Reference

NBS

PUBLICATIONS

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NAT'L INST. OF STAND & TECH

A111DS T7M327

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NBS TECHNICAL NOTE 1196

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

NBS Solar Collector

Durability/Reliability

Test Program:

Final Report
Tm he National Bureau of Standards

1 was established by an act of Congress on March 3, 1901. The

m 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 performed by the National Measurement Laboratory, the National

Engineering Laboratory, the Institute for Computer Sciences and Technology, and the Center for Materials

Science.

The National Measurement Laboratory

Provides the national system of physical and chemical 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, in- dustry, and commerce^ provides advisory and research services to- other

Government agencies; conducts physical and chemical research; develops,

produces, and distributes Standard Reference "Materials; and provides

calibration services. The Laboratory consists of the following centers:

• Basic Standards2 • Radiation Research

• Chemical Physics

• Analytical Chemistry

The National Engineering Laboratory

Provides technology and technical services 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 Engineering2 Manufacturing Engineering

Building Technology

Fire Research

Chemical Engineering2

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

The Center for Materials Science

Conducts research and provides measurements, data, standards, reference

materials, quantitative understanding and other technical information fundamental to the processing, structure, properties and performance of materials; addresses the scientific basis for new advanced materials technologies; plans

research around cross-country scientific themes such as nondestructive

evaluation and phase diagram development; oversees Bureau-wide technical programs in nuclear reactor radiation research and nondestructive evaluation; and broadly disseminates generic technical information resulting from

its programs. The Center consists of the following Divisions:

Inorganic Materials Fracture and Deformation3 Polymers

Metallurgy Reactor Radiation

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address

Gaithersburg, MD 20899.

2 Some divisions within the center are located at Boulder, CO 80303.

3 Located at Boulder, CO, with some elements at Gaithersburg, MD.
KATlOKAt BUREAU OP fftAl*rt»BP» UBRABt NBS Solar Collector a^°

Durability/Reliability Test Program: '%yj( ^t

Final Report /??c/

David Waksman

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

William C. Thomas

Department of Mechanical Engineering

Virginia Polytechnic Institute and State University

Blacksburg, VA 24061

Elmer R. Streed 1

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

1 Now retired.

Prepared for: Office of Solar Heat Technologies

U.S. Department of Energy

Washington, DC 20585

/ v %

00

5753 %*,

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0, 1196 M 55 J ^cAskJratA /1/IC&-

go^l . DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

IONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued September 1984
National Bureau of Standards Technical Note 1196

Natl. Bur. Stand. (U.S.), Tech. Note 1196, 148 pages (Sept. 1984)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1984

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

Efforts in the development of reliability/durability tests for solar collectors and their materials

have been hampered by the lack of real time and accelerated degradation data that can be correlated

with in use conditions. The focus of this report is on research undertaken at the National Bureau

of Standards (NBS) to help generate the data required to develop methods for predicting the long term

durability and reliability of flat-plate solar collectors and their materials.

In this research, eight different types of flat-plate solar collectors were exposed outdoors at four

sites located in different climatic regions. Small scale cover and absorber materials coupon

specimens consisting of samples taken from a collector of each of the eight types used and a number

of additional materials were exposed concurrently with the full-size collectors. Periodic measurements were made of collector and materials performance as a function of outdoor exposure time. In- door laboratory aging tests were conducted concurrently on specimens of the same materials to provide

a basis for comparison with the outdoor exposure tests.

This report presents the results obtained in this test program. Recommendations are made regarding

the use and limitations of performance measurements and environmental exposure tests for assessing

the durability of solar collectors and absorber and cover materials.

Key words: absorber materials; accelerated aging; cover materials; durability; environmental exposure;

solar collectors; solar materials; stagnation testing; thermal performance.

iii
ACKNOWLEDGEMENTS

The work discussed in this publication was funded by the Office of Solar Heat Technologies

of the United States Department of Energy. Outdoor thermal performance testing and exposure

of solar collectors and materials were carried out by DSET Laboratories (DSET), the

Florida Solar Energy Center (FSEC), the Lockheed Research Laboratory (LMSC), and the

National Bureau of Standards (NBS). Indoor solar simulator tests were performed at the

Kent Space Center of the Boeing Company and by Wyle Laboratories. The cover and absorber

optical property data reported were measured by DSET, LMSC, and NBS. Assistance in data

analysis and evaluation was provided by the Department of Mechanical Engineering of

the Virginia Polytechnic Institute and State University (VPI&SU). Optical and scanning

electron microscope studies and PMMA molecular weight measurements were performed at

NBS.

Personnel from the above mentioned organizations who participated in the test program

are listed below. Apologies are expressed by the authors for any omissions.

Test Site Managers

DSET: Richard D. Whitaker

FSEC: James C. Huggins and Ross McCluny

LMSC: Roger K. Wedel

Boeing: Al R. Lunde

Wyle: Wallace R. Youngblood

Collector Exposure and Thermal Performance Testing

DSET: William J. Putnam

FSEC: James C. Huggins and James D. Rowland

LMSC: Ronald Dammann

NBS: John R. Jenkins and Donn F. Ebberts (of NBS),

Ken Shih and Mark Rallas (of Wyle)

Boeing: Al R. Lunde

Wyle: Wallace W. Youngblood

Outdoor Exposure of Materials

DSET: Thomas E. Anderson

FSEC: James C. Huggins and James D. Rowland

LMSC: Ronald Dammann

NBS: James F. Seiler

Cover and Absorber Optical Property Measurements

DSET: Thomas E. Anderson

LMSC: Roger K. Wedel

NBS: Willard E. Roberts and Catharine D. Kelly

Data Analysis and Evaluation

VPI & SU: Aaron G. Dawson, III, Donald S. Culkin,

Daniel E. Douro, and Mark A. Monroe

Optical and Scanning Electron Microscopy

NBS: W. Eric Byrd

PMMA Molecular Weight Measurements

NBS: Brian Dickens and Sharyl J. Santema

IV
SI CONVERSION UNITS

The metric SI system of units is used throughout this report-. This table is included to assist in

converting from SI metric units to common U.S. units which are presently used by the building industry

in the United States.

LENGTH

1 in = 0.0254 meter (exactly)

1 ft = 0.3048 meter (exactly)

AREA

1 in2 = 6.45 x 10"4 meter2

1 ft 2 = 0.09290 meter2

VOLUME

1 in3 = 1.639 x 10~5 meter 3

1 gal (U.S. liquid) = 3.75 x 10-3 meter3

MASS

1 ounce-mass (avoirdupois) = 2.835 x 10-2 kilogram

1 pound-mass (avoirdupois) = 0.4536 kilogram

PRESSURE OR STRESS (Force/Area)

1 inch of mercury (60°F) = 3.377 x 10 3 pascal

1 pound-force/inch2 (psi) = 6.895 x 10 3 pascal

1 pound-force/foot 2 (psf) = 47.88 pascal

ENERGY

1 foot-pound-force (ft-lbf) = 1.356 joule

1 Btu (International Table) = 1.055 x 10 3 joule

POWER

1 Watt = 1 x 10' erg/second

1 Btu/hr = 0.2931 Watt

TEMPERATURE

t°C = 5/9 (t°F - 32)

HEAT

1 Btu/in/hr«ft2 «°F = 1.442 x 10_1 W/m«K (thermal conductivity)

1 Btu/lbm«°F = 4.187 x 10 3 J/kg«K (specific heat)

1 langley = 4.184 x 104 J/m2 = 1 cal/cm2 = 3.69 Btu/ft 2

TABLE OF CONTENTS

Page

ABSTRACT t±±± iii

ACKNOWLEDGMENTS iv

CONVERSION UNITS v

1

INTRODUCTION 1

2. TEST PROGRAM OVERVIEW 2 2.1 Solar Collector Tests 2 2.1.1 Outdoor Exposure Conditions 2 2.1.2 Collector Description 4 2.1.3 Measurements and Observations 4 2.2 Collector Materials Tests 6 2.2.1 Exposure Conditions 6 2.2.2 Test Specimens 6 2.2.3 Measurements and Observations 6

3. RESULTS AND DISCUSSION: MATERIALS EXPOSURE STUDIES 15

3.1 Cover Materials Testing 15

3.1.1 Full-Size Collector Stagnation Testing 15

3.1.2 "Real Time" Outdoor Mini-Box Testing 18

3.1.3 Accelerated Outdoor Testing 27 3.1.4 Temperature Testing 30

3.1.5 Temperature and Humidity Testing 30

3.1.6 Temperature and Xenon Arc Radiation Testing 34

3.1.7 Comparison and Assessment of Test Procedures 39

3.2 Absorber Materials Testing 42

3.2.1 Full-Size Collector Stagnation Testing 42

3.2.2 "Real Time" Coupon Specimen Testing 42

3.2.3 Accelerated Outdoor Testing 48

3.2.4 Temperature Testing 48

3.2.5 Temperature and Humidity Testing 48

3.2.6 Temperature and Xenon Arc Radiation Testing 53

3.2.7 Thermal Cycling 53

3.2.8 Comparison and Assessment of Test Procedures 53 3.3 Additional Collector-Level Observations 56

4. RESULTS AND DISCUSSION: COLLECTOR THERMAL PERFORMANCE AND EXPOSURE STUDIES 57 4.1 Summary of Previously Reported Research Findings of This Program 57 4.1.1 Collector Thermal Performance Test Data Uncertainty 57 4.1.2 Analysis of Measurement and Calculation Procedures for Incident Angle

Modifiers for Flat-Plate Solar Collectors 57 4.1.3 Evaluation of Absorber Stagnation Temperature as an Indicator of

Changes in Solar Collector Materials Performance 58 4.1.4 Comparison of Solar Simulator and Outdoor ASHRAE Standard 93 Thermal

Performance Tests 58 4.2 Outdoor Slope and Intercept Data 59

4.3 Analysis of Slope and Intercept Data 85

4.3.1 Variation Within and Between Test Sites 88

4.3.2 Influence of Experimental Apparatus 38 4.3.3 Environmental Factors 95 4.3.4 Effect on FR(xa) of Linearized Efficiency Curves 104

4.4 Collector Thermal Performance Dependence on Material Properties 104

4.5 Absorber Stagnation Temperature Measurement Results 10°

4.5.1 General Considerations 1°8 4.5.2 Temperature H°

4.5.3 Combined Irradiance and Ambient Temperature HO

4.5.4 Use of Stagnation Temperature for Monitoring Changes in Thermal

Properties HO

4.6 General Observations on Outdoor Test Methods for Measurement of Collector

Performance Degradation 115

vn
TABLE OF CONTENTS (Continued)

Page

5. CONCLUSIONS AND RECOMMENDATIONS 12 o 5.1 Cover Materials 120

5.2 Absorber Materials 121

5.3 Solar Collectors 123

6. REFERENCES 125

APPENDIX A: SOLAR RADIATION EXPOSURE SUMMATIONS 129

Vlll
LIST OF TABLES

Page

Table 2.1.1 Summary Description of Field Test Series for Solar Collectors 3

Table 2.1.2 Test Collector Specimen Description 5

Table 2.2.1 Exposure Tests for Cover Materials 7

Table 2.2.2 Exposure Tests for Absorber Materials 7

Table 2.2.3 Cover Test Materials 8

Table 2.2.4 Absorber Test Materials 9

Table 3.1.1 Transmittance of Full-Size Collector Cover Material After Exposure 19

Table 3.1.2 Effect of "Real Time" Outdoor Exposure on Cover Sample Transmittance ..... 19

Table 3.1.3 Effect of Accelerated Outdoor Tests on Cover Sample Transmittance 29

Table 3.1.4 Effect of Temperature Exposure on Cover Sample Transmittance 29

Table 3.1.5 Effect of Temperature and Humidity on Cover Sample Transmittance

Table 3.2.8 Effect of Temperature on Absorber Coatings

Table 3.2.9 Effect of Temperature and Humidity and Temperature and Xenon Arc Radiation

on Absorber Coatings

Table 4.3.1 Statistical Analysis of Efficiency Curve Intercept and Slope Parameters

Based on Initial and Final Tests, All Collectors and Test Sites,

Series 1 and 2

36

Table 3.1.6 Effect of Temperature and Xenon Arc Radiation on Cover Sample

Transmittance 36

Table 3.1.7 Effect of Environmental Exposure on the Molecular Weight of Poly(methyl

methacrylate) 41

Table 3.2.1 Absorptance of Full-Size Collector Absorber Coatings After Exposure 44

Table 3.2.2 Emittance of Full-Size Collector Absorber Coatings After Exposure 45

Table 3.2.3 Effect of Outdoor Exposure on Absorber Coating Absorptance-Selective

Side 46

Table 3.2.4 Effect of Outdoor Exposure on Absorber Coating Emittance-Selective Side .. 47

Table 3.2.5 Effect of Outdoor Exposure on Absorber Coating Absorptance-Nonselective

Side 49

Table 3.2.6 Effect of Outdoor Exposure on Absorber Coating Emittance-Nonselective

Side 50

Table 3.2.7 Effect of Accelerated Outdoor Exposure on Absorber Coatings •• 51

52

54

Table 3.2.10 Effect of Thermal Cycling on Absorber Coatings 55

Table 4.2.1 Summary of all reported Fr(tci) and F^Ul Values 60

91

Table 4.3.2 Residual Standard Deviations for Collector B, Test Series 1, Linear

Efficiency Curve Correlations

ix
LIST OF TABLES (Continued)

Page

Table 4.3.3 Comparison of Frequency of Extreme Values for F^(xa) Reported by Test Sites 92

Table 4.3.4 Correlations of Pyranometers and Test Stands with Experimental Results

Outside Probable Measurement Uncertainty Ranges, Site 1, All Series

through 120-day Retests 93

Table 4.3.5 Reference Environment Used in Compensation for Environmental Dependence of

Reported Results 96

Table 4.4.1 Base Case Collector Parameters for Calculating Thermal Performance

Dependence on Material Properties 107

Table 4.4.2 Effects of Material Property Changes on Fr(toi) and FrUl for Collector D .. 108

Table 4.4.3 Effects of Material Property Changes on FR

(xa) and FRUL for Collector H .. 109

Table 4.5.1 Hours of Exposure with Conditions Exceeding Corresponding Ambient

Temperature and Irradiance Levels, Site 1, Phoenix, Arizona, July 1979 to June 1980 113

Table 4.5.2 Hours of Exposure with Conditions Exceeding Corresponding Ambient

Temperature and Irradiance Levels, Site 2, Cape Canaveral, Florida,

November 1979 to October 1980 113

Table 4.5.3 Hours of Exposure with Conditions Exceeding Corresponding Ambient

Temperature and Irradiance Levels, Site 3, Palo Alto, California,

July 1979 to June 1980 H4

Table 4.5.4 Hours of Exposure with Conditions Exceeding Corresponding Ambient

Temperature and Irradiance Levels, Site 4, Gaithersburg, Maryland,

April 1979 to March 1980 I 14

Table A.l Exposure Summation for All Collectors 131

Table A.

2

Exposure Summation for Material Samples at Phoenix J-33

Table A.

3

Exposure Summation for Material Samples at Cape Canaveral -*

Table A.

4

Exposure Summation for Material Samples at Palo Alto 13->

Table A.

5

Exposure Summation for Material Samples at Gaithersburg 13°
LIST OF FIGURES

Figure 3.1.2 Spectral transmit tance curves for cover samples from full-size collectors

after 480 exposure days (^-17,000 kJ/sq-m-day)

Figure 3.1.4 Spectral transmittance curves for cover samples exposed to "real time"

and accelerated outdoor exposure on mini-boxes

Figure 3.1.5 MLcrocracking of cover samples after accelerated exposure on mini-boxes

equivalent to » 2 years "real time"

Figure 3.1.6 Blistering of cover material H after 2000 h at 125°C

Figure 3.1.7 Spectral transmittance curves of cover samples after temperature aging ..

Figure 3.1.9 Spectral transmittance curves of cover samples after temperature and

humidity aging

Figure 3.1.10 Spectral transmittance curves of cover samples after temperature and xenon arc radiation aging

Figure 4.2.1 Collector A, normalized Fr(toi) vs retests, series 1 and 2, all test sites ,

Figure 4.2.2 Collector B, normalized Fr(toO vs retests, series 1 and 2, all test sites

Figure 4.2.4 Collector D, normalized Fr(toi) vs retests, series 1 and 2, all test sites ,

Page

10 Figure 2.2.1 Cover exposure mini-box

Figure 2.2.2 Accelerated exposure cover mini-box H

Figure 2.2.3 Accelerated exposure test plate "

Figure 2.2.4 Outdoor absorber exposure box

Figure 2.2.5 Accelerated exposure absorber mini-box

13

14

Figure 3.1.1 MLcrocracking of cover samples from full-size collectors after 480

exposure days (^-17,000 kJ/sq-m-day) *"

20

Figure 3.1.3 MLcrocracking of cover samples after "real time" exposure on mini-boxes

for 480 exposure days (^17,000 kJ/sq-m-day) 2

24

28

31

32

Figure 3.1.8 Surface changes in cover material H after 500, 1000, and 2000 h at 90°C

and 95% RH 35

37

40

Figure 3.2.1 Whitish deposits on absorber material A sample from a full-size collector

after 480 exposure days (^17,000 kJ/sq-m-day) 43

Figure 3.2.2 Corrosion of absorber material D sample from a full-size collector after

480 exposure days (^17,000 kJ/sq-m-day) 43

61

62

Figure 4.2.3 Collector C, normalized Fr(toi) vs retests, series 1 and 2, all test

sites 63

64

Figure 4.2.5 Collector E, normalized Fr(toi) vs retests, series 1 and 2, all test

sites 65

Figure 4.2.6 Collector F, normalized Fr(toi) vs retests, series 1 and 2, all test

sites 66

xi
LIST OF FIGURES (Continued)

Page

Figure 4.2.7 Collector G, normalized Fr(toi) vs retests, series 1 and 2, all test sites 67

Figure 4.2.8 Collector H, normalized Fr(toi) vs retests, series 1 and 2, all test

sites 68

Figure 4.2.9 Collectors A and B, normalized Fr(toi) vs retests, series 3 and 4,

test sites 1 and 2 69

Figure 4.2.10 Collectors C and D, normalized Fr(to() vs retests, series 3 and 4, test

sites 1 and 2 70

Figure 4.2.11 Collectors E and F, normalized Fr(tci) vs retests, series 3 and 4, test

sites 1 and 2 '1

Figure 4.2.12 Collectors G and H, normalized Fr(toc) vs retests, series 3 and 4, test sites 1 and 2 ,

Figure 4.2.13 Collector A, normalized FrUl vs retests, series 1 and 2, all test

sites

Figure 4.2.14 Collector B, normalized FrUl vs retests, series 1 and 2, all test

sites ,

Figure 4.2.15 Collector C, normalized FrUl vs retests, series 1 and 2, all test sites ,

Figure 4.2.16 Collector D, normalized FrUl vs retests, series 1 and 2, all test sites

Figure 4.2.17 Collector E, normalized FrUl vs retests, series 1 and 2, all test

sites ,

Figure 4.2.18 Collector F, normalized FrUl vs retests, series 1 and 2, all test sites ,

Figure 4.2.19 Collector G, normalized FrUl vs retests, series 1 and 2, all test sites

Figure 4.2.20 Collector H, normalized FrUl vs retests, series 1 and 2, all test sites ,

Figure 4.2.21 Collectors A and B, normalized FrUl vs retests, series 3 and 4, test

sites 1 and 2 ,

Figure 4.2.22 Collectors C and D, normalized FrUl vs retests, series 3 and 4, test

sites 1 and 2 ,

72

73

74

75

76

77

78

79

80

81

82

Figure 4.2.23 Collectors E and F, normalized FrUl vs retests, series 3 and 4, test sites 1 and 2 83

Figure 4.2.24 Collectors G and H, normalized FrUl vs retests, series 3 and 4, test sites 1 and 2 84

Figure 4.3.1 Collector B, aggregate plot of measured efficiency, series 1, test site 1, all retests 86

Figure 4.3.2 Collector E, aggregate plot of measured efficiency, series 1, test site 1, all retests 86

Figure 4.3.3 Collector F, aggregate plot of measured efficiency, series 1, test site 1, all retests 87

xii
LIST OF FIGURES (Continued)

Figure 4.3.7 Collector B, efficiency curve slope parameters vs exposure time, all test

sites, series 1

Figure 4.3.9 Comparison of calculated and measured efficiencies for collector B,

site 1, test series 1, all retest data ,

Figure 4.3.11 Values of FrUl for collector B, site 4, test series 1 vs exposure time,

adjusted to standard test conditions

Figure 4.3.12 Reported values of Fr(toi) for collector B, site 4, test series 1 vs exposure time

Figure 4.3.13 Values of Fr(toi) for collector B, site 4, test series 1 vs exposure time,

adjusted to standard test conditions

Figure 4.3.14 Reported values of FrUl for collector B, sites 1, 2, and 3, test series 1 vs exposure time

Figure 4.3.16 Reported values of Fr(tcx) for collector B, sites 1, 2, and 3, test series 1 vs exposure time

Page

Figure 4.3.4 Collector B, efficiency curve intercepts vs exposure time, test site 1,

all series 89

Figure 4.3.5 Collector B, efficiency curve slope parameters vs exposure time, test

site 1, all series 89

Figure 4.3.6 Collector B, efficiency curve intercepts vs exposure time, all test

sites, series 1 90

90

Figure 4.3.8 Typical pyranometer calibrations vs time 94

97

Figure 4.3.10 Reported values of FrUl for collector B, site 4, test series 1 vs exposure time 98

98

99

99

100

Figure 4.3.15 Values of FrUl for collector B, sites 1, 2, and 3, test series 1 vs exposure time, adjusted to standard conditions 100

101

Figure 4.3.17 Values of Fr(toi) for collector B, sites 1, 2, and 3, test series 1 vs exposure time, adjusted to standard conditions 101

Figure 4.3.18 Reported values of FrUl for collector E, sites 1, 2, and 3, test series 1 vs exposure time 102

Figure 4.3.19 Values of FrUl for collector E, sites 1, 2, and 3, test series 1 vs exposure time, adjusted to standard conditions 102

Figure 4.3.20 Reported values of Fr(tcx) for collector E, sites 1, 2, and 3, test

series 1 vs exposure time 103

Figure 4.3.21 Values of F^(xa) for collector E, sites 1, 2, and 3, test series 1 vs exposure time, adjusted to standard conditions 103

Figure 4.3.22 Comparison of measured F^(xa) to values extrapolated from linearized

efficiency curves 105

Figure 4.4.1 Calculated effects on thermal performance of material property changes

for collectors D and H 106

Figure 4.5.1 Typical absorber stagnation temperature profiles of exposed collectors,

site 2, clear day Ill

Xlll
LIST OF FIGURES (Continued)

Page

Figure 4.5.2 Daily absorber stagnation temperature profiles for double-glazed

collectors B and D, site 4, clear day Ill

Figure 4.5.3 Comparison of absorber stagnation temperature profiles using solar

irradiance simulators and outdoor exposure, collector D 112

Figure 4.5.4 Days per year above minimum solar irradiance levels for each of the 4 test sites 112

Figure 4.5.5 Sensitivity of normalized absorber stagnation temperature to changes in

material properties 116

Figure 4.5.6 Effect of environmental test conditions on normalized absorber stagnation

measurements 117

Figure 4.5.7 Sensitivity of the all-day integration method to a 0.10 change in plate

absorptance for collector B 118

xiv
1 . INTRODUCTION

Public Law 93-409, the "Solar Heating and Cooling Demonstration Act of 1974" [1]* and Public

Law 93-473, the "Solar Energy Research, Development and Demonstration Act" [2] authorized a vigorous

Federal program of research, development, and demonstration to help establish solar energy as a viable energy resource for the nation. The primary goal of the program, as stated in the National

Program for Solar Heating and Cooling of Buildings (ERDA 76-6) [3], is to work with industry in the

development and early introduction of economically competitive and environmentally acceptable solar

energy systems to help meet national energy requirements.

The National Bureau of Standards (NBS) and other Federal agencies are cooperating with various

private sector organizations in the development of consensus methods for the installation, performance, and testing of solar equipment. This effort has resulted in the publication of several

procedures that can be used to assess the thermal performance and durability of solar energy systems

and the components and materials used in their construction. The American Society of Heating,

Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standards 95-1981 [4], 94-77 [5], and 93-77

[6] for testing the thermal performance of residential hot water systems, thermal storage devices,

and solar collectors, respectively, were based to a considerable extent on technical data generated

by NBS. Similarly, the American Society for Testing and Materials (ASTM) has developed several

standards concerned with the reliability and durability of solar collectors [7], cover plate

materials [8, 9, 10, 11], absorptive materials [12, 13], rubber hoses and seals [14, 15, 16, 17,

18], metal-heat transfer fluid compatibility [19, 20], and polymeric containment material-heat

transfer fluid compatibility [21].

In 1977, a program was initiated by NBS to help provide an experimental basis for the development of consensus standards for assessing the reliability and durability of solar collectors and their

materials. In this program, eight different types of flat-plate solar collectors and small-scale

cover and absorber materials specimens, representative of those in use at that time, were exposed

outdoors at four sites located in different climatic regions. Periodic measurements were made of

their performance as a function of exposure time. Laboratory aging tests were conducted concurrently

on specimens of the same materials to provide a basis for comparison with the outdoor exposure tests.

This report presents and discusses the results obtained for these outdoor and laboratory aging tests.

Several additional publications resulted from this program. These include:

• an overall program test plan [22, 23]; • an analysis of thermal performance data uncertainty for liquid-heating flat-plate solar

collectors [24, 25]; • a discussion of the determination of incident angle modifiers for flat-plate solar

collectors [26, 27]; • a comparison of outdoor and solar simulator solar collector thermal performance tests [28]; • a discussion of preliminary test program results [29]; • an evaluation of the use of absorber stagnation temperature as a parameter for determining

changes in flat-plate solar collector performance [30, 31]; and

• an analysis of an integrated day-long stagnation temperature technique for detecting changes in

solar collector performance [32].

Key research findings contained in these publications are discussed in section 4.1 of this report.

This work is intended to be complementary to other collector materials test methods development

projects underway at NBS and elsewhere by providing a basis for comparing the results of small-scale

materials level tests with changes taking place in full-size commercially available solar collectors.

Throughout this report, reference is made to days of outdoor exposure. Unless otherwise specified,

the term "days" refers to days with solar radiation levels of 17,000 kJ/m^ (1500 Btu/ft^) or greater

measured in the plane of the test specimen. These 17,000 kJ/m^ days were used for accounting purposes rather than calendar days in an attempt to make the solar radiation exposure at the various

outdoor exposure test sites more comparable.

* Numbers in brackets indicate references given in section 6 of this report,
2. TEST PROGRAM OVERVIEW

The tests and exposure procedures described herein are intended to determine the influence of

environmental exposure parameters that could affect the degradation of solar collectors and their

materials. They are also intended, to the extent possible, to provide a correlation between changes

that occur at the materials and the collector component levels.

A more complete description of the test procedures summarized in this section is given in NBS

Technical Note 1136 [22].

2.1 SOLAR COLLECTOR TESTS

2.1.1 Outdoor Exposure Conditions

The four different collector test series selected for use in the program and the purpose of each test

series are summarized in table 2.1.1.

Series 1 and 2 were intended to evaluate the effects of "normal" stagnation conditions. The

collectors in series 1 were allowed to stagnate dry, whereas the collectors in series 2 were filled,

allowed to stagnate under filled conditions with a pressure relief valve set to the maximum allowable

collector pressure, and subjected to thermal shock/cold fill and thermal shock/water spray tests at specified time intervals. The purpose of exposing filled collectors to stagnation conditions was to evaluate the combined effects of the temperatures and pressures that would occur under stagnation

conditions.

Series 3 was intended to determine whether or not changes in collector performance will occur under

the reduced absorber plate temperatures characteristic of operational conditions. Augmentation

reflectors were used to amplify the solar radiation to which stagnating collectors were exposed in

series 4. This series was intended to determine the effects that such reflectors would have when

they are used in actual systems and to determine whether or not reflectors are a practical way of

accelerating the degradation of stagnating collectors.

These exposure conditions were intended to subject the solar collectors and their materials to different thermal stress levels at each outdoor exposure site. The thermal stress level that would

occur in normal use most likely lies between the series 1 and 2 and the series 3 test conditions,

with some stagnation being a normal occurrence.

The stagnation testing of solar collectors is often considered to be an accelerated test in that it

exposes solar collectors to temperatures that would not occur with the heat transfer fluid flowing.

However, solar collectors may frequently be exposed to stagnation conditions in normal service.

This can occur either when the collectors are initially installed, before system start-up, or when

the system is shut down for maintenance or for seasonal considerations. In commercial buildings,

time periods of up to a year between the installation and start-up of equipment have been

experienced. Thus, only that portion of stagnation exposure time which would not be attributed to normal service can be considered to represent accelerated aging.

Four outdoor exposure test sites were selected which represent both the median and extreme United

States climatological conditions. These test sites can be briefly described as follows:

Climatological Extremes

Site 1. hot, dry

high solar radiation

(high UV radiation would accompany these conditions)

The hot, dry condition can be found in southwestern states (i.e., Arizona, Nevada, and New Mexico).

DSET Laboratories, Inc., located in Phoenix, Arizona was selected as this test site.

Site 2. hot, humid

high solar radiation

(low to moderate UV radiation would accompany these conditions)

The hot, humid condition can be found either in Florida or along the Gulf Coast in the states of

Alabama, Mississippi, Louisiana or Texas. The Florida Solar Energy Center, located in Cape

Canaveral, Florida was selected as this test site.
Table 2.1.1 Summary Description of Field Test Series for Solar Collectors

Collector Conditions

Test Performance for Weathering Purpose of Series Measurement Exposure Test Series**

Series 1 Initial measurement in Each collector preconditioned 1. Observation of effects

"dry stagnation" accordance with ASHRAE for each weathering exposure of dry stagnation

93-77 except delete by purging with dry air to collector performance

3 day pre-exposure and remove the remaining heat and other characterismeasurement of time transfer fluid. Successive tics for various

constant weathering exposure between

performance retests provide

weathering times.

Performance retest cumulative exposures of 3, 2. Provide data for comparafter 3, 15, 30, 60, 15, 30, 60, 120, 240, and ing initial performance

120, 240 and 480 day 480 days.* without 3 day pre-expo- exposures.* sure per ASHRAE 93-77.

Series 2 Initial measurement in Collectors preconditioned for 1. Observation of effects

"no-flow accordance with ASHRAE weathering exposure by filling of no-flow stagnation on stagnation" 93-77 after 3 day pre- per NBSIR 78-1305A [33], collector performance and exposure. Delete capping and allowing to boil other characteristics. measurement of time dry. Weathering exposures

constant. same as in series 1 except 2. Observation of effects

that Thermal Shock Tests per of Thermal Shock Tests Performance retests NBSIR 78-1305A will be per- representing: (a) filling

same as in series 1formed during first 30 day a hot collector with cool exposure on series 2 test heat transfer medium,

collectors only. and (b) rain on a hot collector.

3. Observation of static pressure leakage after

30 and 120 days of exposure

Series 3 Performance of test During weathering exposure, 1. Observation of effects of "controlled collectors measured in heat transfer flow rate main- normal operation on colflow" accordance with ASHRAE tained at 25% of operational lector performance and 93-77 , taking only 3 flow rate for liquid. other characteristics.

points. Delete 3 day pre-exposure and time constant measurement. Performance retests

up to 240 days, same as series 1

Series 4*** Initial measurement Preconditioning and weathering 1. Observation of effects of "dry stagnation same as in series 1

exposures same as in series 1 , dry stagnation on collecwith augmenta- except that a reflector was tor performance and other

tion reflectors" Performance retests used on each collector characteristics with

same as in series 1

during each day of weathering solar radiation amplified

exposure.* Solar radiation by a reflector. measurements required both

with and without reflector. 2. Obtaining temperature

history within collectors

for most severe exposure

conditions.

* Individual days with solar radiation of 17,000 kj/m2, day or greater as measured in the plane of the collector aperture without the influence of a reflector.

