0~| ON ON ON ON ON ON On m

ON mON P^ooi^oof^i>»o0oooor^f^r^^i/Nr^'^ro onononononononononononononooooonon

00 CM o m m vo m

ON ON On On On ON ON

v£> ONNO

ON r^r^t^r^r^r^r^r^r^r^r^r^fOONOfOco

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33

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1

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00 U 0

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C3 C3 3 3 01 0> 00 y (U y y 0) 0) MJ 4J MJ CJN

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1

no a a no no ns CO CO CO

•fH •H o o •H •H •H 01 y 3

1—

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i-i iH N N •H •H •H •H •H •H 2 3 to ns ns ns O O o Cl, 3i CM, iH rH o 0 o (U (U •H •H (U M > u > V4 > 33 33 33 ns ns nS •H O U o CU 3 Ml Ml (A U M no no o. o o >N o o >N O 33 +J 4J >N >N >N -H •H •H • • • O O NM C|-l 33 to O O O Cu tt) CO rH 3 rH 3 rH 3 d) 03 0) 3«S M M o o O rH rH fH ns ns Ml Ml <3 S3 •H C C3 o iH rH O rH o rH O iH Ml Ml Ml I—

I

1—

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rH CM CM Dr rH •H O o O O O ns ns <3 <3 CJ CO OQ 04 U4 CL, IX| CM Cl, tZ3 to to <3 C <3 W W 03 W o> C/3 S3 S3 S3 CM CM (U

CO

CU

to O 0

o. a

X X u w

CM

24
TABLE

12.

RESULTS

OF

OUTDOOR

EXPOSURE

AT

MIAMI,

FLORIDA

(Uo

a

nJ

4-) iJ w

COH

Z

§

w

COo

D-i X

w

0)o

c

to 4Ja

u

o

CO C

CNl

CM

CO

fO

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r-H T3 s a 'O t3 T3 CO to CO •H •H o o •H •H •H tu tu tu iH O X I—

1

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X X X d d

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1

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40 to o O O a cd tO f-4 3 rH 3 cH 3 tu CU (U o o o rH rH rH '3 t3 TO 54 <J Z •H C3 (3 o iH iH o iH O i-H O iH >.) 1.4 Ui r-4 H a a. a •H •H •H O O o O O

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33 sfoo-^^o0’^<fco -H<f00^<f0OCMOO J J J s Z z Z CM C^

25

Exposed

from

6/78

to

10/78.

Exposed

from

11/78

to

5/79

(with

a2

mo.

non-exposuro

period).

Exposed

from

7/79

to

11/79.
TABLE 13. RESULTS OF OUTGASSING TESTS AT 250^0

%

Code

Number

Absorber

Material

%

Condensibles

Loss in

Transmittance

A8 Nickel Foil 0 0

B6 Anodized Aluminum 0 0

C5 Copper Oxide 0 0

D2 Black Chrome 0 0

HI Polyvinylidene 0.01 0.4

Fluoride

11 Urethane 0.1 1.8

J1 AIkyd 0.2 5.5
I

I

Figure I- Test Box Used for Outdoor Exposures

27
33NVl331J3d

2b

Figure

2.

Reflectance

of

Exposed

and

Unexposed

Copper

Oxide
33NV1331J3H

29

Figure

3.

Reflectance

of

Black

Chrome

on

Copper

Before

and

After

Oven

Aging
I

1

'1

Figure 4. Blistering of Nickel Foil During Oven Aging

I

30
Figure 5. Appearance of Copper Oxide Specimens Before and After 250°C

Exposure

Figure 6. Appearance of Polyvinylidene Fluoride (G9) on Galvanized Steel

Before and After 250°C Exposure

31
Figure 7

Figure B

Appearance of Urethane on Copper Before and After 250°C

Exposure

OVEN AGiNG

Appearance of Urethane on Aluminum Before and After 250°C

Exposure

32
OV\H M'JUU

j'ji, C

Figure 9. Appearance of Alkyd on Copper Before and After 250°C

Exposure

Figure 10. Appearance of Alkyd on Aluminum Before and After 250°C

Exposure

33
CM

csi

33NVl331J3d

34

Figure

11.

Reflectance

of

Exposed

and

Unexposed

Alkyd

on

Steel
\

I

I

Figure 12. Loss of Adhesion of Modified Polyester Coating During Thermal

Cycling Exposure

Figure 13. Appearance of Alkyd on Steel Before and After Exposure at

92°C and 97 Percent R. H.

