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•'brERENCE Publications

NBSIR 84-2845

Test Methods and Standards

Development for Active Solar

Heating and Cooling Systems

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Washington, DC 20234

May 1 984

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

LC Active Heating and Cooling Division

130 Washington, DC 20585

.1155

84-2345

1934

-J

NBSIR 84-2845

TEST METHODS AND STANDARDS

DEVELOPMENT FOR ACTIVE SOLAR

HEATING AND COOLING SYSTEMS

JfATU OF

Library

ia.tr

qC

i

Heinz R. Trechsel 1 Belinda L. Collins2

1 H. R. Trechsel Associates

P.O. Box 211

Germantown, MD

2 U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Washington, DC 20234

May 1984

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

ABSTRACT

Since test methods and standards for active solar heating and cooling systems

did not exist in 1976, the Department of Energy sponsored research at the

National Bureau of Standards (NBS) and other laboratories to aid in the development of research-based standards. This research was intended to facilitate a sound data base for the development of national consensus standards and test

methods. In the present report, research by NBS and other laboratories is

described for solar domestic hot water systems, solar collectors, thermal

storage devices and collector materials. For collectors, the report describes

research and test methods for determining the performance of cover plates,

absorber materials, collector insulation, gaskets and sealants, rubber hose,

containment materials, and heat transfer fluids.

Key words: absorbers; collectors; materials; consensus standards; domestic hot

water; durability; heat transfer fluids; reliability; solar energy;

test methods; thermal performance; thermal storage devices.

iii
ACKNOWLEDGMENTS

The authors gratefully acknowledge the project guidance and review comments

provided by Robert Dikkers, Research Coordinator, Solar Technology, National

Bureau of Standards (NBS). The authors also thank Larry Masters, Joseph Greenberg,

and Stanley Liu, NBS, for their helpful review comments. The support of this

work by the Active Heating and Cooling Division, Office of Solar Heat Technologies,

U.S. Department of Energy is also appreciated.

iv
TABLE OF CONTENTS

Page

ABSTRACT iii

ACKNOWLEDGMENTS iv

LIST OF FIGURES viii

LIST OF TABLES xi

1 . INTRODUCTION 1

1.1 BACKGROUND 1 1.2 SCOPE 3

1.3 DEFINITION OF STANDARDS 3

2. SOLAR HOT-WATER HEATING SYSTEMS 5

2.1 THERMAL PERFORMANCE 5 2.2 ASHRAE STANDARD 95-81 7 2.3 EXPERIMENTAL VALIDATION OF ASHRAE 95-81 8

3. THERMAL STORAGE DEVICES 12

3.1 THERMAL PERFORMANCE 12

3.2 ASHRAE STANDARD 94-77 14

3.3 EVALUATION OF ASHRAE STANDARD 94-77 16

4. SOLAR COLLECTORS 20

4.1 INTRODUCTION 20

4.2 THERMAL PERFORMANCE OF SOLAR COLLECTORS 21

4.2.1 Initial Evaluation and Standards Development 21

4.2.2 NBS Research 21

4.3 ASHRAE STANDARDS FOR THERMAL PERFORMANCE 22

4.3.1 ASHRAE 93-77 22

4.3.2 ASHRAE 96-80 30

4.4 VERIFICATION AND EVALUATION OF ASHRAE STANDARD PROCEDURES 30

4.4.1 Round-Robin Test Using Interim Procedures 30

4.4.2 Evaluation of ASHRAE 93-77 36

4. 4. 2.1 NBS Test Facility and Research 36

4. 4. 2.

Comparison of ASHRAE and BSE Procedures 40

4. 4. 2.

Evaluation of Unglazed Flat-Plate Collectors ... 46

4.5 DURABILITY AND RELIABILITY OF SOLAR COLLECTORS 48

4.5.1 Proposed Evaluation Criteria 48

4.5.2 Durability Research Program 53

4. 5. 2.1 Research Plan 53

4. 5. 2.

Statistical Uncertainty in Thermal Performance

Data 57

4.6 FLAMMABILITY OF SOLAR COLLECTORS 65

v
TABLE OF CONTENTS

Page

5. COLLECTOR MATERIALS RESEARCH 69

5.1 COVER PLATES 69

5.1.1 Introduction 69

5.1.2 NBS Research on Cover Plates 69

5. 1.2.1 Calculation of Solar Transmittance 71

5. 1.2. 2 Determination of Impact Resistance 71

5.1.3 ASTM Standards 72

5. 1.3.1 ASTM E 765-80 72

5. 1.3. 2 ASTM E 782-81 73

5. 1.3. 3 ASTM E 881-82 75

5. 1.3. 4 ASTM E 822-81 75

5. 1.3. 5 Proposed Revisions to ASTM E424-71 77

5.2 ABSORBER MATERIALS

5.2.1 Introduction 77

5.2.2 NBS Research 78

5.2.3 ASTM Standards 80

5. 2. 3.1 ASTM E 744-80 80

5. 2. 3. 2 ASTM E 781-81 81

5.3 COLLECTOR INSULATION 83

5.3.1 Introduction 83

5.3.2 NBS Research 84

5.3.3 ASTM E 861-82 85

5.4 GASKETS AND SEALANTS 87

5.4.1 Introduction 87

5.4.2 NBS Research 87

5.4.3 ASTM Standards 89

5. 4. 3.1 ASTM D 3667-78 89

5. 4. 3. 2 ASTM D 3771-79 91

5. 4. 3. 3 ASTM D 3832-79 94

5. 4. 3. 4 Proposed test method for outgassing 94

5. 4. 3. 5 Proposed recommendations for sealing joints with

rubber seals 96

5.5 RUBBER HOSE 96

5.5.1 Introduction 96

5.5.2 NBS Research 97

5.5.3 ASTM Standards 102

5. 5. 3.1 ASTM D 3952-80 102

5. 5. 3. 2 Proposed Revision to ASTM Standard D 3952-80 ... 102

5.6 CONTAINMENT MATERIALS AND HEAT TRANSFER FLUIDS 105

5.6.1 Introduction 105

5.6.2 NBS Research 105

5.6.3 ASTM Standards 117

5. 6. 3.1 ASTM E 712-80 117

5. 6. 3. 2 ASTM E 745-80 119

5. 6. 3. 3 ASTM E 862-82 121

vi
TABLE OF CONTENTS

Page

6. CONCLUSIONS 122

6.1 SUMMARY 122

6.2 RESEARCH NEEDS 122

7. REFERENCES 125

vii
LIST OF FIGURES

Page

1. Schematic diagram of a solar domestic water heating system 6

2. Hourly hot water draw profile used in the TRNSYS simulation 6

3. Correlation of solar fraction for the solar domestic water heating

system for Santa Maria, CA 6

4. Single-tank direct system schematic 9

5. Solar hot water system with the irradiated collector array replaced

with an electric heat source 9

6. Solar hot water system with the irradiated collector array replaced

with a non-irradiated collector array with downstream heat source .. 10

7. Representative test configuration for a thermal storage device

using air as the transfer fluid 15

8. Time-temperature variation of transfer fluid during a charge/

discharge test cycle 15

9. Suggested plot of transfer fluid temperature 17

10. Schematic representation of the two basic methods for determining

solar collector thermal efficiency 23

11. Testing configuration for the solar collector when the transfer

fluid is a liquid 26

12. Testing configuration for the solar collector when the transfer

fluid is air 26

13. Efficiency curves for two flat-plate collectors using air as the

transfer fluid 27

14. Open-loop testing configuration for the solar collector when heattransfer fluid is liquid and fluid is not supplied continuously .... 29

15. Open-loop testing configuration for use when fluid is supplied

continuously 29

16. Climatic location of testing facilities 32

17. Schematic of flat-plate liquid-heating collector No. 1 32

18. Schematic of flat-plate liquid-heating collector No. 2 33

viii
LIST OF FIGURES

Page

19. Frequency distribution of F’CioOe values for round-robin

collectors . 33

20. Uncorrected results from 12 facilities for collector No. 1 tests ... 35

21. Uncorrected results from 10 facilities for collector No. 2 tests ... 35

22. Solar collector testing equipment being raised to ground level at

NBS, Gaithersburg, MD 37

23. Solar collector testing equipment in place at NBS, Gaithersburg, MD 37

24. Schematic diagram of an NBS test loop for liquid-heating solar 38

collectors

25. Schematic of the NBS test stand for air-heating solar collectors ... 38

26. Collector efficiency using BSE procedure vs. the difference

between collector mean fluid temperature, tm , and ambient air

temperature, t a 44

27. Closed loop testing configuration for comparing BSE and ASHRAE

test methods 44

28. Schematic of axial fan wind simulator 45

29. Environmental simulators 45

30. Schematic of fluid test loop used in experimental work on unglazed

flat-plate liquid collectors 47

31. Diagrams of unglazed collector 1 and 2 absorbers 47

32. Comparison of ASHRAE standard 96-80 and modified BSE efficiency

curves (for collector 1) 49

33. Uncertainty associated with the ASHRAE-93-77 efficiency

measurements for a single-glazed selective absorber-type

collector 63

34. Collector mounting configurations 67

35. Schematic of fire test apparatus 67

36. Sketch of mini-collector (weathering box) for natural weathering

exposure 76

ix
LIST OF FIGURES

Page

37. Outline of test method options 82

38. Schematic of circulatory system for performance test apparatus 98

39. Pressure vessel for immersion tests at 150°C 106

40. Schematic of apparatus used in test B 108

41. Isometric drawing of apparatus used in test D 109

42. Schematic drawing of apparatus for test F 109

43. Schematic of the apparatus used to study the degradation of

ethylene and propylene glycol 110

44. Schematic of simulated service loop system Ill

45. Plots of solution pH vs. time for aerated solutions of ethylene and

propylene glycol at 100°C 113

46. Plots of solution pH vs. time for deaerated solutions of ethylene

and propylene glycol in contact with aluminum and copper at 100°C .. 114

47. Plots of solutions pH vs. time for aerated solutions of ethylene and

propylene glycol in contact with aluminum and copper at 100°C 114

48. Outline of testing sequence for screening plastic containment

materials 115

x
LIST OF TABLES

Page

1. Recommended Priorities for the Development of Test Methods for

Active Solar Systems 2

2. Description of Collectors Used in Comparison of BSE and ASHRAE

Test Methods 41

3. Environmental Test Conditions Allowed Within the ASHRAE 93-77 and

BSE Collector Test Procedures 43

4. Typical Computation of All-Day Solar Efficiency 51

5. Summary Description of Field Test Series on Solar Collectors 55

6. Exposure Tests for Cover Materials 56

7. Exposure Tests for Absorber Materials 56

8. Solar Collector Test Specimen 58

9. Cover Test Materials 58

10. Absorber Test Materials 59

11. Test Collector Specimen Description 60

12. Standard Deviation and Coefficient of Variation of Intercept

Within and Between Test Sites 62

13. Standard Deviation and Coefficient of Variation of Slope Within

and Between Test Sites 62

14. Incident Angle Modifier Standard Deviation Values Within and

Between Test Sites 64

15. Fire Test Collector Description 66

16. Flow Chart of Sequence for Testing Response to Aging 74

17. Exposure Test Conditions 82

18. Tests and Procedures for Evaluating Thermal Insulation Materials for

Solar Collectors 86

19. Results of Outgassing Tests for Materials Used to Seal Flat-Plate

Solar Collectors 90

xi
LIST OF TABLES

Page

20. Requirements for Class PS Materials Used to Seal Flat-Plate Solar

Collectors 92

21. Requirements for Class SC Material Used to Seal Flat-Plate Solar

Collectors 93

22. Elongation Requirements for Rubber Seals In Liquid Heat-Transport

Systems 95

23. Other Requirements for Rubber Seals in Liquid Heat-Transport

Systems 95

24. Hose Dimensions, Specimen Failures, and Leaks at Clamps 98

25. Compression Set 100

26. Effect of Hose On Metal Plugs In Water Vapor Transmission Tests .... 100

27. Bursting Pressures 101

28. Requirements for Tube/Cover Assembly 103

29. Requirements for Composite Hose 104

30. Summary of Test Results 108

31. Summary of Simulated Service Test Conditions 110

32. Summary of Simulated Service Test Results 115

33. Plastic Containment Materials Studied by NBS 116

34. Summary of Active Solar System, Component, and Materials Standards

and Related Research 123

xii
I . INTRODUCTION

1.1 BACKGROUND

The purpose of voluntary consensus standards for solar heating and cooling

applications is to provide industry and government with a technical basis for:

• Measuring and comparing performance (thermal, durability, safety),

• Evaluating new and advanced materials, components, and systems,

• Selecting and specifying materials, components and systems, and

• Establishing rating and certification programs.

The development of consensus standards, which include test methods, recommended

practices, and specifications, for solar applications is complex. It involves

the interaction of many different entities, from the Federal government to

lending institutions to consumers, as well as designers and researchers. As a result, in 1976, the National Bureau of Standards (NBS) recommended the development of an ANSI (American National Standards Institute) Steering Committee

on Solar Energy Standards Development. The purpose of this committee as established was to identify needs and formulate specific tasks leading to the development of national consensus standards for using solar energy for heating and

cooling. The committee also had the responsibility of assigning standards

development activities to standards-writing organizations and maintaining an overview of their activities to avoid duplication of effort or conflicting

standards

In a related activity, the National Bureau of Standards (NBS), under sponsorship

of the Energy Research and Development Administration (ERDA), initiated a planning exercise for the development of standards for solar heating and

cooling. This plan, published by NBS in 1976 [1]^ and revised in 1978 [2], was designed to identify needed standards and to coordinate the development

and implementation of voluntary consensus standards on a systematic basis.

In addition, a Federal government program was undertaken to develop research

data and technical information which could provide a sound basis for developing

national, voluntary consensus standards. This program involved NBS, standards

developing organizations such as American Society for Testing and Materials

(ASTM), American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) , as well as industry and consumers. The ANSI steering committee,

which represented at least 22 organizations, approved the original NBS standards

development plan given in [1], and revised in [2], The plans attempted to

identify those standards known to require further development as well as to

assign priorities for their development. Recommended practices for the development of thermal performance and durability/reliability test procedures are given in table 1. This table does not list applicable standards which did

1 Refers to numbers for the references given at the end of the text.

1
Table

1.

Recommended

Priorities

for

the

Development

of

Test

Methods

for

Active

Solar

Systems

(from

Ref.

2) 5o

iJ * 2

In

descending

order,

priorities

are:

High-1,

High-2,

Higli-3,

Medium

or
not appear to need modification. The plans also suggested the likely responsible

organization and an estimated time schedule.

1 .2 SCOPE

Since 1976-78 when the planning process was initiated, a large body of research

has been accomplished. Where possible, much of this research has been translated into voluntary standards, often at an accelerated pace, since many of the

research reports also contained draft standards that could be immediately

considered for consensus approval. The present report will identify those areas

listed in the planning document where standards were actually promulgated, and

document the research that led to their development or validation. Although

considerable research was accomplished at a number of different national laboratories and research institutions, the present report will focus on research at

the National Bureau of Standards. (Companion reports are currently being

developed to document research performed elsewhere.) The NBS research efforts

generally followed the following framework:

• Identification of the performance requirements.

• Identification and assessment of existing test methods.

• Development of new test methods, as needed for evaluation.

• Performance of laboratory and field research to evaluate the

effectiveness of the test methods in predicting performance.

• Preparation of a draft standard for consideration as consensus standard,

based on data and test method evaluation.

• Submittal of draft standard to relevant consensus standard organization.

• Validation of provisions in standard by further research, as needed.

The present report documents the research efforts that led to the development

of at least 21 consensus standards for solar heating and cooling applications

during the period 1976-78 and 1982. These standards were developed for evaluating the thermal performance of domestic hot water systems, thermal storage

devices, and solar collectors. In addition, standards were developed to evaluate the performance of individual materials used in collectors, such as cover

plates, absorbers, insulation, gaskets and sealants, rubber hose, and containment materials. Each research project is discussed in detail to provide a synopsis of the procedure and results. The resulting standard is then reviewed

to demonstrate how the research procedures were translated into standard testing and evaluation procedures. As a result, the document provides a ready

reference for someone interested in the evaluation procedures, test results,

and voluntary standards developed as a result of NBS research on solar energy

applications. The NBS research also was successful in establishing valid,

reliable evaluation procedures which were suitable for consideration as consensus standards. The success of this research can be seen in the number of

standards and speed with which they were adopted.

1.3 DEFINITION OF STANDARDS

While the American Society for Testing and Materials (ASTM) defines four major

types of standards, the standards of interest in the present report are those

related to test methods. ASTM defines a "test method" as the following (ASTM,

3
1977) [3]: "A form of standard that covers sampling and describes the subsequent testing procedures used in determining the properties, composition, or performance for materials, products, or services that may be specified. A test

method shall not include the kind of numerical limits for the properties, com- position, or performance that should normally be included in a specification."

A "specification" sets forth a set of requirements that a material, product,

etc. should satisfy, while a "classification" defines an arrangement of objects

into groups with similar characteristics. Finally, a "definition" sets forth

the meaning of terms used in the standards. ASTM also defines a "practice" as

a "procedure guide or service which may or may not be auxiliary to a test

method or specification."

Such standards are typically developed through the voluntary consensus process

which uses the consensus of a broad range of interested parties (government,

industry, producers, consumers, institutions, and individuals) to ensure that

the standards will have widespread acceptance and use. Thus ASTM defines a consensus standard as: "A standard produced by a body selected, organized, and

conducted in accordance with the procedural standards of 'due process'. In

standards development practice, a consensus is achieved when substantial

agreement is reached by concerned interests according to the judgment of a duly-appointed review authority" [Ref. No. 3, pg.8] . For further information

about standards developing bodies, the reader is referred to NBSIR 78-1 143A

[2], which delineates the various organizations involved in the development

of building codes and standards. For the present report, the primary

standards-developing organizations of interest include ASHRAE, ASTM, and ANSI.

4
2. SOLAR HOT-WATER HEATING SYSTEMS

2 . 1 THERMAL PERFORMANCE

Some of the first commercial applications of solar technology included solar

domestic hot water systems. For example, such water heaters were introduced in

Florida in 1923. During their peak use (1936 to 1941), an estimated 60,000

solar units were in service—more than twice as many solar units as electric

[4], However, lower energy costs, increased hot water use, utility company

promotion, and poor performance, particularly in the area of durability, caused

a sharp decline in the use of solar domestic water heaters in the 40s and 50s

[4], With increased energy costs after the oil embargo in 1973, and with the

development of new technology and materials, solar domestic hot water systems were again considered the most economically competitive of all the possible

solar energy applications [5,6]. The increasing interest in solar hot water

systems led ASHRAE to form Standards Project Committee 95 in 1977. Because

solar domestic hot water systems tend to be designed, assembled, and sold as packaged systems, this committee was charged with developing standard test

methods for the thermal performance of the whole system. NBS conducted research

which supported the activities of this committee.

An initial NBS research effort was the analysis of the thermal performance of

typical solar domestic hot water systems through simulations using the University of Wisconsin computer program TRNSYS [7,8]. The major objective of this

analysis was to determine the extent to which short-term outdoor exposure tests

could be used to predict long-term performance.

The results of the initial effort were published by Liu and Hill [9]. Figure 1 shows the diagram of the solar domestic hot water system, including the flatplate collectors used in the simulation. Figure 2 provides the hot water draw

profile used in the TRNSYS simulations. These figures show that the thermal performance of the domestic hot water heater studied could be adequately described

by a plot of solar fraction f (fraction of hot water load provided by solar

energy) against F (a combination of variables which include primary thermal

characteristics of the collector, incident solar radiation, ambient air temperature, and hot water load). The results shown in figure 3 indicate that the

correlation for monthly and yearly periods is adequate, but not very satisfactory for daily values. The types of correlations used were shown to be valid

for an actual in-service installation.

The main conclusion drawn from the exploratory computer simulation was that

actual outdoor exposure could be used for predicting long-term thermal performance, but that relatively long exposure periods (10 to 20 days) would be

required. As a result, the ASHRAE project committee decided to give prime

consideration to alternate testing procedures that could be conducted entirely

within a laboratory [10]. The continuing NBS technical support for the development of a testing standard for rating solar domestic hot water systems is

described in [10] and [11].

5
PYRANOMETER

• TEMPERATURE SENSOR

FLOW METER

Figure 1. Schematic diagram of a solar domestic water heating system

(from Ref. 9)

Figure 2. Hourly hot water draw profile used in the TRNSYS simulation

(from Ref. 9)

Figure 3. Correlation of solar fraction for the solar domestic water heating

system for Santa Maria, CA (from Ref. 9)

6
Concurrent with the exploratory study, an analytical model [11] was developed

to predict the thermal performance of a solar domestic hot water system under

prescribed conditions. Each component of the system (i.e., solar collectors,

storage tank, heat source, piping, etc.) was modeled, and equations for

calculating relevant thermal parameters were developed. Parameters, such as the collector efficiency, useful energy delivered to the storage tank, and

daily fractional energy savings, were predicted by the model and compared

with results obtained from testing a single tank direct system for four selected

days. Analytical results from the model were in close agreement with the

experimental results.

2.2 ASHRAE STANDARD 95-81

Formed in 1977, ASHRAE Project Committee 95-P evaluated different methods of

measuring the thermal performance of solar domestic hot water systems. The

first draft standard was published in 1979, with the final standard (95-81)

being promulgated in March 1981 [12]. Early in the development of the standard, the ASHRAE project committee considered the following options for

laboratory test procedures:

a. Irradiate the solar collector panels of the water heater with a solar simulator as specified in ASHRAE 93-77 [13]. (Although

this option was included in standard 95-81 , it was not anticipated

that this option would be used frequently, since solar simulators

were not widely available. NBS did not evaluate this option.)

b. Determine the thermal performance of the irradiated solar collectors

using ASHRAE 93-77, and replace the collector array with a conventional heat source. If the flow rate of the transfer fluid through

the collector during the systems test is different from the value

used in the collector test, the correction given in ASHRAE 95-81

must be made.

c. Determine the thermal performance of the irradiated solar collectors

and replace the irradiated array with a conventional heat source in

series with a nonirradiated array. The conventional heat source

can be located either upstream or downstream of the nonirradiated

array.

d. Determine the thermal performance of the irradiated array and

replace this array with strip heaters attached to a nonirradiated

solar collector. One advantage of this method is that it can be

readily used to test thermosyphon systems which cannot be tested

with procedures b or c. Because this procedure was proposed by

NBS after the standard had been approved, it has not yet been

incorporated into the standard.