** All series include provision of data for comparisons between test series, test sites (climatic

regions), collector designs, etc.

*** This series was terminated prematurely after 60 days* due to a serious lack of uniformity in radiation caused by the augmentation reflectors.
Median Climatological Conditions

Site 3. moderate temperature

dry

high solar radiation

moderate UV radiation

The moderate, dry condition can he found primarily in parts of California. The Lockheed Research

Laboratory, located in Palo Alto, California was selected as this test site.

Site 4. moderate temperature

humid

moderate solar radiation

moderate to low UV radiation

The moderate, humid condition can be found in the Pacific Northwest, Midatlantic and Midsouth regions

of the United States. The NBS test facility, located in Gaithersburg, Maryland served as this test site.

All four test series, each having a sample of the eight collector types described in section 2.1.2, were conducted at sites 1 and 2. Only series 1 and 2 were conducted at sites 3 and 4.

2.1.2 Collector Description

Eight types of liquid-heating flat-plate type solar collectors were selected for use in the test program. The designs chosen were representative of commonly used materials and types of construction.

All collectors from each manufacturer were from the same production lot. The collector cover and

absorber materials, their pertinent optical properties, and average collector areas (gross and

aperture) are listed in table 2.1.2. The absorber material optical property data are based upon

measurements of at least ten samples taken from an actual absorber of each collector type prior to aging. The solar transmittance of the glass materials was obtained using the full cover and a pyranometer (ASTM Standard E 424, Method B [34]). The solar transmittance of nonglass cover materials

and the solar absorptance values were obtained from spectral measurements with an integrating sphere

(ASTM Standard E 424, Method A [34]). Emittance was measured using a portable-type instrument

employing a thermopile and infrared reflectance technique, in accordance with ASTM Standard E 408,

Method A [35]

.

Detailed descriptions of each collector's construction dimensions and pertinent material properties

useful for thermal analytical modeling are listed in Appendix B of NBS Technical Note 1140 [25].

Type T thermocouples were attached to the center of the underside of the collector absorber plates

for the measurement of stagnation temperatures.

Collectors were identified by a coding scheme which will be used in later sections of this report in

conjunction with test data. In this scheme, a letter identifying the collector type (A through H)

is followed by a number identifying the test site (1 to 4), which in turn is followed by another

number identifying the test series (1 to 4). For example, G14 represents collector type G exposed

at test site 1 to the series 4 exposure conditions.

2.1.3 Measurements and Observations

Thermal performance measurements were made on the collectors in each test series in accordance with

the schedule summarized in table 2.1.1. The ASHRAE Standard 93-77 thermal performance test procedure

[6] was used in this program both as a full test (four temperatures, four data points at each temperature) and as a three-point performance retest (three temperatures, four data points at each temperature). The retest was conducted to demonstrate the magnitude of changes in the intercept and slope

of the efficiency curve as a function of environmental exposure time. The measurements were required

to be spread over the range of 0.02 to 0.07 o C*m2/W. The term "performance retest" will be used

throughout this document when the three-point test is specified.

Data collected in addition to collector thermal performance included the following:

• Key environmental parameters; high and low daily ambient temperature, total and diffuse solar

radiation, peak hourly solar radiation, wind velocity, precipitation, and visual sky and weather

conditions, on a daily basis.
Table 2.1.2 Test Collector Specimen Description

Collector

Code

Cover Material Absorber Material Average Areas*

Outer Inner Solar

Transmittance Material

Solar

Absorptance Emittance

Gross

(m2

) Aperture

(m2

|

A Water White

Glass

0.90 Black Nickel 0.87 0.13 2.150 1.831

B Low Iron Glass Low Iron Glass

0.88 (ea) Black Velvet

Paint

0.97 0.96 1.732 1.602

C Plate Glass Thin Film

Heat Trap

0.86 Black Velvet

Paint

0.98 0.92 2.589 1.924

D Etched

Glass

Etched

Glass

0.96 (ea) Black Chrome 0.97 0.07 1.655 1.402

E FRP**

(Type I) 0.85 Lacquer

Primer

0.95 0.87 1.892 1.720

F Water White

Glass

0.90 Copper Oxide 0.96 0.75 1.922 1.769

G FRP**

(Type II) FEP***

Film

0.84 ** 0.96 *** Porcelain

Enamel

0.93 0.86 2.563 2.188

H PET  Film

pgp** *

Film

0.85 ****

0.96 *** SiliconizedPolyester

Paint

0.95 0.89 2.916 2.641

* Average of values reported by the four test sites.

** Glass fiber reinforced plastic

*** Fluorinated (ethylene propylene) copoylmer.

**** Poly(ethylene terephthalate)
• Maximum daily collector absorber plate temperature (for all collectors).

• Daily profiles of absorber plate temperature (for collectors D and H) , ambient temperature,

irradiance, and wind velocity at all four outdoor exposure sites for a period of one year.

• Visual observations of changes in collector appearance during outdoor exposure and of

disassembled collectors following the completion of outdoor exposure and thermal performance

testing.

• Optical property measurements on coupon samples of polymeric cover materials and absorber

materials taken from disassembled exposed and unexposed collectors and microstructural studies

on specimens showing degradation.

2.2 COLLECTOR MATERIALS TESTS

2.2.1 Exposure Conditions

Coupon specimens of cover plate and absorber materials were subjected to several different types of

laboratory and outdoor environmental exposure tests. These specimens consisted of samples taken

from the eight types of full-size collectors used in the test program and of several additional

materials of interest. Changes in the optical properties of these materials were measured as a function of exposure time. Microstructural studies were conducted on materials showing visible

degradation.

Outdoor exposure conditions at the materials coupon specimen level included "real time" exposure in

simulated collectors and exposure to concentrated radiation in machines, described in ASTM E 838

[36]. Indoor laboratory tests conducted included exposure to: (1) temperature, (2) combined temperature and humidity, (3) combined temperature and radiation, and (4) thermal cycling (absorber materials

only). Additional materials level exposure tests were conducted in xenon arc and tungsten lamp solar

simulators.

The outdoor "real time" materials level exposures were conducted concurrently with those on full-scale collectors at the four outdoor exposure test sites discussed in section 2.1.1.

The exposure conditions used for the cover and absorber materials are summarized in tables 2.2.1 and

2.2.2, respectively. The exposure conditions summarized in these tables are intended to simulate a broad range of environmental stress conditions. Primary emphasis was placed on exposure to temperature, solar radiation, and moisture. Other degradation factors such as hail, pollutants, and dust

are localized in nature and were assessed via their occurrence at the "real time" outdoor exposure

test sites participating in the program.

2.2.2 Test Specimens

Materials level tests were conducted on the cover and absorber materials listed in tables 2.2.3 and

2.2.4. Materials specimens having code letters "A" through "H" were cut from solar collectors of

the same type and batch as those exposed outdoors as full-size collectors. Figures 2.2.1 through

2.2.5 depict the apparatus in which test specimens were mounted for outdoor exposure.

2.2.3 Measurements and Observations

Data collected included:

Integrated solar transmittance and absorptance per ASTM E 424, Method A.

Emlttance per ASTM E 408, Method A.

Normal and hemispherical spectral transmittance curves measured on spectrophotometers with

and without integrating spheres.

Visual observation of changes.

Microscopic evaluations using both optical and scanning electron microscopes.

Key environmental parameters, as for solar collector exposure.

Test apparatus exposure temperatures on a daily basis.
Table 2.2.1 Exposure Tests for Cover Materials

Exposure Condition Value or Range Exposure Time

Temperature (indoor) a) 70°C

b) 90°C

c) 125°C

500, 1,000, and 2,000 h

Temperature and Humidity (indoor) a) 70°C and 95% RH

b) 90°C and 95% RH

500, 1,000, and 2,000 h

Temperature and Radiation (indoor) Xenon arc weathering

machine

a) 70°C

b) 90°C

500, 1,000, and 2,000 h

Solar Simulator a) Tungsten

b) Xenon simulators

with irradiance

of ~ 950 W/m2 and ~ 70°C

30, 60, and 120 cycles*

"Real Time" Outdoor 1 sun at ~ 60°C 80, 160, 240, and 480

days**

Accelerated Outdoor ~ 6 suns at ~ 70°C 6, 12, and 24 equivalent

months***

* Each cycle consists of 5 h irradiation and 1 h cooling

** Days having a minimum radiant exposure of 17,000 kj/m2 .

*** One equivalent month equals 6.625 x 10° J/m2 (15,835 Langleys)

Table 2.2.2 Exposure Tests for Absorber Materials

Exposure Condition Value or Range Exposure Time

Temperature (indoor) a) 150°C

b) 175°C

1,000 and 2,000 h

Temperature and Humidity (indoor) 90°C and 95% RH 1,000, and 2,000 h

Thermal Cycling (indoor) -10°C to 175°C 5, 15, and 30 cycles

Temperature and Radiation (indoor) Xenon arc weathering

machine at 90°C

1,000 and 2,000 h

Solar Simulator a) Tungsten

b) Xenon simulators

with irradiance

of ~ 950 W/m2 and ~ 130°C

30, 60, and 120 cycles*

"Real Time" Outdoor 1 sun at ~ 140° C and ~ 160°C

80, 160, 240, and 480 days**

Accelerated Outdoor ~ 6 suns at ~ 150°C 6, 12, and 24 equivalent

months***

* Each cycle consists of 5 h irradiation and 1 h cooling

** Days having a minimum radiant exposure of 17,000 kj/m2 .

*** One equivalent month equals 6.625 x 10** J/m2 (15,835 Langleys).
Table 2.2.3 Cover Test Materials

Code 1 Cover Material

Solar

Transmit tance (Controls)

E FRP4 Type la 0.85

G FRP4 Type II 0.84

H

2 PET5 / FEP6 (outer) / (inner)

0.85/0.96

J Polycarbonate 0.88

K Poly(vinyl fluoride) 0.89

L FRP4 Type lb 0.84

M FRP4 Type III 0.78

N Poly(methyl methacrylate) 0.90

2 Glass 7 / Poly(vinyl fluoride)

(outer) / (inner)

0.86/0.89

1 Code letters E, G, and H indicate materials coupon specimens cut from

solar collectors E, G, and H. Codes, J, K, L, M, N, and tested at the materials level only.

2 Materials exposed as a combination in the cover mini-boxes and in the accelerated exposure cover mini-boxes. Materials exposed individually

in all other tests. Glass and FEP materials were not exposed

individually.

3 These properties are dependent on the formulation and manufacturing

processes used. Other products within a generic class of materials

may have significantly different properties.

* Glass fiber reinforced plastic.

5 Poly(ethylene terephthalate)

.

6 Fluorinated (ethylene propylene) copolymer.

' Ordinary plate glass.
Table 2.2.4 Absorber Test Materials

Code 1

Absorber Material Optical Properties 2

Coating Substrate

Solar

Absorptance Emittance 3

A Black Nickel Steel 0.87 0.13

C Black Velvet Paint Copper 0.98 0.92

D Black Chrome Steel (nickel

flashed)

0.97 0.07

E Black Lacquer Primer Copper 0.95 0.87

F Copper Oxide Copper 0.96 0.75

G Black Porcelain

Enamel

Steel 0.93 0.86

H Black Siliconized

Polyester Paint

Aluminum .-. 0.95 0.89

I Black Chrome Stainless Steel 0.88 0.19

J Black Chrome Aluminum 0.98 0.14

L Lead Oxide Copper 0.99 0.29

M Oxide Anodized Aluminum 0.94 0.10

N Oxide Conversion

Coating

Aluminum 0.93 0.51

P Black Chrome Copper (nickel

flashed)

0.96 0.08

1 Code letters A through H indicate materials coupon specimens cut from solar

collectors A through H. Codes I through P tested at the materials level only.

2 These properties are dependent on the formulations and manufacturing processes

used. Other products within a generic class of materials may have significantly

different properties.

3 Average values based on a minimum of ten test specimens.
RAW SHIELD

OUTER COVER

TEST SPECIMEN

SPACER FRAME

MNER COVER

TEST SPECIMEN

SILICONE RUBBER

SEALANT

STAINLESS STEEL

PAN

BSORBER PLATE

E T THERMOCOUPLE

GLASS FIBER INSULATION

Notes:

Stainless Steel Pan: 22 x 12 x 10 cm without rim

Glass Fiber Insulation: 64 kg/iiP density

Bottom thickness 10 cm Edge thickness 2.5 cm wide x 2.5 cm thick

Baked out at 230°C for 24 hours

Absorber Plate: Black chrome on copper

Silicone Rubber Sealant: Between covers and pan, and covers and spacer

Cover Test Specimens: 26 x 16 cm

Rain Shield: 16 ga stainless steel, clamped to pan

Spacer Frame: 6 mm thick aluminum

Figure 2.2.1 Cover exposure mini-box

10
COVER TEST SPECIMEN

SILICONE RUBBER SEAL

ALUMINUM PAN

TYPE T THERMOCOUPLE

ALUMINUM FOIL ABSORBER

Notes:

Cover Size: 7.6 x 5.1 cm Aluminum Pan: 11.5 x 6.5 x 2 cm

Aluminum foil painted black and baked at 230°C for 24 hours

Figure 2.2.2 Accelerated exposure cover mini-box

11
ALUMINUM PLATE

310><140mm (painted black

& baked @ 230°C for

24 hrs.)

COVER TEST SPECIMEN

51 x 38mm

ii i t

CERAMIC INSULATOR

12.7mm long

TYPE T THERMOCOUPLE

Figure 2.2.3 Accelerated exposure test plate

12
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FILM COVER ABSORBER TEST SPECIMEN

ALUMINUM PAN

TIEDOWN WIRE

TYPE T THERMOCOUPLE

Notes:

Film Cover: Polytetraf luoroethylene

Absorber Test Specimen: 7.5 x 5.1 cm Aluminum Pan: 11.5 x 6.5 x 2 cm Interior painted black and baked at 230°C for 24 hours

Figure 2.2.5 Accelerated exposure absorber mini^box

14
3. RESULTS AND DISCUSSION: MATERIALS EXPOSURE STUDIES

The sections which follow present results obtained through the outdoor and indoor laboratory testing

of solar collector cover and absorber materials. Additional materials-related observations made

during the disassembly of full-size solar collectors following outdoor exposure testing are also

presented. The properties of materials discussed in this section are dependent on the formulations

and manufacturing processes used. Other products within a generic class of materials may have

significantly different properties.

3.1 COVER MATERIALS TESTING

In this section, the results of outdoor and indoor laboratory exposure testing of cover materials are presented and the extent to which accelerated outdoor and indoor laboratory tests simulate "real

time" outdoor exposure is discussed. Materials used for these comparisons included: poly(ethylene

terephthalate) - material code H, polycarbonate - material J, poly(vinyl fluoride) - material K,

poly(methyl methacrylate) - material N, and several types of glass fiber reinforced plastics - materials

E, G, L, and M. Materials E and L were essentially the same, with the exception of specimen thickness. Fluorinated (ethylene propylene) copolymer film was also investigated as an inner glazing,

however, it is not discussed in detail in this section since there were no obvious changes observed

in any of the exposure tests performed. The same is true for glass materials used as an outer glazing.

In addition to the small-scale tests which were performed on all materials, samples of cover materials

E, G, and H were cut from full-size solar collectors having the same code letter designations and

evaluated following the completion of collector exposure testing as described in section 2 of this

publication. The types of small-scale outdoor and indoor laboratory tests performed on cover materials

are also summarized in section 2. The reader should note that visual observations of yellowing in

materials E and L are somewhat biased towards the low side since these materials contained a blue

additive which could conceal a considerable amount of yellowing on visual inspection.

Both hemispherical and normal spectral transmittance measurements were made in the UV-visible range

using spectrophotometers with and without integrating spheres, respectively . The integrating sphere

data are representative of the amount of radiation that would reach a flat-plate solar collector

absorber surface. The normal measurements made without an integrating sphere are a more sensitive

indicator of changes in scattering, especially in materials which do not contain reinforcing fibers.

Integrated solar transmittance values given in the tables contained in this section were calculated

in accordance with ASTM E 424, method A using the air mass 2 solar spectrum.

3.1.1 Full-Size Collector Stagnation Testing

Visual Inspection : Cover material E showed a slight amount of yellowing after "real time"

exposure for 480 exposure days at all four outdoor exposure sites. There was also evidence of minor

surface dulling at the Cape Canaveral and Palo Alto sites. A moderate amount of yellowing was observed for material G after 480 days at all four sites. In addition, the exposed surfaces of this

glazing became quite dull and readily absorbed liquids such as ink and there was some resin erosion

which exposed glass fibers. Cover material H showed a considerable amount of top surface dulling

after "real time" exposure for 480 days at all four outdoor exposure sites. The material also became

quite brittle as a result of this exposure and snapped readily when it was bent so that the exposed

surface was in tension. It was much more resistant to breaking when the bottom (unexposed) surface

was in tension on flexing. The covers on several of the H collectors tore from the stresses imposed

when the collectors were moved from exposure racks to test stands for periodic thermal performance

measurements and had to be patched. In addition, the covers of all of the H collectors on exposure

at the Phoenix test site were punctured in several places when exposed to impact from 1.2 cm maximum

diameter hailstones. Testing previously conducted at NBS on unaged material indicated that this

unaged material was capable of withstanding impact by 2.5 cm hailstones without puncture.

Microscopic Examination : Examination of the exposed surfaces of materials G and H with optical and

scanning electron microscopes revealed that the surface dulling was primarily due to the formation

of micro-cracks (see figure 3.1.1). The microcracking was more extensive at the Cape Canaveral and

Palo Alto sites than at the Phoenix and Gaithersburg sites for both materials. There was evidence

of directional stresses in the microcrack patterns observed for material H. A minor amount of

surface pitting was observed optically on those material E specimens which showed surface dulling.

The same type of microcracking observed in this program for material H was also observed by the

authors in samples of the same type of material taken from solar collectors that were installed on a building in Colorado for about 5 years.

15
, 50 /u m |

(a) Material H - Phoenix (b) Material H - Cape Canaveral

(c) Material H - Palo Alto (d) Material H - Gaithersburg

Figure 3.1.1 Microcracking of cover samples from full-size collectors after

480 exposure days (5-17,000 kj/sq-m-day)

16
(e) Material G - Cape Canaveral

(f) Material G - Gaithersburg

Figure 3.1.1 continued

17
Optical Measurements : Integrated solar transmlttance values are given in table 3.1.1 for cover samples cut from full-size collectors E, G, and H following exposure for 480 days at the four test

sites. Material E had minor changes that are probably due to variations in sample homogeneity.

Material G showed a substantial amount of change, with the greatest changes occurring at the Cape

Canaveral site and the least at the Phoenix site. Transmittance curves for samples of materials G and H cut from the series 2 collectors at all four sites are shown in figure 3.1.2. The changes

shown in the curves for material G are most likely due to a combination of yellowing (evidenced by

the shift in the absorption edge* of the curve to longer wavelengths) and the extent of microcracking, which was greater at the Cape Canaveral site than at the Phoenix and Gaithersburg sites.

Material H showed the greatest changes at the Cape Canaveral and Palo Alto sites. Examination of

the transmittance curves for samples of material H taken from series 2 collectors exposed at all four

sites and scanning electron microscope photos (see figure 3.1.1) shows that these changes were most

likely due to the extent of microcracking, which was greatest at the Cape Canaveral and Palo Alto

sites

.

3.1.2 "Real Time" Outdoor Mini-Box Testing

Visual Inspection : Outdoor "real time" exposure of materials E and L on cover mini-boxes caused

slight yellowing after 480 days at the Cape Canaveral and Palo Alto sites with no visual signs of

degradation after 480 days at the Phoenix or Gaithersburg sites or at shorter exposure times at any

of the sites. A slight to moderate amount of yellowing and exposed surface dulling was observed for

material G after 480 days at all four sites with the most severe changes at the Cape Canaveral and

Palo Alto sites. Exposure for 160 and 240 days caused slight yellowing at the Cape Canaveral,

Phoenix, and Palo Alto sites, but no visual evidence of surface dulling. A slight increase in

yellowing and a minor loss in surface gloss was also observed for material M after 480 days.

Exposure of material H caused top surface dulling and embrittlement after 480 days at all four sites,

but not after shorter exposure times. In addition, the same hail damage noted in the previous section for full-size collectors also occurred for the material H mini-box specimens at Phoenix. Very

slight yellowing was observed in material J after 80 days at all four sites which progressed to slight to moderate after 480 days. In addition, surface dulling was observed after 240 and 480 days at all four sites. Exposure at the Palo Alto and Cape Canaveral sites caused noticeably more severe

changes. With the exception of embrittlement after 480 days at the Phoenix site, there were no other visible signs of degradation of material K. Outdoor "real time" exposure of material N caused

a minor amount of yellowing and a noticeable increase in embrittlement on breaking in flexure after

480 days at all sites. This yellowing could only be observed in material N by viewing the edge of

the test specimens.

Microscopic Examination : Examination of the exposed surfaces of materials G, H, and J with optical

and scanning electron microscopes revealed that the surface dulling observed visually, without magnification, was primarily due to the formation of microcracks (see figure 3.1.3). In general, the

microcracking was more extensive at the Cape Canaveral and Palo Alto sites than at the other sites

and had more open crack structure. In addition to the microcracking, the samples of material H exposed at the Palo Alto site and of material J at the Palo Alto and Cape Canaveral sites had evidence of surface etching. The microcracks observed for material J were not as open and extensive

as those in materials G and H.

Optical Measurements : Integrated solar transmittance values are given in table 3.1.2 for cover samples exposed on collector mini-boxes for up to 480 days at all four outdoor exposure sites. With

the exception of material G at all of the outdoor sites, material H at Cape Canaveral, and material

M at the Gaithersburg site, the changes measured for integrated transmittance were not substantial.

Examination of the spectral transmittance curves for materials G, H, J, K, and N revealed the changes

shown in figure 3.1.4. The hemispherical transmittance curves for material G at both the Cape

Canaveral and Phoenix sites were virtually identical for up to 240 days. After 480 days of exposure,

the transmittance curves dropped substantially with the greatest change occurring at the Cape Canaveral

site. This most likely is related to the extend of microcracking occurring with these two materials.

Examination of the curves obtained for material H using an integrating sphere showed no changes

after 480 days at Phoenix or for up to 240 days at Cape Canaveral. After 480 days of exposure at Cape Canaveral, there was a drop in the transmittance curve for material H. Once again, this is

most likely related to the size and extent of microcracking. There also was a slight shift in the

* The term "absorption edge" as used in this report refers to the long wavelength side of the intense

absorption band typically occurring in the 300 to 400 nm wavelength region in the spectral

transmission curves shown in this report.

18
Table 3.1.1. Transm i ttance of Full-Size Collector Cover Material after Exposure.

Site

Exposure

Collector Series Control Days** Phoenix Cape Canaveral Palo Alto Gaithersburg

E 1 0.85

2

3

it

G* 1 0.84

2

3

4

H* 1 0.85

2

480 0.78 0.81 0.81 0.80

480 0.80 0.78 0.82 0.81

240 0.83 0.77 -- --

60 0.79 0.82 -- --

480 0.71 0.76 0.77 0.75

480 0.67 0.54 0.63 0.80

240 0.60 0.67 -- --

60 0.74 0.66 -- --

480 0.85 0.79 0.80 0.84

480 0.86 0.81 0.80 0.84

* Outer Cover

** Days with a Minimum Solar Radiation Level of 17.000 kJ/sq m

Table 3.1.2. Effect of 'Real Time' Outdoor Exposure on Cover Sample Transm i ttance

Phoenix Cape Canaveral Palo Alto Gaithersburg

Days Exposure* Days Exposure Days Exposure Days Exposure

Sample Control 80 160 240 480 80 160 240 480 80 160 240 480 80 160 240 480

0.83 0.83 0.80 0.84 0.84 0.81 0.81 0.83 0.85 0.82 0.80 0.77 0.82 0.81 0.80 0.81

0.80 0.80 0.83 0.68 0.81 0.79 0.79 0.63 0.81 0.80 0.80 0.73 0.82 0.80 0.80 0.75

0.85 0.85 0.85 0.85 0.84 0.85 0.85 0.79 0.84 0.85 0.83 0.84 0.84 0.86 0.87 0.85

0.87 0.86 0.86 0.85 0.85 0.85 0.84 0.84 0.84 0.85 0.83 0.83 0.86 0.86 0.85 0.84

0.89 0.90 0.90 0.90 0.90 0.90 0.84 0.90 0.90 0.89 0.91 0.89 0.90 0.90 0.90 0.90

0.83 0.83 0.83 0.84 0.84 0.84 0.85 0.85 0.84 0.84 0.84 0.82 0.80

0.80 0.74 0.78 0.78 0.78 0.77 0.73 0.75 0.80 0.81 0.74 0.80 0.74 0.75 0.76 0.70

0.89 0.88 0.89 0.89 0.88 0.88 0.88 0.90 0.89 0.89 0.88 0.89 0.89 0.89 0.89 0.88

0.89 -- -- 0.89 -- -- -- 0.89 — — — -

-

* 17000 kJ/sq-m (1500 Btu/sq-ft) Minimum Days

19

E 0.85

G 0.84

H 0.85

J 0.88

K 0.89

L 0.84

M 0.78

N 0.90

0.89
300 500 700 900 1100 1300 1500

WAVELENGTH (nm)

u

<

£ 60

E

I 50 <

« 40

52 30 -

20 -

10

MATERIAL H

(with integrating sphere)

Control Phoenix

Gaithersburg

Cape Canaveral

Palo Alto

J I I L J I L 300 600 700 900 1100 1300 1500

WAVELENGTH (nm)

Figure 3.1.2 Spectral transmittal curves for cover samples from full-size collectors after 480 exposure days (^17,000 kJ/sq-m-day)

20
(a) Material H - Phoenix (b) Material H '- Cape Canaveral

(c) Material H - Palo Alto (d) Material H - Gaithersburg

Figure 3.1.3 Microcracking of cover samples after "real time" exposure on mini-boxes for 480 exposure days (>17,000 kJ[/sq-m-day)

21
(e) Material J - Phoenix (f) Material J - Cape Canaveral

(g) Material J - Palo Alto (h) Material J - Gaithersburg

Figure 3.1.3 continued

22
(i) Material G - Cape Canaveral

Figure 3.1.3 continued

23
100 MATERIAL G (a)

(with integrating sphere)

90

80 / /

^^^ ^\^\ /•

|5

UJU

z

70

/ 1 -

v J

v./"' <

1- 60

' i /"

* '' —

.•' E / / 00 / / Z / s^ . s "". ec 50 ~~

(i

' ^ '

's/~~' V /

—

i

< Control o

E

40 ~

V' Phoenix & Cape Canaveral

Ltl X 'l (80. 160. & 240 days)

M 30 ;/ Accelerated aging (=2 yr) EUJ -

J Phoenix (480 days) 20 Cape Canaveral (480 days)

10 Ji i i

1 1 I 1 1 I III

300 500 700 900 1100 1300 1500

WAVELENGTH (nm)

MATERIAL H

(with integrating sphere)

Control Phoenix (80. 160, 240. & 480 days)

Cape Canaveral (80. 160,

& 240 days)

Cape Canaveral (480 days) Accelerated aging (=2 yr)

300 400 500 600

WAVELENGTH (nm)

700

100

90

80

E 70

UJU

< 60 -

i-E

g 50 <

ec

t-

=! 40 h

Eec1 30

20

10 -

340

- (c)

- 1 MATERIAL H - PHOENIX — y

(without integrating sphere)

Control — 80 days

160 days - 1 240 days

- j

480 days

J i i i i

100

90

80

70

60

50

40

30

20

10

(d)

MATERIAL H - CAPE CANAVERAL

(without integrating sphere)

400 460 520

WAVELENGTH (nm)

580 340 400 460 520

WAVELENGTH (nm)

580

Figure 3.1.4 Spectral transmittance curves for cover samples exposed to "real time" and

accelerated outdoor exposure on mini-boxes

24
100

90

80

ft*-* B 70 z

<

| 60 -

V)zk 50

_l<

a 40

u

x

| 30

UJx

20

10

300

(e)

J_

MATERIAL J - PHOENIX

(with integrating sphere)

Control 80 days

160 days

240 days

480 days

Accelerated aging

(=2 yr)

400 500 600

WAVELENGTH (nm)

700

100 r90

80

g 70

UJu

< 60

i-| 50 <az*Z 40 <

E

i 30

20

10

(f)

MATERIAL J - PHOENIX

(without integrating sphere)

Control 80 days

160 days

240 days

480 days

340 400 460 520

WAVELENGTH (nm)

580 640

100 r

90

80

H 70

uiu

< 60

| 50 <

BE<

E

g 30 z

20

10

(9)

MATERIAL J - CAPE CANAVERAL

(without integrating sphere)

-Control 80 days

160 days

240 days

480 days

340 400 460 520

WAVELENGTH (nm)

580 640

100

90

80

70

g

UIuz

< 60

t

E00 50 z<

cc1- 40 <o

BEUJX 30

0.

V) 20

10

(h)

—'7

MATERIAL K

(with integrating sphere)

Accelerated aging (=2 yr) Control & Cape Canaveral

(80. 160. 240. & 480 days)

& Phoenix (80. 160. & 240

days) Phoenix (480 days)

300 400 500 600

WAVELENGTH (nm)

700

Figure 3.1.4 Continued

25
100

90

80

LUU 70 z

<

tt- 60 z

COz

<

50

h-

_i<

u 40

| 30

LUz

20

10

(i)

MATERIAL N

(with integrating sphere) —Control —Phoenix (80. 160. 240,

& 480 days) & Cape

Canaveral (80, 160, 240,

& 480 days) & Accelerated

aging (=2 yr)

_L 300 400 500 600

WAVELENGTH (nm)

700

Figure 3.1.4 Continued

26
absorption edge towards shorter wavelengths which may be due to loss of UV inhibitor. Normal spectral

transmittance measurements showed systematic changes in the transmittance of material H at both the

Phoenix and Cape Canaveral sites with the greatest changes occurring at the Cape Canaveral site

where the microcracking was more extensive. Hemispherical spectral transmittance curves measured

for samples of material J exposed at Phoenix and Cape Canaveral showed identical shifts in the

absorption edge to longer wavelengths as a function of exposure time (see figure 3.1.4e.). The

similarity of these two curves is probably due to the fact that the microcracking in material J was

not as open or extensive as that observed with materials G and H. Normal spectral transmittance

curves showed greater changes for material J as a function of exposure time at Cape Canaveral than

at Phoenix. With the exception of a slight shift in the absorption edge of material K to shorter

wavelengths after 480 days of exposure at the Phoenix site, outdoor "real time" exposure of material

K at the Phoenix and Cape Canaveral sites did not cause significant changes in the hemispherical

transmittance. Measurements made without an integrating sphere on specimens from the two sites

showed small decreases in transmittance as a function of exposure time and the same slight shift in

the absorption edge towards shorter wavelengths for the 480 day Phoenix specimen. The significance

of this shift in absorption edge will be discussed in section 3.1.3. A shift in the absorption

edge of material N towards longer wavelengths was observed with spectral measurements made both with

and without integrating spheres. In addition, a slight increase in scattering as a function of

exposure time in Cape Canaveral was detected using the spectrophotometer without an integrating

sphere. The shift in the absorption edge of material N to longer wavelengths is most likely due to photodegradation of the UV inhibitor as reported by Newland and Tamblyn [37],

3.1.3 Accelerated Outdoor Testing

Only the results obtained for cover samples mounted in mini-boxes will be discussed here because of

peeling problems experienced with the absorber paint used in the board mount configuration.

Visual Inspection : Accelerated outdoor exposure of material E for a period equivalent to 2 years

"real time" gave rise to slight yellowing. Similar exposure of material G caused slight to moderate

yellowing and evidence of exposed surface dulling. Material M had a slight increase in yellowing and

a minor loss in surface gloss when exposed to these conditions. Accelerated outdoor exposure equivalent to 2 years "real time" caused noticeable top surface dulling and embrittlement of material H

and a slight amount of yellowing and loss in gloss of the exposed surface in material J. Exposure

under these conditions also caused a very slight amount of yellowing in material N which could only

be observed by viewing the edge of the specimen and did not cause any visible changes in material K.

