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No. 1132

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TMENT OF commerce/ National Bureau of Standards

Solar Energy SystemsStandards for Cover Plates

For Flat Plate Solar Collectors
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act ot Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods ot measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities' — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering- — Mechanical

Engineering and Process Technology' — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gailhersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. Some divisions within the center are located at Boulder, CO 80303.
Solar Energy Systems—Standards for Cover Plates

for Flat Plate Solar Collectors

Elizabeth J. Clark

Willard E. Roberts

John W. Grimes

Edward J. Embree

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

Prepared for:

Department of Energy

Office of Solar Applications for Buildings

Office of the Assistant Secretary Conservation and Solar Applications

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued December 1980
National Bureau of Standards Technical Note 1132

Nat. Bur. Stand. (U.S.), Tech. Note 1132, 157 pages (Dec. 1980) CODEN: NBTNAL

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1980

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $5.50

(Add 25 percent for other than U.S. maiUng)
ABSTRACT

Laboratory studies were performed to obtain data needed for the development

of standards to evaluate the performance and durability of cover plates for

flat plate solar collectors used in solar heating and cooling systems. Ten

cover plate materials were evaluated to assess their durabilty after exposure

to heat aging, natural weathering and accelerated weathering. Laboratory

tests included measurement of solar energy transmittance, linear dimensional

stability, warpage and the effect of the dirt retention. The temperatures

cover plate materials attain on solar collectors were determined by measurement and by computer simulations. A procedure was developed for the natural

weathering exposure of cover plate materials at elevated temperatures which

aproximate stagnation conditions of solar collectors.

The results of the laboratory tests are presented and draft standards for

evaluating cover plate materials for flat plate solar collectors are

proposed.

Key words: Cover plate durability; cover plate materials; cover plate

standards; standards; weathering of cover plates.

Ill
PREFACE

The primary purpose of the research described in this report was to obtain

laboratory data needed to develop the technical basis for standards for

solar collector cover plate materials, and not to evaluate commercially

available cover plate materials. However, in order to develop the needed

data, it was essential that a variety of cover plate materials be tested.

No attempt was made to test every type of cover plate material on the

market. The materials selected were typical of those commercially available in 1976. Many cover plate materials are marketed and, in order to

improve their performance, manufacturers sometimes modify the materials.

The properties of a material can be changed by alterations in minor

constituents (i.e. iron oxide content, stabilizers, plasticizers, antioxidants) or in processing techniques. The properties of materials currently

on the market may differ from those described in this report. The reader

is cautioned against direct application of these data to materials currently

on the market.

iv
ACKNOWLEDGEMENT

The authors gratefully acknowledge the contributions of Mr. James Seller and

Ms. Anita Sweigert of NBS for assisting with the laboratory tests described

in this report. The authors also thank Mr. Aaron Dawson of Virginia Polytechnic Institute for performing the computer calculations to estimate

cover plate temperatures of plastic and glass materials and the calculations of the heat loss coefficients and variable angle exposure in

Appendix B. The authors are indebted to Mr. Larry Kaetzel of NBS for his

assistance in linking the spectrophotometer with the computer and his

computer program development in handling data.

In addition, the valuable review comments on draft standards provided by

members of ASTM Committee E44 Solar Energy Conversion are gratefully acknowledged. The authors also thank the personnel at South Florida Test Services,

Inc. and at DSET Laboratories, Inc. for their cooperation in maintaining the

outdoor exposure boxes. The authors also thank the many manufacturers of cover plate materials who supplied materials utilized in this research.
Table of Contents

Page

ABSTRACT iii

PREFACE Iv

ACKNOWLEDGEMENT v

1

.

INTRODUCTION 1

1 .

1

Background 1

1.2 Objectives and Scope • 2

2. PROBLEM ASSESSMENT 3

2.

1

Field Problems 3

2.2 Performance Requirements 5 2.3 Cover Plate Properties 6

2.3.1 Primary Properties 6

2.3.1.1 Solar Energy Transmlttance 6

2.3.1.2 Dimensional Stability 6

2.3.1.3 Tensile Properties 7 2.3.1.4 Hall Impact Resistance 7

2.3.2 Secondary Properties 7

2.3.2.1 Transmlttance of Long Wavelength Radiation .... 7 2.3.2.2 Abrasion Resistance 8

2.3.2.3 Static Electric Charge Buildup 8

2.4 Degradation Factors 8

2.4.1 Solar Radiation 8

2.4.2 Temperature 8

2.4.3 Synergistic Reactions 10

2.4.4 Other Factors 10

3. LABORATORY STUDIES and FIELD EXPOSURES 11

3.1 Introduction 11

3.2 Aging Procedures to Simulate Degradation Factors 11

3.2.1 Heat Stability Aging Procedure 11

3.2.2 Artificial Weathering with Xenon Arc Light 12

3.2.3 Natural Weathering Exposure ,, 12

VI
Table of Contents (con't)

Page

3 .3 Property Tests 14

3.3.1 Solar Energy Transmit tance 14

3.3.1.1 Method A - Integrating Sphere Spectrophotometer 15

3.3.1.2 Method B - Outdoor with Pyranometer 15

3.3.1.3 Effect of Dirt Accumulation on Solar Energy

Transmittance 16

3.3.2 Linear Dimensional Stability 16

3.3.3 Warpage 16

3.4 Materials 18

3.4.1 Selection 18

3.4.2 Preparation of Test Specimens 18

3 .4 .3 Control Specimens 18

3.5 Schedule of Aging Procedures and Property Evaluation Tests .... 20

4. RESULTS AND DISCUSSION 20

4.1 Control Test Specimens 20

4.1.1 Solar Energy Transmittance 20

4.1.1.1 Method A - Spectrophotometer 20

4.1.1.2 Method B - Outdoor with Pyranometer 23

4.1.1.3 Comparison of Results of Method A and

Method B 23

4.1.2 Infrared Transmittance 23

4 .2 Heat Aging 27

4.2.1 Solar Energy Transmittance 27

4.2.1.1 Exposure at 150°C (302°F) 27

4.2.1.2 Exposure at 125°C (257°F) 31

4.2.1.3 Exposures at 100°C (212°F) and 75°C (167°F) ... 31

4.2.1.4 Comparison of Exposures 37

4.2.2 Linear Dimensional Stability 37

4.2.3 Warpage 44

Vll
Table of Contents (con't)

Page

4.3 Artificial Weathering with Xenon Arc Light 46

4.3.1 Solar Energy Transmittance 46

4.3.2 Warpage 50

4.4 Natural Weathering 50

4.4.1 Temperatures oz Minicollectors 50

4.4.2 Cumulative Solar Radiation 52

4.4.3 Visual Observation 52

4.4.3.1 Minicollector Boxes 52

4.4.3.2 Test Specimens 52

4.4.4 Solar Energy Transmittance 57

4.4.4.1 Gaithersburg, Maryland Exposures 57

4.4.4.2 Miami, Florida Exposures 63

4.4.4.3 New River, Arizona Exposures 72

4.4.4.4 Comparison of Exposures 72

4.4.5 Warpage 85

5

.

CONCLUSIONS 85

5.1 Measurement of Transmittance 85

5.2 Accelerated Laboratory Testing 86

5.3 Outdoor Weathering 87

6. ADDITIONAL RESEARCH NEEDS 87

7. PROPOSED STANDARDS 88

7.1 Standard Practice for Exposure of Cover Materials for Solar

Collectors to Natural Weathering Under Conditions Simulating

Stagnation Mode 88

7.2 Standard Practice for Evaluation of Cover Materials for Flat

Plate Solar Collectors 88

8

.

REFERENCES 89

APPENDIX A Determination of Cover Plate Temperatures Al

APPENDIX B Proposed Standard Practice for Exposure of Cover Materials

for Solar Collectors to Natural Weathering Under Conditions

Simulating Stagnation Mode Bl

APPENDIX C Proposed Standard Practice for Evaluation of Cover Materials

for Flat Plate Solar Collectors CI

viii
List of Figures

Page

1. Damage to 0.13 mm (0.005 in) Poly (ethylene terephthalate) Film

Caused by Hailstorm in New River, Arizona 4

2. Spectral Distribution of Sunlight at Air Masses 1,2 and 3 9

3. Minicollector for Natural Weathering Exposure 13

4. Apparatus for Warpage Measurement 17

5a. Spectral Transmittance of Control Test Specimens of Two Glass

Materials 21

5b. Spectral Transmittance of Control Test Specimens of Four Plastic

Films 21

5c. Spectral Transmittance of Control Test Specimens of Two Clear

Plastic Sheet Materials 22

5d. Spectral Transmittance of Control Test Specimens of Two Fiber

Reinforced Plastics 22

6a. Infrared Transmittance of Control Test Specimens of Two Glass

Materials 25

6b. Infrared Transmittance of Control Test Specimens of Two Plastic

Films 25

6c. Infrared Transmittance of Control Test Specimens of Two Plastic

Films 25

6d. Infrared Transmittance of Control Test Specimens of Two Plastic

Sheet Materials 26

6e. Infrared Transmittance of Control Test Specimens of Two Fiber

Reinforced Plastics 26

7a. Spectral Transmittance of 1.5 mm (0.06 in) Fiber Reinforced Plastic

Exposed to 150°C (302°F) for Four Time Periods 29

7b. Spectral Transmittance of 0.10 mm (0.004 in) Poly(vinyl fluoride)

Exposed to 150°C (302°F) for Four Time Periods 29

8. Appearance of Poly(vinyl fluoride) After Exposure at 150°C (302°F)

for Various Periods 30

9a. Rate of Solar Energy Transmittance Loss of Five Cover Plate

Materials Exposed at 150°C (302°F) 32

ix
List of Figures (con't)

Page

9b, Rate of Solar Energy Transralttance Loss of Five Cover Plate

Materials Exposed at 125°C (257°F) 34

10a. Spectral Transmittance of 0.10 mm (0.004 in) Poly (vinyl fluoride)

Exposed to Four Temperatures for Approximately 2000 Hours 38

10b, Spectral Transmittance of 0.13 mm (0.005 in) Poly (ethylene

terephthalate) Exposed to Four Temperatures for Approximately

2000 Hours 38

10c. Spectral Transmittance of 0,076 mm (0,003 in) Acrylic Film

Exposed to Three Temperatures for Approximately 2000 Hours 39

lOd. Spectral Transmittance of 1.5 mm (0.06 in) Poly(methyl

methacrylate) Exposed to Three Temperatures for Approximately

2000 Hours 39

lOe, Spectral Transmittance of 1,0 mm (0,04 in) Polycarbonate Exposed

at Two Temperatures for Approximately 2000 Hours 40

lOf, Spectral Transmittance of 1,0 mm (0,04 in) Fiber Reinforced

Plastic Exposed at Four Temperatures for Approximately 2000

Hours 40

lOg, Spectral Transmittance of 1.5 mm (0.06 in) Fiber Reinforced

Plastic Exposed at Four Temperatures for Approximately 2000

Hours 41

11a. Linear Dimensional Changes of Fluorinated(ethylene propylene)

Copolymer Exposed at 150°C (302°F) 43

lib. Linear Dimensional Changes of Acrylic Film Exposed at 150*'C

(302°F) 43

12a, Appearance of Poly(methyl methacrylate) After 150°C (302°F) Heat

Aging 45

12b, Appearance of Polycarbonate After 150°C (302°F) Heat Aging 45

13a. Spectral Transmittance of 1.0 mm (0,04 in) Polycarbonate After

Exposure in a Xenon Arc Artificial Weathering Device 48

13b. Spectral Transmittance of 1.0 mm (0.04 in) Fiber Reinforced

Plastic After Exposure in a Xenon Arc Artificial Weathering

Device 48
List of Figures (con't)

Page

13c. Spectral Transmittance of 0.13 nun (0.005 in) Poly(ethylene

terephtalate) After Exposure in a Xenon Arc Artificial Weathering

Device 49

14. Minicollectors Exposed in Gaithersburg, Maryland 54

15. A Double Cover Minicollector with Poly (vinyl fluoride) Covers

After 18 Months Exposure at New River, Arizona 56

16. Comparison of the Spectral Transmittance of Control with Washed

and Unwashed 0.10 mm (0.004 in) Poly(vinyl fluoride) Exposed

Two Years in Gaithersburg, Maryland 60

17a. Spectral Transmittance of Washed 3.2 mm (1/8 in) Glass (0.01%

iron oxide) After Two Years Outdoor Weathering 79

17b. Spectral Transmittance of Washed 3.2 mm (1/8) Glass (0.10% iron

oxide) After Two Years Outdoor Weathering 79

17c. Spectral Transmittance of Washed 0.025 mm (0.001 in) Fluorinated

(ethylene propylene) Copoljrmer After Two Years Outdoor

Weathering 80

17d. Spectral Transmittance of Washed 0.10 mm (0.004 in) Poly(vinyl

fluoride) After Two Years Outdoor Weathering 80

17e. Spectral Transmittance of Washed 0.13 mm (0.005 in) Poly(ethylene

terephthalate) After Two Years Outdoor Weathering 81

17f. Spectral Transmittance of Washed 1.5 mm (0.06 in) Poly(methyl

methacrylate) After Two Years Outdoor Weathering 81

17g. Spectral Transmittance of Washed 1.0 mm (0.04 in) Polycarbonate

After Two Years Outdoor Weathering 82

17h. Spectral Transmittance of Washed 1.0 mm (0.04 in) Fiber

Reinforced Plastic After Two Years Outdoor Weathering 82

17i. Spectral Transmittance of Washed 1.5 mm (0.06 in) Fiber

Reinforced Plastic After Two Years Outdoor Weathering 83

18. Comparison of Spectral Transmittance of 1.0 mm (0.04 in)

Polycarbonate Exposed Two Years in Arizona 84

19. Comparison of Spectral Transmittance of 1.0 mm (0.04 in)

Polycarbonate Single Covers Exposed in Florida 84

xi
List of Figures (con't)

Page

Al. Collector on Which Temperature Measurements Were Made A2

A2, Location of Thermocouples on Collector During Stagnation

Temperature Moni torlng A2

A3, Cover Plate Temperatures Measured June 23, 1977 on Collector in

Stagnation Condition A4

A4, Temperatures Measured September 18, 1977 in Center of Collector in

Stagnation Condition A5

A5a. Maximum Stagnation Temperatures in a Collector with a Flat

Black Absorber (a=0.9, e=0.9 and Two Glass Cover Plates

(1=1070 W/m2 (340 Btu/ft^'h), No Wind) A7

A5b. Maximum Stagnation Temperatures in a Collector with a Selective

Absorber (a=0.9, e=0.1) and Two Glass Cover Plates (1=1070

W/m2 (340 Btu/ft2»h), No Wind) A7

A5c. Maximum Stagnation Temperatures in a Collector with Flat Black

Absorber (a=0.9, e=0.9) and One Glass Cover Plate

(1=1070 W/m^ (340 Btu/ft^'h), No Wind) A8

A5d. Maximum Stagnation Temperatures in a Collector with a Selective

Absorber (a=0.9, e=0.1) and One Glass Cover Plate (1=1070

W/m^ (340 Btu/ft^'h), No Wind) AS

Bl. Top View of Exposure Test Fixture B4

B2 . Natural Weathering Box Assembly B5

CI. Sequence of Testing C5

xii
List of Tables

Page

1. Schedule for Variable Angle Exposure 14

2. Solar Energy Transmlttance of Control Test Specimens as Measured

with an Integrating Sphere Spectrophotometer ,.„ 19

3. Solar Energy Transmlttance of Control Test Specimens as Measured

Outdoors 24

4. Solar Energy Transmlttance of Cover Plate Materials After Heat Aging

at 150°C (302°F) 28

5. Solar Energy Transmlttance of Cover Plate Materials After Heat Aging

at 125°C (257°F) 33

6. Solar Energy Transmlttance of Cover Plate Materials After Heat Aging

at 100°C (212°F) 35

7. Solar Energy Transmlttance of Cover Plate Materials After Heat Aging

at 75°C (167°F) 36

8. Percent Change in Linear Dimensions of Cover Plate Materials After

Heat Aging 42

9Warpage of Test Specimens After Heat Aging 46

10. Solar Energy Transmlttance of Cover Plate Materials After Artificial

Weathering with Xenon Arc Light 47

11. Temperatures of Absorber and Cover Plate Materials Exposed to

Natural Weathering on Minicollectors in Gaithersburg,

Maryland 51

12. Natural Weathering Exposure Data for Minicollectors 53

13. Solar Energy Transmlttance of Cover Plate Materials Exposed as Single

Covers to Natural Weathering in Gaithersburg, Maryland 58

14. Solar Energy Transmlttance of Cover Plate Materials Exposed as Double

Covers to Natural Weathering in Gaithersburg, Maryland 59

15. Solar Energy Transmlttance Loss of Cover Plate Materials Exposed as Single Covers to Natural Weathering in Gaithersburg, Maryland 61

Xlll
List of Tables (con't)

Page

16, Solar Energy Transmittance Loss of Cover Plate Materials Exposed as Double Covers to Natural Weathering in Gaithersburg, Maryland ....... 62

17, Solar Energy Transmittance Loss Due to Dirt on the Surface of the

Cover Plate Materials After a Single Cover Exposure in Gaithersburg,

Maryland 64

18, Solar Energy Transmittance Loss Due to Dirt on the Surface of the

Cover Plate Materials After Double Cover Exposure in Gaithersburg,

Maryland 65

19, Solar Energy Transmittance of Cover Plate Materials Exposed as Single Covers to Natural Weathering in Miami, Florida 66

20, Solar Energy Transmittance of Cover Plate Materials Exposed as Double Covers to Natural Weathering in Miami, Florida 67

21, Solar Energy Transmittance Loss of Cover Plate Materials Exposed as Single Covers to Natural Weathering in Miami, Florida 68

22, Solar Energy Transmittance Loss of Cover Plate Materials Exposed as Double Covers to Natural Weathering in Miami, Florida 69

23, Solar Energy Transmittance Loss Due to Dirt on the Surface of Cover

Plate Materials After Single Cover Exposure in Miami, Florida 70

24, Solar Energy Transmittance Loss Due to Dirt on the Surface of Cover

Plate Materials After Double Cover Exposure in Miami, Florida 71

25, Solar Energy Transmittance of Cover Plate Materials Exposed as Single Covers to Natural Weathering in New River, Arizona 73

26, Solar Energy Transmittance of Cover Plate Materials Exposed as Double

Covers to Natural Weathering in New River, Arizona 74

27, Solar Energy Transmittance Loss of Cover Plate Materials Exposed as Single Covers to Natural Weathering in New River, Arizona 75

28, Solar Energy Transmittance Loss of Cover Plate Materials Exposed as Double Covers to Natural Weathering in New River, Arizona 76

xiv
List of Tables (con't)

Page

29. Solar Energy Transmittance Loss Due to Dirt on the Surface of Cover

Plate Materials After a Single Cover Exposure in New River,

Arizona 77

30. Solar Energy Transmittance Loss Due to Dirt on the Surface of Cover

Plate Materials After Double Cover Exposure in New River,

Arizona 78

Al. Description of Collector on Which Temperatures Were Measured Al

A2, Properties of Glass Cover Plates Used in Computer Simulation of

Maximum Collector Temperatures in Interim Performance Criteria .... A6

A3. Collector and Weather Parameters Used in Computer Calculations .... AlO

A4. Cover Plate Properties Used in Computer Calculations All

A5. Cover Plate and Absorber Plate Temperatures Reached on Stagnating

Collectors with an Ambient Air Temperature of 38°C (100°F) and a Solar Irradiance of 1070 W/m^ (340 Btu/ft^'h) A12

A6. Summary of Cover Plate Temperature Data for Stagnation

Conditions A13

A7. Cover Plate and Absorber Plate Temperatures Reached on Stagnating

Collectors with an Ambient Air Temperature of 25°C (77 °F) and a Solar Irradiance of 945 W/m^ (300 Btu/ft^'h) A15

AS. Variation in Cover Plate Temperatures for Stagnating Collectors

with No Wind When the Weather Conditions Are Altered A16

A9. Comparison of Measured Temperatures of Glass Cover Plates on Flat

Plate Solar Collectors under Stagnation Conditions with Estimated

Temperatures from Computer Simulations A17

Bl. Examples of Exposure Test Fixtures with Combined Heat Loss

Coefficient for Back and Edge Losses Less Than 1.5 W/m^'^C

(0 .264 Btu/ (f 1 2 'h* °F )) B3

B2. Variable Angle Rack Adjustment Schedule Utilizing Four Changes per

Year B9

B3. Variable Angle Rack Adjustment Schedule Utilizing Ten Changes per

Year B20

B4. Variable Angle Rack Adjustment Schedule Utilizing Six Changes per

Year B20

XV
List of Tables (con't)

Page

CI. Summary of Property Tests and Aging Procedures C4

C2. Total Energy Below 400 nm for Artificial Weathering Apparatus with

Xenon Arc Light C12

C3. Average Total Energy Below 400 nm for Natural Solar Irradiance on a South Facing Tilt Angle in Arizona and Florida C13

XVI
1. INTRODUCTION

1.1 BACKGROUND

Increasing emphasis is being focused on the development of solar collectors

to permit solar energy to be utilized for space heating and cooling and to

supply domestic hot water. The success of a solar collector is strongly

dependent upon the overall performance of the materials from which it is con- structed. Solar collectors should be constructed of durable materials that do

not degrade prematurely resulting in decreased collector efficiency. Solar

collectors are designed to maximize absorption of solar radiation and minimize

heat losses. The principal heat loss mechanisms from the front surface of a collector are by convection and reradiation. These may be controlled by the use of one or more transparent covers [1]*. Cover plates also protect the

internal components of the collector from the external environment, i.e.,

rain and snow. A glazing or cover plate is a major functional element of most

solar collectors.

Glass is the most commonly used cover plate material. It is a durable material, highly resistant to deterioration resulting from the thermal and environmental conditions encountered in solar collectors. Its optical properties

are well characterized and include a relatively high transmittance of solar

radiation which is likely to remain stable for the projected life of the solar

collector. The primary disadvantages of glass are its ease of breakage, weight,

and cost. Ordinary annealed glass can be strengthened by tempering. This

treatment significantly increases the strength and impact resistance of

glass.

Alternative materials are constantly being considered in search of the ideal

cover plate material. A number of plastics have been employed as cover plate

materials; these include thermoplastics and thermosetting resins and range

from thin film to rigid sheet. Thermoplastics soften when heat is applied and

exhibit considerable creep under load, particularly at elevated temperatures.

Thermosetting materials (once cured) do not soften appreciably on the application of heat; they normally exhibit higher strength and have less tendency

to creep than thermoplastics. However, they are more brittle. Compared to

glass, plastics are generally lower in cost, lighter in weight, easier to

fabricate, and some have a higher impact resistance [1, 2]. However, the

mechanical properties of plastics are much more time and temperature dependent

and their long-term durability is less well known than that of glass. Plastics

are more readily affected by environmental factors such as solar radiation,

temperature, humidity, airborne abrasive particles, and air pollutants. The

degradation resulting from exposure of plastic materials to such environmental

factors can reduce transmittance, induce shrinkage and cause other changes,

e.g., in mechanical properties such as brittleness and tendency to sag.

*Figures in brackets refer to references in Section 8.

1
Questions frequently arise over which Is the best cover plate material, or which should be used In a specific situation. Answers are not simple

because many factors are Involved. Crucial considerations include optical

properties, physical properties, mechanical properties and the durability of

the material. However, all too often the optimization of one property of a cover plate material (e.g. impact resistance) occurs at the expense of other

properties (e.g., transmlttance and weight).

Ideally the long-term performance of any solar collector material should be

assessed in service, i.e., on a solar collector, where it is simultaneously

subjected to all aspects of weathering including relatively high temperatures, cyclic variations of temperature between day and night, and variations

in humidities and intensities of solar radiation. Due to the time involved

in such long-term testing, and the variation in weather conditions, shortterm methods to measure the performance and durability of cover plate materials are needed. The established test methods and standards for glass and

plastic used in conventional construction applications, such as windows and

doors, seldom adequately evaluate durability or take into account the kind

of service conditions encountered by solar collectors. Standards for cover

plates must Include the weathering factors encountered both in the external

environment and within the solar collector. Standards should identify both

initial and long-term performance properties and test methods for evaluating

these properties to measure the performance of the cover plate materials.

Cover plate standards will provide a means of comparing various materials

and lead to the development of a uniform data base of materials performance

and durability.

1.2 OBJECTIVES AND SCOPE

This study, which was sponsored by the Department of Energy (DoE), had the

following objectives:

1. To identify performance requirements for cover plate materials used

in flat plate nonconcentrating solar collectors,

2. To identify and assess existing test methods for evaluating the

performance and durability of cover plate materials and to modify or develop new methods as needed,

3. To obtain data needed as the technical basis for standards by evaluating commercially available cover plate materials according to the

performance requirements, and

4. To prepare draft standards for cover plate materials for consideration as consensus standards by the American Society for Testing and

Materials (ASTM) and other organizations.

The laboratory and field studies conducted for the project include: the

modification and development of aging procedures which simulate environmental

degradation factors, the measurement of critical properties of typical cover

plate materials, and the assessment of material durability by comparison of
material property values before and after aging procedures. The environmental

degradation factors studied on this project were heat aging and solar radiation. Three aging procedures which simulated these factors were identified

and developed. They were heat stability exposure in an oven, artificial solar

radiation exposure in a laboratory apparatus, and natural weathering exposure

on a minicollector. Both simulated weather exposures and natural weathering

exposures were performed to permit comparison of the exposures and to determine

if the effects on the materials were independent or synergistic. The material

properties that were evaluated were solar energy transmittance, linear dimensional changes, warpage and the effect of dirt retention on solar transmittance.

2. PROBLEM ASSESSMENT

2.1 FIELD PROBLEMS

The performance of cover plate materials on solar energy systems in the field

was reviewed to identify the problems that have been encountered. Many problems have been previously documented in two major studies [3, 4] which address

materials performance in solar collector systems.

Breaking, rupturing, and cracking of cover plates has occurred in some systems

[3, 4, 5, 6]. While such cover plate damage has not been widely observed, it

remains a concern. Both glass and plastic materials are susceptible to such

damage. Some of the causes of this damage are thermal stresses, accidental

impact, and environmentally induced embrittlement. The breakage has ranged

from damage on a single collector to failure of 90 percent of the covers in a single solar heating system. Annealed glass was used in the collectors which

encountered 90 percent breakage, and the failure was reportedly caused by

thermal stress on the glass [5]. In this system reflectors were used on part

of the collector array. The incidence of breakage on the collectors subjected

to added reflected radiation was higher than the collectors without the reflectors. In recent years most glass cover plates have been tempered glass which

is more resistant to failure caused by thermal stresses and impact.

Impact damage can be caused by vandalism, accidents or hail. To date there has

been little documented damage resulting from vandalism. Damage incurred during

handling or by accidental impact has occurred. Due to the brittle nature of

glass, impact damage results in the glass breaking into multiple pieces. Plastic materials, which are more flexible, may crack, tear or rupture. Tears and

holes, apparently caused by impact, have been observed in polycarbonate, fiber

reinforced plastics, and film cover plate materials [3].

Although there have not been many reports of hail damage to solar collectors

[6], the risk of hail damage has been of particular concern. Hail storms occur

in most regions of the United States and severe storms have caused considerable

damage to roofs and other building elements [7]. Figure 1 shows damage incurred by a 0.13 mm (5 mil) poly(ethylene terephthalate) cover material exposed
FIGURE 1 Damage to 0.13r1i1 (0.005 in) PolyCethylene Terephthalate)

Film Caused by Hailstorm in New River, Arizona.
at a test site in New River, Arizona. The material had been exposed 17 months

prior to the hail storm. Personnel in the area estimated the hail size to be

13-19 mm (1/2-3/4 in.) in diameter.

Embrittlement and cracking, apparently caused by environmenal conditions such

as sunlight and elevated temperature, have been reported [3, 4]. Stress cracking was observed in polycarbonate cover plates that had been designed with

indented sections to provide structural rigidity. Approximately 60-70 percent

of the indentations exhibited stress cracking to some degree. Poly (vinyl

fluoride) film exhibited embrittlement and yellowing after exposure on collectors that had been subjected to stagnation conditions for several months due

to a system failure. Later laboratory examination of the material indicated

approximately a 3.5 percent loss of solar transmittance. Poly (ethylene

terephthalate) film also exhibited brittleness after exposure to sunlight, see Figure 1.

Warpage and distortion of cover materials have been reported on several systems

[3]. In one case the polycarbonate cover plate had distorted so much that it was touching the absorber at one location while nearby it had buckled above

the plane of the collector surface. Sagging of some film materials has also

been observed.

Dirt on both the inner and outer surfaces of cover plates, unidentified

deposits, and moisture condensation on the inner surfaces have been observed

on many solar collector systems [3, 4, 8]. In some cases, the deposits have

produced a film over the entire inner surface, apparently due to outgassing

from other collector materials.

Field problems mentioned above have been identified through visual examination.

Other changes in materials which cannot be detected by visual inspection may

also cause reductions in optical, physical and mechanical properties but would

require laboratory testing. However, since the cover plate is an integral part

of the solar collector, it has been difficult to obtain samples for laboratory

evaluation.

2.2 PERFORMANCE REQUIREMENTS

The primary purposes of the cover plate are to minimize heat losses from the

front surface of the collector and to protect the interior of the collector

from the external weather. The cover plate should perform these functions

while maintaining the maximum possible transmittance of the incident solar

radiation. It reduces heat losses by insulating the absorber from the outside

environment with an air gap and by trapping infrared (IR) radiation from the

absorber surface (if the glazing is opaque to the reradiated IR energy). It

also protects the interior of the collector from elements such as rain and

snow, shields the absorber from air pollutants and reduces the amount of

ultraviolet (UV) radiation falling on the absorber.

To perform these intended functions, the cover plate should have good optical,

physical and mechanical properties when installed on the solar collector and

retain these properties after long-term exposure to service conditions. In
addition, the cover plate should be resistant to deterioration caused by

sunlight and the temperatures encountered in use, and it should be compatible

with adjacent materials such as sealants, gaskets and framing. In short the cover material must be durable.

2.3 COVER PLATE PROPERTIES

Cover plate properties may be divided into primary and secondary properties.

Primary properties are those material properties considered critical to the

performance of the solar collector which are likely to be affected by weathering on a solar collector. Primary properties of a cover plate included solar

energy transmittance, dimensional stability, i.e. linear dimensions and warpage, tensile properties, and hail impact resistance.

Secondary properties are those which have some bearing on the performance of

the material as a cover plate but are less critical to collector performance.

These included transmittance of long wavelength radiation, abrasion resistance,

and static electric charge buildup.