The final version of the standard applies to laboratory tests for the thermal

performance of a domestic solar hot water system with a storage capacity of

0.45 m^ (120 gal) or use demands on the order of 0.38 m^ (100 gal) of hot water

per day. It does not specify the test conditions to be used for obtaining a

7
standard rating. The test procedure in the standard employing a nonirradiated

solar collector array in series with a conventional heat source does not apply

to integral collector storage systems or to thermosyphon systems. Test procedures are given for assessing the performance of three categories of solar

domestic hot water systems: solar only, solar-preheat, and solar-plussupplemental. In these tests, the system is tested until performance is the same for two sucessive days. Either the solar fraction, f, or the fractional

energy savings, e, are determined. The standard provides for tests using

solar simulators or tests using a conventional heat source in series with a nonirradiated collector array (after the array is tested following ASHRAE

93-77). Procedures are also given for determining the energy delivery capacity

of the system during continuous draw-down.

2.3 EXPERIMENTAL VALIDATION OF ASHRAE 95-81

NBS built an experimental apparatus to assess the proposed standard test methods

of ASHRAE 95-81 for solar domestic hot water heaters. This apparatus, which is

described in [11], contained both an irradiated system (figure 4) and one using

the thermal simulators mentioned above (figures 5 and 6).

In the experimental assessment, the outdoor irradiated system was operated

normally for several days, to serve as a baseline for comparison. Tests were

then conducted on the nonirradiated system with power supplied to the heat source as determined by analysis of the recorded irradiance and temperature

from the baseline operation. This procedure allowed a direct comparison of the

thermal performance of the two systems. In no case did the daily fractional

energy savings for the systems differ by more than three percent between the

measurements for the irradiated array and for the simulated energy input. Similar agreement was achieved in tests with an electric heat source only, and for

the nonirradiated array with attached strip heaters. Of particular concern

was the sensitivity of the test procedures to stratification within the storage

tank. Since this stratification is influenced by the tank design and dimensions, as well as the velocity and manner in which fluids are introduced into

the tank, several return-tube designs were tested. (The return-tube introduces

the solar heated water into the storage tank.) In these tests, the storage

tank itself and the flow rate were not altered. These tests showed that the

alteration of the return tubes produced different levels of stratification,

resulting in differences of up to 8.7 percent in fractional energy savings.

(Maximum stratification resulted in the greatest savings.) The tests also

showed that the test procedures of ASHRAE 95-81 are sensitive to the degree of

stratification which occurs in storage tanks.

An analytical model [14] was also used to determine the quantitative effect of

various test methods for the solar domestic hot water heaters. For tests

conducted on two different days, the analysis indicated that data from the

various test procedures (irradiated array, nonirradiated array with downstream

heat source, nonirradiated array with upstream heat source, and nonirradiated

array with strip heaters) were all in good agreement. The effect of wind speed

and thermal losses from the heat source were each found to be less than one percent .

8
Figure 4. Single-tank direct system schematic (from Ref. 11)

Figure 5. Solar hot water system with the irradiated collector array

replaced with an electric heat source (from Ref. 11)

9
INDOOR ENVIROMENT

POWER

INPUT

Figure 6. Solar hot water system with the irradiated collector array replaced

with a nonirradiated collector array with downstream heat source

(from Ref. 11)

10
In summary, both the analytical and the experimental work indicated that the

thermal performance of domestic solar hot water systems can be duplicated

under indoor laboratory conditions without solar simulators. The research

indicated further that outdoor stagnation conditions cannot be duplicated by

using an electric heat source only. Strip heaters must be attached to the

back of absorber plates within nonirradiated collector arrays, or an electric

source must be used with a nonirradiated collector array. The research also

indicated that the test methods prescribed in ASHRAE 95-81 are sensitive to

temperature stratifications within the storage tank. Finally, test result

repeatability was found to be excellent for the experimental technique using a nonirradiated collector array in series with a downstream heat source.

Recommendations made by NBS for future work included the development of

correlations of ASHRAE 95-81 results with meteorological conditions, the conduct of experiments to validate suggested test methods for thermosyphon systems,

and an investigation of the effects of liquid flow rates, with the objective

being to eliminate the need to use the same fluid in testing both the collector

and the entire system.

11
3. THERMAL STORAGE DEVICES

3.1 THERMAL PERFORMANCE

With support from the National Science Foundation (NSF) , NBS undertook the

development of standardized test procedures for thermal storage devices,

beginning in 1974. This effort continued in parallel with the development of

NBS-recommended procedures for evaluating the performance of solar collectors.

The NBS research, its development into an ASHRAE standard, and the experimental

evaluation of the effectiveness of this standard are outlined below, along with

some additional research performed by the Argonne National Laboratory.

Initially, NBS personnel reviewed the available literature on the analysis of

the performance of thermal storage devices and applicable test methods, and

developed a framework for evaluation. Thermal storage devices for solar heating and cooling systems can be classified as either sensible-heat or latentheat devices. In sensible-heat devices, the heat absorbed or removed from the

unit changes the temperature of the storage medium, with no change of phase.

Typical storage media include water (pressurized or unpressurized), rock,

brick, or concrete. In latent-heat devices, the heat (added or removed) changes

the enthalpy of the storage medium through a change of phase in the storage

medium—typically inorganic salt hydrates or organic materials. The choice of

thermal storage device is frequently related to the type of solar collector, so that storage devices for water-heating collectors are usually the sensible-heat

type, while a storage device for an air-handling collector could be a latent-heat device.

Performance of thermal storage devices is governed by:

1) Thermal capacity;

2) Operational temperature range;

3) Means of addition or removal of heat and the associated temperature

differences

4) Temperature stratification in the unit;

5) Power requirements for the addition or removal of heat;

6) Containers or other structural elements associated with the system;

7) Thermal losses;

8) Cost.

The emphasis in the NBS research program was placed on developing a testing

procedure for evaluating thermal storage devices on the basis of thermal

performance—as was also done with solar collectors. The test procedures for

thermal storage devices differ from those for solar collectors in one significant area. Because solar collector performance is largely determined by its

characteristics under steady or "quasi-steady"* state conditions, the ASHRAE

Quasi-steady is the term used to describe the state of the solar collector

test when the flow rate and temperature of the fluid entering the collector

is constant, but the exit temperature changes gradually due to the normal

change of insolation that occurs with time for clear sky conditions.

12
standard test is conducted under "quasi-steady" conditions. Performance of a thermal storage device, however, is determined by its characteristics under

transient operating conditions.

In the research program, NBS evaluated the effectiveness of three different

methods of determining the thermal performance of thermal storage devices

(both sensible and latent heat), used primarily in conjunction with solar

space heating, cooling, and domestic hot water supply.

The first, and most commonly used, method for evaluating the thermal performance

of storage devices is to change the temperature (by a known amount) of the

transfer fluid entering the device, and then measure the temperature of the

fluid leaving the device. By integrating this temperature difference over the

testing period and multiplying the result by the mass-flow rate and specific

heat of the transfer fluid, the amount of heat added or removed during the

testing period can be calculated.

In the second method, the heat transfer fluid is subjected to a constant influx

of heat and the time-dependent outlet temperature is measured. While this

method is a close simulation of the real interaction between a collector and a storage device, it does not allow measurement of the energy storage and removal

capacity of the unit.

In the third method, a time-varying influx of heat is applied to the transfer

fluid, and the outlet temperature is measured as a function of time. This

method simulates the output of a collector, and the response of the storage

device over several days. Neither method 2 or 3 allows ready comparison of

different devices, however.

While methods 2 and 3 simulate more closely the real interaction between solar

collectors and heat storage devices, they do not allow the measurement of the

energy storage and removal capacity of the units. As a result, the comparison

of the performance of different units is very difficult. Method 1 allows easy

comparison between units, and appears to be the most fundamental approach.

As a result, NBS devloped a test procedure which uses the approach outlined in

method 1 . This procedure specifies a series of tests for determining thermal

performance. This includes:

1) One test to determine a heat-loss factor for the storage unit;

2) A series of tests to determine the response characteristics of the

device to a step increase in the temperature of the entering fluid

(energy addition);

3) Another series of tests to determine the response to a step decrease

in the temperature of the entering fluid (energy withdrawal).

This procedure was contained in a draft test method [15,16,17] submitted by NBS

to ASHRAE for consideration as a voluntary consensus standard. At the same

time, the draft was submitted to a broad two-step review by experts during

NSF/RANN (Research Applied to National Needs) workshops at Colorado State University on August 23, 1974, and at Charlottesville, VA, on April 16-18, 1975.

13
3.2 ASHRAE STANDARD 94-77

In 1977, ASHRAE promulgated standard 94-77 (ANSI Standard B199.1, 1977) [18],

entitled "Methods of Testing Thermal Storage Devices Based on Thermal

Performance." The ASHRAE standard closely follows the format and technical

requirements of the NBS-developed draft.

ASHRAE Standard 94-77 applies to both sensible and latent-heat type storage

devices in which a transfer fluid enters the device through a single inlet and

leaves through a single outlet. It describes the required accuracy and precision, the apparatus and test configuration (see figure 7), the test procedures,

the data to be recorded, and the test report format. It also specifies the

instrumentation (such as thermopiles, thermometers, or thermistors) for measuring the temperature of the transfer fluid, the liquid or air flow, pressure,

and time and mass. Test procedures are given for both air and liquid as the

transfer fluid.

The test procedures consist of three elements:

a) A heat-loss rate test, to be performed before any other test;

b) Two charge tests; and

c) Two discharge tests.

These tests allow the determination of both the heat-loss rate and the overall

storage capacity. All tests require that the temperature of the storage

medium be uniform at the desired temperature, and that there be a steady flow

of transfer fluid through the storage system during the test.

For both the charge and discharge tests, the transfer fluid, at a constant

temperature, is passed through the storage device until the device is brought

to a uniform initial temperature. After adjusting the flow to the charge test

flow rate, the temperature of the transfer fluid entering the storage device

is then increased in a step-like manner, and maintained until the temperature

of the transfer fluid leaving the storage device no longer changes with time

(see figure 8). The standard recommends inlet temperature steps of 15°C (27°F).

During the charge and discharge test time, the differences between the temperatures of the transfer fluids entering and leaving the storage device is to be

recorded and integrated over time, and the charge or discharge capacities

determined as:

Cc wc

ct f /

(tj. nt ouj-)dx and

o

T d

^d = ^dCtf / (tin “ t out)dx

o

Where

Cc = Charge capacity J(Btu)

= Discharge capacity J(Btu)

14
DIFFERENTIAL PRESSURE MEASURING DEVICE

+ CALIBRATED DRY BULB TEMPERATURE MEASURING DEVICE

-H- CALIBRATED WET BULB TEMPERATURE MEASURING DEVICE

Figure 7. Representative test configuration for a thermal storage device

using air as the transfer fluid (test configuration for liquid

as transfer fluid is similar) (from Ref. 18)

TIME qn , INLET TEMPERATURE

«out. OUTLET TEMPERATURE

Cc a * cif A c * CHARGE CAPACITY

Cd-w c,f Ad» DISCHARGE CAPACITY :

' MASS FL

?

W RATE}oF TRANSFER FLUID

c

tf * SPECIFIC HEAT J

Tc = CHARGE TIME Td » DISCHARGE TIME

Figure 8. Time-temperature variation of transfer fluid during a charge/discharge test cycle (from Ref. 18)

15
Wc = Mass flow rate of transfer medium during charge test, kg/s (£bm per

hr)

W<j = Mass flow rate of transfer medium during discharge test, kg/s (£bm

per hr)

C t f = Specific heat of transfer fluid (J/kg°C), (Btu per £b°F)

t c = Charge test time, s(hr)

= Discharge test time, s(hr)

T = Time, s(hr)

The method also requires that the time variation in outlet temperature of the

transfer fluid be plotted as shown in the graph in figure 9.

3.3 EVALUATION OF ASHRAE STANDARD 94-77

NBS performed a number of experimental studies to evaluate provisions of ASHRAE

94-77 [19,20,21], In the first, a preliminary analytical model was developed;

in the second, procedures for testing water storage tanks were assessed; and

in the third, both a pebble-bed and a phase change material were evaluated.

In a preliminary report, Peavy and Dressier [19] evaluated sensible heat changes

in porous thermal storage material with fluid transpiring through it. Using the

test methods described in ASHRAE 94-77, they subjected a porous bed of sand,

with water flowing through it at a uniform temperature, to a sudden change in

the entering fluid temperature. The transient temperature distribution was determined until steady-state conditions were again achieved. The analytical

model which was developed for predicting the thermal performance of

transpiration energy storage units showed good agreement with the experimental

data.

In a second study, Hunt, Richtmyer, and Hill evaluated the thermal performance

of a water tank used as a thermal storage device [20]. These tests were con- ducted on a 1.9 m^ (500 gal) water tank built into a complete solar heating and

cooling system at NBS. The tests, which followed the procedures outlined in

ASHRAE 94-77, determined that the ASHRAE methods allowed an accurate quantification of the heat loss characteristics, and of the energy charged and discharged in sensible heat storage devices (water tanks) for liquid systems. The

research also determined that a number of minor modifications could be made to

the standard to improve the ease of testing. These modifications included:

1) A precise 15°C (27°F) temperature step change, as recommended by the

standard (see 2.3.2), does not appear to be necessary. Adequate

results were obtained without an exact 15°C (27°F) step, provided

these results were presented in dimensionless terms.

2) The dimensionless plots described in reference [18] should be used in

the analysis of the results. Use of such plots allows a means for

comparing the thermal performance of different storage devices.

16
Figure 9

At = TEMPERATURE STEP CHANGE

Tc - CHARGE TEST TIME

Td « DISCHARGE TEST TIME

Suggested plot of transfer fluid temperature (from Ref. 16)

17
3)

A "Stagnant Heat Loss Test" as described in [18] should be incorporated

into the standard as an alternate technique to the heat loss test

prescribed

Another study conducted at NBS [21] evaluated the effectiveness of ASHRAE 94-77

for storage devices using air as the transfer fluid. This study again used the

the procedures given in the standard to evaluate the thermal performance of a

7 m^ (250 ft^) pebble bed, and of a similarly-sized phase change unit (264 MJ or 250,000 Btu) , which used 726 plastic trays containing a glauber salt-water

mixture (sodium sulfate decahydrate) [21]. Air was the transfer fluid in both

test series.

Although the tests were found to permit the quantification of heat loss and the

amounts of energy charged into or discharged from the devices, a number of

problems were encountered. These included problems with variations in temperature difference across a pebble bed, and major difficulties in quantifying the

exchange of moisture with the air stream in pebble beds. Problems for phase

change devices included heat loss due to air leakage, temperature stratification in the trays containing the phase-change material, and difficulties in

calculating discharge capacities. As a result, a number of recommendations

were suggested for the ASHRAE standard. These included:

1) The flow rate for devices using air as the transfer fluid should be

changed to reflect the flow rates normally measured in installed solar

systems

2) The temperature difference for the heat loss test should be increased

from 25°C to 35°C (77°F to 95°F) so that this test can easily be

conducted in conjunction with the transient charge and discharge

tests

3) Dimensionless plots should be used for the analysis of the test results

(as was recommended for water tank tests.)

4) Establishment of the relationship between test results and real-world

system performance, although the test results should allow a relative

ranking of different thermal energy storage devices.

5) Other specific suggestions included: reporting requirements for

latent-heat storage devices; specification of inlet air conditions

for devices using air as the transfer media; and the basing of charge/

discharge capacities only on sensible-heat exchange for pebble-bed

devices

In addition to the work performed at NBS, ANL also conducted research to

evaluate the effectiveness of the ASHRAE standard for evaluating devices which

use latent-heat thermal storage (an area not addressed in detail by NBS).

Experience since 1977 had demonstrated that ASHRAE 94-77 did not yield reliable

results for latent-heat thermal energy storage (LHTES) devices. As a result,

Argonne identified problems areas in the standard and developed a proposed test

procedure for use with latent heat units [22].

18
The modifications proposed by ANL refer only to LHTES units with one inlet and

one outlet for the heat transfer fluid. This fluid may be either a nonevaporating liquid or a noncondensing gas. The proposed procedure specifies the

test apparatus and instrumentation given in ASHRAE 94-77, but requires the

installation of additional thermocouples to measure the unit’s surface and

storage medium temperature.

The proposed test procedure calls for a minimum of six tests.

These include;

• Cycling

• Full Capacity Charge

• Heat Loss

• Full Capacity Discharge

® Partial Capacity Charge

• Partial Capacity Discharge.

A major departure from ASHRAE 94-77 is the addition of the cycling test. The

proposed procedure specifies 50 continuous charge/discharge cycles of 2 hours

each. The proposed method also introduced and defined parameters for comparing

the performance of different LHTES units. These parameters, which are to be

determined from the test data include;

• Effectiveness

• Charge/discharge capacity per unit volume

• Effectiveness degradation.

ANL indicated that this proposed test method for LHTES units is preliminary

and requires further validation in both bench and full-scale tests. It also

attributed the problems with the ASHRAE standard to the combination of test

procedures for both sensible-heat and latent-heat thermal-storage devices into

a single format. As a result, the method cannot provide either representative

charge or discharge capacities, or a base for comparing different LHTES units.

As a result, ANL proposed the changes mentioned above to the ASHRAE standard

for latent-heat storage devices. These changes were submitted to ASHRAE for

consideration at the June 1983 meeting.

19
4 . SOLAR COLLECTORS

4 . 1 INTRODUCTION

Various standards have been developed to rate the thermal efficiency and

provide for the safety of conventional fuel—burning equipment. Such standards

and rating systems allow the establishment of design guidelines, as well as the systematic comparison of component performance. In the early 1970s, solar

space and domestic hot water heating systems began to receive serious attention.

Yet, because there were no standardized test procedures, there was no way to

rate various components or to compare the performance and safety of different

products. The lack of standardized test procedures, although true to a degree

for all solar components, was particularly noticeable for solar collectors.

Their thermal performance varies as a function of many parameters, such as operating temperature, fluid flow rate, insolation, orientation, tilt, time of

day, day of year, wind, temperature, sky condition, etc, making standardized

evaluation difficult.

In an effort to assist the young industry and to pave the way for increased use

of solar space- and domestic-hot-water heating systems, the National Bureau of

Standards (NBS) began a program in 1974 to develop procedures for testing the

thermal performance of solar collectors. The testing procedures for thermal

performance were intended to be submitted to ASHRAE for consideration as voluntary consensus standards. In the research program, NBS constructed a set

of test facilities at the NBS-Gaithersburg, MD site, conducted a set of roundrobin tests, and completed a series of comparison and validation tests using

test procedures from both the United States (U.S.) and Europe. The test program

for evaluating solar collectors was sponsored initially by NSF, and later by

the Department of Energy (DoE), and the Department of Housing and Urban

Development (HUD).

While thermal performance was the primary criterion for evaluating any spaceor domestic-hot-water system or component, NBS also recognized that safety and

durability should be evaluated. As a result, NBS studied the operational performance of solar collectors in terms of overall durability, snow and wind

loads, hail, over-pressure, and fire. Appropriate existing test methods were identified for various characteristics, while modifications or new methods were

outlined as needed. Many of the test methods for durability and safety were submitted to ASTM for consideration as voluntary standards. In the following

pages the NBS research and the applicable ASHRAE and ASTM standards for solar

collector performance will be discussed. In section 4, the performance of the

solar collector as a unit is described, while in section 5, the performance of

individual materials or components of solar collectors is discussed. Section 4

treats thermal performance first, followed by durability and reliability

considerations .

20
4.2 THERMAL PERFORMANCE OF SOLAR COLLECTORS

4.2.1 Initial Evaluation and Standards Development

The NBS research began with an extensive review of the literature on methods of

analysis and testing of collector thermal performance. In the project, it was

soon realized that the performance of solar collectors depends to some extent

on the system in which they are used. Yet, it was also realized that evaluating

the collector within a total system was impractical because of the many ways

that solar energy could be used within a building. As a result, the NBS efforts

focused on developing test procedures for evaluating solar collectors as individual components of the system.

Based on the literature review and preliminary research, NBS drafted test

procedures for evaluating the thermal performance of solar collectors. These

procedures were reviewed by participants at three NSF/RANN workshops, held

during 1974 and 1975. These procedures were also contained in a document

entitled "Interim Performance Criteria for Solar Heating and Cooling Systems"

[23] prepared by NBS for HUD as part of its implementation of the Solar

Heating and Cooling Demonstration Act [24]. The proposed test procedures were

then submitted to ASHRAE for consideration as a standard.

4.2.2 NBS Research

The development of the interim ASHRAE standard, and the investigations

preceding it, were discussed in NBS Technical Note 899 [16]. This report discussed methods of analysis that provide the theoretical basis for the thermal

performance test methods contained in the interim ASHRAE standard.

NBS classified solar collectors as either flat-plate or focusing (concentrating)

types. In a flat-plate collector, the absorber surface is basically flat, and

the aperture size for incident radiation is equal to the absorber area. In a focused collector, the absorber area is smaller than the aperture so that

energy is concentrated onto its surface. This results in higher temperatures

for the transfer fluid.