Microscopic Examination : Examination of the exposed surfaces of materials G, H, and J revealed the

presence of microcracking (see figure 3.1.5). The microcracks observed with accelerated outdoor

aging generally appeared to be finer and less open than those observed with outdoor "real time"

aging.

Optical Measurements : Integrated solar transmittance values are given in table 3.1.3 for cover

samples exposed outdoors on an accelerated weathering machine for a period of time equivalent to 2 years "real time." Materials G, H, and M were the only materials which showed substantive changes in

their transmittance in this time period. Examination of hemispherical spectral transmittance curves measured on materials G, H, J, K, and N revealed a number of changes resulting from this accelerated

exposure (see figures 3.1.4a, b, e, h, and i) . The transmittance curve for material G was somewhat

higher than those measured on "real time" samples exposed at Phoenix and Cape Canaveral for 480 days;

however, it was lower than the curves of the 240 day samples. This is probably due to the less

developed microcrack pattern that occurred on the samples of material G exposed to accelerated aging.

The curve for material H was very similar to that obtained for the 480 day Cape Canaveral sample.

Both of these samples had a well developed microcrack structure. The slight shift of the absorption

edge towards shorter wavelengths that occurred with "real time" exposure of material H also occurred

with the accelerated aging sample. As a result of the accelerated aging, the absorption edge of

material K showed a large shift towards shorter wavelengths. This was probably due to the loss of UV

inhibitor which permitted the material to transmit further into the ultraviolet.

The same shift of the absorption edge of material N towards longer wavelengths observed with "real

time" exposure also occurred with accelerated exposure. As was previously mentioned, this is

probably due to photodegradation of the UV inhibitor into products which absorb at longer

wavelengths

27
(a) Material H - Sample 1 (b) Material H - Sample 2

«i9 3X£3

flf9S.'-'

:'

wU Li Hi

(c) Material G (d) Material J

Figure 3.1.5 Microcracking of cover samples after accelerated exposure on mini-boxes equivalent to » 2 years "real time"

28
Table 3.1.3. Effect of Accelerated Outdoor Tests on Cover

Sample Transmi ttance

Sample l,Mi ni-box Sampl e 2, Mi ni-box

Days Exposure* Days Exposure

Sample Control 30 60 120 30 60 120

E 0.83 0.83 0.84 0.63 0.63 0.83 0.83

G 0.84 0.83 0.63 0.78 0.81 0.82 0.74

H 0.85 0.8t4 0.85 0.81 0.84 0.85 0.81

J 0.87 0.86 0.66 0.84 0.84 0.66 0.85

K 0.90 0.93 0.92 0.92 0.92 0.92 0.91

L — — -- — — — —

M 0.82 0.77 0.79 0.76 0.81 0.79 0.80

N 0.91 0.89 0.89 0.89 0.89 0.69 0.89

* Equivalent Days; 5 Equivalent Days = 1 Month Real Time

= 6.625E+5 kj/sq m

Table 3.1.4. Effect of Temperature Exposure on Cover Sample Transm i ttance

Temperature: 70 C 90 C 125 C Hours Exposi re Hours Exposure Hours Exposure

Sample Control 500 1000 2000 500 1000 2000 500 1000 2000

E 0.85 0.83 0.85 0.81 0.84 0.84 0.82 0.82 0.81 0.79

G 0.84 0.82 0.80 0.79 0.73 0.73 0.78 0. 70 0.66 0.64

H 0.85 0.84 0.85 0.84 0.84 0.84 0.85 0.85 0.85 0.84

J 0.88 0.87 0.87 0.87 0.87 0.87 0.87 0.87 0.87 0.86

K 0.89 0.91 0.91 0.91 0.90 0.91 0.91 0.89 0.87 0.85

L 0.84 0.83 0.81 0.83 0.83 0.80 0.83 0.81 0.80 0.80

M 0.78 0.81 0.79 0.80 0.75 0.64 0.66 0.68 0.63 0.55

N 0.90 0.89 0.90 0.90 0.90 0.90 0.89 0.89 0.90 0.91

29
3.1.4 Temperature Testing

Visual Inspection: Laboratory exposure of materials E and L to temperature caused a slight amount of

yellowing after 2000 h at 70°C and after 500, 1000, and 2000 h at 90°C. At 125°C, slight yellowing

was observed after 500 h which became progressively deeper after 1000 h and 2000 h of exposure.

Exposure to a temperature of 70°C caused very slight yellowing of material G after 1000 h which

became slightly deeper at 2000 h. A moderate amount of yellowing occurred in this material at 90°C

after 500, 1000, and 2000 h of exposure. At 125°C, this material had very severe yellowing after

500 h which became progressively worse at 1000 and 2000 h. Laboratory exposure of material M to a temperature of 70°C caused a slight increase in yellowing after 500 h that increased slightly after

2000 h of exposure. A slight to moderate amount of yellowing occurred at 90° C after 500 h, which

deepened to moderate at 2000 h. At 125°C, material M had severe yellowing after 500 h which became

progressively worse at 1000 h and 2000 h. The top surface also exhibited a wrinkled pattern.

Laboratory exposure of material H to temperatures of 70°C, 90°C, and 125°C did not cause any visible

changes with the exception of possible very slight yellowing after 2000 h at 125°C. Material J showed no visible changes after exposure for up to 2000 h at 70°C and 90°C and 125°C. Laboratory

exposure of material K to temperatures of 70°C and 90°C did not cause any visible changes in 2000 h. At 125°C, there was moderate yellowing after 500 h which became progressively more severe with exposures up to 2000 h; however, the film still remained flexible. Similarly, laboratory exposure of

material N for up to 2000 h at 70°C and 90°C did not cause any visible changes. After 1000 h at 125°C, there was noticeable sagging and warping and a minor amount of yellowing that could only be seen by looking at the edge of the specimen.

Microscopic Examination : Examination of the surface of a specimen of material H, heated for

2000 h at 125°C, with a scanning electron microscope revealed a considerable number of small surface

blisters (see figure 3.1.6). Changes were not observed for any of the other cover materials exposed

to temperature in this program.

Optical Measurements : Integrated solar transmittance values are given in table 3.1.4 for cover samples exposed for up to 2000 h at temperatures of 70°C, 90°C, and 125°C. Material G was the only

material to show a change at 70°C. At 90°C, materials G and M had substantial changes. At 125°C,

materials G and M had greater changes than those observed at lower temperatures and materials E, K,

and L showed slight changes. Spectral transmittance curves are shown for materials G, H, J, and K as

a function of exposure temperature and exposure time in figure 3.1.7. The absorption edge of

material G, measured on a spectrophotometer with an integrating sphere, shifts to longer wavelengths

as temperature and exposure time increase. This accounts for the changes in yellowing observed

visually. The absorption edge of material H had a substantial shift to shorter wavelengths at 125°C.

As was previously mentioned, this shift is most likely due to loss of UV inhibitor. Material J shows a slight rounding off of the top of the absorption edge as a function of exposure temperature

and time. This is probably indicative of the onset of degradation. Measurements made on material K

using an integrating sphere show an initial shift in the absorption edge to shorter wavelengths

followed by a gradual shift back towards longer wavelengths. Measurements made on material K on a spectrophotometer without an integrating sphere show that this shift back towards longer wavelengths,

following the loss of the UV inhibitor at temperatures as low as 70°C may be at least partially due

to either an increase in scattering or film shrinkage. The spectral curves measured for material N using an integrating sphere did not show any signs of change after 2000 h at 125°C.

3.1.5 Temperature and Humidity Testing

Visual Inspection : Exposure to combined moisture and temperature caused moderate whitening of

materials E and L at 70°C and a combination of severe whitening, blister formation, and yellowing at 90°C which became progressively worse with exposure time (500 h to 2000 h) . Exposure of material G to combined moisture and temperature caused severe whitening after 500 h at 70°C which became more

severe at longer times. At 90°C with moisture, there was a combination of very severe whitening and

yellowing which progressed from moderate to very severe as the exposure time increased from 500 h to 2000 h. The surfaces remained glossy but changed in appearance from the unexposed specimens. Exposure of material M to combined moisture and temperature caused severe whitening after 500 h at 70°C

and evidence of large blisters on the bottom surface. The top surface became very sticky after this

exposure. At 90°C with moisture, there was a combination of severe whitening and yellowing which

progressed from moderate after 500 h to a deep tan after 2000 h. There was also a noticeable change

in top surface texture and evidence of large blisters on the bottom surface. Exposure of material H

to combined moisture and temperature caused a slight amount of yellowing after 1000 at 70°C; however,

the material remained flexible. At 90°C with moisture, there was slight yellowing and material H became brittle after 500 h of exposure; after 1000 h, there was moderate yellowing and some clouding

of the material in addition to the embrittlement and after 2000 h the material broke into small

pieces. Exposure of material J to combined moisture and temperature caused no visual changes at

30
Figure 3.1.6 Blistering of cover material H after 2000 h at 125°C

31
100 — (a)

100

(b)

90 - 90

80

5?

- 80

g 70 z

<

t 60

§ 1

'{' /j MATERIAL G

\j I j"jj (with integrating sphere)

[/ /// Control

[1 // 70"C-500h

j !// 70°C-1000h

B 70 z

<

| 60

c/s z

2 50 <

ii

MATERIAL H E

COz

2 50 -

_

<

| 40

_ .//

ii,

-in

(with integrating sphere)

Control 70°C-2000h

90"C-2000h

LUx | U\ 70°C-2000 h

UJX i/i 125T-1000 h & 2000 h 1 30 / /'/' 90"C-500 h

a.£ 30 J/ i 125°C-500h

EUJX

20 —

j ilj 125°C-500 h

/< 125°C-1000 h

\j< 125°C-2000h

H]

SUJX

20

! f !

1 i 10 10 1

i

j —

tr i i i i i i i i

i i

i fcft/l 1 1 1 1 1 1 1 1 1 1 1 300 500 700 900 1100

WAVELENGTH (nm)

1300 1500 300 500 700 900 1100

WAVELENGTH (nm)

1300 1500

100

90

80 E

uj 70 z

<

I

60

V)z

S 50 <£ 40

I 30

20

10 LJ

(C)

100

MATERIAL J

(with integrating sphere)

Control 125°C-2000h

- 70°C-500. 1000, &

2000 h & 90°C-500.

1000. & 2000 h&

125°C-500& 1000 h

J I I L J I L 300 500 700 900 1100

WAVELENGTH (nm)

1300

u

z<

50

£ 40

Q.£ 30

20

10 i

: I

J

MATERIAL K

(with integrating sphere)

Control 70°C-2000h&

90-C-2000 h 125"C-500h

125'C-IOOOh

125°C-2000h

_L 300 500 700 900 1100

WAVELENGTH (nm) 1300

Figure 3.1.7 Spectral transmittance curves of cover samples after temperature aging

32
80

70

60

UJU

z

<1- 50

h-Z

V)z 40 <

ce

t—_i 30 <

£

cco 211 z

10

MATERIAL K

(without integrating sphere) (e)

Control 70°C-500 h 70X-1000 h 70°C-2000

h

200 300 400 500

WAVELENGTH (nm)

§00 700

Figure 3.1.7 Continue^

33
70°C. At 90°C with moisture, the material started to whiten after 1000 h, and was considerably

whiter after 2000 h. The material also became progressively more brittle as a result of this

exposure; however, its surfaces remained glossy. Exposure of material K to moisture at temperatures

of 70°C and 90°C for up to 2000 h caused no visible changes. Material N showed no visible signs of

change after exposure to moisture at 70°C. After 500 h of exposure to moisture at 90°C, the material

started to take on a cloudy white appearance, which became progressively more severe at longer exposure times. The cloudy white appearance was accompanied by a noticeable increase in embrittlement

when specimens were broken in flexure; however, the specimen surfaces remained glossy indicating

that surface microcracking was probably not the cause of this whitening.

Microscopic Examination : Examination of materials E, G, L, and M with an optical microscope. revealed

that the whitening of these materials, when exposed to temperature in combination with moisture, was probably due to delamination between the glass fibers and the resin in the bulk of the material.

This type of resin-glass fiber separation was not observed with specimens exposed outdoors to either

"real time" or accelerated conditions or in any other type of laboratory exposure used. Surface

microcracking was observed in specimens of material G after 500 h of exposure to moisture at 90°C;

but not at 70°C with moisture. The change in surface texture observed visually for material M at 90°C with moisture was also found to be due to the formation of surface microcracks. Examination

with a scanning electron microscope of specimens of material H, exposed to moisture at 90°C, indicated

a considerable amount of moisture etching; but no microcracking (see figure 3.1.8). Similar moisture

etching was also observed on the Palo Alto "real time" exposure test specimen after 480 days. Examination of the surfaces of materials J, K, and N after exposure to combined moisture and temperature

showed no noticeable changes.

Optical Measurements : Integrated solar transmittance values are given in table 3.1.5 for cover samples exposed for up to 2000 h at temperatures of 70°C and 90°C with a relative humidity of approximately 95 percent. With the exception of the sample of material H which disintegrated after 2000 h

at 90°C and could not be measured, all of the materials showed very large changes in their transmittance at this temperature. After 1000 h at 70°C, materials H, K, and N did not show changes in

integrated transmittance. The remainder of the materials showed substantial changes which were

somewhat less than those which occurred at 90°C with humidity. Spectral transmittance curves are shown for materials G, H, J, K, and N in figure 3.1.9. Material G showed a drastic decrease in

spectral transmittance as a result of exposure to temperature and humidity. The other glass fiber

reinforced plastics performed somewhat better, but also showed substantial decreases in performance.

Measurements of material H using an integrating sphere showed a slight shift in the absorption edge

to shorter wavelengths at 90°C. The curves measured for this material without an integrating sphere,

which are shown in figure 3.1.9b, show a substantial decrease in transmittance, probably due to scattering, after 1000 h of exposure to humidity at 90°C; the 2000 h samples broke into small flakes.

Materials J and N showed similar changes in their spectra, both with and without an integrating

sphere. These changes, which were much greater without an integrating sphere, are most likely due to scattering which would account for the cloudy white appearance observed visually for these materials.

Material K showed the same loss in UV inhibitor observed for exposure to temperature without humidity

with a shift of the absorption edge towards shorter wavelengths.

3.1.6 Temperature and Xenon Arc Radiation Testing

Visual Inspection : Temperature and xenon arc radiation exposure for up to 2000 h at 70°C caused no visual signs of degradation in materials E and L. After 500 h at 90°C with xenon arc radiation, a slight amount of yellowing was observed in these materials which became progressively more severe with exposure time. Similarly, material G showed no visible signs of degradation with xenon arc radiation at 70°C; however, at 90°C there was severe yellowing after 500 h of radiation exposure

which became progressively more severe with exposure time. Temperature and xenon arc exposure of

material M for 500 h at 70°C caused a slight increase in yellowing after 500 h which became slightly

more intense with exposure time. At 90°C with xenon arc radiation, there was slight yellowing after

500 h which progressed to severe yellowing after 2000 h. No visible signs of degradation occurred

in material H after exposure to temperature and xenon arc radiation for 2000 h at 70°C and after

1000 h at 90°C; after 2000 h of exposure at 90°C, there was a slight amount of yellowing and the

material became quite brittle. In the case of material J, temperature and xenon arc radiation caused

a moderate amount of yellowing after 500 h at both 70°C and 90°C. This yellowing appeared to remain

fairly constant for exposure times up to 2000 h. Material K showed no visible signs of degradation

after radiation exposure for 2000 h at 70°C and 500 h at 90°C. After 1000 h at 90°C, there was a slight amount of yellowing and after 2000 h of xenon arc radiation exposure at 90°C, material K

became moderately yellow and brittle. Exposure of material N to temperature and xenon arc radiation

caused no visible signs of degradation after 2000 h at 70°C. After 500 h of radiation exposure at 90°C, a slight amount of yellowing could be seen on viewing the edge of the test specimen. Specimens

34
(a) 500 hours

(b) 1000 hours

(c) 2000 hours

Figure 3.1.8 Surface changes in cover material H after 500, 1000, and 2000 h at 90°C and 95% RH

35
Table 3.1.5. Effect of Temperature & Humidity on Cover

Sample 7 ransm i ttance

70 C and 95% RH 90 C and 95% RH

Hours Exposure Hours Exposure

Sample Control 500 1000 2000 500 1000 2000

E 0.85 0.72 0.64

C 0.814 0.15 0.13

H 0.85 0.85 0.85

J 0.88 0.87 0.87

K 0.89 0.90 0.91

L 0.84 0.54 0.51

M 0.78 0.35 0.31

N 0.90 0.88 0.89

0.50 0.46 0.30

0.10 0.08 0.07

0.84 0.83 —

0.86 0.84 0.74

0.91 0.91 0.87

0.31 0.25 0.21

0.24 0.23 0.18

0.76 0.82 0.64

Table 3.1.6. Effect of Temperature & Xenon Arc Radiation

on Cover Sample T ransm i ttance

70 C and Xenon Arc 90 C and Xenon Arc

Hours of Exp osure Hours of Exposure

Samp I

e

Contro

I

500 1000 2000 500 1000 2000

E 0.85 0.85 0.85' 0.84 0.78 0.79 0.77

G 0.84 0.80 0.78 0.81 0.78 0.78 0.74

H 0.85 0.85 0.85 0.85 0.86 0.86 0.85

J 0.88 0.86 0.86 0.86 0.85 0.85 0.84

K 0.89 0.92 0.93 0.92 0.91 0.89 --

L 0.84 0.82 0.84 0.83 0.83 0.83 0.80

M 0.78 0.79 0.78 0.80 0.74 0.73 0.70

N 0.90 0.89 0.89 0.90 0.89 0.88 0.88

36
100

90

80

70

t 60 E

CO 50 <

H 40

ffi 30

20

10

(a)

(with integrating sphere)

Control 70°C & 95%RH-500 h 70°C&95% RH-IOOOh

90°C & 95% RH-500 h 90°C& 95% RH-IOOOh

90°C & 95% RH-2000 h

1£t

-:\ /. -x " Sfl«^%//:- *&

J L I I I I 300 500 700 900 1100

WAVELENGTH (nm)

1300 1500

CJz

<

<

ee

90 (b)

80 s^^^^^

70

// // 60 //

//

/

/

/

/ 50

....

40

' '''

' /'' MATERIAL H

{without integrating sphere)

30 / /' Control

/

'

/ f 90"C & 95% RH-500 h 20

10

/ /

// 1

/ 7

90°C& 95% RH-IOOOh

i i i i 340 400 460 520

WAVELENGTH (nm)

580 640

100

90

80

LUo

70 z

<(-

1— 60 £

CO

&.< 50

1-_

<u 40

ecUJz

0.CO 30

1=2 LUX

20

10

(c)

MATERIAL J (with integrating sphere)

Control 70°C&95%F?

90°C & 95% RH-500 h 90°1&95%RH-1000 h 90°C& 95% RH-2000 h

J L J L J L J L 300 500 700 900 1100

WAVELENGTH (nm)

1300

100

90

80 B 70

LUu

< 60 ~ ,„ ^

40

30

20

10

(d)

MATERIAL J (without integrating sphere)

f/^-90"-

400 460 520 580 640

WAVELENGTH (nm)

Figure 3.1.9 Spectral transmittance curves of cover samples after temperature and humidity aging

37
100

90

80

70

60

50

40

30

20

10 h

MATERIAL N

(with integrating sphere)

Control 70°C & 95% RH-500 & 1000 h 90"C & 95% RH-500 h 90°C&95%RH-1000h

90°C&95%RH-2000h

J_ 300 500 700 900 1100

WAVELENGTH (nm)

1300

u

z

<

E

in

<E

cco

100

90

80

70

60

50

40

30

20

10

(f)

MATERIAL N

(without integrating sphere)

Control 90°C& 95% RH-500 h 90°C&95%RH-1000h

90°C&95%RH-2000h

/

/

/"

300

K^

400 500 600

WAVELENGTH (nm)

700

100 - (g)

90

80 4

s~'

. -^..^/

J?£ 70

ast 60

E

V)

as i

'!

MATERIAL K

(with integrating sphere)

Control g 50 "I 70°C & 95% RH-500 h

_l<

/

i 70°C&95%RH-1000h

a 40

ce 1 90°C & 95% RH-500 & 1000 h

HEMISPHE w

1 90°C & 95% RH-2000 h

20 —

10 \ i i i i i i i i i i 300 500 700 900 1100 WAVELENGTH (nm)

1300

80

70 H 60

UJU

z

<1— 50

1-S

z 40

«t

cci— 30 <

E

cco XII z

10

(h)

MATERIAL K

(without integrating sphere)

Control 90"C & 95% RH-2000 h 90'C&95%RH-1000h

90°C& 95% RH-500 h

220 280 340 400

WAVELENGTH (nm)

460 520 580

Figure 3.1.9 Continued

38
exposed for 1000 h and 2000 h under these conditions became noticeably more brittle on breaking in

flexure in addition to showing this slight yellowing.

Microscopic Examination: Surface raicrocracking was observed in the specimen of material J exposed to xenon arc radiation at 90°C for 2000 h. No other changes were observed for test specimens exposed

to xenon arc radiation in conjunction with temperature.

Optical Measurements : Integrated solar transmittance values are given in table 3.1.6 for cover

samples exposed to xenon arc radiation for up to 2000 h at temperatures of 70°C and 90°C. Material G was the only material which showed a change at 70° C. At 90° C, materials E, G, J, L, and M showed

significant changes in their integrated transmittance. Examination of spectral curves obtained for

material G showed shifts in the absorption edge to longer wavelengths as a function of exposure time

and temperature (see figure 3.1.10a). This shift probably accounts for the yellowing observed

visually for this material. Material H showed a slight shift in its absorption edge towards shorter

wavelengths similar to that observed with the exposure tests discussed in preceding sections of this

report.

Changes in the absorption edge of material K, similar to those previously found for exposure to temperature and temperature and humidity, were also found after exposure to temperature and xenon arc radiation. Material N exhibited the same spectral shift in its absorption edge observed previously

in outdoor exposure (see figure 3.1.41). Material J showed a slight shift in its absorption edge

which increased with the exposure time and temperature (see figure 3.1.10b). This probably accounts

for the yellowing observed visually for this material.

3.1.7 Comparison and Assessment of Test Procedures

The "real time" outdoor mini-box testing and accelerated outdoor testing both appear to be good ways

of duplicating the types of changes observed with full-size collectors. However, outdoor "real

time" exposure of 480 days having a minimum solar radiation level of 17,000 kJ/m^/day is required with

the cover mini-boxes to make many of the changes observed at shorter times with full-size stagnating

collectors evident. The accelerated outdoor test appears to be capable of doing this in 120 actual

exposure days. This is most likely because the temperatures of cover samples mounted on the miniboxes were typical of those measured on operating solar collectors, whereas those of samples in

the accelerated test apparatus were typical of those measured on the covers of stagnating collectors.

The changes used for comparative purposes included spectral changes, microcracking, embrittlement

on bending, visual yellowing, and in the case of PMMA, molecular weight measurements (see

table 3.1.7). The amount and extent of microcracking observed in outdoor "real time" exposure

appeared to be closely related to the amount of moisture and condensation present at the exposure

site with the greatest changes observed at sites with high prevailing humidity, i.e., Cape

Canaveral rather than Phoenix. The cumulative amounts of solar radiation received at all four sites were roughly comparable (see Appendix A). The microcrack patterns observed with accelerated outdoor

testing appeared to be finer and less open than those observed with outdoor "real time" exposure. This

is probably due to the higher stress levels and loading rates caused by exposure to concentrated

solar radiation in conjunction with an intermittent water spray.

The indoor laboratory tests were able to duplicate some but not all of the changes observed outdoors.

The temperature and xenon arc radiation tests appeared to be reasonable methods for determining

changes due to these parameters. However, the indoor temperature and humidity testing produced

changes that were not observed in outdoor exposure under conditions representative of those occurring in actual solar collectors. The primary value of this type of long-term test would be for

glazings for trickle down collectors and polymeric water storage tanks where continuous exposure to moisture at elevated temperatures is likely. None of the indoor laboratory tests duplicated the

extensive microcracking observed outdoors. Some of the glass fiber reinforced plastics produced

microcracking as a result of temperature and moisture exposure; however, as mentioned above, the

exposure conditions used in this study for humidity testing are believed to be too severe, i.e.,

there was extensive delamination between the glass fiber reinforcement and the resin in all of the

glass fiber reinforced plastics subjected to this test.

With regard to optical property measurements, emphasis in current ASTM methods concerned with the

durability of cover materials [8, 9, 11] has been placed on the use of integrated spectral transmittance values. These integrated values are not sensitive to spectral changes that occur in a limited

part of the solar spectrum, i.e., at short wavelengths in many polymers. Since little or no energy is found in the solar spectrum in this short wavelength region, integrated solar spectral

transmittance values are of little value in detecting these changes which are sensitive indicators of

degradation in many polymers. More emphasis should be placed on the analysis of normal and

hemispherical spectral transmittance curves.

39
100

90

80

70

60

50

40

30

20

10

(a)

MATERIAL G

(with integrating sphere)

Control 70°C & Xenon arc - 500 h 70°C & Xenon arc - 1000 h 70°C & Xenon arc - 2000 h 90°C & Xenon arc - 1000 h S0°C & Xenon arc - 2000 h

j_ J_ _L 300 500 700 900 1100

WAVELENGTH (nm)

1300 1500

u

z<

z

100 (M

90

to 1

V w

70 - II

f MATERIAL J

60

/J n

i

i

j

(with integrating sphere)

Control 70°C & Xenon arc - 500, 1000. 50 & 2000 h 90°C & Xenon arc - 500 h 40 — 90°C & Xenon arc - 1000 h 90"C & Xenon arc - 2000 h 30 -

20 -

10 "i i i

I I i I I I I l l I 300 500 700 900 1100

WAVELENGTH (nm)

1300 1500

Figure 3.1.10 Spectral transmit tance curves of cover samples after temperature and xenon

arc radiation aging

40
m oo n m — r*. m en en

»* en co n mm en en en m en qo -a cm

(SI cm —

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m m m

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3.2 ABSORBER MATERIALS TESTING

In this section, the results of outdoor and indoor laboratory exposure testing of absorber materials

are presented and the extent to which accelerated outdoor and indoor laboratory tests simulate "real

time" outdoor exposure tests are discussed. A complete listing of materials used for these comparisons and their code letters are presented in table 2.2.4 of this publication. These materials were

selected as being representative of the broad variety of absorber materials being used in flat-plate

solar collectors when this test program was initiated. In addition to the small-scale tests which

were performed on all materials, samples of those absorber materials having code letters A through H were cut from full-size solar collectors and evaluated following the completion of collector exposure

testing as described in section 2 of this publication. The types of small-scale outdoor and indoor

laboratory tests performed on absorber materials are also summarized in section 2.

3.2.1 Full-Size Collector Stagnation Testing

Visual Inspection : Whitish areas appeared on the absorptive coatings of several of the type A

collectors within the first few months of outdoor exposure (see figure 3.2.1). There were no obvious environmental trends with regard to the occurrence of this whitening, i.e., one of the two type A collectors exposed at the Palo Alto site had extensive whitening and the other showed no evidence of this phenomenon. The source of this whitening has been identified by Moore as a crystalline zinc salt [38]. Large dark gray irregular areas appeared on the absorptive coatings of most

of the type C collectors exposed outdoors. These collectors had considerable evidence of water

leakage and condensation which may have been the source of the problem. Pitting and, in some cases, extensive corrosion occurred on the absorbers of most of the type D collectors (see figure 3.2.2).

Once again, there was no consistent trend that could be observed with regard to outdoor exposure

site location and the degree of pitting and corrosion may have been caused by water leakage which

could vary with the construction and assembly of individual collectors. The absorbers of collector

types E, F, G, and H showed no visual signs of deterioration.

Optical Measurements : Tables 3.2.1 and 3.2.2 present values for absorptance and emittance,

respectively, measured on samples taken from the absorber plates of full-size collectors A through H following the completion of outdoor exposure and thermal performance testing. The nonselective

coatings did not show any significant changes in optical performance as a result of exposure for 480

days. The absorptance of coating A changed from an original value of 0.87 to as low as 0.79 and the

emittance from 0.13 to as high as 0.30 for the series 1 and series 2 collectors. These changes in

emittance appeared to be associated with the whitish deposits observed visually, with the largest

change occurring in the series 2 collector exposed in Palo Alto. The series 4, Type A, collector

exposed to stagnation conditions with radiation augmentation reflectors in Phoenix had a larger

change in emittance to 0.43; however, the whitish deposits on this sample did not visually appear to be as extensive as those on the Palo Alto Series 2 collector. Changes in emittance for coating A

were minimal at the Gaithersburg site, which correlated with the lack of visual signs of change for

samples exposed there. Moore [38] measured values in emittance of 0.18 in the whitish areas of

absorber samples taken from type A collectors used in an operational system for 2 years. As observed

by Moore, variations in the color of absorptive coating A from tan to bluish-purple did not affect

the optical properties of this material. Such colors were observed on both exposed and unexposed

materials in this program. Increases in the emittance of absorber material D from 0.06 to as much

as 0.17 are related to the amount of corrosion that occurred. Significant changes in absorptance

were not observed for this material. The emittance of material F improved from 0.75 to as low as 0.50. Accompanying this improvement in emittance was a decrease in absorptance from 0.96 to as low

as 0.88. Samples of absorber material P, which were taken from collectors used in a solar energy

system in Gaithersburg, Maryland, showed no significant changes in optical properties. These collectors had been in use for 5 years with a little more than a year of that time consisting of

stagnation exposure.

3.2.2 "Real Time" Coupon Specimen Testing

Visual Inspection : The only materials which showed visual evidence of changes were material N, which

changed in color from dark brown to tan within 80 days of exposure at all four sites, and material H, which showed large gray areas lighter than the original coating after 480 days of exposure at the

Palo Alto site. All of the paints used as absorptive coatings gave off condensible outgassing

products. This resulted in the appearance of cloudy areas on the glazing of the absorber exposure

box directly above the paint coupon specimens. There were no visible signs of the whitening and

corrosion observed for full-size collectors.

Optical Measurements : Tables 3.2.3 and 3.2.4 present values for absorptance and emittance,

respectively, measured as a function of exposure time for coupon specimens exposed on the selective

42
Figure 3.2.1 Whitish deposits on absorber material A sample from a full-size collector

after 480 exposure days (^17,000 kJ/sq-m-day)

Figure 3.2.2 Corrosion of absorber material D sample from a full-size collector after

480 exposure days (^-17,000 kJ/sq-m-day)

43
Table 3.2.1. Absorptance of Full-Size Collector Absorber Coatings after Exposure,

S i te

Series Contro

1

Exposure

Co 1 1 ector Days** Phoen ix Cape Canavera 1 Palo Alto Gaithersburg

A 1 0.87 480 0.79 0.83 0.86 0.84

2 0.87 480 0.83 0.82 0.82 0.84

3 0.87 240 0.86 0.84 -- --

4 0.87 60 0.88 0.83 -- --

B 1 0.98 480 0.98 0.97 0.97 0.97

2 0.98 480 0.98 0.97 0.97 0.97

3 0.98 240 0.97 0.96 -- --

4 0.98 60 0.98 0.97 -- --

C 1 0.98 480 0.96 0.96 0.95 0.97

2 0.98 480 0.96 0.96 0.96 0.97

3 0.98 240 0.96 0.96 -- --

4 0.98 60 0.97 0.97 -- --

D 1 0.97 480 0.95 0.95 0.95 0.95

2 0.97 480 0.95 0.95 0.96 0.96

3 0.97 240 0.95 -- -- --

4 0.97 60 -- -- -- --

E 1 0.95 480 0.96 0.95 0.96 0.96

2 0.95 480 0.96 0.95 0.96 0.95

3 0.95 240 0.95 0.94 -- --

4 0.95 60 0.96 0.95 -- --

F 1 0.96 480 0.91 0.90 0.90 0.88

2 0.96 480 0.90 0.91 0.89 0.89

3 0.96 240 0.94 0.93 -- --

4 0.96 60 0.91 0.90 -- --

H 1 0.95 480 0.95 0.95 0.95 0.95

2 0.95 480 0.95 0.95 0.95 0.95

3 0.95 240 0.95 0.95 -- --

4 0.95 60 0.95 0.95 -- --

PI - 0.96 * 0.93

P2 ~ 0.96 X- 0.96

* Samples from NBS Townhouse Collectors after Approximately 5 Years Exposure.