2.3.1 Primary Properties

2.3.1.1 Solar Energy Transmittance

Solar energy transmittance is the key factor in the admittance of energy

through the cover plate to the absorber of the solar collector. At sea level

the incident solar radiation extends from about 300 nm to 2600 nm-'-4 A reduction of the solar energy reaching the absorber can be caused by a change in

the transmittance, absorbance or reflectance of the cover plate material, or by dirt or other substances on the cover plate which block the passage of solar

energy. A reduction of the energy reaching the absorber decreases the collector efficiency and results in a decline in overall system thermal performance.

2.3.1.2 Dimensional Stability

Dimensional stability is essential if the cover plate is to perform its primary

functions. The cover plate should remain intact and maintain its original size

and shape. Excessive changes in the linear dimensions such as shrinkage or expansion should not occur. Shrinkage can cause the material to pull away

from the edge restraints resulting in a tear or a gap between the edge of the cover plate and the collector frame. This would permit heat losses and allow

rain and other weather elements to enter the collector. Expansion of the

material can lead to breaking, sagging, or warping, possibly into contact with

an inner cover plate or the absorber plate. In addition, deviations from flatness (e.g., by warp, bow, buckle, and sag) reduce the effective insulating value

of the air gap between the absorber and the cover material.

'-' nm = nanometer = 1 x 10 " meters.
2.3.1.3 Tensile Properties

The tensile stress-strain curve provides an Indication of the strength, toughness and elasticity of the material. Changes in the stress-strain behavior

also indicate whether a material is becoming more elastic or more brittle.

2.3.1.4 Hail Impact Resistance

The Department of Housing and Urban Development (HUD) Minimum Property Standards [9] and the DoE Interim Performance Criteria [10, 11] require collectors

to be resistant to the damage caused by hall expected for the particular geographic location. Other solar collector buyers have also placed hall impact

requirements in purchase specifications. The desired level of hail Impact

resistance must be related to the size of hail reasonably expected at the

location of the solar energy system. Cllmatological data provide an indication

of the geographical distribution of hall and some history of the Intensity and

severity of hall storms. Some regions of the United States rarely have hall

storms, while other regions have had an average of eight storms per year in a 20 year period [7]. Not only do the number of storms vary, but the size of

hall also has a wide range. While rare, hail stones with a diameter 70 mm

(2-3/4 Inches) and larger have occurred in the United States [6, 12, 13].

The damage to cover plate materials caused by hall is directly related to the

size and velocity of the hail at impact. Since cover plate materials may be

rigid, semirigid, or flexible materials, the type of damage resulting from

hall is dependent upon the material composition. For brittle materials, hail

impact may cause the material to break or shatter. For flexible or semirigid

materials, hall impact may tear, puncture, or Indent the material. Such damage

to the material could prevent the cover plate from performing its primary functions. Breakage or rupture of the cover plate would permit heat losses to occur and expose the absorber to the weather elements. Hall Impact resistance

of cover plates, in particular the polymeric materials, may be reduced by

exposure to sunlight and high temperature if the materials become brittle.

2.3.2 Secondary Properties

2.3.2.1 Transmittance of Long Wavelength Radiation

The heat losses through the cover plate are related to the long wavelength

transmittance, the emittance of the cover material, and the convectlve heat

loss at the cover. The reradlation of thermal energy from the absorber and

subsequent heat loss from the collector correspond to the ability of a cover

plate material to transmit long wavelength (infrared) radiation (wavelengths

greater than 2.8 ym^'). This radiation will be lost through the cover material to the surrounding cooler sky if the solar collector cover is not

opaque to this radiation. The infrared energy absorbed by the cover plate is

-'' ym = micrometer = 1 x 10 ^ meters = 100 nm.
also reradiated to the sky and collector as a function of cover plate emittance

and its temperature. Convective heat losses are related to wind speed and

direction, heat transfer coefficient, and ambient temperature.

2.3.2.2 Abrasion Resistance

Abrasion resistance of a cover plate material is the ability to resist scratching or marring of the surface. Abrasion damage could reduce the transmittance

of the cover material due to scatter of the solar energy or an increase in the

dirt retained on the roughened surface. Abrasion of thin cover materials may

also result in a decrease in the tensile and/or impact properties.

2.3.2.3 Static Electric Charge Buildup

Certain plastic materials, in particular films, develop static electric charge.

As a cover plate, such a material may attract or pull dust and dirt particles

out of the air and hold the particles on the surface. These particles could

Interfere with the passage of sunlight and reduce the solar transmittance of

the cover plate.

2.4 DEGRADATION FACTORS

The durability of a material is related to its ability to resist degradation

by its exposure environment. The foremost degradation factors encountered by

a solar collector cover plate are solar radiation and elevated temperature

(due to the close proximity of the absorber plate). Other factors which may

contribute to degradation include synergistic reactions resulting from the

combination of solar radiation with moisture or temperature or both. The

resulting stress fatigue may induce a gradual impairment of material properties.

In addition, thermal cycling, moisture, air pollutants, wind flutter, and particulate matter such as dirt, dust, or sand may also degrade cover plate materials.

2.4.1 Solar Radiation

The spectral distribution of sunlight is illustrated in figure 2, and the

ranges of ultraviolet (UV), visible (VIS) and near infrared (NIR) energy are indicated. Degradation due to ultraviolet radiation has long been of great

concern to people designing or using products exposed to sunlight. The solar

radiation spectrum band below 400 nm is detrimental to most poljnners [14].

The radiation may cause crosslinking, breaking of chemical bonds, and reduction

in molecular weight [15, 16]. The poljoner damage may be reflected by changes

in appearance or in chemical, physical, mechanical or optical properties [17].

To offset degradation due to UV radiation, UV stabilizers are frequently added

to polymers or coated on their surface to absorb the UV radiation and inhibit

bond breakage.

2.4.2 Temperature

It is unlikely that temperatures reached in a flat plate solar collector would

damage glass cover plate materials by degradation, although they could increase
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the solarization rate of Impure glasses. However, elevated temperatures may

cause degradation of plastic cover plates. The temperature conditions which

may lead to degradation include extended exposure to operational temperatures

and to stagnation^' temperatures. Cover materials are exposed to operational

temperatures for 6-10 hours a day. Stagnation temperatures may occur in sev- eral circumstances: 1) after the collector is put in place but prior to system

startup, 2) when the system is shutdown for maintenance or repair, or 3) when

the system is not operated because the heat is not needed (e.g., a space heating system during the summer).

2.4.3 Synergistic Reactions

Plastic materials are susceptible to damage caused by the combination of

sunlight, heat, oxidation, and moisture [14, 18, 19]. The deterioration of

polymers exposed to outdoor weathering is related to bond breakage caused by

photooxidation (i.e., UV radiation and oxidation). Heat also aids in the

degradation process, but in the presence of UV radiation and oxygen, it seems primarily to increase the rate of photooxidation. "Both 'weathering' at high

temperature (i.e. > 38°C (100°F)) and photochemical impurities speed the photodegradation of such polymers. Generally, photochemical reaction rates are doubled for each 10°C (18°F) rise in temperature" [20].

2.4.4 Other Factors

Thermal cycling, can affect a cover material in two ways. First, if the

material is a composite, the thermal cycling may cause delamination of the

components. Second, differences in the thermal expansion coefficients of

the cover and adjacent materials may impose cyclic stress at the edges where

the cover material is joined to the frame. The resulting damage would be

dependent on the method of joining. A cover plate mechanically held in place,

such as with a screw, may stress crack at the screw hole. Cover plate/sealant

joints may pull apart.

Moisture (e.g., humidity, condensation, or rainfall) and air pollutants (e.g.,

ozone, oxides of nitrogen, or sulfur dioxide) may cause surface degradation of cover plates. Wind flutter of thin films may fatigue the material and alter

mechanical properties. Cleaning with harsh detergents or cleaning agents could

subject the outer cover plate surface to abrasive chemical attack. Contact of

the cover material with the heat transfer fluids may also initiate chemical

attack. However, compatibility between the cover material and the heat transfer fluid is not required unless the collector design permits contact.

1/ Stagnation is a condition obtained when the heat transfer fluid is not

flowing through the collector, however, the collector is exposed to the

amount of solar radiation that it would receive under normal operational

conditions. The heat transfer fluid may be in the collector but it is not

flowing.

10
3. LABORATORY STUDIES AND FIELD EXPOSURES

3.1 INTRODUCTION

The laboratory and field studies conducted for the project included: the

measurement of critical properties of typical cover plate materials, the

exposure of cover plate materials to aging procedures which simulate environmental degradation factors in solar collectors, and the assessment of material

durability by comparison of material property values before and after aging

procedures. This report addresses the measurement of solar energy transmittance, linear dimensional stability, warpage, and dirt retention, and evaluates

the effects of heat aging and solar radiation on these properties. The subjects of hail impact and tensile properties will be described in a later

report. This chapter contains a description of the test methods and aging

procedures utilized in the project and a description of the cover materials

which were studied. Test results and data to evaluate the durability of cover

plate materials are incorporated in chapter 4.

3.2 AGING PROCEDURES TO SIMULATE DEGRADATION FACTORS

Heat aging and solar radiation were the environmental degradation factors

studied on this project. The three aging procedures utilized were: heat

aging in an oven, artificial solar radiation in a laboratory apparatus, and

natural weathering on a minicollector. In the laboratory the materials were

exposed separately to heat and artificial solar radiation, whereas in the

natural weathering they were exposed to natural sunlight while the material

temperatures were raised by the minicollector. Both simulated and natural

weathering exposures were performed to permit comparison of the exposures

and to assess the simulated weathering.

3.2.1 Heat Stability Aging Procedure

Cover plate temperatures on flat plate solar collectors vary considerably

depending on the weather and operating conditions, however, it is the maximum

temperature to which materials are exposed that is most important. Studies were performed to determine cover plate temperatures under stagnation conditions. These studies are described in Appendix A, Determination of Cover Plate

Temperatures. Temperatures for the heat aging tests were based upon maximum

cover plate stagnation temperatures which are summarized in table A9 of Appendix A. Heat aging test temperatures were established as 75°C (167°F), 100°C

(212°F), 125°C (257°F), and 150°C (302°F).

Test specimens were suspended vertically from a single point at the center of

the top edge and were exposed in a mechanical convection oven. Such an oven

is described in ASTM Method D 794, Recommended Practice for Determining Permanent Effect of Heat on Plastics [21]. Separate sets of test specimens were

used for each of the temperatures. Test specimens were removed at time intervals ranging up to 2,000 hours and were evaluated to determine the effect of

the heat exposure. Once a test specimen was removed for evaluation it was not

returned to the oven. Properties measured after the heat aging were solar

transmittance, warpage, and linear dimensions. One set of test specimens

11
(115 by 230 mm (4.5 by 9 in.)) was used for the solar transmlttance and

warpage evaluations. A separate set of test specimens of the sizes described

in Section 3.3.2 was measured for linear dimensions. These were placed in a horizontal position while in the oven.

3.2.2 Artificial Weathering with Xenon Arc Light

Properly filtered xenon arcs have been shown to have a spectral energy distribution which closely simulates the spectral distribution of sunlight at the

surface of the earth [22, 23, 24]. The procedure used to expose the cover

plate materials to artificial solar radiation was ASTM Method D 2565, Recommended Practice for Operating Xenon Arc-Type (Water-Cooled) Light-Exposure

Apparatus With and Without Water for Exposure of Plastics [25]. A Type B

apparatus was used with the irradiance controlled through the use of a con- tinuously controlling monitor that automatically maintained uniform intensity

at preselected wavelengths of 340, 420 and 580 nm. A borosilicate glass filter

with suitable transmlttance was used to filter the light and to provide radiation which simulated the solar spectrum at sea level. Test specimens were exposed to a continuous light cycle without water spray. The ambient temperature within the chamber was approximately 40°-50°C (104°-122°F).

Test specimens were mechanically fastened with a clip to a metal backing which

was placed in the vertical specimen rack. The test specimen size was about

320 by 75 mm (12.5 by 3 in.). Periodically, test specimens were repositioned

to assure uniform radiation. They were removed for evaluation at time intervals ranging from 250 hours to 4,000 hours and were not replaced in the

apparatus.

3.2.3 Natural Weathering Exposure

Existing outdoor exposure procedures did not provide for sufficient elevation

of the material temperature above ambient conditions. To obtain the elevated

temperature, a small box was designed to hold the cover plate materials during

natural weathering. The box resembled a small collector. The minicollector

consisted of a wooden box containing insulation and an absorber plate. For

double cover exposures a spacer was also utilized. Figure 3 illustrates the

construction of the minicollector. To minimize outgassing resulting from

deterioration of the organic components during exposure to elevated temperatures, the insulation and the wooden boxes and spacers were heated in an oven

before the minicollectors were assembled. The wooden components were permitted

to regain moisture equilibrium prior to assembly. After this was completed,

the minicollectors were assembled. The test specimen size for natural weathering exposure was 115 by 230 mm (4.5 by 9 in.). For single cover exposures, the

test specimen was adhered to the box using sealant. For double cover exposures,

the inner cover was adhered directly to the box with sealant and then the outer cover was adhered to the double cover spacer which in turn was adhered to the

inner cover and box. Staples were used to provide mechanical fastening of the

spacer to the side of the box. Sealant was then used to fill any voids such

as between the spacer and the box, and to coat remaining unpainted areas. The

weep and drain holes were not filled.

12
FIGURE 3 Minicollector for Natural Weathering Exposure

OUTER DOUBLE

COVER

SPACER FOR OOUBLE

COVER

SINGLE COVER OR INNER DOUBLE COVER

BOX

ABSORBER PLATE

INSULATION

13mm (3/4")

3mm(3'/4'

MATERIALS:

BOX AND SPACER:

19mm (3/4") WOOD PAINTED WHITE ON OUTSIDE

INSULATION:

50mm (2") FIBERGLASS BLANKET

ABSORBER:

BLACK PORCELAIN ENAMEL

ON STEEL. a= ,93,6=85

SEALANT:

RTV SUITABLE FOR OUTDOOR

EXPOSURE.

NOTE:

VENT HOLES WERE DRILLED

IN THE BOTTOM OF THE BOX

AND THE END OF THE SPACER

TO MINIMIZE MOISTURE

ACCUMULATION

140mm /

(5'/2") ^>^

13
The mlnlcollectors were weathered at three sites: New River, Arizona; Miami,

Florida; and Gaithersburg, Maryland. The minicollectors were mounted in an unbacked condition on weathering racks such as those described in ASTM D 1435

Recommended Practice for Outdoor Weathering of Plastics [26]. They were mounted side by side with the long sides touching and the a narrow end of

the box toward the ground. A variable angle exposure was utilized to maximize

solar radiation incident upon the minicollector. The schedule used for the

variable angle exposure is contained in table 1. The exposures were initiated

September 1, 1977 on seven sets of minicollectors. (A set of minicollectors

consisted of one single cover and one double cover minicollector for each of

ten materials.) A set of test specimens was removed for evaluation at 3, 6,

12, 18, and 24 month intervals.

Table 1

Schedule for Variable Angle Exposure

Miami New River Gaithersburg

Rack Tilt Angle 26° latitude 33° latitude 39°latitude

5° April-August April-August May-August

latitude September September September

45° October-February October-February October-March

latitude March March April

After removal of the first sets of specimens, new test specimens were placed

on the minicollectors to obtain exposures for similar time intervals. At this

time (August, 1980), two of the original sets of minicollectors remain on exposure at each location.

3.3 PROPERTY TESTS

The material properties evaluated in this project were solar energy transmittance, the effect of dirt retention on solar energy transmittance, linear

dimensional stability, and warpage.

3.3.1 Solar Energy Transmittance

Two methods for the measurement of solar energy transmittance are included

in ASTM E 424-71, Standard Test Methods for Solar Energy Transmittance and

Reflectance (Terrestrial) of Sheet Materials [27]. Method A is performed in

a laboratory utilizing an integrating sphere spectrophotometer. Method B is

conducted outdoors where the transmittance is measured using a pyranometer

14
in an enclosure and the sun as the energy source. Test specimens for Method

A are limited in size by the geometry of the spectrophotometer while Method B

requires a specimen 0.61 m (2 ft) square.

3.3.1.1 Method A - Integrating Sphere Spectrophotometer

The spectral transmittance of the cover plate materials was measured utilizing

a Gary 17D Spectrophotometer-'-' with a 76 mm (3 in) diameter integrating sphere.

Measurements of spectral transmittance relative to air were made over the

spectral range from 300 to 2150 nm. The illumination and viewing mode were normal-diffuse. The transmittance measurements were made by placing the test

specimen in direct contact with the sphere aperture so that the incident monochromatic radiation was normal to the plane of the specimen. The sphere aperture had approximate dimensions of 25 by 10 mm (1 by 3/8 in.). The incident

beam was approximately 24.4 by 6.2 mm (0.96 by 0.24 in.) and it intersected

the test specimen near the center. The solar energy transmitted was obtained

by integrating over the solar energy distribution, as reported by Parry Moon

[28], for sea level and air mass 2 (AM 2). The weighted ordinates calculation

method from ASTM E 424 was used to integrate the solar energy distribution at

50 nm intervals, normalized to 100. The spectral transmittance data were digitized by the spectrophotometer and fed directly into a computer which

performed the integration calculations after correcting for the baseline. Due

to the optics associated with the integrating sphere and moisture in the atmosphere, the 100 percent baseline contained several absorption bands for which

corrections were made.

A change in an integrated solar transmittance of a material does not indicate

how the material changed, however the spectral transmittance curve provides

this information. Most commonly, solar transmittance losses result from

increased absorption (i.e. transmittance decrease) in the UV-VIS region,

although in some cases a fairly uniform transmittance loss across the full

spectrum occurs. Increased absorption in the UV-VIS range is due to damage

to the bulk material as caused by heat or solar radiation, while a loss across

the spectrum indicates changes in surface properties. Dirt, surface deposits,

a change in refractive index, and delamination of the fibers in fiber reinforced

plastic are examples of such changes in surface properties.

3.3.1.2 Method B - Outdoor with Pyranometer

Method B of ASTM E 424 was utilized to measure the total transmittance of

selected sets of cover plate test specimens. The apparatus included a box

having an open side with outside dimensions of 0.66 m (26 in) square. This

side of the box supported a 0.61 m (24 in) square test specimen. The interior

1/ Certain trade names and company products are identified in order to

adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of

Standards, nor does it imply that the products are necessarily the best

available for the purpose.

15
of the box was painted flat black. A Class 1 pyranometer, as classified by

the World Meteorological Organization, was placed in the center of the box

about 50 mm (2 in) below the bottom plane of the test specimen. Film materials

were placed in a frame which held the material taut. During the transmittance

measurements the box was positioned so that the plane of the test specimen was essentially perpendicular to the incident solar radiation. The solar energy

transmittance of the test specimens was the ratio of the energy measured when

the test specimen was in place on the box between the sun and the pyranometer

to the energy measured with no test specimen in place.

3.3.1.3 Effect of Dirt Accumulation on Solar Energy Transmittance

To determine the solar transmittance loss due to dirt accumulated on the sur- face of a cover plate and to differentiate this from the loss due to degradation of the material caused by the weathering, the transmittance of the test

specimens from the natural weathering minicollectors was measured twice. After

removal of the cover plates from the minicollectors, the transmittance was measured as described in Section 3.3.1.1, the test specimens were washed and

the transmittance was remeasured using the same method.

The cleaning procedure consisted of immersing the test specimens in a 0.1 percent solution of detergent in distilled water. A soft brush was used to clean

both sides of the test specimen which was then rinsed with distilled water and

air dried. Care was taken to avoid scratching or stretching the plastic

materials.

3.3.2 Linear Dimensional Stability

Linear dimensional changes were determined according to the procedures in ASTM

D 1042, Test for Linear Dimensional Changes of Plastics [29], and ASTM D 1204,

Test for Linear Dimensional Changes of Nonrigid Thermoplastic Sheeting or Film

at Elevated Temperatures [30]. ASTM D 1204 was used for four film materials

which had a thickness of 0.25 mm (0.01 in) or less. The other materials were measured in both the length and width directions using ASTM D 1042. The size

of test specimens was about 120 by 150 mm (4.75 by 6 in) for ASTM D 1042 and

254 mm (10 in) square for ASTM D 1204. A set of triplicate test specimens was used for each test.

3.3.3 Warpage

For sheet materials, deviation from flatness was measured and reported as warpage. The test specimens were 115 by 230 mm (4.5 by 9 in). The apparatus

consisted of a table with a movable platform, a probe which scanned the profile

of the test specimen, and graph paper to record the profile. Figure 4 illustrates the apparatus. The heavy framed table had a motorized aluminum platform with variable speed control. An inverted L-shaped support frame extended

upward from one side of the platform. A plastic shaft with a small wheel on

the lower end extended from an end of the support frame. The shaft was positioned above the center of the platform with a manually operated gear rod. A

pen was mounted on the vertical shaft and positioned in contact with the graph

paper.

16
FIGURE ^ Apparatus for Warpage measurement

graph of profile

of test specimen

movable

platform

shaft

test

specimen

17
The procedure was as follows: 1. The graph paper was placed in the proper

location and the plane of the platform surface was drawn on the paper, 2. The

test specimen was placed concave side down on the platform with one end against

the positioning bar and in a position to assure the shaft would pass over the

location with the maximum warpage, 3. The platform was adjusted and the shaft was lowered until the small wheel came in contact with one end of the upper

surface of the test specimen, 4. The platform was activated which slowly moved

the wheel over the surface of the specimen as the pen graphed a profile of the

test specimen.

The maximum warpage was determined by measuring the distance between the plane

of the platform surface and the profile of the test specimen. The distance was corrected for the thickness of the test specimen.

3.4 MATERIALS

3.4.1 Selection

The cover plate materials utilized in this study were typical of those commercially available in 1976. The properties of materials currently on the market

may differ from those described in this report. Properties of cover plate

materials can be altered by changes in minor constituents (i.e. iron oxide

content, stabilizers, plasticizers, antioxidants) or in processing techniques.

Many cover plate materials are marketed, and in order to improve their performance, manufacturers sometimes modify the materials. The reader is cautioned

against direct application of the data in this report to materials currently

marketed.

The cover plate materials selected for testing were representative of the

materials and thicknesses used in solar collectors. The ten materials are listed in Table 2. Two of the materials were glass sheet. They differ in

iron oxide content which directly affects the solar energy transmittance.

Four of the materials were plastic films, and four materials were plastic

sheet, two of which were fiber reinforced plastics (FRP). These FRP materials

were from separate manufacturers and contained different resins. They will be

distinguished in this report by their thicknesses. All of the materials had

smooth surfaces, except the glass with the lower iron oxide content. On this

material one surface was textured.

3.4.2 Preparation of Test Specimens

Test specimens were cut from either sheet or rolls of material. Prior to

initiation of aging procedures, all test specimens were cut to the dimensions

required for the tests. A random number selection procedure was used to assign

test specimens to each aging procedure. This precaution was taken to minimize

any bias in test results for a particular test due to material inhomogenieties.

3.4.3 Control Specimens

A set of three test specimens (115 by 230 mm (4.5 by 9 in)) was utilized as control specimens for all tests except the Method B transmittance test. The

18
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19
control test specimens for Method B transmlttance measurements were 0.61 m

(24 In) square. The control test specimens were stored in the dark under

typical laboratory conditions of 23 + 2°C (74 + 4°F) with 50 + 10 percent

relative humidity.

3.5 SCHEDULE OF AGING PROCEDURES AND PROPERTY EVALUATION TESTS

To establish the initial material properties for unaged materials, the solar

transmlttance and warpage properties were measured on sets of control test

specimens. Then to evaluate durability, sets of test specimens were exposed

in each aging procedure followed by remeasurement of material properties.

After the heat aging the solar transmlttance was measured on each test specimen

while linear dimensions were measured on test specimens from the 100° and 150°C

(212° and 302°F) exposures. Selected test specimens from 125° and 150°C (257°

and 302°F) exposures were evaluated for changes in warpage. Test specimens

exposed in the xenon arc artificial weathering had solar tansmittance measured

after the aging. The test specimens exposed to natural weathering were evaluated by measurement of solar energy transmlttance and the effect dirt retention

on solar transmlttance. Method A of E 424 was used to measure transmlttance

after aging.

4. RESULTS AND DISCUSSION

4.1 CONTROL TEST SPECIMENS

4.1.1 Solar Energy Transmlttance

4.1.1.1 Method A - Spectrophotometer

The solar energy transmlttance for the control set of test specimens for air mass 2 at sea level is listed in table 2. The average solar energy transmlttance of the control test specimens was used when comparing aged and unaged

materials. For all but two materials the standard deviation of the three

control test specimens was 0.1 percent or less. Therefore three additional

test specimens of the glass (0.01 percent iron oxide) and the 1.5 mm FRP were measured to establish a better basis for their averages. At least two factors

may have contributed to the variation in solar transmlttance: first, material

thickness; and second, material inhomogeneities. The thickness of a number

of pieces of the glass (0.01 percent iron oxide) was measured and found to

vary from 3.02 to 3.43 mm (0.119 to 0.135 in), a difference of more than 10

percent. There were no apparent inhomogeneities in the glass other than the

textured surface. The solar transmlttance of the 1.5 mm FRP material varied

by 7.1 percent among the six test specimens. Their thickness ranged from 1.37

to 1.52 mm (0.054 to 0.060 in), greater than a 10 percent difference. An irregular fiber distribution was evident upon visual inspection and undoubtedly

contributed to transmlttance variations.

The spectral transmlttance curves from 300 to 2150 nm are shown in figures

5a - 5d for control specimens. The differences between the two types of glass

are illustrated in figure 5a, while the four films are displayed in figure 5b.

20
FIGURE 5a Spectral Transmittance of Control Test Specimens of Two Glass Materials

100

90

80

70

t 50

S

z 40

30

20

10

Glass (0.01% iron oxide) 3.2 mm (1/8 in]

Glass (0.f^ iron oxide) 3.2 mm (1/8 inl

_L 400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

FIGURE 5b Spectral Transmittance of Control Test Specimens of Four Plastic Films

100

90

80

70

60

50

40

30

20

10

Fluorinated (ethylene propylene) copolymer 0.025 mm (0.001 in)-

' fi

Poly(ethylene terephthalate) 0.13 mm (0.005 in)

' ^ Poly (vinyl fluoride) 0.10 mm (0.004 in)

Acrylic 0.076 mm (0.003 in) r '/

?

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

21
FIGURE 5c Spectral Transmittance of Control Test Specimens of Two

Clear Plastic Sheet Materials

"1 1

1 1 1 1 r

Poly(methyl methacrylate) 1.5 mm (0.06 in)

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

FIGURE 5d Spectral Transmittance of Control Test Specimens of Two

Fiber Reinforced Plastics

Fiber reinforced plastic 1.0 mm (0.04 in)

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

22
The two clear rigid plastic sheet materials, are shown in figure 5c, whereas,

the two fiber reinforced plastics are compared in figure 5d. Among the various

materials it is noteworth to observe the differences in the location of the

absorption band in the ultraviolet region (below 400 nm). Absorbing the ultraviolet portion of solar energy can result in deterioration of the material.

A. 1.1. 2 Method B - Outdoor with Pyranometer

The solar energy transmittance for a control set of test specimens as measured

outdoors is listed in table 3. The table lists the solar transmittance of each

control specimen, their average, and the standard deviation. The standard deviation of the 1.5 mm fiber reinforced plastic was 4.6 percent. For the other

materials it ranged from 0.2-0.9 percent.

4.1.1,3 Comparison of Results of Method A and Method B

The solar energy transmittance values in tables 2 and 3 were used to compare

Methods A and B. With Method B, the standard deviation was greater for each

material. Also with Method B, the average solar transmittance values for six

materials (i.e. both glasses, fluorinated (ethylene propylene) copolymer,

poly(vinyl fluoride), poly(ethylene terephthalate) , and 1,5 mm fiber reinforced

plastic) were higher than with Method A. An explanation for this may lie with

the fact that with Method A (spectrophotometer) the calculations were made

using AM 2, while with Method B (outdoor) the air mass was about 1.2, Figure 2 illustrates the differences in energy distribution for the air masses. Air mass 1 has more energy in the ultraviolet (below 400 nm). Consequently materials having their absorption band well below 400 nm (Figures 5a-5d) have a higher transmittance with Method B compared to Method A. Conversely, materials

having their absorption band closest to 400 nm (i.e. acrylic film, poly(methyl

methacrylate) , polycarbonate and the fiber reinforced plastics) do not transmit

the additional energy contained in AM 1 and consequently have a lower solar

energy transmittance when comparing the values obtained from Method B with

Method A.

The solar energy transmittance determined by Method B may vary from day to day

due to atmospheric conditions, changes in the spectral distribution of sunlight,

and variation in the direct/diffuse ratio of sunlight. Method A has a constant

energy source and a high degree of precision, consequently small changes in

transmittance can be detected. For these reasons, Method A was used to evaluate the changes in transmittance caused by aging procedures,

4.1,2 Infrared Transmittance

Near normal infrared transmittance measurements from 2,5 to 40 pm were made to

characterize the cover plate materials and to determine if the materials would

transmit the long wavelength energy reradiated by the absorber. Figures 6a to

6e contain the spectral transmittance curves in the infrared region for control

test specimens. The poly (methyl methacrylate) and the two fiber reinforced

plastics, figures 6d and 6e, transmit almost no infrared (IR) energy. The two

types of glass, figure 6a, and the polycarbonate, figure 6d, have some IR

transmittance below 4,5 ym but overall the IR transmittance is low. Conversely,

23
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24
FIGURE 6A Infrared Transmittance of Control Test Specimens of Two Glass Materials

Glass (0.01% iron oxide) 3.2 mni[0.125in)

Glass 10.10% iron oxide) 3.2 mni(0.125in)

5.0 6.0 7.0 8.0 9.0 10 WAVELENGTH |MICRONS]

FIGURE 6b Infrared Transmittance of Control Test Specimens of Two Plastic Films

X

Poly(vinyl fluoride) 0.10 mm (0.004in)

Fluorinated (ethylene propylene) copolymer 0.025 mmlO.OOlin

15 20 30

2.5 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10

WAVELENGTH (MICRONS)

15 20 30 40

FIGURE 6c Infrared Transmittance of Control Test Specimens of Two Plastic Films

Acrylic 0.076 mm (0.003in) ^y-Poly(ethylene terephthalate) 0.13 mm (0.005in)

/vI \

5.0 6.0 U7.0 WAVELENGTH (MICRONS)8.0 9.0 10 30 40

25
100

80

FIGURE 6d Infrared Transmittance of Control Test Specimens of Two Plastic Sheet Materials

i 60

40

20

0.