NBS identified fundamental characteristics of collectors were identified as:

heat loss coefficient, transmittance-absorptance product, and short- or longterm efficiency. These characteristics can be evaluated by two basic test

procedures: instantaneous and calorimetric. Calorimetric procedures are limited to collectors which use liquid rather than gas as the heat transfer

medium. Collector efficiency was defined as:

n = [Actual Useful Energy Collected]

[Solar Energy Incident Upon or Intercepted by the Collector]

or in equation form n =

21
where qu

= rate of useful energy extracted from solar collector, W

A = cross-sectional area, m^ , of the collector

I = total solar energy incident upon the plane of the solar collector

per unit time per unit area, W/m^

In the instantaneous method of calculating collector efficiency, the mass flow

rate of the transfer fluid, the difference in fluid temperature between the

inlet and outlet, and the insolation, are all measured simultaneously and under

steady-state conditions. The instantaneous efficiency is then determined by:

n -1"* m c tf (t£e t fi )

where: m

c tf

working fluid mass flow rate through the collector per unit

cross-sectional area, kg/(s»m2)

specific heat of transfer fluid, j/(kg*°C)

temperature of fluid entering collector, °C

temperature of fluid leaving the collector, °C.

The apparatus used for the instantaneous method is an open-system type, while

the apparatus used for the calorimetric method is a closed-system type. Figure

10 provides a schematic representation of these two types of testing apparatus.

NBS also reviewed specific state-of-the-art procedures used for determining

thermal efficiencies. This extensive review included some ten different

approaches used throughout the world:

• Procedures for determining collector efficiency (South Africa)

• Studies of heat transfer (Israel)

® Procedures for assessing collector performance (Australia)

o Testing of plastic bag solar hot water heaters (Japan and U.S.)

® Modified instantaneous testing procedures (Jet Propulsion Lab)

• Use of supplementary heating (University of Pennsylvania)

• Determination of daily average efficiency (U.S.)

• Use of Solar Calorimeter (Arthur D. Little)

• Collector performance testing (NASA Lewis Research Center), and

® Testing of air solar collectors (South Africa and the U.S.)

4.3 ASHRAE STANDARDS FOR THERMAL PERFORMANCE

4.3.1 ASHRAE 93-77

Based on the literature review [16], NBS developed a test method for rating

solar collectors based on their thermal performance [25]. This method was submitted to ASHRAE and adopted in large measure as ASHRAE Standard 93-77.

This standard provides test methods for determining the thermal performance of

solar collectors for heating fluids for thermal systems. Some of its

provisions will be discussed briefly here.

22
v 1 5

1 I OPEN SYSTEM - INSTANTANEOUS PROCEDURE

Governing Equation : Q

* mc^ (t^

e”

C f i^ Where Q

= Heat Delivered by Collector,

f - Fluid Heat Capacity

Governing Equation: Q => me

p dr Where m » Total System Mass,

» Time Rate of Change of Temperature

Figure 10. Schematic representation of the two basic methods for determining

solar collector thermal efficiency. (Adapted from: Thomason, Jr.,

"Solar Houses/Heating & Cooling Progress Report," Solar Energy,

Vol . 15, No. 1, pp. 27-40, 1973.)

23
The standard applies only to collectors in which fluid enters through a single

inlet and leaves through a single outlet, or to collectors with more than a single inlet/outlet in which the piping can be so connected as to effectively

provide a single inlet and outlet. The transfer fluid may be either a liquid or

a gas. The standard does not apply to collectors in which the thermal storage

unit is an integral part so that the collector and storage processes cannot be

separated for the purpose of measurement. The method is applicable only for

determining steady-state efficiency and not transient responses. The standard

further specifies mounting precautions, average irradiation, collector orientation, and wind velocity measurements as appropriate for specific tests.

Instrumentation is required for measuring: shortwave radiation (pyranometer)

direct normal irradiation (pyrheliometer) ; temperature and temperature differences; liquid flow; air flow; pressure; time and mass; and wind velocity.

The standard provides three test configurations for testing liquid solar

collectors. It specifies mounting tilt, ambient temperature sensor location,

measurement of solar radiation, temperature difference and pressure drop

measurements across the solar collector. It also specifies procedures for

measuring air as the transfer fluid. Allowable tolerances and levels of accuracy are given for all required instrumentation and measurements.

Parameters to be measured during the testing procedure include: temperature,

temperature differences, direct pressures, air flow measurements , air leakage,

and air reconditioning. The standard further provides procedures for indoor

testing with a solar simulator.

The test procedures require experimental measurement of the rate of incident

solar radiation onto the solar collector, as well as the rate of energy addition to the transfer fluid as it passes through the solar collector. From a combination of values of incident radiation, ambient temperature, and inlet

fluid temperature, values of instantaneous efficiency can be obtained. In

addition, tests are specified for determining the time response characteristics

of the collector and the way in which steady-state thermal efficiency varies

with the incident angle between the direct beam and the collector.

Performance equations are given for calculating collector thermal efficiency,

collector time constant (for evaluating transient behavior), and collector

incident angle modifier.

The testing procedure provides for a sequence of tests. First is the

experimental determination of the collector time constant (by one of two

methods). Secondly, a series of thermal efficiency tests are conducted to

determine the governing efficiency curve. Finally, the value of the collector

incident angle modifier is determined experimentally. This latter procedure

is not necessary for flat-plate collectors whose angular response characteristics are known. Following the experimental determination, a sequence of

computations are to be performed. These include computation of collector time

constant, collector thermal efficiency, collector incident angle modifier, air

flow rate, nozzle Reynolds number, and theoretical power requirements.

Equations are given for each computation.

24
The standard specifies that tests are to be conducted only on days having

weather conditions in which the 15 minutes integrated average solar radiation

is a minimum of 630 W/m2 (200 Btu/h*ft 2 ). Further restrictions are given for

incident angle and the range of ambient temperatures for test points comprising

the efficiency curve.

Figure 11 gives the testing configurations to be used with the solar collector

when the transfer fluid is a liquid, and figure 12 shows the configuration for

a collector using air as the transfer fluid.

The standard details the instrumentation requirements including those for the

pyranoraeter and its location, and for flow metering in air systems. The test

procedure and calculations call for obtaining values of instantaneous combination of values of incident solar radiation, ambient temperatures, and inlet

fluid temperatures. This requires measuring the rate of solar radiation

incidence onto the solar collector, as well as the rate of energy addition to

the transfer fluid as it passes through the collector under quasi-steady conditions. Based on the definition of collector efficiency given above it can

be stated that:

‘f.l + C f,e .

- F * ( Ta)e* F

' U

L

Where: F' = Solar collector efficiency factor

(xa) e = Effective transmission - absorptance factor for the solar

collector

Ul = Heat transfer loss coefficient for the solar collector,

W/m2 *°C)

t a = Ambient air temperature

tf^jtf e ; I = As defined previously

This equation indicates that if the efficiency is plotted against an appropriate

At/I, a straight line will result where the slope is some function of Ul, and

the n intercept is some function of (ia) e

. Figure 13 shows typical results of

tests performed on air heaters with single-glass cover plates [26], For determining the efficiency curve, at least four different values of inlet fluid

temperature must be used, and for each inlet fluid temperature, at least four

data "points" must be taken, two preceeding and two symmetrically following

solar noon. Each data "point" is to represent 15 minutes integrated efficiency

value

While ASHRAE Standard 99-77 follows closely the NBS interim standard, it is a further development and thus differs in several significant ways. The most

significant of these are:

• Additional tests are required to determine the collector time constant

and the incident angle correction factor;

• It allows for indoor testing using a solar simulator.

25
Figure 11. Testing configuration for the solar collector when the

transfer fluid is a liquid (from Ref. 25)

Figure 12. Testing configuration for the solar collector when the

transfer fluid is air (from Ref. 25)

26
COLLECTOR

EFFICIENCY,

%

1.23 m by 0.76 m Air Heaters

single glass cover plate

.15 m glass wool edge insulation

.05 m glass wool back insulation

o corrugated galvanized iron absorber

surface, carbon black paint

• corrugated aluminum absorber surface,

commercial chimney paint

»f,i + »f,e

to

2 <*C Sec — 1 - I Langley

Figure 13. Efficiency curves for two flat-plate collectors using air

as the transfer fluid (from Ref. 26)

27
« The specimen shall be preconditioned by stagnation heat in a nonoperational, dry condition for three days.

Specific technical changes include:

® Modifications to the test loop to add a storage tank with by-pass

for dampening-out thermal transients.

® Rearrangement of the air heater test loop to "pull" the air through

rather than blowing it through.

® Tightening-up the requirements for measuring solar radiation.

® Requirement that the incident angle must be less than 30 degrees

(instead of 45 degrees).

® Reduction of the time period for integrating energy quantities for

computing efficiency values from 15 minutes to either 5 minutes or one

time constant, whichever is larger.

® Use of gross frontal area of the collector for computing efficiency

instead of aperture area.

® Dividing by the incident solar radiation rather than the average

fluid temperature in drawing the efficiency curve which plots

efficiency versus the difference between inlet fluid temperature and

ambient temperature.

In addition, the standard also provides information on open loop testing

configurations for liquid transfer fluids, both with (figure 14) and without

(figure 15) continuous fluid supply.

The preconditioning consists of stagnation heat in nonoperational mode under

dry conditions for three days with a cumulative mean incident solar radiation of

not less than 4722 (W«hr)/(m^*day) [ 1500 Btu/(ft^*day) ]

For indoor testing with a solar simulator, the standard specifies spectral

qualities to duplicate average North American irradiation as closely as possible,

and best represented by an air-mass 2 solar spectrum [27]. It further requires

a uniformity of illumination over the collector aperture of not more than ± 10

percent, collimation such that at least 95 percent of the energy output is

within a subtended angle of 12 degrees or less, and maintenance of a substantially uniform air flow across the collector of at least 3.5 m/s (7.6 mph) . It

also specifies that the collector configuration factor between the solar simulator

surface and the solar collector not exceed 0.05.

To evaluate the transient behavior of the collector, the standard requires that

the collector time constant be determined. The time constant is defined as the

time required for the fluid leaving a solar collector to attain 63.2 percent of

its steady-state value following a step change in irradiation or inlet fluid

28
Figure 14. Open-loop testing configuration for the solar collector when

heat transfer fluid is liquid and fluid is not supplied

continuously (from Ref. 30)

Figure 15. Open-loop testing configuration for use when fluid is

continuously (from Ref. 30)

supplied

29
temperature. Two methods are provided for the experimental determination of

the collector time constant.

To permit the prediction of the collector performance under a wide range of

conditions and/or time of day, with varying angles of incidence, the standard

requires the determination of the collector incident angle modifier, using

either of two experimental methods.

4.3.2 ASHRAE 96-80

While the scope of ASHRAE Standard 93-77 does not specifically exclude unglazed

flat-plate collectors operating with a liquid as the transfer fluid, such

collectors have special performance characteristics when used in low temperature applications such as swimming pool heaters or heat pumps. Unglazed

collectors have a greater sensitivity to environmental conditions and to operating flow rates. Also, the liquid heat transfer fluid may have a temperature

lower than the ambient air. These characteristics need to be considered in

developing test procedures for unglazed flat-plate solar collectors. Accordingly, a new standard, ASHRAE 96-80, Methods of Testing to Determine the

Thermal Performance of Unglazed Flat-Plate Liquid-Type Solar Collectors

[28], was promulgated. This standard closely follows ASHRAE 93-77, with the

exceptions noted below. Accordingly, the new standard:

• has tighter requirements on collector test loop stability;

® requires greater instrumentation accuracy;

• restricts allowable environmental variables during the tests;

• requires that, during the tests, the wind does not exceed 1.3 m/

(3 mph);

o specifies that the range of ambient temperatures for all reported test

points making up the efficiency curve must be less than 10°C (18°F).

Further differences between ASHRAE 96-80 and 93-77 are that ASHRAE 96-80 applies

only to flat-plate non-concentrating collectors using liquid heat transfer media,

while ASHRAE 93-77 applies to both liquid and air collectors, as well as concentrating and flat-plate collectors. These differences in scope made it possible

to omit the requirement for determining the collector time constant from

ASHRAE 96-80.

4.4 VERIFICATION AND EVALUATION OF ASHRAE STANDARD PROCEDURES

4.4.1 Round-Robin Test Using Interim Procedures

A round-robin test program was conducted at a number of test facilities in the

United States to determine the comparability of thermal performance data for

flat-plate liquid-heating solar collectors. When the test series was planned,

ASHRAE 93-77 had not yet been adopted so that the NBS interim test procedure

was specified for the round-robin. Some testing facilities, however, did use

30
the more restrictive requirements set forth in the final standard (which was adopted during the course of the round-robin). The results of the round-robin

series were reported in an NBS publication [29] on the "Results and Analysis of

a Round-Robin Test Program for Liquid-Heating Flat-Plate Solar Collectors."

A total of 21 laboratories participated in the round-robin. These laboratories

were located geographically as shown in figure 16. Two commercially available

solar collectors were tested at each site. Collector No. 1 had two glass cover

plates and a flat-black coated aluminum absorber plate assembled into a sealed

unit which was attached to a sheet-metal box containing glass-fiber insulation,

as shown in figure 17. Collector No. 2 consisted of a single glass cover

plate and a steel absorber with a black chrome selective coating. The absorber

was mounted on a thermal insulator, backed with glass fiber insulation. The

entire assembly was mounted in a steel frame with a sheetmetal back and aluminum

glazing frame, as shown in figure 18.

Each participant in the test series reported the test conditions for each data

point and also plotted collector efficiency, as follows:

£ f,e +

_ t

2 a

n versus

I

The data were used to determine values for the collector efficiency factors

F'(Ta) e and for the collector heat removal factor F’Ul by a first-order leastsquare fit to all measured data points. Mean and standard deviation values

were calculated for these two parameters from each laboratory. This statistical analysis indicated important deviations, particularly for collector No. 2.

Figure 19 presents an indication of the spread and distribution of F'(Ta) e values for each collector. The greater frequency of values about the mean

(smaller deviation) for collector No. 2 was attributed to the fact that more

data were taken at smaller angles of incidence, than for collector No. 1.

The value F'(Ta) e showed a relatively large and unexpected standard deviation

of 7.7 percent for collector No. 1. A major source of the deviation appeared

to be the use of a "black and white" pyranometer by eight participants. Fewer

participants used this pyranometer for collector No. 2. These data confirmed

that "black and white" pyranometers are sensitive to tilt angle. Accordingly

ASHRAE Standard 93-77 was modified to contain a requirement for using only a first class pyranometer or phyrheliometer as classified by the World

Meteorological Organization (WMO) [13,30].

The statistical analysis also indicated a large deviation in values for F'Ul

for collector No. 1, apparently due to variations in insulation at the back and

edges of some collector mountings, the use of different mass flow rates from

that prescribed, and incidence angles greater than 45 degrees. A significantly

lower overall loss coefficient observed for collector No. 2 appeared to be

due to better thermal insulation for the absorber plate and the use of a selective coating. Data from the round-robin tests were also analyzed statistically

to determine the effect of environmental conditions [31]. The data were examined

31
Aw - Tropical Savanna

BSh - Tropical and Subtropical Steppe

BSk - Middle Latitude Steppe

BWh - Tropical and Subtropical Desert

BWK - Middle Latitude Desert

Cs - Mediterranean

Cb - Marine West Coast

Caf - Humid Subtropical, no dry season

Da - Humid Continental (Warm Summer)

Daf - Humid Continental (Warm Sumner), no dry season

Db - Humid Continental (Cool Summer)

Dbf - Humid Continental (Cool Summer), no dry season

Figure 16. Climatic location of testing facilities

(from Ref. 29)

3.18 mm GLASS

0.953 cm AIRSPACE

3.18 mm GLASS

0.933 cm AIRSPACE

ALUMINUM SOLAR ABSORBER

FIBERGLASS INSULATION

GALVANIZEO PAN TO COVER INSULATION

Figure 17. Schematic of flat-plate liquid-heating collector No. 1 (from Ref. 29)

32
TEMPERED GLASS

T= -90

BLACK CHROME COATING

a s = .95, e = .12

1. INSULATING MOUNTING BLOCK

2. FIBERGLASS INSULATION

3. GALVANIZED STEEL COLLECTOR BOX

Figure 18. Schematic of flat-plate liquid-heating collector No. 2

(from Ref. 29)

F fra) COLLECTOR No. 1

«

.66 .61 .70 .72 .74 .76 78 .80 .82 14 J6 .88 90 F(Va) COLLECTOR No. 2

e Figure 19. Frequency distribution of F'(Ta) e values for round-robin

collectors (from Ref. 29)

33
to determine consistency across test conditions. The statistical analysis

referenced all measured thermal efficiencies to a common set of environmental

conditions by using analytical models for the two flat-plate collectors tested.

The analysis indicated that the referencing of efficiencies to a common environment is feasible and significantly reduces the differences in measured

efficiencies reported by the various participants.

Figures 20 and 21 show considerable scatter in the data, although the data for

an individual facility were generally consistent and showed very little scatter.

Correcting the data for differences in environmental conditions between facilities only reduced the scatter by about 30 percent. This small reduction appears

to indicate that either the test requirements were not restrictive enough or

that no accounting was made for systematic errors. B. D. Wood conducted a further analysis [29] on the effect of random and systematic errors. After

analyzing the data from all test facilities, he concluded that the data scatter

actually occurring (after correcting for operating and environmental conditions)

among those facilities that reportedly adhered to the ASHRAE 93-77 procedures

was much larger than should be expected. Based on the available data, he

hypothesized that a combination of systematic errors resulted in data scatter.

Possible sources of systematic error included:

• Systematic Facility Errors

+ Heat transfer between test apparatus and collector.

+ Reduced heat losses to ambient environment due to shielding of

collector support stand.

+ Test apparatus not at steady- or "quasi-steady-" state condition

during test.

+ Change in specific heat of the transfer fluid.

+ Inexperienced technicians conducting the tests.

• Systematic Instrument Errors

+ Conduction errors in thermocouple installations.

+ Pyranometer calibration error.

+ Flowmeter calibration error.

The significance of the variation in the reported performance for a particular

collector was illustrated by calculating the expected system performance of two

sample solar systems. The first sample was a residential, combined solar

domestic hot water and space heating system in Madison, Wisconsin, with a hot

water demand of 0.32 m^ per day and an average monthly energy requirement for

the year of about 1.7 x 10^J. It was assumed to consist of 80 m2

(861.14 sq. ft.) of collector No 1 . In the second example, a solar domestic

34
o

Figure 20. Uncorrected results from 12 facilities for collector No. 1 tests (from Ref. 29)

I W

Figure 21. Uncorrected results from 10 facilities for collector No. 2 Tests

(from Ref. 29)

35
hot water system was simulated in five different cities across the country. It was assumed that 5.37 m2 (57.80 sq. ft.) of collector area No. 2 was installed.

The results of the analysis indicated that the use of the mean, "best", and

"worst" values obtained in the round-robin test series resulted in a total

spread of about 22.1 percentage points about the mean for the first example.

For the second example, the maximum spread of 17.8 percent occurred in

Washington, D.C.

In conclusion, data from the round-robin testing indicated that most of the

reported differences in measured efficiency were due to experimental error or systematic differences between facilities. The data indicated that the preparation and mounting of collector No. 1 resulted in large variability in the test

results. The results further indicated the importance of using a pyranometer

which meets the class 1 requirements of WMO. In addition, differences in measured efficiencies between sites due to environmental conditions can be reduced

by referencing all test results to a common environment using a straightforward

analytical procedure. Finally, greater consistency in test results was observed

for those participants who met the more restrictive ASHRAE 93-77 requirements.

4.4.2 Evaluation of ASHRAE 93-77

4. 4. 2.1 NBS Test Facility and Research

During the round-robin testing, NBS also constructed a facility for evaluating

solar collector performance and assessing individual provisions of ASHRAE 93-77.

The NBS test facility consisted of two test loops for evaluating liquid collectors and one for evaluating air collectors. The test equipment was located on

an elevator which allowed the test facility to be exposed to the outdoor

environment only when tests were run, and to conduct indoor tests of heat loss

characteristics without disassembling any part of the equipment for the specimen

mounting. Figures 22 and 23 show the facility in the process of being raised

to ground level, and in the full raised condition, respectively. The details

of the test loops and the specifications for the instrumentation are given in an NBS publication entitled, "Experimental Verification of a Standard Test

Procedure for Solar Collectors" [30]. The use of solar simulators was not

evaluated during this particular study.

Two separate, essentially identical test loops (see figure 24) were built for

evaluating water heating collectors, so that it would be possible to test two

individual collectors independently, and to make adjustments to one loop while

not affecting the other. The collector mounting could be adjusted for tilt

angles from 0 to 70 degrees, and for azimuth angles of 0 to 360 degrees.

Figure 24 is a schematic drawing of the liquid collector test loop. This loop

was similar to the closed loop shown in ASHRAE standard 93-77 [13] and was a development from the loop discussed in reference [25]. In the design of the

test apparatus, particular care was given to the selection and installation of

measurement instrumentation and controls for the temperature of the transfer

fluid. Different flow meters were used for the two different loops.

36
Figure 22. Solar collector testing equipment being raised to ground level

at NBS, Gaithersburg, HD (from Ref. 30)

Figure 23. Solar collector testing equipment in place at NbS,

Gaithersburg, MD. (Test stand with solar collector in middle

foreground is not normally a part of the equipment)

(from Ref. 30)

37
Figure 24. Schematic diagram of an NBS test loop for liquid-heating

solar collectors (from Ref. 30)

ANEMOMETER BAROMETRIC

POWER

CONTROLLER Figure 25. Schematic of the NBS test stand for air-heating solar

collectors (from Ref. 30)

38
Only one test loop was built for assessing air collectors. It is shown in

figure 25. The collector stand could accomodate collector arrays up to 10

in area. It could be tilted from 0 to 65 degrees and fully rotated for

orientation

As with the design and control of the test loops, the accuracy of the

instrumentation used to make the meteorological measurements met, or exceeded,

the requirements of ASHRAE 93-77. Ambient air temperature, wind speed, wind

direction, total solar radiation, sky temperature, barometric pressure, and

sun angle were measured. Originally a "black and white" pyranometer

was used to measure total solar radiation, but this instrument was found to be

sensitive to tilt angle. Errors of up to 7.4 percent were observed for

typical collector tilt angles of 56 degrees, even though the same instrument

agreed to within 1.5 percent in the horizontal position. This finding is

consistent with those of other researchers. As a result, a first class

pyranometer as classified by WMO, was substituted for the NBS testing and

required in the ASHRAE standard.