** Days with a Minimum Solar Radiation level of 17,000 kj/sq m

44
Table 3.2.2 Emittance of Full-Sized Collector Absorber Coatings after Exposure.

S i te

Ser i es Cont ro

1

Exposure

Co 1 lector Days** Phoen i x Cape Canavera I Palo Alto Gaithersburg

A 1 0. 13 480 0. 12 0. 15 0. 18 0. 14

2 0.13 480 0. 17 0. 13 0.31 0. 12

3 0. 13 240 0. 12 0. 13 -- --

4 0. 13 60 0.43 0. 13 -- --

B 1 0.91 480 0.91 0.91 0.91 0.91

2 0.91 480 0.91 0.91 0.91 0.91

3 0.91 240 0.91 0.91 -- --

4 0.91 60 0.91 0.91 -- --

C 1 0.92 480 0.90 0.91 . 90 0.90

2 0.92 480 0.90 0.91 0.91 0.91

3 0.92 240 0.91 0.91 -- --

4 0.92 60 0.91 0.90 -- --

D 1 0.07 480 0.06 0.06 0.06 0.14

2 0.07 480 0.06 0. 10 . 06 0.06

3 0.07 240 0.06 — -- --

4 0.07 60 -- -- -- --

E 1 0.87 480 0.88 0.87 0.88 0.87

2 0.87 480 0.89 0.88 0.88 0.87

3 0.87 240 0.86 0.86 -- --

4 0.87 60 0.87 0.87 -- --

F 1 0.75 480 0.56 0.55 0.50 0.53

2 0.75 480 0.60 0.58 0.57 0.53

3 0.75 240 0.63 0.54 -- --

4 0.75 60 0.55 0.50 -- --

H 1 0.89 480 0.89 0.88 0.89 0.88

2 0.89 480 0.89 0.89 0.89 0.88

3 0.89 240 0.88 0.89 -- --

4 0.89 60 0.89 0.88 -- --

P1 - 0.08 •* 0.07

P2 - 0.08 w 0.08

* Samples from NBS Townhouse Collectors after Approximately 5 Years Exposure,

** Days with a Minimum Solar Radiation Level of 17,000 kJ/sq m.

45
Table 3.2.3. Effect of Outdoor Exposure on Absorber Coating Absorptance - Selective Side

Phoenix Cape Canaveral Palo Alto Gaithersburg

Days Exposure* Days Exposure Days Exposure Days Exposure

Sample Control 80 160 240 480 80 160 240 480 80 160 240 480 80 160 240 480

A 0.87 0.87 0.86 0.88 -- 0.88 0.85 0.86 0.86 0.88 0.86 0.87 0.88 0.87 -- 0.87 0.87

C 0.98 0.97 0.97 0.97 0.97 0.97 0.98 0.98 0.97 0.97 0.97 0.98 0.97 0.97 0.97 0.97 0.97

D 0.97 0.96 0.95 0.94 0.94 0.95 0.96 0.96 0.94 0.96 0.96 0.96 0.95 0.96 0.96 0.96 0.94

E 0.95 0.96 0.96 0.96 0.96 0. 96 0. 96 0. 96 . 96 . 96 0. 96 0. 96 0. 96 . 96 0. 96 0. 96 . 96

F 0.96 0.92 0.92 0.90 0.89 0.93 0.91 0.91 0.89 0.93 0.90 0.90 0.89 0.93 0.92 0.91 0.89

G 0.93 0.92 0.90 0.92 0.93 0.93 0.93 0.93 -- 0.93 0.93 0.93 -- 0.93 0.93 0.93 --

H 0.95 0.96 0.95 0.96 0.94 0.95 0.96 0.96 0.95 0.95 0.96 -- 0.95 0.95 0.96 0.96 0.95

I 0.88 -- -- 0.87 -- -- -- 0.88 0.87 — -- 0.88 0.86 -- -- 0.87 0.87

J 0.98 0.97 0.97 0.97 0.96 0.97 0.97 0.98 0.97 0.97 0.97 0.91 0.90 0.97 0.98 0.97 0.97

L 0.99 0.98 0.97 0.97 0.96 0.98 0.98 0.99 0.97 0.99 0.99 — 0.98 0.99 0.98 -- 0.97

M 0.94 0.94 0.92 0.91 — 0.96 0.93 0.96 0.95 0.91 0.96 0.92 0.92 0.94 -- 0.93 0.93

N 0.93 0.76 -- 0.77 -- 0.74 -- 0.74 0.73 0.76 -- 0.78 0.76 0.77 -- 0.77 0.76

P 0.96 0.95 0.95 0.94 0.94 0.95 0.95 0.95 0.94 0.95 0.94 0.95 0.95 0.95 0.95 0.95 0.94

* 17000 kJ/sq-m (1500 Btu/sq-ft) Minimum Days

4b
Table 3.2.4. Effect of Outdoor Exposure on Absorber Coating Emittance - Selective Side

Phoen ix Days Exposure*

Sample Control 80 160 240 480

Cape Canavera I Days Exposure

80 160 240 480

Pa lo A I to Days Exposure

80 160 240 480

Ga i thersburg

Days Exposure

80 160 240 480

0.13 0.10 0.11 0.18 0.22 0.10 0.12 0.11 0.17 0.11 0.13 0.14 0.18 0.14

0.92 0.91 0.92 -- 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91

0.07 0.05 0.10 -- 0.07 . 06 . 09 . 06 . 06 0. 05 0. 07 0. 08 0. 07 0.06

0.87 0.89 0.85 — 0.90 0.89 0.90 0.90 0.90 0.88 0.89 0.89 0.90 0.88

0.75 0.65 0.61 -- 0.64 0.66 0.66 0.62 0.63 0.67 0.63 0.62 0.62 0.68

0.86 0.86 0.86 -- 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85

0.89 0.87 -- -- 0.86 0.88 0.85 0.86 0.86 0.88 0.89 0.86 0.86 0.88

0.19 -- -- 0.17 0.16 -- -- 0.18 0.17 -- -- 0.18 0.18

0.14 0.13 0.15 -- 0.11 0.10 0.11 0.11 0.12 0.12 0.12 0.10 0.09 0.12

0.29 0.39 -- -- 0.44 0.39 0.42 0.36 0.44 0.27 0.35 0.30 0.31 0.29

0.10 0.11 0.08 0.08 0.07 0.12 0.10 0.13 0.13 0.08 0.13 0.11 0.09 0.09

0.51 0.48 -- 0.49 0.50 0.45 — 0.47 0.47 0.47 -- 0.47 0.48 0.46

0.08 0.07 0.10 -- 0.09 0.07 0.07 0.07 0.07 0.08 0.08 0.09 0.07 0.07

0.11 0.11 0.11

0.91 0.91 0.91

0.06 0.06 0.06

0.89 0.89 0.90

0.65 0.63 0.64

0.85 0.85 0.85

0.88 0.87 0.86

- - 0.170.19

0.11 0.13 0.12

0.35 -- 0.34

-- 0.09 0.09

-- 0.48 0.48

0.08 0.06 0.07

* 17000 kJ/sq-m (1500 Btu/sq-ft) Minimum Days

47
side of the absorber box described in section 2. Values for specimens exposed on the nonselective

side are given in tables 3.2.5 and 3.2.6. With the exception of absorber material H, which had a slight, but consistent, improvement in emittance from 0.88 to 0.86 after 480 days on the selective

side of the absorber box, there were no changes in the optical properties of the nonselective

absorber coupon specimens studied. Material A increased in emittance from about 0.13 to values of

0.22, 0.17, and 0.18 after 480 days of selective side exposure at the Phoenix, Cape Canaveral, and

Palo Alto sites, respectively. No significant changes were observed for this material when exposed

on the selective side at the Gaithersburg site or for the nonselective side at any of the outdoor

exposure sites. Material D had a slight tendency towards lower absorptance values, decreasing from

0.97 to 0.94. Material F had decreases in absorptance from 0.96 to 0.89 (selective side) and about

0.91 (nonselective side) and improvements in emittance from 0.75 to approximately 0.63 (selective

side) and 0.66 (nonselective side). The major portion of the changes occurred within 80 days of

exposure. Material J decreased in absorptance from 0.97 to 0.90 and improved in emittance from 0.12

to 0.09 when exposed on the selective side of the absorber box at Palo Alto for 480 days. Similar

changes were not observed at the other exposure sites. Material L increased substantially in emittance from 0.29 to as much as 0.51 with a slight decrease in absorptance. The changes were greatest

at the Cape Canaveral site and the least at the Palo Alto and Gaithersburg sites. Material N

decreased in absorptance from 0.93 to as low as 0.73 and improved slightly in emittance from 0.51 to

as low as 0.45 within 80 days of outdoor exposure and then showed no further changes. Changes in

the optical properties of materials, I, M, and P were not obvious from examination of the absorptance

and emittance data.

3.2.3 Accelerated Outdoor Testing

Visual Inspection : One of the two samples of absorptive coating material D after exposure equivalent

to 2 years "real time" had several small spots which, on microscopic evaluation, were found to be pits

due to corrosion. After the same exposure, one of two samples of material J exhibited several small

surface spots, one of two samples of material L showed considerable lightening and a light gray

mottled appearance, and both specimens of material N changed from dark brown to tan. No other changes were observed.

Optical Measurements : Table 3.2.7 presents values for absorptance and emittance for absorber

materials exposed outdoors in accelerated weathering machines with temperatures simulating stagnation

conditions. Material C was the only nonselective absorber material exposed since prior "real time"

outdoor exposure did not cause significant optical property changes in this class of materials.

Changes similar to those which occurred in the "real time" outdoor exposure occurred for materials

D, F, L, N, and one of two samples of material A. The optical property changes in material J, which

were observed for "real time" exposure of coupon specimens only at the Palo Alto site (and not at the other three "real time" outdoor exposure sites), were not observed following accelerated outdoor

exposure.

3.2.4 Temperature Testing

Visual Observations : With the exception of material N, which changed from dark brown to light tan within 1000 h of exposure at 150°C, none of the other materials showed visual signs of degradation

after exposure for up to 2000 h at this temperature. Material A darkened considerably after 1000 h

at 175°C and became a darker brown after 2000 h at this temperature. After 2000 h at 175°C,

material L had a slight lightening in color and material G took on a slight purplish tinge.

Material N changed from dark brown to tan within 1000 h of exposure at this temperature. No other

visual changes were observed.

Optical Measurements : Table 3.2.8 presents values for absorptance and emittance for absorber

materials exposed in the laboratory for up to 2000 h at temperatures of 150°C and 175°C. Absorber

coating materials F, L, and N were the only materials which showed substantial changes within 2000 h

of exposure at 150°C. These changes were consistent with those reported for the outdoor exposure

tests and occurred prior to the 1000 h time period at which samples were removed for measurement.

After 2000 h at 175°C, material A had a substantial increase in emittance from 0.13 to 0.52.

Materials F, L, and N also showed substantial changes at this temperature. A slight change in

emittance was observed for material G after 2000 h.

3.2.5 Temperature and Humidity Testing

Visual Observations : After 1000 h at 90°C and 95 percent RH, the absorptive coating on material A

had disintegrated, exposing the galvanized substrate beneath it. There was also some corrosion

through the galvanized layer; after 2000 h, there was severe corrosion. Material C had no visual

signs of degradation after 2000 h of exposure. After 1000 h, a considerable amount of the black

48
Table 3.2.5. Effect of Outdoor Exposure on Absorber Coating Absorptance - Nonselective Side

Phoenix Cape Canaveral Palo Alto Gaithersburg

Days Exposure* Days Exposure Days Exposure Days Exposure

Sample Control 80 160 240 480 80 160 240 480 80 160 240 480 80 160 240 480

A 0.87 0.87 0.87 0.87 -- 0.87 0.87 0.88 0.87 0.87 0.86 0.87 0.87 0.88 0.87 0.86 0.86

C 0.98 0.97 0.97 0.98 -- 0.97 0.97 0.97 -- 0.97 0.97 0.97 -- 0.97 0.97 0.97 --

D 0.97 0.96 0.96 0.96 -- 0. 96 0. 95 0. 96 0. 94 0. 96 0. 96 0. 96 0. 95 0. 96 0. 96 0. 94 0. 94

E 0.95 0.96 0.96 0.96 -- 0.96 0.96 0.96 -- 0.96 0.96 0.96 -- 0.96 0.96 0.96 --

F 0.96 0.93 0.93 0.93 -- 0.93 0.92 0.93 0.91 0.93 0.92 0.93 0.92 0.94 0.93 0.93 0.91

G 0.93 0.92 0.93 0.92 -- 0.93 0.93 0.93 -- 0.93 0.93 0.93 -- 0.93 0.93 0.93 --

H 0.95 0.95 0.95 0.96 -- 0.95 0.95 0.96 0.95 0.95 0.95 — 0.95 0.95 0.95 0.95 0.95

I 0.88

J 0.98

L 0.99 0.99 0.99 0.99 -- 0.98 0.97 0.97 0.96 0.99 0.98 -- 0.98 0.98 0.98 0.98 0.97

M 0.94 0.91 0.95 0.95 --

' 0.94 0.91 0.92 0.92 0.96 0.94 0.94 0.94 0.95 0.95 0.94 0.93

N 0.93 -- -- 0.77 -- -- -- 0.75 0.74 -- -- 0.78 0.78 -- -- 0.78 0.77

P 0.96 0.95 0.95 0.95 -- 0.95 0.95 0.96 0.94 0.96 0.95 0.95 0.95 0.95 0.95 0.95 0.94

* 17000 kj/sq-m (1500 Btu/sq-ft) Minimum Days

49
Table 3.2.6. Effect of Outdoor Exposure on Absorber Coating Emittance - Nonselective Side

Phoen i x Days Exposure*

Sample Control 80 160 240 480

Cape Canavera

Days Exposure

80 160 240 480

Pa lo Alto

Days Exposure

80 160 240 480

Ga i thersburg

Days Exposure

80 160 240 480

0.14 0.11 0.11 0.12 0.14 0.10 0.13 0.13 0.14 0.14 0.15 0.13 0.11 0.11 0.10 0.10

0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.90 0.91 0.91 0.91 0.91 0.91

0.09 0.07 0.06 0.06 0.07 0.05 0.06 0.06 0.07 0.06 0.08 0.06 0.06 0.06 0.06 0.06

0.88 0.89 0.89 0.90 0.87 0.89 0.89 0.90 0.87 0.89 0.89 0.90 0.87 0.89 0.89 0.89

0.66 0.66 0.66 0.65 0.66 0.65 0.66 0.66 0.67 0.66 0.67 0.68 0.68 0.67 0.67 0.66

0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85

0.88 0.88 0.88 0.87 0.88 0.88 0.88 0.87 0.88 0.88 0.88 0.87 0.88 0.88 0.88 0.87

0.29 0.32 0.31 0.37 0.44 0.42 0.46 0.51 0.33 0.38 0.34 0.32 0.32 0.30 0.30 0.31

0.08 0.11 0.10 0.10 0.10 0.08 0.09 0.10 0.13 0.12 0.11 0.10 0.11 0.10 0.09 0.10

-- 0.49 0.50 -- -- 0.46 0.47 -- -- 0.48 0.49 -- -- 0.47 0.46

0.07 0.07 0.07 0.07 0.07 0.08 0.07 0.08 0.07 0.07 0.09 0.08 0.07 0.07 0.07 0.07

A 0.13

B 0.92

D 0.07

E 0.87

E 0.75

G 0.86

H 0.89

I 0. 19

J 0. 14

L 0.29

M 0. 10

N 0.51

P 0.08

* 17000 kj/sq-m (1500 Btu/sq-ft) Minimum Days

50
Table 3.2.7. Effect of Accelerated Outdoor Exposure on Absorber Coatings

Absorptance Emittance

Control Sample 1 Sample 2 Control Sample 1 Sample 2 Coating 36* 60 120 36 60 120 36 60 120 36 60 120

A 0.87 0.87 0.87 0.88 0.87 0.87 0.86 0.13 -- -- 0.18 -- -- 0.12

C 0.97 0.96 0.97 0.97 0.97 0.98 0.97 0.92 -- -- 0.91 -- -- 0.91

D 0.96 0.96 0.97 0.93 0.95 0.95 0.94 0.07 -- -- 0.07 -- -- 0.07

F 0.94 0.95 0.92 0.88 0.91 0.91 0.91 0.75 — -- 0.64 -- -- 0.66

J 0.97 0.97 0.97 0.96 0.97 0.97 0.96 0.14 -- -- 0.14 -- -- 0.12

L 0.97 0.97 0.97 0.95 0.97 0.97 0.96 0.29 — — 0.47 -- -- 0.36

M 0.94 0.93 0.94 0.93 0.94 0.94 0.93 0.10 -- -- 0.11 -- -- 0.10

N 0.91 0.78 0.77 0.73 0.78 0.76 0.74 0.51 -- -- 0.43 -- -- 0.42

P 0.94 0.95 0.95 0.94 0.94 0.94 0.94 0.08 -- -- 0.08 -- -- 0.07

* Equivalent Days; 5 Equivalent Days = 1 Month 'Real Time' = 6.625E+5 kJ/sq m

51
Table 3.2.8. Effect of Temperature on Absorber Coatings

Temperature

:

150 C 175 C Ab sorptance Em i ttance Absorptance Emi ttance

Sample Control 1000* 2000 Contro

I

1000 2000 1000 2000 1000 2000

A 0.87 0.86 0.86 0. 13 0.09 0. 13 0.87 0.91 0. 13 0.52

C 0.96 0.97 0.97 0.92 0.91 0.92 0.97 0.97 0.91 0.91

D 0.97 0.96 0.96 0.07 0.06 0.06 0.95 0.96 0.07 0.07

E 0.95 0.96 0.96 0.87 0.86 0.87 0.96 0.96 0.89 0.89

F 0.96 0.91 0.90 0.75 0.66 0.65 0.89 0.89 0.67 0.70

G 0.93 0.93 0.93 0.86 0.85 0.85 0.94 0.95 0.86 0.82

H 0.95 0.96 0.96 0.89 0.88 0.88 0.95 0.95 0.88 0.87

1 0.88 -- -- 0. 19 0.19 -- 0.87 -- 0. 18 --

J 0.98 0.98 0.98 0. 14 0. 12 0. 12 0.97 0.97 0. 13 0. 13

L 0.99 0.99 0.99 0.29 0.37 0.35 0.98 0.99 0.41 0.38

M 0.94 0.95 0.91 0.10 0. 12 0.08 0.93 0.92 0. 12 0. 12

N 0.93 0.85 -- 0.51 0.49 -- 0.80 -- 0.50 --

P 0.96 0.95 0.95 0.08 0.07 0.06 0.95 0.95 0.08 0.10

* Hours Exposure

52
chrome absorptive coating had flaked off absorber material D, exposing the nickel layer beneath it,

and there was some pitting and corrosion of the steel substrate through the nickel layer in the areas where the flaking occurred. This pitting and corrosion became more severe after 2000 h; however,

it did not have the same appearance as that which occurred on the absorbers of full-size collectors.

Coating E lightened to mid-gray after 1000 h at 90°C and 95 percent RH and remained the same color

after 2000 h. Several small mid-gray specks appeared on the coating of material F after 1000 h of

exposure which became somewhat larger after 2000 h. There was severe surface etching of the porcelain enamel coating of material G after 1000 h and some rust spots; this remained about the same after 2000 h of exposure. Extensive broken paint blisters were observed in coating H after 1000 h

and 2000 h of exposure. Coating I had no visual signs of degradation after 1000 h of exposure. A

considerable amount of light gray specks and streaking was observed in coating J after 1000 h and

2000 h of exposure. Material L was severely corroded and took on a white, mottled gray appearance

after 1000 h and 2000 h at 90°C and 95 percent RH. The absorptive coatings of materials M and N were

removed, exposing the bare metal substrate after 1000 h of temperature and moisture exposure. There

were large gray mottled areas on the coating of material P after 1000 h of exposure and the coating

appeared to be washed away.

Optical Measurements : Table 3.2.9 presents values for absorptance and emittance for absorber

materials exposed in the laboratory for up to 2000 h at 90°C and 95 percent RH. The nonselective

absorber materials (C, E, G, and H) were not significantly affected by this exposure. With the

exception of material I, which was only exposed to these conditions for 1000 h and material F, all

of the selective absorber materials (A, D, J, L, M, N and P) had substantial (>0.03) changes in

their optical properties.

3.2.6 Temperature and Xenon Arc Radiation Testing

Visual Observations : Absorber coating materials A, C, D, E, H, I, J, M, and P had no visual signs

of change after exposure to xenon arc radiation at 90°C for up to 2000 h. Material F changed from

black to dark gray after 2000 h of exposure. There was a dull gray surface film on material G,

after 1000 h and 2000 h of exposure, which could be wiped off. Material L lightened to a medium

gray from a dark gray after 1000 h and 2000 h of exposure and material N changed from dark brown to tan.

Optical Measurements : Table 3.2.9 presents values for absorptance and emittance for absorber

materials exposed in the laboratory for up to 2000 h in conjunction with xenon arc radiation simulating the solar spectrum. Materials F, I, L, N, and P showed substantial changes in their optical

properties as a result of this exposure.

3.2.7 Thermal Cycling

Visual Observations : White spots, similar in appearance to those observed on full-size collectors,

were observed on absorber coating material A after 15 cycles from -10°C to 175°C. This whitening

became progressively more severe after 30 cycles. After thermal cycling, material D had small spots

which looked like the early stages of corrosion observed on full-size collectors. Light specks

appeared on the surface of the material J after 15 and 30 cycles. Material L had a mottled light to medium gray appearance after 15 cycles and the surface of material N changed from dark brown to tan

after 15 cycles. Visual changes were not observed for materials C, E, F, G, H, I, M, and P.

Optical Measurements : Table 3.2.10 presents values for absorptance and emittance for absorber

materials exposed in the laboratory for up to 30 cycles from -10°C to 175°C. Materials A, F, L, M. and N had significant changes in their optical properties.

3.2.8 Comparison and Assessment of Test Procedures

The test boxes used for the outdoor "real time" exposure of absorber coupon specimens appear to be

a good way of exposing a large number of samples to determine their thermal stability under stagnation conditions. However, the boxes, which were carefully constructed and designed to be watertight,

did not have the moisture and condensation problems that were observed for virtually all of the

full-size collectors. The problem of how to determine the proper test conditions for assessing the

moisture stability of absorber materials is very complex, i.e., the presence of moisture on the

inner surface of the glazing of a collector that is stagnating on a clear day does not mean that the

relative humidity in the vicinity of the absorber is anywhere near as high as that in the vicinity

of the glazing since absorber temperatures are much higher than cover temperatures. In addition, the

presence of porosity in many absorptive coatings means that moisture can condense in these pores at humidities lower than 100 percent RH. It is more likely that moisture would condense out on the

absorber at night, when it is cool, rather than in the daytime. The thermal cycling test was the test

53
Table 3.2.9. Effect of Temperature & Humidity and Temperature & Xenon Arc Radiation on Absorber Coatings

Temperature & Humidity, 90 C & 95% RH Temperature & Radiation, 90 C & Xenon Lamp

Absorptance Emittance Absorptance Emittance

Sample Control 1000* 2000 Control 1000 2000 1000 2000 1000 2000

A 0.87 0.73 0.84 0. 13 0.70 0.53

C 0.98 0.97 0.97 0.92 0.91 0.91

D 0.97 0.95 0.95 0.07 0. 13 0.19

E 0.95 0.92 0.92 0.87 0.88 0.89

F 0.96 0.93 0.93 0.75 0.76 0.77

G 0.93 0.93 0.94 0.86 0.89 0.87

H 0.95 0.94 0.94 0.89 0.90 0.90

1 0.88 0.88 -- 0. 19 0. 17 --

J 0.98 0.96 0.96 0. 14 0.22 0.26

L 0.99 0.85 0.81 0.29 0.81 0.85

M 0.94 0.59 0.54 0. 10 0.62 0.65

N 0.93 0.50 -- 0.51 0.81 --

P 0.96 0.95 0.95 0.08 0. 12 0. 14

0.86 0.87 0. 10 0. 11 0.97 0.98 0.91 0.91

0.96 0.96 0.06 0.06

0.96 0.97 0.89 0.89

0.90 0.89 0.73 0.71

0.94 0.94 0.86 0.86

0.95 0.96 0.87 0.87

0.85 -- 0.15 --

0.98 0.98 0. 12 0. 11 0.98 0.90 0.37 0.39

0.94 0.95 0. 10 0. 11 0.79 -- 0.45 --

0.95 0.96 0. 11 0.32

* Hours Exposure

54
Table 3.2.10. Effect of Thermal Cycling on Absorber Coatings

Absorptance Emittance

Thermal Cycling (Simulated Daily Cycles)*

Sample Control 5 15 30 Control 5 15 30

A 0.87 -- 0.87 0.87 0. 13 0.16 0.25

C 0.98 -- 0.97 0.97 0.92 0.91 0.91

D 0.97 -- 0.95 0.96 0.07 0.06 0.06

E 0.95 -- 0.96 0.96 0.87 0.86 0.85

F 0.96 -- 0.91 0.91 0.75 0.64 0.65

G 0.93 -- 0.92 0.93 0.86 0.85 0.85

H 0.95 -- 0.95 0.95 0.89 0.88 0.88

I . 88 -- 0.88 -- 0. 19 0.17 --

J 0.98 -- 0.97 0.97 0. 14 0.16 0. 12

L 0.99 -- 0.95 0.94 0.29 0.45 0.43

M 0.94 -- -- 0.94 0.10 -- 0.14

N 0.93 -- 0.85 -- 0.51 0.53 --

P 0.96 -- 0.95 0.95 0.08 0.09 0.09

10 C to 175 C

55
which most closely simulated the types of corrosion and other changes that were observed in full-size

collectors. In this test, coupon specimens were removed from a chamber at -10°C and allowed to equilibrate at room temperature prior to being placed in an oven at 177 °C. During this equilibration

process, moisture condensed on the test specimen surfaces; this most likely led to the corrosion

observed. The accelerated outdoor test, in which test specimens were exposed to concentrated solar

radiation and stagnation temperatures, appeared to provide results compariable to the "real time"

coupon specimen exposure; but it was not obvious that the concentrated solar radiation accelerated

the photolytic degradation of absorber materials. Laboratory testing at stagnation temperatures

brought out a number of thermal stability problems; however, these problems and a number of additional

ones were also brought out by the thermal cycling test. The temperature and humidity testing caused

corrosion problems different from those observed in "real time" exposure and is believed to be unduly

severe. Exposure to temperature and xenon arc radiation made evident some appearance changes not

observed with other tests. The importance of this test will be determined to a large extent by the

spectral transmittance of the collector glazing used in conjunction with the absorber.

3.3 ADDITIONAL COLLECTOR-LEVEL OBSERVATIONS

During the outdoor exposure of the full-size collectors, visual observations were made to identify

potential problems. Condensed moisture on the Inner surfaces of glazing materials was a common

occurrence, especially after periods of rainfall. In several cases, there were obvious openings in

the collector cases. The corners of the cases of several of the collectors used in the test program

were found to be unsealed on disassembly following outdoor exposure and testing. In addition,

several of the glazing gasket designs used would allow ponding of water between the glazing and the

gasket on the lower part of the collector during a rainstorm. It is unlikely that the water condensation observed is due only to normal collector breathing and cooling down at night in view of differences observed in the corrosion of duplicate collectors exposed outdoors at the same exposure

sites. All of the exposed collectors had visible evidence of the buildup of outgassing products on

their glazings; however, these products did not appear to substantially affect the collector thermal

performance as will be discussed in section 4. As discussed in section 3.2.2, all of the paints

used as absorptive coatings gave off condensible outgassing products when exposed in the absorber

test box as coupon specimens. Another probable source of outgassing is the insulation used in

several of the collectors as will be discussed later in this section. Outgassing was especially

obvious in collector H where the buildup of condensible material on the inner glazing resulted in a change in transmittance from an original value of 0.96 to as low as 0.79. The amount of change measured varied from collector to collector and depended on the location within the collector from

which the sample was taken. The fluorinated (ethylene propylene) copolymer film used as an inner

glazing in collector H and as a heat trap in collector C tended to sag and touch the absorber plates

when the collectors were heated by the sun. Support wires were used in collector G to prevent

this from happening. The film did not appear to be affected by a few years of contact with the

absorbers of the Type C collectors, which reached temperatures as high as 160°C under stagnation

conditions. Absorber plate buckling was also observed in several collector types; however, it did

not appear to be serious enough to cause failure. There was also evidence of cracking in the rubber

grommets used to seal the absorber plate inlet and outlet tubes.

Disassembly of the solar collectors following the completion of outdoor stagnation exposure for 480

days revealed a number of problems. The paper facing material used on mineral wool insulation in

collector A charred badly on the side in contact with the absorber plate. There was a considerable

amount of browning and shrinkage of the foam insulation used in collector C, despite the fact that

there was a 2.5 cm layer of glass fiber insulation between the absorber plate and the foam material.

The binder in the top layer of glass fiber insulation turned from yellow to tan. The binder in the

glass fiber insulation used in collector B turned from yellow to brown in the layer closest to the

absorber plate. The binder was completely burned out of the glass fiber insulation used in collector D to a depth of about 2.5 cm on the side closest to the absorber plate. Absorber plate stagnation temperatures of 220°C were measured for this collector on clear days. The foil-faced foam

insulation board used in collectors E, G, and H, and in collector F with a top layer of glass

fiber insulation 2.5 cm thick showed little evidence of change.

56
4. RESULTS AND DISCUSSION: COLLECTOR THERMAL PERFORMANCE AND EXPOSURE STUDIES

The sections which follow: 1) summarize previously published research findings resulting from this

test program, 2) present the collector thermal efficiency test results obtained during the test program and analyze possible sources of the scatter observed in these measurements, and 3) analyze

the sensitivity of collector thermal efficiency and absorber plate stagnation temperature measurements to changes in collector materials performance.

4.1 SUMMARY OF PREVIOUSLY REPORTED RESEARCH FINDINGS OF THIS PROGRAM

The research findings summarized in the following sections are discussed in detail in the

publications referenced.

4.1.1 Collector Thermal Performance Test Data Uncertainty

Statistical analyses of the thermal performance data sets obtained on eight liquid-heating,

flat-plate solar collectors at four test sites broadly dispersed in the U.S. have confirmed that the

total experimental uncertainty is about the same as the probable estimate of random error predicted

for the use of ASHRAE Standard 93-77 [24, 25]. Measurement error is believed to be the major con- tributor to the "within" test site variability (repeatibility) , and environmental effects a significant additional factor influencing the slope (loss coefficient) data obtained "between" test sites

(reproducibility). No evidence of product variability was discerned for the various collector types.

On the basis of first-order curve fits of the data points, an average "within" test site coefficient

of variation, percent cvr , of 2.1 percent and "between" test site coefficient of variation, percent

cvr, of 2.4 percent was calculated for the intercept [F^(Ta) e ].

Similar data for the overall heat loss coefficient, FrUl, indicated a "within" (percent cvr ) coefficient of variation value of 5.93 percent and a "between" (percent cvr) of 8.37 percent. A

coefficient of variation (percent cvr) for incident angle modifier (IAM) data of 34 percent was

found based upon measurements at three test sites.

The data uncertainties, when used in appropriate analytical models, resulted in the following

application uncertainties:

• Material degradation equivalent to about a 0.10 change in cover transmittance or absorber

solar absorptance and emittance

• Variations in collector all-day thermal output of from i 17 to ± 68 percent for specific

winter operating conditions resulting in significant ranking changes

• Variations in residential annual heating and domestic hot water solar fraction of i 6 to 7 percent.