Polycarbonate 1.0 mm (0.04in) ^Poly (methyl methacrylate] 1.5 mm (O.OOin) / ,^ ^H^-^-fk

2.5 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10 WAVELENGTH (MICRONS)

15 20 30 40

100 _ 80

5^

FIGURE 6e Infrared Transmittance of Control Test Specimens of Two Fiber Reinforced Plastics

-

-

li 60

1—

1—

- -

^ 40 - -

ee. y'V^Fiber reinforced plastic 1.0 mm(0.04in) - 20 - L^ Fiber reinforced plastic 1.5 mm (O.OBinJ .'-\ ^ ^

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2.5 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10 15 20 30 40 WAVELENGTH (MICRONS]

26
the four film materials, figures 6b and 6c, (i.e., fluorinated (ethylene

propylene) copolymer, poly(vinyl fluoride), poly(ethylene terephthalate) , and

acrylic) transmit significant portions of the infrared energy.

IR transmittance measurements were made on selected film test specimens after

some of the aging procedures. No significant changes relating to the IR long

wavelength transmittance or emittance were observed.

4.2 HEAT AGING

A. 2.1 Solar Energy Transmittance

A. 2. 1.1 Exposure at 150°C (302°F)

The test specimens exposed to heat aging at 150°C (302°F) were removed at time

intervals ranging from 10 to 1825 hours. From visual inspection it was evident

that many materials were affected by the heat. Some materials (i.e., poly

(vinyl fluoride) and the fiber reinforced plastics) visibly yellowed while

other materials were dimensionally unstable. Table A contains the solar energy

transmittance for air mass 2 at sea level. The greatest decrease occurred in

the 1.5 mm fiber reinforced plastic which lost approximately half of the initial transmittance. The decrease occurred rapidly and took place across the

entire spectrum from 375 to 2150 nm, as is shown in figure 7a. The solar transmittance of the 1 mm fiber reinforced plastic also dropped significantly during

the exposure i.e. 13.5 percent. Like the 1.5 mm fiber reinforced plastic, this

material lost transmittance across the entire spectrum, although the decrease

was greatest below 700 nm for both materials.

The solar energy transmittance of the poly(vinyl fluoride) declined about 20

percent during the exposure. The transmittance loss occurred primarily below

900 nm, as is shown in figure 7b. The test specimens gradually darkened during

the exposure until they were nearly brown. Figure 8 provides a comparison of

the appearance of the poly(vinyl fluoride) after various times at 150°C

(302°F). The poly(ethylene terephthalate) showed a slow but steady decrease

in transmittance resulting in about 5 percent loss during the exposure. The

gradual decrease took place across the entire spectrum. The heat aging caused

a 2.5 percent decline in the polycarbonate. Although this is relatively small,

it was caused by increasing absorption below 600 nm.

The poly(methyl methacrylate) was seriously distorted and the acrylic film was

so shrunken by the heat such that it was difficult to find an adequate flat area on which to make the measurement. The solar energy transmittance of both

the poly (methyl methacrylate) and acrylic film decreased. In part, this was caused by increasing absorption in the ultraviolet-visible region, although

the distortion of the test specimens and the resulting thickness deviations

may also have contributed. After 10 hours, the fluorinated (ethylene propylene)

copolymer had a decrease of about 1 percent which remained relatively constant

for the remainder of the exposure. Examination of the transmittance curves showed little, if any, absorption change in the ultraviolet region.

27
Table 4

Solar Energy Transmittance of Cover Plate Materials After Heat Aging at 150°C (302°F)^

____Exposure Time (hours) Material ' —_______^^ 10 100 225 500 750 1000 1250 1500 1825

Glass (0.01% Iron oxide) 90.7 91.1 89.9

90.0

90.2 89.989.7

89.8 89.8 88.8 90.1 91.6 89.3

91.6

Glass (0.10% iron oxide) 87.4 86.9

86.5

86.9

88.9 86.7 86.6

86.7 86.8 86.5 86.2 87.1 86.9

87.0

Fluorinated (ethylene

propylene) copolymer

96.0 95.0 95.095.1 94.994.9

95.2 95.2 94.9 95.0 95.2 94.5 94.8 94.8

Poly(vinyl fluoride) 92.5 91.2 91.2 87.2

87.4 87.987.9

86.8 82.4 80.5 75.0 71.7 72.0

72.5

Poly (ethylene terephthalate) 86.3 84.9 86.0

85.8

86.0

85.5 84.8 84.0 83.1 81.5 82.4 82.3 81.6

80.7

Acrylic 90.7 hJ

h/ b/ 86.9

87.7

88.1 87.6 88.4 88.0 87.2 8|.0

Poly(methyl methacrylate) 91.0 90.1 90.4

b/ b/

89.9 89.5 89.3 89.4 y i/

b/

Polycarbonate 88.1 87.2 86.887.2

87.1 86.7 86.4 86.6 86.2 86.3 85.7

85.6

Fiber reinforced plastic (1.0 ram) 86.3 77.3

74.5

7b.

A

76.1 77,2

b/ 77.0 75.2 74.5 72.7 70.2 74.3

70.9

Fiber reinforced plastic (1.5 mm) 78.5 64.1

77.2

60.1 60.752.1

56.1 52.2 43.6 39.7 39.7 44.6 39.7

39.4 ^ Transmittance measurements were made on a Gary 17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's Standard solar energy distribution curve using weighted ordinates. The initial solar energy transmittance values

listed are the average of the control test specimens. Each transmittance value for exposed materials represents

a separate test specimen. Transmittance is expressed in percentage. y Test specimen was too deformed from the heat to make a transmittance measurement.

28
FIGURE 7a Spectral Transmittance 1,5mm(0,06in) Fiber Reinforced Plastic Exposed to .150°C(302°F) for Four Time Periods

1 1 —I

Control - no heat exposure

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

FIGURE 7b Spectral Transmittance O.IOmmCO.OOIin) Poly(vinyl fluoride)

Exposed to 150°C(302°F) for Four Time Periods 1

1

1 1 1 r

Control - no heat exposure

10 hours 225 hours 1000 hours 1825 hours

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

29
Figure 8 Appearance of Poly(vinyl fluoride) after Exposure at

150°C{302°F) for Various Periods

noexposure

10 h 225 h 1250 h 1825 h

30
The two glass materials had some scatter in the solar energy transmittance

results. The irregularities may be attributed to two causes: 1) slight variations in the thickness of the test specimens; and 2) deposit of a film on the

glass. Since glass and plastic test specimens were simultaneously exposed in

the same oven, products from outgassing or deterioration of the plastic materials may have been deposited on other materials thereby causing slight changes

In transmittance.

The rate of solar energy transmittance loss varied among the materials, as shown in figure 9a. The 1.5 mm fiber reinforced plastic decreased rapidly

until about 1500 hours exposure when it leveled off. For the 1.0 mm fiber

reinforced plastic material, there was a large immediate loss followed by a gradually increasing loss. These two fiber reinforced materials were not the same resin and were produced by different manufacturers which is the reason

for the differences in the test results. The poly(vinyl fluoride) had a fairly

uniform rate of transmittance loss while the poly (ethylene terephthalate) and

polycarbonate changed more slowly.

4.2.1.2 Exposure at 125°C (257°F)

The materials exposed to 125''C (257°F) were removed from the oven at time

increments ranging from 100 to 2000 hours. While yellowing and deformation

of the materials did occur, it was less pronounced than for the 150°C (302°F)

exposure. The solar energy transmittance of the cover plate materials after

heat aging at 125°C (257°F) is listed in table 5. As was expected, the transmittance losses were less for the 125''C (257''F) exposure than for the 150°C

(302°F) exposure. The 1.5 mm fiber reinforced plastic again had the greatest

solar transmittance loss, declining approximately 30 percent, whereas the 1 mm

fiber reinforced plastic lost 7 percent solar energy transmittance. The poly

(vinyl fluoride) decreased an average 8 percent after 2000 hours, whereas the

poly (ethylene terephthalate) fell about 2.5 percent. Although the transmittance losses for the polycarbonate and poly (methyl methacrylate) were small,

examination of the transmittance curves revealed that absorption had started

to increase below 500 nm. Very little difference was observed in the spectra

for the fluorinated (ethylene propylene) copolymer. The deviations in the

transmittance of the glass materials appear to be due to variations in the

actual glass thickness or film deposits caused by deterioration of other materials. The acrylic film test specimens were too deformed from shrinkage to

allow transmittance measurements to be made.

The rate of transmittance loss at 125°C (257°F) is plotted for several materials in figure 9b. Although the rates are lower than for 150°C (302°F) (figure

9a), the general trends are the same for both heat aging exposures.

4.2.1.3 Exposures at 100°C (212°F) and 75°C (167°F)

The cover materials were exposed to 100°C (212°F) and 75°C (167°F) heat aging

for periods of 10 to 2000 hours. The solar energy transmittance for these

materials is tabulated in tables 6 and 7. The decreases in solar energy

transmittance are less than the decreases caused by the 125°C (257°F) exposures for comparable times. From the data, it was obvious that the two fiber

31
FIGURE 9a Rate of Solar Energy Transmittance Loss of Five Cover

Plate Materials Exposed at 150°C(302°F)

Polycarbonate 1.0 mm (0.04 in)

250 500 750 1000 1250

HOURS EXPOSURE

1500 1750 2000
Table 5

Solar Energy Transmlttance of Cover Plate Materials After Heat Aging at 125°C (257°F)^

Exposure Time (hours)

Material ^~ 100 250 500 750 1000 1500 2000

Glass (0.01% iron oxide) 90.7 89.4

89.7

89.9 89.3 89.6 89.7 89.7 89.5

89.6

Glass (0.10% iron oxide) 87.4 86.9

86.7

86.7 86.8 86.8 85.8 86.3 86.7 86,4

Fluorinated (ethylene

propylene) copolymer

96.0 95.0

95.3

95.2 95.4 95.2 95.4 95.3 95.2

95.1

Poly(vinyl fluoride) 92.5 90.5

90.6

90.4 90.3 86.1 88.0 85.9 87.2 83.4

Poly( ethylene terephthalate) 86.3 86.0

85.7 85.0 84.6 84.7 84.9 83.7 82.3

84,9

Acrylic 90.7 87.4

87,7

88,0 b/ b/ y 89.8 V

Poly(methyl methacrylate) 91.0 89.3 89.7 90.3 90.2 90.1 90.2 90,4

90.2

Polycarbonate 88.1 87.5

87.5

87.5 87.3 87.7 87.1 87.0 86.9 86.9

Fiber reinforced plastic (1.0 mm) 86.3 83.2

83.3

82.6 80.3 80.6 77.8 79.2 79,2

79.1

Fiber reinforced plastic (1.5 mm) 78.5 73,7

68.1 63.8 56.9 51.2 55.8 47.0 55,7

45,5

a/

y

Transmlttance measurements were made on a Gary 17D integrating sphere spectrophotometer according

to ASTM E 424-71, Method A, The solar energy transmlttance for air mass 2 at sea level was obtained

by integrating over Parry Moon's Standard solar energy distribution curve using weighted ordinates.

The initial solar energy transmlttance values listed are the average of the control test specimens.

Each transmlttance value for exposed materials represents a separate test specimen. Transmlttance

is expressed in percentage.

Test specimens too deformed from the heat to make a transmlttance measurement.

33
FIGURE 9b Rate of Solar Energy Transmittance Loss of Five Cover Plate

Materials Exposed at 125°C(257°F)

35

30

CO

CO

o

—J 25

UJ

CJ2

<

t 20

S

CO^

<oc

»— 15

>-CD

OC

UJ2

UJ

10

QC

3

CO

Fiber reinforced plastic 1.0 mm (0.04 in)

fiiinride) 0.1 mm (0.04 in)

"terephthalate) 0.13 mm (0.005 in)

Polycarbonate 1.0 mm (0.04 in)

750 1000 1250

HOURS EXPOSURE

1750 2000

34
Table 6

Solar Energy Transmittance of Cover Plate Materials After Heat Aging at 100°C (212°F)^ ——____Exposure Time (hours) Material -_______^ 10 100 250 550 750 1050 1300 1700 2000

Glass (O.OU iron oxide) 90.7 89.8

89.9

90.0 89.8

89.7 88.7 89.4 89.5 88.8 88.6 89.4 89.9

Glass (0.10% iron oxide) 87.4 86.5

86.6 86.986.8

86.7 86.286.4 86.5 86.7 86.1 86.9 87.3

86.9

Fluorinated (ethylene

propylene) copolymer

96.0 95.4

95.3

95.5 94.995.4

95.9

95.6 95.8 95.8 95.5 95.7 96.0

95.6

Poly(vlnyl fluoride) 92.5 91.2 91.0

91.3 89.491.5 91.391.5 91.6 91.5 90.6 90.7 90.5

90.4

Poly( ethylene terephthalate) 86.3 85.7 85.585.6

85.6 86.385.8

86.0 85.3 85.3 84.6 85.6 85.3

85.6

Acrylic 90.7 89.9

89.5

88.7

88.2 89.8

89.5 89.7 89.1 88.8 87.7 88.1 88.888.7

Poly(methyl methacrylate) 91.0 90.2 90.090.4

90.0 89.989.8

90.2 90.3 90.3 89.7 90.1 89.9

90.0

Polycarbonate 88.1 86.9

87.2 86.887.1 87.6

88.1 87.1 87.0 87.0 87.0 87.3 87.387.7

Fiber reinforced plastic (1.0 mm) 86.3 81.4

82.3

82.9

83.0 83.384.0

84.0 84.3 83.6 82'. 7 79.6 81.0

79.7

Fiber reinforced plastic (1.5 mm) 78.3 67.4

72.2 65.968.4 70.5

69.5 78.0 71.0 63.6 61.9 59.6 61.9

65.1

a/ Transmittance measurements were made on a Gary 17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's

Standard solar energy distribution curve using weighted ordinates. The initial solar energy transmittance values

listed are the average of the control test specimens. Each transmittance value for exposed materials represents

a separate test specimen. Transmittance is expressed in percentage.

35
Table 7

Solar Transmittance of Cover Plate Materials After Heat Aging at 75°C (167°F) ^

~~——~_____Expo s ur e Time (hours) Material " —______^ 10 100 250 500 750 1000 1500 2000

Glass (0.01% iron oxide) 90.7 89.9

90.0

90.1 90.390.3 90.2 90.6 90.2 90.2 90.5 90.4

Glass (0.10% iron oxide) 87.4 86.1 86.2 86.5

87.2 86.6 86.7 86.6 87.1 86.6 87.1 87.0

Fluorinated (ethylene

propylene) copolymer

96.0 96.0

96.0

95.6 95.495.5 95.7 95.4 95.5 95.1 95.1 95.1

Poly(vinyl fluoride) 92.5 91.6 91.2

90.2 91.4

90.3 91.3 91.2 91.5 90.1 91.0

91.0

Poly( ethylene terephthalate) 86.3 85.6 85.9

86.0

85.1 84.4 85.9 85.5 84.8 84.5 85.0

85.0

Acrylic 90.7 89.4

90.4 88.6 89.0

89.8 89.9 89.2 89.3 89.7 89.5

90.2

Poly(methyl methacrylate) 91.0 90.3 90.590.4

90.4

90.2 90.7 90.4 90.4 90.4 90.8 90.6

Polycarbonate 88.1 87.4

87.3

87.2 87.487.1 87.2 88.1 86.8 86.9 87.1 87.3

Fiber reinforced plastic (1.0 mm) 86.3 81.3 87.385.3

83.4

83.3 81.0 83.8 83.8 82.5 83.4 82.4

Fiber reinforced plastic (1.5 mm) 78.3 74.2 74.6 75.3

77.3

79.9 69.3 72.1 69.3 66.6 71.8

71.4

—a/

' Transmittance measurements were made on a Gary 17D integrating sphere spectrophotometer according to ASTM E 424-71 Method A. The solar energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's

Standard solar energy distribution curve using weighted ordinates. The initial solar energy transmittance values

listed are the average of the control test specimens. Each transmittance value for exposed materials represents

a separate test specimen. Transmittance is expressed in percentage.

35
reinforced materials suffered deterioration which had caused a loss in transmittance, however, decreases in the transmittance of the other materials were

generally less than 2 percent. To determine if the losses were caused by the

heat, the spectral transmittance graphs were inspected. They indicated that

absorption had increased in the ultraviolet-visible region for poly(vinyl

fluoride), acrylic film, poly(ethylene terephthalate) , and poly(methyl

methacrylate). The increased absorption observed in the graphs for these

materials indicated that the solar energy transmittance losses, though small, were caused by deterioration of the material. For polycarbonate, small variations in transmittance existed across the entire spectrum, however, there was

little change in the location of the absorption band in the ultraviolet region,

The spectral transmittance curves for the fluorinated (ethylene propylene)

copolymer showed minor variations when compared with the control test specimens. The transmittance variations of the glass materials were irregular and

did not appear to be caused by the heat.

4.2.1.4 Comparison of Exposures

Except for glass and fluorinated (ethylene propylene) copolymer, two conclusions can be made from the data in tables 4-7. First, the solar energy transmittance decreased as the cover material was exposed to longer periods in a heat aging, and second, as the temperature was increased, the solar energy

transmittance decreased. The rate of transmittance loss was related to the

exposure temperature. Figures lOa-lOg provide a comparison of the transmittance curves of the control test specimens and the plastic materials after

approximately 2000 hours in the four heat aging exposures. The 2000 hours is

roughly equivalent to one year with the temperature elevated six hours per day.

Since only relatively small changes were observed with glass and fluorinated

(ethylene propylene) copolymer, their transmittance curves are not compared.

4.2.2 Linear Dimensional Stability

Linear dimensional stability was evaluated after 10 and 100 hours of heat

aging at 100°C (212°F) and 150°C (302°F). Table 8 contains the results of

the measurements. The data are the average from three test specimens. As was expected, no change was observed for the glass materials or the fiber

reinforced plastic materials (which were thermosets). However, the other six

plastic materials exhibited anisotropy, i.e., unequal changes in length and

width. Although the film test specimens were square prior to exposure, examination of the exposed test specimens indicated that the changes were not proportional across the material. Figures 11a and lib illustrate the irregular

shape of some film test specimens after heat aging. The dimensional

measurements were made on the centerline in each direction.

Only small differences were observed in the data for the 10 and 100 hour

exposure. Comparing the 100 hour data for the two temperatures showed that

the materials behaved similarly at both temperatures although greater expansions or contractions took place at 150°C (302 °F). Poly(vinyl fluoride) and

polycarbonate were exceptions to this. At 100°C (212°F) poly(vinyl fluoride)

37
FIGURE 10a Spectral Transmittance O,10MM(O,O0iJiN) Poly(vinyl fluoride)

Exposed to Four Temperatures for Approximately 2000 Hours

Control - no heat exposure

75"C (167°F) 100°C (212°F)

125°C {2brf)

150°C (302°F)

"V^

1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

FIGURE 10b Spectral Transmittance of 0.13mm(0,Q05in) Poly(ethylene

terephthalate) Exposed to Four Temperatures for Approximately

20Q0 Hours

(/} 40

30 [t

20

10 If

T

Control - no heat exposure

125''C (257°F)

— 100°C (212°F)

75°C (167°F)

150°C (302 ^^r^

400 600 800 1000 1200 1400 1600

WAVELENGTH (nm)

1800 2000 2200

38
FIGURE 10c Spectral Transmittance of 0.076mm(0.003in) Acrylic Film Exposed to Three Temperatures for Approximately 2000 Hours

100

90

80

70

S5

LUo 60

Z

^

t 50

Sv»<

40

QC 30

20

10

1 1 1 r

Control - no heat exposure

75°C (167°F)

100°C (212°F)

150°C (302°F)

J

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

FIGURE lOd Spectral Transmittance of 1 5nnm(0.06in) Poly(methyl methacrytate) Exposed to Three

Temperaturesfor Approximately 2000 Hours

100

90

80

70

60

t 50

s

V) 40

<QC

1— 30

20

10

1 1 1 1 —Control - no heat exposure

bJ-^

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

3g
FIGURE IOe Spectral Transmittance of I.OhmCO.O^in) Polycarbonate

Exposed at Two Temperatures for Approximately 2000 Hours

100 -

1 1 1 r

1 1 1

1 1

1 Control - no heat exposure

90 - J- >A X^-^

80 - W 1 -^1^ -

\ ^100°C (212°F) ^ V S

70 - ^

125°C (257°F)

S 60

r V

z

<

t 50

1-0

oc

1- 30

-

r

20 - -

10 -

J \ L.. 1 1 1 1 1 1 400 600 800 1000 1200 1400 1600

WAVELENGTH (nm)

1800 2000 2200

FIGURE IOf Spectral Transmittance 1,0mm(0,04in) Fiber Reinforced

Plastic Exposed at Four Temperatures for Approximately

2000 Hours

100

1 1 1 1 1 1 I 1 I

1 Control - no heat exposure

90 / ^75°C (167°F)

80 //^^^25X(257°F) ^-^"l .^ /^

70 // / \^100°C (212°F) ' fW \

S5 1 / ^150°C (302°F) I \ y 60

E 50

s

% 40

oc

1- 30

-

\

20 -

10 - j W 1 1 1 1 1 1 1 1 400 600 800 1000 1200 1400 1600

WAVELENGTH (nm)

1800 2000 2200

an
FIGURE IOg Spectral Transmittance of l,5rw(0.06iN) Fiber Reinforced

Plastic Exposed at Four Temperatures for Approximately

2000 Hours

I 1 1

1 \ T

Control - no heat exposure - 7bx mr?)

400 600 1000 1200 1400 1600

WAVELENGTH (nm)

1800 2000 2200

41
Table 8

ff Percent Change in Linear Dimensions of Cover Plate Materials After Heat Aging^

100°C (212''F) 150°C (302°F)

Material Direction

10 hours 100 hours 10 hours 100 hours

Glass (0.01% iron

oxide)

length

width

Glass (0.10% iron oxide) length

width

Fluorinated (ethylene

propylene) copolymer

length

width

-1.9 +0.9

-1.9 +0.9

-3.2 +1.7 -3.2 +1.7

Poly(vinyl fluoride) length

width

-2.9 +0.6-2.9

+0.6

-4.1 -1.8-4.5

-2.4

Poly(ethylene

terephthalate)

length

width

-0.3

-0.1 -0.3-0.1 -1.0-0.3 -1.3-0.5

Acrylic length

width

-8.4

-5.9 -8.6-6.3 -45.0

-13.1 -45.4-13.5

Poly(methyl

methacrylate)

length

width

0.32 0.380.32 0.381.09

2.11

1.09 2.11

Polycarbonate length

width

+0.35

+0.26

+0.52 +0.26 y^ y^

Fiber reinforced

plastic (1.0 mm)

length

width

Fiber reinforced

plastic (1.5 mm)

length

width

-^ The fluorinated(ethylene propylene) copolymer, poly(vinyl fluoride), poly(ethylene terephthalate) and acrylic

film were measured using ASTM D 1204. The other materials were measured with ASTM D 1042. The values in the table are an average of three test specimens.

y Polycarbonate warped and deformed during the heat exposure. Although the test specimens were flattened

for measurement, the measurements on these test specimens are approximate due to permanent deformation.

42
FIGURE 11a Linear Dimensional Changes of Fluorinated (ethylene propylene)

Copolymer Exposed at 150°C(302°F) -4-,

10 hours exposure

Initial shape —*

FIGURE 11b Linear Dimensional Changes of Acrylic Film Exposed at 150°C(302°F)

- InKial shape

I—10 hours exposure r- ""--"------,

'— 100 hours exposure '

I it- I

i^^^^^

J

43
expanded 0.6 percent in the width direction, whereas, it contracted 2.4 percent

at 150°C (302°F). The polycarbonate was similar although the contraction was greater (i.e. five percent) at 150°C.

The acrylic film proved to be the most dimensionally unstable material. It

shrank substantially in length and width at both temperatures. After 100 hours

at 150°C (302°F) it had contracted 45.4 percent in length and 13.5 percent in

width, see figure lib. Similar shrinkage, although not as large, occurred at

100°C (212°F). The fluorinated (ethylene propylene) copolymer expanded in

width while shrinking in length, but the test specimen was not rectangular

after the exposure. Figure 11a illustrates the shape after 150°C (302°F)

aging. The poly (ethylene terephthalate) contracted in both length and width

although the shrinkage was not equal in both directions.

Although there are small differences for some materials between the 10 and 100

hour data, it appears that measurement of the linear dimensions after 10 hours

exposure at a particular temperature is sufficient to establish trends and

determine approximate values for a temperature. The properties of viscoelastic

materials such as plastics are a function of the time at a particular temperature because of stress relaxation constants. To determine total change at a given temperature, the dimensions should be measured at increasing time intervals until no additional dimensional change is induced by continued heat

exposure.

4.2.3 Warpage

Warpage and shape distortion were observed on test specimens from the 150°C,

125°C, and 100°C (302°F, 257°F, and 212°F) heat stability exposures. Films were not evaluated in this test since a material had to be self supporting to

measure warpage. Materials exhibiting warpage were poly (methyl methacrylate)

,

polycarbonate, and 1.0 mm fiber reinforced plastic. The test specimens were suspended vertically during the heat stability test and constrained only by

the paper clip in the center of the short edge. As a result, the materials

were free to move. Table 9 provides the ranges in warpage measured after the

heat aging.

The poly(methyl methacrylate) became extremely distorted during the 150°C

(302°F) heat aging and meaningful warpage measurements were not possible

•on these specimens. Figure 12a shows the appearance of some of these test

specimens. At the lower heat aging temperatures the poly(methyl methacrylate)

suffered some warpage, however it was not as drastic. The amount of warpage

(i.e. deviation from flatness) was not directly related to exposure time. The

heat aging period varied from 100 to 2000 hours. The edges of the polycarbonate test specimens curled up from the heat. The resulting warpage is

illustrated in figure 12b. Apparently the elevated temperature soften the

polycarbonate enough such that stresses developed during the manufacture of

the sheet were relieved. As with the poly(methyl methacrylate), the amount

of warpage was not directly related to the exposure time.

44
FIGURE 12a Appearance of Poly(methyl methacrylate) after 150°C

(302°F) Heat Aging.

'i:

' #

a*-

y-^mifuS

FIGURE 12b Appearance of Polycarbonate after 150°C (302°F)

Heat Aging.

45
Table 9

Warpage of Test Specimens After Heat Aging

Material^/ 150^0 (302°F) 125°C (257°F) 100°C (212»F)

poly(methyl methacrylate)

polycarbonate

fiber reinforced

plastic 1.0 mm

too distorted to

measure

35.3-56.6 mm

(1.39-2.23 in)

6.1-11.8 mm

(0.24-0.46 in)

6.4-20.2 mm

(0.25-0.80 in)

5.1-15.2 mm

(0.20-0.60 in)

slight-8.4 mm

(slight-0.33 in)

4.8-17.5 cm (0.19-0.69 in)

8.3-14.0 mm

(0.33-0.55 in)

slight-8.3 mm

(slight-0.33 in)

i/Test specimens were about 115 mm by 230 mm (4.5 by 9 in) and were suspended vertically from the center of the short edge during the heat aging. Aging periods

ranged from 100 to 2000 hours, however the amount of warpage was not directly

related to aging time. Measurement of the warpage is described in Section 3.3.3.

The 1.0 mm fiber reinforced plastic exhibited slight distortion after the heat

aging. Although the distortion was slight (see table 9), it was observable and can account for the wavy appearance of fiber reinforced plastic glazed collectors reported by some observers.

4.3 ARTIFICIAL WEATHERING WITH XENON ARC LIGHT

4.3.1 Solar Energy Transmittance

Test specimens were exposed to the xenon arc light in an artificial weathering

device for time increments ranging from 250 to 4000 hours. The solar energy

transmittance of the materials after this artificial weathering is tabulated in

table 10. The integrated solar energy transmittance of the materials changed

little due to the exposure. Inspection of the corresponding spectral transmittance curves (e.g. figures 13a & 13b) provided a better indication of the

effect of the artificial solar radiation on the materials.

Polycarbonate had the largest solar energy transmittance loss (3.7 percent)

after 4000 hours. The loss was primarily due to increased absorption in the

ultraviolet and the visible region below 700 nm, as shown in figure 13a. A

2.5 percent solar energy transmittance decrease for the 1.0 mm fiber reinforced

plastic resulted from a decline in transmittance across the entire spectrum, as

is illustrated in Figure 13b. The 1.5 mm fiber reinforced plastic also had

transmittance variations across the entire spectrum, however, except for the

4000 hour data, the solar energy transmittance values are within the range of

the variation seen in the control test specimens (table 2). As the exposure

46
Table 10

a/ Solar Energy Transmittance of Cover Plate Materials After Artificial Weathering with Xenon Arc Light—'

____Exposure Time (hours) Material ___^_^ 250 500 750 1000 2000 3000 4000

Glass (0.01% iron oxide) 90.7 89.9

90.1 90.0 90.2 89.2 89.1 87.6 88.987.5

Glass (0.10% iron oxide) 87.4 87.0

87.0 87.0 87.0 87.0 86.9 86.1 85.986.3

Fluor inated (ethylene

propylene) copolymer

96.0 95.1 95.495.5 95.4 95.4 95.0 95.0 95.2 94.8

Poly(vinyl fluoride) 92.5 91.4 91.691.4 91.4 91.4 91.1 91.4 91.5

91.3

Poly( ethylene terephthalate) 86.3 85.9

85.1 85.5 85.3 85.4 85.5 85.3 84.9 84.9

Acrylic 90.7 90.0

90.0 89.9 90.3 90.1 90.0 89.2

89.8

Poly(methyl methacrylate) 91.0 89.3

89.4

89.2 89.1 88.9 89.2 88.7 89.0

88.9

Polycarbonate 88.1 87.8

87.5 87.0 86.6 86.4 85.3 85.6 84.584.3

Fiber reinforced plastic (1.0 mm) 86.3 85.5

85.1 86.6 85.5 86.2 84.7 85.2 84.083.6

Fiber reinforced plastic (1.5 mm) 78.5 79.7

76.4

77.7 79.0 81.3 79.8 79.3 71.6

70.2

a/ Transmittance measurements were made on a Gary 17D integrating sphere spectrophotometer according to ASTM E 424-71 Method A. The solar energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution curve using weighted ordinates. The initial solar energy transmittance values listed are the average of the control test specimens. Each transmittance value for exposed materials represents a separate test specimen. Transmittance is expressed in percentage.