To verify ASHRAE 93-77 experimentally, NBS tested one air-heating and five

water heating solar collectors [30]. A major objective of these tests was the

determination of whether the tests for time constant, the near-normal incidence

efficiency, and the incident angle modifier, as specified by ASHRAE 93-77, could

be carried out readily and accurately. It was found that all three tests could

be conducted with little difficulty. In addition, a comparison of data from

three all-day tests of collector efficiency showed good agreement with calculated efficiencies. Other conclusions reached by NBS included the following:

• The time constant can also be determined for concentrating collectors

having a low concentration ratio. Method 1 of ASHRAE 93-77 is much

easier to complete than Method 2.

• The incident angle modifier test as specified is applicable only to

flat-plate collectors. An entirely different procedure, which depends

on the optical characteristics of the collector, is required for

concentrating collectors. Method 2 of ASHRAE 93-77 (section 8.3.3),

involving an all-day test with the collector stationary facing south,

is preferable to Method 1, which requires several test days and can produce erroneous results for collectors with large time constants.

® Thermal efficiency data must be collected symmetrically with respect to

solar noon. Near-normal incidence efficiency data can and should always

be collected to reduce scatter.

® Aperture of absorber area should be used instead of gross collector

area

® A class 1 pyranometer, as specified by WMO, is required for accuracy.

• Air leakage may be a significant source of error in tests of air-heating

collectors. If the collector is tested under positive pressure, the

air flow rate should be measured downstream of the collector; if under

39
negative pressure, air flow should be measured on both sides of the

collector. An open loop is preferable to a closed loop in testing air

collectors

® The all-day collector performance of flat-plate collectors on clear

sunny days can be predicted with reasonable accuracy using test results

obtained in accordance with ASHRAE 93-77.

® Tests on flat-plate collectors having a time constant of less than five

minutes can be completed in six test days.

The recommendations for future work included the following:

® The feasibility of adapting the standard to concentrating collectors

including procedures for determining incident angle modifiers for such

collectors

® The relationship between long-term collector performance in service and

the performance indicated by tests.

• The combination of outdoor exposure for insolation/solar heating gain

and indoor tests for heat loss should be examined, as it would reduce

the time and cost of the tests. This concept was proposed by Smith

[32], Symons [33], and by the German Solar Energy Industries

Association [34].

4. 4. 2.

Comparison of ASHRAE and BSE Procedures

One of the recommendations resulting from the NBS support work on ASHRAE 93-77

[13] as described by Hill, Jenkins, and Jones [30] was that a combination of

outdoor exposure for insolation/solar heat gain and indoor tests for heat loss

should be examined for glazed flat-plate liquid collectors. A procedure for

such outdoor/indoor tests was developed by the German Solar Energy Association

and adopted by the Bundesverband Solar Energy (BSE) [35]. Implementing this

procedure, NBS conducted tests on five solar collectors (see table 2) using

both the BSE and ASHRAE procedures [36], and compared the results. The NBS

facility, described in reference [30], was particularly well-suited for this

work, since it allowed the entire testing apparatus to be used both outdoors

and indoors.

The BSE testing procedure prescribes a series of tests for determining

collector efficiency by independently measuring optical efficiency and thermal

losses through a combination of outdoor and indoor testing. These tests are described by Jenkins and Hill [36]. To determine the collector instantaneous

efficiency according to BSE, the useful energy output and incident solar radiation are monitored on a collector installed outdoors. While the collector

operates under quasi-steady operational conditions, the mass flow rate and

temperature rise across the collector are measured. With the mean fluid temperature regulated to within ± 10°C of ambient air temperature, heat loss from

the collector to the surrounding environment is negligible. (If the mean fluid

40
Table 2. Description of Collectors Used in Comparison of BSE and ASHRAE Test Methods (from Ref. 36)

Collector

No. 1 Collector

No. 2 CollectorNo. 3 Collector

No. 4 CollectorNo. 5

Manufacturer Commercial Solar Energy

PPG Industries Chamberlain Mfg.

Company Lennox LMSC 18-1 Commercial Solar

Energy

Cross Area (m 2

) Aperture Area (m 2

) 2.47 2.29

1.65

1.60

1.96

1.79 1.53

1.40 2.472.29

Glazing

Material Plate glass Tempered glass Tempered lowiron glass

Tempered low-iron

glass with antireflective coatings

Polyf luoroethylene

Number 2 2 1 2 1

Solar Transmittance 0.87 (single glass)

0.85 0.90 0.96 0.92

Absorber

Material Copper Foil Aluminum Roll

Bond Mild Steel Mild Steel Copper Foil

Flow configuration 10 parallel

risers

13 parallel

risers

Pillow absorber

completely wetted

surface

10 parallel 10 parallel

risers

Coating Flat Lacquer Flat Lacquer Black Chrome

Nickel Substrate

Black Chrome

Nickel Substrate

Flat Lacquer

Solar Absorptance 0.95 0.94 0.94 0.94 0.95

I.R. Emittance 0.92 0.92 0.12 0.10 0.92

Insulation

Material Low Density

Polyurethane

Glass fiber Glass fiber Fiberglass-board Low Density

Polyurethane

Thickness (cm) 6.35 7.62 7.00 7.9 6.35

*Fr (ro) e 0.58 0.74 0.80 0.72 0.63

*F

r

U

l W/(m 2 .°C) -4.71 -5.14 -4.43 -3.62 -6.56

**F» 0.74 0.97 0.95 0.86 0.75

**UL W/(m2 »°C) 6.06 5.44 4.63 4.24 9. 10

* Derived from slope or intercept of ASHRAE 93-77 efficiency curve

** Determined from F^(Ta) e and FrUl and MWB analytical formulations of (ta) e and F'

41
temperature is not within * 10°C, a correction must be added.) The incident

solar radiation absorbed by the absorber surface, Q0 , is then:

Q

o

dx

T

2 ~ Tj

where

•m = mass flow rate through collector

Cp = specific heat of the collector working fluid

t = time

tf o

= collector outlet fluid temperature

tf ^

= collector inlet fluid temperature

The thermal losses are determined within a controlled indoor laboratory under

zero solar irradiation. The working fluid is circulated in reverse through the

collector over a range of operating mean temperatures above ambient at levels

of 30°, 50°, 70° and 90°C, while the flow rate and temperature drop across the

collector are monitored. The rate of thermal energy dissipated to the environment, Ql, is then:

Q

L

T 2

mC, T

1 (t f,o - c f,i> dT

To “ T

Using the results of both outdoor and indoor tests, a family of efficiency

curves can then be developed, as shown in figure 26.

The BSE and ASHRAE test requirements regarding environmental conditions,

measurement uncertainties, and range of operating conditions are shown on table 3. The requirements for outdoor testing following the BSE and ASHRAE

are nearly identical in all categories.

To compare the BSE and ASHRAE tests, it is essential that these results be

presented in a common format. Since the BSE and ASHRAE procedures prescribe

different formats for reporting these results, the BSE efficiency curves were transposed into an ASHRAE efficiency curve. In transposing the BSE data, each

of the curves is based on the average outdoor solar irradiance experienced

during the ASHRAE 93-77 test on the same collector.

The test loops used for both tests are essentially identical and are similar to

those used for the tests described in BSS 117 [30]. Figure 27 shows a diagram

of the closed-loop configuration, which was designed to control and stabilize

the collector fluid inlet temperature to within ± 0.5°C (± 1.0°F) and the fluid

flow rate to within ± 1 percent.

For conducting the BSE indoor tests, two types of wind simulators were used.

The axial fan wind simulator is shown schematically in figure 28, and two

42
Table 3. Environmental Test Conditions Allowed Within the ASHRAE 93-77

and BSE Collector Test Procedures (from Ref. 36)

Environmental Parameter

ASHRAE Standard

93-77

BSE*

nQ - Determination,

Outdoor Testing

BSE* •Ql - Determination

Indoor Testing

Ambient air temperature range < 30°C no limits 15 - 25°C

Wind velocity across collector

should be < 4.5

m/

s

> 4 m/s > 4 m/ s

Total solar irradiance

within collector plane

> 630 W/m2 no minimum < 1 W/m2

Beam solar irradiance

incident angle

< 30° < 30°

Foreground reflectance

Apparent environmental

radiance temperature

< 0.20 < 0.20

< t a ± 3°K

* BSE Guidelines and Directions for Determining the Usability of Solar Collectors, [35].

43
Figure 26. Collector efficiency using BSE procedure vs. the difference

between collector mean fluid temperature, t m , and ambient

air temperature, t a (from Ref. 36)

Figure 27. Closed-loop testing configuration for comparing BSE

and ASHRAE test methods (from Ref. 36)

44
A - AXIAL FAN OIRECTED HORIZONTALLY AT COLLECTOR MIDSECTION

B - AXIAL FAN DIRECTED LONGITUDINALLY ALONG COLLECTOR PLANE

Figure 28. Schematic of axial fan wind simulator

(from Ref. 36)

PRESSURE GAUGE

Figure 29. Environmental simulators

(from Ref. 36)

45
environmental simulators are shown in figure 29. The environmental simulators

provide a more uniform wind velocity across the collector, and permit the

investigation of low "sky" temperature during indoor testing for thermal loss.

The test results indicated that:

• For all five collectors tested, the differences between the efficiency

curves determined according to BSE and ASHRAE procedures were less

than the total uncertainties associated with ASHRAE Standard 93-77.

• Uncertainty in determining collector thermal efficiency is reduced and

repeatability improved when using the BSE collector test procedure com- pared with the ASHRAE procedure.

o The determination of near-normal incidence instantaneous collector

efficiency using the BSE procedure can be completed in two days,

only one of which is dependent on outdoor environmental conditions

Four days of outdoor tests are normally required for ASHRAE tests

using a fixed orientation stand.

® The environmental simulators were successful in increasing the

uniformity of indoor environmental conditions. Axial fan wind

simulators were also sufficient for simulating controlled wind

conditions

4. 4. 2. 3 Evaluation of Unglazed Flat-Plate Collectors

NBS also conducted experiments to determine the differences in the thermal

performance of unglazed flat-plate liquid solar collectors using both ASHRAE

96-80 [28] and BSE [35] test procedures.

The BSE procedures, unlike the ASHRAE, require both indoor (nonirradiated) and

outdoor (irradiated) testing to determine collector optical efficiency and

thermal loss characteristics independently. Instantaneous thermal performance

for the expected operating conditions is calculated from these separately

determined properties. Because the BSE procedure determines collector heat

loss under zero-irradiance conditions, unlike the ASHRAE, it was not apparent

that the thermal efficiency determined by the two separate procedures would be

equivalent

As a result, two unglazed flat-plate liquid heating collectors were tested

using both BSE and ASHRAE procedures [37],

As with the earlier work, the tests were conducted at the NBS laboratories in

Gaithersburg. The fluid test loop used in the tests is shown schematically in

figure 30. Figure 31 gives a diagram of the two collector specimens which were

tested by both ASHRAE and BSE procedures.

As mentioned in the previous section, the major difference between the ASHRAE

and BSE procedures is that in the ASHRAE test, the collector heat loss is

determined during outdoor exposure of the specimen, whereas in the BSE test,

the heat loss is determined under controlled indoor conditions. Since unglazed

46
rrifip n INK irnrcf

FLUID TYPE T THERMAL LIQUID H.L

OUTLET THERMOPILE GUARD

Figure 30. Schematic of fluid test loop used in experimental work

on unglazed flat-plate liquid collectors (from Ref. 37)

Figure 31. Diagrams of unglazed collector 1 and 2 absorbers

(from Ref. 37)

47
collectors are more sensitive to the influence of wind, ASHRAE 96-80 restricts

the average wind speed for each test to no more than 1.3 m/ s (3.0 mph) , a factor of three lower than the ASHRAE 93-77 requirement for glazed collectors.

For the comparative tests, the BSE procedure was slightly modified to allow for

direct comparison between the ASHRAE and BSE test results. Specifically, gross

collector area instead of net aperture area was used, and the BSE wind speed

specifications were changed to determine losses at several wind speeds from

0 m/s (still air) to 3.9 m/ s (0 - 8.7 mph).

The test results again indicated that the differences between the thermal

efficiency curves determined through the BSE and ASHRAE 96-80 procedures were

less than the uncertainties associated with the curves. As can be seen in

figure 32, the BSE-determined efficiency curves demonstrated the strong sensitivity of the unglazed collector thermal performance to wind. Although

results from the two procedures were very similar, NBS concluded that the modified BSE procedure was preferrable to the ASHRAE method. The modified BSE

procedure eliminates thermal loss uncertainties resulting from transient outdoor test conditions, permits thermal performance to be determined under

selected environmental conditions, and is less time consuming.

Because of the strong dependence of test results on wind speed, and the results

of the BSE investigation, it was suggested that wind simulators could also be

used in outdoor tests. Although not tried in the reported test series, the use

of such simulators could result in more controllable wind conditions during

ASHRAE 96-80 tests.

4.5 DURABILITY AND RELIABILITY OF SOLAR COLLECTORS

4.5.1 Proposed Evaluation Criteria

The significance of collector durability and long-term performance is

demonstrated by the 30 percent decrease in efficiency found in one U.S. collector system over a period of 18 years [38]. Such measurements of performance

over time are quite rare, however. As a result, there was a great need to

develop testing methods which would measure and simulate long-term performance.

The "Solar Heating and Cooling Demonstration Act of 1974," Public Law 93-409

[24] provided for the development of performance criteria and standard testing

methods for use by industry in evaluating materials, components, and systems.

In 1977, DOE instituted a comprehensive research program to provide an accurate

data base for developing durability criteria and test methods.

One early step in the development of durability and reliability criteria was

the publication in 1977 of a set of proposed test methods for evaluating solar

collectors [39], This publication, which was revised in 1978 [40], was intended to serve as a resource document in the development of consensus standards, a background document for organizations developing certification programs, and the basis for a testing program sponsored by DOE. The test methods

and provisional rating criteria contained in it address issues of thermal

performance, durability/ reliability , structural integrity, and fire safety. In

48
Figure 32. Comparison of ASHRAE Standard 96-1980 and modified BSE

efficiency curves (for collector 1) (from Ref. 37)

49
the document, the selection of test methods was based on a review of over 400 consensus and other industry standards.

Individual test methods include thermal performance tests (in general

accordance with ASHRAE 93-77), and calculation of all-day solar collector

efficiency (see table 4). One modification to ASHRAE 93-77 is a revised

preconditioning exposure. The thermal performance tests also include baseline

and post-stagnation thermal performance tests. Other thermal performance tests

include one for no-flow 30-day degradation to identify potential problems in

collector materials or construction resulting from prolonged exposure to

natural elements. Other tests are concerned with identifying potential

durability/reliability and safety problems. They include:

1. Thermal Shock Tests —to determine the ability of a heated collector

to withstand thermal shock caused by heavy rain, and to evaluate

collector reliability after being subjected to thermal shock and

boiling stresses induced by filling a hot collector with cool

transfer fluid during start-up;

2. Rain Test—to determine collector resistance to water penetration

when subjected to wind-driven rain. One recommended test is based

on ASTM E 331 [41].

3. Thermal Cycling—to determine whether collectors will perform

reliably after exposure to freezing conditions.

4. Live Loads—to determine the collector's ability to function after

being subjected to snow loads, or positive, negative or combination

wind loads with the loads being established according to ANSI A-58.1

[42], and the tests being conducted according to ASTM E 72 [43].

5. Longitudinal Loads—to determine the ability of the collector

mountings to withstand cyclic loads in the plane of the longitudinal

axis of the collector.

6. Hail Loads—to determine the ability of the cover plate to withstand

the impact of hailstones and other flying objects.

7. Air Collector Rupture and Collapse Tests—to determine the ability of

the air collector to meet the pressure requirements established by

Underwriters Laboratories (UL) [44].

8. Static Pressure Leakage Tests—to determine the resistance of air

collectors to air leaks using the UL procedures given above, and to

determine the resistance of liquid filled collectors to static overpressure using ASME hydrostatic and pneumatic tests [45].

9. Fire Resistance Tests —to determine the relative fire hazards created

by roof-mounted solar collectors exposed to a fire sources using

ASTM E 108-75 [46].

50
Table 4. Typical Computation of All-Day Solar Efficiency (from Ref. 40)

Calculation Steps Hour of the day, solar time Daily

6-7 7-8 8-9 9-10 10-11 11-12 12-1 1-2 2-3 3-4 4-5 5-6 Total

1. Inlet fluid temp to collector, tf ^ , °C 49 49 49 49 50 50 51 52 54 54 54 54

2. Ambient air temp,

t a , °c 10 10 10 13 15 16 16 22 24 24 18 18

3. Incident radiation

on collector plane,

1T , W/m2

, (Table A2

ASHRAE 93-77) 90 186 454 693 876 993 1031 993 876 693 454 186 7525

4. Collector thermal

efficiency at normal

incidence, determined

in accordance with

Sections 8.3.2 and 8.5 of ASHRAE 93-77

and using data from

lines 1, 2, and 3. CM O .15 on 00 CM • .46 .46 .46 .43 CM .18 .02

5. Incident angle between

the direct solar beam

and outward drawn

normal to the collector

plane, 0. 90 75 60 45 30 15 15 30 45 60 75 90

6. Incident angle modifier,

determined in accordance

with Sections 8.3.3 and

8.6 of ASHRAE 93-77 and using the value of 6 from line 5. 00 00 CM .93 .97 1.0 1.0 .97 .93 00 CM oo

7. Energy output from the collector, W/m^, (line 3) x(line 4 + [FR {To} e n l x [ {line 6}-l]) 0 0 229 350 457 474 437 333 180 0 2460

8. Collector thermal

efficiency, line 7/line 3 .33 1

51
The proposed rating criteria do not provide end-point criteria for thermal

performance, but require that such performance be determined in accordance with

ASHRAE 93-77, and that all-day collector efficiency be calculated and reported

as shown in table 4. Measurements made after the 30-dajt no-flow degradation

test are to be used to determine performance. For the 30-day. jno-f low degradation, the criteria limit the change in intercept or slope to that which would

not cause a degradation in thermal efficiency of more than 10 percent.

The proposed criteria provide more specific guidelines for determining failure

for durability/reliability, safety, and fire tests. For these tests, the major

rating criteria include:

1. Thermal Shock (water spray/cold fill) Tests: No visual observation of

catastrophic or structural failure, and no visual evidence of leakage.

2. Rain Tests: No visual observation of water penetration into the

collector and no increase in collector weight greater than 1 percent.

3. Thermal Cycling Tests: No visual observation of catastrophic or structural failure.

4. Static Pressure Tests: For air collectors, the total volume of air

is not to exceed 20 multiplied by the volume of the collector panel

assembly for the test period. No pressure drop or visual observation

o# leakage for liquid collectors.

5. Wind Load Tests: For positive, negative, and combined loads,

procedures are given for determining a "Design Load Rating" U, as follows

U - Rm (1 - ov)

Where: Rm = Mean resistance (average maximum load resistance by

collector)

,

a = A factor dependent on the number of specimens tested,

and

v = Coefficient of variation.

6. Longitudinal Load Test: No visual evidence of physical damage or permanent distortion.

7. Hail Load Test: Collectors which are not damaged by 4 cm (1-1/2 in.)

diameter hailstones have a high probability of withstanding most hailstorms without damage. The reader is referred to the intermediate

minimum property standards* to determine the mean annual days with

hail in different regions of the U.S.

V

* Intermediate Minimum Property Standards supplement, Solar Heating and Domestic

Hot Water Systems, 1977 Edition, HUD 4930.2, prepared for HUD by NBS.

52
8. Air Collector Rupture and Collapse: No visual evidence of physical

damage or permanent distortion.

9. Fire Performance: Meet Class C rating [46] for intermittent flame,

spread of flame, and burning brand tests, except that only one series

of tests needs to be performed. Meet ASTM E 84 guidelines for flame

spread classification index for all insulation materials.

In addition, the provisional test procedures [39,40] provide guidelines for the

selection and characterization of test specimens, as well as for shipment

inspection. Recommendations are also given for test sequencing which considers

technical considerations such as the need to induce degradation of materials

before the appropriate test and minimize the probability that testing would

damage a collector so that further testing would be impossible. Thus, the

following sequence is suggested:

1. Static Pressure Leakage Test—to detect leaks in both air and liquid

collectors

2. Thermal Tests

a. Preconditioning exposure

b. Thermal Performance Baseline

c. No-Flow 30-day Degradation — (including d. and e.)

d. Thermal Shock/Water Spray

e. Thermal Shock/Cold Fill

f. Post-Stagnation Thermal Performance

3. Rain Test.

4. Thermal Cycling including Positive Live Load; Negative and Combination

Wind Loads; Longitudinal Load; and Hail Load; in any desired order

following test No. 1.

5. Air Collector—Rupture and Collapse—To be done near the end of

testing.

6. Fire Test—To be conducted at the end of testing since it will damage

collector.

Throughout all collector testing, extensive documentation and calibration

records are required.

4.5.2 Durability Research Program

4. 5. 2.1 Research Plan

The NBS research plan for determining collector durability is described in NBS

TN 1136 entitled "NBS Solar Collector Durability/Reliability Test Program

Plan" [47], This research program was designed to correlate the results of

laboratory, accelerated field, and simulated operational exposure tests with

53
actual operating performance of collectors used for building heating and

cooling applications.

The research described in [47] was designed to assess the influence of

environmental exposure parameters that could affect the degradation of solar

collectors and materials. Data obtained during the research program were

intended to lead to more meaningful durability/reliability tests of collectors

and materials. The testing program was designed to accomplish the following

goals

• Evaluate test procedures for predicting material deterioration rates

and collector performance reliability;

® Evaluate the effects of collector material deterioration on collector

thermal performance;

® Determine the influence of environmental exposure conditions on collector material deterioration;

© Compare thermal performance measurements of outdoor natural exposure

with those obtained from exposure to solar simulators;

• Correlate data for tests from accelerated and normal operation of

collectors and materials;

© Determine the influence of thermal shock and moisture penetration.