4.1.2 Analysis of Measurement and Calculation Procedures for Incident Angle Modifiers for Flat-Plate

Solar Collectors

The test procedure used to determine incident angle modifiers (IAM) for flat-plate collectors was shown to give an IAM curve with a large uncertainty [26, 27]. The problem results from measuring

collector efficiency at non-normal incident angles where the measurement uncertainty is of the same order as the efficiency reduction attributable to these off-normal angles. The effects of side and

end shading of the absorber are of the same order of importance as the effects of non-normal angles

on the optical properties of the cover assembly. Consequently, even flat-plate collectors can have a significant bidirectional angular response.

Comparing the advantages and limitations of analytical versus experimental procedures for determining

incident angle modifiers for flat plate collectors of simple geometry, analytical methods are con- sidered to be as adequate as current experimental procedures and considerably less expensive. A

simplified analytical procedure for calculating the IAM for these type collectors is described.

Further consideration is needed, however, for collectors with additional directional characteristics,

such as convex covers, honeycomb grid convection suppressors, and nontracking radiation

concentration features.

57
4.1.3 Evaluation of Absorber Stagnation Temperature as an Indicator of Changes in Solar Collector

Materials Performance

Results obtained from nearly steady-state measurements (actually, time-average results for a relatively short duration) showed that the technique based on measuring the absorber stagnation

temperature rise above ambient resulting from a given solar irradiance is at least as, and possibly

more, sensitive than thermal efficiency measurements for detecting small changes in material properties with a much simpler test apparatus and experimental procedure [30, 31]. While the measured

results using the new method appear reliable and less expensive to obtain, problems were identified

which can be attributed to the strong effects on measured absorber temperature of transients resulting from changes in solar irradiance and environmental conditions. Short term transients in the

solar irradiance profile, such as would be caused by intermittent cloud cover, and in other environmental parameters can reduce the reproducibility of the data significantly. A solar irradiance

simulator could solve the problems arising from such transients and nonrepeating environmental con- ditions. A limitation associated with simulators, however, is the possible sensitivity of material

properties to the spectral distribution of the irradiance simulator.

A subsequent investigation [32] showed that an all-day integration method is a viable approach for

outdoor experimental determination of degradation in the thermal performance of solar collector

materials. This method is at least as accurate as the energy output method, provided that appropriate

limits are placed on variations in environmental conditions (primarily wind), and much simpler to perform experimentally. The all-day integration technique resolves problems associated with the

method based on steady-state temperatures such as short-term transients in solar irradiance and

wind speed, which are serious limitations. The graphical presentation of data resolves the

difficulties of comparing results obtained on days with different levels in solar irradiation.

4.1.4 Comparison of Solar Simulator and Outdoor ASHRAE Standard 93 Thermal Performance Tests

Standard test methods for determination of solar collector thermal performance permit the use of

solar simulators. An evaluation of available outdoor and solar simulator data [28] showed that the

thermal efficiency of flat-plate solar collectors can be substantially higher when measured in a solar simulator. The data and analytical studies indicated that heated optics and a relatively large

view factor can cause excessive infrared radiation exchange with the collector cover and result in

higher efficiencies for some collectors. Modifications to the simulator to change the spectral distribution, incident angle and direct/diffuse ratio are possible but careful monitoring of

the uniformity, spectral distributions, and effective environmental radiance temperature is necessary

to maintain consistent test conditions.

A more extensive experimental and analytical evaluation of solar simulator characteristics was recommended to determine the minimum requirements of adequate simulation and to provide a better

understanding of the effect of individual environmental parameters experienced in outdoor testing.

58
4.2 OUTDOOR SLOPE AND INTERCEPT DATA

In this section, collector thermal performance results are presented and compared. The bases of

comparison are the intercept and (negative) slope of the linearized curves which correlate measured

efficiency values.

The most commonly used method of characterizing collector performance is in terms of the thermal

efficiency curve. The useful energy gain may be determined from a Hottel-Whillier-Bliss analysis

for collectors of the type used in the present study. Based on this theory, the thermal efficiency

may be expressed as

n = FR

(xa) - FR

U

L [t.-ta ]/G (4.2.1)

If the parameters F (xa) and F U are assumed constant, the efficiency is a linear function of the

parameter [t.-t ] /G where F (xa) is the intercept and -F U is the slope of the efficiency curve.

i a R R L The assumptions of constant F (xa) and F U are reasonable for correlating efficiency test results.

The transmittance-absorptance product, (xa) , is a measure of collector optical characteristics and

is relatively insensitive to the test environment. The overall loss coefficient U depends more

strongly on environmental factors such as wind speed, ambient and sky temperature, and mean absorber

plate temperature. The heat removal factor F is a weak function of U and, consequently, depends

somewhat on collector temperature. Previous investigations [26, 39, 40] have shown, however, that

any errors inherent in assuming constant F (xa) and constant F U are generally overshadowed by

experimental error and variations caused by different test environments. In a later section the

suitability of linearized efficiency curves for comparing results is further validated.

The values reported for the intercept and slope of the efficiency curves for all tests conducted in

the program are presented in table 4.2.1. The collector type, test site, and test series, respectively, are designated by the codes in the first column. The other eight column pairs from left to right show the performance parameters for the initial (0- or 3-day) through final tests. This

tabulation of measured results is the data base used for all investigations of possible thermal degradation in the collectors. In order to investigate possible trends more clearly, the values of the

parameters F (xa) and F U are presented in bar graph type plots for each retest . These results are shown in figures 4.2.1 through 4.2.24. The parameters are normalized with initial (0-day) test values for test series 1, 3, and 4. For the series 2 tests, the results are normalized with initial

values which were obtained after 3 days of exposure.

Several observations follow from an examination of the slope and intercept data. Few, if any,

general conclusive statements can be made with regard to long-term degradation trends. The figures

show an apparent random scatter in results of about the same order of magnitude as any general

change in performance. While the plots of F U vs exposure exhibit far more scatter than the

corresponding plots for F (xa) , even the latter show significant and inconsistent variations within

and between test sites, collectors, and retests.

A greater scatter in the F U plots is expected because this parameter is affected more strongly by

environmental and test conaitions than is F (xa) . Additionally, visual inspection reports from the

participating laboratories suggest that the insulation in some collectors might experience a curing

process which would affect F U . On the other hand, visual inspections suggested some insulations were degraded by the presence of moisture within the collector. The effect of wind speed on the

loss coefficient is strong. Small differences between laboratories in measurement location and wind

conditions preceding the actual test can affect measured efficiency results.

The parameter F (xa) does not exhibit as strong an environmental dependence as FrU*l and should be

a more repeatable measure of collector thermal performance. As a result, more emphasis is placed on investigating the changes in F (xa) with exposure as a characteristic index of performance. In a later section, the effect of variations in environmental conditions on both slope and intercept are investigated.

Considerine series 1 and 2 results, only collectors F, G, and H show a distinct systematic decrease

in the intercept parameter. These collectors all have plastic or glass fiber reinforced plastic (FRP) covers. Collector B, in fact, shows a general improvement in the intercept parameter with exposure.

This collector with glass covers and a flat-black absorber is evidently quite stable. As expected,

there are no general trends in the slope parameter F U which is more sensitive to changes in

environmental conditions.

An examination of figures 4.2.1 through 4.2.24 shows one other apparent trend in the intercept

parameters. This parameter tends to peak between 30 and 60 exposure days and thereafter decrease

59
Table 4.2.1 Summary of all Reported F (ta) and F UT Values K R L

COLLECTOR RETEST DAYS

) 3 15 30 50 120 240 480

FRTA FRUL FRTA FRUL FRTA FRUL FRTA FRUL FRTA FRUL FRTA FRUL FRTA FRUL FRTA FRUL

A-1-1 0.602 4.562 0.593 4.308 N/A N/A 0.599 4.023 0.606 3.978 0.590 4.060 0.591 4.381 0.583 4.366

A-1-2 N/A N/A 0.596 4.440 N/A N/A 0.605 4.092 0.628 4.318 0.613 3.960 0.596 4.505 0.600 4.440

A- 1-3 0.622 4.549 N/A N/A 0.623 3.910 0.636 4.244 0.623 4.987 0.598 3.766 0.596 4.585 N/A N/A A-1-4 0.601 4.429 N/A N/A 0.610 4.148 0.603 4.307 0.591 5.107 N/A N/A N/A N/A N/A N/A

A-2-1 0.609 5.250 0.609 4.850 0.610 4.610 0.623 4.280 0.602 4.550 0.607 4.870 0.617 4.392 0.623 »4.679 A-2-2 N/A N/A 0.605 4.980 0.610 5.100 0.628 4.11-0 0.626 5.630 0.607 5. 190 0.588 4. 184 0.600 •4.394

A-2-3 0.612 4.490 N/A N/A 0.637 5.120 0.622 4.560 0.610 5.150 0.610 5.020 0.604 4.305 N/A N/A A-2-4 0.581 4.427 N/A N/A 0.629 4.967 0.617 4.893 0.608 5.113 N/A N/A .N/A N/A N/A N/A

A-3-1 0.627 3.910 0.633 3.810 0.605 3.600 0.628 4.310 0.629 4.120 0.641 3.980 0.641 4.360 0.646 3.901 A-3-2 N/A N/A 0.612 3.660 0.636 4.450 0.612 4.390 0.614 4.250 0.629 4.030 0.626 4.420 0.607 3.544

A-4-1 0.611 4.349 0.660 5.167 0.614 4.389 0.601 4.583 0.598 3.805 N/A N/A 0.617 4.024 0.643 3.974

A-4-2 N/A N/A 0.655 5.445 0.595 4.571 0.604 4.702 0.598 4.350 N/A N/A 0.607 4.430 0.654 4.505

8-1-1 0.654 5.460 0.654 5.437 N/A N/A 0.649 5.001 0.670 5.078 0.670 5.490 0.630 5.022 0.659 5.479

B-l-2 N/A N/A 0.640 5.428 N/A N/A 0.661 5.306 0.668 5.074 0.669 5. 170 0.656 5.594 0.651 5.440

8-1-3 0.652 5.476 N/A N/A 0.652 5.350 0.688 5.420 0.656 5.624 0.647 5.173 0.648 5.610 N/A N/A

B-1-4 0.666 5.459 N/A N/A 0.669 5.536 0.681 5.539 0.636 5.038 N/A N/A N/A N/A N/A N/A

B-2-1 0.654 5.380 0.640 5.384 0.665 6.105 0.648 5.648 0.673 5.705 0.666 5.780 0.668 5.254 0.683 »5.472 B-2-2 N/A N/A 0.646 5.696 0.659 6.209 0.651 5.693 0.669 5.636 0.669 5.770 0.688 5.891 0.685 »5.349

B-2-3 0.648 5.588 N/A N/A 0.666 5.852 0.668 6.011 0.662 5.364 0.668 5.320 0.671 5.608 N/A N/A

B-2-4 0.648 5.730 N/A N/A 0.651 5.737 0.655 5.869 0.653 5.824 N/A N/A N/A N/A N/A N/A

B-3-1 0.652 5.433 0.653 5.390 0.683 5.460 0.656 5.174 0.663 5.417 0.674 4.990 0.686 5.540 0.689 5.200

B-3-2 N/A N/A 0.636 5.180 0.654 5.150 0.653 5.159 0.668 5.520 0.675 5.060 0.694 5.750 0.690 5.349

8-U-l 0.672 5.212 0.696 5.865 0.709 5.746 0.719 5.723 0.662 4.616 0.651 4.719 0.688*5.440 0.673 5.007

B-4-2 N/A N/A 0.680 6.200 N/A N/A 0.687 5.383 0.654 4.452 0.647 4.753 0.705*5.881 0.665 4.984

C-1-1 0.545 4.377 0.528 4.076 N/A N/A 0.528 3.889 0.534 3.627 0.531 4.059 0.511 3.834 0.492 3.838 C-1-2 N/A N/A 0.520 3.969 N/A N/A 0.538 3.915 0.529 3.717 0.527 4.172 0.512 3.591 0.493 3.929

C-1-3 0.538 3.908 N/A N/A 0.524 4. 105 0.522 3.566 0.530 4.283 0.524 3.714 0.510 4.457 N/A N/A C-1-U 0.540 4.465 N/A N/A 0.496 3.765 0.485 3.345 0.465 3.828 N/A N/A N/A N/A N/A N/A

C-2-1 0.524 4.531 0.522 4.324 0.541 4.578 0.542 4.488 0.539 4.098 0.541 4.294 0.537 3.942 0.529 3.813

C-2-2 N/A N/A 0.511 4.268 0.524 4.513 0.525 3.929 0.517 3.968 0.512 4.173 0.498 3.840 0.501*3.983

C-2-3 0.557 4.739 N/A N/A 0.552 4.105 0.549 4.586 0.546 4.456 0.552 4. 131 0.557 4.420 N/A N/A C-2-4 0.541 4.802 N/A N/A 0.540 4.604 0.540 4.549 0.521 4.022 N/A N/A N/A N/A N/A N/A

C-3-1 0.549 4.228 0.530 3.919 0.540 3.889 0.531 3.867 0.528 3.799 0.541 3.747 0.536 3.776 0.556 3.907

C-3-2 N/A N/A 0.517 4.044 0.531 3.782 0.530 4.026 0.523 3.725 0.543 3.600 0.538 3.839 N/A N/A

C-4-1 0.552 4. 151 0.573 4.849 0.540 3.975 0.546 4.713 0.532 3.833 0.524 3.311 0.507*3.832 0.483 2.880

C-4-2 N/A N/A 0.549 4.060 0.552 3.992 0.532 4.537 0.525 4.491 0.548 3.958 0.518*3.311 0.540 3.389

D-l-1 0.648 3.468 0.629 3.152 N/A N/A 0.632 2.654 0.642 2.648 0.644 3.180 0.620 2.969 0.617 3.060

0-1-2 N/A N/A 0.633 3.179 N/A N/A 0.639 3.059 0.649 2.790 0.666 3.220 0.635 2.781 0.622 3.015

0-1-3 0.626 2.833 N/A N/A 0.613 2.982 0.637 3.174 0.618 3.090 0.634 2.902 0.619 3.229 N/A N/A

D-1-4 0.640 3.200 N/A N/A 0.623 2.968 0.632 2.790 0.605 3.013 N/A N/A N/A N/A N/A N/A

0-2-1 0.640 3.410 0.650 3.200 0.647 3.490 0.634 3.250 0.637 3.300 0.651 3.820 0.610 3.789 0.657*3.498

0-2-2 N/A N/A 0.659 3.010 0.656 3.290 0.646 3.470 0.641 3.400 0.647 3.720 0.625 3.164 0.635*3.578

0-2-3 0.655 3.450 N/A N/A 0.660 3.400 0.663 3.280 0.640 2.950 0.662 3.280 0.636 3.564 N/A N/A

0-2-4 0.636 3.385 N/A N/A 0.658 3.434 0.645 3.364 0.643 3.485 N/A N/A N/A N/A N/A N/A

0-3-1 0.647 2.555 0.641 2.410 0.619 2.610 0.639 2.940 0.648 3.040 0.632 2.425 0.634 2.740 0.650 3. 140 0-3-2 N/A N/A 0.635 3.640 0.629 2.620 0.652 3.100 0.657 3.110 0.646 2.487 0.645 2.870 0.661 3. 134 0-4-1 0.636 3.055 0.657 3.276 0.688 3.946 0.691 3.583 0.642 2.953 0.616 3.021 0.656*3.353 0.633 2.950

D-4-2 N/A N/A 0.670 3.384 0.678 3.390 0.664 3. 100 0.645 2.646 0.625 2.396 0.647*3.281 0.634 2.822

£-1-1 0.613 7.465 0.605 5.997 N/A N/A 0.624 5.866 0.625 6.260 0.600 5.770 0.585 6. 158 0.571 6.479

£-1-2 N/A N/A 0.598 6.443 N/A N/A 0.624 6.269 0.619 6.129 0.605 5.480 0.596 6.645 0.569 6.359

E-1-3 0.599 6.801 N/A N/A 0.595 6.063 0.609 6.222 0.616 6.470 0.604 6.680 0.578 6.032 N/A N/A

E-1-H 0.590 6. 149 N/A N/A 0.619 5.805 0.618 5.574 0.595 7.101 N/A N/A N/A N/A N/A N/A

E-2-1 0.588 6.488 0.576 5.816 0.605 6.700 0.602 6.233 0.595 6.679 0.590 6.750 0.612 6.185 0.600*6.546

E-2-2 N/A N/A 0.619 6.580 0.630 7.639 0.616 6.515 0.613 6.769 0.612 6.450 0.611 6.737 0.601*6.633

E-2-3 0.602 7.027 N/A N/A 0.614 6.963 0.610 5.970 0.609 6.780 0.605 6.810 0.620 6.330 N/A N/A

E-2-4 0.603 6.045 N/A N/A 0.595 6.448 0.588 6.234 0.569 6.653 N/A N/A- N/A N/A N/A N/A

E-3-1 0.621 6.306 0.596 5.630 0.602 6.600 0.616 6.547 0.621 5.820 0.625 6.820 0.642 5.945 0.657*6. 179 E-3-2 N/A N/A 0.595 5.584 0.598 6.560 0.626 6.696 0.617 5.830 0.633 6.880 0.637 5.871 0.65.1*6.332

E-4-1 0.619 6.257 0.598 5.990 0.597 6.331 0.594 6.172 0.618 6.451 N/A N/A 0.611 5.837 0.619 5.050

E-4-2 N/A N/A 0.602 6.217 0.583 5.996 0.585 5.792 0.614 6.451 N/A N/A 0.610 6.016 0.618 4.937

F-1-1 0.665 6.504 0.648 5.152 N/A N/A 0.668 5.152 0.664 5.859 0.633 5.590 0.616 5.655 0.631 6.433

F-1-2 N/A N/A 0.638 6.728 N/A N/A 0.659 5.798 0.644 5.528 0.642 5.627 0.615 5.291 0.621 6.200

F-1-3 0.646 5.802 N/A N/A 0.632 5.927 0.672 5.680 0.653 6.473 0.651 5.337 0.620 6.289 N/A N/A

F-1-4 0.644 6.093 N/A N/A 0.639 5.682 0.631 5.401 0.617 6.312 N/A N/A N/A N/A N/A N/A

F-2-1 0.652 6.799 0.666 6.280 0.678 5.895 0.675 6. 101 0.661 6.167 0.651 5.921 0.624 6.068 0.648*6.270

F-2-2 N/A N/A 0.661 6.256 0.646 6.676 0.684 6.084 0.653 6.566 0.664 6.528 0.650 6.820 0.662*6.496

F-2-3 0.668 6.068 N/A N/A 0.656 6.081 0.687 6. 100 0.665 6. 143 0.664 6.376 0.650 6.319 N/A N/A

F-2-4 0.653 6.141 N/A N/A 0.652 6.171 0.658 6.047 0.626 5.746 N/A N/A N/A N/A N/A N/A

F-3-1 0.679 5.350 0.674 4.936 0.651 5.916 0.663 6.058 0.657 5.479 0.653 5.394 0.668 5.570 0.666 5.405

F-3-2 N/A N/A 0.660 4.967 0.627 5.922 0.642 6. 172 0.655 5.826 0.646 5.388 0.652 5.684 0.653 5.530

F-4-1 0.657 5.860 0.631 5.337 0.651 5.769 0.637 5.383 0.655 5.798 N/A N/A 0.651 5.370 0.648 5.354

F-4-2 N/A N/A 0.623 5.655 0.627 5.740 0.616 5.451 0.634 6.651 N/A N/A 0.622 5. 136 0.690 7.331

C-1-1 0.572 5.905 0.548 5.322 N/A N/A 0.563 5.438 0.584 5.025 0.567 5.672 0.548 5.276 0.528 5.877

G-1-2 N/A N/A 0.567 5.429 N/A N/A 0.568 5.431 0.566 5.010 0.565 5.066 0.514 5.305 0.537 5.899

G-1-3 0.553 5.480 N/A N/A 0.539 5.347 0.581 5.039 0.552 5.538 0.546 4.550 N/A N/A N/A N/A

G-1-4 0.563 5.463 N/A N/A 0.549 4.948 0.503 4.933 0.538 5.884 N/A N/A N/A N/A N/A N/A

G-2-1 0.550 5.540 0.556 5.253 0.553 5.238 0.559 5.168 0.560 5.455 0.560 5.928 0.516 5.325 N/A N/A G-2-2 N/A N/A 0.534 5.201 0.541 5. no 0.514 4.590 0.541 5.543 0.547 5.638 N/A N/A N/A N/A

G-2-3 0.546 5.397 N/A N/A 0.566 5.052 0.587 4.968 0.563 5.539 0.564 5.458 0.521 5.578 N/A N/A G-2-4 0.529 5.367 N/A N/A 0.569 5.528 0.563 4.824 0.555 5.645 N/A N/A N/A N/A N/A N/A

C-3-1 0.586 5.329 0.548 4.614 0.557 5.281 0.573 5.241 0.572 4.906 0.569 5.599 0.572 4.974

C-3-2 N/A N/A 0.549 4.550 0.559 5.604 0.576 5.587 0.570 4.997 0.572 5.723 0.582 5.139

G-lt-1 U.588 5.587 0.601 5.865 0.572 5.258 0.569 5.429 0.571 4.952 N/A N/A 0.550 4.438 0.558 5.204

C-4-2 N/A N/A 0.595 5.894 0.557 5.002 0.553 5.281 0.559 4.975 N/A N/A 0.539 4.824 0.554 5.429

H-l-1 0.636 5.776 0.615 5.672 0.627 5.380 0.613 4.907 0.656 5.230 0.633 4.790 0.608 5.522 0.594 5.286

H-l-2 N/A N/A 0.641 5.483 0.618 5.338 0.638 5. 177 0.648 5.068 0.638 4.590 0.594 5.423 0.581 5.229

H- 1-3 0.632 5.770 N/A N/A 0.624 5. 180 0.654 5.455 0.643 5.990 0.627 5.821 0.623 5.575 N/A N/A H-1-4 0.643 5.505 N/A N/A 0.617 4.405 0.628 4.901 0.584 5.444 N/A N/A N/A N/A N/A N/A

H-2-1 0.625 6.014 0.618 5.654 0.630 5.902 0.638 5.575 0.618 5.540 0.615 5.350 0.518 5.851 0.575" 5.096

H-2-2 N/A N/A 0.629 5.710 0.626 5.887 0.632 5.295 0.624 5.620 0.629 5.490 0.62O 5.680 0.534« 4.356

11-2-3 0.623 6. 108 N/A N/A 0.659 6.344 0.663 5 . 904 0.624 5. 790 0.643 5.670 0.628 5.208 N/A N/A

H-2-4 0.607 5.930 N/A N/A 0.637 6.049 0.622 5.258 0.597 5.677 N/A N/A N/A N/A N/A N/A

H-3-1 0.638 5.620 0.647 5.590 0.665 5.630 0.634 5.29 3 0.629 5.440 0.638 5.040 0.628 5.510 0.616 4.941 H-3-2 N/A N/A 0.627 5.571 0.653 5.450 0.639 5.599 0.630 5.650 0.639 5.010 0.645 5.740 0.636 5.117

H-l|-1 0.640 5. 144 0.648 5.218 0.635 5. 190 0.652 6.212 0.585 5.655 0.605 4.072 0.593*4.769 0.583 4.467

H-4-2 N/A N/A 0.653 5.320 0.653 5. 116 0.643 5.678 0.623 5.309 0.633 4.464 0.503*4.993 0.620 5.072

INDICATES THAT RETEST WAS PERFORMED AT EXPOSURE SOMEWHAT OIFFERENT FROM THAT SHOWN.

)TE3: 1) ALL COEFFICIENTS BASED ON COLLECTOR CROSS AREA.

2) UNITS OF FRUL ARE DECREES! C ) /W/SQ H .

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84
somewhat. This general trend is observed in more than one-half of the results shown. Attempts to correlate the 30 - 60 exposure day peak with particular collector materials were not successful.

One possible factor, reported in recent literature [41], is that the transmittance values of polymer covers exhibit an initial increase followed by a monotonic decrease with exposure. However,

the peak was observed for collectors with glass as well as polymer covers. It is also possible

that the peak behavior could occur as a result of two or more compensating mechanisms [42]. It appears that further examination of the 30 - 60 exposure day peak behavior may be warranted since

the mechanisms responsible for this pattern may be significant in investigating the long-term

durability of collector materials.

Considering the series 3 and 4 tests, the intercept peaks are more distinct at 30 to 60 days except

for the collectors G and H. Generally, the series 4 exposure procedure, which was discontinued

after 60 days, appears to be much more severe than the other three series. Although there are many

exceptions, the results obtained in series 4 after 60 days are approximately equal to the results

obtained in the other three test series after 480 days. This observation suggests the possibility

of designing accelerated exposure tests based on concentrated solar radiation.

Of the four test sites, the collectors exposed at site 1 show apparent degradation most

frequently. As previously noted, this site is in a desert environment with a relatively high

ambient temperature and with clear sky conditions common. This combination of environmental

conditions results in higher collector stagnation temperatures than collectors tested at the other

four sites. On the other hand, the cumulative solar irradiation received by collectors at this

site in 480 days with a minimum of 17,000 kJ/m -day is less than the cumulative total received at the other sites. The reason is that this laboratory receives amounts of solar irradiation in excess of the qualifying 17,000 kJ/m -day nearly every day. These facts suggest that stagnation

temperature may be the key parameter in inducing accelerated degradation.

The reported results from site 4 had greater variability than the results obtained at the other

three sites. It is not apparent from the information available whether this trend resulted from

differences in climatic conditions or test procedures.

The series 4 tests also provide further evidence that elevated stagnation temperature may give

accelerated degradation. In series 4 tests, augmented exposure through the use of mirrors showed

the most consistent pattern of performance degradation through the first 60 exposure days. The use

of reflectors resulted in levels of solar irradiaton higher than would otherwise be experienced.

As a consequence of augmentation, higher stagnation temperatures were achieved in collectors

undergoing this series of tests. Unfortunately, difficulties in achieving uniform solar irradiation

on the collectors arose from the use of reflectors. This problem along with other experimental

difficulties resulted in the cancellation of this test series after 60 exposure days.

4.3 ANALYSIS OF SLOPE AND INTERCEPT DATA

As noted in the previous section, it is difficult to find obvious trends among the data from the

test program. The measured values of F (xa) and F

R UT exhibit considerable scatter and it does not

appear justified to present and analyze all the collector results in depth. Therefore, collectors

with typical results will be selected to analyze slope and intercept data statistically.

Figures 4.3.1 through 4.3.3 show typical efficiency curves for three collectors. Collector B is an extremely stable collector and the 0-day through 480-day efficiencies show a normal spread but no evidence of materials degradation. Collector E shows significant apparent degradation in thermal

performance after an initial exposure period of approximately 30 days. Figure 4.3.2 shows that the

30-day retest curve represents the highest overall collector efficiency and that thermal

performance decreased for subsequent exposure. (The reason that the 0-day curve is the lowest is

unknown.) The results for collector F are shown in figure 4.3.3. A similar decline in thermal

performance is evident in the figure. The absorber samples tested for this collector exhibited

consistent degradation in absorptance and a slight improvement in emittance as shown elsewhere in

the report. Correlations between materials sample degradation and collector efficiency results are analyzed in a later section.

The results in the previous section showed that generally changes in the two primary collector

performance parameters were of about the same order of magnitude as the nearly random scatter in

measured results. Consequently, it is in order to investigate further the accuracy associated with
o

.

co

r\i_ COLLECTOR B

SITE 1 SERIES - DRY

+ 3

- DOT

X 30 - DRY — — « 60 - DAT

* 120 - DRY

X 240 - DRY - z 480 - DRY

0.00000 0.08000

TI-TR/G CC/N/SQ H)

Figure 4.3.1. Collector B aggregate plot of measured efficiency, series 1, test site 1, all retests

CD

CM COLLECTOR E

SITE 1 SERIES 1 A -- DRY

+ 3

-- DRY

X 30 -- DRY

60 -- DRY

120 -- DRY

X 240 -- DRY

z 480 -- DRY

0.00000 0.08000

TI-TR/G (C/W/S0 M:

Figure 4.3.2 Collector E aggregate plot of measured efficiency, series 1, test site 1, all retests

86
CO ^

COLLECTOR F

SITE 1 SERIES -

3

- DOT

DRY

X 30 - DAY

o 60 - DRY

* 120 - DRY

X 2140 - DRY — _ z 1480 - DRY

0.00000 0.08000

TI-TR/G (C/W/SQ Ml

Figure 4.3.3 Collector F aggregate plot of measured efficiency, series 1, test site 1, all retests

87
the experimental facilities, variations in measured results within and between test sites, and the

effects of environmental factors.

4.3.1 Variation Within and Between Test Sites

Variations in the reported values of the collector parameters result from experimental error in

measuring collector energy output as well as from differences in collector material parameters. In order to emphasize differences associated with experimentation and test specimens of the same collector, results for an extremely stable collector (B) are investigated initially. Measurements of

both collector efficiency and material properties, before and during exposure, showed that the

performance of collector B was unchanged during the entire test program.

Figures 4.3.4 and 4.3.5 show typical variations of slope and intercept values within a given site for

all four test series. The apparent peaking of F (xa) and other characteristics of these measured

parameters follow the same general trends as noted earlier. Figures 4.3.6 and 4.3.7 show the primary

collector performance parameters plotted versus time for test series 1 at all test sites. The results

on these two plots are typical of between-site differences. As expected, the results are less con- sistent between sites with correspondingly higher standard deviations. Within a given test site, the

standard deviation is about the same as estimated measurement uncertainty [24,25]. For a given

collector and test series, the calculated standard deviations are higher for the results which

include all four test sites. These two observations apply to stable collectors such as collector B.

Table 4.3.1 shows the results of a statistical analysis of the initial and final values of the two performance parameters. The results shown are based on test series 1 and 2 for each collector as reported by all four test sites. For series 2, the 3-day tests are used as the initial results. The

final data are the 480-day results or the last test in those cases where the exposure period was shortened somewhat because of time limitations. Only collectors G and H, and possibly D, show a definite decrease in the mean value of the efficiency curve intercept. On the other hand, the loss

coefficient parameter, F U generally shows a slight decrease for most of the collectors. The

coefficient of variation for the intercept parameter generally increased for the collectors after

exposure, but no clear trend is shown for the slope parameter variation. It should be noted that the

above comparison includes unequal aging effects for the same model collector at different test sites. An increase in the coefficient of variation for the intercept parameter can be interpreted as meaning

that the collector performance changes in a different manner at individual sites. Changes in the

slope parameter, and to a lesser extent, the intercept parameter are undoubtedly masked by differences

in test procedures, apparatus, and prevailing climatic conditions.

Table 4.3.2 shows that the scatter of measured efficiencies around the linear correlating curve was

less for the final test as compared to the initial test for a typical stable collector. The table

shows that the residual standard deviation decreased approximately 50 per cent for collector B under

the series 1 test procedure. (The values of F (tcO and F

R U in table 4.3.2 included all reported

data. These values may be somewhat different from those in table 4.2.1 based on results reported

by the test laboratories who often used a subset of all efficiency values reported to generate an efficiency curve.) The results shown in table 4.3.2 indicate that the test laboratories improved

their procedures with repetition and experience.

4.3.2 Influence of Experimental Apparatus

Generally, changes in the two primary performance parameters were of about the same order of magnitude

as the random scatter in measured results for unexposed collectors. Consequently, the accuracy

associated with the experimental facilities and possible site bias should be investigated.

There are no obvious degradation trends with exposure time observed in much of the reported data.

Increases in Fr(tci) and decreases in F^Ut were frequently observed. Several collectors, especially

those tested at site 3, experienced a consistent increase in F^(xa) or a decrease in F^U^ throughout

the entire exposure period. In other words, based on the reported results, after nearly three years

of exposure the performance of several collectors was apparently better than the original performance.

Physical reasoning dictates that such results be viewed with suspicion. Because of these anomalies,

the experimental apparatus and procedures were investigated. An analysis of the effect of changes in

pyranometer sensitivity with time was carried out. The temperature dependence of pyranometer performance was also examined. Finally, correlations were performed to determine whether excessive data

scatter or suspicious results were associated more with particular pyranometers or test stands.