47
400

FIGURE 13a Spectral Transmittance of I.OmmCQ.OIin) Polycarbonate

After Exposure in a Xenon Arc Artificial Weathering Device

1000 1200 1400 WAVELENGTH (nm)

2000 2200

100

90

80

70

se 60

LUo

z

<r 50

h-1— 40 z

<

K

t- 30

20

10

FIGURE 13b Spectral Transmittance of l.OMMCO.OiiiN) Fiber Reinforced

Plastic After Exposure in a Xenon Arc Artificial Weathering

Device T T T T

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENTH (nm)

13
FIGURE 13c Spectral Transmittance of 0,13mm(0.005in) PolyCethylene

terephthalate) After Exposure in a Xenon Arc Artificial

Weathering Device

400 600

4000 hours

Control - no exposure

250 hours

3000 hours

800 1000 1200 1400

WAVELENGTH (nm)

1600 1800 2000 2200

tl9
time increased, the transmittance below 500 nm declined. The integrated solar

energy transmittance data mask the increased absorption at the ultraviolet

end of the spectrum.

During the artificial weathering exposure the poly(ethylene terephthalate)

suffered a small but definite decline in the transmittance below 700 nm and

the absorption band moved toward the visible range. This change is displayed

in figure 13c. For poly(methyl methacrylate) the absorption band shifted more

than 50 nm toward the visible region in the first 250 hours; after this,

decreases in transmittance were small. The acrylic film also had observable

absorption increases along the absorption band in the ultraviolet-visible

region.

The transmittance of the f luorinated(ethylene propylene) copolymer fell off

slightly below 700 nm but the absorption band remained below 325 nm. The

transmittance of the poly (vinyl fluoride) dropped somewhat from 375 to 700 nm

but increased in the region of the absorption band between 300 and 375 nm. The glass with the 0.10 percent iron oxide content showed no significant change

in the transmittance curves whereas the low iron glass had small deviations in

the transmittance across the spectrum. This seemed to indicate possible variations in the glass thickness.

4,3.2 Warpage

No significant warpage was observed in the test specimens after artificial

weathering.

4.4 NATURAL WEATHERING

4.4.1 Temperatures of Minicollectors

The temperatures of the cover plate and absorber materials were measured on

one set of minicollectors exposed in Gaithersburg, Maryland. Thermocouples

were installed in the center of the back of the absorber of a complete set of

minicollectors. In addition, thermocouples were installed on one cover plate

of the minicollectors having cover plates made of glass (0.01 percent iron

oxide), poly (vinyl fluoride), poly (methyl methacrylate), and both fiber reinforced materials. For the double cover minicollectors this thermocouple was

on the inner cover plate. The thermocouples on the cover plates were shielded

from direct radiation from the sun and the absorber. The choice of the cover materials for temperature measurement was based upon optical properties, (i.e.

initial solar and infrared transmittance) which affect temperature. The set

of minicollectors had approximately 80 days weathering prior to the measurements. The temperatures of the materials are tabulated in table 11. The

absorber temperatures range from 95-109°C (203-228°F) for single cover exposures and 99-117''C (210-243°F) for the double cover exposures. The cover plate

temperatures varied from 59-63°C (138-145°F) for single covers and 78-85°C

(172-185°F) for the inner double cover. In comparing these temperatures to

those calculated for similar cover plates on stagnating collectors (tables A6

and A7 in Appendix A), it was observed that these were lower. However, since

these temperatures were measured three hours after solar noon, when the solar

50
>o T3

4J

C

to X) rH

(U t^

CO I-l o CO asXw M60

CO )-i iH 3

to XI

•H CO Vj I-l (U (U •U x:

CO •w S •H

CO

0)

u

u

to c

iH •w

lILi ^H to -H I-l I-l (U o

0) > •u iH O o

XI

U CU

to iH H t3 M C o

to o

•H

^1 c

<u •H xa s

;jo c

CO o

rO< onc

<A-i •H O Vi

(U

CO x:

(U

w

^4 CO 3 m

4J S COU .H

<u CO o. U § 3

4-1

H CO 2

QJ I-l ^ (U 3 >

to o o CJ a tn 00

m <T> <yi

C

X ^ 00 1 1 l->. 1 ^1 00 1 r^ r-~ o w tu c u (3 OJ OJ •

tu f—1

0)

X

:2i >o X

H

XI

CO •

Csl

CU

CO+

<u

X

X CO

r-( I-l r-» vO r-^

m ^D r-«

m <3^ ^ C3^ 4-1 ^^^

0)

X o 1—I

i-H o o O o o O o 0^ CO CO 00

Vj 3 CO ,-_( ^H f—i -H r-i rH 1—1

rH 1—1IH 3 • 3 o X O 60 rH u Q < 4-1 to O XI X u CU c

QJ <-i -H

u

a 01

I-l 3

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to

rH & O

CJ 4-1 o H COo CU X H 00

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a (T\ CN CM CN CO

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u LO 1 1 vO 1 1 vO 1 vO vD •H rH

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51
irradiance was lower than that used for the calculated temperatures, it is

likely that the temperatures reached near solar noon by the materials on these

minicollectors would have been in the range of stagnation temperatures in solar

collectors

.

4.4.2 Cumulative Solar Radiation

The exposure schedule and the solar radiation accumulated by the test specimens

during exposure are summarized in table 12. Comparing the solar radiation

accumulated at the three sites during simultaneous exposure, it is observed

that the test specimens received approximately 50 percent more solar radiation

at New River than at Gaithersburg, and about 25 percent more solar radiation at

New River than at Miami. Similarly, the test specimens at Miami received about

20 percent more solar radiation than those at Gaithersburg.

4.4.3 Visual Observation

4.4.3.1 Minicollector Boxes

The minicollector boxes were examined as they were returned to the lab after

the exposures. Although the boxes had been sealed prior to exposure, problems

did occur. From the first set returned, it was evident that there were problems with the boxes, i.e. peeling paint, water accumulation in some boxes,

swelled and cracked wood. The problems occurred at all three exposure sites

and were apparently initiated by moisture intrusion into the box. Whether the

initial moisture penetration occurred through the paint or through voids is

uncertain; however, once the moisture began to accumulate in the box, peeling

paint and swelled wood followed. Expansion and contraction of the wood caused

it to crack and, in some cases, the bottom of the boxes split. Weep holes were drilled in the bottom of the boxes to prevent accumulation of water, and additional efforts were made to seal the boxes, where possible. Nevertheless,

deterioration of the boxes has continued to be a problem. Wood rot has

occurred on some of the boxes exposed in Miami.

It is probable that these problems could be reduced in a number of ways:

1) Extending the test specimen to the edges of the box; 2) Placing an "L"

shaped molding over the edge of the test specimen and sealing it to the box;

3) Using paints or sealants that are impervious to moisture; 4) Using metal or plastic for the box instead of wood.

Examination of the boxes during exposure determined that some of the test specimens, in particular poly(methyl methacrylate) and polycarbonate, partially

lost adhesion to the sealant and lifted up. When this occurred, the test

specimens were reattached to the boxes. The minicollectors on exposure in

Gaithersburg are pictured in figure 14. For many of the minicollectors it is

evident that there is moisture on the interior of the cover plates.

4.4.3.2 Test Specimens

The test specimens were examined prior to removal from the minicollector boxes.

Some of each type of glass test specimen had broken during the exposure. This

52
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53
FIGURE 1^\ HlNICOLLECTORS EXPOSED IN GaITHERSBURG. MARYLAND

x«J;^i^' I 1 'f^fffifBiiiiiiiiiiiiii

if ffffiii^iiiiavitfiiiiiiiiii

54
breakage was apparently due to the manner in which the boxes were held on the

exposure rack. It was not due to impact. One test specimen did crack during a hail storm in Arizona. Both types of glass were annealed. The test specimens

had been cut from large sheets and the edges had not been treated to reduce the

stress levels. It is doubtful cracking would have occurred with tempered

glass.

With the fluorinated (ethylene propylene) copolymer, there was little change in

appearance other than an accumulation of dirt. However, several of the double

cover minicollectors (some from Miami and some from Gaithersburg) developed

small holes 20-40 mm (0,75 - 1.5 in) from the edge. The holes were reported

to be caused by birds pecking at the cover plates. The minicollectors in

Arizona suffered damage due to a hail storm during the 29th month of exposure.

Damage ranged from dents to holes of 13-20 mm (0.5-0.75 in) diameter.

The poly (vinyl fluoride) became extremely taut across the boxes, apparently

caused by shrinkage of the material The test specimens gradually yellowed

as the exposure continued. In addition, the inner cover of the double cover

minicollectors exposed for 18 and 24 months in Arizona and for 24 months in

Florida became so brittle that they ruptured in place. Figure 15 shows the

appearance of one of these minicollectors. The behavior of this material

is similar to the deterioration of poly(vinyl fluoride) cover materials

exposed on stagnating collectors near Denver, Colorado [3]. One of the four

minicollectors in Arizona at 29 months exposure developed dents, but not

holes, during a hail storm.

The poly (ethylene terephthalate) outer covers exposed for one year and longer

in Miami developed a white, opaque appearance. Although the material was transparent prior to exposure, it became a milky white. Closer examination of

the test specimens found that the white material was present only on the exterior surface. Apparently the environmental factors in Miami, e.g. more moisture

and an industrial atmosphere, caused changes in the surface of the poly

(ethylene terephthalate) exposed directly to the weather. The white material

was not present on the interior side of the outer cover nor on the inner cover. Examination of outer surface with a scanning electron microscope indicated

minute cracking on the surface.

The poly(ethylene terephthalate) test specimens from Arizona were damaged

apparently by a hailstorm during the 17th month of exposure (see figure 1).

All of the outer double cover and single cover poly (ethylene terephthalate)

test specimens on exposure at that time had holes and cracks in them. The

material had become extremely brittle. The inner cover remained intact. All

but one of the minicollectors with poly (ethylene terephthalate) from Arizona

were terminated after 18 months due to this damage. The remaining minicollector suffered similar serious damage in a hail storm during the 29th month of

exposure. None of the other cover plate materials were damaged by the hailstorm during the 17th month of exposure.

The acrylic film was the first material to physically deteriorate during the

exposure. It cracked and broke into pieces at all three exposure sites. The

material failed most rapidly in Arizona, followed by Florida and Maryland.

55
FIGURE 15 A Double Cover Minicollector with Poly(vinyl fluoride)

Covers after 18 Months Exposure at New River, Arizona.

The inner cover has ruptured.

56
The outer cover test specimen deteriorated more rapidly than the inner one. After 18 months exposure in Arizona and Florida, the outer covers had com- pletely disintegrated and blown away. The inner cover and single cover test

specimens exhibited somewhat less damage. All minicollectors with the acrylic

film have been terminated due to material failure.

The poly(methyl methacrylate) appeared to be somewhat less dirty than most of

the rest of the materials after 24 months. On the minicollectors from Arizona,

the inner cover sagged in the center, apparently due to the elevated temperature.

The polycarbonate material visibly yellowed as it was exposed. In addition,

the test specimens exposed in Florida developed deposits on the outer surface.

The deposits were not washed off during the cleaning and were apparently due

to the industrial atmosphere near the exposure site.

Both of the fiber reinforced materials yellowed during the outdoor exposure,

with the 1.5 mm FRP changing the most. The resin of this material also progressively developed a whitish appearance when weathered in Florida. This

change may be attributed to either moisture or the industrial atmosphere.

After 24 months exposure the 1.5 mm fiber reinforced material also exhibited a significant amount of "fiber bloom" on the outer covers while the 1.0 mm fiber

reinforced material showed a small amount. "Fiber bloom" occurs when fibers

break away from the resin and protrude from the surface. The "fiber bloom"

occurred most on the test specimens from Arizona.

4.4.4 Solar Energy Transmittance

4.4.4.1 Gaithersburg, Maryland Exposures

The solar energy transmittance after the natural weathering in Gaithersburg is

tabulated in table 13 for the single cover test specimens and table 14 for the

double cover test specimens. In all cases, the solar energy transmittance

declined; however the loss varied among the materials. The solar transmittance

loss had two causes: 1) material deterioration due to weathering, and 2) dirt

accumulation on the cover plate. The combined solar energy transmittance loss

due to both weather and dirt can be determined by comparison of the average

value for the controls with the data in tables 13 and 14. Dif ferentration

between the causes was accomplished by measurement of the transmittance before

and after washing the materials. The spectral transmittance of a control and

a test specimen before and after washing are given in figure 16 to illustrate

these differences. The solar energy transmittance loss due to weathering was determined by comparison of the average solar energy transmittance of the

controls with the value of the washed test specimen. The solar energy transmittance loss due to dirt was obtained by comparing the values obtained before

and after washing.

The solar energy transmittance losses due to weathering in Gaithersburg are tabulated in tables 15 and 16. The transmittance losses for the single covers

and outer double covers followed the same general trend for all the materials.

However, for the inner double covers the losses were much lower for all

57
Table 13

Solar Energy Transmittance of Cover Plate Materials Exposed

as Single Covers to Natural Weathering in Gaithersburg, MarylandS/

^ Exposure Period (months) 3 6 12^ 12^ 18 24

Material ^\Solar Radiation (GJ/m^)^ 1.176 2.251 4.633 5.477 7.680 10.720

glass (0.01% iron oxide) 90.7 90.4 90.5 87.7 85.5 86.2 84.0

glass (0.10% iron oxide) 87.4 86.1 85.3 81.6 82.9 84.4 81.2

fluorinated (ethylene propylene)

copolymer

96.0 92.0 93.8 83.3 84.8 84.0 86.0

poly(vinyl fluoride) 92.5 89.5 86.9 80.8 86.1 87.3 85.8

poly(ethylene terephthalate) 86.3 84.7 82.7 75.8 80.9 80.4 75.3

acrylic 90.7 87.7 85.7 79.0 83.2 dy d/

poly(methyl methacrylate) 91.0 88.1 86.5 80.4 85.4 84.9 81.9

polycarbonate 88.1 87.3 _ 78.3 83.0 80.3 78.5

fiber reinforced plastic 1.0 mm 86.3 85.1 82.3 74.7 80.0 80.0 77.4

fiber reinforced plastic 1.5 mm 78.5 75.0 69.9 66.2 64.9 70.8 65.6

d

\J

Materials were exposed on minicollectors. Transmittance measurements were made on a Gary

17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The solar

energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution curve using weighted ordinates. Transmittance

was measured after exposure but before washing. Each transmittance value represents a separate test specimen. Average solar transmittance of the control test specimens is given

for comparison. Transmittance is expressed in percentage.

Two sets of minicollectors were exposed for the same length of time but different dates. ^ Solar radiation values in GJ/m are converted to Btu/ft by: Btu/ft = (GJ/m x 1.135 x 10 ), d/ Material failed prior to this time.

58
Table 14

Solar Energy Transmittance of Cover Plate Materials Exposed as Double Covers to Natural Weathering in Gaithersburg, Maryland-iJ

uter Cover

^n. Exposure Period (months) 3 6 12b/ 12^ 18 24 Material^ Solar Radiation (GJ/m^)^ 1.176 2.251 4.633 5.477 7.680 10.720

glass (0.01% iron oxide) 90.7 89.3 88.3 82.3 88.8 87.9 85.2

glass (0.10% iron oxide) 87.4 86.6 85.4 79.2 84.2 86.1 83.1

fluorinated(ethylene propylene)

copolymer

96.0 94.5 92.4 82.3 86.7 86.9 84.7

poly(vinyl fluoride) 92.5 89.6 83.3 82.2 86.0 88.7 83.6

poly(ethylene terephthalate) 86.3 84.8 80.1 79.7 81.4 82.0 74.1

acrylic 90.7 88.1 84.2 82.9 84.5 83.5 d/

polydnethyl methacrylate) 91.0 88.6 86.3 81.9 86.3 86.0 _

polycarbonate 88.1 86.6 85.0 81.1 82.0 81.3 77.1

fiber reinforced plastic 1.0 mm 86.3 84.4 81.6 76.6 80.9 79.0 77.8

fiber reinforced plastic 1.5 mm 78.5 71.5 76.6 71.1 71.9 65.5 67.3

Inner Cover

^S. Exposure Period (months) 3 6 i2ty 12b/ 18 24

Material^ . Solar Radiation (GJ/m^)^ 1.176 2.251 4.633 5.477 7.680 10.720

glass (0.01% iron oxide) 90.7 89.3 _ 89.3 91.8 90.1 89.8

glass (0.10% iron oxide) 87.4 87.0 _ 86.5 86.3 86.8 85.7

f luorinated(ethylene propylene)

copolymer

96.0 95.5 94.9 95.2 95.5 95.1 94.4

poly(vinyl fluoride) 92.5 90.6 89.7 91.5 91.5 91.9 91.7

poly(ethylene terephthalate) 86.3 85.7 85.5 84.6 85.3 85.1 85.5

acrylic 90.7 90.0 89.2 89.6 89.9 89.6 d/

poly(methyl methacrylate) 91.0 90.2 89.6 90.1 89.8 91.1 _

polycarbonate 88.1 88.0 86.9 87.1 87.4 87.6 87.3

fiber reinforced plastic 1.0 mm 86.3 86.0 84.7 81.0 82.8 84.8 84.5

fiber reinforced plastic 1.5 mm 78.5 75.0 67.8 67.0 70.4 67.1 62.8 d Materials were exposed on minicollectors. Transmittance

before washing. Transmittance measurements were made on spectrophotometer according to ASTM E 424-71, Method A. air mass 2 at sea level was obtained by integrating over distribution curve using weighted ordinates. Each transm

test specimen. Average solar transmittance of the contro comparison. Both the inner and outer covers on each mini Transmittance is expressed in percentage.

was measured after exposure but

a Gary 17D integrating sphere

The solar energy transmittance for Parry Moon's standard solar energy ittance value represents a separate

1 test specimens is given for collector were the same material.

by Two sets of minicollectors were exposed for the same length of time but different dates. ^ Solar radiation values in GJ/m are converted to Btu/ft by: Btu/ft^ = (GJ/m^ x 1.135 x 10^^).

d Material failed prior to this time.

59
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60
Table 15

Solar Energy Transmittance Loss of Cover Plate Materials Exposed

as Single Covers to Natural Weathering in Gaithersburg, Marylanda,b/

^\ Exposure Period (months) 3 6 11^ 12^ 18 24

Material^ Solar Radiation (GJ/m^)^ 1.176 2.251 4.663 5.477 7.680 10.720

glass (0.01% iron oxide) 1.6 0.6 -0.6 0.5 -0.4 2.6

glass (0.10% iron oxide) -0.8 0.6 -0.1 -0.4 0.3 1.5

fluorinated (ethylene propylene)

copolymer

-0.3 0.8 0.4 2.6 2.4

poly(vinyl fluoride) 3.0 3.0 5.2 0.6 3.3 4.9

poly(ethylene terephthalate) 1.1 3.3 3.5 0.6 1.7 9.7

acrylic 0.6 3.4 4.7 3.4 ej ey

poly(methyl methacrylate) 1.5 1.9 1.8 1.7 2.9 5.2

polycarbonate -0.1 _ 1.1 0.9 2.5 6.5

fiber reinforced plastic 1.0 mm 0.6 2.1 1.8 1.9 3.8 6.0

fiber reinforced plastic 1.5 mm 3.5 6.6 1.9 9.0 4.0 12.9

—a/

' Materials were exposed on minicollectors . Transmittance measurements were made on a Gary 17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The

solar energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution curve using weighted ordinates. Each

transmittance value represents a separate test specimen. Transmittance is expressed

in percentage.

b/

c/

d/

e/

The solar energy transmittance loss is the difference between the average for the control

test specimens (Table 11) and the values obtained after the test specimen was washed.

Negative values represent a gain in comparison with the average for the controls. This

solar energy transmittance loss does not Include the loss due to dirt on the surface.

Losses due to dirt are tabulated in Table 17.

Two sets of minicollectors were exposed for the same length of time but different dates.

9 9 9 Solar radiation values in GJ/m are converted to Btu/ft by: Btu/ft = (GJ/m^ X 1.135 X 10^^).

Material failed prior to this time.

61
Table 16

Solar Energy Transmlttance Loss of Cover Plate Materials Exposed

as Double Covers to Natural Weathering in Gaithersburg, Maryland^^—

Out er Cover \v Exposure Period (months) 3 6 ^^d lid 18 24

Material ^ Solar Radiation (GJ/m^)^ 1.176 2.251 4.633 b.kii 7.680 10.720

glass (0.01% Iron oxide) 1.1 0.6 0.0 -0.8 0.4 1.5

glass (0.10% iron oxide) -0.4 0.2 0.0 -1.6 -0.5 -0.5

fluorinated(ethylene propylene)

copolymer

0.2 0.7 0.2 0.5 2.9 4.3

poly(vinyl fluoride) 2.0 3.2 4.8 -0.3 0.3 5.9

poly(ethylene terephthalate) 0.7 1.7 2.8 0.6 1.0 15.0

acrylic 1.6 3.1 2.9 2.9 3.0 £/

poly(methyl methacrylate) 0.4 2.0 1.2 1.4 2.3 _

polycarbonate -0.8 1.5 0.4 1.1 1.7 6.9

fiber reinforced plastic 1.0 mm 1.1 2.4 3.0 2.8 5.8 8.2

fiber reinforced plastic 1.5 mm 5.2 0.9 1.7 5.5 9.7 9.8

Inner Cover >v Exposure Period (months) 3 6 12£/ ,2d 18 24

MaterlalSv Solar Radiation (GJ/m^)^ 1.176 2.251 4.633 5.477 7.680 10.720

glass (0.01% Iron oxide) 1.4 2.9 -0.3 -1.1 0.4 0.6

glass (0.10% iron oxide) -0.9 0.5 0.2 -1.5 0.4 -0.7

f luorinated(ethylene propylene)

copolymer

0.1 0.4 0.2 0.0 0.1 -0.5

poly(vinyl fluoride) 1.1 1.8 0.8 0.7 -0.3 0.5

poly(ethylene terephthalate) 0.3 1.4 0.7 0.4 0.6 0.3

acrylic 0.4 0.7 1.2 0.4 0.8 sJ

poly(methyl methacrylate) 0.1 1.2 0.0 0.3 0.2 _

polycarbonate 0.2 1.0 -0.2 0.2 -0.2 0.2

fiber reinforced plastic 1.0 mm 1.7 1.2 4.7 3.2 2.7 3.6

fiber reinforced plastic 1.5 mm 3.8 11.0 11.4 10.4 12.7 16.8

3/ Materials were exposed on minicollectors. Transmlttance measurements were made on

er according to ASTM E 424-71, Method A.

at sea level was obtained by integrating

ibution curve using weighted ordinates.

e test specimen. Transmlttance is

a Gary 17D integrating sphere spectrophotomet6

The solar energy transmlttance for air mass 2 over Parry Moon's standard solar energy distri

Each transmlttance value represents a separate expressed in percentage.

by

c/

The solar energy transmlttance loss is the di control test specimens (Table 11) and the val washed. Negative values represent a gain in controls. This solar energy transmlttance lo; on the surface. Losses due to dirt are tabul

fference between the average for the

ue obtained after the test specimen was comparison with the average for the

ss does not include the loss due to dirt

ated in table 18.

Two sets of minicollectors were exposed for the same length of time but different

dates. d Solar radiation values In GJ/m^ are converted to Btu/ft^ by: Btu/ft^ = (GJ/m^ x 1.135 lo'^). ^ Material failed prior to this time.

62
materials except the 1.5 mm fiber reinforced plastic. The damage suffered by

this material was most likely caused by the temperature encountered in the

inner cover exposure.

The smallest transmittance losses occurred with the two glass materials, the

fluorinated (ethylene propylene) copol5mier, and the acrylic film. The 1.5 mm

fiber reinforced plastic suffered the greatest loss as both an outer and inner

cover. This indicates the material is susceptable to damage by both solar

radiation and temperature. The losses in the poly (vinyl fluoride), poly

(ethylene terephthalate) , poly(methyl methacrylate) , polycarbonate, and 1.0 mm

fiber reinforced plastic were due to a reduction in transmittance across the

entire solar spectrum but were greatest below 700 nm as the absorption band in

the UV-VIS moved toward the visible region. Between 18 and 24 months, significant transmittance losses occurred with the single and outer cover test specimens.

The solar energy transmittance losses due to dirt are listed in table 17 for

single covers and table 18 for double covers. All materials lost transmittance

due to dirt accumulation on the surface. In most cases the dirt on the inner

cover plates reduced transmittance less than one percent. For outer and single

covers the transmittance losses generally ranged from three to five percent,

although in one case the fluorinated (ethylene propylene) copolymer lost 12.3

percent. It appeared that after the first 12 months the losses did not increase

significantly.

4.4.4.2 Miami, Florida Exposures

The solar energy transmittance after natural weathering on the minicollector

in Miami is tabulated in table 19 for the single cover test specimens and in

table 20 for the double cover test specimens. Transmittance losses due to

weathering (not dirt retention) are listed in tables 21 and 22. In the inner cover application the solar transmittance changes were generally less than one percent, except for the fiber reinforced materials which were more susceptible

to damage caused by the temperature of the inner cover application. For the

outer double cover and the single cover applications the greatest losses also

occurred in the fiber reinforced materials. The glass materials showed only

small changes. The other plastic materials exhibited transmittance losses

across the entire spectrum and the absorption band in the UV-VIS region shifted

toward the visible, an indication of material degradation.

The solar energy transmittance losses due to dirt on the surface of the cover

plate materials are given in tables 23 and 24. For the outer and single

covers, the transmittance losses were generally three to seven percent although

the poly (ethylene terephthalate) lost more than ten percent in three cases. For the inner covers, the dirt caused decreases of one to two percent for the

exposures 12 months and less. However for the 18 and 24 month exposures, the

transmittance losses were three to four percent with the losses for several

materials exceeding five percent. This increase in transmittance loss due to

dirt on the inner cover is probably resulting from decomposition of the wooden

boxes and from dirt leaking in when the covers had loosened.

63
Table 17

Solar Energy Transmittance Loss Due to Dirt on the Surface of the Cover

Plate Materials After a Single Cover Exposure in Gaithersburg, Maryland a/

Material^\^ Exposure Period (months) 3 6 12b/ 12^ 18 24

glass (0.01% iron oxide) -1.3 -0.2 3.6 4.7 4.9 4.1

glass (0.10% iron oxide) 2.1 1.5 5.9 5.9 2.3 4.7

fluorinated(ethylene propylene)

copolymer

4.3 1.4 12.3 1.2 9.4 7.6

poly(vinyl fluoride) 1.2 2.6 6.5 5.8 1.9 1.8

poly(ethylene terephthalate) 0.5 0.4 8.0 4.8 4.2 1.3

acrylic 2.4 1.6 7.0 4.1 8.6 3.7

poly(methyl methacrylate) 1.4 2.6 8.8 3.9 3.2 3.9

polycarbonate 0.9 _ 8.7 4.2 5.3 3.1

fiber reinforced plastic 1.0 mm 0.6 1.9 9.8 4.4 2.5 2.9

fiber reinforced plastic 1.5 mm 2.4 10.4 4.6 3.9 4.3

BJ Materials were exposed to natural weathering on minicollectors. Transmittance

measurements were made on a Cary 17D integrating sphere spectrophotometer according

to ASTM E 424-71, Method A. The solar energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution curve using weighted ordinates. Each transmittance value represents a separate

test specimen. The solar energy transmittance loss due to dirt is the difference

between the values obtained before and after washing the test specimen. Negative

values represent a loss in transmittance after washing. Transmittance is expressed

in percentage.

b/ Two sets of minicollectors were exposed for the same length of time but different

dates.

64
Table 18

Solar Energy Transmittance Loss Due to Dirt on the Surface of the Cover Plate Materials After Double Cover Exposure In Gaithersburg, Maryland^

Outer Cover

Material^^^ Exposure Period (Months) 3 6 12b/ 12^ 18 24

glass (0.01% iron oxide) 0.3 1.8 8.4 2.7 3.2 4.0

glass (0.10% iron oxide) 1.2 1.8 8.2 4.8 1.8 4.8

f luorinated(ethylene propylene)

copolymer

1.3 2.9 3.5 8.8 6.2 7.0

poly(vinyl fluoride) 0.9 6.0 5.5 6.8 3.5 3.0

poly(ethylene terephthalate) 0.8 4.5 3.8 4.3 3.3 2.8

acrylic 1.0 3.4 4.9 3.3 4.2 d

poly(methyl methacrylate) 2.0 2.7 7.9 3.3 2.7 _

polycarbonate 2.3 1.6 6.6 5.0 5.1 4.1

fiber reinforced plastic 1.0 mm 0.8 2.3 6.7 2.6 1.5 0.3

fiber reinforced plastic 1.5 mm 1.8 1.0 5.7 1.0 3.3 1.4

Inner Cover

Material^^^ Exposure Period (months) 3 6 12b/ 12b/ 18 24

glass (0.01% iron oxide) _ 1.7 0.2 0.3

glass (0.10% iron oxide) 1.3 _ 0.7 2.6 0.2 2.4

fluorinated(ethylene propylene)

copolymer

0.4 0.7 0.6 0.5 0.8 2.9

poly(vinyl fluoride) 0.8 1.0 0.2 0.3 0.9 0.3

poly(ethylene terephthalate) 0.3 -0.6 1.0 0.6 0.6 0.5

acrylic 0.3 0.8 -0.1 0.4 0.3 d

poly(methyl methacrylate) 0.8 0.2 0.9 0.9 -0.3 _

polycarbonate -0.1 0.2 1.2 0.5 0.7 0.6

fiber reinforced plastic 1.0 ram -1.4 0.5 0.6 0.3 -0.8 -1.8

fiber reinforced plastic 1.5 mm -0.3 -0.2 0.1 -2.3 0.3 -1.1

a/ Materials were exposed to natural weathering on minicollectors

measurements were made on a Gary 17D integrating sphere spectro

to ASTM E 424-71, Method A. The solar energy transmittance for level was obtained by integrating over Parry Moon's standard so tion curve using weighted ordinates. Each transmittance value

test specimen. The solar energy transmittance loss due to dirt between the solar energy transmittance measurements made before

the test specimen. Negative values represent a loss in transmi

Transmittance is expressed in percentage.

Transmittance

photometer according

air mass 2 at sea lar energy distriburepresents a separate

is the difference

and after washing

ttance after washing.

b/ Two sets of minicollectors were exposed for the same length of time but different

dates. d Material failed prior to this time.