35
Figure 14. Appearance of Alkyd on Copper Before and After Exposure at

92°C and 97 Percent R. H.

*• K

Figure 15. Appearance of Alkyd on Aluminum Before and After Exposure

at 92°C and 97 Percent R. H.

36
I

I

Figure 16. Appearance of Epoxide on Steel Before and After Exposure at

92®C and 97 Percent R. H.

Figure 17. Appearance of Epoxide on Aluminum Before and After Exposure

at 92°C and 97 Percent R. H.

37
Figure 18. Cross-Section of Unexposed Epoxide Coating with Phenolic

Butyrate Primer on Aluminum (300 x)

Figure 19. Cross-Section of Epoxide Coating and Primer on Aluminum

Following 21 Weeks Exposure at 92 °C and 97 Percent R. H.

38
Figure 20. Appearance of Polyvinylidene Fluoride on Galvanized Steel

(G9) Before and After Exposure at 92°C and 97 Percent R. H.

39
Figure 21. Cross-Section of Unexposed Polyvinylidene Fluoride Coating

with Epoxide Primer on Galvanized Steel (G9) (1000 x)

Figure 22. Cross-Section of Polyvinylidene Fluoride with Primer on Galvanized Steel (G9) Following 21 Weeks Exposure at 92°C and

97 Percent R. H.

40
I

Figure 23. Appearance of Alkyd on Aluminum Before and After Cumulative

Exposure (12 Weeks at 200°C Followed by 28 Temperature Cycles

from 200°C to -10°C)

Figure 24. Appearance of Urethane on Copper Before and After Cumulative

Exposure (12 Weeks at 2U0°C Followed by 28 Temperature Cycles

from 2U0°C to -10°C)

41
33NV1331J3U

42

Figure

25.

Reflectance

Curves

of

Black

Chrome

on

Steel

Before

and

After

Outdoor

Exposure

at

Gaithersburg,

Maryland
33NV1331d3d

43

WAVELENGTH

|/zmj

Figure

26.

Reflectance

Curves

of

Black

Chrome

on

Copper

Before

and

After

Outdoor

Exposure

at

Gaithersburg,

Maryland
M

(U 4J

4-1 Cc

nJ

0)o

4-1(U CQ

(UaC4ooco6

4i XiuXo

nj rH

oq

4-1o

CO

cu>

i-l 3u

(Uo

C3

cd

4-1 o

(U

i-H 4-1(U OS

CM

(U

>-l D

60

*H

33NV1331J3U

44

Outdoor

Exposure

at

Miami,

Florida
APPENDICES

Appendix A

Appendix B

- Standard Practice for Evaluating Solar Apsorptanc

Materials for Thermal Applications

- Standard Practice for Determining The Effect of

Outgassing on the Transmittance of Glass Test

Plates Which Simulate Solar Collector Covers
i

p

\

'-’aA •:'> i J-'fi iqqA 'C^ari'^'rfT itolt ?r.hH?»Ji.

. v'iH3 „>rn’ jrfirt»^«aal»C to t''«3oi:,n^o*i b-rftbfrnil' -a^T sHiiXO 1,0 aoiTbJJltf.^a'.vr i/rfj no go -0 loJaaUfO ^O?. ? oirfw 0Si/;r«r

\

i;

<!

fe s ^

0*1 CM

'

'

v'i

>..• ,f„-- «. .« ./•'•“». V,

ft.; '«J^: ^ \

'i: <
APPENDIX A

STANDARD PRACTICE FOR EVALUATING SOLAR ABSORPTIVE MATERIALS FOR THERMAL

APPLICATIONS

1 . SCOPE

1.1 This practice provides a testing methodology for evaluating absorptive

materials used in flat plate or concentrating collectors, with concentrating

ratios not to exceed 5, for solar thermal applications. This practice is

not Intended to be used for the evaluation of absorptive surfaces that are

1) used in direct contact with, or suspended in, a heat transfer liquid (l.e. , trickle collectors, direct absorption fluids, etc.), 2) used in evacuated

collectors or 3) used in collectors without cover plate(s).

1.2 Test methods included in this practice are property measurement tests

and aging tests. Property measurement tests provide for determination of

various properties of absorptive materials, e.g., absorptance, emittance and

appearance. Aging tests provide for exposure of absorptive materials to

environments that may induce changes in the properties of test specimens.

Measuring properties before and after an aging test provides a means of

de .erminlng the effect of the exposure.