The research plan [47] describes the test procedures to be followed. Where

possible, procedures using consensus standards outlined in [39,40] are to be

followed. Table 5 identifies the tests for solar collectors, while tables 6

and 7 outline the exposure tests for cover and absorber materials, respectively.

The plan also described the exposure conditions to be considered in the

selection of outdoor tests sites. These include:

• Annual solar radiation and seasonal distribution,

© Humidity,

• Ultraviolet radiation in solar flux,

• Average and extreme high-low temperatures,

® Total seasonal precipitation,

® Average and maximum wind velocities,

® Pollutants, including particulate matter, oxidants, and acid gases.

Four sites were selected for testing solar collector performance based on desired environmental conditions and availability. These included: Phoenix,

AZ (hot and dry); Cape Canaveral, FL (hot and humid); Palo Alto, CA (moderate

and dry); and Gaithersburg, MD (moderate and humid).

Needed test procedures were identified for both the whole collector and

collector materials to determine the influence of environmental exposure parameters. Tests results for the collector as a unit are discussed in section

4, while test results for collector materials and components are discussed

in section 5.

54
Table 5. Summary Description of Field Test Series on Solar Collectors

(from Ref. 47)

Test Collector Performance Conditions for Weathering Properties of

Series Measurement Exposure Test Series***

Initial measurement in Each collector pre-condi- 1. Observation of collector

Series accordance with ASHRAE tioned for each weathering performance and other

1 93-77. exposure by purging with characteristics for vardry air to remove the ious weatherizing times.

"dry Performance re-test remaining heat transfer

stagnation" after 3, 15, 30, 60, fluid 2. Provide data for

120 and 240 day performance data.

exposures.*

3. Provide data for

comparing collector.

Initial measurement in Collectors same as in 1 . Observation of effects

accordance with ASHRAE Series 1 except that of no-flow stagnation on

Series 93-77. Tests per NBSIR 78-1305A collector performance

2 Performance re-tests

will be performed during

Series 2 test collectors

and other characteristics.

"no-flow same as in Series 1. only. 2. Observation of effects

stagnation" of Thermal Shock Tests

representing (a) filling

a hot collector with

cool heat transfer medium and (b) summer rain

on a hot collector.

3. Observation of static

pressure leakage after

30 and 120 days of

exposure.

Performance of test During weathering exposure 1 . Observation of effects of

Series collectors measured in heat transfer for liquid. normal operation on collec3 accordance with ASHRAE tor performance and other

93-77. characteristics

"controlled

flow" Performance re-tests

same as in Series 1.

Initial measurement Pre-conditioning and 1. Observation of effects of same as in Series 1. weathering exposures same dry stagnation on collecSeries as in Series 1 except that tor performance and other

4 Performance re-tests a reflector** will be used characteristics with

same as in Series 1. on each collector during solar radiation amplified

"dry each day of weathering by a reflector.

stagnation exposure*. Solar radiawith tion measurements required 2. Obtaining temperature

augmentation both with and without history within collectors

reflectors" reflector. for most severe exposure

conditions

Individual days with solar radiation of 17,000 kj/m^.day or greater as measured in the plane *

t the collector aperture without the influence of a reflector.

The reflector is described in reference 47.

All series include provision of data for comparisons between test series, test sites (cl lm.it i. regions), collector designs, etc.

55
Table 6. Exposure Tests for Cover Materials

(from Ref. 47)

Exposure Conditions Value of Range Exposure Time

Temperature (indoor) a) 70°C

b) 90°C

c) 125° C

500, 1,000 and 2,000 hr.

Temperature and Humidity

(indoor)

a) 50° C and 98% RH

b) 70°C and 95% RH

c) 90°C and 95% RH

500, 1,000 and 2,000 hr.

Temperature and Radiation

( indoor)

Xenon arc weathering

machine

a) 70°C

b) 90°C

500, 1,000 and 2,000 hr.

Solar Simulator a) Tungsten

b) Xenon simulators

with irradiance

of ~950 W/m^ and

~70°C

30, 60 and 120 cycles*

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

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

months***

* Each cycle consists of 5 hrs. irradiation and 1 hr. cooling, as defined in

reference 47.

** Radiant exposure must exceeded 17,100 kj/m^ for each day.

*** One equivalent month equals 6.625 x 10® J/m^ (15,835 Langleys)

Table 7. Exposure Tests for Absorber Materials

(from Ref. 47)

Exposure Conditions Value of Range Exposure Time

Temperature (indoor) a) 150°C

b) 175° C 500, 1,000 and 2,000 hr.

Temperature and Humidity

( indoor)

90° C and 95% 500, 1,000 and 2,000 hr.

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

Temperature and Radiation

(indoor) Xenon arc weathering

machine at 90°

500, 1,000 and 2,000 hr.

Solar Simulator a) Tungsten

b) Xenon simulators

with irradiance

of ~950 W/m^ and

~130°C

30, 60 and 120 cycles*

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

~160° C 80, 160 and 240 days**

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

months***

* Each cycle consists of 5 hrs. irradiation and 1 hr. cooling, as defined in

reference 47.

** Radiant exposure must exceeded 17,100 kj/m^ for each day.

*** One equivalent month equals 6.625 x 10® J/m^ (15,835 Langleys)

56
Five criteria were used to select collectors for testing. These included

commercial availability and representativeness of the materials and

construction practices commonly available. The collectors selected were

flat-plate, aqueous-liquid types with one or two covers of glass, sheet plastic,

or film, and selective or nonselective absorber coatings. Tracking concentrating collectors, collectors with nonaqueous or gaseous heat-transfer fluids

did not meet the criteria, and so were not included in the testing. Table 8

outlines the key features of the eight collector specimens selected. Tables

9 and 10 present the cover and absorber materials included in the materials

tests

The testing program called for data to be collected on thermal performance,

material properties, and environmental conditions. Computerized data reduction

routines were advocated to deal with the large volume of data, and to identify

correlations between relevant variables. The resulting data analysis would

focus on three major areas of performance:

• Analysis of measured collector efficiency to correlate thermal

performance changes with elapsed time and collector environment.

• Analysis of changes in material properties, including the correlation

of property changes with exposure time and environmental conditions,

and the effect of changes in material properties on collector thermal

performance

® Investigation of stagnation temperatures of collector materials. This

includes the development of a mathematical model for predicting stagnation temperatures. Experimentally verified stagnation temperatures

could then be used in developing accelerated aging procedures.

4. 5. 2. 2 Statistical Uncertainty in Thermal Performance Data

The round-robin study of collector performance described earlier (4.4.1)

demonstrated that corrections for environmental difference such as wind and

solar irradiance reduced variability in the data between sites only slightly.

It appeared that this variability was due more to systematic measurement

problems arising from differences in instrument calibration, sensor installation, and general procedures.

As a result, NBS conducted a series of tests and analytical studies to determine

the overall uncertainty in thermal performance data for solar collectors. This

effort was designed to illustrate how the uncertainty in individual tests can affect the rating and selection of collectors, as well as to determine degradation with exposure or operating time [48,49]. Eight liquid flat-plate collectors representative of commonly used materials and construction types were selected for the tests (see tables 8 and 11). All collectors from each manufacturer were from the same production lot. The collectors were tested at four

test sites representing climatological extremes.

57
Table 8. Solar Collector Test Specimen

(from Ref. 47)

Code Cover Material^ Absorber Material^

Outer Inner Coating Substrate

A Glass! — Black Nickel Steel

B Glass2 Glass2 Black Velvet Paint Copper

C Glass3 FEP Film

Heat Trap

Black Velvet Paint Copper

D Glass2 >4 Glass2 »4 Black Chrome Steel

(nickel flashed)

E FRP - Type la — Flat Black Paint

(lacquer primer)

Copper

F Glass 1 — Copper Oxide Copper

G FRP - Type II FEP Film Porcelain Enamel Steel

H Polyester Film FEP Film Flat Black Paint

(siliconized polyester)

Aluminum

* Water White Glass

2 Low Iron Glass

2 Ordinary Plate Glass

^ Etched Anti-Reflection Treatment

5 Obtained from collector manufacturer literature

Table 9. Cover Test Materials

(from Ref. 47)

Code 1 Cover Material Transmit tance^ (Controls)

E FRP Type la 0.85

G FRP Type II 0.84

H

2 Polyester/FEP

(outer) (inner)

0.85/0.96

J Polycarbonate 0.88

K Polyvinyl Fluoride 0.89

L FRP Type lb 0.84

M FRP Type III 0.78

N Polymethyl methacrylate

(acrylic)

0.90

O

2 Glass^/Polyvinyl 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 0 tested at the materials

level only.

2 Materials to be exposed as a combination in the cover mini-boxes and in the accelerated weathering machine.

Materials to be exposed individually in all other tests. Glass and FEP materials not to be used for individual tests because of proven stability.

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.

^ Ordinary plate glass.

58
Table 10. Absorber Test Materials

(from Ref. 47)

„ . 1 Absorber Material

o Optical Properties

Code

Coating Substrate Absorptance^ Emittance^

A Black Nickel Steel 0.87 0.13

C Flat Black Paint Copper 0.98 0.92

D Black Chrome Steel

(nickel flashed)

0.97 0.07

E Flat Black Paint Copper 0.95 0.87

F Copper Oxide Copper 0.96 0.75

G Black Porcelain Enamel Steel 0.93 0.86

H Flat Black 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 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.

- These properties are dependent on the formulations and manufacturing processes used.

Other products within a generic class of materials may have significantly different

properties

^ Average values based on a minimum of ten test specimens.

59
Table

11.

Test

Collector

Specimen

Description

(from

Ref.

48) 33

S-i c

a)bO3

S-i 3

<:ou

a)n3 ^ r—

CN CN

O 05 00 rH

4-1 CN CO o CN o CN 50 00

s-i

a 00 50 05 rH 50

0)

w • • • • • • • • 9?<1 rH r-H i-H r—

rH rH CN CN

05 o CN 05

n CN CN CO 50

05 ---v

m co 00

n 05 CN 50 i-H OCN rH r^. n 50 00 05 in 05

s-i

a » • • • • • • • 35 w CN *-h CNJ rH rH rH CN CN

asoc

cd cn 50 CN

m 50 05

u rH 05 05 o 00 00 00 u • • • • • • • • •H o o o O o o o o aw

cuoc

s-i co 00

m 50 CO

m

Cd 4-J 00 05 05 05 05 05 05 05

rH D-i • • • • • • • • O H o o o o o o o o

C/5

O

05 -O<

l-l 0) rH 4J 4-1 0) a 3

cu 01 0)

a •H

XX

rH OS > > o S-l •H 3

cd o rH rH

M CL, * 3 N S-i •H •H 0) <u -3 o •H •H 3

S-i 2 > > CJ> s-l 3 3 4-1 3 CU s-l rH rH O C/5 u M 4-» OS 4-1 M 3 3 3 3 3 3 4J

cd o 3 C

O 3 CU cr

a 3 a •H >5 3

cd cd -H cd •H cd 3 3u S-i 3 rH rH •H rH rH rH cd rH 3 o O 3 •H o 3

00 PQ Oi PQ Ol PQ

H o 04

W C/5 o. 04

0)oc

cd

4-1

S-i 4-1 cd *H o 00 50 50 in o 50

n 50

i—i a 05 00 00 05 00 05 00 05 00 05 O 05 • • • • • • • • • •

C/5

C o o O o o o o o o o cd

S-l H

a

a c rH cd

S-l o •H S-i cu 1 S-i PH H X) 1 1 o 1 M 05 05 05 1 1 e 1 05 c 4-J s: 05 1 1 a

a

M & cd •i-i cd cu cd 04 rH 04 rH O »—

J3 cu 4-4 rH

w •H PH

h-J o H

WO PH PH PH PH

cu 05 3

4-1 05 4-» •H cd •H S-l

S-l 2 3 rH /TS

n

M 3

as 3 O o MM 4-1

4-J S-i X) C/5 3 S-4 05 M 05 cu 01 05 3 s-l w 3 3 o (U 05 05 4-1 p3 05 a 3 CO SO >5

a

4-1 Cd !S cd cd 3 3 O, >5 4-J 3 04 >5 rH rH

CO rH

O i—

rH 4-1 rH 2 H 3 rH 2 Eh o 3 O H O O4 W O MH V--/ 3 o V-/ 04 PH

cu X)o <3 PQ CJ

Q

w PH o pc

C_5

60

Average

of

values

reported

by

four

test

sites
Two categories of uncertainty were identified in the thermal performance data

for solar collectors. These included:

• Measurement uncertainties related to the individual and cumulative

instrument calibration and operational errors. Measurement accuracy

problems were identified for solar irradiance, fluid flow rate, temperature and working fluid properties, and the data acquisition system.

• Meteorological uncertainties related to variations in the site

environment including ambient air temperature, wind speed, total solar

irradiance, beam solar ir radiance incident angle, and reflected solar

irradiation.

The data analysis was performed using the statistical concepts and terminology

recommended by ASTM [50]. This analysis was performed for both thermal efficiency and incident angle modifier (IAM) measurements. Variability was calculated for measurements made "within" a single test site and "between" various

test sites. Tables 12 and 13 present the intercept, mean, standard error of

the mean, standard deviation, and coefficient of variation for the intercept

and slope respectively for data obtained "within" and "between" test sites. As

might be expected, the "between" standard deviations are somewhat larger than

those for "within" site data.

Figure 33 illustrates the calculated measurement uncertainty. The solid curves

represent the difference in calculated thermal performance for a single glasscover, selective-absorber type collector exposed to the allowed extreme

environmental conditions. (Calculations are given in reference [29].) Experimental verifications performed with a Xenon arc solar simulator [49] are indicated by black squares bracketing the curve for average outdoor data.

Table 14 presents the incident angle modifier data.

Based on the results of the experimental tests and statistical analysis, it was concluded that the total experimental uncertainty is about the same as the ran- dom error predicted by ASHRAE 93-77. Furthermore, measurement error was believed to be the major contributor to the "within" site variability, while

environmental effects appeared to be a significant factor affecting the loss

coefficient data obtained "between" sites (reproducibility). Heat loss coefficient data were analyzed before and after testing to examine degradation problems. This analysis indicated that application of uncertainty values for

thermal performance data can have significant impact on evaluating collector

degradation, collector rating, and seasonal efficiency. The following

conclusions were drawn:

• Material degradation is about equivalent to a 0.10 change in cover plate transmittance or absorber solar absorptance and eraittance.

• Collector all-day thermal output varies from ± 17 to i 68 percent for

specific winter operation conditions, which results in significant

ranking changes.

61
Table 12. Standard Deviation and Coefficient of Variation of Intercept Within and Between Test

Sites (from Ref. 48)

Collector Intercept

Mean

Standard Error of Mean

Standard Deviation and Coefficient of Variation

Within Between

Value Percent s r %cvr

S R %cvR

A 0.610 0.0061 0.99 0.0159 2.60 0.0176 2.88

B 0.652 0.0073 1.11 0.0105 1.60 0.0167 2.56

C 0.537 0.0042 0.78 0.0146 2.71 0.0146 2.71

D 0.643 0.0036 0.56 0.0125 1.94 0.0215 1.94

E 0.603 0.0039 0.56 0.0118 1.94 0.0119 1.97

F 0.651 0.0066 1.01 0.0155 2.37 0.0181 2.78

G 0.560 0.0101 1.80 0.0120 2.13 0.0222 3.98

H 0.632 0.0053 0.83 0.0080 1.25 0.0123 1.94

Average Coefficient of Variation Within (%cv R) = 2.07%

Average Coefficient of Variation Between (%cv R) = 2.39%

Table 13. Standard Deviation and Coefficient of Variation of Slope Within and Between Test Sites

(from Ref. 48)

Collector

Slope

Mean

(W/°C*m2

) Standard Error of Mean

Standard Deviation and Coefficient of Variation

Within Between

Value Percent s r %cv rS R %cvR

A 4.531 0.236 5.20 0.375 8.22 0.556 12.3

B 5.535 0.078 1.41 0.267 4.87 0.267 4.87

C 4.287 0.120 2.81 0.233 5.48 0.307 7.12

D 3.095 0.216 6.98 0.238 7.71 0.471 15.26

E 6.439 0.167 2.59 0.466 7.22 0.500 7.80

F 5.996 0.254 4.51 0.363 6.07 0.613 10.20

G 5.405 0.145 2.69 0.278 5.16 0.363 6.77

H 5.661 0.145 2.56 0.153 2.70 0.312 5.54

Average Coefficient of Variation Within (%cv R) = 5.93%

Average Coefficient of Variation Between (%cv R) = 8.37%

62
I

' w

Figure 33. Uncertainty associated with the ASHRAE 93-77 efficiency measurement

for a single-glazed selective absorber-type collector (from Ref. •-/

63
Table

14.

Incident

Angle

Modifier

Standard

Deviation

Values

Within

and

Between

Test

Sites

(from

Ref.

48)

Average %cv

i—

CO

II

l-l >o

ens

sO• oCNJ

II

14 >o

s-s

<3- CO

II

cd >o

S'?

1^ o i-H 00 CO uo

CM COm i-H CM

sc i—

o O 1 l 1 1 i—

o o • • • as | 1 1 1 • • • 00 o o o co o o o CO

CO CO *-H

O o 00 CO 00 as o m ON CO CM CM CNl sO o CM o o r-H o o ^H o O • • • m • • • CM • • • o o o o o o o CM o o o

00

m SO o o 00 co CNl 1 Csl OS CM CM i-H CO in

&4 o o o o o o i-H o o • • • CO • • • r-H • • • i-H o o o CO o o o CO o o o UO

oj&, 00 o CNl uo i-H co

So, sO CO

m sO CM

Ed r*H o o 1 ! 1 1 •-H o o • • • r-H 1 i 1 I • • • sO

X o o o co o o o CM oo

0) o i-Hm in UO UO o 1—1 CM CO CN| CO so

r—

O i—

o o ^H o o r-H o o o • • • • • • sO • • • o o o o CS o o o CM o o o CM

00 00 00 o o 00 i-H sO

CM CM CM sO i-H co

C_> CM o o CN] o o CM o o • • • as • • • O • • • o o o i-H O o o i-H o o o i—

so COm

m as o o o CM i-H CM

CQ ^H o o CM 1 1 1 | ^H o o • • • • 1 1 1 i • • • CM o o o o o o CM o co CM o o as CM

r-H CNJ o r»H r-H as CM

m

<d o o o ^H o o o o o • • • m • • • -o- • • • r- o o o uo o o o »-H o o o U0

s-a> c c C

4— 03 03 o 03 03

O 03 03 o

0) l-i •H

X 14 1-4 14 14 •H E tO CO 4-1 CO CO 4-1 cO CO 4-J tO 03 I-l 03 cO 03 I-l 03 CO 03 14 03 cO

i- e C o (S •H

u C3 (S o C -H |4 e e o IS •H cd

CO cO CO I-l CO > > CO cO I-l CO > > co CO |4 CO > > a 0) 4-1 i-l 4J ai o <u 43 I-l 4-i 05 a 05 4-) I-l 4-) a) o s CO w CO o S'? X CO w CO Q ss CO Ed CO Q e-s

<u

c.

i- a) <15 <15 i 4-1 4-) 4-1 CO c i •H •h •H

CO CO CO CO 1«

•—

CM

to 4-J 4J 4-J CM

05

W CO

CO <15 05 05 * c H H H i-H

64
• Residential annual heating and domestic hot water solar fraction can vary by as much as - 6 to 7 percent annually.

The analysis of uncertainty in solar collector data indicated that variability

collector performance continues to be a real problem.

4.6 FLAMMABILITY OF SOLAR COLLECTORS

As part of the DOE collector test program (section 4.5.1), research was initiated

to evaluate the flammability of solar collectors to determine applicable test

methods. As a possible rating basis, references 19 and 20 suggested that

solar collectors meet Class C rating for intermittent flame, spread of flame,

and burning brand tests of ASTM E 108 "Fire Tests of Roof Coverings" [46],

NBS directed an experimental program at two different laboratories to:

(1) evaluate the use of ASTM E 108 as a means for determining the influence of

roof-mounted collectors on the fire characteristics of roof coverings;

(2) determine modifications in the test procedure that might be required to

make it applicable to roof mounted collectors; and (3) determine the influence

the flat-plate solar collectors constructed of various materials might have on

the fire characteristics of roof coverings [51]. The study did not include

the rating of collectors tested, potential self-ignition of collectors, or

the fire resistance of roof assemblies with solar collectors when exposed to

interior fires.

The experimental work was conducted at two private testing laboratories. The

tests were conducted on 11 collector designs as outlined in table 15 and the four

mounting configurations as shown in figure 34. The test roofs were wood decks

covered with Class C asphalt-impregnated organic-felt shingles. Burning brand,

intermittent flame, and spread of flame tests were conducted.

For the burning brand tests. Class A, B, or C brands were placed on the

collector covers and, for collectors mounted on standoffs, the brands were

placed beneath the covers directly on the roof surface. For the intermittent

flame test, the test deck (including collector) was subjected to a luminous

gas flame—of a temperature of 700° to 760° C ± 30°C (1300° to 1400° F ± 50°F)

depending on class approximately the width of the deck, which uniformly bathed

the surface. The flame was applied intermittently for a specified period

ranging from 1 minute on and 2 minutes off to 2 minutes on and 2 minutes off,

in 3 to 15 cycles. For the spread of flame test, the same type of flame was applied continuously for 10 minutes for Classes A and B and for 4 minutes for

Class C. Figure 35 shows a schematic drawing of the test apparatus.

The Class C brands ignited the one collector cover with acrylic glazing and

destroyed the collector, but had no effect on either tempered or annealed glass

and caused only a small area of blackening on fiber reinforced plastic (FRP) covers. Both Class A and B brands readily ignited FRP covers, shattered tempered glass glazing and broke annealed glass, but the absorber plate prevented

further fire penetration of the collector.