65
Table 19

Solar Energy Transmittance of Cover Plate Materials Exposed

as Single Covers to Natural Weathering in Miami, Florida^

>s^ Exposure Period (months) 3 6 12^/ 12^ 18 24

Material N^ Solar Radiation (GJ/m^)^ 1.455 2.864 6.344 6.523 9.333 13.125

glass (0.01% iron oxide) 90.7 87.0 87.8 87.2 86.2 85.0 84.9

glass (0.10% iron oxide) 87.4 86.4 85.2 82.2 84.1 81.9 82.9

f luorinated(ethylene propylene)

copolymer

96.0 93.2 88.2 86.4 89.0 86.5 84.0

poly(vinyl fluoride) 92.5 89.5 89.6 84.6 86.7 84.4 85.7

poly(ethylene terephthalate) 86.3 83.8 82.5 71.5 78.0 71.3 75.8

acrylic 90.7 87.2 84.9 84.8 84.5 85.4 d/

poly(methyl methacrylate) 91.0 88.6 86.9 84.5 87.3 81.7 83.5

polycarbonate 88.1 85.9 85.1 78.2 81.7 77.6 75.4

fiber reinforced plastic 1.0 mm 86.3 82.6 82.4 77.7 79.8 74.3 76.6

fiber reinforced plastic 1.5 mm 78.5 69.2 71.0 66.3 63.8 53.0 56.5

a/

b/

d

d/

Materials were exposed on minicollectors. Transmittance measurements were made on a Gary 17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's stan- dard solar energy distribution curve using weighted ordinates. Transmittance was measured after exposure but before washing. Each transmittance value for the exposed materials represents a separate test specimen. Average solar transmittance of the control test specimens is given for comparison. Transmittance is expressed in percentage.

Two sets of minicollectors were exposed the same length of time but different dates.

Solar radiation values in GJ/m are converted to Btu/ft by: Btu/ft = (GJ/m x 1.135 x 10 ).

Material failed prior to this time.

66
Table 20

Solar Energy Transmittance of Cover Plate Materials Exposed

as Double Covers to Natural Weathering in Miami, Florida^

Outer Cover

^*Nv^ Exposure Period (months) 3 6 12b/ 12b/ 18 24 Material^V^ Solar Radiation (GJ/m^)^ 1.455 2.864 6.344 9.333 9.333 13.125

glass (0.01% iron oxide) 90.7 87.6 89.3 89.0 83.6 83.6 87.0

glass (0.10% iron oxide) 87.4 86.0 85.4 81.1 82.6 82.6 84.5

fluorinated(ethylene propylene)

copolymer

96.0 90.3 90.2 88.5 88.6 88.6 88.5

poly(vinyl fluoride) 92.5 89.6 90.6 84.3 88.1 85.9 86.4

poly(ethylene terephthalate) 86.3 84.4 83.5 77.6 70.2 70.2 77.9

acrylic 90.7 87.4 87.4 86.1 d/ d/ d/

poly(methyl methacrylate) 91.0 91.1 87.0 87.2 84.0 84.0 84.5

polycarbonate 88.1 88.7 85.2 79.9 77.0 77.0 77.9

fiber reinforced plastic 1.0 mm 86.3 85.9 83.0 76.9 73.4 73.4 69.1

fiber reinforced plastic 1.5 mm 78.5 76.2 76.6 52.5 50.2 50.2 53.7

Inner Cover

>«. Exposure Period (months) 3 6 12b/ 12b/ 18 24

MateriaTS^ Solar Radiation (GJ/m^)^ 1.455 2.864 6.344 6.523 9.333 13.125

glass (0.01% iron oxide) 90.7 87.9 89.1 89.1 88.2 82.1 88.5

glass (0.10% iron oxide) 87.4 87.1 85.8 86.1 86.2 83.0 73.1

f luorinated(ethylene propylene)

copolymer

96.0 95.4 95.2 95.9 95.4 94.9 93.4

poly(vinyl fluoride) 92.5 90.6 91.3 88.5 91.1 89.1 89.1

poly(ethylene terephthalate) 86.3 85.6 85.5 85.3 85.4 85.4 85.0

acrylic 90.7 90.0 89.6 89.6 89.5 86.8 d/

poly(methyl methacrylate) 91.0 93.0 89.8 87.9 90.0 81.2 89.5

polycarbonate 88.1 89.9 87.4 81.8 87.2 81.7 82.0

fiber reinforced plastic 1.0 mm 86.3 85.1 85.0 84.4 81.8 77.7 71.2

fiber reinforced plastic 1.5 mm 78.5 64.8 69.9 58.8 67.3 64.5 63.7

a/

b/

Materials were exposed on rainicollectors. Transmittance measurements were made on a Gary 17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The

solar energy transmittance for air mass 2 at sea level was obtained by integrating over

Parry Moon's standard solar energy distribution curve using weighted ordinates. Each transmittance value represents a separate test specimen. Average solar transmittance of the control test specimens is given for comparison. Both inner and outer covers on each minicollector were the same material. Transmittance is expressed in percentage.

Two sets of minicollectors were exposed for the same legth of time but different dates. ^ Solar radiation values in GJ/m^ are converted to Btu/ft^ by: Btu/ft^ = (GJ/m^ x 1.135 x lO^"'). d/ Material failed prior to this time.

67
Table 21

Solar Energy Transmittance Loss of Cover Plate Materials Exposed

as Single Covers to Natural Weathering in Miami, Floi

lis Expose

)rida5:ii/

^s^ Exposure Period (months) 3 6 i2cy lid 18 24 Material\ Solar Radiation (GJ/m^)^ 1.455 2.864 6.344 6.523 9.333 13.125

glass (0.01% iron oxide) 0.6 0.7 0.9 0.2 1.6 0.3

glass (0.10% iron oxide) -0.6 0.4 -1.2 0,4 1.5 0.1

fluorinated(ethylene propylene)

copolymer

0.3 0.7 0.8 1.1 3.6 4.3

poly(vinyl fluoride) 0.6 1.1 4.0 0.8 5.2 4.6

poly(ethylene terephthalate) 0.2 4.9 7.7 4.8 4.8

acrylic 1.9 1.0 3.2 2.0 3.4 d

poly(methyl methacrylate) 0.7 1.9 2.2 2.3 4.1 4.9

polycarbonate -0.3 0.1 2.5 3.5 4.6 7.9

fiber reinforced plastic 1.0 mm 1.4 1.3 4.1 1.9 9.1 0.9

fiber reinforced plastic 1.5 mm 5.6 7.0 8.5 7.9 19.6 15.9

a/

k/

d

Transmittance measurements were made on a Cary 17D integrating sphere spectrophotometer

according to ASTM E 424-71, Method A. The solar energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution

curve using weighted ordinates. Each transmittance value represents a separate test specimen. Transmittance is expressed in percentage.

The solar energy transmittance loss is the difference between the average for the control

test specimens (Table 11) and the values obtained after the test specimen was washed. Negative values represent a gain in comparison with the average for the controls. This solar

energy transmittance loss does not include the loss due to dirt on the surface. Losses due

to dirt are tabulated in table 23.

Two sets of minicollectors were exposed for the same length of time but different dates.

d Solar radiation values in GJ/m^ are converted to Btu/ft^ by: Btu/ft^ = (GJ/m^ x 1.135 x 10^ ), d Material failed prior to this time.

68
Table 22

Solar Energy Transmittance Loss of Cover Plate Materials Exposed

as Double Covers to Natural Weathering in Miami, Florida?^—

/

Outer C over

>>^ Exposure Period (months) 3 6 12C/ i2cy 18 24 Material^ Solar Radiation (GJ/m^)^ 1.455 2.864 6.344 6.523 9.333 13.125

glass (0.01% iron oxide) -0.3 0.4 -1.7 0.6 1.4 1.5

glass (0.10% iron oxide) -0.3 0.8 0.6 0.9 0.9

f luorinated(ethylene propylene)

copolymer

-0.5 1.3 0.1 4.5 1.4

poly(vinyl fluoride) 0.9 1.4 3.0 1.0 4.4

poly(ethylene terephthalate) -0.4 0.9 7.7 5.2 5.2 3.9

acrylic 3.2 2.6 2.1 2.7 d £/

poly(methyl methacrylate) 1.0 1.8 2.8 2.0 3.3 3.1

polycarbonate -0.1 0.8 2.4 2.5 5.0 5.1

fiber reinforced plastic 1.0 mm -2.2 0.8 7.3 9.1 8.5 15.0

fiber reinforced plastic 1.5 mn -2.4 0.3 19.7 10.6 24.3 24.8

Inner C over ^w Exposure Period (months) 3 6 12^ 12£/ 18 24 Materia]\^ Solar Radiation (GJ/m^)^ 1.455 2.864 6.344 6.523 9.333 13.125

glass (0.01% iron oxide) -0.2 -0.6 0.1 0.5 -0.9

glass (0.10% iron oxide) -0.3 0.2 -0.6 0.5 0.1 -1.0

fluorinated( ethylene propylene)

copolymer

-0.4 -0.1 0.7 0.3 0.2 -0.1

poly(vinyl fluoride) 0.4 0.1 0.7 0.8 0.7 0.8

poly(ethylene terephthalate) -0.1 0.6 1.3 0.5 0.8 0.1

acrylic 1.0 -0.4 1.0 0.4 0.7 e/

poly(methyl methacrylate) -0.4 -0.5 0.6 0.6 -0.3

polycarbonate -0.2 -0.3 0.9 0.6 0.8 -0.1

fiber reinforced plastic 1.0 mm -1.6 0.2 1.9 3.3 6.9 1.5

fiber reinforced plastic 1.5 mm 11.5 6.4 19.8 9.9 10.1 12.5

a/ Materials were exposed on 17D integrating sphere sp' energy transmittance loss Moon's standard solar ene tance value represents a

minicollectors. Transmittance measurements were made on a Gary ectrophotometer according to ASTM E 424-71, Method A. The solar

for air mass 2 at sea level was obtained by integrating over Parry

rgy distribution curve using weighted ordinates. Each transmit- separate test specimen. Transmittance is expressed in percentage.

i2/ The solar energy transmittance loss is the difference between the average for the cone 11) and the value obtained after the test specimen was washed.

a gain in comparison with the average for the controls. This

e does not include the loss due to dirt on the surface. Losses

in table 24.

trol test specimens (Table Negative values represent

solar energy transmittance

due to dirt are tabulated

£/ Two sets of minicollectors were exposed for the same length of time but different dates.

-' Solar radiation values in GJ/m are converted to Btu/ft by: Btu/ft = (GJ/m x 1.135 x 10 ).

e/ Material failed prior to this time.

69
Table 23 Solar Energy Transmlttance Loss Due to Dirt on the Surface of Cover Plate Materials After Single Cover Exposure

in Miami, Florida^

MaterlaF'\_ Exposure Period (months) 3 6 12b/ 12b/ 18 24

glass (0.01% Iron oxide) 3.1 2.2 2.6 3.3 4.1 5.5

glass (0.10% iron oxide) 1.6 1.9 3.5 3.8 4.0 4.4

fluorinated(ethylene propylene)

copolymer

2.5 7.1 6.2 8.5 5.9 7.7

poly(vinyl fluoride) 2.4 1.8 1.8 7.1 2.9 2.2

poly (ethylene terephthalate) 2.5 3.6 3.4 7.1 10.2 5.7

acrylic 1.6 4.8 3.0 3.9 1.9 d

poly(methyl methacrylate) 1.7 2.2 1.5 4.2 5.2 2.6

polycarbonate 2.5 2.9 3.9 6.4 5.9 4.8

fiber reinforced plastic 1.0 mm 2.3 2.6 3.4 6.7 2.9 -1.1

fiber reinforced plastic 1.5 mm 3.9 0.5 6.2 4.3 5.9 6.5

b/

d

Materials were exposed to natural weathering on minicollectors. Transmlttance measurements were made on a Cary 17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmlttance for air mass 2 at sea level was obtained

by integrating over Parry Moon's standard solar energy distribution curve using weighted

ordinates. Each transmlttance value represents a separate test specimen. The solar energy transmlttance loss due to dirt is the difference between the value obtained before and

after washing the test specimen. Negative values represent a loss in transmlttance after

washing. Transmlttance is expressed in percentage.

Two sets of minicollectors were exposed for the same length of time but different dates.

Material failed prior to this time.

70
Table 24

Solar Energy Transmittance Loss Due to Dirt on the Surface of Cover Plate Materials After Double Cover Exposure in Miami, Florida^

Outer Cover

Material ^^-^v^ Exposure Period (months) 3 6 i2by i2by 18 24

glass (0.01% iron oxide) 3.4 1.0 3.3 1.1 5.7 2.2

glass (0.10% iron oxide) 1.7 2.1 1.8 5.7 3.9 2.1

fluorlnated(ethylene propylene)

copolymer

6,2 5.8 6.2 7.4 2.9 0.9

poly(vinyl fluoride) 2.0 0.5 1.4 7.2 6.6 1.7

poly(ethylene terephthalate) 2.3 1.9 1.0 10.9 10.9 4.5

acrylic 0.1 0.7 2.5 2.7 d d

poly(methyl methacrylate) -1.1 2.2 1.0 5.0 3.7 3.4

polycarbonate -0.5 2.1 5.8 6.3 6.1 5.1

fiber reinforced plastic 1.0 mm 2.6 2.5 2.1 6.7 4.4 2.2

fiber reinforced plastic 1.5 mm 4.7 1.6 6.3 4.6 4.0 6.1

Inner C over

Material ^^^^ Exposure Period (months) 3 6 12^ 12^ 18 24

glass (0.01% iron oxide) 3.0 1.6 3.2 2.4 8.1 3.1

glass (0.10% iron oxide) 0.6 1.4 1.9 0.7 4.3 15.3

f luoTinated(ethylene propylene)

copolymer

1.0 .9 -0.6 0.2 0.9 2.7

poly(vinyl fluoride) 1.5 1.1 2.3 0.6 2.7 2.6

poly(ethylene terephthalate) 0.8 0.2 -0.3 0.4 0.1 1.2

acrylic -0.3 1.5 0.1 0.8 3.2 c/

poly(methyl methacrylate) -1.6 1.7 3.1 0.4 9.2 1.8

polycarbonate -1.6 1.0 7.2 0.3 5.6 6.2

fiber reinforced plastic 1.0 mm 2.8 1.1 1.2 1.7 3.6

fiber reinforced plastic 1.5 mm 2.2 2.2 -0.1 1.3 3.9 2.3

a/

d

d

Materials were exposed to natural weathering in minicollectors. Transmittance measurements

were made on a Cary 17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmittance for air mass 2 at sea level was obtained by inte- grating over Parry Moon's standard solar energy distribution curve using weighted ordlnates.

Each transmittance value represents a separate test specimen. The solar energy transmittance

loss due to dirt is the difference between the solar energy transmittance measurements made

before and after washing the test specimen. Negative values represent a loss in transmittance

after washing. Transmittance is expressed in percentage.

Two sets of minicollectors were exposed for the same length of time but different dates.

Material failed prior to this time.

71
4.4.4.3 New River, Arizona Exposures

The solar energy transmlttance of the materials exposed In New River, Arizona

Is provided In table 25 for single cover applications and table 26 for double

covers. The solar energy transmlttance losses caused by weathering (not dirt

retention) are given in tables 27 and 28. In the inner cover applications the

only materials to have significant solar transmlttance losses were the two

fiber reinforced plastics. These same materials suffered the greatest losses

in applications as outer and single covers. The glass materials and the

fluorinated (ethylene propylene) copolymer exhibited changes of generally less

than one percent. Losses for the other plastic materials were in the range of one to three percent.

The loss of solar energy transmlttance due to dirt retention on the cover

plate materials is listed in tables 29 and 30. The outer and single cover

losses varied from two to five percent in the first 12 months. The transmlttance loss increased for the 18 and 24 month exposures. The increase may

be due to surface roughess caused by material deterioration or abrasion from

wind blown sand. The inner covers generally had losses less than two percent

until the 24 month exposure when most of the losses exceeded this.

4.4.4.4 Comparison of Exposures

The materials followed the same general trends at all three locations. The

greatest solar energy transmlttance losses due to weathering occurred at either

Florida or Maryland, with specimens exposed in Arizona showing the least transmlttance loss. This is probably due to deterioration caused by the combination

of moisture and sunlight. Figures 17a-17i illustrate the spectral transmlttance of the outer double covers after 24 months at the three exposure sites.

After weathering the transmlttance of the single covers was similar to that of

the outer double covers but the inner double covers generally showed little

change. A comparison of the transmlttance of inner and outer double covers

and single covers after 24 months exposure in Arizona is shown in figure 18

for polycarbonate. The gradual loss of transmlttance over time occurred for

both single and outer double covers. A typical example of the decrease is

represented in figure 19 which illustrates the transmlttance of polycarbonate

exposed in Florida for periods up to two years. For the inner cover application, the two fiber reinforced materials appear to be the only materials having

significant transmlttance losses due to weathering (not dirt). The other materials changed very little. The fiber reinforced plastic materials also had

significant losses in outer cover and single cover applications.

The solar energy transmlttance losses caused by dirt seemed to be slightly

greater at Miami than Galthersburg with the losses being least at New River.

The amount of dirt retained on the surface is directly related to the most

recent rain or snow fall. Consequently, comparisons should be limited to

materials simultaneously exposed at a single location. In evaluating the data,

it appears that after about one year the solar energy transmlttance losses due

to dirt do not increase significantly except in Arizona. Solar energy transmlttance losses of three to five percent due to dirt are common for all cover

72
Table 25

Solar Energy Transmittance of Cover Plate Materials Exposed

as Single Covers to Natural Weathering in New River, Arizona-a/

\\ Exposure Period (months) 3b/ 3b/ 6b^ 6^ 12by 12b/ 18 24

Materialise Solar Radiation (GJ/m^)^ 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370

glass (0.01% iron oxide) 90.7 89.4 89.5 88.5 87,5 89,6 85.5 87.5 86.8

glass (0.10% iron oxide) 87.4 85.5 87.6 85.4 85.7 86.6 85.9 84.0 82.6

fluorinated(ethylene propylene)

copolymer

96.0 93.3 93.5 92.9 94.5 91.6 90.9 91.6 90,1

poly(vinyl fluoride) 92.5 91.0 89.4 90.2 90.6 89.6 86.7 87.7 87.5

poly(ethylene terephthalate) 86.3 84.6 83.2 83.9 83.9 82.5 71.0 76,8 d/

acrylic 90.7 89.1 87.9 88.8 88.8 88,1 79.0 d/ d/

poly(methyl methacrylate) 91.0 88.7 88.6 87.1 86.8 86.8 85.1 84,7 83,9

polycarbonate 88.1 87.2 86.1 85.9 84.2 _ 84.1 80.9 84.0

fiber reinforced plastic 1.0 mm 86.3 83.5 84.3 83.8 82.3 80.9 81.2 75,0 77.2

fiber reinforced plastic 1.5 mm 78.5 73.8 72.4 68.5 66,7 68.6 63.0 64.7 61.2 ^ Materials were exposed on minicollectors. Transmittance measurements were made on a Cary 17D integrating

sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmittance for air mass

2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution curve using

weighted ordinates. Transmittance was measured after exposure but before washing. Each transmittance value represents a separate test specimen. Average solar transmittance of the control test specimens is given

for comparison. Transmittance is expressed in percentage.

b/

4/

Two sets of minicollectors were exposed for the same length of time but different dates.

Solar radiation values in GJ/m are converted to Btu/ft by: Btu/ft = (GJ/m x 1.135 x 10 )•

Material failed prior to this time.

73
Table 26

Solar Energy Transmlttance of Cover Plate Materials Exposed

as Double Covers to Natural Weathering in New River, Arizona^

Outer Cover ^\^ Exposure Period (months) 3h/ 3hy 6l^ (>^ I2W i2by 18 24

Material>v^ Solar Radiation (GJ/m^)'^ 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370

glass (0.01% iron oxide) 90.7 90.4 89.7 89.2 86.3 96.7 88.1 87.2 85.2

glass (0.10% iron oxide) 87.

A

86.9 86.1 86.1 84.8 87.1 83.6 84.3 83.8

fluorinated (ethylene propylene)

copolymer

96.0 93.1 93.6 93.5 94.4 93.1 91.8 91.5 94.4

poly(vinyl fluoride) 92.5 91.0 90.4 89.0 90.5 88.7 87.5 88.3 87.2

poly (ethylene terephthalate) 86.3 85.6 84.6 84.1 83.8 83.8 1/ 11.1 d/

acrylic 90.7 88.2 88.2 89.5 88.4 88.8 ^ d/ i/

poly(methyl methacrylate) 91.0 89.3 87.4 87.9 87.4 86.5 86.3 86.1 84.5

polycarbonate 88.1 86.5 86.2 86.4 84.5 83.9 84.8 82.6 82.5

fiber reinforced plastic 1.0 mm 86.3 85.7 83.4 84.0 80.5 83.3 77.6 79.5 78.0

fiber reinforced plastic 1 . 5 mm 78.5 77.5 76.7 74.0 69.1 70.1 72.7 68.2 61.8

Inner Cover ^v^ Exposure Period (months) 3hy ah/ 6l^ 6i^ 12t/ I2U/ 18 24 Material\ Solar Radiation (GJ/m^)'^ 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.125

glass (0.01% iron oxide) 90.7 91.1 89.3 89.2 89.0 87.1 90.2 89.3 87.8

glass (0.10% iron oxide) 87.4 87.9 86.7 86.6 86.9 88.2 86.5 87.0 85.9

fluorinated (ethylene propylene)

copolymer

96.0 95.7 95.3 95.5 95.6 94.2 95.2 95.1 93.7

poly(vinyl fluoride) 92.5 91.5 90.1 91.1 91.4 89.8 91.4 90.2 d

poly(ethylene terephthalate) 96.3 85.6 85.3 85.0 85.0 84.0 d/ 84.7 Al

acrylic 90.7 89.8 88.9 89.7 90.1 88.9 Al 89.2 AJ

poly (methyl methacrylate) 91.0 90.7 89.7 90.6 89.6 89.2 90.1 89.4 88.0

polycarbonate 88.1 87.2 87.2 87.4 87.4 85.5 87.2 85.8 83.1

fiber reinforced plastic 1.0 mm 86.3 84.2 84.2 85.2 82.7 79.9 82.4 74.1 74.2

fiber reinforced plastic 1.5 mm 78.5 71.4 69.0 68.6 64.2 64.3 56.4 56.9 64.1 ^ Materials were exposed on minicollectors. Transmlttance was measured after exposure but before washing. Transmlttance measurements were made on a Gary 17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmlttance for air mass 2 at sea level was obtained by integrating

over Parry Moon's standard solar energy distribution curve using weighted ordinates. Each transmlttance

value represents a separate test specimen. Average solar tramsraittance of the control test specimens is given for comparison. Both the inner and outer covers on each minicollector were the same material. Transmlttance is expressed in percentage.

hy Two sets of minicollectors were exposed for the same length of time but different dates.

'-^ Solar radiation values in GJ/m are converted to Btu/ft by: Btu/ft = (GJ/m x 1.135 x 10 ).

<U Material failed prior to this time. 74
Table 27

Solar Energy Transmittance Loss of Cover Plate Materials Exposed

as Single Covers to Natural Weathering in New River, Arizona^—'

^S^ Exposure Period (months) 3^ 3^ 6^ 6^ 12^/ 12^7 18 24 Material*^ Solar Radiation (GJ/m^)^ 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370

glass (O.OU iron oxide) -0.1 1.6 1.0 -1.7 -0.8 0.7 0.9 0.7

glass (0.10% iron oxide) 0.5 0.3 -1.9 0.2 0.3 -0.6 0.2

fluorinated (ethylene propylene)

copolymer

0.2 0.7 0.7 0.4 1.3 0.8 0.6 0.5

poly(vinyl fluoride) 1.1 1.8 1.5 1.2 2.0 1.2 1.8 1.1

poly (ethylene terephthalate) 1.4 1,0 0.4 1.4 -0.5 3.2 6.6 e/

acrylic 1.6 0.9 0.3 0.2 1.0 3.9 d d

poly(methyl methacrylate) 1.5 1.6 2.2 -0.3 1.8 3.2 2.5 1.6

polycarbonate 0.2 0.8 0.5 1.7 _ 3.2 3.1 1.8

fiber reinforced plastic 1.0 mm 1.0 1.9 1.8 2.7 3.7 6.1 4.7 1.9

fiber reinforced plastic 1.5 mm 4.6 4.2 8.6 12.5 12.7 1.9 10.3 8.9 d

d

d

Materials were exposed on minicollectors. Transmittance measurements were made on a Cary 17D integrating

sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmittance for air mass

2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution curve using

weighted ordinates. Each transmittance value represents a separate test specimen. Transmittance is expressed in percentage.

The solar energy transmittance loss is the difference between the average for the control test specimens

(Table 11) and the value obtained after the test specimen was washed. Negative values represents a gain in comparison with the average of the controls. This solar energy transmittance loss does not include loss due to dirt on the surface. Losses due to dirt are tabulated in Table 29.

Two sets of minicollectors were exposed for the same length of time but different dates. d Solar radiation values in GJ/m^ are converted to Btu/ft^ by: Btu/ft^ = (GJ/m^ x 1.135 x 10^^).

d Material failed prior to this time.

75
Table 28

Solar Energy Transmittance Loss of Cover Plate Materials Exposed

as Double Covers to Natural Weathering in New River, Arizona^*—

liter Cover ^\^^ Exposure Period (months) 3C/ 3C/ b-J 6^ 12£/ lid 18 24 Material\^ Solar Radiation (GJ/m^)^ 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370

glass (0.01% iron oxide) 0.1 0.9 -0.4 -0.4 -0.3 -0.5 -0.5 -0.5

glass (0.10% iron oxide) 0.3 0.8 0.3 -2.0 -1.5 0.3 0.3 -0.5

fluorinated (ethylene propylene)

copolymer

0.4 0.8 0.3 0.4 0.1 0.2 1.1 0.5

poly(vinyl fluoride) 0.7 1.1 2.8 0.8 1.8 1.2 _ 3.0

poly(ethylene terephthalate) 0.6 0.8 0.3 1.3 1.1 e/ 1.5 sJ

acrylic 0.9 0.7 -0.1 1.5 £/ e/ sJ

poly(methyl methacrylate) 1.7 1.6 1.7 -0.5 1.0 2.7 2.0 1.7

polycarbonate 0.5 0.2 0.3 1.6 2.2 2.3 2.1 3.0

fiber reinforced plastic 1.0 mm -0.2 1.8 1.8 2.1 2.3 3.2 3.3 3.5

fiber reinforced plastic 1.5 mm 0.2 1.0 0.7 6.5 6.3 2.8 6.5 7.6

I nner Cover ^\^^ Exposure Period (months) id 3C/ 6£/ (.d lid 12^ 18 24 Material\^ Solar Radiation (GJ/m^)^ 1.422 2.012 3.487 5.123 8.006 8.259 11.642 16.370

glass (0.01% iron oxide) 0.1 2.4 0.6 -1.6 0.4 0.3 0.6 1.0

glass (0.10% iron oxide) -0.1 0.9 -2.0 -2.5 0.4 0.3 0.9

fluorinated (ethylene propylene)

copolymer

0.3 0.3 2 0.4 -0.3 0.2 0.1

poly(vinyl fluoride) 0.7 1.4 0.8 1.1 0.4 0.7 £/ £/

poly (ethylene terephthalate) 0.6 0.8 1.8 1.4 0.6 e/ 1.2 £/

acrylic 0.6 0.2 0.4 -0.1 ey -0.3 e/

poly(methyl methacrylate) 1.1 0.3 0.4 -0.3 0.3 0.4 0.6 0.2

polycarbonate 0.7 0.3 0.3 0.5 0.5 0.6 0.9

fiber reinforced plastic 1.0 imn 2.1 3.3 1.8 3.6 5.6 3.9 8.5 6.2

fiber reinforced plastic 1.5 mm 7.8 10.6 9.0 14.5 7.8 2.7 19.4 10.1

a/-^ Materials were exposed on minicollectors. Transmittance measurements were made on a Gary 17D integrating

sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution

curve using weighted ordinates. Each transmittance value represents a separate test specimen. Transmittance is expressed in percentage.

b/

U

The solar energy transmittance loss is the difference between the average for the control test specimens

(Table 11) and the value obtained after the test specimen was washed. Negative values represent a gain

in comparison with the average for the controls. This solar energy transmittance loss does not include

the loss due to dirt on the surface. Losses due to dirt are tabulated in Table 30.

Two sets of minicollectors were exposed for the same length of time but different dates. d Solar radiation values in GJ/m are converted to Btu/ft by: Btu/ft = (GJ/m x 1.135 x 10 ). ^e/ Material failed prior to this time.

75
Table 29

Solar Energy Transmittance Loss Due to Dirt on the Surface

terials After a Sing New River, Arizona^

of Cover Plate Materials After a Single Cover Exposure in

Material ^s,. Exposure Period (months) 3b/ 3b/ 6^7 6b-/ 12b/ 12b-/ 18 24

glass (0.01% iron oxide) 1.4 -0.4 1.2 4.9 1.9 4.5 2.3 3.2

glass (0.10% iron oxide) 0.4 -0.2 1.7 3.6 0.6 1.2 4.0 4.6

fluorinated (ethylene propylene)

copolymer

2.5 1.8 2.4 1.1 3.1 4.3 3.8 5.4

poly (vinyl fluoride) 0.4 1.3 0.8 0.7 0.9 4.6 3.0 3.9

poly (ethylene terephthalate) 0.3 2,1 2.0 1.0 4.3 12.1 2.9 d

acrylic 1.9 1.6 1.7 1.6 7.8 d d

poly (methyl methacrylate) 0.8 0.8 1.7 4.5 2.4 2.7 3.8 5.5

polycarbonate 0.7 1.2 1.7 2.2 _ 0.8 4.1 2.3

fiber reinforced plastic 1.0 mm 1.8 0.1 0.7 1.3 1.7 1.0 6.6 7.2

fiber reinforced plastic 1.5 mm 0.1 1.9 1.4 -0.7 0.2 13.6 3.5 8.4

d

d

Materials were exposed to natural weathering on minicollectors. Transmittance measurements were made

on a Cary 170 integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The solar

energy transmittance for air mass 2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution curve using weighted ordinates. Each transmittance represents a separate test specimen. The solar energy transmittance loss due to dirt is to the difference between

the solar energy transmittance measurements made before and after washing the test specimen. Negative

values represent a loss in transmittance after washing. Transmittance is expressed in percentage.

Two sets of minicollectors were exposed for the same length of time but different dates.

Material failed prior to this time.