1.3 The assumption is made that solar radiation, elevated temperature, temperature cycles and moisture are the primary factors that cause degradation of

absorptive materials. Aging tests are described for exposure of specimens to

these factors. For some geographic locations, other factors such as salt

spray and dust erosion, may be important.

2. SIGNIFICANCE AND USE

2.1 The methods in this practice are intended to aid the assessment of longterm performance by comparative testing of absorptive materials. The results

of the methods, however, have not been shown to correlate to actual in-service

performance.

2.2 The testing methodology in this practice provides two testing methods

(See Figure A-1). Method 1, which aims at decreasing the time required for

evaluation, uses a series of individual tests to simulate various exposure

conditions. Method 2 utilizes a single test of actual outdoor exposure under

conditions simulating thermal stagnation. Equivalency of the two methods has

not yet been established.

3. APPLICABLE DOCUMENTS (Latest editions shall apply unless otherwise specified)

3.1 ASTM DOCUMENTsL/

i/ Available from ASTM, 1916 Race Street, Philadelphia, PA 19103

A-1
B537 - Rec. Practice for Rating of Electroplated Panels Subjected to Atmospheric Exposure

EA08 - Test for Total Normal Emittance of Surfaces Using Inspection-Meter

Techniques

E42A - Test for Solar Energy Transmittance and Reflectance (Terrestrial) of

Sheet Materials

EA3A - Test for Calorimetric Determination of Hemispherical Emittance and the

Ratio of Solar Absorptance to Hemispherical Emittance Using Solar

Simulation

G23 - Rec. Practice for Operating Light and Water-Exposure Apparatus (CarbonArc Type) for Exposure of Nonmetallic Materials

G26 - Rec. Practice for Operating Light-Exposure Apparatus (Xenon-Arc Type)

with and without Water for Exposure of Nonmetallic Materials

3.2 ANSI DOCUMENTS^/

Z97. 1-1975 - Performance Specifications and Methods of Testing for Safety

Glazing Material Used in Buildings

A. TEST SPECIMENS

A.l Test specimens shall be either the coating applied to a specific substrate or the absorber material (for materials other than coatings).

A. 2 The specimens shall be prepared in accordance with procedures and conditions used (or expected to be used) in commercial practice or in accordance

with the recommendations of the coatings or material supplier.

5. CONDITIONING

5.1 Specimens shall be measured and tested as received without additional

processing or pre-conditioning.

6. METHODS OF TEST

6.1 PROPERTY MEASUREMENT TESTS

Perfora all property measurement tests at room temperature unless otherwise

specified.

2/ Available from the American National Standards Institute, Inc., 1A30

Broadway, New York, N.Y. 10018

A-2
6 . 1.1 Absorptance

Method E424, Method A, unless otherwise specified.

6.1.2 Emittance

Method E434 or Method E408, unless otherwise specified.

6.1.3 Appearance

Method B537, unless otherwise specified.

6.2 AGING TESTS

6.2.1 Effect of Solar Radiation (Light Fastness)^/

Note 1 - Expose materials with a cover plate between the light source and

the test specimen if 6.2. 1.1 or 6.2. 1.2 are used. The use of a tempered glass cover plate with spectral transmission characteristics approximating those of low iron glass, as shown in

Figure A-2, is recommended. Transparent plastics or other types

of glass may be used for the cover plates if the absorber material

under test is used with these cover plate materials in actual

solar collectors.

6. 2. 1.1 Outdoor Exposure Under Simulated Stagnation Conditions

Expose test specimens for a minimum period of twelve months using ASTM , Standard Practice for Evaluating Absorptive Solar Receiver Materials when

Exposed to Conditions Simulating Stagnation in Collectors with Cover

PlLe(s)^/.

Note 2 - It may be desirable to continue exposures of test specimens

beyond the time period recommended to obtain additional rate

data or to obtain data on mechanisms of degradations.

Note 3 - This method is not applicable to concentrating collectors with

a concentration ratio greater than 5.

6. 2. 1.2 Outdoor Exposure Using Fresnel Concentration

Expose test specimens to direct sunlight reflected from a Fresnel concentrator

as referenced in ANSI Z97. 1-1975, paragraph 4.3.2, in accordance with the

conditions stipulated in Table A-1.

Incident radiation should be monitored using a sensor meeting the requirement of a WMO 2nd class pyranometer.