65
Table 15. Fire Test Collector Description (from Ref. 51)

Materials

Collector

Code

Case Sides Case Back Glazing Insulation Absorber

Approximate Dimensions

Length x Width x Depth

29-L Aluminum Aluminum Glass

(tempered) Foam plastic * Aluminum 259 x 127 x 17.7 cm

(102 x 36 x 3 1/2 in)

32-L Wood Plywood FRP 2 Fiberglass Aluminum 305 x 127 x 17.7 cm

(120 x 48 x 5 in)

41-L Wood Plywood Double Glass

(tempered)

Urethane foam Copper 239 x 91 x 8.6 cm

(94 x 36 x 3 3/8 in)

43-L Molded FRP Molded FRP FRP Urethane foam Copper 244 x 91 x 14.6 cm

(96 x 36 x 5 3/4 in)

45-L Aluminum Aluminum FRP Foam plastic Aluminum 300 x 96 x 7.6 cm

(118 x 37 3/4 x 3 in)

46-L Wood Hardwood Glass

(tempered)

Isocyanurate

foam

Copper 236 x 89 x 8.9 cm

(93 x 35 x 3 1/2 in)

54-L Wood Plywood Glass

(annealed)

Fiberglass Aluminum 251 x 126 x 8.9 cm

(99 x 49 3/4 x 3 1/2 in)

103-L Aluminum Aluminum Acrylic Polystyrene

(loose fill)

Aluminum 244 x 122 x 19.1 cm

(96 x 48 x 7 1/2 in)

108-Steel Steel FRP Fiberglass Steel 220 x 102 x 7.6 cm

(86 1/2 x 40 x 3 in)

126-L Aluminum Aluminum Glass

(tempered)

Fiberglass Copper 196 x 89 x 12.7 cm

(77 x 35 x 5 in)

78-L Aluminum Aluminum FRP None Aluminum 500 x 61 x 18.4 cm

(196 3/4 x 24 x 7 1/4 in)

1

* Rigid foam plastic, type unknown

66
Figure 34. Collector mounting configurations

(from Ref. 51)

Figure 35. Schematic of fire test apparatus

(from Ref. 51)

67
In the intermittent flame tests conducted on three collectors constructed of

FRP, wood, and aluminum, neither the shingles nor the collector were ignited.

During the flame spread tests, both FRP and acrylic collector covers were

ignited within less than 4 minutes. The spread of flame under collectors was shown to be primarily a function of separation or standoff distance between

the collector and the roof covering. The rate of flame travel did not depend

on the material (wood, steel, aluminum) of the collector.

From the test results, it was concluded that the burning brand test of ASTM

E 108 could be applied with only minor modifications to solar collectors. It was also determined that collectors meeting the burning brand test and the

spread of flame test also met those for the intermittent flame test. The

presence of the collector on the roof appears to have the most significant

impact on the flame spread test. The results indicate that collectors mounted

on roofs with a standoff greater than approximately 4 cm (1-1/2 inch) above a Class C roof covering will result in flame travel under the collector greater

than that allowed by the test criteria, regardless of collector construction.

The results also indicate that collectors with combustible glazing could provide

a path for rapid flame spread from one area of roof to another. This could be

a particular problem for thermoplastic materials.

The researchers suggested that further investigations should be conducted to

determine the suitability of small-scale flammability tests such as ASTM D 635

for evaluating the combustibility of glazing materials. In addition, the

possibility of fire penetration into the building air duct system from burning

collector covers should be investigated.

68
5. COLLECT 'R MATERIALS RESEARCH

The preceeding section summarized various evaluations of the performance

of a solar collector as a unit. Collector performance can also be analyzed in

ter-.s of the performance of individual materials such as glazing or insulation.

In section 5, research and test methods for cover plates, absorber materials,

collector insulation, gaskets and sealants, rubber hose, containment materials,

and heat transfer fluids are reviewed. The bulk of this research dealt with

issues such as durability and stability over time, rather than thermal performance, with the majority of the resulting standards being promulgated by ASTM

rather than ASHRAE. Yancey [52] summarized NIBS materials research activities

in a companion document.

5.1 COVER PLATES

5.1.1 Introduction

In a flat-plate solar collector, cover plates perform a number of important

functions. They minimize heat losses from the front surface of the solar

collector, while protecting the internal components of the collector from

adverse weather conditions such as rain, snow, and hail. The cover plates

must also maintain the maximum possible transmittance of incident solar

radiation.

The properties of most commonly used cover plate material, glass, are wellknown and understood. Glass is subject to breakage, relatively heavy, and

costly, so that alternative materials such as plastics have been used. Plastics

are generally less costly, lighter in weight, easier to fabricate, and can have

higher impact resistance. On the other hand, the long-term durability of

plastic is not well documented. Exposure of plastic to sunlight, airborne

abrasives, and aging can reduce transmittance and induce brittleness. As a result, a number of research studies were performed at NBS and elsewhere of

documented field problems [53, 54, 55, 56] to determine methods for evaluating

the optical and mechanical properties of cover plates, as well as durability

and resistance to adverse weather such as hall. The studies reviewed in sec- tion 5.1 [62, 63, 64, 65] formed the technical basis for ASTM Standards

E 765-80 [57], E 782-81 [58], E 881-82 [59], and E 822-81 [60], as well as a set of proposed revisions to ASTM E 424-71 [61].

5.1.2 NBS Research on Cover Plates

In 1976, NBS initiated a series of laboratory and field studies to deter-ine

the optical, mechanical, and durability properties of different cover plate

materials [62] . This research was designed to assess the performance

requirements for cover plate materials, identify test methods for assess inc

performance, collect data on performance, and prepare draft standards for

consideration by ASTM Committee E-44 on Solar Energy Conversion.

N3S conducted research in several areas: cover plate performance after •

- and accelerated weathering [62, 63]; determination of cover plate pr per::*.-

69
including stability, warpage, and mechanical response [62]; measurement of

optical properties [64]; and determination of procedures for assessing hail

impact on the durability of cover plate materials [65]

In the first study [62], researchers identified field problems, determined

performance requirements for and properties of cover plate materials, and

identified degradation factors. The following potential problems with different cover plate materials were identified: breaking, rupturing, and cracking

caused by thermal stresses, accidental impact, and environmentally induced

embrittlement; impact damage from hail, accidents, or vandalism; loss of solar

transmittance due to yellowing, mechanical distortion, dirt, or moisture con- densation on either surface of the plate material. These problems were then

evaluated through research which determined the properties of cover plate

materials and their response to various degradation factors. Laboratory and

field exposure tests were conducted on selected typical cover plate materials

commercially available in 1976.

Primary cover plate properties include solar transmittance, dimensional

stability, tensile properties, and hail impact resistance. Secondary properties include the transmittance of long wavelength radiation, abrasion resistance, and static electric charge build-up (which could attract and hold dust

or dirt particles, thereby reducing solar transmittance.) The most important

degradation factors include solar radiation (primarily ultraviolet (UV) radiation) and elevated temperatures. In addition, synergistic reactions from a combination of solar radiation, high temperatures, and moisture, appear to be

significant factors in the degradation of plastic materials. Furthermore,

thermal cycling, moisture, and air pollution may cause surface degradation.

The materials were subjected to aging procedures which simulated common degradation factors. These aging procedures included: heat stability aging

given in ASTM D 794 [66] , artificial weathering with xenon arc lamp (ASTM D 2565)

[67], and natural weathering exposure. For natural weathering, insulated

wooden "mini-collectors" were used in tests conducted in Gaithersburg, MD;

Miami, FL; and New River, AZ. After exposure, the specimens were tested for

solar energy transmittance, linear dimensional stability, and warpage. One

objective of the tests was the comparison of results from screening tests for

solar energy transmittance using two different procedures recommended in ASTM

E 424 [61]. The various tests were conducted on ten materials, including

glass, plastic films, and plastic sheets.

The results from the laboratory and field studies [62] indicated that:

® Accelerated laboratory testing procedures offer an effective means of

screening cover plate materials.

• Heat aging at 150°C (302°F) for 500 hours is an effective means of

identifying materials which will develop problems in outdoor weathering.

• The mini-collectors provide useful data on outdoor exposure.

70
• Factors which contribute to solar energy transmittance losses include

solar radiation, moisture, and air pollutants, as well as dust and dirt

retention. Solar radiation dosage, however, appears to contribute to

decreases in the mechanical and physical properties of materials.

• Method A of ASTM E 424, which uses a spectrophotometer, has better

precision and reliability than Method B, for determining solar energy

transmittance of cover plate materials.

5. 1.2.1 Calculation of Solar Transmittance

NBS also conducted a parametric study of the procedures for measuring the

spectral transmittance of cover plate materials [64], Method A of ASTM E 424

allows two procedures for calculating the spectral transmittance of a solar

collector cover from data obtained with an integrating sphere spectrophotometer.

The obtained data may be integrated using the weighted ordinate method or the

selected ordinate method. The NBS results indicate that these two methods do

not yield identical results, with the weighted ordinate method producing higher

solar transmittance values than the selected ordinate method (up to one percent

higher .

)

Method A also assumes that the solar energy spectral distribution or air mass

is equal to 2.0 (the value obtained when the sun reaches an angle of 60 degrees

between the zenith and the line of sight). Most solar collectors, however,

are exposed to more hours of radiation that approximates air mass 1.5. As a result, Roberts, Masters, and Clark [64] assessed the effect of varying air

mass value upon the transmittance and reflectance values of typical materials.

They determined that the three values of air mass studied, 1.0, 1.5, and 2.0,

could vary the obtained transmittance values by as much as 4 percent. This

effect is greatest for those materials with the least transmittance in the

ultraviolet range.

Consequently, NBS recommended to ASTM Committee E44 that ASTM E 424 be revised

to specify only one integration method for calculating transmittance, since

the two calculation methods do not agree. It also recommended that the solar

energy distribution used in Method A be changed to air mass 1.5, as this appears

to be a more representative value.

5. 1.2. 2 Determination of Impact Resistance

In addition, NBS studied the effect of simulated hail impact on solar collectors

[65,68]. These studies were concerned with estimating the maximum size of hail

[68], hail impact loadings and stresses, simulating hail by ice balls, and

selecting launching equipment. They identified means of mounting cover plate

test specimens, determining suitable points of vulnerability, predicting appropriate ice ball velocity and direction, and assessing damage to the collector

cover plate. These experiments confirmed the feasibility of using ice balls

propelled from a compressed-air launcher to simulate hail.

71
5.1.3 ASTM Standards

Based on the laboratory and field studies, NBS drafted several proposed methods

for evaluating cover plate materials and submitted them to ASTM Committee E44.

The committee promulgated the four standard practices described below, which

closely follow the draft proposals. It is currently considering revisions to

ASTM E 424 in line with NBS test results. Each of the four standards will be

reviewed below in some detail to provide an indication of the kinds of

information covered by the standard.

5. 1.3.1 ASTM E 765-80

Standard Practice for Evaluation of Cover Materials for Flat Plate Solar

Collectors — ASTM E 765-80 — provides evaluation procedures for measuring the

primary properties of cover plate materials and for assessing the durability

of cover plates by comparing initial property measurements with those obtained

after exposure to aging procedures [57].

The standard specifies the number and geometry of the test specimens. It

indicates that tension test specimens should be cut from the same piece of

material that was subjected to the aging procedures. It states further that

optical property tests should not be performed on specimens damaged from the

physical property tests. The specimens should be conditioned for at least 40

hours prior to testing. Both conditioning and testing should occur at 23 ± 2°C and 50 ± 5 percent relative humidity.

The property measurement tests for ASTM E 765-80 involve measurement of solar

energy transmittance, effect of dirt retention on solar transmittance, dimensional stability, tensile properties, and impact resistance. Four measurements

of solar energy transmittance are to be made by the appropriate method from

ASTM E 424. Dirt retention is determined by measuring solar energy transmittance after exposure to natural weathering. The specimen is first measured,

then cleaned, and then remeasured. The difference in solar transmittance

before and after cleaning is a measure of the dirt retained during natural

weathering. Linear dimensional stability is measured by ASTM Method D 1042

for rigid and semi-rigid materials, and by ASTM D 1204 for non-rigid materials.

Warpage for rigid and semi-rigid materials is measured by D 1181. Tensile

properties, which must be measured only after careful conditioning, include

tensile strength, percent elongation at yield and at break, and elasticity.

Method ASTM D 638 is specified for use with rigid and semi-rigid plastic

materials, and ASTM D 822 for flexible membrane materials. Finally, impact

resistance is to be measured by ASTM E 822 (which was only a draft standard

when E 765 was adopted.)

Aging procedures include heat stability, natural weathering, and optional

accelerated weathering procedures. Heat stability is measured separately for

outer and inner cover materials using Practice D 794 for 240 hours. The test

temperature is 75 ± 2°C for outer and 125 ± 2°C for inner cover materials.

To measure natural weathering, the cover test specimen is exposed to natural

conditions for at least 12 months using Practice E 782. Although accelerated

weathering should not be substituted for natural weathering tests, it may be

72
used as a screening tool. (If accelerated weathering test results can be shown

to correlate with natural weathering data, then accelerated weathering may be

effective for obtaining long-term durability information in a short time.) Two

different procedures may be used for accelerated weathering—a laboratory

procedure or an outdoor procedure with concentrated natural solar radiation.

For the laboratory procedure, specimens are exposed according to Procedure A

of ASTM practice D 2565. Three increments of exposure length are specified,

and vary from 700 to 3800 hours. Moisture conditions should be simulated by a water spray for 30 minutes following 90 minutes of light exposure. In the outdoor exposure, specimens are exposed to concentrated natural solar radiation

machines such as referenced in ANSI Z97. 1-1975, until they have received a specified level of total incident radiant exposure. Again, the application of

a water spray at specified levels and intervals is recommended for materials

exposed to moisture in service.

Determination of cover plate properties includes measurement of solar

transmittance and effect of dirt on solar transmittance, as well as determination of dimensional stability, warpage, tensile properties, and hail impact

resistance. Measurement of durability is specified by aging procedures, such

as heat stability, natural and accelerated weathering, and outdoor exposure to

concentrated natural solar radiation.

The standard also specifies procedures and sequencing for the various property

measurement tests. Solar transmittance, tensile strength, impact resistance,

initial warpage, and initial linear dimensions are to be measured before aging.

After aging, solar transmittance, tensile strength, effect of dirt retention,

warpage, and final linear dimensions are to be measured. Impact resistance

should be measured after aging unless sample size limitations prevent this

measurement. Solar transmittance, impact resistance, dimensional stability,

and dirt retention should be measured on a minimum of three separate specimens

at each exposure increment for each aging procedure. Table 16 provides a flow

chart of the sequence for testing both properties and response to aging. The

standard also requires the development of a test report which documents the

solar collector cover plate materials, and the data obtained from the various

testing procedures.

5. 1.3. 2 ASTM E 782-81

Standard Practice for Exposure of Cover Materials for Solar Collectors to

Natural Weathering under Conditions Simulating Operational Mode [58] sets forth

a procedure for evaluating cover plate materials, including both glass and

plastics, that are exposed to natural weathering conditions which approximate

normal operating (rather than stagnation) conditions. It outlines the use of a weathering box test fixture to elevate cover plate temperature (similar to the

"mini-collectors" used by NBS [62]). Elevated temperatures are used with solar

radiation since this combination may cause some cover plate materials to degrad.*

more rapidly, thus allowing a more realistic assessment of material stability.

The practice also provides guidelines for fabricating the weathering box to

provide uniform exposure to minimize unwanted variation. The weathering box

should be made of corrosion resistant metal with a weep hole for drainage. Thu

interior should be coated with a flat black nonselective coating. Organic

73
Table 16.

Property tests

Aging procedures

Property tests

Flow Chart of Sequence for Testing Response to Aging

(from Ref. 62)

74
materials should be eliminated from the interior to minimize outgassing.

Mounting procedures are given for rigid and semi-rigid glazing, as well as films. Assembly procedures for the weathering box are also given.

The weathering boxes are to be mounted on weathering racks such as those

described in ASTM Practice D 1435. The racks should be capable of angular

adjustment with the axis of rotation on an east-west line. Rack tilt should be

adjusted seasonally (at least four times a year) to maximize incident solar

radiation. Although a period of exposure is not specified, the standard notes

that repeated exposure at different seasons over a period of more than 1 year

is needed. ASTM E 765 is referenced as a means of evaluating weathering effects.

The practice also provides a procedure for mounting the weathering boxes on weathering racks capable of adjustment to variable angles throughout the year.

Finally, it outlines the data to be collected by the weathering box procedure.

These include identification of the collector plate material, material test

configuration, mounting procedures, geographical and climatic factors, exposure

duration, radiant exposure, monthly temperature mean and range, visual inspection data, and description of control specimens. Additional, optional data

include monthly relative humidity range and mean, monthly and daily rainfall

or dew, daily radiant energy and temperature range, wind direction and velocity,

atmospheric conditions including air pollution, ultraviolet radiation, and

maximum absorber plate temperature. Again, the reporting procedures are specified.

5.1 .3.3 ASTM E 881-82

Standard Practice for Exposure of Solar Collector Cover Materials to Natural

Weathering Under Conditions Simulating Stagnation Mode [59] provides a procedure

for evaluating cover plate materials exposed to natural weathering at elevated

temperatures approximating stagnation conditions. As such, it is a companion

document to ASTM E 782-81, for evaluating both plastic and glass materials

under normal operating conditions. It also uses a weathering box technique

similar to that described in E 782-81. The practice sets forth the configuration for the weathering box, including a detailed appendix for determining the

back and edge loss coefficient. Figure 36 presents a sketch of the weathering

box. Procedures for natural weathering (under stagnation conditions) are given

along with another appendix for calculating variable angle exposure schedule.

5. 1.3. 4 ASTM E 822-81

Standard Practice for Determining Resistance of Solar Collector Covers to Hall

by Impact with Propelled Ice Balls [60] describes a procedure for assessing

the ability of collector cover plates to withstand the impact of falling hail.

Propelled ice balls are used to simulate hail. The data obtained by using the

practice can be used to evaluate the impact resistance of a single material or collector, or to compare several collector covers, or to provide common data

for selecting covers in a particular area. They can also be used to assess the

effects of environmental conditions on changes in impact resistance.

75
MATERIALS

BOX AND SPACER 19mm (34") WOOD PAINTED WHITE ON OUTSIDE

INSULATION 50mm (2") FIBERGLASS BLANKET

ABSORBER

BLACK PORCELAIN ENAMEL

ON STEEL. a= . 93. 6= 85

SEALANT

RTV SUITABLE FOR OUTDOOR

EXPOSURE

NOTE

VENT HOLES WERE DRILLED

IN THE BOTTOM OF THE BOX AND THE END OF THE SPACER

TO MINIMIZE MOISTURE ACCUMULATION

Figure 36. Sketch of mini-collector (weathering box) for natural

weathering exposure (from Ref. 62)

76
Standard Practice E 822-81 applies to rigid, semi-rigid, and flexible cover

plate materials. It describes use of a launcher to propel an ice ball at a desired, calculated velocity with an accuracy of within ± 2.5 cm of the intended

impact point. A test specimen may consist of a complete glazing assembly,

complete solar panel, or section of cover plate material. Test base material

and arrangement are specified. A pre-conditioning period of not less than

24 hours at 23 ± 2°C and 50 ± 5 percent relative humidity is specified for

test materials. Testing must occur within one hour of preconditioning.

Ice ball size should correspond to hail size typical of a given location (as

determined from local weather records or various publications referenced in the

standard). Uniform, spherical, crack-free ice balls are made in molds stored

in a freezer maintained at -12 ± 5°C. The velocity used in launching the ice

ball must correspond to the ice ball diameter, however. A formula is given for

calculating the resultant velocity. The launcher should be positioned so that

the path of the ice ball will be essentially perpendicular to the cover plate

material. Four impact points are specified, each in a corner 15 cm from both

supporting edges. The test should be terminated when the cover plate material

is totally destroyed. Launch velocity, iceball weight and diameter, and all

observable effects should be recorded.

5. 1.3. 5 Proposed Revisions to ASTM E 424-71

Standard Test Methods for Solar Energy Transmittance and Reflectance

(Terrestrial) of Sheet Materials [61] specifies the procedures for measuring

the solar energy transmittance and reflectance of materials in sheet form by

two measuring instruments—a spectrophotometer and a pyranometer. Revisions to

the calculations performed on the data collected with the spectrophotometer

have already been discussed. These revisions are currently being considered by

the E44 committee.

5.2 ABSORBER MATERIALS

5.2.1 Introduction

Absorber materials in solar heating and cooling systems are used to absorb

radiant energy from the sun and convert it into thermal energy. For maximal

efficiency, absorber materials should have high absorptance and low eraittance,

not only when first installed, but throughout the life of the solar collector.

Significant problems with absorber materials occurred in the solar demonstration

programs conducted by the HUD and DOE. Field inspections by NBS personnel also

revealed various problems [53]. Many of these problems can be attributed to

prolonged exposure at stagnation conditions, as well as to thermal cycling,

high levels of ultraviolet radiation, and moisture. As a result, NBS conducted

a series of studies [69] which evaluated methods for assessing and screening

absorber materials.

In these studies, Masters, Seiler, Embree, and Roberts [69] identified the

prime performance requirements for absorber materials as:

77
• High absorption in the wavelength range of 0.3-3. 5 nm

9 Ability to maintain high absorption over extended time-periods

Resistance to degradation resulting from: —elevated temperatures —temperature cycles —ultraviolet radiation —moisture

• Chemical and physical compatibility with other materials.

Degradation products should not affect performance of other collector

materials

The functional key properties of absorber materials include solar absorptance

(a) and infrared emittance (e). Key degradation factors include elevated

temperature, temperature cycling, ultraviolet radiation, and moisture. Secondary degradation factors include freezing, oxygen, ozone, and airborne contaminants. These latter degradation factors were not included in the NBS research,

however.

5.2.2 NBS Research

In the evaluation of performance requirements for absorber materials, NBS

conducted laboratory (accelerated) and field (stagnation) weathering tests on

12 absorber materials used in solar applications [69] . These materials

included:

® Nickel coated foil

• Anodized aluminum

® Copper oxide

® Black chrome

® Polyvinylidene fluoride (2 samples)

® Urethane

® Alkyd

® Silicone

• Modified polyester

• Epoxide

® Porcelain enamel

Substrates included steel, copper, and aluminum, but not all absorber materials

were tested on all substrates. In the testing phase, properties of the

materials were measured before and after a series of aging (conditioning) tests.