77
Table 30 Solar Energy Transmlttance Loss Due to Dirt on the Surface of Cover Plate Materials After Double Cover Exposure in New River, Arizona^

Outer Cover

MateriaF"***,^ Exposure Period (months) ,y 3b/ 6^ 6^ 12V 12b/ 18 24

glass (0.01% iron oxide) 0.2 0.1 1.9 5.8 0.3 2.1 3.0 5.4

glass (0.10% iron oxide) 0.2 0.5 1.0 4.6 1.8 3.5 2.8 4.1

fluorinated (ethylene propylene)

copolymer

2.5 1.7 2.2 1.2 2.8 4.0 3.4 1.1

poly(vinyl fluoride) 0.8 0.7 1.2 2.0 3.8 _ 2.3

poly(ethylene terephthalate) 0.1 0.9 1.9 1.2 1.4 £/ 7.6 qJ

acrylic 1.6 1.8 1.3 2.3 0.4 cj c/ d

poly(methyl raethacrylate) 2.0 1.4 3.1 3.5 2.0 2.9 4.8

polycarbonate 1.1 1.7 1.4 2.0 2.0 1.0 3.4 2.6

fiber reinforced plastic 1.0 mm 0.8 1.1 0.5 3.7 0.7 5.5 3.5 4.8

fiber reinforced plastic 1.5 mm 0.8 0.8 3.8 2.9 2.0 3.0 3.8 9.1

Inner Cover

Material^~^»^ Exposure Period (months) 3t/ 312/ ^y 6^/ 12^ I2W 18 24

glass (0.01% iron oxide) -0.5 -1.0 0.9 3.3 3.2 0.2 0.8 1.9

glass (0.10% iron oxide) -0.4 0.7 -0.1 2.5 1.7 0.5 0.1 0.6

fluorinated (ethylene propylene)

copolymer

0.4 0.3 2.1 0.8 0.7 2.2

poly(vinyl fluoride) 0.3 1.0 0.6 2.3 0.4 _ _

poly(ethylene terephthalate) 0.1 0.2 -0.5 -0.1 1.7 cj 0.4 d

acrylic 0.9 1.2 0.8 0.2 1.9 d 1.8 d

poly(methyl raethacrylate) -0.8 1.0 1.7 1.5 0.5 1.0 2.8

polycarbonate 0.2 0.6 0.4 0.7 2.1 0.4 1.7 4.1

fiber reinforced plastic 1.0 mm -1.2 0.3 0.8 3.7 5.9

fiber reinforced plastic 1.5 mm -0.7 -1.1 0.9 -0.2 6.4 9.4 2.2 4.3 d

d

Materials were exposed to natural weathering on minicollectors. Transmlttance measurements were made

on a Cary 17D integrating sphere spectrophotometer according to ASTM E 424-71, Method A. The solar energy transmlttance for air mass 2 at sea level was obtained by integrating over Parry Moon's standard solar energy distribution curve using weighted ordinates. Each transmlttance value represents a separate test specimen. The solar energy transmlttance loss due to dirt is the difference

between the solar energy transmlttance measurements made before and after washing the test specimen.

Negative values represent a loss in transmlttance after washing. Transmlttance is expresed in percentage.

Two sets of minicollectors were exposed for the same length of time but different dates.

c/—' Material failed prior to thi

78
FIGURE 17a Spectral Transmittance of Washed 3.2mm(1/8in) Glass (0,01%

Iron Oxide) After Two Years outdoor Weathering

UJU

I

00

100 h90

80

70

60

50

40

30

20 -

10

Control Arizona

Florida Maryland

J. ± ± ± ± ±

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

c/)

FIGURE 17b Spectral Transmittance of Washed 3.2mm (1/3 in) Glass (0.107. 100 I

1''°'^ Oxide) After Two Years Outdoor Weathering

90 —r/e^r^^^^^^

80

70

60

50

40

30

20 t

Arizona

Control Maryland

Florida

400 600 800 1000 1200 1400 1600 WAVELENGTH (nm)1800 2000 2200

79
i

CO

FIGURE 17c Spectral Transmittance of Washed 0,025fiM(0,001iN) Fluorinated

(eihylene propylene) Copolymer After Two Years Outdoor Weathering

/

f100

90

80

70

60

50

40

30

20 —

10

/

Arizona

Control ~ Maryland

Florida

400 600 800 1000 1200 1400 1600 1800

WAVELENGTH (nm)

2000 2200

FIGURE 17d Spectral Transmittance of Washed O.IOmmCO.OO^in) Poly(vinyl

fluoride) After Two Years Outdoor Weathering

C/3 z

100

90

80

70

60

50

40

30

20

10

Control Arizona Maryland

Florida

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

80
FIGURE 17e Spectral Transmittance of Washed 0,13m(0,G05iN) Poly(ethylene

terephthalate) After Two Years Outdoor Weathering

z

400 600 800 1000 1200 1400 1600 1800 2000 2200 WAVELENGTH (nm)

FIGURE 17f Spectral Transmittance of Washed l,5nM(0,06iN) Poly(methyl

methacrylate) After Two Years Outdoor Weathering

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

Bl
FIGURE 17g Spectral Transmittance of Washed 1.0mm(0.04in) Polycarbonate

After Two Years Outdoor Weathering

100 —

C/9

2200 WAVELENGTH (nm)

FIGURE 17h Spectral Transmittance of Washed 1,0mm(0 QiJiN) Fiber Reinforced Plastic After Two Years Outdoor Weathering

100 ::::f^^^^^

1000 1200 1400 1600 WAVELENGTH (nm)

1800 2000 2200

82
g

C/9

FIGURE 17r Spectral Transmittance of Washed 1,5mm(0.06in) Fiber Reinforced Plastic After Two Years Outdoor Weathering

400 600 800 1000 1200 1400 1600 WAVELENGTH (nm)

1800 2000 2200

8^
100

90

80

70

60

50

40

30

20

10

FIGURE 18 Comparison of Spectral Transmittance of 1.0mm(0,04in)

Polycarbonate Exposed Two Years in Arizona

liHIMiJ II" '*^J

Control Double inner cover

Single cover Double outer cover

U \ L

400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (nm)

I

100 -

FIGURE 19 CoMPARisiON of Spectral Transmittance of 1,Omm(G.O^) Polycarbonate Single Covers Exposed in Florida

400 1000 1200 1400 1600

WAVELENGTH (nm)

8ii

2200
plate materials in outer cover and single cover applications. Some cover materials have even greater losses, e.g. poly(ethylene terephthalate) in

Miami, fluorinated (ethylene propylene) copolymer in Gaithersburg, both fiber

reinforced plastics in Miami and New River.

Although the inner cover was not expected to accumulate dirt, the data indicate

that in some cases the inner cover does get dirty and lose solar energy transmittance. This was particularly true after 18 and 24 months in Miami and New

River. This could result from decomposition of the wood during the exposures

or from infiltration of dirt into boxes which were not well sealed or where

the cover had loosened.

4.4.5 Warpage

Warpage was observed on some of the test specimens after outdoor exposures.

The warpage occurred with the inner cover in the double cover minicollectors.

Since the test specimens were adhered to the box on all four edges, the warpage occured primarily as sagging below the plane of the cover, although

slight buckling above the plane was also observed. Although the edges were

constrained, many test specimens of poly(methyl methacrylate) sagged in the

center. For example, the test specimens exposed in a double cover box in

Arizona for six months had a warpage of 6.4 mm (1/4 in) near the center. The

polycarbonate exhibited slight sagging and buckling similar to warpage observed

on full size solar collectors. Some fiber reinforced test specimens had a slight shape distortion after exposure. This may account for the wavlness

reported for some collectors glazed with fiber reinforced plastics.

5. CONCLUSIONS

5.1 MEASUREMENT OF TRANSMITTANCE

1

Method A (spectrophotometer) of ASTM E424 has better precision and repeatability than Method B (outdoor) and is therefore preferrable to Method B

for determining solar energy transmittance of cover plate materials . Method A has a constant energy source whereas with Method B the sun's

energy varies with time of day and year, weather conditions, sky conditions

and geographic location. The range in solar energy transmittance values

of the control specimens was greater using Method B (Table 3) than Method

A (Table 2). Small solar energy transmittance changes of less than one percent can be easily measured with Method A whereas, with Method B, this is

not possible. For example, in this study, there was frequently a variation of greater than one percent among the results for the controls when

using Method B.

2. When the integrated solar energy transmittance loss is less than three

percent, the spectral transmittance curve should be reviewed to determine

the cause of the transmittance loss . While integrated solar transmittance

values are helpful in evaluating material changes, when the solar transmittance change is small it is difficult to interpret the results. The

85
spectral transmittance should be examined to determine if there is a shift in the absorption band in the UV-VIS region, or whether the transmittance curve has declined across the entire spectrum.

5.2 ACCELERATED LABORATORY TESTING

1

Accelerated laboratory testing procedures, such as those performed in this

study and included in the proposed standard (Appendix C), offer an effective means of screening candidate cover plate materials .

2. Heat aging at an elevated temperature (150°C (302°F)) for a relatively

short time (500 hours) is an effective means of identifying materials which

will develop problems in outdoor weathering . The cover plate materials

having solar energy transmittance losses greater than two percent after only

500 hours at 150°C (302°F) (i.e. poly(vinyl fluoride), poly(ethylene

terephthalate), acrylic film and fiber reinforced plastics) also developed

problems in natural weathering. The poly (vinyl fluoride) inner covers cracked and disintegrated in less than 24 months at all three outdoor

exposure sites. As outer and single covers, the solar energy transmittance

loss of poly(vinyl fluoride) was generally greater than four percent after 24

months. The poly(ethylene terephthalate) turned white in Florida and also

became very brittle at all three sites. The acrylic film cracked and disintegrated in 12-18 months at each site. Both the 1.5 and the 1.0 mm fiber

reinforced plastics yellowed and lost transmittance at all three sites.

3. The heat aging had little effect on the glass and the fluorinated (ethylene propylene) copolymer .

4. For the materials included in this study other than the glass and fluorinated (ethylene propylene) copolymer, the rate of transmittance loss was related to the aging temperature and was accelerated by higher temperatures. The solar energy transmittances decreased as the cover plate materials were exposed to longer periods of heating aging. Also, as the temperature was increased the solar energy transmittances decreased.

5. Measurement of linear dimensions after ten hours exposure at a specific

temperature is sufficient to establish trends and determine approximate

dimensional changes due to that specific temperature .

6

Art ificia l weathering with a xenon arc light is helpful in distinguishing

materials sensitive to solar radiation . Polycarbonate, the fiber reinforced plastics, poly (ethylene terephthalate), and the acrylic film suffered

transmittance losses in the artificial weathering. In the natural weathering the polycarbonate and the fiber reinforced materials yellowed and lost

transmittance. The poly(ethylene terephthalate) and acrylic showed smaller

decreases due to the xenon arc exposure but significant changes in physical

and mechanical properties when exposed outside.

86
5.3 OUTDOOR WEATHERING

1

.

The mlnlcollectors increased the temperature of the cover plate materials

adequately to simulate operating collector temperatures and to approach

stagnation temperatures of some flat plate solar collectors. They proved

to provide data which were useful in evaluating candidate cover plate

materials .

2. Solar radiation is not the only weathering factor causing solar

energy transmittance losses . Although Arizona specimens received the

most solar radiation and experienced the highest temperatures, the cover

plate materials from the Arizona exposures did not have the greatest

solar transmittance losses. Apparently moisture, air pollutants, and

possibly other weather factors caused greater permanent transmittance

losses at the Florida and Maryland locations. In addition, the dirt and

dust caused greater temporary transmittance losses (which can be regained

after cleaning) at these sites.

3. Dirt and dust retention is a major cause of solar energy transmittance

decrease in all cover plate materials . Solar transmittance losses due to

dirt and dust (which can be washed off) are generally from three to five

percent and occassionally higher. These transmittance losses frequently

equalled or exceeded losses due to natural weathering factors. Dirt and

dust accumulation seemed somewhat material dependent (e.g. fluorinated

(ethylene propylene) copolymer usually had higher transmittance losses due

to dirt than other materials). Also, as the surface of a material

roughened during exposure the dirt retained on the surface increased.

4

.

Decreases in the mechanical and physical properties of materials included

in this study seemed related to solar radiation dosage. For example, the

deterioration of the acrylic film, poly(vinyl fluoride) as the inner cover,

and the poly(ethylene terephthalate) occured most rapidly at Arizona

followed by the other sites.

6. ADDITIONAL RESEARCH NEEDS

In addition to these studies reported here, other valuable data could be

obtained by further studies. The following research needs are identified:

1. Artificial weathering exposures with xenon arc light should be performed

at higher temperatures to simulate the temperatures encountered in stagnating collectors. This experiment would combine the two major factors

causing degradation of plastics in solar collectors. The data should

be particularly useful in correlating with outdoor exposures.

2. The effect of moisture on cover plate materials should be studied and a laboratory procedure to evaluate this effect should be identified or developed.

87
3. Various methods for cleaning cover plate materials should be evaluated and

a standard practice for cleaning cover plate materials and collectors

should be developed.

4. Studies to further evaluate the interaction of dirt and dust with the

cover plate materials should be performed.

5. The surfaces of materials exposed in Maryland and Florida should be

examined to better characterize them and to determine if the increased

transraittance losses are due to deposits on the surfaces or to degradation of the materials by weathering factors.

7. PROPOSED STANDARDS

Based upon the laboratory work reported in Chapters 3 and 4, two standards for

use in evaluation of cover plate materials for flat plate solar collectors

were drafted and submitted for consideration for acceptance as consensus standards by ASTM Committee E44 on Solar Energy Conversion, Subcommittee

E44.04 on Materials Performance. The proposed standards are included in

Appendices B and C of this report.

These proposed standards, while based upon NBS laboratory studies, incorporate the comments of many E44 members and reflect the results of many hours of

discussion of the E44.04.02 Cover Plate Task Group.

7.1 STANDARD PRACTICE FOR EXPOSURE OF COVER MATERIALS FOR SOLAR COLLECTORS

TO NATURAL WEATHERING UNDER CONDITIONS SIMULATING STAGNATION MODE

(APPENDIX B) .

This is a proposed standard practice for the exposure of cover materials to

natural weathering. It provides a procedure for the natural weathering of cover materials for flat plate solar collectors on a box which has heat loss

coefficients similar to a flat plate collector. The box should elevate the cover plate temperature to approximate stagnation conditions of a solar collector. A variable angle exposure is used. Provisions are made for exposure of

single and double cover assemblies to accomodate the need for exposure of both

inner and outer solar collector cover materials. This document is based upon

the concept of the minicollector (with modifications) described in Section

3.2.3. The standard practice provides guidelines for the calculation of the

heat loss coefficient and guidelines for determining a variable angle exposure

schedule.

7.2 STANDARD PRACTICE FOR EVALUATION OF COVER MATERIALS FOR FLAT PLATE

SOLAR COLLECTORS (APPENDIX C)

This is a proposed standard practice for evaluation of cover materials for flat

plate solar collectors. Much of this proposed standard practice is based upon

the studies described in this report. The proposed standard includes both

aging procedures and material property measurement tests. It describes evaluation procedures for assessment of the durability of solar collector covers by

88
comparison of initial material property values with those obtained after

exposure to aging procedures. The aging procedures encompass exposures for

heat stability, natural weathering and accelerated weathering. Property test

methods are included for measurements of solar energy transmittance, linear

dimensional stability, warpage, impact resistance, tensile strength and the

effect of dirt retention.

8 . REFERENCES

[1] Grimmer, D. P. and Moore, S. W., "Practical Aspects of Solar Heating:

A Review of Materials Use in Solar Heating Applications," presented at

Society for the Advancement of Materials and Process Engineering, Oct.

14-16, 1975.

[2] "Component Spotlight - Glazing," Solar Heating and Cooling, Vol. 5, No.

1, p. 16, (January 1980).

[3] Skoda, L. F. and Masters, L. W., "Solar Energy Systems - Survey of

Materials Performance," National Bureau of Standards Report, NBSIR

77-1314, (1977). Available from NTIS, Order No. PB 273 305.

[4] Lorriman, D. B., "An Assessment of Problems Experienced With Operating

Solar Systems in Canada and the Northern United States", National

Research Council Technical Series No. 1, (1978). Available from Canadian

Government Publishing Centre, Supply and Services Canada, Hull, Quebec,

Canada.

[5] Hobbs, Norman P., "Investigation of Glass Breakage in Solar Panels at

Timonium School", Burlington, Mass., Kaman Avldyne, A Division of Kaman

Sciences Corp, prepared for the U.S. Dept. of Energy Research and

Development Administration under contract NSF-C871, P.O. WA-76-5330,

(1976).

[6] Lof, G.O. and French, R.R., "Hail Resistance of Solar Collectors with

Tempered Glass Covers", Preconference Proceedings - Solar Heating and

Cooling Systems - Opertional Results, Colorado Springs, CO, November

27-30, 1979. Available from Solar Energy Research Institute Report

No. TP-245-430.

[7] Baldwin, J. L., Climates of the United States , National Oceanic and

Atmospheric Administration, Washington, D.C., (1973). Available from

the Government Printing Office, Washington, D.C. 20402.

[8] Mendelsohn, M.A., Yeoman, F.A., Luck, R.M., Navish, R.W. and Meier, J.F.,

"Collector Sealants and Breathing", Westinghouse Electric Corporation,

Pittsburgh, PA, Mid-Term Report Sept. 1978 - May 1979, for U.S. Dept. of

Energy, Report No. ALD-153624.

[9] "Intermediate Minimum Property Standards Supplement-Solar Heating and

Domestic Hot Water Systems", U.S. Department of Housing and Urban and

89
[10] Holton, J. K., et al., "Interim Performance Criteria for Solar Heating

and Cooling Systems in Residential Buildings, Second Edition", National

Bureau of Standards Report NBSIR 78-1562, (1978). Available from

Nationaal Technical Information Service, Springfield, VA 22161,

Order No. PB 289 767.

[11] "Interim Performance Criteria for Solar Heating and Cooling Systems in

Commercial Buildings", National Bureau of Standards Report NBSIR 76-1187,

(1976). Available from National Technical Information Service,

Springfield, VA 22161, Order No. PB 262 114.

[12] Mathey, R. C, "Hail Resistance Tests of Aluminum Skin Honeycomb Panels

for the Relocatable Lewis Building, Phase II", National Bureau of

Standards Report 10193 (1970). Available from National Bureau of

Standards, Washington, D.C. 20234.

[13] Fuchs, E. R., "Hailstorm in Montana", American Roofer, (Nov. 1955).

[14] Kalmal, M. R. and Saxon, R., "Recent Developments in the Analysis and

Prediction of the Weatherability of Plastics", in Applied Polymer

Symposia No. 4, Weatherability of Plastics, M. R. Kalmal Ed., pp. 1-28,

(Interscience, New York, N.Y. 1967).

[15] Jellinek, H. H. C, "Fundamental Degradation Processes Relevant to

Outdoor Exposure of Polymers", in Applied Pol5nner Symposia No. 4,

Weathering of Plastics, M. R. Kamal, Ed., pp. 41-59, (Interscience,

New York, N.Y. 1967).

[16] Winslow, F. H., and Hawkins, W. L., "Some Weathering Characteristics

of Plastics", in Applied Polymer Symposia No. 4, Weatherability of

Plastics, M. R. Kamal, Ed., pp. 29-39, (Interscience, New York, N.Y.

1967).

[17] Gray, V.E. and Cadoff, B.C., "Survey of Techniques for Evaluating Effects

of Weathering on Plastics", in Applied Poljniier Symposia No. 4, Weathering

of Plastics, M.R. Kamal, Ed., pp. 85-95, (Interscience, New York, N.Y.

1967).

[18] Carlson, D.J. and Wiles, D.M., "The Photooxidative Degradation of Polypropylene. Part I. Photooxidation and Photoinitiation Processes",

J. Macromol. Sci., Rev. Macromol. Chem. , C14, 65 (1976).

[19] Day, M. and Wiles, D.M., "Photochemical Degradation of Poly(ethylene

terephthalate)", I., II., and III. J. Appl. Polymer Sci., 16, 175, 191,

203 (1972).

[20] Zerlaut, G. A., "Fundamental Materials Considerations for Solar Collectors", Chpt. 13, Critical Materials Problems in Energy Production,

Academic Press, (New York: 1976).

90
[21] ASTM D 794, Recommended Practice for Determining Permanent Effect of Heat

on Plastics, Annual Book of ASTM Standards, Book 35, American Society for

Testing on Materials, 1916 Race Street, Philadelphia, PA 19103.

[22] Searle, N. Z. Giesecke, P., Kinraonth, R. and Hirt, R. "Ultraviolet

Spectral Distributions and Aging Characteristics of Xenon Arcs and

Filters", Applied Optics, APOPA, Vol. 3, No. 8, pp. 923-27, (August

1964).

[23] Boettner, E. A. and Miedler, L. J., "Simulating the Solar Spectrum with

Filtered High Pressure Xenon Lamp", Applied Optics, APOPA, Vol. 2, No.

1, pp. 105-108, (January 1963).

[24] Hirt, R. C, Schmitt, R. G. Searle, N. Z. and Sullivan, A. P.,

"Ultraviolet Spectral Distribution of Natural Sunlight and Accelerated

Test Light Sources," Journal of the Optical Society of America, JOSAA,

Vol. 50, No. 7, pp. 706-13, (1960).

[25] ASTM D 2565, Recommended Practice for Operating Xenon Arc-Type

(Water-Cooled) Light-and Water-Exposure Apparatus for Exposure of Plastics,

Annual Book of ASTM Standards, Book 35, American Society of Testing and

Materials, 1916 Race Street, Philadelphia, PA 19103.

[26] ASTM D 1435, Recommended Practice for Outdoor Weathering of Plastics,

Annual Book of ASTM Standards, Book 35, American Society of Testing and

Materials, 1916 Race Street, Philadelphia, PA 19103.

[27] ASTM E 424, Standard Test Methods for Solar Energy Transmittance and

Reflectance (Terrestrial) of Sheet Materials, Annual Book of ASTM

Standards Book 46, American Society of Testing and Materials, 1916 Race

Street, Philadelphia, PA 19103.

[28] Smithsonian Physical Tables, Table 1, Vol. 815, p. 273, (1954).

[29] ASTM D 1042, Standard Test Method for Linear Dimensional Changes of

Plastics, Annual Book of ASTM Standards, Book 35, American Society of

Testing and Materials, 1916 Race Street, Philadelphia, PA 19103.

[30] ASTM D 1204, Standard Test Method for Linear Dimensional Changes of

Nonrigid Thermoplastic Sheeting of Film at Elevated Temperatures,

Annual Book of ASTM Standards, Book 35, American Society of Testing

and Materials, 1916 Race Street, Philadelphia, PA 19103.

91

APPENDIX A. DETERMINATION OF COVER PLATE TEMPERATURES

A.l Introduction

Prior to performing heat aging exposures in the laboratory, it was necessary to

determine the temperatures to which cover plate materials are exposed on flat

plate solar collectors. Two approaches were used to determine the temperatures. The first involved the installation of thermocouples on a collector

to measure cover plate temperatures. The second involved computer simulations

of collectors to estimate the temperatures reached.

A. 2 Measurement of Cover Plate Temperatures

To measure material temperatures, thermocouples were installed at various locations on a typical flat plate solar collector. A description of the collector

is contained in table Al and the collector is shown in figures Al and A2. Type

'T' , copper constantan thermocouples were installed on the cover plate and the

absorber in the locations indicated in figure A2.

Table Al

Description of Collector on Wh ich Temperatures Were Measured

Exterior Size 1.93 X 0.86 m (76 x 34 in)

Two Cover Plates,

each cover: 3.2 mm (1/8 in) glass

» 88% solar transmittance

Absorber Coating a « 0.92, e « 0.92

Insulation 80 mm (3 in) fiberglass blanket

Spacing between Absorber

and Inner Cover 9 mm (0.35 in)

Spacing between Covers 9 mm (0.35 in)

Thermocouples were attached to the cover adhesive tape. The size of the strip of

provide secure contact with the surface

tioning of the foil backed tape, the the

from direct exposure to either the rerad

or direct solar radiation. Installation

plate was accomplished by embedding the

the 3.2 mm (0.125 in) thick aluminum abs

into a hole drilled at an angle in the a peening.

plates using aluminum foil backed

tape used was just large enough to

of the cover plate. By proper posirmocouple junctions were protected

iation of heat from the absorber plate

of the thermocouples on the absorber

thermocouple junction in the back of

orber. The thermocouple was inserted

bsorber and it was held in place by

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Solar radiation was recorded by a pyranometer mounted at the same tilt angle

as the solar collector. A Class 1 pyranometer, as classified by the World

Meteorological Organization, was used. The solar radiation data from the

pyranometer was collected with an integrator. Monitoring the test was accomplished by scanning the thermocouples and solar radiation integrator

at intervals (e.g., 10 or 15 minutes) using an automatic data logging system.

The solar collector was exposed at Gaithersburg, Maryland facing south.

Temperatures were monitored with the collector in a stagnation condition.

Figure A3 is an illustration of the cover plate temperatures on June 23,

1977. The highest inner cover plate temperature measured was about 120''C

(248°?), while the maximum outer cover plate temperature was 70°C (158°F).

The slight wind during the day would have had some cooling effect, especially

on the outer cover plate. Examination of the temperatures across the diagonal

of the inner cover plate indicated a temperature differential of approximately

30*'C (55°F) from the center of the collector to the upper right or lower

left corners of the cover plate. This provides an indication of the temperature differences to which the inner cover was exposed.

Figure A4 illustrates the temperatures on September 18, 1977 of the absorber

plate along with the inner and outer cover plates in a stagnating collector

and the ambient air. The temperatures were measured at positions B, D and E

shown in Figure A2. The maximum absorber plate temperature reached on this

day was approximately 150°C (302°F) while the corresponding inner and outer

cover plate temperatures were 130°C (265°F) and 75°C (167°F). When the

maximum temperatures were measured, the instantaneous solar irradiance was approximately 945 W*h/m (300 Btu/ft ), the average wind speed was 10.9

km/h (6.8 mph) and the ambient air temperature was 28''C (82°F).

The data in figure A4 show that the inner cover plate was approximately

20°C (37°F) cooler than the adjacent absorber while the difference between

the inner and outer cover plates was approximately 55°C (98°F). The outer

cover plate was also about 47°C (85°F) above the ambient temperature. Similar temperature differences were also found in the data in figure A3.

A. 3 Computer Simulations to Estimate Cover Plate Teiiq)eratures

A. 3.1 Interim Performance Criteria Calculations

The Interim Performance Criteria for Solar Heating and Cooling Systems in

Residential Buildings^ contains computer generated graphs representing

maximum temperatures for cover plate and absorber plate materials under

1/ Holton, J.K. et al. , "Interim Performance Criteria for Solar Heating

and Cooling Systems in Residential Buildings, Second Edition", National

Bureau of Standards Reprot NBSIR 78-1562, (1978). Available from

National Technical Information Service, Springfield, VA 22161,

Order No, PB 289 967.

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stagnation conditions. The graphs were based upon calculations for typical

flat plate solar collectors having two types of cover plates, i.e., a typical

window glass and a more transparent glass with a low iron oxide content.

Temperatures of glass cover plates and absorber plates were plotted as

a function of ambient temperature for exposure to a solar radiation flux

of 1070 W/m^ (340 Btu/ft^*h). Data were shown for collectors with one and

two cover plates for the two cover glass materials and with spectrally

flat and selective black absorber coatings. The calculations were performed

with a transient thermal analysis model using the following conditions:

1) air space thickness of 9.5 mm (3/8 in) between the two cover plates

and between the absorber surface and inner cover plate; 2) zero edge and

back losses; 3) incident solar radiation normal to the collector; and

A)the properties of glass cover plates contained in table A2 . Four graphs,

figures A5a-A5d, illustrate the cover plate temperatures estimated by this

computer simulation.

Table A2

Properties of Glass Cover Plates Used in Computer Simulation of

Maximum Collector Temperatures in Interim Performance Criteria

Conventional Glass Low Iron Oxide Glass

(0.1% iron oxide) (0.01% iron oxide)

thickness 3.2 mm (1/8 in) 3.2 mm (1/8 in)

aborptance (a) 0.06 0.02

extinction coefficient 2.76 X 10-2/mm 7.87 X 10-3/mm

(0.70/in) (0.20/in)

index of refraction 1.52 1.52

thermal conductivity 1.019 W/m°C 1.019 W/m-°C

(0.589 Btu/(ffh-°F)) (0.589 Btu/(ffh-°F))

solar energy

transmittance 86% 90%

emittance (e) 0.86 0.86

With the ambient temperature at 27°C (81°F), the maximum temperature estimated

for the conventional glass cover plates was 130-135°C (266-275°F) as an inner

cover, 80-85°C (176-185°F) as an outer cover and 88-93°C (190-200°F) as a single cover plate. The temperatures of the conventional glass and the low

iron glass varied by less than 5°C (9°F). Comparing the temperatures of the

single and double glazed collectors, the data indicate that the cover plate on

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a single glazed collector is approximately the same temperature as the outer

cover plate on the double glazed collector, with the outer cover plate being

perhaps 10°C (18°F) lower. Note that the temperature of the cover plates

on collectors having flat black absorbers were only about 5-10°C higher

than cover plates on similar collectors having selective absorber surfaces.

This appears to be the case for both single and double glazed collectors.

This temperature difference can be attributed to the selective surface

reradlating less of the thermal energy than the flat black absorber.

It should be noted that the exposure conditions used for these comparisons

represent typical maximum conditions. Higher solar radiation flux can be

experienced under some atmospheric conditions or by the use of external

reflectors. The cover plate temperature would also increase as the ambient

temperature rises. For example, if the ambient temperature rose from 27°C

(81°F) to 38°C (100°F), these graphs predict the temperature of the cover would rise 5-8°C (9-14°F).

A. 3. 2 Calculations for Plastic Cover Plate Materials

Plastic cover plate materials have different material properties than those

used for the glass covers in the computer calculations described in the

preceding section. To estimate the temperatures reached by various

types of plastic cover plate materials, additional computer calculations

were made. Cover plate and absorber plate temperatures were determined

for two sets of weather conditions for stagnating collectors having

single and double cover plates and with spectrally flat and selective black

absorber surfaces. Temperatures were calculated for seven cover materials

which included three plastic films, (i.e., f luorinated(ethylene propylene)

copolymer, poly(vinyl fluoride), and poly(ethylene terephthalate)) , three

plastic sheet materials, (i.e., poly(methyl methacrylate) , polycarbonate,

and fiber reinforced plastic) and one glass. Temperatures were calculated

for each cover plate material on a single cover collector and for double

cover collectors having both covers made of the same material. Additional,

calculations were made using glass as the outer cover with each of the

three films as the inner cover. A computer analysis technique was used

to determine the temperatures. Collector and weather parameters which

were utilized in the calculations are contained in table A3. The cover plate materials and their properties are listed in table A4.