This test method is currently being developed by ASTM E44.04. It provides for outdoor exposure of absorber specimens in a box which simulates

a solar collector.
6. 2.

1.3

Xenon Arc Exposure

Expose the specimens to radiation from a filtered Xenon arc lamp in accordance with the conditions stipulated in Table A-1.

6.2.

1.4

Carbon Arc Exposure

Expose test specimens to radiation from a filtered Carbon arc lamp in accordance with the conditions stipulated in Table A-1.

6.2.2 Effect of Elevated Temperatures

Test specimens shall be exposed to heat aging for a period of 500 hours at

the stagnation temperature to which they will be exposed in actual service.

Note 4 - The maximum service temperature will normally be obtained when the

collector is receiving its maximum level of solar radiation and the

heat transfer fluid is not flowing through the collector. Data on stagnation temperatures are normally available from collector manufacturers. Also, stagnation temperature data for flat plate collectors are included in NBSIR 78-1562, "Interim Performance Criteria

for Solar Heating and Cooling Systems in Residential Buildings,"

National Bureau of Standards, Washington, D.C. 20234.

6.2.3 Effect of Moisture

Test specimens for 30 days at 90 + 5‘*C and 95 + 5 percent relative humidity.

6.2.4 Effect of Thermal Cycling

Test specimens shall be exposed to 30 cycles of heating and cooling with each

cycle consisting of the following steps:

1. Heat for one hour at the stagnation temperature to which they

will be exposed in actual service (see Note 4).

2. Cool to room temperature over a 30 minute period.

3. Cool at -10°C and 70+5 percent r.h. for one hour.

4. Warm to room temperature over a 30 minute period.

6.2.5 Outgasslng

Release of volatiles from the absorber material, in some cases, may be a significant factor that can affect the transmittance of a cover plate, or

the properties of the absorber itself, or both. A generally applicable

procedure is not presently available to evaluate effects of outgasslng

products. It is essential, however, that this factor be recognized.
7 . TEST PROCEDURE

7.1 Select either Method 1 (7.2) or Method 2 (7.3).

7.2 Method 1 (Laboratory Exposure)

7.2.1 Select 5 sets of test specimens for each absorber material to be evaluated. Each set shall consist of at least three identical test specimens.

7.2.2 Determine the absorptance and emittance of the specimens in accordance

with 6.1.1 and 6.1.2. Also document the appearance of the specimens in accordance with 6.1.3.

7.2.3 Store Set 5 specimens (controls) at ambient laboratory conditions.

7.2.4 Expose Set 1 specimens in accordance with 6.2.2.

7.2.5 Expose Set 2 specimens in accordance with 6.2.4.

7.2.6 Expose Set 3 specimens in accordance with 6.2.3.

7.2.7 Expose Set 4 specimens in accordance with one of the exposures described in 6.2. 1.2, 6.2. 1.3 and 6.2. 1.4.

7.2.8 Determine absorptance and emittance and document appearance as in 7.2.2.

7.3 Method 2 (Outdoor Exposure)

7.3.1 Select 2 sets of test specimens for each absorber material to be evaluated. Each set shall consist of at least three identical test specimens.

7.3.2 Determine the absorptance and emittance of the specimens in accordance

with 6.1.1 and 6.1.2. Also document the appearance of the specimens in accordance with 6.1.3.

7.3.3 Store the Set 2 specimens (controls) at ambient laboratory conditions.

7.3.4 Expose Set 1 specimens in accordance with 6.2. 1.1.

7.3.5 Determine absorptance and emittance and document appearance as in 7.3.2.

8. REPORT

8.1 The report shall Include the following:

8.1.1 Complete Identification of the absorber material tested.

8.1.2 Identification of which Method, 1 or 2, was used. If Method 2 was

used, the report shall include data required by ASTM EXXXX (see footnote 4).

A-5
8.1.3 Complete identification of the substrate(s) used for absorber coatings^

including composition and thickness.

8.1.4 Complete identification of and spectral transmission characteristics

of cover plate material used in 6.2. 1.1 or 6.2. 1.2.

8.1.5 Method of preparing test specimens.

8.1.6 Location and dates of exposure if 7.3, Method 2 (Outdoor Exposure) is

used.

8.1.7 Total radiation exposure received by test specimens in 6.2. 1.1, length

of time of exposure and documentation of Instrument and procedure for measur'*

Ing radiation exposure.