88
5 i. C o b

ID CODE

OB-1-1

AB-1-2 + B-1-3

XB-l-H

RESULTS RS REPORTED

UNCERTRINTY. ± 0.01 MERN VRLUE. 0.66

STD DEVIRTION.0.01

~l 1 1

1-

3 IS 30 60

EXPOSURE TIME ( DAYS ) —

I

-

120 —

I

240 480

Figure 4.3.4 Collector B efficiency curve intercepts vs exposure time, test site 1, all series

O

°.-\

RESULTS RS REPORTED

3 o

o

ID CODE

OB-1-1

AB-1-2 + 8-1-3

XB-1-4

UNCERTRINTY. ± 0.20

MERN VRLUE. 5.36

STD DEVIATION. 0.20

-, ,

, ,_ 3 IS 30 60

EXPOSURE TIME ( DRYS ) —r~

120 240 480

Figure 4.3.5 Collector B efficiency curve slope parameter vs exposure time, test site 1, all series

89
i

»

a o' i

»

a o"

fc

RESULTS AS REPORTED

ID CODE

OB-1-1

AB-2-1 + B-3-1

XB-1-1

UNCERTAINTY. ± 0.01 MEAN VALUE. 0.67

STD DEVIRTION.0.02

1 15 EXPOSURE

1 r

30 60 TIME ( DAYS ) 120 240 180

Figure 4.3.6. Collector B efficiency curve intercepts vs exposure time, all test sites, series 1

a

o

RESULTS AS REPORTED

o

00o

r-~ o

u>~

*. X

*

X

+ X

., J-.... A© * © o

s 6 ©

X

+X

£o

+

o

sr~ o ID CODE

en OB-1-1

ab-2-1 + B-3-1

UNCERTRINTY. * 0.20

MEAN VALUE. 5.35

STD DEVIATION.0.91 o

ruo

XB-M-1

O

O

o 1 I 3

1 15i 30

1 60

1 120

1 240

1 480

EXPOSURE TIME (DAYS ) Figure 4.3.7. Collector B efficiency curve slope parameter vs exposure time, all test sites, series 1

90
Table 4.3.1. Statistical Analysis of Efficiency Curve Intercept and Slope Parameters

Based on Initial and Final Tests, All Collectors and Test Sites, Series

1 and 2

Intercept F (ret) R

-Slope CW

Std Error of mean Std Error of mean

Coef of

Variation

Coef of

Collector Mean Value Per cent Mean Variation Value Per cent

A i* 0.615 3.034 0.00659 1.073 4.575 13.640 0.2206 4.823

f* 0.620 4.185 0.00917 1.480 4.225 8.994 0.1344 3.180

B i 0.654 2.302 0.00532 0.814 5.499 5.911 0.1149 2.090

f 0.674 2.184 0.00521 0.772 5.285 3.784 0.0707 1.338

C i 0.533 3.174 0.00598 1.122 4.204 4.459 0.0663 1.576

f 0.513 5.461 0.01060 2.064 3.677 10.950 0.1522 4.139

D i 0.646 2.006 0.00458 0.709 3.213 10.626 0.1207 3.757

f 0.639 2.495 0.00563 0.882 3.150 8.302 0.0924 2.935

E i 0.607 2.094 0.00449 0.740 6.418 8.123 0.1843 2.872

f 0.611 5.328 0.01151 1.884 6.189 8.733 0.1911 3.087

F i 0.654 2.621 0.00606 0.927 6.015 11.089 0.2358 3.921

f 0.652 3.264 0.00753 1.154 6.127 10.997 0.2382 3.888

G i 0.568 3.831 0.00769 1.354 5.429 7.982 0.1532 2.822

f** 0.544 2.600 0.00708 1.300 5.602 6.116 0.1713 3.058

H i 0.636 1.437 0.00323 0.508 5.580 4.849 0.0957 1.714

f 0.592 5.397 0.01130 1.908 4.946 7.008 0.1225 2.478

* i denotes initial test: 0-day for series 1, 3-day for series 2 f denotes final test : 480-day or somewhat less

** based on 4 data sets available

Table 4.3.2. Residual Standard Deviations for Collector

B Test Series 1 Linear Efficiency Curve

Correlations

Site Test

id

No. of

data pairs

Intercept

(percent)

C-) Slope

CW/m

2 -°C)

Residual std

deviation

1 0-day 16 65.3 5.46 0.992

480-day 16 65.9 5.49 0.544

2 0-day 16 65.5 5.49 1.904

480-day 16 68.3 5.47 1.092

3 0-day 16 66.2 5.61 0.544

480-day 16 70.0 5.46 0.659

4 3 -day 12 69.9 5.86 1.107

480-day 40 67.3 5.01 0.630

91
The investigation of possible interlab differences in reported results was conducted to determine

whether any test site consistently reported values of efficiency curve intercepts or slope which

were higher or lower than the mean of the results from the other laboratories. The approach taken

was to plot the values of the parameters from each of the four sites on a common graph vs exposure

using series 1 and 2 results. Table 4.3.3 shows the results as the percentage of times that a particular test site reported extreme values of F (xa) The table shows the times that a particular

Table 4.3.3. Comparison of Frequency of Extreme Values

for F (xa) Reported by Test Sites

R

Site

Percentage of times a site reported values:

highest lowest highest or lowest

1 10.9 32.9 21.5

2 14.7 36.3 25.5

3 43.9 7.9 25.9

4 30.5 23.7 27.1

site reported either the highest, lowest, or highest or lowest values. The latter is a measure of

the excess spread in reported results.

As shown in the table, sites 1 and 2 reported the lowest readings most often while sites 3 and

4 reported the highest readings more often. Caution is necessary in interpreting these results.

The tabulation considers only the relative magnitudes of the reported values, i.e., the highest and

lowest readings were tabulated but the quantitative amount by which a reading was the highest or lowest is not reflected in the tabulations. Also, the results presented may be affected by prevailing climatic conditions. The effects of environmental conditions on reported results are con- sidered in a later section. Based on the last column in table 4.3.3, extreme readings, on the

average, were essentially evenly distributed between the four test sites. This observation suggests

relative consistency between the sites.

An analysis was made to determine if a particular pyranometer or test stand within a test site could

be identified as a major source of the scatter in data. This evaluation was carried out by

identifying values of F (xa) which fell outside the calculated experimental uncertainty limits as measured from linear curve fits of F (xa) as a function of exposure time for a particular collector

- series combination. The experimental uncertainty in determining F (xa) resulting from instrumentation tolerances was taken to be 0.01 as calculated from a Kline-McClintock type analysis by

Culkin [43]. Any values of F (xa) beyond this uncertainty from the linear fit were tabulated along

with the pyranometer serial number and test stand number within the test site used for that result.

The number of times each pyranometer and test stand were used was also tabulated in order to prevent

an unjust bias of the data. For data falling outside the measurement uncertainty band, the percentage of times a particular pyranometer or test stand was used was then tabulated. Table 4.3.4

shows the results of this analysis through 120 exposure days.

The tabulated results show that no one pyranometer or test stand can be identified as a primary

source of suspect data. It should be noted that of the points identified outside the permissible

band, as many were found to be high as were found to be low. This results also held true for

individual pyranometers . In other words, the points identified as suspect were randomly distributed.

The pyranometers and test stands yielding such data were random as well.

An analysis was performed to examine the effect of pyranometer sensitivity changes on the reported

results. From pyranometer calibration histories provided by two of the participating laboratories

and the pyranometer manufacturers, it was observed that pyranometer sensitivity generally decreases

with time and increasing ambient temperature. The net effect of a pyranometer sensitivity decrease

is to charge the collector with less solar irradiance than it actually receives. Consequently,

measured values of F (xa) would be greater than actual. On the other hand, pyranometer sensitivity

R

92
changes with time have no effect on measured values of F U provided such changes are negligible

during a given test.

The reported values of solar irradiance in the plane of the collector must be corrected in order to compensate for changes in pyranometer sensitivities. Generally, the pyranometers used in the test program were calibrated on an average of two times per year rather than at the time of each retest.

Therefore, a method of interpolating calibration results was necessary to obtain an approximation

for the sensitivity at the time of the test. A linear interpolation procedure was used.

Table 4.3.4. Correlations of Pyranometers and Test

Stands with Experimental Results Outside

Probable Measurement Uncertainty Ranges,

Site 1, All Series through 120-day Retests

Pyranometer

Serial No.

or Test

Stand

Times

Used

Percentage of times used

When values beyond

Uncertainty Range

14317 16 18.75

14318 25 12.00

14319 13 38.46

14320 5 40.00

14321 21 9.52

14322 10 30.00

17349 25 20.00

18273 14 42.86

Stand 1 12 25.00

Stand 2 15 26.67

Stand 3 25 16.00

Stand 4 23 13.04

Stand 5 24 25.00

Stand 6 32 31.25

The reported values of F (xa) from site 2 were particularly appropriate for examining the effects of

pyranometer sensitivity changes. This site used pyranometer calibration constants from calibrations

performed at the beginning of the test program for data reduction throughout. The other participating laboratories generally used updated calibration constants from periodic recalibrations

.

Consequently, the values of the intercept parameter reported by site 2 represent a limiting worst

case with regard to possible errors caused by pyranometer sensitivity changes.

Typical pyranometer sensitivity curves are presented in figure 4.3.8. Generally, maximum changes of

from two to three percent in the reported values of F (xa) resulted from pyranometer sensitivity

corrections. This correction had no significant effect on reducing the data scatter in the reported

results. While the sensitivity changes introduced a minor consistent error in the reported results,

this source of error was found to be too small to mask any significant degradation trends . After

correction, the values of F (xa) still exhibited random fluctuations of at least the same order of

magnitude as any general degradation trend.

A correction methodology was also applied to account for instrument sensitivity to ambient temperature. For a given solar irradiance, the voltage signal output of pyranometers decreases with

increasing ambient temperature. The instrument sensitivity to operating temperature was assumed to be invariant with time and taken from the manufacturer's calibration information. It was found that

temperature compensation did not result in any significant changes in the reported results.

Generally, the effect of temperature dependence on pyranometer output was insignificant.

93
o

rvi

in X -

azo

<no

u

—iu

Q

m

O) .

CALIBRATION CURVE FOR PTRANOMETER 1W321

.40 -10 —T" 20 50

CALIBRATION CURVE FOR

PTRflNOMETER 14322

-40 -10 20

CALIBRATION CURVE FOR PTRRNOMETER 14392

Q

cm

azo

m

o -

COz

o

cr a O 01 a

Id

fvi

en2

O

u

—' ' I—'—'—I—'—'—

I

-40 -10 20 SO MONTHS (FROM JAN.

CflLIBRflTION CURVE FOR

PTRflNOMETER 17349

1979)

-40 -10 20

I 50

in

o

o .

CflLIBRflTION CURVE FOR

PTRflNOMETER 19241

— — CflLIBRflTION CURVE FOR

PTRflNOMETER 15839 o

o- 9—

a

—

in V o

a \

a.—"—-1 | 1 , 1 r

1 -40 -10 MONTHS

20 50 (FROM JAN. 1973! mom

OJCflLIBRflTION CURVE FOR

PTRflNOMETER 18273

O CC

VS M0NUM

-40 •10 —

T" 20 —

1

50

in

01 -

CALIBRATION CURVE FOR PTRflNOMETER VPI. (REr.)

T—i ' 1

' ' 1' ' 1 -40 -10 20 SO MONTHS (FROM JAN. 1979)

-r-40 to

MONTHS

—

i

-

20 50 (FROM JRN. 1979)

Figure 4.3.8. Typical Pyranometer calibrations vs time

94
4.3.3 Environmental Factors

The measurement of collector efficiency, which is the basis for investigating the thermal

performance of collectors in the present study, may be affected by variations in test environments

within and between the four test sites. The results of calculations to determine the expected

effect on measured efficiency of different environmental parameters, such as wind speed, ambient

temperature, total and scattered solar irradiance, and beam incident angle, are presented in this

section. Such parameters can substantially affect both the heat transfer and optical

characteristics of various collectors.

The parameter F„U-r is much more sensitive to variations in the test environment than is F (xa). The

overall loss coefficient IL represents the sum of the top, edge, and back loss conductions per unit

aperture area. For well designed collectors including those used in the test program, most heat

loss occurs through the top cover assembly. The top loss coefficient is a function of the

convective heat transfer coefficient which, in turn, depends on wind speed, ambient temperature, and mean absorber plate temperature. The top loss coefficient also depends indirectly on the total and

scattered solar irradiance since these parameters affect the mean absorber temperature.

Consequently, U, and F

RUT depend on the prevailing environmental conditions at the time of each

efficiency test.

Since variations in the test environment were suspected to cause some of the random variations noted

in the two collector parameters, an analytical procedure was used to adjust the results of

efficiency tests under any particular set of test conditions. The motivation for correlating test results to a standard set of conditions was twofold. First, the spread in reported values

of F (m) and F„Ut which may be attributed to different environmental conditions should be

reduced. Secondly, adjustment to a standard set of test conditions provides a common basis to compare results for the same model collector at different test sites. If the measured results

correlate with test conditions, a correction of this type is necessary to determine whether

interlaboratory differences in the testing procedures occurred.

An analytical procedure was developed earlier to effect correction of test data to a standard test condition. A detailed description of the correction procedure and mathematical model used is presented

in Reference [40]. The correction is based on using an analytical model of the collector to calculate

efficiencies at standard and at actual test conditions for a particular [t^ - t a

]/G. The difference

between these calculated efficiencies is then added to the measured efficiency, i.e.,

n(X, std) = n (X, test) + [n , (X, std) - n , (X, test)] meas calc caic

where

X = [t. - t ]/G

1 a

is held constant. As shown in Reference [40] , the experimental verification of this procedure has

been reasonably successful provided the collector configuration fits the analytical model used.

The information required to apply the correction methodology includes pertinent weather parameters,

operating conditions during testing, and physical dimensions of the collector. In addition, heat

transfer and optical characteristics of the various collector components are necessary. The

standard test environment used for the present investigation is shown in table 4.3.5. The values

shown in this table represent approximately median weather conditions of the test conditions

experienced throughout the test program.

The correction procedure as outlined was used on the reported test results for collectors B and E. These collectors were selected for analysis because they are typical of the most prevalent designs

and employ construction materials that are commercially available. Both collectors have parallel

flow through the absorber which is easier to model than the serpentine case. Collector E has a single FRP cover with a flat-black painted copper absorber. Collector B has a double, low-iron,

glass cover assembly with a flat-black copper absorber. The double-covered collector was expected

to be influenced less by variations in environmental conditions. The physical characteristics and

heat transfer properties used in the correction were based on measurements obtained at the time of

initial exposure. These values are tabulated in Reference [25].

95
Table 4,3.5. Reference Environment Used in Compensation

for Environmental Dependence of Reported

Results

Condition Value

Ambient temperature 20°C

Solar: Irradiance 1000 W/m2 Beam angle 15 deg

Diffuse fraction 0.15

Wind speed 3.0 m/s

Test fluid water

Flow rate 0.02 kg/s-m2

Collector slope 45.0 deg

Figure 4.3.9 shows a comparison between the calculated efficiency curve and an aggregate plot of all

measured instantaneous efficiencies for collector B, series 1 at site 1 (B-l-1 0- through 480-day

retests) . This figure shows the typical agreement between the model and data which is judged to be

satisfactory for calculating efficiency differences attributed to environmental variations. The

correction procedure was then carried out to determine whether a significant amount of the scatter

in the reported values of F (to) and F U could be attributed to variations in environmental factors.

Plots of the "corrected" values vs exposure time were prepared and comparisons of these were made to the plots of the "as reported" results. Figures 4.3.10 through 4.3.21, typical plots of this type,

show that compensation for environmental differences at the time and location of the test does

not significantly alter the reported results.

An analysis of the graphical results shows that no detectable changes in trends, or lack of trends,

in the values of F (xa) and F U resulted from taking into account different environmental test conditions. The mean values of F (xa) generally remained within 0.01 efficiency per cent of the mean values of the reported results by individual sites or by series. The mean values of F U actually increased somewhat. This slight increase probably resulted from the standard value of wind

speed being somewhat higher than the average wind speed experienced during testing. More importantly, the standard deviations about the mean values were not reduced significantly for either F U

or F (xa) . In fact, the standard deviation in F (xa) increased as a result of the analytical

correction procedure. This unexpected result is attributed to a lack of responsiveness or precision

in the test procedure. As shown in the tabulations of thermal performance data in appendix C of

Reference [25], the measured efficiency trends do not always follow those expected when environmental

parameters change. For example, there are cases where the wind speed increases somewhat while all

the other operating conditions remain constant; however, the measured efficiency does not always

decrease in these cases. It appears that the time response of the collectors along with inherent

experimental uncertainties associated with the test method may be contributing to this anomaly

.

While the plots of the corrected values of F (xa) and F U revealed no new trends, the weather

compensation procedure did not affect the observed 30 - 60 exposure day peak behavior in F (xa)

discussed earlier. This behavior, therefore, does not appear to be caused by environmental changes

during the testing.

The analysis of variations in measured results caused by variations in environmental conditions show

that such changes have no significant effect on the two primary efficiency curve parameters.

96
o

o

.

O)o

CO

>-

<_> to z

UJ

t—

I

o

(_) in

t—i Lu

|J- o

LU rro

roo

OJ

LEGEND

o 0-DF1 T x 60-DflY

a 3-DHY o 120-DAY

15-DAY ^-240-DAY

+30-DAY XMOO-DRY

zMODEL

1 0.01000 0. 10000

(TI-TR)/HT (C/H/M**2)

Figure 4.3.9. Comparison of calculated and measured efficiencies for collector

test series 1, all retest data

site 1,

97
o

a-, o

en' o

CD"

~ O U

. w

E °. O in' 3 o

ID CODE

OB-4-I

RESULTS RS REPORTED

T15

UNCERTAINTY. * 0.20

MEAN VfiLUE. 5.29

STD DEVIATIONS. >18

-

r~ 30

1^ 60 120 240 180

EXPOSURE TIME ( DAYS )

Figure A. 3. 10. Reported values of F U for collector B, site 4, test series 1 vs exposure time

U

ADJUSTED TO STANDARD

ID CODE

OB-M-1 UNCERTAINTY, t 0.20 MEAN VALUE. 5.37

STD DEVIATIONS. 50

1 15 EXPOSURE —i r

30 60

TIME ( DAYS ) —

r

-

120 240 480

Figure 4.3.11. Values of F U for collector B, site 4, test series 1 vs exposure time, adjusted to standard test conditions

98
ab

RESULTS AS REPORTED

ID CODE

OB-1-1 UNCERTAINTY. « 0.01 MEAN VfiLUE. 0.68

STD DEVIATION.0.03

~i 1 1 r

3 15 30 60

EXPOSURE TIME ( Dfi-YS )

I 120 210 180

Figure 4.3.12. Reported values of F (ta) for collector B, site 4, test series 1 vs exposure time

ADJUSTED TO STANDARD

_© ©

S

-

a. o

ID CODE

OB-q-1

UNCERTAINTY. * 0.10

MEAN VALUE. 0.68

STD DEVIATION. 0.02

Figure 4.3

1 1 1 v~

!5 30 60 120 EXPOSURE TIME ( DAYS )

240 480

.13. Values of F„(,-) for collector B, site 4, test series 1 vs exposure time, adjusted

to standard test conditions

99
RESULTS AS REPORTED

10 CODE OB-l-1

AB-2-1 + B-3-I

UNCERTAINTY, i 0.20 MERN VALUE. 5.37 STD DEVIRTION.0.31

-j i

1 r

3 15 30 60 EXPOSURE TIME ( ORYS I —r~

120 —I

210 —

I

180

Figure 4.3.14. Reported values of F U for collector B, site 1, 2, and 3, test series 1 vs exposure time R L

ADJUSTED TO STANDARD

3 o

+ w zro ID CODE

L. cn~ OB-l-l o AB-2-1

f\J + B-3-I

UNCERTRINTY. * 0.20 MERN VRLUE. 5.M7 STD DEVIATION. 0. 34

~l 1

i

1 1 1 1 3 15 30 60 120 210 180 EXPOSURE TIME (DAYS I

Figure 4.3.15. Values of F U for collector B, sites 1, 2, and 3, test series 1 vs exposure

time, adjustea to standard conditions

100
RESULTS AS REPORTED

o.

in b a

a*

m ID CODE o

OB-I-I

c\i AB-2-1 o f B-3-1

UNCERTAINTY. » 0.01 MEAN VALUE. 0.66 STD DEVIATIONS. 01

~1 1 1 r3 IS 30 60 EXPOSURE TIME I DAYS »

—T~

120 —r~210 -1 180

Figure 4.3.16. Reported values of F (ia) for collector B, sites 1, 2, and 3, test series 1 vs exposure time

ADJUSTE0 TO STANDARD

ID CODE OB-I-I

AB-2-1 + B-3-I

UNCERTAINTY. > 0.01 MEAN VALUE. 0.66 STD DEVIATIONS. 02

"I 1 1 f~ 3 IS 30 60 EXPOSURE TIME ( DAYS I —I I20 180

I 210

Figure 4.3.17. Values of F (tcx) for collector B, sites 1, 2, and 3, test series 1 vs exposure

time, adjusted to standard conditions

101
O m

3 o

RESULTS AS REPORTEO

10 CODE OE-I-I

AE-2-1

+ £-3-1

UNCERTAINTY. « 0.20 HERN VALUE. 6.31 STD DEVI AT I ON. 0.12

1

1 r

IS 30 60 EXPOSURE TIME ( DAYS I -1—

2-IO

"~1 120 180

Figure 4.3.18. Reported values of F U for collector E, sites 1, 2, and 3, test series 1 vs exposure time

ID CODE © E-l-l a E-2-1 + E-31

ADJUSTED TO STANDARD

UNCERTAINTY. > 0.20 MEAN VALUE. 6.63 STD DEVIHTION.0.35

1 1 1 1 1 r 1 3 15 30 60 120 210 180 EXPOSURE TIME ( OAYS I

Figure 4.3.19. Values of F U for collector E, sites 1, 2, and 3, test series 1 vs exposure

time, adjusted to standard conditions

102
ID CODE OE-I-I

AE-2-1 + E-3-I

RESULTS AS REPORTED

UNCERTAINTY. > 0.01 Hern value. o.6i STD DEVI AT I ON. 0.02

! 1 1-

15 30 60 EXPOSURE TIME I DAYS I

~~

T~"~ 120 210 180

Figure 4.3.20. Reported values of F (tcc) for collector E, sites 1, 2, and 3, test series 1 vs exposure time

cJ1

ADJUSTED TO STANDARD

m ID CODE o O E-l-l A E-2-I + E-3-I

UNCERTAINTY. « 0.0I HE AN VALUE. 0.60 STD DEVIATION. 0.02

T 1 1 1- 3 IS 30 60 EXPOSURE TIME I DAYS I 120 —f—

210 ~

1

160

Figure A. 3. 21. Values of F (tcc) for collector E, sites 1, 2, and 3, test series 1 vs exposure time,

adjusted to standard conditions

103
4.3.4 Effect on F da) of Linearized Efficiency Curves

K

The analysis of the reported values of the efficiency curve parameters is based on the assumption

that efficiency can be adequately represented by a linear function of [t^ - t a ]/G. Actually, the

elope of collector efficiency curves continually decreases with increasing temperature since the

loss coefficient increases with absorber temperature. In this section, the validity of comparing

values of F da) based on a linear efficiency curve as the characteristic parameter is established.

A comparison is made of the values of F da) extrapolated from the linearized model with those

obtained by instantaneous efficiency measurements where the inlet temperature was equal to the ambient temperature. The "directly measured" values were taken to be all the instantaneous

efficiencies obtained with [t. - t a ]/G less than or equal to 0.001°C/Wm2

. The

measured values are shown in figure 4.3.22 as the abscissa with the extrapolated values shown as

the ordinate. Perfect agreement would correspond to the case where the points would lie on the

solid line. The extrapolated values are slightly and consistently higher than those measured

directly. Each extrapolated value, however, is within 1.0 efficiency per cent of the measured

value.

Based on figure 4.3.22, representing collector efficiency as a first-order function in [t. - t ] /G

introduces a small but consistent error in the values of F (to) for collectors of the type used in

the test program. Since this error is consistent and small, however, all efficiency curves may be

assumed to be affected by about the same amount. Consequently, use of a linear efficiency

correlation does not contribute to the scatter in reported values of F da) and has a negligible

effect on the comparisons presented.

4.4 COLLECTOR THERMAL PERFORMANCE DEPENDENCE ON MATERIAL PROPERTIES

A mathematical model for collector thermal performance was used to calculate expected changes in

efficiency curve parameters as a result of arbitrary and measured changes in several key material

properties. The results are shown for changes in absorber plate emittance and absorptivity, cover

normal beam transmittance, and conductivity of thermal insulation. The results calculated for

typical changes in measured properties are then compared with measured changes of the collector

efficiency curve parameters.

Collectors D and H were selected for the theoretical investigation. The efficiency curve parameters

and material properties for these two collectors showed significant changes after exposure. Both collectors could be mathematically modeled in a straight-forward manner. Collector D has two glass

covers and a selective absorber. Tests on actual samples of the absorber plate showed that the emittance changed appreciably during the exposure period for some of the collectors exposed. Collector H

has an outer cover of poly(ethylene terephthalate )(PET) and an inner fluorinated (ethylene propylene)

copolymer (FEP) cover with a flat-black absorber. Tests showed that significant changes had occurred

in the solar transmittance of the two cover system primarily due to outgassing deposits.

The mathematical model used to calculate the thermal performance is based on the Hottel-WhillierBliss analysis with an extension to account for the serpentine flow configuration of collector H. A detailed description of the mathematical model is given in Reference [40], Table 4.4.1 shows the

base case parameters and dimensions required by the analytical model for the two collectors.

Tables 4.4.2 and 4.4.3 show calculated efficiencies for the base case and arbitrary changes in

properties. The calculations shown are for changing only one property with others held at the base

case value for a range of [tj - t ]/G. In order to compare with measured results, a linear curve

was fit to the calculated efficiency values shown. The abscissa values were selected in accordance

with ASHRAE 93-77 [6] as approximately 0.1, 0.3, 0.5, and 0.7 of [t ± - t a ]/G at stagnation

conditions. The F da) and F IL values shown in the tables are then the intercept and negative

slope of the correlating curves. The mean residual errors for the curve fit based on these four

sets of values are also included in the table. In the tables, x , designates the solar beam

transmittance of the inner cover (1) and outer cover (2). The results shown in the two tables are also depicted graphically in figure 4.4.1 where the parameters F da) and F

R

U. have been normalized

by the case values.

Several observations follow from examining the effect on F U da) and F

R

IL of changes in the four

materials. As would be expected, changes by 0.01 W/m-°C in the thermal conductivity from the base

case value have a strong effect on the slope parameter but a small effect on the intercept

parameter. While conductivities were not measured before and after exposure, this property could

change as a result of compaction, moisture entering fibrous insulation (Collector D) , thermal damage,

or moisture entry into open pores of organic foam insulation (Collector H)

104
AVERAGE DIFFERENCE « +0.617/100 —. 1

1 ,

1 ,

1 ,

1 1 , 0.54 0.5S 0.58 0.60 0.62 0.64 0.66

MEASURED

Figure 4.3.22. Comparison of measured F (xa) to values extrapolated

from linearized efficiency curves

105
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Table 4.4.1 Base Case Collector Parameters for Calculating

Thermal Performance Dependence on Material Properties

Dimension or Property Units

Absorber

Flow Configuration

Effective Length

Effective Width

Flow Tubes: Number

O.D.

Hydraulic Diameter

Wetted Perimeter

Thickness

Thermal Conductivity

Emittance

Solar Absorptance

Cover Assembly

Number of Covers

Air Space: under cover 1/under cover 2 Infrared Emittance: cover 1/cover 2 Infrared Transmittance: cover 1/cover

Index of Refraction: cover 1/cover 2 Extinction Coefficient: cover 1/cover

Thickness: cover 1/cover 2

Insulation

Thickness: Back

Edge

Conductivity: Back

Edge

Aperture Area

Gross Area

mm

mm

mmmmmm

W/m°C

Collector

parallel serpe

1.726 2.337

0.813 1.130

10 8 8.1 15.88

4.93 12.70

15.49 39.90

0.90 1.778

45.00 200.0

0.07 0.89

0.97 0.96

mm 37.0/25.0 25.4/12.7

- 0.84/0.84 0.33/0.76

2 0.02/0.02 0.60/0.12

- 1.30/1.30 1.33/1.68

2 mm 0.0021/0.0021 0.0275/0.1874 mm 3.18/3.18 0.0254/0.1778

mm 88.9 25.4 mm 25.4 25.4

W/mOC 0.04 0.02

W/m°C 0.04 0.02

1.39

1.67

2.64

2.93

An increase in the absorber plate emittance would be expected to increase the loss coefficient U^

and have an insignificant effect on the optical parameter (to) as shown since this property controls

long wavelength radiation from the absorber. The increase in e p by 0.1 increases FR

UL much more for

collector D than for collector H since the former has a selective surface with a relatively low

emittance value. Actual tests of collector D absorber plate samples showed in some cases that the

plate emittance changed by approximately this amount in going from an initial value of 0.07 to 0.17

after exposure. The emittance of test samples from collector H decreased slightly after exposure from

0.89 and 0.86.

A decrease of 0.1 in the solar absorptance of the plate decreases the intercept parameters of both

collectors by about 8 per cent and has a negligible effect on the efficiency curve slopes. Tests on absorber samples from both collectors showed the solar absorptance was essentially unchanged from

the base case value after exposure.

The effect on F (xa) and F U of changes in the solar transmittance of the cover system is

consistent with expectations. The decreases in transmittance decrease F (tot) proportionately but

decrease F^U^ very little. The calculations are based on the assumption that the cover reflectance

is unchanged; a decrease in transmittance is assumed to increase the cover absorptance by the same

amount. Consequently, the covers are somewhat warmer than in the base case and slightly reduce heat

loss from the absorber. While the glass cover material of collector D would not degrade with

exposure, the antiref lective etching can lose its effectiveness, e.g., due to the buildup of outgassing

condensation or dust. The cover system of collector H is more susceptible to degradation from

exposure. Table 4.4.3 shows the effects of arbitrary decreases in solar beam transmittance of 0.10

from the base case value. The last four columns show additional results calculated for actual

measured changes in transmittance for the two covers. Two samples of the outer cover, an ultraviolet-resistant PET film, experienced a decrease in transmittance from 0.85 to 0.84 and 0.81.

107
Table 4.4.2. Effects of Material Property Changes on

FD (ia) and F_UT for Collector D R R L

base k + 0.01 k - 0.01 e +

P

10 a - 0.10

P

x -. . - 0.10

sbl x , - 0.10

sb case (W/m-°C) (Wm-°C) (inner) (both)

At/G Calculated efficiency (aperture area basis)

(°C-m

2 /W) (%)

0.02 71.2 69.3 73.2 70.3 64.5 66.0 58.9

0.05 58.3 55.1 61.6 56.6 51.6 53.3 46.3

0.08 45.8 41.3 50.4 43.0 38.9 40.7 33.6

0.12 26.5 20.2 33.0 21.7 19.6 21.4 14.4

Fr

(tc<) 0.806 0.796 0.816 0.806 0.738 0.754 0.683

fr\

(W/m -°C)

4.458 4.902 4.006 4.847 4.470 4.450 4.445

o x 100 0.564 0.583 0.552 0.749 0.539 0.597 0.6032

n (%)

a is the mean residual error of the linear correlating curve

n

2 Base case stagnation At/G rise = 0.17°C-m /W

Samples of the inner cover , FEP film, showed apparent changes from 0.96 to 0.81 and 0.79 after 480

days exposure. The transmittance changes in the inner cover resulted from an outgassing deposit on

the inner surface which could be removed by washing. The results for the four degraded

transmittance combinations are shown in the table.