A9
Table A3

Collector and Weather Parameters Used in Computer Calculations

Collector Slope 45°

Incident Angle 0.0°

Number of Covers One or Two

Space Between Absorber

and Cover 25 mm (1 in)

Space between Inner and Outer Covers 25 mm (1 in)

K-Insulation 0.035 W/m-°C (0.20 Btu/(ffh- °F))

(No Edge Losses)

Insulation Thickness 76.2 mm (3 in)

Plate Absorptance (a) 0.95

Plate Emittance (e) 0.20 or 0.90

Solar Radiation 945 W/m2 (300 Btu/ft^-h)

or 1070 W/m2 (340 Btu/ft^'h)

Diffuse Fraction 0.10

Ambient Temperature 25°C (77°F) or 38°C (100°F)

Sky Temperature Ambient Temperature

Wind Speed 0.0 m/s

Table A5 represents the cover plate and absorber temperatures expected

for a stagnating collector with an ambient temperature of 38°C (100°F)

and a solar irradiance of 1070 W/m^ (340 Btu/ft^'h). These conditions

represent clear sky conditions which can be reached in most parts of

the United States, although a higher solar irradiance may be achieved,

especially if reflectors are used. From the data in table A5, the highest predicted cover plate temperatures for a collector under these

stagnation conditions would occur with low iron glass, although generally

the temperatures of the plastic sheet materials were just a few degrees

lower. Since film materials are generally transparent to reradiated

long wavelength energy, film temperatures are lower than the corresponding temperatures for glass and plastic sheet materials (which are opaque

to the long wavelength radiation).

For single cover applications, glass and plastic sheet materials had essentially the same maximum temperatures with the range being from 98-102 °C

(208-216°F). The films had lower temperatures spanning from 81-96°C (178-

205°F). For the double cover collector, the maximum temperature of the

inner cover ranged from 143-153°C (289-307°F) for the glass and plastic

sheet materials while the film temperatures varied between 110° and 131 °C

(230° and 268°F). The outer cover temperatures ranged from 93°-98°C

(199-208°F) for glass and plastic sheet materials and from 76-87°C (169-

189°F) for films. Comparing the double cover collectors having the glass/

film cover plate combination with those having two glass covers, the outer

glass cover plate remained essentially the same temperature, while the inner

film cover plate was 3-14°C (5-25°F) lower. Table A6 lists the range of

maximum stagnation temperatures along with average temperatures and the

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Cover Plate and Absorber Plate Temperatures^' Reached on Stagnating Collectors with an Ambient

Air Temperature of 38°C (100°F) and a Solar Irradiance of 1070 W/m^ (340 Btu/ft^-h)

Flat Black

absorber'^ 1=0.95, e=0.90

Single Cover Collector

Selective

absorber"'

=0.95, e=0.20

Double Cover Collector

Flat Black absorber"'

=0.95, €=0.90

Selective absorber

0=0.95, e=0.20

Cover Material

Absorber Cover

Plate Plate

°C °C

Absorber Cover

Plate Plate

°C °C

Inner Outer Absorber Cover Cover

Plate Plate Plate

"C °C °C

Absorber

Plate

°C

Inner Outer

Cover Cover

Plate Plate

°C °C

Fluorinated (ethylene

propylene) copolymer

Poly(vinyl fluoride)

Poly (ethylene terephthalate)

Poly(methyl methacrylate)

Polycarbonate

Fiber reinforced plastic

Glass (0.01% Fe203)

Glass (0.01% Fe203) - outer fluorinated (ethylene

propylene) copolymer - inner

Glass (0.01% Fe203) - outer poly(vinyl fluoride) - inner

Glass (0.01% Fe203) - outer poly (ethylene

terephthalate) - Inner

134

141

145

157

154

154

159

93

101

100

101

102

207

206

202

212

208

207

213

96

94

95

100

98

100

101

148

162

167

189

184

184

194

172

178

180

110 76

123 84

130 87

148 96

145 94

147 97

153 98

139

142

144

100

98

96

222

223

218

239

231

229

242

237

235

232

125 78

127 83

131 86

146 95

143 93

145 96

151 96

148

145

144

98

97

95

^' Temperatures in Centigrade may be converted to Fahrenheit using the formula: °F = CC x 9^) + 32°

'^ a = solar absorptance

e = emittance

A12
Table A6

Summary of Cover Plate Temperature Data for Stagnation Conciitions^'^f

Single Cover Collector Double C over Collector

Flat Black Absorber

°C

Selective Absorber

°C

Inner Cover Outer C over

Flat Black Absorber

°C

Selective Absorber

°C

Flat Black Absorber

°C

Selective Absorber

"C

Range of maximum

temperatures 81-102 94-101 110-153 125-151 76-100 78-98

Average temperature

all data

95 98 138 141 93 92

Average temperature

excluding collectors with film as outer covert 101 100 145 146 97 96

Standard deviation

of average temper- ature of all data^ 8.1 2.8 13.3 9-7 7.7 6.9

Standard deviation of average temperature

excluding collectors with film as outer coverS^ 0.8 1.3 4,3 2.7 1.9 1.6 ^ These temperatures are extracted from table A5. Temperatures are based upon a solar irradiance of 1070 W/m

(340 Btu/ft^'h) at an ambient temperature of SS'C (100°F). For the flat black absorber a = .95, c = .90, while for the selective absorber a = .95, e =.20.

b/ Centigrade temperatures can be converted to Fahrenheit by the formula: °F = (°C x -2,) + 32°.

£/ Films are generally not used in outer cover applications as their resistance to puncture and impact is very low. This average temperature data excludes the use of fluorinated (ethylene propylene) copolymer,

poly(vinyl fluoride) and poly(ethylene terephthalate) in applications as single covers and outer double

covers.

A13
standard deviations. Since films are generally not used in outer cover applications, the average temperatures were calculated for all collector

cover plate combinations and for the collector cover combinations that

excluded films in applications as a single cover or outer double cover. When the temperature data for films in these applications was excluded,

the average cover plate temperature increased but the standard deviation

of the average temperatures significantly decreased. Thus, these average

temperatures (excluding films as outer or single covers) are probably the

temperatures typically reached by most collectors. It can be noted from

table A6 that the type of absorber (i.e., flat black or selective) has no significant effect on the cover plate temperature. It can also be observed

that single covers are only about 4°C (7°F) higher than the outer double

cover.

Table A7 represents cover plate and absorber plate temperatures expected

from a stagnating collector with an ambient temperature of 25°C (77 °F)

and a solar irradiance of 945 W/m^ (300 Btu/ft^*h). Table A7 has cover plate temperatures similar to the data in table A5, however, the ambient

temperature and solar irradiance are lower. These conditions are typical

of average summer weather for larger parts of the United States but do

not produce maximum stagnation temperatures obtained under the higher

ambient temperatures and solar irradiance. These calculations were done

to compare the calculated temperatures with those which were measured

on the stagnating collector (Section A. 2) and to determine the effect

of changing both the ambient temperature and solar irradiance. The variation in cover plate temperatures obtained by increasing the ambient temperature from 25° to 38°C (77-100°F) and the solar irradiance from 945

W/m^ to 1070 W/m^ (300 Btu/ft^'h to 340 Btu/ft^'h) is summarized in

table A8. Although the ambient temperature was increased 13 °C (23 °F) and

the solar irradiance by 125 W/m (40 Btu/ft *h), the estimated increase

in the cover plate temperature was only 14-19°C (25-34°F).

A. 4 Comparison of Cover Plate Temperature Data

Glass cover plate temperatures measured and those obtained from the computer

simulations are summarized in table A9. Although the weather parameters

varied somewhat, the temperatures measured for the glass cover plates on

the double cover collector are in the range of those predicted by the

computer models when the ambient temperatures were comparable. The slightly

lower outer cover plate temperature in the measured data can be explained

by the 10.9 km/h (6.3 mph) wind while the computer simulations had no wind

conditions. The bottom line of temperatures in table A9 probably represents

the maximum stagnation cover plate temperatures that would be achieved in

most parts of the United States.

A14
Table A7

AmbiCover Plate anej Absorber Plate Temperatures^ Reached on Stagnating Collectors with an bient Air Temperature of 25°C (77°F) and a Solar Irradiance of 945 W/m^ (300 Btu/ft h)

Single Cover Collector Double Cover C ollector

Flat Blad^

absorber—' Selectiveabsorber-^

Flat Black

absorber—' Selective

absorber—' 0=0.95, e=0.90 0=0.95, e:=0.20 0=0.95, e=0.90 0=0.95, £=0 20

Inner Outer Inner Outer

Absorber Cover Absorber Cover Absorb er Cover Cover Absorber Cover Cover Plate Plate Plate Plate Plate Plate Plate Plate Plate Plate

Cover Material °C °C °C °C °C °C "C °C °C °C

Fluorinated (ethylene

propylene) copolymer 117 67 185 80 132 95 61 200 109 63

Poly(vlnyl fluoride) 124 73 183 79 144 107 69 201 111 68

Poly( ethylene terephthalate) 127 77 180 79 150 114 72 196 115 71

Poly(methyl methacrylate) 139 85 188 83 170 131 80 216 129 79

Polycarbonate 136 84 185 82 165 127 78 208 125 77

Fiber reinforced plastic 136 85 184 84 165 129 81 206 127 80

Glass (0.01% Fe203) 141 86 190 85 175 134 81 219 132 80

Glass (0.01% Fe203) - outer fluorinated (ethylene

propylene) copolymer - inner — — 154 122 84 214 131 82

Glass (0.01% Fe203) - outer poly(vinyl fluoride) - inner — — 159 125 82 212 128 81

Glass (0.01% Fe303 - outer poly(ethylene

terephthalate) - Inner — — — 162 126 80 209 127 79

3/

b/

Temperature in Centigrade may be converted to Fahrenheit by the formula: °F=(°C x ^) H- 32°

5 a = solar absorptance

e = emittance

A15
Table A8

Variation in Cover Plate Temperatures for Stagnating Collectors with No Wind When the Weather Conditions Are Altered^

Single Cover Collector

Double Cover Collector

Inner Cover Outer Cover

Flat Black

a=.95, E=0.90 Selectivea=.95, e=.20

Flat Black

a=.95, c=.90 Selectivea=.95, e=0.20

Flat Black

a=.95, e=.90 Selective

a=.95, c=0.20

Cover Materials °C °C °C °C °C °C

FluorinatedC ethylene

propylene) copolymer 14 16 15 16 15 15

Poly(vinyl fluoride) 15 15 16 16 15 15

PolyC ethylene

terephthalate) 16 16 16 16 15 15

PolyCmethyl

methacrylate) 16 17 17 17 16 16

Polycarbonate 16 16 18 18 16 16

Fiber reinforced

plastic 16 16 18 18 16 16

Glass (0.01% Fe203) 16 16 19 19 17 16

Glass (0.01% FejOj)

outer, fluorinated

(ethylene propylene)

copolymer- inner 17 17 17 16

Glass (0.01% Fe203)

outer, poly(vinyl

fluoride) - inner 17 17 16 16

Glass (0.01% Fe203)

outer, poly(ethylene

terephtahalate) - inner 18 17 16 16

a/ These data were obtained by comparing the temperatures in Tables A5 and A7. The weather conditions were

altered from 25°C (77°) ambient tsnperature with 945 W/m (300 Btu/ft h) solar irradiance to 38° C (100°F) ambient temperature with 1070 W/m (340 Btu/ft 'h) solar irradiance.

A16
Table A9

Comparison of Measured Temperatures of Glass Cover Plates on Flat Plate Solar Collectors

under Stagnation Conditions with Estimated Temperatures from Computer Simulations

Single Cover Temperatures^ Double Cover a/ Temperatures—'

Outer Inner

°C °C °C

Measured on a flat plate 75 130 collector at 28°C (820°C)

ambient air. 930 W/m^

(295 Btu/ft"'h) solar

irradiance, 10.9 km/h

(6.3 mph) wind, flat black

absorber (see figure A4).

From computer simulations, 85-86 80-81 132-134

In IPC described in

section A. 3. 2. 3. 27°C

(81°F) ambient air, 1070 W/m^ (340 Btu/ft^'h) solar

irradiance, no wind (see

figure A5).

From computer simulations 88-93 80-85 130-135

described in section A. 3. 2.

25''C (77''F) ambient air, 9A5 W/m^ (300 Btu/ft^'h)

solar irradiance, no wind

(see table A7).

From computer simulations 101-102 96-98 151-153

described in section A. 3. 2. 38° (100°F) ambient air,

1070 W/m^ (340 Btu/ft^*h)

solar irradiance, no wind

(see table A5).

a/ Temperatures in Centigrade may be converted to Fahrenheit by the formula:

°F = (°C X 9/5) + 32°.

A17

APPENDIX B

PROPOSED STANDARD PRACTICE FOR EXPOSURE OF COVER MATERIALS FOR

SOLAR COLLECTORS TO NATURAL WEATHERING UNDER CONDITIONS

SIMULATING STAGNATION MODE

1

.

SCOPE

1.1 This standard practice provides a procedure for the exposure of cover materials for flat plate solar collectors to the natural weather environment at temperatures which are elevated to approximate stagnation

conditions.

1.2 This standard practice is suitable for exposure of both glass and plastic

solar collector cover materials. Provisions are made for exposure of

single and double cover assemblies to accomodate the need for exposure

of both inner and outer solar collector cover materials.

1.3 This standard practice does not apply to cover materials for evacuated

collectors or photovoltaics.

2. APPLICABLE DOCUMENTS

2.1 ASTM Standards

D 883 Definitions of Terms Relating to Plastics

D 1435 Recommended Practice for Outdoor Weathering of Plastics

E XXX Standard Practice for Evaluation of Cover Materials for Flat

Plate Solar Collectors-'-/

2.2 Other Documents:

Federal Specification HH-I-558B, Amendment 3, Insulation Blocks, Boards,

Felts, Sleeving (Pipe and Tube Covering), and Pipe Fitting Covering

Thermal (Mineral Fiber, Industrial Type) August 1976.^'

3. SIGNIFICANCE AND USE

3.1 This standard practice describes a weathering box test fixture and

establishes limits for the heat loss coefficients. Uniform exposure

guidelines are provided to minimize the variables encountered during

outdoor exposure testing.

'' A copy of this proposed Standard Practice is contained in Appendix C of

this report.

II Available from General Services Administration.

Bl
3.2 Since the combination of elevated temperature and solar radiation may

cause some solar collector cover materials to degrade more rapidly than

either alone, a weathering box which elevates the temperature of the cover materials is utilized.

3.3 This standard practice is intended to assist in the evaluation of solar

collector cover materials in the stagnation mode. Insufficient data

exist to obtain exact correlation between the behavior of materials

exposed according to this recommended practice and actual in-service

performance.

3.4 This standard practice may also be useful in comparing the performance

of different materials at one site and/or the performance of the same material at different sites.

3.5 Means of evaluation of effects of weathering are provided in Method E XXX,

Standard Practice for Evaluation of Cover Materials for Flat Plate Solar

Collectors and in other ASTM test methods which evaluate material properties.

3.6 Exposures of the type described in this standard practice may be used to

evaluate the stability of solar collector cover materials when exposed

outdoors to the varied influences which comprise weather. Exposure

conditions are complex and changeable. Important factors are material

temperature, climate, time of year, presence of industrial atmosphere,

etc. Generally, because it is difficult to define or measure precisely

the factors influencing degradation due to weathering, results of outdoor exposure tests must be taken as indicative only. Repeated exposure

testing at different seasons and over a period of more than one year is

required to confirm exposure tests at any one location. Control samples

must always be utilized in weathering tests for comparative analysis.

4. WEATHERING BOX TEST FIXTURE

4.1 Test Fixture Requirements

4.1.1 The weathering box test fixture should be constructed such that the

combined back and edge loss coefficient is less than 1.5 W/m'^»°C

(0.264 Btu/Cft^. h.°F)) . (Note 1) (The method for determining this

coefficient is outlined in Appendix XI of this Standard). The distance

between the absorber and the closest cover plate should be between 13

and 38 mm (1/2 and l-l/2in). For a double cover exposure the separation

between the inner and outer cover should be between 13 and 38 mm (1/2

and 1-1/2 in). Not more than 10 percent of the absorber plate area

should be shaded when the sun is at a 30° angle with the plane of the

front surface of the exposure box.

4.1.2 Boxes that meet the requirements of Section 4.1.1 are described in

Table Bl. Figures Bl and B2 illustrate the exposure test fixtures.

Although figure Bl shows a square box, any shape is permitted if the

B2
Table Bl . Examples of Exposure Test Fixtures with Combined Heat Loss Coefficient for Back and Edge Losses Less than 1.5 W/m2*°C

(0.264 Btu/(ft2-h-°F))

Container Material

Insulation Material

I, length of aperture inside

edge insulation

w, width of aperture inside

edge insulation

h, distance from top of absorber to bottom of cover plate

A , area of aperture of test fixture A^ = (ixw)

Av, Area of back insulation

A^ = Uxv)

A , area of edge insulation

Ag = 2(Jl+w)h

d^,, thickness of back insulation

d^, thickness of box

dg, thickness of edge insulation

Kv, conductivity of back

insulation

Example 1 Example 2

steel aluminum

glass fiber glass fiber

0.25 m

(9.8 in)

0.61 m

(24 in)

0.13 m

(5.2 in)

0.61 m

(24 in)

0.013 m

(0.5 in)

0.038 m

(1.5 in)

0.033 m^

(51 in2)

0.372 m^

(576 In^)

0.033 m^

(51 in2)

0.372 m2

(576 in^)

0.01 m^

(15 in^)

0.093 m^

(144 in^)

0.077 m

(3 in)

0.05 m

(2 in)

0.001 m

(0.04 in)

0.002 m

(0.08 in)

0.013 m

(0.5 in)

0.025 m

(1 in)

0.038 W/m'^C

(0.22 Btu/(ft2-h- 'F))

0.038 W/m"

(0.022 Btu/

K^ , conductivity of box 43 W/m'^C

(24.9 Btu/(ft2-h«°F)

204 W/m"°C

(118 Btu/(ft2-h-°F))

Kg, conductivity of edge

insulation

0.038 W/m°C

(0.022 Btu/(ft2'h-°F)

0.038 W/m°C

(0.022 Btu/(ft2-h-°F)

Ab/Aa

Ae/Aa

1

0.305

1

0.25

h^S

^c/\

dg/Kg

"L.Back •" ^L.Edge

2.03 m^-'C/W

(11.4 (ft2-h'°F)/Btu)

2.33 X

10~5 m^*°C/W

1.32 m^-°C/W

(7.5 (ft2-h"°F)/Btu)

9.8 X 10"^ m^'C/W

(1.32 X 10"'^ (ft2'h-°F)/Btu) (5.6 x lO'^ (f t^ -h- °F)/Btu)

0.342 m2-°C/W 0.658 m^'^C/W

(1.94 (ft2-h-°F)/Btu) (3.74 (f t^ -h- °F)/Btu)

1.38 W/m2-°C

(0.243 Btu/(ft2-h-°F))

1.14 W/m'^-'C

(0.201 Btu/(ft2-h-°F))

B3
FIGURE Bl Top View of Exposure Test Fixture

n w

^^,^:^^^'->•/v •:•-" ^^./^^^'.^V'* "';

I = length of aperture inside

edge insulation

w = width of aperture inside

edge insulation

dg = thickness of box

de = thickness of edge

B4
FIGURE B2 Natural Weathering Box Assembly

DOUBLE GLAZED SINGLE GLAZED

BOX

GLAZING FRAME

OUTER COVER PLATE

COVER PLATE

INNER COVER PLATE

ABSORBER

BOX

TOP

TAPE

GLAZING FRAME

INSULATION

OUTER COVER PLATE

TOP

TAPE

GLAZING

FRAME

INSULAT10N

INNER COVER

PLATE

INSULATION

OUTER COVER PLATE

TAPE^ _TOP ^

SPACER

GLAZING

FRAME

BOX

B5
requirements in Section 4.1.1 are met. Appendix XI of this standard

describes the method for determining the combined back and edge loss

coefficients.

NOTE 1: A good flat plate solar collector has a combined back and edge loss

coefficient of less than about 1.5 W/m2°C (0.264 Btu/(ft^.h* °F))

.

4.2 The weathering box test fixture should consist of 1) a box, 2) insulation,

3) absorber, 4) a box top, 5) a spacer, 6) a glazing frame, and 7) adhesive tapes.

4.2.1 The box may have any dimensions and be made of any material as long as the requirements in Section 4.1.1 are met. A weephole should be drilled

at the lower end of the bottom of the box to provide drainage and to

minimize moisture accumulation.

NOTE 2: It is desirable that the box and box top be made of a material which

will be unaffected by the exposure environment. A metal resistant

to corrosion encountered the environment would be suitable. If wood

is used, it should be painted or treated on the exterior to make it

impervious to moisture. In certain climates only rot resistant wood

should be utilized to minimize deterioration during exposure.

4.2.2 The insulation should be a material suitable for use at high temperatures, (e.g., 150°C (302°F)).-^/

NOTE 3: Insulation materials having resins or binders should not be used

because elevated temperatures may cause the resin or binder to

deteriorate and outgas. Outgassing products condense on the cover material causing changes in the solar transmittance of the solar

collector cover material.

4.2.3 The absorber should be of an adequate size to cover the interior surface

of the weathering box aperture. The absorber shall have a flat black

nonselective coating having an absorptance not less than 0.90 after

exposure.

4.2.4 The box top should be of an adequate size to fit over the top of the

box.

NOTE 4: The box top is intended to protect the edges of the test specimen

in contact with the box from reaching excessively high temperatures,

to minimize exposure of the adhesive tape to sunlight, and to minimize moisture penetration into the exposure test fixture.

1/ Federal Specification HH-I-558B has several classes of insulation material

intended for high temperature use.

B6
4.2.5 The glazing frame in intended to hold the cover plate material. The

glazing frame should have dimensions similar to the perimeter of the

container. For a double cover exposure the frame should provide a separation between the two cover plates of not less than 13 mm (1/2 in) nor greater than 38 mm (1-1/2 in). Exact dimensions of the frame are related to the requirements in Section 4.1.1. A vent hole may be

drilled at one end of the spacer to provide drainage and to minimize

moisture accumulation.

4.2.6 The spacer is intended to provide a separation of 13 to 38 mm (1/2 to

1-1/2 in) between the absorber and the closest cover plate. Exact

dimensions of the spacer are related to the requirements in Section

4.1.1.

NOTE 5: Certain designs of weathering boxes may eliminate the need for the

spacer.

4.2.7 The adhesive tapes-'-' should be stable when exposed to moisture and

elevated temperatures. They should be compatible with the specific

materials from which the box, glazing frame, box top, and cover plate

are made.

4.2.8 Organic materials are potential sources of outgassing and should be

eliminated from the interior of the weathering box where possible. For

example, metallic parts should be cleaned to remove traces of grease or other foreign matter. Other possible sources of outgassing include

coatings and sealants. Test fixture components containing organic

materials (e.g. absorber coatings or insulation) should be heated in an over at 150°C (302°F) for 24 hours before the test fixture is assembled.

This should minimize outgassing which results from deterioration of the

organic components exposed to elevated temperatures.

4.3 Test Specimen

4.3.1 The test specimen should be of an adequate size to cover the aperture

of the box or glazing frame and to permit suitable attachment.

NOTE 6: Adequate allowances should be made for materials that will undergo

dimensional changes due to temperature.

•' 3M Weather Resistant Adhesive Tape //838 or its equivalent has been found

suitable for attaching the box top to the box. 3M Adhesive Transfer Tape

and Densil Silicone Pressure Sensitive Tape #2078 or the equivalent have

been found suitable for attaching glazings to the glazing frame in solar

collectors. The 3M tape is available from the 3M Company, 3M Center,

St. Paul, MN 55101. The Densil tape is available from the Dennison

Manufacturing Company, Coated Paper Division, Framingham, MA 001701.

B7
4.3.2 The test specimen Identification marks shall not interfere with either

the exposure or the subsequent testing.

4.4 Sample Mounting

4.4.1 Rigid and Semirigid Glazings

4.4.1.1 Test specimens for single cover exposure may be laid directly on either the spacer or the glazing frames. If used, the frame is then

placed on the spacer in the weathering box (see figure B2).

4.4.1.2 Test specimens for inner cover exposure on a double cover assembly

should be either laid on the spacer or attached to the glazing frame

before the glazing frame is placed in the box (see figure B2).

4.4.1.3 Test specimens for outer cover exposure on a double cover assembly

should be laid on the top of the glazing frame (see figure B2),

4.4.2 Films

4.4.2.1 Film test specimens should be placed on the glazing frame using adhesive transfer tape to hold the test specimens taut. The frame should

then be placed in the box similar to Sec. 4.4,1,1, 4.4.1.2 and

4.4.1.3.

4.5 Assembly of Weathering Box

4.5.1 The various parts of the weathering box test fixture should be slid into

position. The outer glazing must be roughly flush with the top side of

the box. The position of an inner glazing, if used, will be nearest the

bottom of the box.

4.5.2 After assembly, the frame and outer glazing should be sealed in place

with an adhesive tape to prevent moisture intrusion. The box top is

then placed on the box covering the tape (see figure B2).

5. NATURAL WEATHERING EXPOSURE

5.1 The weathering boxes shall be mounted in a backed condition on weathering

racks such as those described in ASTM D 1435. The racks should be capable

of having the angles adjusted and have their axis of rotation on an east/

west line.

5.2 A variable angle exposure should be utilized to maximize solar radiation

incident upon the weathering box. Racks shall be adjusted according to

the schedule given in Table B2 . Positive rack angles face south. The

angles shall be so chosen that the weathering boxes are never closer to

the horizontal than by 5 degrees. Other variable exposure schedules utilizing more than four adjustments per year may be utilized. The method to

determine the variable angle exposure schedule is described in Appendix X2

of this standard.
5.3 When a number of weathering boxes are exposed simultaneously, the boxes

should be mounted side by side with the sides not touching.

5. A The solar collector cover materials should not be cleaned during exposure.

Table B2, Variable Angle Rack Adjustment Schedule

Utilizing Four Changes Per Year^ »"/

Calendar Period ~|

Rack Tilt Angle

Degrees

Dates Days of Year

Latitude + 2.5°

(Latitude - 16°) + 2.5°

Latitude + 2.5°

(Latitude + 16°) + 2.5°

3/2 to 4/11

4/12 to 8/31

9/1 to 10/10

10/11 to 3/1

61 to 101

102 to 243

244 to 283

284 to 60 ^ This exposure schedule may be used in both northern and

southern hemispheres. The latitude in the southern

hemisphere is negative. Positive rack angles face south.

^

The incident angle of beam radiation (e) at solar noon

for a south facing collector is _< 8°.

6. REPORT

6.1 The report shall include the following:

6.1.1 Description of the weathering box test fixture and its calculated

combined back and edge loss coefficient.

6.1.2 Complete identification of the solar collector cover material.

6.1.3 Whether the solar collector cover materials are exposed as a single or double cover configuration and whether the test specimen was the inner

or outer cover.

6.1.4 A description of the test specimen attachment and mounting procedures.

6.1.5 Latitude, longitude, altitude and address of the testing site including

a description of the type of climate.

6.1.6 Exposure data.

B9
6.1.6.1 Calendar dates of exposure.

6.1.6.2 Variable angle rack adjust schedule.

6.1.7 Climatological data.

6.1.7.1 Radiant exposure (GJ/m^) measured in the plane of the weathering

boxes.

6.1.7.2 Monthly maximum, minimum and mean temperatures.

6.1.8 A visual inspection of test specimens, noting any obvious changes in

appearance should be made at regular scheduled intervals (once per

month minimum).

6.1.9 Description of control specimens.

6.1.10 Any deviation from this standard practice.

6.2 Other data which are desirable to report, if available are:

6.2.1 Optional climatological data.

6.2.1.1 Daily maximum, minimum and mean percent relative humidity.

6.2.1.2 Daily hours of wetness, both dew and rain.

6.2.1.3 Daily total inches of rainfall.

6.2.1.4 Daily maximum and minimum ambient temperature.

6.2.1.5 Daily radiant energy.

6.2.1.6 Wind direction and velocity.

6.2.2 Type of atmosphere, e.g., industrial, and level of air pollutants.

6.2.3 Ultraviolet radiation.

6.2.4 Maximum absorber plate temperature.

BIO
APPENDIX XI

CALCULATION OF EXPOSURE TEST FIXTURE HEAT LOSSES

XI. 1 SCOPE

XI. 1.1 This appendix outlines the method for determining the combined back and

edge loss coefficients for an exposure test fixture as referenced in

Section 4.1.1 of this standard.

XI. 2 PROCEDURE

XI. 2.1 Assumptions:

1. One-dimensional heat transfer (neglect corner effects)

2. The outside surface of the container is equal to the ambient temperature.

3. The inside surface of the edge insulation is equal to the absorber plate

temperature. (A conservative assumption; the inside edge temperature

would average less than the absorber plate temperature.)

XI. 2. 2 Abbreviations

QlOSS total ~ Total heat loss of test fixture

QlOSS back

~ Heat loss of back of test fixture

QlosS TOP

~ Heat loss of top of test fixture

QlOSS EDGE ~ Heat loss form the edges of test fixture

Ul = Combined loss coefficient of back, edge and top of test fixture

Ul g

= Loss coefficient of back of test fixture

Ul £

= Loss coefficient of edges of test fixture

^L T

~ Loss coefficient of top of test fixture

h = Distance from top of absorber to bottom of cover plate

£ = Length of aperture inside edge insulation

w = Width of aperture inside edge insulation

A^ = Area of aperture of test fixture

A^ = Area of back insulation (A^ = Jlxw)

Bll
Ag = Area of edge insulation (Ag = 2 (A +w)h)

d^ = Thickness of back insulation

dg = Thickness of edge insulation

d^ = Thickness of container

K^ = Thermal conductivity of back insulation

Kg = Thermal conductivity of edge insulation

Kf. = Thermal conductivity of container

Tp = Temperature of absorber plate

T^ = Temperature of ambient air

Tj, = Temperature of container

XI. 2. 3 Heat Losses

XI. 2. 3. I General Equations

QlOSS, TOTAL = QlOSS,BACK "•" QlOSS,EDGE ^ QlOSS.TOP (Eq. 1.0)

orAaUL(Tp - Ta) = A^UL^gCTp - T^) + A^Ul^eCTp " T^) + A^Ul^xCTp - T^) (Eq. 1.1)

Dividing by A^CTp-T^),

Ul = Ul,b + Ul,E + Ul,T (Eq. 1.2)

To keep different sizes of the test fixtures thermally equivalent, the sum of

the loss coefficients, U^ g, Uj^ e» ^^^ ^L T

n^^st remain constant. The top loss

coefficient can be held fairly constant by keeping the cover distance above the

absorber plate between 13 and 38 mm (1/2 and 1-1/2 in). With this constraint,

the sum of the edge loss coefficient, Uj^ e> ^^'^ ^^^ back loss coefficient,

Uj^ B, must remain constant.