8.1.8 Length of time of exposure in 6.2. 1.2, 6.2. 1.3, or 6.2. 1.4 as appropriate.

8.1.9 Identification of filters used in 6.2. 1.3 or 6. 2. 1.4

8.1.10 Temperatures used in the exposures described in 6.2. 1.2, 6.2. 1.3,

6. 2. 1.4, 6.2.2, 6.2,3 and 6.2.4, Also heating and cooling rates and number

of cycles used in 6.2.4,

8.1.11 Moisture conditions used in the exposure described in 6.2.3.

8.1.12 Number of specimens tested.

8.1.13 Test results (absorptance, emittance and appearance) before and after

aging (mean and range) for exposed specimens and for controls.

8.1.14 Documentation from specified procedures /conditions (agreed upon or unitentional).

8.1.15 Any readily apparent change in appearance or condition (fading, blistering, rusting, etc.

)

of individual specimens or groups of specimens*

A-6
Table A-1. Exposure Test Conditions

Reference Paragraph 6.2. 1.2 6.2. 1.3 6.2. 1.4

Radiation Source Concentrated Sunlight Xenon Arc Carbon Arc

Standard Practice (ASTM) — G26 G23

Method A A

Apparatus Type Fresnel Mirror

(8x)

A, AH, B, BH e(0, eh

Filters Borosilicate^ Borosilicate^^^

Irradiance at 340nm, W/m^ .55

Radiant Energy Interval Uncontrolled Continuous Continuous

Black Panel Temperature, “C 90 + 5 90 + 5 90 + 5

Relative Humidity Uncontrolled Uncontrolled^ Uncontrolled(^)

Irradiation at 340nm, kJ/m^ 2970

Exposure Interval, hours 1100 1500 1200

Water or air heaters may be required to maintain temperature

Corning 7740 glass for Xenon Arc and 7058 glass for Carbon Arc is satisfactory.

Operate with humidifier turned off.
Figure

A-1.

Outline

of

Test

Method

Options

SELECT

EITHER

METHOD

1

OR

METHOD

2

A-8

DETERMINE

ABSORPTANCE,

DETERMINE

ABSORPTANCE,

EMITTANCE

AND

APPEARANCE

EMITTANCE

AND

APPEARANCE

AS

APPROPRIATE

AS

APPROPRIATE
PHOTON

ENERGY

|eV)

(%) BONVillNSNVdl

A-9

Figure

A-2.

Transmittance

of

Single

Strength

Low

Iron

Glass

APPENDIX B

STANDARD PRACTICE FOR DETERMINING

THE EFFECT OF OUTGASSING ON THE TRANSMITTANCE

OF GLASS TEST PLATES WHICH SIMULATE SOLAR COLLECTOR COVERS

1.

Scope

1.1 This method covers the determination of the effect of outgasslng on

the transmittance of glass test plates which simulate cover plates used

in solar energy collectors. It is applicable to polymeric materials,

such as rubber seals, insulation and absorptive coatings, which may

release condensible materials upon heating.

1.2 This method can be used to compare the relative effect of volatile

condensible degradation products of specific polymeric materials on the

transmittance of glass test plates.

2. Summary of Method

2.1 The material to be evaluated is heated in a test apparatus as described in Section 5.1. The condensible decomposition products are collected on a glass plate. The transmittance of the glass plate is

measured from 0.3 to 2.1 pm before and after testing to determine

any change in transmittance resulting from the deposited products. The mass loss of the polymeric test specimen and the mass gain of the glass

plate are also determined.

3. Applicable Documents

ASTM E424, Test for Solar Energy Transmittance and Reflectance

(Terrestrial) of Sheet Materials.

4. Description of Terms

4.1 Outgassing - The emission of gases by materials and components,

usually during exposure to elevated temperature or reduced pressure.

5. Apparatus

5.1 Outgassing apparatus - a test apparatus containing a controlled

temperature heating device, a temperature monitor, an enclosed specimen

chamber, a test specimen holder, a glass plate upon which volatiles are condensed and a filtered air stream directed toward the external surface

of the glass plate. Figure B-1 is a diagram of the Individual components

of the appartus and Figure B-2 shows the assembled apparatus.

5.2 Spectrophotometer - a spectrophotometer as described in ASTM E424.

B-1
6.

Test Specimen

6.1 The test specimen shall consist of a sample of the material.

6.2 Specimens of absorptive coatings, which are applicable to metallic

substrates, shall consist of both the coating and the substrate.