Comparing calculated with the measured changes in the efficiency curve parameters (section 4.2), the

observed variations in many cases for F (xa) are of approximately the same order. In many other

cases, particularly for F U , the measured changes are not consistent with those expected solely

from changes in collector material properties. Data scatter in the collector efficiency measurements most likely obscured the observation of these changes.

4.5 ABSORBER STAGNATION TEMPERATURE MEASUREMENT RESULTS

The stagnation, or no-flow, absorber plate temperature was routinely monitored with embedded

thermocouples located in the center of the plate for the collectors used in the series 1 tests. Separate research was also carried out at VPI&SU to investigate the potential of alternative test procedures to detect thermal performance changes based on such temperature measurements. The data

obtained were used to investigate the actual effects on collector components and to compare relative

advantages of test methods based on energy output or stagnation temperature measurement.

4.5.1 General Considerations

Temperature (both ambient and collector component), solar radiation (flux and spectral distribution),

and moisture availability are considered to be the significant environmental factors contributing to corrosion or degradation in optical properties of the key components of solar collectors. Therefore,

the selection of exposure conditions (laboratory or field) , the development of test procedures (realtime and accelerated) , and the evaluation of material properties after exposure to test conditions

must include characterization of these factors. Long-term average and weather tape climatic data for

specific sites in the United States are available from the NOAA [44]. Local differences in conditions

between the test site and the weather bureau instrumentation, variations in real weather from yearto-year and the need for supplemental information to correlate with the particular collector com- ponents resulted in the need to perform measurement of real-time conditions at each site, including

irradiance simulators and laboratory tests.

108
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109
4.5.2 Temperature

The absorber plate is considered to be one of the components of the collector most susceptible to damage because of the magnitude and range of temperatures experienced during operational and

nonoperational periods. The temperature of the absorber plates during a typical nonoperational

exposure day at site 2 and site 4 are shown in figures 4.5.1 and 4.5.2, respectively. The

influence of absorber material and other collector construction details on the peak temperature is

illustrated in figure 4.5.1 by the range from about 120°C for collector B to about 210°C for

collector D. Similar temperature profiles for collectors B and D at site 4 obtained under summer

and winter conditions indicate the large variations with ambient temperature and wind conditions

for a non-selective, double-glazed collector with relatively large heat loss coefficients as compared to the essentially identical plate temperature exhibited by a double-glazed, selectivecoated absorber with a low loss coefficient.

A comparison of the absorber plate temperatures for collectors D when exposed at sites 1, 2, and 4 and to the xenon and tungsten simulators is shown in figure 4.5.3. The higher peak temperature

obtained in the tungsten simulator is attributed to the deviation in spectral distribution from the sun (larger infrared portion) and the higher environmental temperature. The higher peak

temperature obtained in the xenon simulator is partially the result of the higher peak flux of 1100

W/m as compared to about 1000 W/m experienced outdoors. The sharper rise and fall of the

temperature in the simulator results from the abrupt turn-on and shut-off of the simulator lamps.

Absorber plate and cover material temperatures and degradation are dependent upon the solar

radiation flux and spectral distribution. Measurements of the spectral distribution were beyond

the scope of this program except for the radiometric measurement of solar radiation below

383 nm (ultra-violet) performed at site 1. Nonoperational exposure tests [33, 45-48] typically

require exposures of at least 30 days at an irradiation level of 17,000 kJ/m2 (1500 Btu/ft 2

) per day

or greater. Additionally, one period of 4 hours or more with an irradiance greater than 947 W/m

(300 Btu/hr-ft ) is generally required. Continuous solar irradiance measurements were made at each

site and the total number of accumulated days in which the total daily radiation exceeded various

levels was calculated for a one-year period as shown in figure 4.5.4. This figure can be used to estimate the probability and length of exposure time required to meet exposure tests with a wide

range of locations in the contiguous United States. The figure shows that site 4 (Gaithersburg,

MD) has a considerably lower potential for such tests as compared to the other three locations.

Complete data for the measured daily solar irradiance are included in the appendix for all four test sites throughout the duration of the reliability/durability test program.

4.5.3 Combined Irradiance and Ambient Temperature

The current nonoperational test procedure requires a number of hours of simultaneous exposure to solar irradiance and ambient temperature [7], In order to evaluate the capability of the various

sites to provide these minimum conditions and to obtain a data base for recommending

modifications to current practice, measurements of irradiance and ambient temperature were analyzed

over a one-year period for each site. The number of hours in which the exposure condition were

less than the values of irradiance for 950, 1000, 1050, and 1100 W/m2 and 15, 20, 25, and 30°C are shown for each site in tables 4.5.1 through 4.5.4. These data show that site 4 meets the ASTM E 823

exposure condition of an irradiance of 950 W/m with an ambient temperature of 25°C only 9 hours

per year.

4.5.4 Use of Stagnation Temperature for Monitoring Changes in Thermal Properties

An analytical and experimental investigation was carried out to evaluate an alternative to the

energy output method for measuring thermal degradation of materials used in flat-plate collectors.

The method, originally proposed by Birnbreier [49], is based on measuring the temperature of the

absorber under a stagnation condition before and after prolonged' exposure. For a given solar

irradiance level, the measured absorber stagnation temperature depends on cover transmittance, solar

absorptance and infrared emittance of the absorber, and the collector loss coefficient. The method,

test procedures, and results are discussed in detail in References [30-32, 50], References [30-31] are concerned with applying the stagnation temperature methods under nearly steady-state conditions

while Reference [32] considers an averaging method to account for transients in the solar irradiance

profile.

110
O COLLECTOR D, A COLLECTOR B, A COLLECTOR H. • COLLECTOR E.

10 II 12 I 2 DAY TIME

Figure 4.5.1. Typical absorber stagnation temperature profiles of exposed collectors, site 2, clear d ay

250

l 1 i

' ,

. ,. T. __ ...^ —r-

A 9/9/79 O 2/3/80

peok I »22"C peok l --3*C

225 - peak I, * 1041 W/m* peok I, »I028 W/m2 - o

—

—^^^T^-tt^^^ COLLECTOR 0, LJ 200 -

^^*» """•oC^C/ - CC J^ \ TSw 3 ,x </ ^**w. <

or: LJ 175

Jr V\ -

Q. \ \

2 _^ \ \ UJ1- 150 ^^

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O 125 /' / / ^V \ \ —

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CD<

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i —° «^ \\ \

100 / 1 /

1 1 / / 1 /

^ V^ \ \ - y \ ^

75 7/ °" 6/

1 X i i i i i .

-

8 9 10 II 12 13 14 15 16 17 18 TIME

Figure 4.5.2. Daily absorber stagnation temperature profiles for double-glazed collectors B and D, site 4, clear day

111
UJ

q:

h-<

tK

i'u'Ji Q.s

hi

m

o

to

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-

l 1 i T~ t 250 -

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200

175

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125

'/I

///

J \

• • TUNGSTEN SIMULATOR COLLECTOR D, A—-A SITE #2 5/12/80

(t » 26°C,I,(peak) = 1034 W/m') A A SITE # 1 1/31/80

(l = 23<> C,l,(peak) =1041 W/m*) D—D SITE # 1 9/20/79

(» = 37o C,I,(p«0k) =1021 W/m*)

3 \ V

\\ k \\ A

\\\ * \\

V

:

100

75,

- / // //

i

— SITE * 4 2/3/80

(t = -3 C,I,(peok)= 1028 W/m2

)

i i i \

10 12 TIME

Figure 4.5.3. Comparison of absorber stagnation temperature profiles using solar irradiance simulators

and outdoor exposure, collector D

ACCUMULATED DAYS ABOVE MINIMUM DAILY SOLAR

RADIATION LEVELS FOR EACH SITE.

PHOENIX, ARIZONA

>4700 >5000 >5500 >6000 >6500 >7000 >7500 >8000 >8500 >9000

TOTAL DAILY RADIATION (Wh/m*)

Figure A. 5. 4. Days per year above minimum solar irradiance levels for each of the 4 test sites

112
Table 4.5.1 Hours of Exposure with Conditions Exceeding Corresponding Ambient Temperature and

Irradiance Levels. Site 1, Phoenix,

Arizona, July 1979 to June 1980.

2 Irradiance (W/m )

Ambient

Temperature (°C) 950 1000 1050 1100

15 360.5 113.25 20.75 2.75

20 335 106.25 17.5 1.75

25 228.75 70.5 10.5 1.75

30 134.25 38.5 7.75 1.5

Table 4.5.2 Hours of Exposure with Conditions Exceeding

Corresponding Ambient Temperature and

Irradiance Levels, Site 2, Cape

Canaveral, Florida, November 1979 to October 1980.

2 Irradiance (W/m ) Ambient

Temperature (°C) 950 1000 1050 1100

15 389 177 42.5 14.0

20 324.5 136 27.5 12.0

25 230.5 77 12.0 7.5

30

.—.—.——

_

94.5 19.5 3.0

113
Table 4.5.3 Hours of Exposure with Conditions Exceeding

Corresponding Ambient Temperature and

Irradiance Levels, Site 3, Palo Alto, California, July 1979 to June 1980.

2 Irradiance (W/m )

Ambient

Temperature (°C) 950 1000 1050 1100

15 453 128 4.5

20 291 92 3.0

25 173 60 2.0

30 38 11.5

Table 4.5.4 Hours of Exposure with Conditions Exceeding

Corresponding Ambient Temperature and

Irradiance Levels, Site 4, Gaithersburg,

Maryland, April 1979 to March 1980.

2 Irradiance (W/m )

Ambient

Temperature (°C) 950 1000 1050 1100

15 56.25 22 1.5 0.25

20 35.5 13.5 1.0 0.25

25 9.0 4.0 — —

30 2.5 1.0 — —

114
The advantages and limitations of the proposed method are briefly summarized here; the reader is

referred to the references for detailed information. The investigations showed that the proposer!

method is as sensitive to small changes in collector material properties as the currently used

method based on measuring the energy output. Figure 4.5.5 shows the sensitivity of the new method to

a range of property changes for typical collectors. In this figure, the abscissa is the stagnation

temperature rise of the absorber above ambient normalized by the initial value. Property changes on

the order of 0.1 in plate absorptance, cover solar transmittance, and plate emittance would be

detectable using the new method with the exception of cover transmittance for collector E which would

be only marginally detectable. While the measurements required in the stagnation temperature method

are much simpler than those required to measure energy output, other factors, such as nonisothermal

absorbers, variations in environmental conditions, and transient response, must be taken into

account. Figure 4.5.5 assumes that the environmental conditions are essentially constant and that

steady-state conditions exist. Figure 4.5.6 shows the variation of normalized stagnation temperature

rise as a function of environmental parameters for collectors D and E. Clearly, wind speed, ambient

temperature, and solar irradiance have the largest effect on measured stagnation temperature. These

parameters, however, primarily affect U rather than (tcx) .

The investigation showed that even small transients in the solar irradiance profile greatly complicates the determination of small property changes. Reference [32] describes a method for reconciling

problems arising from short-term transients in the solar irradiance profile as well as long-term

variations in daily solar radiation. This method is based on measuring the absorber temperature

continuously over a period of several days along with the total daily solar irradiation. The

absorber temperature rise above ambient is then integrated to determine a daily value. The measurements provide data for a graph of the integrated absorber temperature parameter vs total daily

irradiation in the collector aperture. Comparison of two such graphs, based respectively on data

obtained over a period of several days before and after a prolonged durability exposure, show if the

thermal properties of the collector have changed significantly. Using this approach,'

calculations showed that the effect of short-term transients in the daily irradiance profile is

insignificant. Figure 4.5.7 shows expected results from using all-day integrated parameters for

stagnation temperature and solar irradiation for a typical change in plate absorptivity.

The investigation showed that the all-day integration method is a viable approach for detecting

changes in material properties that has advantages over alternative test methods based on steadystate measurements of either absorber stagnation temperature or collector energy output.

Previously, the temperature measurement approach was limited to short time periods near solar noon

on clear days outdoors or to using solar irradiance simulators indoors. Outdoor test data over a relatively short period of time was obtained to validate the calculated results and depict typical

expected results using the test procedure. The principle limitation of both stagnation temperature

methods, however, is the strong effect of other environmental conditions, particular wind speed, on

the test results. It should also be noted that neither the temperature nor energy output

measurement method can directly identify the particular material property that changed in a collector. With limitations on wind speed and for relatively clear days, the preliminary

investigation suggests that stagnation temperature rise is a reproducible collector performance

parameter which is at least as sensitive as the ASHRAE 93-77 method and much less expensive to measure. The method is applicable to a broad range of collector designs and can be extended to form

the basis for simple comparative materials tests.

4.6 GENERAL OBSERVATIONS ON OUTDOOR TEST METHODS FOR MEASUREMENT OF COLLECTOR PERFORMANCE

DEGRADATION

Except for catastrophic failures and some obvious problems with a few plastic glazing materials, the test program collectors, which are typical of modern commercial equipment, were quite durable in

thermal performance and held up very well under unusually adverse nonoperational conditions over a period of approximately 3 years.

Neither the energy output measurement method or alternative methods based on measuring stagnation

temperature are entirely satisfactory for determining the typically small property changes in

materials that occurred during the test period. The stagnation temperature method involved simpler

measurements and would be less expensive. However, this method has neither been validated extensively

nor subjected to broad field experience. While the energy output measurement is widely used and

accepted by the solar energy community for assessing solar collector thermal performance, variations

in measured performance parameters are higher than is desirable for direct comparison of results to determine degradation in collector material properties.

115
1.0

0.9

x X

0.8 -

0.7

-

1 1 1

-

-*C^\ Afc

, COLLECTOR E

— Aep

, COLLECTOR D -

- COLLECTOR D N,

fc

= 0.94. a

p »0.94. €

p

= 0.25 S. n\ Aap

, COLLECTOR D

- COLLECTOR E >v Aap

. COLLECTOR E

- fc

= 0.86, 5

p *0.95 N.

- \ ATC

, COLLECTOR D

-

1

I 1 0.0 0.1 0.2 0.3 PROPERTY CHANGE (-Aa

p ,-ATC

, A€p

)

0.4

Figure 4.5.5. Sensitivity of normalized absorber stagnation temperature to changes in material properties

116
CO X

0.9 -

IVV

CO X\X

0.9 - I J L

15 20

y/«,)g(DEG)| 20 30 40 50

SLOPE(DEG).

25

60

f

5

i 10

1 15i 20 25

, T (C) , 30

1 35i 40i

i 1

2 3 A ,V>A) ,

5

i 6

i

7

I -25

i -20

i

-15 -10 -5

,(Tt -T )(C),

i

5

1

10

I 30 35

70

800 650 900 950 1000 1050 1100 1150 6(W/m2

) Figure 4.5.6. Effect of environmental test conditions on normalized absorber

stagnation measurements

117
o

H°

ib"

1 1 1 1 1 1 COLLECTOR B THEORETICAL VALUES

1000

$«40°, T =20C, Vw »3.0m/8

O INITIAL PERFORMANCE r—ap =096 sy

D DEGRADED PERFORMANCE

_ j^^^ L-ap =0.86 -

—

500

-

1 1 1

i i i 10

/

24 h Gdt

20

(MJ/m )

30

Figure 4.5.7. Sensitivity of the all-day integration method to a 0.10 change

in plate absorptance for collector B

118
The precision of the measured thermal performance parameters, F (ta) and FrU^, obtained in this

reliability/durability test program are consistent with the experimental error inherent in the

current ASHRAE 93-77 test method [6], For comparison, the variation in F ( to) and F^Ut is somewhat

better than in the original roundrobin test program [39] conducted by NBS. In that program, the

coefficient of variation (standard deviation in percent of mean value) was 4.6 - 7.7 percent

for F

r

(tcx) and 16 - 25 percent for FrU^. In a subsequent Department of Energy comparability test program [51], coefficients of variation for F (tot) ranged from 2.0 - 6.0 percent and ranged from 3.0

- 18 percent for F^U^. In the current test program, the coefficient of variation for the initial

testing was 1.5 - 4.0 percent for F R(ta) and 5-13 percent for F R

UL

. The data from these three

test programs and the supporting analysis of the various factors which could affect the data show

that the variations in the thermal performance results presented herein are within the precision

limits of the current ASHRAE 93-77 test standard.

119
5. CONCLUSIONS AND RECOMMENDATIONS

In this section, the significant observations and recommendations resulting from this test program are summarized. These findings are primarily concerned with an evaluation of test methods for determining

the thermal performance and durability of solar collectors and their materials. During the course of

this investigation, significant research findings have been published in the open literature and presented to appropriate organizations concerned with the development of standards for solar collectors

and their materials such as ASTM and ASHRAE.

5.1 COVER MATERIALS

• Outdoor exposure at sites having a combination of high prevailing humidity and high solar

radiation generally produced more severe changes in polymeric cover materials than the

other test sites. The most extensive microcracking and optical transmission losses

occurred during the outdoor exposure of full-size solar collectors and cover mini-boxes at the test sites located in Cape Canaveral and Palo Alto. Next in order of severity were

changes observed at the Gaithersburg site followed by those at the Phoenix site. This is

consistent with the findings of Clark and Roberts [52] for cover specimens exposed in

Florida and Arizona. It is most likely due to the combined effects of photodegradation

and cyclic shrinking and swelling of the polymer surface due to moisture wetting and drying.

It Is essential that outdoor exposure testing include the combinations of environmental

exposure parameters that will occur in normal use.

2 • Outdoor "real time" exposure on cover mini-boxes for 480 days (^ 17,000 kJ/m /day)

is required for many polymeric materials in order to induce degradation that is

detectable without sophisticated analysis. This is equivalent to two or more calendar

years. Changes such as microcracking and embrittlement were not readily observable in

shorter periods of time for many materials.

• Accelerated outdoor exposure of polymeric cover materials for 120 calendar days produced

changes similar to those occurring in 480 days of "real time" exposure (>/ 17,000 kJ/m^/day )

.

The cover specimens were mounted on the accelerated exposure cover mini-boxes shown in

figure 2.2.2 of this report and exposed to concentrated sunlight (~ 6 suns) and an intermittent water spray using the apparatus described in ASTM E 838 [36].

• Outdoor exposure of cover materials at elevated temperatures representative of both

operational and stagnation conditions is needed to assess the durability of polymeric

covers of solar collectors . Elevated temperatures have an accelerating influence on the

degradation of polymeric materials. In addition, exposure to stagnation temperatures

resulted in the loss of ultraviolet radiation screening additives from several of the

materials studied. Another consideration is that many polymers are more susceptible to degradation when heated above their glass transition temperature. The cover temperatures

in stagnating solar collectors are higher than the glass transition temperatures of many

polymers. The "real time" cover mini-boxes used in this test program produced cover temperatures representative of operational conditions in full-size flat-plate solar

collectors. By controlling the cooling air flow in the accelerated outdoor test apparatus,

it was possible to attain cover temperatures representative of stagnation conditions.

• Currently recognized indoor laboratory exposure tests are not capable of reproducing

many of the changes observed in this program for polymeric materials exposed outdoors . The indoor laboratory exposure testing conducted in this test program, which used test

procedures similar to those in ASTM E 765 [8], did not duplicate the extensive microcracking observed outdoors. Other testing for 1200 hours using a xenon arc weathering

machine and an intermittent water spray, as specified in ASTM E 765, was also unsuccessful

in duplicating this microcracking. In addition to causing light transmission losses,

microcracking is of particular importance when thin film polymers are used as glazing

because of its influence on mechanical properties. Yamasaki and Blaga [53] have had

some success in using an indoor test to duplicate the surface degradation occurring outdoors

on glass-fiber reinforced polyester sheets. However, several thousand hours of exposure

in a weathering machine is required.

120
• Heat stability testing of cover materials at a temperature of 90° C in addition to the

temperatures currently specified in ASTM E 765 [8] is desirable. ASTM E 765

currently specifies the use of two temperatures for screening purposes; 75° C for outer

and single cover materials and 125° C for inner cover materials. Mathematical modelling has

shown that the covers of single-glazed flat-plate solar collectors can reach 90°C under

stagnation conditions with zero wind speed and solar radiation levels of 1000 W/nr . Since

the degradation of polymeric materials is accelerated by exposure to elevated temperatures,

it is desirable to test covers for single-glazed collectors at 90°C.

• Accelerated aging of cover materials using xenon arc weathering machines should be

performed at temperatures representative of stagnation conditions . Materials exposed to xenon arc radiation at 90° C, the temperature of single glazing in stagnating flat-plate

solar collectors, had considerably more degradation than samples of the same materials

exposed at 70° C. Another consideration is the loss or degradation of ultraviolet radiation

screening additives at elevated temperatures.

• Exposure testing of polymeric glazings using elevated temperature and humidity conditions

produced changes considerably different from those caused by outdoor exposure of samples

of the same materials. The primary value of this type of long-term test would be for

glazings of trickle down collectors and polymeric storage tanks where continuous exposure

to moisture at elevated temperatures is likely. As was previously mentioned, there is

a need for the development of an aging test which takes into account the synergistic

effects of moisture, temperature, and sunlight. Cyclic moisture exposure to cause shrinking and swelling of the polymer should be considered as part of such a test.

• Normal and hemispherical spectral transmittance curves measured in the ultraviolet - visible spectral region are more sensitive indicators of cover materials degradation than

integrated spectral transmittance values determined in accordance with ASTM E 424 [34].

Emphasis in current ASTM methods concerned with the durability of cover materials [8, 9,

11] has been placed on the use of integrated spectral transmittance values for measuring

optical property changes in glazings. These integrated values are indicators of changes

in the engineering properties of glazing materials and provide the type of data required

by solar collector designers. However, they are not sensitive to spectral changes that occur in a limited part of the solar spectrum, i.e., at short wavelengths in many

polymers. Since little or no energy is found in the solar spectrum in this short

wavelength region, integrated spectral transmittance measurements are of little value in

detecting these changes which are sensitive indicators of degradation in many polymers.

More emphasis should be placed on the analysis of spectral curves.

• Normal ultraviolet-visible spectral transmittance measurements have greater sensitivity

to chemical and/or physical changes in cover materials than hemispherical transmittance

spectra . The normal measurements are a sensitive indicator of changes in light scattering.

• There is a need for data on the infrared spectral properties of organic materials; e.g.

emittance and diffuse transmittance . These data are needed for the mathematical modelling

of collector designs and the calculation of exposure temperatures.

5.2 ABSORBER MATERIALS

• Outdoor exposure of small-scale absorber samples mounted on the absorber of a simulated

collector is an effective method for determining the thermal stability of large numbers

of samples under stagnation conditions; however, it is not a valid test for determining

the stability in the presence of moisture of these materials when used in solar

collectors . In both ASTM E 781 [13] and this test program, care was taken to prevent

moisture penetration at joints, seams, and seals. However as was observed in this test program, moisture penetration in full-size solar collectors appears to be a common occurrence. This moisture penetration resulted in corrosion and appearance changes not observed with small-scale specimens.

• Both the simulated solar collectors discussed above and the accelerated exposure miniboxes used in this test program (see figure 2.2.5) are useful for exposing small

absorber test specimens to stagnation temperatures; however, unlike the case with

121
cover materials, it is not clear that exposure to concentrated solar radiation accelerated

the photolytic degradation of absorber materials. Similar changes were observed for test specimens exposed to concentrated sunlight in the mini-boxes and to "real time" conditions

in the simulated collectors.

• Indoor laboratory exposure of absorber test specimens to temperatures characteristic of

stagnating solar collectors is an effective method for determining their thermal stability.

Temperature exposure produced thermal changes similar to those which occurred in the

outdoor exposure of absorber test specimens.

• Exposure testing of absorber materials with continuous elevated temperature and humidity

exposure conditions, such as those used in this test program, produced changes that are considerably more severe than those produced by the outdoor exposure of materials of the same composition in full-size solar collectors and exposure test boxes. The moisture

exposure conditions specified in ASTM E 744 [12], 90°C and 95 percent relative

humidity for 30 days, need to be reexamined since these conditions, which are similar

to those used in this test program, may be unduly severe. Materials which are severely degraded as a result of exposure using these test conditions may still be

capable of providing adequate service as absorbers in flat-plate solar collectors.

• Exposure to xenon arc radiation at elevated temperature produced significant optical

property changes in several of the selective absorber materials studied . Exposure

testing with xenon arc radiation should be performed at temperatures representative

of solar collector operating conditions since degradation rates are generally

accelerated by elevating the exposure temperature. The spectral transmission

characteristics of the covert s) used in a solar collector will control the amount,

and spectral distribution, of the solar radiation reaching the absorber surface. For

this reason, testing should be performed with the glazings to be used in the actual collector installed between the light source and the absorber surface, or with the worst-case

configuration possible. The light source should be filtered to match the solar

spectrum.

• The thermal cycling test most closely simulated the types of corrosion and other

changes that were observed in full-size solar collectors . In this test, coupon

specimens were removed from a chamber at -10°C and allowed to equilibrate at room temperature prior to being placed in an oven at 177°C. During this equilibration

process, moisture condensed on the test specimen surfaces; this wetting most likely led

to the corrosion observed. Because of the apparent importance of the condensed

moisture, the humidity needs to be controlled during the equilibration process.

• Research needs to be conducted to define the extent to which the absorber materials

in operational solar collectors are exposed to moisture . Factors such as wetting time,

absorber temperature, diurnal breathing, and environmental exposure conditions need

to be considered in this research. The problem of how to determine the proper test conditions for assessing the moisture stability of absorber materials is very complex,

i.e., the presence of moisture on the inner surface of the glazing of a collector that is

stagnating on a clear day does not mean that the relative humidity in the vicinity of the

absorber is anywhere near as high as that in the vicinity of the glazing, since absorber

temperatures are much higher than cover temperatures. In addition, the presence of

porosity in many absorber coatings means that moisture can condense in these pores at humidities lower than 100 percent relative humidity. It is more likely that moisture would

condense out on the absorber at night when it is cool rather than in the daytime. The

problem is further exacerbated by the presence of moisture due to water leakage in many

solar collectors, in addition to condensation caused by diurnal collector breathing.

• Spectral reflectance curves are a more sensitive indicator of absorber materials

degradation than solar absorptance values determined by integrating over a standard

solar energy distribution curve . Current ASTM standards concerned with the durability

of absorber materials [12, 13] rely on the use of integrated values determined in

accordance with ASTM E 424 [34], Spectral reflectance changes in the near infrared

region are a sensitive indicator of early degradation in many absorber materials. Since

the solar spectrum contains very little energy in this region, integration tends to conceal these changes.

122
• Absorber materials generally exhibited larger changes in emittance than in integrated

solar absorptance for both the outdoor exposure and indoor laboratory tests . Of the

thirteen different types of absorber materials studied, the aluminum oxide conversion

coating, material N, was the only exception to this trend.

• The absorber samples tested must duplicate the full-scale collector material in

substrate, preparation, and coating application techniques to provide valid test results . The variability of black chrome performance with processing parameters is an example of

this type of consideration.

5.3 SOLAR COLLECTORS

• Neither energy output measurements, based on ASHRAE Standard 93-77, nor stagnation

temperature measurement methods are satisfactory for determining changes in material

properties observed after outdoor exposure . Collector thermal performance measurements

are not sufficiently sensitive and precise under typical test environments to detect the

relatively small changes in efficiency curve slope and intercept resulting from

changes in material properties (cover transmittance, absorber absorptance and emittance,

and insulation thermal conductivity) caused by 480 days of outdoor exposure

(> 17,000 kJ/m2 /day).

• The absorber stagnation temperature methods evaluated during this investigation are

at least as good an indicator of solar collector thermal performance changes, with

the exception of bond conductance, as the ASHRAE 93-77 test method and are less

expensive to implement.

• Initial reliability/durability screening tests for materials considered for use in

solar collectors should be performed both in the laboratory and outdoors on smallscale samples using materials exposure tests such as those used in this program . These small-scale tests can be used to narrow down the choice of materials. Careful

field measurement of peak stagnation temperatures for full-size collectors or extrapolation of thermal performance curves should be used as a basis for determining

appropriate exposure test conditions.

• The final selection of materials for use in solar collectors should be based both on

the results of small-scale materials tests and on an evaluation of the properties of

materials samples taken from full-size solar collectors exposed outdoors . Materials

should be capable of withstanding stagnation conditions without property changes greater

than those allowed for in the collector design. Exposure testing of full-size collectors

makes possible an evaluation of materials interactions that may not be obvious from smallscale materials tests. Additional consideration needs to be given to changes in the mechanical properties of solar collector materials; especially thin film glazings.

The uncertainty in the measured efficiency curve intercept and slope parameters are as expected considering the inherent precision in the ASHRAE 93-77 test method . The

coefficients of variation for the initial baseline tests of the eight collector types,

from all test sites, were from 1.5 to 4.0 percent for the intercept parameter and 5 to 13 percent for the slope parameter. Statistical analyses of the test data showed no systematic

trends, either higher or lower, with the site performing the testing or with individual

collector test stands at a particular test site.

Intercept values determined from linear first-order correlations of efficiency data

provided good agreement with those obtained from second-order correlations . The effect

on the intercept was less than one percent in all cases, with the linear curve always

giving an intercept higher than the second-order curve.

For flat-plate collectors of conventional design, calculation procedures are capable

of giving results at least as good as those obtained with the ASHRAE 93-77 measurement procedure for the incident angle modifier . A coefficient of variation for the

incident angle modifier parameter of 34 percent was found for measurements made on solar collectors of the same type at three test sites.

• A comparison of results obtained under natural outdoor test conditions with those

obtained using tungsten-halogen and xenon arc simulators showed that higher

123

•

•
thermal performance results can be obtained with the solar irradiance simulators for

collectors having polymeric covers . The highest results were obtained with the xenon arc simulator.

• Stagnation exposure testing of flat-plate solar collectors intended for use in systems

with solar radiation augmentation reflectors should be conducted with these reflectors

in place . Stagnation exposure testing of flat-plate solar collectors, in this

program, for 60 days with reflectors (^ 17,000 kj/m^/day measured in the plane of the

collector without reflectors) appeared to cause more severe thermal degradation than

480 days (^ 17,000 kj/m2 /day) of stagnation exposure of the same types of collectors

without the reflectors.

® The peak stagnation temperature for flat-plate solar collectors should be measured at a distance approximately one-fourth of the way below the top of the collector and with the

collector tilted so that its aperture is normal to the sun . The magnitude of the peak

temperature needs to be taken into account for both safety and durability considerations

when selecting materials to withstand stagnation conditions. Measurements made as part of

this test program [50] have shown that absorber and cover stagnation temperatures vary

as a function of tilt angle and position on the absorber. This is most likely due to the combination of thermal stratification of air inside the collector and edge losses.

• Over a period of approximately three years, pyranometer sensitivity changes, if not

calibrated out, could give rise to errors comparable to the uncertainty in the measured

collector efficiency . These pyranometer calibration changes are not linear with respect

to exposure time. Temperature dependence of the pyranometers, which had built-in

temperature compensation circuits, was not found to be significant.

• There is a need for a water leakage test for flat-plate solar collectors similar to the

test described in ASTM E 331 [54] for exterior windows, curtain walls, and doors . This

method includes the use of a pressure differential, similar to that occurring with wind

driven rain, to enhance the penetration of water into the assembly being tested. This test method could be simplified for flat-plate collectors by applying a negative pressure inside the collector case to induce the pressure differential.

Several of the collectors used in this test program and others observed in the field showed

signs of excessive internal moisture which appeared to result from leakage rather than

condensation caused by diurnal collector breathing. The presence of moisture and

resulting degradation of materials appears to be strongly dependent on both the design

of collector joints and seams and the quality of workmanship in assembling the

collectors. In several cases where two collectors of the same type were exposed side

by side, only one of the two absorbers showed significant corrosion. Materials used in

flat-plate collectors should be moisture resistant unless the collector is hermetically

sealed.

• Outgassing deposits were observed on the glazings of virtually all of the solar collectors

exposed in the test program . However, changes due to this outgassing could not be

discerned by thermal performance measurements. The outgassing was especially severe

in the case of collector H. All of the paint absorber coatings studied in this program

caused a buildup of outgassing deposits on the glazing of absorber test box in which

they were exposed. Other potential sources of outgassing include insulation materials,

gaskets and sealants, and adhesives.