Therefore,

^L B

"•" ^L E

^ Constant (Eq. 2)

XI. 2. 3. 2 Determination of Heat Loss Coefficient (Uj^ g) for Back of Test

Fixture

The heat loss through the back of a test fixture is equal to:

B12
QlOSS,BACK = AaUL^B(Tp " T^) = AbCKfe/db) (Tp - T^)

= Ab(Kc/dc) (Tc - Tg) (Eq. 3.0)

Reduction of equation 3.0 yields

A /A

Ut R =

^—^ (Eq. 3.1)

This reduction is accomplished by:

(Tp - Ta) = (Tp - T^) + (Tc - T^) (Eq. 3.2)

Substituting quantities from equation 3.0 into equation 3.2

^LOSS.BACK ^ QlOSS^BACK ^ QlOSS.BACK /g 3 3)

Aa Ul,B Ab(Kb/db) Ab(Kc/de)

Dividing by qLOSS,BACK

1 d^ d^

^ = L + £- (Eq. 3.4)

AaUL,B AbKb AbK^

Then,

Au/A,

Ut R = °—^

(Eq. 3.1) L>^ (d^/K^) + (d^/K^)

XI. 2. 3. 3 Determination of Heat Loss Coefficient (Ul g) for Edge of Test Fixture

The heat loss through the edge of the test fixture is equal to:

QlOSS.EDGE = A^Ul^e (Tp - W = Ae(Ke/de) (Tp - T^)

= Ae(Kc/dc) (Tc - T^) (Eq. 4.0)

Reduction of equation 4.0 yields;

Ut F = — ^^—^ (Eq. 4.1)

This reduction is accomplished by

(Tp - Tg) = (Tp - T^) + (T^ - Ta) (Eq. 4.2)

B13
Substituting quantities from equation 4.0 into equation 4.2

Qloss,edge ^ Qloss,edge ^ Qloss.edge /g , 2)

Aa Ul,E Ae(Ke/de) AgCKc/de) ^ q. . ;

Dividing by QlOSS.EDGE

1 d d - ^ + _£_ (Eq. 4.4)

Aa Ul,E AeKg AeK^

XI. 2. 3. 4 Combined Heat Loss Coefficient for Back and Edge Losses from Test

Fixture

The combined heat loss coefficient for back and edge losses from the test

fixture is found by adding equations 3.1 and 4.1.

Then

Ab/Aa . Ae/*a

For most designs Ay^/A^ » 1, and d^/K^ and dg/Kg >> d(,/K^.

Therefore, Ul^b + U^.E = (K^/dt) + (Ag/A^) (Kg/dg) (Eq. 5.1)

XI. 2. 3. 5 Examples:

This is an example of how to determine the combined heat loss coefficients and

shading factor for the exposure test fixtures described in Table Bl in Section

4.1.2 of the standard.

XI. 2. 3. 5.1 For Example 1 Test Fixture from Table Bl,

If A^/A^ = 1, Ag/A^ = 0.305, and d^/K^ = 2.03 m^«°C/W (11.4 (ft^»h» °F)/Btu),

d^/K^ = 2.33 X 10"^ m^»°C/W, (1.32 x

10"'^ (f t^«h« °F)/Btu)

,

dg/Kg = 0.342 m2»°C/W (1.94 (f t2.h.°F)/Btu)

B14
Then d^/Kb » d^/K^ and dg/K^ » d^/K^,

Therefore, equation 5.1 can be used.

Ul,B + Ul,E = (Kb/db) + (Ae/A^) (K^/de) (Eq. 5.1)

= 0.49 W/m2.''C (0.088 Btu/(f t^-h* °F)

+ 0.89 W/m^-'C (0.155 Btu/(f t^.h* °F))

Ul B

+ Ul E = 1.38 W/m2«°C (0.243 Btu/(f t^»h. °F))

To determine the shading of the absorber:

% shade = z«h«tan 6-100% (Eq. 6.0)

z»y

where

z = north-south dimension of absorber

y

= east-west dimension of absorber

h = height from absorber to top of outer cover plate

9 = solar beam angle of incidence (15° = 1 hour from solar noon)

If 6 = 30°, z = 0.25 m (9.8 in)

y = 0.13 m (5.2 in)

h = 0.013 m (0.5 in)

% shade = (0.25m) (0.013m) tan 30 x 100%

(0.25m) (0.13m)

% shade =5.8%

XI. 2. 3. 5. 2 For Example 2 Test Fixture from Table Bl,

U, 3, + Ut F = - ^-^ + ^—^ : ^ (Kq. 5.0) L»B L,E idjKO + (d./Kj (d /K ) + (d /K ) b' b'^ ^ c' C^ e' e^ c' c'

If A^/A^ = 1, kjk^ = 0.25 and d^^/K^ = 1.32 m2»°C/W (7.5 (f t^'h»°F)/Btu)

,

d^/K^ = 9.8 X 10"^ m2'°C/W (5.6 x

10"^ (f t^.h* ''F)/Btu)

,

^e^^e " ^'^^^ m^'°C/W (3.74 (f t2»h«°F)/Btu)

Then d^/Kb » d^/Kc, and dg/Kg » dc/K^

B15
Therefore, equation 5.1 can be used.

Ul,B + Ul^e = Kb/db + (Ae/Aa) (Ke/de)

= 0.7S x/m2.°C (0.134 Btu/(f t2.h.°F))

+ 0.38 W/m.°C (0.067 Btu/(f t2.h.°F))

^L B

"•" ^L E

" ^'^^ W/m^.°C (0.201 Btu/(f t^ .h.°F)

)

To determine the shading of the absorber, equation 6.0 is used.

If 6 = 30°, z = 0.61 m (24 in)

y = 0.61 m (24 in)

h = 0.038 m (1.5 in)

% shade = (0.61m) (0.038in) tan 30 x 100%

(0.61m) (0.61m)

% shade = 3.6%

B16
APPENDIX X2

DETERMINATION OF VARIABLE ANGLE EXPOSURE SCHEDULE

The direction of beam solar radiation can be determined by equations provided

In Duffle and Beckman.^/ The geometric relationships between a plane of any

particular orientation relative to the earth at any time (whether that plane

is fixed or moving relative to the earth) and the incoming beam solar radiation, that is, the position of the sun relative to that plane, can be described

in terms of several angles. These angles, and the relationship between them

are:

<() = latitude (north positive);

6 = declination (i.e., the angular position of the sun at solar noon

with respect to the plane of the equator) (north positive);

s = the angle betwen the horizontal and the plane (i.e., the slope)

(facing south is positive);

Y

= the surface azimuth angle, that is, the deviation of the normal to

surface from the local meridian, the zero point being due south,

east positive, and west negative;

0) = hour angle, solar noon being zero, and each hour equaling 15° of

longitude with mornings positive and afternoons negative (e.g.,

0) = +15 for 11:00, and w = -37.5 for 14:30);

9 = the angle of incidence of beam radiation, the angle being measured

between the beam and the normal to the plane.

The declination, 6, can be found from the approximate equation

6 = 23.45 sin [360 (284_±JI)] (Eq. 1)

365

where n is the day of the year.^/

'/ Duffy, John and Beckman, William, Solar Energy Thermal Processes, New

York, John Wiley and Sons, (1974).

^1 Declination can also be conveniently determined from charts.

B17
The relation between 6 and the other angles is given by

cos 6 = sin 6 sin <j) cos s - sin 6 cos (j) sin s cos Y

+ cos 6 cos (() cos s cos 0)

+ cos 6 sin (j) sin s cos y cos w

+ cos 6 sin s sin y sin (o (Eq. 2)

Equation 2 reduces to the following for a south facing collector:

cos 9 = sin ((|)-s) sin 6 + cos (<j)-s) cos 6 cos o) (Eq. 2.1)

At solar noon w =0 and cos w = 1 , therefore

cos 6 = sin ((|)-s) sin 6 + cos (((>-s) cos 6 (Eq. 2.2)

Using the identity: cos (A-B) = sin A sin B + cos A cos B, equation 2,2

becomes:

cos 6 = cos [((t)-s)-6] (Eq. 2.3)

Therefore, 6 = <J) - s - 6 (Eq. 2.4)

In order to make 9 =0, the following must be true

Sopt = * - 5 where (Eq. 3)

^opt ~ optimal collector slope

<t> = latitude

6 = declination of sun

To maximize the incident solar radiation upon the cover plate materials, the

angle of incidence of the beam solar radiation, 9, should be as close to as possible at solar noon. This can be achieved by periodically adjusting the

slope of the exposure fixture. The optimal slope may be determined by

equation 3.

Example:

Find optimum slope for Gaithersburg, Md. on May 3.

(j) = 39.1° (latitude)

n = 123 (day of year for May 3)

B 18
From Equation 1

,

6 = 23.45 sin [ii£ (284 + 123)]

365

= 15.5 o

Therefore,

Sopt = (}) - 6 = 39.1" - 15.5° = 23.6°

Using equation 3, Tables B3 and B4 were developed for variable angle exposure

schedules necessary to keep the angle of incidence of the beam solar radiation,

(e), less than 4° and 6°. Other exposure schedules may be calculated using

this approach.

B19
Table B3. Variable Angle Rack Adjustment Schedule Utilizing

Ten Changes per Ypar^»t>/

Rack Angle (Degrees

)

Calendar Pe riod

Dates Days of Year

latitude +12° 2-7 to 3-1 38 to 60

latitude +4° 3-2 to 3-21 61 to 80

latitude -4° 3-22 to 4-11 81 to 101

latitude -12° 4-12 to 5-4 102 to 124

latitude -20° 5-5 to 8-7 125 to 219

latitude -12° 8-8 to 8-31 220 to 243

latitude -4° 9-1 to 9-20 244 to 263

latitude +4° 9-21 to 10-10 264 to 283

latitude +12° 10-11 to 11-2 284 to 306

latitude +20° 11-3 to 2-6 307 to 37

^

This exposure schedule may be used in both northern and southern

hemispheres. The latitude in the southern hemispheres is negative.

Positive rack angles face south.

=/ The incident angle of beam radiation (6) at solar noon for a south

facing collector is < 4°.

Table B4. Variable Angle Rack Adjustment Schedule Utilizing

Six Changes per Ypar^>

V

Rack Angle (Degrees)

Calendar Period

Dates Days of Year

latitude +6°

latitude -6°

latitude -18°

latitude -6°

latitude +6°

latitude +18°

2-19 to 3-21

3-22 to 4-22

4-23 to 8-20

8-21 to 9-20

9-21 to 10-22

10-22 to 2-18

50 to 80

81 to 112

113 to 232

233 to 263

264 to 294

295 to 49

Sy This exposure schedule may be used in both northern and southern hemispheres. The latitude in the southern hemisphere is negative. Positive

rack angles face south.

s/ The incident angle of beam radiation (6) at solar noon for a south

facing collector is < 6°.

B20
APPENDIX C

PROPOSED STANDARD PRACTICE FOR EVALUATION OF COVER MATERIALS FOR FLAT

PLATE SOLAR COLLECTORS

1. SCOPE

1.1 This standard practice is intended for the evaluation of cover materials

for flat plate solar collectors.

1.2 This standard practice includes evaluation procedures for the measurement

of primary properties of materials and for the assessment of durability

of solar collector covers by comparison of initial primary property val4ie«

with those obtained after exposure to aging procedures.

1.3 This standard practice includes both primary property measurement tests

and aging procedures. Property test methods are included for measurments

of: solar transmittance, dimensional stability, impact resistance, tensile strength, and the effect of dirt retention on solar tramsmittance.

Aging procedures are included for exposure to: heat, natural weathering,

and accelerated weathering.

2. APPLICABLE DOCUMENTS

2.1 ASTM STANDARDS

D 618 Conditioning Plastics and Electrical Materials for Testing

D 638 Tensile Properties of Plastics

D 794 Recommended Practice for Determining the Permanent Effect of Heat

on Plastics

D 882 Tensile Properties of Thin Plastic Sheeting

D 1042 Linear Dimensional Changes of Plastics Under Accelerated Service

Conditions

D 1181 Warpage of Sheet Plastics

D 1204 Linear Dimensional Changes of Nonrigid Thermoplastic Sheeting or Film at Elevated Temperature

D 2565 Recommended Practice for Operating Xenon Arc-Type (Water-Cooled)

Light-Exposure Apparatus With and Without Water for Exposure of Plastics

E 424 Solar Energy Transmittance and Reflectance (Terrestrial) of

Sheet Materials

E XXX Standard Practice for Determining Resistance of Solar Collector

Covers to Impact by Hail-^'

^' A copy of this proposed standard is contained in National Bureau of

Standards Report, by D.R. Jenkins and R.G. Mathey which is in preparation.

CI
E XXX Standard Practice for Exposure of Cover Materials for Solar

Collectors to Natural Weathering Under Conditions Simulating Stagnation

Mode-'-/

2.2 ANSI STANDARDS

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

Safety Glazing Material Used in Buildings.^'

2.3 OTHER DOCUMENTS

Recommendations for the Integrated Irradiance and Spectral Distribution

of Simulated Radiation for Test Purposes, CIE Publication No. 20 (TC-2.2),

1972.^/

3. SIGNIFICANCE AND USE

3.1 While this standard practice is addressed to testing of covers for flat

plate collectors, it may be used for testing covers for other solar

collectors where applicable.

3.2 This standard practice is intended to aid the prediction of in-service

performance by short term testing.

3.3 Insufficient data exist to obtain exact correlation between the results

of accelerated weathering tests and actual in service performance.

3.4 Primary properties and their long term stability are critical to the

performance of a solar collector. Property measurement tests provide

for measurement of material properties of solar collector covers. Aging

procedures provide for exposure of solar collector covers to environments

which may induce changes in material properties as shown in property

measurement tests. Property measurement tests performed before and after

aging procedures provide a means of determining the effect of the aging

procedures on the solar collector cover material and assist in estimating

the durability of solar collector covers.

3.5 These tests include only those considered most critical to the performance of the solar collector. Other tests to evaluate materials may also

be performed.

'•' A copy of this proposed standard is contained in Appendix B of this report.

^' Available from American National Standards Institute, 1430 Broadway, New

York, NY 10018

3/ Available from Dr. Jack L. Tech, Secretary of U.S. National Committee of

International Commission on Illumination, National Bureau of Standards,

220 B-306, Washington, DC 20234

C2
3.6 This standard practice is intended for use by material manufacturers,

solar collector manufacturers, testing laboratories and others. The use

of this standard practice will provide material property data and durability data by which a material's suitability for a solar collector cover can

be assessed. It will also provide data for comparing the durability of

various collector cover materials. However, when using the data for com- parison of materials, the user should be aware that the many factors

influencing degradation due to weathering vary from location to location

as well as time of test. Because of this, outdoor exposure data should

be taken as indicative only. For direct comparisons the materials should

undergo the identical exposure conditions. Control samples must be used

in weathering exposures when identical exposure conditions cannot be used.

4. TEST SPECIMENS

4.1 The numbers and types of tests required and the preferred testing sequence

are specified in Table CI and Figure CI.

4.2 Except for thickness, the geometry of the test specimens used to evaluate

the properties of cover materials shall be in accordance with that specified in the applicable documents listed in Section 2. The thickness of

the specimens shall be that thickness intended for use in solar collector

covers. Warpage specimens will be 15 cm by 15 cm (6 in by 6 in) (See

Section 6 .1 .3).

4.3 Tensile test specimens used for evaluating the effects of aging procedures

shall be cut from a larger piece of material that was subjected to the

aging procedure.

4.4 Separate test specimens shall be used for the physical property tests if

the physical property test could change the test specimen prior to measurement of solar transmittance or dirt retention. Optical property

tests shall not be performed on test specimens damaged by physical

property tests.

5. RECOMMENDED CONDITIONING FOR PHYSICAL PROPERTY TESTS

5.1 Conditioning - Unless otherwise specified, the test specimens for physical

property tests shall be conditioned at 23° + 2°C (73° ± 4°F) and 50 +

5 percent relative humidity for not less than 40 hours prior to test in

accordance with Procedure A of ASTM D 618.

5.2 Test Conditions - Conduct tests in the Standard Laboratory Atmosphere of

23° + 2°C (73° + 4°F) and 50+5 percent relative humidity, unless otherwise specified in the methods.

NOTE 1 : In actual practice collector cover materials must have acceptable

physical properties over a wide range of temperature and humidity

conditions. However performing physical property tests above or below ambient conditions is difficult to implement.

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6. METHODS OF TEST

6.1 PROPERTY MEASUREMENT TESTS

6.1.1 SOLAR TRANSMITTANCE

6.1.1.1 Transmittance is an important factor in the admittance of energy

through the solar collector cover.

6.1.1.2 Determine solar energy transmittance by the appropriate method described in ASTM E 424. The same method shall be used for the initial

and final measurements. Four measurements shall be made, rotating

the test specimen 90° after each measurement. The average of the four

values shall be considered the solar transmittance of the material.

If Method A is used, a representative sample of the material must be

supplied for transmittance measurements to establish initial properties.

NOTE 2 ; Method A of ASTM E 424 uses a spectrophotometer with an Integrating

sphere, and is especially useful for precise determinations of minor

changes in clear or uniformly diffusing cover materials. Method B of

ASTM E 424 using a pyranometer may be used for nonuniformly diffusing

thick cover materials.

6.1.1.3 These methods provide a means for measuring solar transmittance under

fixed conditions of incidence and viewing and have been found practical for both transparent and translucent materials.

6.1.2 EFFECT OF DIRT RETENTION ON SOLAR TRANSMITTANCE

6.1.2.1 Dirt on a solar collector cover will block the passage of solar energy

and reduce solar energy transmittance.

6.1.2.2 The effect of dirt retention on the solar transmittance of a solar

collector cover material may be determined on a test specimen exposed

to natural weathering (Sec. 6.2.2). Measure solar transmittance as described in Sec. 6.1.1.2. Then using a very soft brush, clean both

sides of the sample with a 0.1 percent solution of nonionic detergent

(Note 3) in distilled water. Rinse with distilled water, blot remaining water and air dry. Care should be taken to avoid scratching or stretching plastic materials. Remeasure the total solar transmittance. The difference in solar transmittance before and after cleaning is an indication of the amount of dirt retained during natural

weathering.

C6
NOTE 3 ; Suitable nonionic detergents Include Alconox, Triton DF 16 and Triton

xioo.J-/

NOTE 4 ; Since dirt retention in natural outdoor weathering is dependent on when the last rainfall took place, it is difficult to compare dirt

retention on materials that have not undergone identical, simultaneous

weathering.

6.1.3 DIMENSIONAL STABILITY

6.1.3.1 Changes in the shape of a solar collector cover may result from exposure to service conditions. A material may sag, shrink or warp.

6.1.3.2 Changes in linear dimensions shall be determined for rigid and

semirigid materials as described in ASTM D 1042, and for nonrigid

materials as described in ASTM D 1204.

6.1.3.3 Determine warpage of rigid and semirigid plastic materials using the

apparatus described in ASTM D 1181, or an equivalent apparatus.

During heat aging the test specimens shall be suspended vertically

from a single point at the center of the top edge. Measure at the

location having the greatest deviation from flatness. Specimens for

this test shall be 15 cm by 15 cm (6 in by 6 in). Specimens must be

cut to size prior to heat aging.

6.1.4 TENSILE PROPERTIES

6.1.4.1 Tensile properties are an indication of the mechanical properties of

a material. During environmental exposure a material may soften or become more brittle. Tensile properties include tensile strength,

percent elongation at yield and at break, and modulus of elasticity.

6.1.4.2 Since tensile properties are influenced by temperature and humidity,

the test specimens shall be carefully conditioned as per Sec. 5.1

and 5.2 prior to each test.

6.1.4.3 Determine tensile properties of rigid and semi-rigid plastic materials

as described in ASTM D 638 using Speed B with a Type IV specimen. For

flexible membrane materials use ASTM D 882.

-'' Alconox is available from Alconox Inc., 217 Park Ave S., New York, N.Y.

Triton DF16 and Triton XlOO are available from Rohm and Haas, Industrial

Chemical Dept., Philadelphia, PA 19105.

C7
6.1.5 IMPACT RESISTANCE

6.1.5.1 Impact resistance is the ability of a material to resist fracture

under shock force.

6.1.5.2 Determine hail impact resistance of solar collector cover materials

using ASTM E XXX, Standard Practice for Determining Resistance of

Cover Plates for Solar Energy Collectors to Hail Impact.

6.1.5.3 Collector covers are also subject to impact by thrown and wind-blown

solid objects. The evaluation of impact resistance to these objects

will require different Standard Practices than that for hail impact

resistance.

6.2 AGING PROCEDURES

6.2.1 HEAT STABILITY

6.2.1.1 The elevated temperatures reached by solar collector covers may cause changes in properties of the material. The design of the solar collector will influence the temperatures attained by the collector

covers. This heat stability test is intended to be an early screening device to discriminate between materials that are probably not

suitable for solar collector covers and those that may have value in

this application.

6.2.1.2 Using an oven and a specimen rack described in ASTM D 794, Sections

3 and 4, expose the test specimens for 500 hours to 150°C (302°F).

6.2.2 NATURAL WEATHERING

6.2.2.1 On a solar collector the cover is simultaneously exposed to elevated

temperature and solar radiation. The combination may have a more

severe effect on materials than either would independently.

6.2.2.2 Expose solar collector cover test specimens for a minimum period of

twelve months using ASTM E XXX, Standard Practice for Exposure of

Cover Materials for Solar Collectors to Natural Weathering under

Conditions Simulating Stagnation Mode.

6.2.3 ACCELERATED WEATHERING

6.2.3.1 Accelerated weathering exposure procedures shall not replace the

Natural Weathering exposure procedure in Sec. 6.2.2.2. They may be

used as a screening test for cover materials. It is noted that where

correlation to natural weathering is or can be established for a can- didate cover material, accelerated testing using simulated weathering

devices may be an effective means for obtaining reliable long-term

durability information in short periods of time.

C8
NOTE 5 ; When two or more materials are being compared, all materials must be

exposed in the same test.

6.2.3.2 Laboratory Procedure

6.2.3.2.1 Expose the test specimens according to Procedure A of ASTM D 2565.

With the Type A or AH apparatus having a 508 mm (20 in) diameter

rack, the exposure periods shall be 700, 1400 and 2800 hours. With

the Type B or BH apparatus or with the Type A or AH apparatus having

a 648 mm (25.5 in) inclined rack, the exposure periods shall be 950,

1900, and 3800 hours. The black panel temperature should be 70° +

2°C (158°F + 4°F) with a relative humidity of 55 + 5 percent. Solar

Solar collector cover materials exposed to moisture in service shall

be subjected to an exposure cycle of 90 minutes of light only followed by 30 minutes of light with water spray.

NOTE 6 ; In order to approximate the equivalence of hours of exposure in ASTM

D 2565 to years of exposure to natural solar irradiance. Tables C2 and

C3 in Appendix XI of this standard are provided.

6.2.3.3 Outdoor Exposure Using Concentrated Natural Solar Radiation

6.2.3.3.1 Outdoor weathering can be accelerated by concentrating natural

sunlight using reflectors.

6.2.3.3.2 Using concentrated natural solar radiation machines such as those

referenced in ANSI Z97. 1-1975, paragraph 4.3.2, expose the test

specimens to concentrated natural solar radiation. The test specimens shall be framed and off -mounted (between 0.5 and 1 cm (1/4 and

1/2 inch) from the target board. The cooling of the test specimens

shall be adjusted to maintain the surface temperature of the test

specimens to within 15°C (27°F) of identical test specimens exposed

to the nonconcentrated irradiance at a standard fixed angle. The

framing or mounting of the relevant test specimens must be identical. The test specimens shall be exposed until they have received

a total incident radiant exposure of 4.0 GJ/m^ + 2 percent, 8.0

GJ/m^ + 2 percent and 16.0 GJ/m^ + 2 percent.-^' Solar collector

cover materials exposed to moisture in service shall be subjected

to water spray for a period of 8 minutes during each 60 minutes of

sunlight exposure, and four eight minute spray cycles per night.

The water spray should be at a rate of 1.2x10"^ z/s'-at- (1.8xlO~2

gal/min«f f^) + 20 percent.

^' These values are based on a solar exposure of 6 months, 12 months, and

24 months, assuming an average annual radiant exposure of 8.0 GJ/m

(9.1 X 10^3 Btu/ft^) + 2 percent.

C9
7

PROCEDURE

7.1 Some of the property measurement tests are to be performed before the

aging procedures, some after, and some both before and after.

7.1.1 Solar transmittance, tensile strength, impact resistance, initial

warpage and the initial dimensions for the linear dimensional stability

test shall be measured before aging to establish the initial material

properties of a solar collector cover material.

7.1.2 Solar transmittance, tensile strength, the effect of dirt retention,

warpage and final dimensions for the linear dimensional stability test

shall be measured after aging to assess the durability of a solar collector cover material.

7.1.3 After natural weathering exposure, specimens for testing shall be taken

from the center of the test specimen.

7.1.4 The impact resistance may be measured after the aging procedures, unless

sample size limitations prevent this determination.

NOTE 7 ; Many materials lose resistance to impact after exposure to aging

procedures. A remeasurement of impact resistance after exposure to

the elements should detect this loss.

7.2 Tests conducted in accordance with this standard practice shall be as summarized in Table CI and in the sequence as shown in Figure CI.

7.3 The solar transmittance, impact resistance, dimensional stability, and the

effect of dirt retention shall be measured on a minimum of three separate

specimens at each exposure increment of each aging procedure indicated in

Table Cl

7.4 At each exposure test increment, tensile test specimens may be cut from a single piece of cover plate material. At least five test specimens shall

be tested for each material in the case of isotropic materials. At least

ten specimens, five normal to and five parallel with the principal axis of

anisitropy, shall be tested for each material in the case of anisotropic

materials.

8. REPORT

8.1 The test report shall include the following:

8.1.1 Any deviation from the conditions described in this standard practice.

8.1.2 Complete identification of the solar collector cover material tested.

8.1.3 Indicate the method used for measuring solar transmittance.

CIO
8.1.4 For the effect of dirt retention, report the original solar energy

transmittance and the solar transmit tance before and after cleaning.

Report the exposure period (dates).

8.1.5 For linear dimensional stability, the test specimen size and the average

percentage change in linear dimensions.

8.1.6 For warpage, the greatest deviation from flatness.

8.1.7 For tensile properties, whether the material is isotropic or anisotropic

and the data required for the report in ASTM D 638 or D 882.

8.1.8 For impact resistance.

8.1.8.1 The type and size of the test specimen.

8.1.8.2 The size, mass and velocity of each ice sphere and the results of each

impact, e.g., breakage, rupture, tear.

8.1.9 For Natural Weathering, the data required for the report in Method EXXX,

Standard Practice for Exposure of Cover Materials for Solar Collectors

to Natural Weathering Under Condiitons Simulating Stagnation Mode.

8.1.10 For the Accelerated Weathering Laboratory Procedure, the type of artificial weathering apparatus, length of time of exposure and whether

water spray was utilized.

8.1.11 For the Outdoor Exposure Procedure using Concentrated Natural Solar

Radiation, the radiant exposure, i.e, total energy incident during the

time of exposure, period (dates) of exposure, and whether water spray

was utilized.

8.1.12 Description of visible changes in test specimens after exposure to

aging procedures.

8.1.13 Number of specimens tested.

8.1.14 Results of initial property measurement values and property measurement

values determined after each aging procedure.

CU
APPENDIX XI

ENERGY BELOW 400 run FOR ARTIFICIAL WEATHERING APPARATUS

WITH XENON ARC LIGHT AND NATURAL SOLAR IRRADIANCE

Table C2

Total Energy Below 400 ran for Artificial Weathering

Apparatus with Xenon Arc Light

Type of Apparatus^

25K Xenon 65K Xenon

A or AH B or BH

Hours o f Exposure^ MJ/m2 MJ/m2

1 0.356 0.261

700 249

950 248

1400 498

1900 496

2800 997

3800 992

S/ The artificial weathering apparatus types are described in

ASTM D 2565.

fe/ These time increments correspond to the exposure times in

Section 6.2.3.2.

The exposure intervals provided in Table CI are predicted on

constant irradiance throughout the test and are based on the

spectral distribution of the light source as listed in the

table. To assure these data when following Procedure A of

D 2565, the 2.5 kW xenon lamp should be operated to provide

controlled irradiance at 340 mm of 0.8 W/m^ and the 6.5 kW

lamp must provide 0.6 W/m^. With an integrating light monitor the intervals of irradiation at 340 mm and 2052 kJ/m^,

4104 kJ/m^ and 8208 kJ/m^. Irradiance, and thus time, may

vary within reasonable tolerances but the total irradiation

incident on the specimen remains constant. If the irradiation

is not monitored and controlled, the stepped wattage alternative may be expected to lower the precision of reproducibility.

C12
Table C3

Average Total Energy Below 400 nm for Natural Solar Irradlance on

a South Facing Tilt Angle in Arizona and Florida^/

Total Energy (MJ/m^) Below 400 nm

45'

Phoenix, Arizona Miami, Florida

1 year

2 years

3 years

4 years

5 years

492

984

1476

1968

2460

Phoenix, Arizona Miami, Florida

340

681

1022

1363

1704

429

859

1288

1718

2148

378

757

1135

1524

1892

•i/ Arizona and Florida data are based on 5-year (1973-1977) averages and assume

the energy below 400 nm (ultraviolet) is 6.1 percent of the total solar

energy. This assumption is based on data from the International Commission

on Illumination (CIE) published in "Recommendations for Integrated Irradlance and Spectral Distribution of Simulated Radiation for Test Purposes".

C13

ft U. S. CXDVERNMENT PRINTING OFFICE : 1980 340-997/325
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS TN 1132

2. Performing Organ. Report No 3. Publication Date

December 1980

4. TITLE AND SUBTITLE

Solar Energy Systems - Standards for Cover Plates for Flat Plate Solar Collectors

5. AUTHOR(S)

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

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

Department of Energy

Office of Solar Applications for Building

Office of the Assistant Secretary, Conservation and Solar Collectors

Washington, DC 20585

10. SUPPLEMENTARY NOTES

(3J Document describes a computer program; SF-I8S, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Laboratory studies were performed to obtain data needed for the development

of standards to evaluate the performance and durability of cover plates for flat

plate solar collectors used in solar heating and cooling systems. Ten cover plate materials were evaluated to assess their durability after exposure to heat

aging, natural weathering and accelerated weathering. Laboratory tests included

measurement of solar energy transmittance, linear dimensional stability, warpage

and the effect of the dirt retention. The temperatures cover plate materials

attain on solar collectors were determined by measurement and by computer

simulations. A procedure was developed for the natural weathering exposure of cover plate materials at elevated temperatures which approximate stagnation

conditions of solar collectors.

The results of the laboratory tests are presented and draft standards for

evaluating cover plate materials for flat plate solar collectors are proposed.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Cover plate durability; cover plate materials; cover plate standards; standards;

weathering of cover plates.

13. AVAILABILITY

[x] Unlimited

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20402.

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

14. NO. OF

PRINTED PAGES

157

15. Price

$5.50

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