6.3 Specimens of absorptive coatings, which are normally applied to

polymeric materials for use in solar collectors, should be applied on a metallic substrate as in 6.2

6.4 The size of specimen should be approximately proportional to the

ratio of apparatus glass plate area to cover plate area of solar

collector.

7. Conditioning

7.1

Test specimen shall be conditioned at room temperature prior to

testing.

8. Procedure

8.1 Measure the transmittance of the glass plate from 0.3 to 2.1 ym

according to ASTM E424,

8.2 Weigh the test specimen and the glass plate of the test apparatus to

the nearest 0.00001 gm.

8.3 Adjust the temperature of the test apparatus to the desired test

temperature prior to inserting the test specimen and Installing the

glass plate.

8.4 Place the test specimen in the specimen holder and Install the

glass plate over the teflon gasket using the spring clips.

8.5 Turn on the air stream, adjusted to 70 CFH, and direct the flow to

the center of the exterior surface of the glass plate.

8.6 Allow the test to proceed for 3 hours or the time mutually agreed

upon by the purchaser and the seller.

8.7 Stop the flow of the air stream, turn off the hot plate and remove

the glass plate and the test specimen.

8.8 Allow the test specimen and the glass plate to reach room temperature and weigh both.

8.9 Re-measure the transmittance of the glass plate from 0.3 to 2.1 ym

according to ASTM E424,

B-2
9 Calculations

9.1 Integrate the area under the transmittance curves obtained in 8.1

and 8.9 and calculate the difference between the integrated values.

9.2 Calculate the mass gain of the glass plate and the mass loss of the

test specimen.

10. Report

10.1 The report shall include the following:

10.1.1 Identification of the test specimen, including type of material,

source or manufacturer, product identification, batch or lot serial

number.

10.1.2. Identification of metal used if the test specimen is an absorptive coating applied to a substrate.

10.1.3. Conditioning procedure prior to testing.

10.1. A, Identification of type of glass used as the glass plate, test

temperature and test duration.

10.1.5. Test specimen mass before and after test and mass loss.

10.1.6. Glass plate mass before and after test and mass gain.

10.1.7. Integrated transmittance from 0.3 to 2.1 ym of the glass

plate before and after testing and the difference between the two values.

B-3
I.

Figure B-1. Components of the Outgassing Test Apparatus.

Figure B-2. Assembled Outgassing Test Apparatus.

I

iiJ

« B-4
NBS>114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS 81>2232

2. Govt Accession No.

isiSiSigijSsgKiSg

^ 4. TITLE AND SUBTITLE 5. Publication Date January 1981

Solar Energy Systems - Standards for Absorber Materials Perfonnjng Organization Code

7. AUTHOR(S) 8. Performing Organ. Report No. L.W. Masters, J.F. Seiler, E.J. Embree, W. E. Roberts

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSrr«,B<, City, Srafe, ZIP)

Iftilt Ho.

11. Contract/Grant No.

13. Type of Report & Period Covered Department of Energy

Office of the Assistant Secretary

Conservation and Solar Applications

20-Mas s-achusetts Avenoe -----W<a-h4-ftgtont D*

15. SUPPLEMENTARY NOTES

m4&

Final

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or tess factual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here,) Absorber materials used in solar heating and cooling systems absorb energy from the

sun and convert it to thermal energy. These materials must perform their intended

functions both when first installed and after extended use. However, the environment

in which absorber materials are exposed can cause degradation and loss of ability to

function. Numerous problems with absorber materials in solar energy systems have

demonstrated the need for standards to assess their performance and durability.

A study was performed to aid in the development of accelerated test methods needed for

the evaluation of absorber materials and to incorporate the methods into draft

standards for consideration as consensus standards by the American Society for Testing

and Materials (ASTM).

After identifying the performance requirements for absorber materials, laboratory

and field studies were performed to measure performance according to the requirements.

The data obtained, using twelve absorber materials, were used as the technical basis

for two draft standards.

This report presents the results of the research, including the proposed draft

standards

.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Absorber materials; accelerated test methods; durability; solar energy; standards

18. AVAILABILITY QQ Unlimited 19. SECURITY CLASS 21. NO. OF

(THIS REPORT) PRINTED PAGES

1 1 For Official Distribution. Do Not Release to NTIS UNCLASSIFIED

fin r 1 Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20. SECURITY CLASS

(THIS PAGE)

22. Price

20402, SD Stock No. SN003-003- $8.00

Order From National Technical Information Service (NTIS), Springfield, VA. 22161 UNCLASSIFIED

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