The optical reflectance and absorptance of the absorber materials were determined by ASTM E 424-71 Method A [61], Total normal infrared emittance was measured using ASTM E 408-71 [70]. These measurements were complemented with

visual inspections.

The accelerated laboratory exposure tests assessed the effects of elevated

temperature, thermal cycling, ultraviolet radiation, and moisture on absorber

material performance. The results of oven aging at 150°C (302°F) for up to

14 weeks showed little effect on the absorptance of all absorber materials

78
except for copper oxide and black chrome on copper. Exposure at higher

temperatures (200° and 250°C) resulted in greater changes in absorptance and

emittance for some specimens. Nine out of 26 specimens showed emittance changes

of more than 0.01 unit (8 decreased, while 1 increased—polyvinylidene fluoride).

One material—epoxide—decomposed on a copper/aluminum substrate but not on a steel substrate. After 12 weeks of exposure to 250°C, 10 specimens exhibited

little or no change in absorptance, but 13 exhibited degradation or loss of

adhesion. The three remaining specimens showed degradations in absorption,

and two exhibited substantial degradation in emittance.

After 84 cycles of thermal cycling, a moderate reduction in absorption was

observed in two specimens (copper oxide and black chrome on copper) and failure

in nine specimens. Accelerated UV radiation of more than 2000 hours had little,

if any, effect on the optical properties of the absorber materials.

It appears that both moisture tests—continuous exposure at 92° C and 97 percent

relative humidity exposure to a moisture condensation cycle—induced the same degradation mechanisms, but that the condensation cycle was less severe. In

neither test was the degradation in absorptance and emittance severe. However,

the visual appearance of several specimens changed, due to surface corrosion.

In one specimen, the coating lost adhesion after 21 weeks at 92°C and 97 percent

relative humidity.

The cumulative exposure test was designed to study the effect of sequential

exposure of the same specimen to oven aging, thermal cycling, UV radiation, and

moisture. This test, however, was terminated after 28 thermal cycles because

of the large number of failures. After 28 cycles, only specimens of anodized

aluminum, copper oxide, black chrome on steel and on copper, polyvinylidene

fluoride on steel and on aluminum remained intact. All other specimens had

delaminated

A second series of tests measured the effects of outdoor exposure on absorber

materials. At all three sites studied, only small changes in absorptance and

emittance were measured after outdoor exposure periods of up to five months.

These data suggested that the laboratory tests provided more severe conditions.

A test for outgassing showed that outgassing appeared to affect the transmittance of the cover glass only with polyvinylidene fluoride, urethane, and alkyd

absorbers at 0.4, 1.8 and 5.5 percent, respectively.

The data collected in the NBS tests showed that elevated temperature, thermal

cycling, and moisture can degrade some materials used as absorbers. Where

specific degradation factors such as high temperature, thermal cycling, and

moisture are likely to occur, the data indicated that accelerated aging tests

can be used effectively in evaluating and screening absorber materials. The

tests also indicated that degradation which can be observed visually does not

always result in a change in absorption or emittance. Finally, little change

was observed in the absorptance, emittance, and appearance of most test specimens exposed to outdoor weathering at three sites, since these conditions

appeared to be less severe than the accelerated laboratory exposures. Exposure

to elevated temperatures above 150°C (302°F) can identify polymeric materia

79
that are susceptible to degradation from thermal decomposition. Thermal cycling

can identify potential adhesion problems due to expansion and contraction.

Cumulative exposure testing is a more severe test than exposure of test specimens to a single aging procedure. The data also indicate that the substrate

to which the material is applied can affect its resistance to the exposure

conditions.

Masters, Seiler, and Roberts [71] continued the evaluation of the performance of

selected absorptive coatings for outdoor exposure at three sites. This study

was designed to obtain additional data on the effects of stagnation conditions

on absorptive coatings. Results from the additional outdoor exposure were

compared with data from accelerated laboratory exposures reported earlier [69]

In the follow-on NBS research, test specimens (of both selective and

nonselective absorptive coatings) were exposed outdoors at three sites with

different climate conditions for about 24 months. Solar absorptance and infrared emittance were measured before exposure and at approximately four-month

intervals. The same test materials were studied as in the earlier study.

Three specimens of each material were exposed in the NBS mini-collectors,

mentioned earlier. Again, tilt angle was adjusted seasonally, to expose the

test materials to maximum solar radiation.

The authors found that the outdoor exposure data indicated that simulation

stagnation conditions do degrade some materials used as absorber coatings.

Degradation of optical properties and appearance was similar during both outdoor exposure and accelerated weathering. Some changes in properties for some coatings were observed during accelerated laboratory weathering, but not during

outdoor exposure. These included moisture exposure, oven aging, and thermal

cycling. The authors concluded that the accelerated laboratory procedures

produce more severe conditions than the outdoor exposures. These data confirm

that ASTM E 781-81 adequately addresses the problem of measuring the response

of absorber coatings to outdoor weathering conditions.

5.2.3 ASTM Standards

5. 2. 3.1 ASTM E 744-80

Based on the results from the laboratory and field exposure tests, NBS

submitted drafts for two proposed standards to Committee E44. The drafts con- tained methods for evaluating solar absorptive materials for thermal applications (E 744-80) and a procedure for determining the effect of outgassing on the

transmittance of glass test plates (No ASTM Designation).

Standard Practice ASTM E 744-80 [72] provides a methodology for evaluating

absorptive materials used in flat-plate or concentrating collectors with concentration ratios not to exceed 5. The practice does not apply to absorptive

surfaces directly in contact with the heat transfer fluid, used in evacuated

collectors, or in collectors without cover plates. The practice includes test

methods for determining properties such as absorptance, emittance, and appearance, as well as aging tests for primary degradation factors as solar radiation,

elevated temperatures, temperature cycles, and moisture. (In some geographical

80
locations other factors, such as salt spray or dust erosion should be

considered.) The practice includes two alternative testing methods: Method A,

consisting of a series of laboratory tests to simulate various outdoor exposure

conditions; and Method B, a single test for actual outdoor exposure. Either

method may be used. Equivalency of the two methods through correlation of

laboratory performance with actual in-service performance was not established,

however. Figure 37 shows an outline for the two test method options. Both

methods require that before and after the aging tests, absorptance, emittance,

and appearance of the specimens be determined by ASTM E 424-Method A, by ASTM

E 434, and by ASTM B 537, respectively. The aging tests in Method A consist

of

• A heating aging test at stagnation temperature for 500 hours.

• A temperature cycling test with stagnation temperature as maximum and

— 10°C as minimum. Thirty cycles at three hours are required.

• A moisture test by storing the specimens for 30 days at 90 ± 5°C and

95 ± 5 percent relative humidity.

• Exposure to ultraviolet light, using either outdoor exposure to direct

sunlight reflected from a Fresnel concentrator, or a xenon arc or a filtered carbon arc lamp. Table 17 establishes the test conditions for

the exposures.

These individual tests are conducted on separate specimens.

The aging test in Method B consists of exposure of the specimen for a minimum

of 12 months to the outdoor environment in a unit simulating stagnation conditions for collectors with cover plates. This test is to be evaluated using

ASTM E 781-81, to be discussed in the next section. Again, absorptance,

emittance, and appearance are to be measured both before and after aging.

NBS also drafted a "Proposed Standard Practice for Determining the Effect of

Outgassing on the Transmittance of Glass Test Plates Which Simulate Solar

Collectors." This draft is discussed in references [69] and [74],

5. 2. 3. 2 ASTM E 781-81

Although NBS did not develop a draft of ASTM E 781-81 [73] , the research

conducted by Masters, Seiler, and Roberts [71] was done specifically to provide

a validation of the procedures described in Standard Practice for Evaluating

Absorptive Solar Receiver Materials When Exposed to Conditions Simulating

Stagnation in Solar Collectors with Cover Plates. Since the research demonstrated that the test procedures were, in fact, valid, they will be discussed

in some detail here.

The practice provides a test procedure for evaluating absorptive solar receiver

materials and coatings used in flat-plate collectors where maximum stagnation

temperatures reach 200°C (392°F). It is intended to be a screening test for

81
Figure 37. Outline of test method options (from Ref. 72)

Table 17. Exposure Test Conditions (from Ref. 72)

Outdoor * Xenon Arc

Conditions exposure exposure

Carbon Arc

exposure

Radiation Source Concentrated Sunlight Xenon Arc Carbon Arc

Standard Practice (ASTM) G26 G23

Method A A

Apparatus Type Fresnel Mirror

(8x)

A, AH, B, BH E^ 1

) , EH

Filters Borosilicate(2) Borosilicate(2)

Irradiance at 340 nm, Wm^ .55 —

Radiant Energy Interval Uncontrolled Continuous Continuous

Black Panel Temperature, °C 90 ± 5 90 ± 5 90 ± 5

Relative Humidity Uncontrolled Uncontrolled^) Uncontrolled^)

Irradiation at 340 nm, kj/m2 2970

Exposure Interval, hours 1100 1500 1200

* using Fresnel concentration

(1) Water or air heaters may be required to maintain temperatures*

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

(3) Operate with humidifier turned off.

82
absorber materials under conditions with single cover plates. It differentiates

between test methods for selective and non-selective absorber materials.

The practice specifies a test apparatus, including an exposure box and

conditions for its pre-exposure. It specifies characteristics for cover

plates, seals, insulation, sample mounting plates, and condensation control.

The practice provides guidelines for actual testing using two options. Option

A allows test specimens to be removed, measured, and then returned to the test

site. Option B calls for providing enough test specimens that can be replicated to permit withdrawals at preselected intervals without replacement. The

standard notes that the cover plate should be cleaned if outgassing occurs.

The practice specifies mounting angles as a function of test site latitude. It

also specifies that test specimens be exposed to a minimum total solar irradiation of 6.2 x 10^ J/M^ (essentially a 12-month period of normal solar radiation.) Solar absorptance, emittance, and general appearance are to be measured

prior to exposure and at preselected intervals. Other measurements include

daily temperature, total incident irradiation using a pyranometer, and cover

plate spectral transmittance, solar absorptance of the test specimen, and

appearance at specified intervals. The practice also calls for periodic

maintenance of the test fixture.

The NBS research, as noted earlier, confirmed that the procedures given for

exterior weathering appear to predict the performance of solar absorptive

materials adequately.

5.3 COLLECTOR INSULATION

5.3.1 Introduction

Insulation for flat-plate solar collectors is designed to minimize heat loss

from the back and sides of the collector enclosure. To do this, the insulation should have low thermal conductivity, dimensional stability, and chemical

compatibility under environmental conditions which include variations of

temperature, humidity, and solar radiation. The insulation must also not

adversely affect other collector materials with which it is in contact or which

it may otherwise affect.

Although there are many test methods for evaluating the performance of thermal

insulation under normal conditions in buildings or industrial applications,

Skoda and Masters [53] determined that the problems which occurred with insulation for solar collector materials were different from those commonly

encountered in building applications. As a result, NBS conducted research to

identify the critical in-service operating conditions and performance requirements, to determine appropriate test methods for evaluating representative

insulation materials, and to draft procedures for screening insulation

materials used in solar collectors.

Godette, Lee, and Fearn [75] identified the critical in-service conditions

under which collector insulation must perform as the following:

83
• Sustained high or low temperatures, which vary from as high as 260°C

(500°F) to as low as -4Q°C (-4Q°F).

• Diurnal temperature - humidity cycling.

® Condensation or accumulation of water within the collector.

® Continuous contact between the insulation and the components of the

collector system.

The key properties of sucessful collector insulations were identified as: mass,

size, density, water absorption, moisture absorption, thermal conductivity, and

friability. The degradation factors were identified as: high and low temperature, water, high relative humidity, mold, and vibration. In addition, contact

compatibility is an important factor.

5.3.2 NBS Research

To identify test methods that are most appropriate for evaluating commonly

used insulation materials, particularly in terms of degradation, NBS studied

21 insulation materials using seven ASTM and four non-standardized tests [75].

The materials covered were:

® Mineral fibrous materials (glass, slag, rock, wool), in both batt and

board form.

® Mineral cellular materials (calcium silicate, foamed glass, perlite,

vermiculite) , in performed tubular and in board form.

• Organic fibrous materials (cellulose, wood fiber), as loose fill and

board.

• Organic cellular materials (polystyrene, polyurethane, ureaf ormaldehyde

foams), as board, preformed tabular, and foamed-in place.

The following ASTM tests were conducted:

o C 167 : "Thickness and Density of Blanket-or Batt-Type Thermal

Insulating Materials."

o C 209: Section 13: "Testing Insulation Board (Cellulosic Fiber),

Structural and Decorative: Water Adsorption."

° C 553: Section 15: "Mineral Fiber Blanket and Felt Insulation

(Industrial Type): Moisture Adsorption."

o C 518: "Steady-State Thermal Transmission Properties by Means of

Heat Flow Meter."

° C 411: "Hot Surface Performance of High- Temperature Thermal

Insulation.

84
o C 356: "Linear Shrinkage of Preformed High- Temperature Thermal

Insulation Subjected to Soaking Heat."

o D 3273: "Resistance to Growth of Mold on the Surface of Interior

Coatings in an Environmental Chamber."

Non-standard tests were conducted for:

o Thermal cycling

o Outgassing

o Corrosion

o Friability (breaking up of material)

The test results indicated that, for the types of materials included, not all

of the above 11 tests were appropriate for screening insulation materials.

Those tests which are appropriate include the ASTM tests for: water absorption, thermal conductivity, linear shrinkage, and hot surface performance; and

the newly developed tests for: corrosion, outgassing, and thermal cycling.

The tests for density, moisture adsorption, resistance to mold growth, and

friability were not found to be necessary, as these characteristics do not

appear to be principal factors in the degradation of insulation.

5.3.3 ASTM E 861-82

Based on the laboratory research, NBS drafted a set of procedures for screening

insulation materials for solar collectors. ASTM promulgated Standard Practice

for Evaluating Thermal Insulation Materials for Use in Solar Collectors (ASTM

E 861-82) [76], which relies heavily on the NBS draft procedure. This ASTM

practice provides a methodology for screening solar collector insulation

materials, in terms of their level of performance and relative stability under

simulated in-service conditions. The test methods provide for the assessment

of both the fundamental properties of the insulation, and the response to aging

(degradation of test materials). The methods, which are designed to apply to

all types of insulation, assess the effects of elevated temperatures,

temperature cycles, and moisture on the degradation of insulation materials.

The practice stipulates the use of at least three representative specimens.

These are to be maintained in a conditioned space at 24°C + 2°C (73 ± 5°F) for

at least 48 hours before testing. The practice provides a set of 10 tests for

evaluating the degree of degradation of the insulation materials after exposure

to simulated in-service conditions. These tests include methods for determining

pH, surface burning characteristics, moisture adsorption and moisture absorption, thermal resistance, linear shrinkage, hot surface performance, chemical

compatibility with adjoining material, outgassing, and durability.

Table 18 lists the tests and procedures to be followed. The purpose of this

standard is to present various tests by which the performance of different

insulation materials may be screened and compared. As a result, no criteria

for performance levels are given.

85
Table 18. Tests and Procedures for Evaluating Thermal Insulation Materials

for Solar Collectors [from Ref 76]

Preparation

Select 3 representative specimens for each sample

Condition at 24°C and 50+5 percent relative humidity for 48 hours

Procedure (no sequence required)

1) Measure pH

2) Determine flame spread and smoke developed classifications

3) Determine moisture adsorption

4) Determine water absorption

5) Determine thermal resistance

6) Determine linear shrinkage (expansion) - at expected maximum

temperatures (including stagnation)

in-service

7) Determine hot-surface performance

8) Determine chemical compatibility with adjoining material following exposure

to 49 °C and 95+3 percent relative humidity for 96 hours

9) Inspect for outgassing (no test method available)

10) Evaluate durability by determinng thermal resistance before and after

accelerated aging

11) Examine specimens and note visible changes such as cracking,

delaraination , decomposition, and dimensional variation, as a aging.

warpage

,

function of

86
5.4 GASKETS AND SEALANTS

5.4.1 Introduction

The primary function of rubber seals in solar energy systems is to seal or weatherproof joints between materials or components, and to do this over the

design service life. Seals and gaskets must perform under exposure to elevated

and depressed temperatures, temperature cycles, solar radiation, oxygen, ozone,

and airborne pollutants. These seals must maintain sufficient elasticity and

adhesion to substrates, and must not affect the performance of other materials

which they contact.

While there are numerous test methods for sealants used in conventional

building construction, none of these is totally adequate for the high temperatures encountered in solar energy applications. Existing standards seldom

consider temperatures of building seals above 70°C. Yet, the temperature of

solar collector materials can range from 150° to 250°C. Current standards

also do not cover hydraulic seals exposed to certain heat transport and storage

liquids at elevated temperatures. Skoda and Masters [53] observed rubber seal

failures, such as loss of elasticity, bond failure, and outgassing, in numerous

operational solar energy systems.

5.4.2 NBS Research

NBS conducted a study [74] to identify the performance requirements for rubber

seals in solar energy systems. This study was also designed to: identify

and assess existing test methods and develop new ones as needed; evaluate

commercially available rubber seal materials; and draft proposed standard test

methods, specifications, or practices for rubber seals for submission to ASTM

for consideration as consensus standards.

The NBS research into the performance of sealants identified the following key

properties and applicable ASTM test methods:

• Ultimate elongation - D 412

• Compression set - D 395, D 1229

• Hardness - D 1415, D 2240

® Tension strength - D 412

• Low temperature flexibility - C 711, C 734, C 765, D 2137

• Volatile content, condensible volatiles - D 865

• Adhesion - C 719

The key factors that affect the above properties and applicable ASTM test

methods were identified as:

o Elevated temperatures - C 771, C 792, D 865

o Depressed temperature - D 1229, D 2137

o Ozone - D 1149

o Ultraviolet radiation - C 732, C 793

o Cyclic joint movement - C 719

o Contact with liquids - D 471

87
Based on the above assessment, three groups of tests were conducted. First,

15 samples of vulcanized sheets were submitted to the ASTM D-series described

above. In addition, a modification of test D 865 was also performed to determine the total volatiles at 150°C and the volatiles condensible at 23°C. In

addition, liquid immersion tests were conducted on rubber used in direct contact

with typical heat transport and storage liquids. The second group of samples

was tested according to modified methods of the ASTM C-series listed above.

The third testing procedure dealt with the problem of outgassing — that is the

degradation of materials within a solar collector and the subsequent deposition

of products on the underside of the collector cover. Although the measurement

of volatile condensibles gives an indication of the amount of material that

might be deposited, it does not provide a means of determining the effect of

the condensibles on the cover plate transmittance. Since no standard test

method existed, a new method needed to be developed.

The new method calls for heating the specimen for 3 to 6 hours at 150°C in an enclosed chamber under a glass plate which is cooled by an air stream, and

measuring the mass of the glass plate and test specimen, as well as the

transmittance of the glass specimen, both before and after heating.

The results of the first series of tests (ASTM designations D) showed that

several samples were not properly formulated to give optimum performance in

solar collectors. On the other hand, several kinds of rubbers, when properly

vulcanized, were shown to be suitable for use as seals in flat-plate solar

collectors. Ethylene-vinyl acetate copolymer (EAM), terpolymer of ethylene,

propylene and a diene (EPDM) (when properly formulated and vulcanized), and

silicone copolymer (VMQ) were found to be suitable for use in all climates.

Acrylate polymer (ACM) and f luoro-rubber (FKM), were found to be suitable for use in warm climates only. Chloro-sulfonyl-polyethelyne (CSM), copolymer of

ethylene oxide and chloromethyl oxirane (ECO), and polychloromethyl oxirane

(CO) have higher compression sets than desirable for performed gaskets. The

polysulfide rubber (EOT) tested was found unsuitable since it melted at 150°C

and in subsequent tests at 121°C. (It may be possible to develop improved

satisfactorily performing formulations for the above unsuitable rubbers.)

In the immersion tests, only f luoro-rubber (FKM) performed satisfactorily in

all liquids. In addition, chloro-sulf ony 1-polyethylene (CSM), terpolymer of

ethylene propylene and a diene (EPDM) (properly vulcanized), and silicone

copolymer (VMQ) performed satisfactorily in polyalkylene glycol ether; silicone

copolymer in polyaromatic and modified terphenyl; acrylate copolymer (ACM),

ethylene-vinyl acetate copolymer (EAM), and terpolymer of ethylene propylene

and a diene (EPDM) (properly formulated) in silicone liquids.

The results of the second group of tests (ASTM designations C) showed lower mass losses for silicone sealants than for polysulfide sealants (3.2 to 8.1

percent versus 4.0 to 13.6 and 11.8 to 21.0 percent, respectively).

The hardness values for silicone sealants were relatively independent from test

temperatures and ranged from 27 to 44 after exposure. The hardness of

88
polyurethane ranged from 45 to 69, and that of polysulfides from 0 to 65, with

with two samples becoming harder and two softer.

Bond loss was rated as greater than or less than 25 percent. This value

represents bond to an aluminum substrate and cohesive bond within the sealant.

Four of six silicone samples had less than 25 percent bond loss, and two of

five tested with glass had less than 25 percent loss. About one-half of the

polysulfides showed less than 25 percent bond loss after exposure to 82°, 100°,

and 121°C (179.6°, 212°, and 249. 8°F). Of two polyurethane samples, one had greater and one less than 25 percent bond loss.

In accelerated weathering tests, none of the silicone sealants developed cracks

after 1,000 hours UV exposure, nor did any cracks develop after the 180-degree

bend tests at a temperature of -26°C. Only one of eight polysulfide sealants

developed cracks after UV treatment, but five of the eight polysulfide samples

developed cracks after the bond tests followed temperature treatments.

The tests indicated that silicone sealants might be the best choice for sealing

joints around solar collectors where temperatures may reach 125°C (257°F).

However, the tests also suggest that bonding properties of some silicone sealants to certain substrates are unsatisfactory. In applications where

temperatures do not exceed 80°C, polysulfide and polyurethane sealants may be

suitable.