9 The thermal shock/water spray test did not cause thermal shock problems in any of the

solar collectors subjected to this test in this program.

124
6 . REFERENCES

1. "Solar Heating and Cooling Demonstration Act of 1974," Public Law 93-409, September 3, 1974.

2. "Solar Energy Research, Development and Demonstration Act," Public Law 93-473, October 26, 1974.

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Research and Development Administration (now Department of Energy), Washington, DC 20545,

November 1976.

4. "Methods of Testing to Determine the Thermal Performance of Solar Domestic Water Heating

Systems," ASHRAE Standard 95-1981, American Society for Heating, Refrigerating, and AirConditioning Engineers, Inc., 1791 Tullie Circle, N.E., Atlanta, GA, December 1981.

5. "Methods of Testing Thermal Storage Devices Based on Thermal Performance," ASHRAE

Standard 94-77, ibid., February 1977.

6. "Methods of Testing to Determine the Thermal Performance of Solar Collectors," ASHRAE

Standard 93-77, ibid., February 1977.

7. "Practice for Nonoperational Exposure and Inspection of a Solar Collector," ASTM E 823-81,

American Society for Testing and Materials, 1916 Race St., Philadelphia, PA 19103.

8. "Practice for Evaluation of Cover Materials for Flat-Plate Solar Collectors," ASTM E 765-80,

ibid.

9. "Practice for Exposure of Cover Materials for Solar Collectors to Natural Weathering Under

Conditions Simulating Operational Mode," ASTM E 782-81, ibid.

10. "Practice for Determining Resistance of Solar Collector Covers to Hail by Impact with Propelled

Ice Balls," ASTM E 822-81, ibid.

11. "Practice for Exposure of Solar Collector Cover Materials to Natural Weathering Under Conditions

Simulating Stagnation Mode," ASTM E 881-82, ibid.

12. "Practice for Evaluating Solar Absorptive Materials for Thermal Applications," ASTM E 744-80,

ibid.

13. "Practice for Evaluating Absorptive Solar Receiver Materials When Exposed to Conditions

Simulating Stagnation in Solar Collectors with Cover Plates," ASTM E 781-81, ibid.

14. "Specification for Rubber Seals Used in Flat-Plate Solar Collectors," ASTM D 3667-78, ibid.

15. "Specification for Rubber Seals Used in Concentrating Solar Collectors," ASTM D 3771-79, ibid.

16. "Specification for Rubber Seals Contacting Liquids in Solar Energy Systems," ASTM D 3832-79,

ibid.

17. "Specification for Rubber Seals Used in Air-Heat Transport of Solar Energy Systems,"

ASTM D 3903-80, ibid.

18. "Specification for Rubber Hose Used in Solar Energy Systems," ASTM D 3952-80, ibid.

19. "Practice for Laboratory Screening of Metallic Containment Materials for Use with Liquids in

Solar Heating and Cooling Systems," ASTM E 712-80, ibid.

20. "Practice for Simulated Service Testing for Corrosion of Metallic Containment Materials for Use

with Heat Transfer Fluids in Solar Heating and Cooling Systems," ASTM E 745-80, ibid.

21. "Practice for Screening Polymeric Containment Materials for the Effects of Heat and Heat Transfer Fluids in Solar Heating and Cooling Systems," ASTM E 862-82, ibid.

22. Waksman, D. , Streed, E. R. , and J. Seiler, "NBS Solar Collector Durability/Reliability Test

Program Plan," NBS Technical Note 1136, National Bureau of Standards, Washington, DC 20234,

January 1981.

125
23. Streed E. R. and Waksman, D., "NBS Solar Collector Durability/Reliability Program," Durability

of Building Materials and Components , ASTM STP 691, P. J. Sereda and G. G. Litvan, Eds.,

American Society for Testing and Materials, 1980, pp. 239-249.

24. Streed E. R. , and Waksman, D., "Uncertainty in Determining Thermal Performance of Liquid-Heating

Flat-Plate Solar Collectors," Journal of Solar Energy Engineering , ASME, Vol. 103, May 1981,

pp. 126-134.

25. Streed E. R. and Waksman, D., "Uncertainty in Determining Thermal Performance of Liquid-Heating

Flat-Plate Solar Collectors," NBS Technical Note 1140, National Bureau of Standards, Washington,

DC 20234, March 1981.

26. Thomas, W. C. , Dawson, A. G., Ill, Waksman, D., and Streed, E. R. , "Incident Angle Modifiers

for Flat-Plate Solar Collectors: Analysis of Measurement and Calculation Procedures," Journal

of Solar Energy Engineering , ASME, Vol. 104, November 1982, pp. 349-357.

27. Thomas, W. C, Dawson, A. G., Ill, Waksman, D., and Streed, E. R. , "Determination of Incident

Angle Modifiers for Flat-Plate Solar Collectors," Proceedings of ASME Solar Energy Division:

Fourth Annual Conference , April 1982, pp. 501-510.

28. Streed, E. R. , Waksman, D., Dawson, A. G., Ill, and Lunde, A., "Comparison of Solar Simulator

and Outdoor ASHRAE Standard 93 Thermal Performance Tests," Proceedings of 1980 Annual Meeting,

American Section of the International Solar Energy Society , June 1980, pp. 405-409.

29. Waksman, D., Streed, E. R. , and Dawson, A. G., Ill, "The Influence of Environmental Exposure on Solar Collectors and Their Materials," Proceedings of 1980 Annual Meeting, American Section of

the International Solar Energy Society , June 1980, pp. 415-419.

30. Dawson, A. G. , III, Thomas, W. C, and Waksman, D., "Evaluation of Absorber Stagnation

Temperature as a Characteristic Performance Parameter of Flat-Plate Solar Collectors," ASME

Paper 82-WA/Sol-5, American Society of Mechanical Engineers, New York, NY 10017, November

1982.

31. Dawson, A. G., Ill, Thomas, W. C, and Waksman, D.. "Solar Collector Durability Evaluation by

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August 1983, pp. 259-267.

32. Thomas, W. C, Dawson, A. G., Ill, and Waksman, D., "Testing Solar Collector Materials

Durability by Integrated Day-Long Stagnation Temperature Measurements," Proceedings of ASME

Solar Energy Division: Fifth Annual Technical Conference , April 1983, pp. 301-307.

33. Waksman, D., Streed, E. R., Reichard, T. W., and Cattaneo, L. E., "Provisional Flat-Plate Solar

Collector Testing Procedures: First Revision," NBS Interagency Report 78-1305A, National Bureau

of Standards, Washington, DC 20234, June 1978.

34. "Test for Solar Energy Transmittance and Reflectance (Terrestrial) of Sheet Materials,"

ASTM E 424-71, op. cit. 7.

35. "Test for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques," ASTM E 408-71,

op. cit. 7.

36. "Practice for Performing Accelerated Outdoor Weathering Using Concentrated Natural Sunlight,"

ASTM E 838-81, op. cit. 7.

37. Newland G. C. and Tamblyn, J. W. , " Mechanism of Ultraviolet Stabilization of Polymers by

Aromatic Salicylates," Journal of Applied Polymer Science , _8, 1964, pp. 1949-1956.

38. Moore, S. W. , "Los Alamos Optical Materials Reliability, Maintainability, and Exposure Testing

Program," Report No. LA 9735 MS, Los Alamos National Laboratory, Los Alamos, New Mexico, 1983.

39. Streed, E. R. , Thomas, W. C, Dawson, A. G., Ill, Wood, B. D., and Hill, J. E., "Results and

Analysis of a Round-Robin Test Program for Liquid-Heating Flat-Plate Solar Collectors," NBS

Technical Note 975, National Bureau of Standards, Washington, DC 20234, August 1978.

126
40. Thomas, W. C, "Solar Collector Test Procedures: Development of a Method to Refer Measured

Efficiencies to Standardized Test Conditions," Report VPI-E-80.23, Virginia Polytechnic

Institute and State University, Blacksburg, VA 24061, November 1980.

41. Cheng, H. and Bannerot, R. B., "On the Weathering of Thin Plastic Films," ASME Paper 8I-WA/S0I-6,

American Society of Mechanical Engineers, New York, NY 10017, November 1981.

42. Douro, D. D., "Investigation of Performance Degradation and Test Methods for Flat-Plate Solar

Collectors," M.S. Thesis, Mechanical Engineering Department, Virginia Polytechnic Institute and

State University, Blacksburg, VA 24061, June 1982.

43. Culkin, D. S., "Analysis of Flat-Plate Solar Collector Durability Test Data," M.S. Thesis,

Mechanical Engineering Department, Virginia Polytechnic Institute and State University,

Blacksburg, VA 24061, June 1982.

44. "Hourly Solar Radiation Surface Meteorological Observations," TD-9724, S0LMET, National Oceanic

and Atmospheric Administration (NOAA), Asheville, NC, 1979.

45. "Test Methods and Minimum Standards for Certifying Solar Collectors," ISCC Document 80-1,

Interstate Solar Coordination Council, December 1980.

46. "Test Methods and Minimum Standards for Certifying Solar Collectors," SRCC Standard 100-81,

Solar Rating and Certification Corporation, 1001 Connecticut Avenue, NW, Washington, DC, March

1982.

47. "Standard for Solar Collectors," ARI Standard 910-81, Air-Conditioning and Refrigeration

Institute, 1815 North Fort Myer Drive, Arlington, VA 22209, 1981.

48. "Intermediate Minimum Property Standards Supplement - Solar Keating and Domestic Hot Water

Systems," 1977 Edition, prepared for HUD by NBS. Available from GPO, Order No. SN 023-000-90161-7.

49. Birnbreier, H. , "Durability Tests by Stagnation Temperature Measurements," International

Energy Agency Task III meeting, Heidelberg, West Germany, December 1978.

50. Dawson, A. G. , III, "Stagnation Temperature Test Methods for Determining Solar Collector

Thermal Performance," Ph.D Dissertation, Mechanical Engineering Department, Virginia Polytechnic

Institute and State University, Blacksburg, VA 24061, June 1981.

51. Kirkpatrick, D. L., "Flat-Plate Solar Collector Performance Data Base and User's Manual,"

Report No. SERI/STR-254-1515, Solar Energy Research Institute, Golden, CO 80401, July 1983.

52. Clark, E. J. and Roberts W. E. , "Weathering Performance of Cover Materials for Flat Plate

Solar Collectors," NBS Technical Note 1170, National Bureau of Standards, Washington, DC,

20234, November 1982.

53. Yamasaki, R. S. and Blaga,, A., "Accelerated Weathering Test for Glass Fiber Reinforced

Polyester (GRP) Sheets," Durability of Building Materials and Components , ASTM STP 691,

P. J. Sereda and G. G. Litvan, Eds., American Society for Testing and Materials, 1980,

pp. 874-889.

54. "Standard Test Method for Water Penetration of Exterior Windows, Curtain Walls, and Doors

by Uniform Static Air Pressure Difference," ASTM E 331-70, American Society for Testing and

Materials, 1916 Race Street, Philadelphia, PA 19103.

127

APPENDIX A: SOLAR RADIATION EXPOSURE SUMMATIONS

Table A.l Exposure Summation for all Collectors

Table A.

Exposure Summation for Material Samples at Phoenix

Table A.Exposure Summation for Material Samples at Cape Canaveral

Table A.

Exposure Summation for Material Samples at Palo Alto

Table A.

Exposure Summation for Material Samples at Gaithersburg

129

Table A. 1. Co I I ector

ID

Exposure Summation for all Collectors (MJ/sq m)

3 15 30 Exposure Days*60 120 240 480

Tota I Days*Tota I Exposure

A- 1-1 82 414 775 1527 1 3293 6594 1 13118 I 485 1 13251 A-1-2 82 414 770 1525 3293 6592 13118 I 485 I 13250 A-1-3 86 411 774 1678 1 3244 6524 I 250 1 6805 A-l-4 80 399 775 1570 I 63 1 1639

B-1-1 81 412 772 1575 3305 6626 13151 I 488 I 13363 B-1-2 78 407 755 1563 3303 6624 13149 485 I 13282

B-1-3 86 411 774 1678 3244 6524 250 I 6805 B-1-4 80 399 775 15 70 63 1639 C-l-1 81 412 769 1525 3263 6591 13139 487 1 13324 C-1-2 78 406 756 1511 3277 6614 13129 484 1 13243 C-1-3 86 411 774 1678 3244 120 3244 C-1-4 80 399 775 31 800

D-1-1 81 408 772 1546 3314 6616 13141 485 13274 D-1-2 66 350 727 1628 3285 6559 468 12806

D-1-3 86 411 774 1678 3244 6524 250 6805

D-1-4 80 399 775 1570 63 1639

E-1-1 84 418 775 1579 3320 6638 13163 486 13322

E-l-2 84 418 775 1625 3328 6628 470 12904

E-1-3 86 411 774 1678 3244 6524 250 6805

E-1-4 80 399 775 1570 63 1639

F-1-1 84 418 778 1553 3288 6479 395 10787

F-1-2 84 418 778 1553 3288 6606 13131 485 13250

F-1-3 86 411 774 1678 3244 6524 250 6805

F-1-1) 80 399 775 1570 63 1639 C-1-1 84 418 778 1531 3300 6598 13122 486 13267 G-1-2 84 418 778 1535 3301 6602 13126 485 13245 G-1-3 86 411 774 1678 3244 6524 250 6805

G-l-4 80 399 775 1570 63 1639

H-1-1 81 413 774 1525 3221 6528 13054 486 1 3198 H-1-2 85 409 758 1520 3219 6525 13043 483 13117

H-1-3 86 411 774 1678 3244 6524 250 6805

H-1-4 80 399 775 1570 63 1639

A-2-1 77 414 841 1693 3336 6499 424 11313 A-2-2 77 414 801 1656 3299 6451 423 11249 A-2-3 72 356 743 1598 3241 6393 247 6581 A-2-4 81 413 799 59 1627 B-2-1 77 414 841 1700 3322 6423 415 11101 B-2-2 77 414 801 1658 3284 6379 415 11057

B-2-3 72 356 783 1642 3264 6348 250 6654

B-2-4 81 413 799 59 1635

C-2-1 77 414 840 1702 3379 6512 419 1 1214 C-2-2 77 414 840 1705 3292 6441 396 10527 C-2-3 72 356 783 1644 3318 6419 247 6650 C-2-4 86 412 831 57 1620

D-2-1 77 414 841 1676 3318 6455 420 11191 D-2-2 77 414 801 1638 3281 6416 419 11127

D-2-3 72 356 743 1581 3223 6358 247 6584

D-2-4 81 413 799 59 1610

E-2-1 77 414 841 1712 3322 6489 426 11369 E-2-2 77 414 801 1677 3283 6454 426 11322

E-2-3 72 356 743 1619 3225 120 3225 E-2-4 81 413 799 59 1655

F-2-1 77 414 841 1677 3317 234 6268

F-2-2 77 414 801 1639 3277 6381 405 10689

F-2-3 72 356 743 1581 3219 233 6146

F-2-4 81 413 799 59 1612 G-2-1 77 414 850 1728 3407 6563 423 11364 G-2-2 77 414 829 1701 3374 6534 422 1 1308 G-2-3 72 356 772 1643 3316 6476 248 6687

G-2-4 81 413 849 59 1698 H-2-1 77 414 841 1677 3303 6401 414 11055 H-2-2 77 414 801 1637 3265 6386 417 1 1088 H-2-3 72 356 743 1580 3209 234 6160

H-2-4 81 413 799 59 1612

A-3-1 65 435 867 1692 3537 6937 479 13680 A-3-2 65 435 867 1692 3537 6937 479 13680

B-3-1 78 362 708 1573 3147 6724 468 13137 B-3-2 72 356 702 1566 3140 6718 468 13131 C-3-1 78 362 708 1578 3184 6733 478 13419 C-3-2 72 356 702 1555 3129 6707 351 9564

D-3-1 65 435 873 1698 3544 6943 479 13687 D-3-2 65 435 873 1698

E-3-1 83 426 836 1707 3583 6668 422 10782

E-3-2 83 426 836 1707 3583 6668 422 10782

F-3-1 65 435 873 1698 3544 6943 479 13687 F-3-2 65 435 873 1698 3544 6943 479 13687

G-3-1 83 426 836 1707 3583 6668 430 12995 G-3-2 83 426 836 1707 3583 6668 430 12995

H-3-1 78 362 708 1573 3147 6724 468 13137 H-3-2 72 356 702 1566 3140 6718 468 13131 A-4-1 108 388 864 1702 3665 7049 361 11103 A-4-2 108 388 864 1702 3665 7049 361 11103

B-4-1 66 360 823 1694 3510 7458 460 14172 B-4-2 66 360 823 1694 3510 7458 460 14172

C-4-1 64 371 842 1802 3558 7375 425 13146 C-4-2 64 371 842 1802 3558 7375 425 13146

D-4-1 66 360 823 1694 3510 7458 460 14172

D-4-2 66 360 823 1694 3510 7458 460 14172

E-4-1 96 534 958 1875 3817 7193 361 11246 E-4-2 96 534 958 1875 3817 7193 361 1 1246

F-4-1 96 534 958 1875 3817 7193 361 11246

F-4-2 96 534 958 1875 3817 I 7193 1 361 1 1246 G-4-1 108 388 864 1702 3665 7049 361 1 1103 G-4-2 108 388 864 1702 3665 1 7049 361 ! 1 1103 H-4-1 64 371 842 1802 3558 1 7375 425 13146 H-4-2 64 371 842 1802 3558 7375 1 I 425 1 13146

Days with a Minimum Solar Radiation Level of 17,000 kj/sq m

131
Table A. 1 . Exposure Summation for all Collectors (Btu/sq ft) Exposure Days*

3 15 30 60 120 240

Co I I ector

ID 480 Tota I Days

Tota I Exposure

A-1-1 7288 36495 1 68259 134509 I 289982 I 580668 1 1155180 I 485 1 1166860 A-1-2 7288 36495 67886 134369 290000 I 580488 1 1155107 I 485 1 1166787 A-l-3 7612 36264 68183 147772 285675 I 574503 1 I 250 1 599264

A-1-4 7122 35169 68319 138256 I 1

I 63 1 144389

B-1-1 7159 36366 68012 138754 291059 I 583514 I 1158071 488 1 1176745

B-1-2 6893 35860 66520 137654 290871 583322 t 1157906 I 485 1 1169586

B-1-3 7612 36264 68183 147772 285675 574503 I 250 1 599264

B-1-4 7122 35169 68319 138256 I I 63 1 144389

C-1-1 7159 36366 67757 134369 287362 580429 I 1157029 487 1 1173271 C-1-2 6893 35834 66627 133107 288562 582411 1156098 484 I 1166112

C-l-3 7612 36264 68183 147772 285675 120 1 285675

C-1-4 7122 35169 68319 31 1 70467

D-1-1 7159 35989 68015 136159 291823 I 582609 1157193 1 485 1 1168873

D-1-2 5823 30874 64081 143390 289324 I 577597 1 468 1 1127655

D-1-3 7612 36264 68183 147772 285675 574503 250 1 599264

D-.1-4 7122 35169 68319 138256 63 1 144389

E-1-1 744 3 36886 68277 139086 292403 584559 1159098 486 1 1173081 E-1-2 7443 36886 68277 143126 293104 583677 470 1 1136334

E-1-3 7612 36264 68183 147772 285675 574503 250 I 599264

E-1-4 7122 35169 68319 138256 63 144389

F-1-1 7443 36886 68532 136761 289587 570586 395 949858

F-1-2 7443 36886 68532 136761 289587 581743 1156282 485 1166745

E-1-3 7612 36264 68183 147772 285675 574503 250 599264

F-1-4 7122 35169 68319 138256 63 144389

G-1-1 7443 36886 68532 134815 290602 581017 1 155501 486 1168223 G-1-2 7443 36886 68532 135229 290702 581388 1155871 485 11663 34 G-1-3 7612 36264 68183 147772 285675 574503 250 599264

G-1-4 7122 35169 68319 138256 63 144389

11-1-1 7159 36381 68241 134322 283658 574898 1 149503 486 1 162225 H-1-2 7498 36089 66749 133857 283484 574573 1 148572 483 1155066

H-1-3 7612 36264 68183 147772 285675 574503 250 599264

H-1-4 7122 35169 68319 138256 63 144389

A-2-1 6857 36487 74097 149119 293778 572275 424 996209 A-2-2 6861 36491 70535 145847 290526 568098 423 990531 A-2-3 6371 31402 65446 140 758 285437 56 3009 247 579571 A-2-4 7174 36388 70432 59 14 33 38 B-2-1 6857 36487 74097 149758 292546 565638 4 15 977500

B-2-2 6861 36491 70535 146077 289204 561784 415 973646

B-2-3 6371 31402 69012 144673 287461 559044 250 585994

B-2-4 7174 36388 704 32 59 143977

C-2-1 6857 36487 740 34 149898 297570 573490 419 987451 C-2-2 6861 36491 74038 150154 289945 567197 396 926953

C-2-3 6371 31402 68949 144813 292209 565304 247 585647

C-2-4 7612 36299 73223 57 142663

D-2-1 6857 36487 74097 147587 292246 568447 420 985492

D-2-2 6861 36491 70535 144315 288960 564973 419 979814

D-2-3 6371 31402 65446 139226 283871 559884 247 579808

D-2-4 7174 36388 70432 59 141806

E-2-1 6857 36487 74097 150763 292590 571414 426 1001165

E-2-2 6861 36491 70535 147689 289120 568372 426 996961

E-2-3 6371 31402 65446 142600 284031 120 284031 E-2-4 7174 36388 70432 59 145792

F-2-1 6857 36487 74097 147731 292123 234 552013

F-2-2 6861 36491 70535 144332 288574 561924 405 941252

F-2-3 6371 31402 65446 139243 283485 233 541246

F-2-4 7174 36388 70432 59 141950

G-2-1 6857 36487 74906 152180 300018 577986 423 1000657

G-2-2 6861 36491 73076 149817 297130 575364 422 995793

G-2-3 6371 31402 67987 144728 292041 570275 248 588872

G-2-4 7174 36388 74807 59 149583

H-2-1 6857 36487 74097 147731 290910 563694 414 973528

H-2-2 6861 36491 70535 144230 287580 562396 417 976371

H-2-3 6371 31402 65446 139141 282569 234 542459

H-2-4 7174 36388 70432 O 59 141950

A-3-1 5794 38379 76371 148998 311492 610851 479 1204670 A-3-2 5794 38379 76371 148998 31 1492 610851 479 1204670

B-3-1 6954 31958 62374 138523 277122 592133 468 1156794

B-3-2 6411 31415 61831 137980 276579 591590 468 1 156251 C-3-1 6954 31958 62374 138980 280432 592933 478 1181613 C-3-2 6411 31415 61831 137007 275606 590617 351 842229

D-3-1 5794 38379 76951 149578 3 12072 61 1431 479 1205250

0-3-2 5794 38379 76951 149578 312072 6114 31 479 1205250

E-3-1 7392 37517 73633 150336 315580 587199 422 949447

E-3-2 7392 37517 73633 150336 315580 587199 422 949447

F-3-1 5794 38379 76951 149578 312072 61 1431 479 1205250

F-3-2 5794 38379 76951 149578 312072 6114 31 479 1205250

G-3-1 7392 37517 73633 150336 315580 587199 430 1 144325 C-3-2 7392 37517 73633 150336 315580 587199 430 1144325

H-3-1 6954 31958 62374 138523 277122 592133 468 1156794 H-3-2 6411 31415 61831 137980 276579 591590 468 1156251

A-4-1 9524 34227 76130 149950 322755 620736 361 977752

A-4-2 9524 34227 76130 149950 322755 620736 361 977752

B-4-1 5820 31735 72488 149222 309135 656724 460 1247989

B-4-2 5820 31735 72488 149222 309135 656724 460 1247989

C-4-1 5713 32752 74214 158700 313355 649423 425 1157650 C-4-2 5713 32752 74214 158700 313355 649423 425 1 157650

D-4-1 5820 31735 72488 149222 309135 656724 460 1247989

D-4-2 5820 31735 72488 149222 309135 656724 460 1247989

E-4-1 8493 47059 84388 165130 336187 633416 361 990331 E-4-2 8493 47059 84388 165130 336187 63 34T6 361 990331

F-4-1 8493 47059 84388 165130 336187 633416 I 361 9903 31 F-4-2 8493 47059 84388 165130 336187 633416 j 361 9903 31 G-4-1 9524 34227 76130 149950 322755 620736 I 361 1 977752

G-4-2 9524 34227 76130 149950 322755 620736 I 361 1 977752

H-4-1 5713 32752 74214 158700 313355 649423 I 425 1 1157650

H-4-2 5713 32752 74214 1 158 700 I 313355 I 649423 1 I 425 1 1 157650

* Days with a Minimum Solar Radiation Level of 1,500 Btu/sq ft

132
Table A. 2. Exposure Summation for Material Samples at Phoenix

A. Cover Samples Miniboxes

Exposure Schedule: Started 5- 8-79 Ended 11-18-80

( 2-12-80 through 3- 3-80 Excluded)

Ca I enda r Exposure Date Tota I So I a r Rad i a t ion

Days Days* Btu/sq ft MJ/sq m

85 80 7-30-79 194587 2209

170 160 10-23-79 371472 4218

283 240 3- 5-80 565881 6426

372 320 6- 2-80 759847 8629

454 400 8-23-80 953122 10823

540 479 1 1-17-80 1127756 12807

B. Absorber Sample Coupons AC12 - AH12, AJ12, AL12, & AP12

Exposure Schedule: Started 6- 4-79 Ended 6-14-81

( 3-24-80 through 8-28-80 Excluded)

Ca I enda r Exposure Date Tota I So I a r Rad i at i on Days Days Btu/sq ft MJ/sq m 82 80 8-24-79 188493 2140

170 160 11-20-79 367469 4173

292 240 3-21-80 565438 6421

381 320 1 1-23-80 743729 8446

487 400 3- 9-81 934281 10609

576 480 6- 6-81 1133008 12866

584 488 6-14-81 1153849 13103

C. Absorber Sample Coupons AI11, AA12, AM12, AN12, AN13, & AA15 - AAP15

Exposure Schedule: Started 6- 4-79 Ended 6-14-81

( 3-24-80 through 1- 3-81 Excluded)

Ca I enda r Exposure Date Tota I So I a r Rad ia t i on Days Days Bt u/sq ft MJ/sq m 82 80 8-24-79 188493 2140

170 160 11-20-79 367469 4173

292 240 3-21-80 565438 6421

397 320 4-16-81 764755 8684

456 376 6-14-81 902626 10250

* Days with a Minimum Solar Radiation Level of 17,000 kJ/sq m

133
Table A. 3. Exposure Summation for Material Samples at Cape Canaveral

A. Cover Sample Miniboxes

Exposure Schedule: Started 4- 6-79 Ended 2-10-81

Ca I enda r Exposure Date Tota I So I a r Rad i a t i Days Days* Btu/sq ft MJ/sq m

97 80 7-11-79 188013 2135

199 160 10-21-79 373420 4240

328 240 2-27-80 571479 6489

435 320 6-13-80 770512 8750

528 400 9-14-80 951532 10805

645 480 1- 9-81 1 141726 12965

677 500 2-10-81 1194989 13570

B. Absorber Sample Coupons

Exposure Schedule: Started 5- 2-79 Ended 10-24-81

( 4-14-8O through 12-20-80 Excluded)

Ca I enda r Exposure Date Tota 1 So 1 a r Rad i at i Days Days Btu/sq ft MJ/sq m

93 80 8- 2-79 18281

1

2076

203 160 11-20-79 372875 4234

330 240 3-26-80 571779 6493

455 320 4- 6-81 781735 8877

541 400 7- 1-81 967635 10988

638 480 10- 6-81 1149935 13059

656 494 10-24-81 1182464 13428

* Days with a Minimum Solar Radiation Level of 17,000 kJ/sq m

134
Table A. 4. Exposure Summation for Material Samples at Palo Alto

A. Cover Sample Mini boxes

Exposure Schedule: Started 4- 6-79 Ended 7-28-81

Ca I enda r Exposure Date Tota 1 So 1 a r Rad i a t i on Days Days* Btu/sq ft MJ/sq m 88 80 7- 2-79 200822 2280

172 160 9-24-79 390452 4434

342 240 3-12-80 612243 6952

437 320 6-15-80 803841 9128

519 400 9- 5-80 989828 1 1240

666 480 1-30-81 1196403 13586

782 560 5-26-81 1399242 15890

845 623 7-28-81 1549724 17599

B. Absorber Sample Coupons

Exposure Schedule: Started 5- 2-79 Ended 7-28-81

( 5-22-80 through 5-29-80 Excluded)

Ca I enda r DaysExposure

Days

Date Tota I So la r Btu/sq ftRad i at i on MJ/sq m

83 80 7-23-79 199394 2264

178 160 10-26-79 391549 4446

341 240 4- 6-80 609729 6924

433 320 7-15-80 803517 9124

517 400 10- 7-80 981376 11144

690 480 3-29-81 1207844 13716

776 560 6-23-81 13981 19 15877

811 595 7-28-81 1481065 16819

* Days with a Minimum Solar Radiation Level of 17,00 kj/sq m

135
Table A. 5 Exposure Summation for Material Samples at Ga i thersburg

A. Cover Sample Mini boxes

Exposure Schedule: Started 5- 5-79 Ended 6- 8-81

Ca I enda r Exposure Date Tota I So I a r Rad i at i Days Days'* Btu/sq ft MJ/sq m

130 80 9-11-79 213024 2419

320 160 3-19-80 443133 5032

443 240 7-20-80 645341 7328

575 320 11-29-80 848138 9631

766 397 6- 8-81 1083173 12300

B. Absorber Sample Coupons

Exposure Schedule: Started 5- 5-79 Ended 6- 8-81

( 8-15-80 through 12- 5-80 Excluded)

Total Solar Radiation

Btu/sq ft MJ/sq m 213024 2419

443133 5032

645341 7328

875453 9941

918297 10428

* Days with a Minimum Solar Radiation Level of 17,000 kj/sq m

Ca I enda r Exposure Date

Days Days

130 80 9-11-79

320 160 3-19-80

443 240 7-20-80

622 320 5- 8-81

653 336 6- 8-81

136
NBS-114A (REV. 2-80) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS/TN-1196

2. Performing Organ. Report No 3. Publ ication Date

September 1984

4. TITLE AND SUBTITLE

NBS Solar Collector Durability /Reliability Test Program: Final Report

5. AUTHOR(S)

David Waksman, William C. Thomas, Elmer R. Streed

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Office of Solar Heat Technologies

U.S. Department of Energy

Washington, DC 20585

10. SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, 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) Efforts in the development of reliability/durability tests for solar collectors and

their materials have been hampered by the lack of real time and accelerated degradation data that can be correlated with in use conditions. The focus of this report

is on research undertaken at the National Bureau of Standards (NBS) to help generate

the data required to develop methods for predicting the long term durability and

reliability of flat-plate solar collectors and their materials.

In this research, eight different types of flat-plate solar collectors were exposed

outdoors at four sites located in different climatic regions. Small scale cover and

absorber materials coupon specimens consisting of samples taken from a collector of

each of the eight types used and a number of additional materials were exposed con- currently with the full-size collectors. Periodic measurements were made of collector and materials performance as a function of outdoor exposure time. Indoor laboratory aging tests were conducted concurrently on specimens of the same materials to

provide a basis for comparison with the outdoor exposure tests.

This report presents the results obtained in this test program. Recommendations are made regarding the use and limitations of performance measurements and environmental

exposure tests for assessing the durability of solar collectors and absorber and cover materials.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

absorber materials; accelerated aging; cover materials; durability; environmental

exposure; solar collectors; solar materials, stagnation testing, thermal performance,

13. AVAILABILITY

[Xj Unlimited

| | For Official Distribution. Do Not Release to NTIS

[Xl Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

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

14. NO. OF

PRINTED PAGES

148

15. Price

USCOMM-DC 6043-P80

*U.S. GOVERNMENT PRINTING OFFICE : 1984 0-420-997/10038

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