The results of the tests on outgassing were insofar unexplainable, as percents

of volatiles and condensibles measured after 3 and 6 hours did not in any way

correlate with the values measured after 70 and 166 hours run according to the

modified ASTM D 865 procedure. Table 19 provides a comparison of the test

results. The results did indicate that the change in transmittance is not

directly related to the gross weight of condensibles collected on the glass and

that the change in transmittance is more related to the chemical composition of

the condensibles.

5.4.3 ASTM Standards

The NBS research [74] determined that the performance of the various rubber

materials varied widely, depending on the laboratory test used. Based on these

results, five proposed standards on rubber seals in solar systems were submitted

to ASTM Committee Dll on Rubber. Three of the standards have subsequently been

accepted. Each standard will be discussed in turn.

5. 4. 3.1 ASTM D 3667-78

Standard Specification for Rubber Seals in Flat-Plate Solar Collectors [77]

defines two types of seals: one to be used in all climates (Type C) and the

other to be used only where minimum temperatures are above -10°C (Type W). It also specifies six hardness grades for preformed rubber seals and three hardness

grades for sealing compounds.

Requirements for preformed seals include assessment of: minimum elongation t >r each grade; compression set after heating at 150°C for 70 hours to determine

89
Table

19.

Results

of

Outgassing

Tests

for

Materials

Used

to

Seal

Flat-Plate

Solar

Collectors

(from

Ref.

74) •ecoo

CMo

4J ooCOeou

4-1

<uoe

co

4->

4-1 •Hs

coC

cOU

a) 4J

cobO

0)

4-4 cM

90
that the seal will be effective for extended service periods; compression set

after cooling at -10°C for 166 hours to determine that rubber crystallization

does not result in air leaks; resistance to heating and ozone to determine

that seal deterioration will be slow; resistance to low temperatures (-40°C)

for Type C seals; and measurement of total volatile and condensible matter (to

determine the amount of material that could cause fogging of the cover plates

and reduce transmittance of solar energy).

While the performance requirements for sealing compounds are similar to those

for preformed seals, there are some differences. First, requirements for

compression set are not applicable. A lower temperature (125°C, 257°F) is

also used to test for resistance to heating of sealing compounds. An adhesion

requirement, however, is included to assure maintenance of the seal during

repeated thermal expansion and contraction within the solar collector.

ASTM Standard D 3667-78 provides general requirements for materials used in

rubber seals for flat-plate, non-vertically mounted solar collectors. Its

requirements pertain only to permissible deflections of rubber during thermal

expansion and contraction, and to tolerances for dimensions. It does not

contain requirements for the fabrication or installation of rubber seals. The

standard classifies materials by: types for a given climate (C for cold

climates and W for warm climates); grades of hardness (7); and classes, whether

preformed (PS) or a sealing compound (SC). It specifies that the permissible

deflection of the seal during thermal expansion and contraction be no more than

25 percent in any direction. It references the RMA Handbook [78] for dimensional tolerances. The standard also addresses workmanship, sampling and

inspection, marking, and packaging requirements.

The standard specifies that class SC materials should be prepared in 150 by

150 by 2 mm sheets, and adhesions specimens in accordance with Method C 719.

Each sheet and specimen should be conditioned for 14 days at a temperature of

23°C (73.4°F) and relative humidity of 50 percent.

Both PS and SC materials should be heated for 166 hours at the appropriate

temperature to determine volatiles lost (by comparing differences in mass both

before and after.) Volatiles condensible at 23°C (73.4°F) following heating

for 166 hours should also be determined. The standard specifies a series of

test methods for both preformed seals and sealing compounds. Test methods

for preformed seals (PS) include: ultimate elongation, compression set as a function of elapsed time and temperature, and resistance to heating, ozone,

and low temperature. Test methods for sealing compounds (SC) address: ultimate elongation, adhesion loss, and resistance to heating, ozone, and low

temperatures. Performance criteria for each test as a function of the grade

of the material are given in table 20 for preformed gaskets and table 21 for

sealing compounds. In addition to the test methods, the standard contains

requirements for number and sizes of specimens and their conditioning.

5. 4. 3. 2 ASTM D 3771-79

Standard Specification for Rubber Seals Used in Concentrating Solar Collectors

[79], which was drafted by NBS, is also very similar to the specification for

rubber seals used in flat-plate collectors. The only major difference is that use of a maximum service temperature is specified instead of a specific

91
Table 20. Requirements for Class PS Materials Used to Seal Flat-Plate

Solar Collectors (from Ref. 77)

Property Grade Method

3_ 4_ 5_ 6 8

Ultimate Elongation-% min 350 300 250 200 150 100 D412

Compression Set-% max

after 70 h at 150°C 30 30 30 30 30 30 D395 a after 166 h at -10°C 60 60 60 60 60 60 D1229 b

Resistance to Heating (166 h at 150° C) c D865

Hardness change, max Ultimate Elongation

10 10 10 10 10 10 D1415 or D2240

Change-% max

Tensile Strength

30 30 30 30 30 30 D412

Change-% max 20 20 20 20 20 20 D412

Volatiles lost-% max Volatiles condensible1 1 1 1 1 1 See TEXT

-% max 0.1 0.1 0.1 0.1 0.1 0.1 See TEXT

Resistance to Ozone D1149

100 mPa, 166 h at 40°C No Cracking

Resistance to Low Temperature D2137

Type C only, °C max -40 -40 -40 -40 -40 -40

a Method B

b Set to be measured at 10 seconds after release. Lubricated plates or polytetraf luoroethylene film is recommended if the rubber adheres to the

metal compression plates during test.

c The test temperature of 150°C is used to test seals for cover plates. A seal

in contact with an absorber plate should be tested at a standard test temperature listed in ASTM D1349 next above the maximum temperature of the absorber

plate in service (which generally occurs under stagnation conditions and maximum radiation flux) but not less than 150°C. The higher test temperatures

are: 175, 200, 225 and 250°C.

92
Table 21. Requirements for Class SC Material Used to Seal

Flat-Plate Solar Collectors (from Ref. 77)

Property Grade Method

2 3 4

Ultimate Elongation, % min 200 150 100 D412

Resistance to Heating D865

(166 h at 125°C)

Hardness Change, max Ultimate Elongation

10 10 10 C661

Change , % max

Tensile Strength

30 30 30 D412

Change, % max 20 20 20 D412

Volatiles Lost, % max Volatiles Condensible,1 1 1 See TEXT3

% max 0.1 0.1 0.1 See TEXT3

Resistance to Ozone D1149

100 mPa, 166 h at 40°C No Cracking

Resistance to Low

Temperature D2137

Type C only, °C max -40 -40 -40

Adhesion loss (-cm^ max)b 9 9 9 C719 c

3 The test is not required if the design precludes condensing of the

volatiles on the cover plates(s) of the solar collector.

k The combined loss in bond and cohesion areas for the three specimens

tested shall not exceed 9 cm^.

c The temperature in 6.3 of ASTM C719 shall be modified to 125°C.

93
temperature level for heat aging in compression set tests, and in resistance to

heating tests for both PS and SC materials. The maximum service temperature

normally occurs when the collector is under stagnation conditions, and is

receiving the maximum radiation flux to which it will be exposed.

5. 4. 3. 3 ASTM D 3832-79

ASTM Specification for Rubber Seals Contacting Liquids in Solar Energy Systems

[80] contains general requirements for materials to be used in preformed

rubber seals which contact circulating liquid in solar energy systems. It

establishes six grades of hardness for preformed rubber seals, and two temperature types (C and W). It provides for three classes: Class A, seals for use with aqueous liquids at a maximum service temperature of 100°C (212°F);

Class AT, seals for use with aqueous liquids at a maximum service temperature

above 100°C (212°F); and Class N, seals for use with nonaqueous liquids.

The standard specifies that test specimens be vulcanized, conform to dimensional

tolerances given in the RMA Handbook, and be free of blisters, checks, cracks,

and similar imperfections. These specimens must be tested for: minimum (ultimate) elongation as a function of grade; compression set after exposure to

elevated temperature for 70 hours and low temperature for 166 hours; resistance

to heating and ozone; resistance to low temperatures for type C materials; and

resistance to immersion in the liquid used for the particular heat transport

system. Performance requirements for these various tests are given in tables

22 and 23.

The standard suggests slightly different temperatures as a function of class.

Thus, Class A seals are to be tested for heat resistance and compression set at

a temperature of 125°C (257°F) and for resistance to liquids at 100°C (212°F).

Class AT and Class N seals are to be tested for heat resistance and compression

set at the standard test temperature (specified by ASTM D 1349 to be 25-49°C

above the maximum service temperature). These seals are to be tested for

liquid resistance at a temperature between 1° to 25 °C above the maximum service

temperature. Test temperatures for Class AT and N materials are always to be

higher than those for Class A seals. The standard specifies procedures for

preparing materials for testing, and provides test methods and criteria.

These test methods include tests for ultimate elongation as a function of

grade, compression set for both high and low temperatures, and resistance to

heating, ozone, low temperature, and liquid. The standard provides temperatures

to be used in determining heat resistance, resistance to liquids, and com- pression set tests. The liquid used for the test is specified to be the same

as that used in service for the particular system.

5. 4. 3. 4 Proposed Test Method for Outgassing

In the course of the research on the performance characteristics of rubber

seals [74] , NBS also developed a method for determining effect of outgassing on

the transmittance of solar collector covers. This test method was submitted to

the ASTM Committee Dll for consideration as a voluntary consensus standard.

This committee is currently working with Committee E21 to draw on the methods

presently being used in the aerospace industry in developing an outgassing test.

94
Table 22. Elongation Requirements for Rubber Seals

in Liquid Heat - Transport Systems

(from Ref. 80)

Grade

Ultimate Elongation

percent, minimum

3

4

5

6

78

350

300

250

200

150

100

Table 23. Other Requirements for Rubber Seals in Liquid

Heat - Transport Systems (from Ref. 80)

Property Unit Requirement ASTM Method

Compression Set

High temperature3 % 30 maximum D 395, Method B

Low temperature^ % 60 maximum D 1229

Resistance to Heating

Hardness change3

Ultimate elongation3

IRHD

% of

10 maximum D 1415

change original 30 maximum D 412

Resistance to 0zone c - no cracking D 1149

Resistance to Low

Temperature3 °C -40 maximum D 2137

Resistance to Liquid3

Volume change % +40 to -10 D 471

Hardness change IRHD ±10 D 1415

a Exposure time and temperature specified in proposed specification.

b After compression for 166 hrs. at -10°C. Set shall be measured at 10 sec. after force is released. Lubrication plates or polytetraf luoroethylene film

is recommended if the rubber adheres to the metal plates during test.

c This requirement does not apply to seals that are not exposed to outside

atmospheres

^ This requirement applies to Type C seals only.

95
The procedure outlined in the method proposed by NBS includes heating the

specimen in a closed container and collecting the condensible decomposition

products on a glass plate. The transmittance of the glass plate is measured

(according to ASTM E 424-71) from 300 to 2100 nm (in increments of 50 nm)

before and after heating. The mass loss of the specimen and the heating gain

of the glass plate must also be determined. Although the test was developed

primarily for evaluating rubber seal materials, it can also be used in the

evaluation of absorber materials (see section 5.2).

5. 4. 3. 5 Proposed Recommendations for Sealing Joints with Rubber Seals

While the various specifications for rubber seals specifically excluded

requirements for the installation and fabrication of sealing materials, NBS

also recognized that correct application methods are essential for proper

performance of installed seals, and that improper installation can lead to poor

seal performance, even for excellent materials.

Because seals used for various solar energy applications are required to

perform under more rigorous conditions than seals generally used in building

construction, NBS developed a draft recommended practice [74], These recommendations apply only to the joints used in solar collectors and require that the

sealants meet Federal Specifications TT-S-00230C (COM-NBS), TS-S-00227E (COMNBS) , and TS-S-001543A (COM-NBS).

The NBS recommendations [74] provide suggested installation practices for

liquid applied seals. The practice stresses the need to ensure that the joint

to be sealed be dry and free of dust, dirt, oil or grease. It states that only

primers recommended by the sealant manufacturer should be used. Along with

recommending installation methods dependent on whether the sealant is in

cartridge or bulk form, the recommendation specifies that the sealant should

not be applied at temperatures below 4.4°C (40°F), and that the joint should be

tooled as soon as possible after application. The practice also requires that

sealants should not be placed in a joint if the sealant material has begun to cure.

5.5 RUBBER HOSE

5.5.1 Introduction

The primary function of rubber hose for solar energy systems is to transport

heat transfer fluid between various components of the collector. Rubber hose

is used because it is flexible, and will easily accommodate some misalignment.

Furthermore, it absorbs vibration, dampens sound transmission, is economical,

and does not place high stresses on the connected components. However, rubber

hose is also susceptible to leakage at connections, has an uncertain life

expectancy in the severe environment of solar energy systems, and may require

maintenance and replacement, which are often difficult. It may also not be

compatible with heat transfer fluids or with connector materials, and is not

suitable for use above certain temperatures.

96
Although there are standards for rubber hose for specific industries such as the automotive or petroleum industries, these standards do not address the

particular needs of solar installations. Therefore, NBS conducted a study [81]

to assess the performance requirements needed for developing standards and test

methods for rubber hose in solar systems.

5.5.2 NBS Research

As a first step, NBS identified the properties that significantly affect the

performance of rubber hose in solar applications. The study also identified

applicable ASTM standards. The most relevant properties and test methods

included [81]:

• Tensile strength and ultimate elongation, both of the inner lining

and the outer rubber cover - D 142, • Flexibility at low temperatures - no standard test,

• Fluid compatibility - no standard test,

• Resistance to air aging - D 573,

• Resistance to ozone - D 380 and D 1149,

• Water vapor transmission - no standard test,

• Compression set - D 395 modified,

® Bursting pressure - D 38

,

® Resistance of reinforcement fiber to degradation from the heat

transfer fluid at service temperature - no applicable standard test.

The laboratory work consisted of conducting tests on 1 1 rubber hoses, and

1 polytetra-f luorethylene (PTFE) hose. None of these hoses were specifically

designed for use in solar energy systems. The specimens, produced by seven manufacturers, were made of Chloroprene rubber (CR)^ (Neoprene), Terpolymer of

Ethylene, Propylene, and a Diene (EPDM), and silicone rubber having both methyl

and vinyl groups on the polymer chain (VMQ)

Since applicable test methods were not currently available for all relevant

properties, NBS developed an apparatus for assessing the performance of rubber

hose. The apparatus consisted of two circulating systems mounted in an enclosure. One system was constructed of steel and stainless steel, the other of

brass and copper. The major components of each system are shown in figure 38.

The concept of the tests was to circulate alternatively, hot (100°C, 212°F),

fluid (Ethylene Glycol - water mixture) under 125 to 150 kPa pressure and cold

(7°C to -40°C) fluid at ambient pressures. The tests were run for slightly

over 7 months. Specimens that failed were replaced by fresh specimens, cut

from the same hose. The results of the tests are given in table 24. As can

be seen, specimens 3, 4, and 5 deteriorated substantially under the conditions of this test. Of the seven hoses that did not fail during the entire

period, all but three showed leaks at the hose lamps. (Four different clamps were used - one of which, a plain clamp, showed markedly more failures.) The

CR hose produced more leaks at the clamp than any other (and for all clamps),

although the specimen did not fail otherwise.

97
nnnnnrifinnnnl t innnnnnnnnnrl

(See host loop for typical eonnaction to manifolds]

A - Air supply connection C - Hose clamps (see test for types]

E - Heat exchanger

F Hose nipples, 16 mm OJ). H - Hose specimens. 350 mm

I • Inlet manifold 0 Outlet manifold

C3

3A - Suspended mass

P • Pump

0 - Reservoir S - Supply plug T • Thermocouples

ti - U fitting

Figure 38. Schematic of circulatory system for performance test

apparatus (from Ref. 81)

Table 24. Hose Dimensions, Specimen Failures, and Leaks

(from Ref. 81)

Hose Type Bore Wall Specimens Leaks at mm mm No. failed Cl b C2

l c EPDM 15.8 3.9 0 1 14

2 EPDM 16.2 4.3 2 1 20

3 EPDM 15.0 4.2 4 0 3

4 EPDM 15.4 3.8 6 0 4

5 EPDM 15.4 4.0 4 3 21

6 EPDM 15.5 5.1 0 0 0

7 C CRd 15.1 5.0 0 2 32

8 VMQ 16.0 4.3 0 0 0

9 VMQ 15.7 4.8 0 0 0

10 VMQ 16.9 5.2 1 2 -

11 PTFE 16.2 1.7 0 e e

12 EPDM 19.9 4.6 0 —

a Cl •- Banded clamp; C2-Plain clamp; C3-Screw clamp; C4-S<

C3 C4

clamp with internal band.

b When a leak occurred at a banded connection, a screw clamp was applied adjacent to the band.

c Hose was purchased locally. Other hoses were supplied by the

manufactures

d CR cover; type of rubber in lining is not known.

e Special fittings are required with the stainless steel braided

hose. None of these connections leaked.

98

r^o

cm

st
Additional tests were conducted on the seven hoses for ozone resistance,

compression set, water vapor transmission, and bursting pressure.

ASTM D 380 and D 1149 were used for ozone resistance. Numerous cracks were

observed on the edge of the CR (Neoprene) hose in tension. No cracking was visible on the other hoses.

A modification of ASTM D 395 was used for compression set. The results were

shown in table 25. As can be seen from the table, the EPDM and CR hoses 1 through 7 had 100 percent compression set. The VMQ hoses 8 and 9 had high set,

but there was still some compression stress after the 94 and 166 hour test.

Only the VMQ hose 10 had low compression set even after the 190 hour test. (A

high resilience EPDM hose was received too late and was submitted only to the

166 hour test; it had a set comparable to the VMQ hose.)

A non-standard method was used for water vapor transmission. The test consisted

of comparing the weight loss of filled and unfilled, sealed hose segments before

and after subjecting them to temperatures of 100 ± 1°C. Readings were taken

daily for 7 days. In two tests, the hoses were filled with distilled water,

and in a third test, an ethylene glycol/water mixture was used. The lowest

transmission losses were observed for PTFE, and the highest for VMQ, approximately 25 times that for PTFE. EPDM showed losses about five times of those

for PTFE. In test 3 (hoses filled with the glycol-water mixture), the water

vapor transmission was similar to the water-filled hoses. For CR, however, the

loss was less than one-half.

Since the hoses in the water vapor transmission test were closed off with plugs

of different metals, the test on hoses filled with distilled water served to

evaluate the compatibility of the metals with the hoses. The results are shown

in table 26.

ASTM D 380 was followed for bursting pressure. Results from the tests which

were conducted on hoses used in the water vapor transmission tests are summarized in table 27. From the table, it can be seen that all rubber hoses lost

strength in the performance test. For example, the values for hose 12 (EPDM)

decreased from 7500 to 550 kPa, a loss of over 90 percent. The values for VMQ

hoses tended to decrease less in the performance test than those for other

hoses. This difference may be attributable to the reinforcement material in

those hoses. Some (but not all) of the EPDM hoses showed a loss of approximately 30 percent for hoses that previously were subjected to the water vapor

transmission tests.

The laboratory work indicated that tests were available, or could be devised,

to assess all performance requirements for rubber hose used in solar energy

systems, except deterioration of reinforcing materials. (The behavior of

fibers under various conditions is known, however, or can be established.

Fibers which deteriorate excessively under service conditions should not be

used as reinforcing materials.)

99
Table 25. Compression Set (from Ref. 81)

Hose Type Compression Set at 100°C - Percent

94 h 166 h 190 h

1 EPDM 100+ 100+ 100+

2 EPDM 100+ 100+ 100+

3 EPDM 100+ 100+ 100+

4 EPDM 100+ 100+ 100+

5 EPDM 100+ 100+ 100+

6 EPDM 100+ 96 100+

7 CR 93 100+ 100+

8 VMQ 99 84 100+

9 VMQ 39 72 100+

10 VMQ 5 32 33

11 PTFE Test is not applicable

12 EPDM 100+ 100+ 100+

13 a EPDM - 87 -

+ Compression set is over 100 percent.

a Hose received after performance test on other hoses were

completed.

Table 26. Effect of Hose on Metal Plugs in Water Vapor Transmission

Tests (166 Hours at 100°C) (from Ref. 81)

Hose Aluminum Brass Steel

Test 2 Test 1 Test 1

123

4

56

78

9

10

11

12

13

cc

b

b

cb

d

a

c

f

ec

c

d

ea

f

f

f

a

c

a clean or nearly clean

b very slight discoloration

c slight discoloration

d dark discoloration

e slight corrosion

f corrosion

100
Hose

Table 27.

Type

Bursting Pressures (from Ref. 81)

Bursting Pressures in kPa

a b c d e f

1 EPDM 2210 1240 1100 1240 970

2 EPDM 7000+ 5930 1100* 6620* 970 970

3 EPDM 2340 1240 1380* 830* 2000* 2340*

4 EPDM 3310 2620 1240* 830* — 1100*

5 EPDM 3030 2620 2260* 1380 2380* 830

6 EPDM 7000+ 7000+ 3100 6760 2140 2620

7 CR 6070 6270 2340 2070 1720 970

8 VMQ 2480 1930 1380 2480 1650 1310

9 VMQ 4830 4270 4410 3450 3720 3720

10 VMQ 7500 7500+ 4900 5790 5790 6760*

11 PTFE not tested

12 EPDM 7500+ 7500+ 550 — 690 —

13 EPDM 3450 3450

a Specimens used as blanks and heated at 100°C for 166 h.

b Specimen used for water vapor transmission and heating at 100°C

for 166 h.

c Specimens connected to inlet manifold of brass and copper system.

d Specimens connected to outlet manifold of brass and copper system.

e Specimens connected to inlet manifold of steel system.

f Specimens connected to outlet manifold of steel system.

£ Replacement specimens.

+ Specimen did not fail at this pressure.

101
5.5.3 ASTM Standards

5. 5. 3.1 ASTM D 3952-80

Based on the laboratory results, NBS drafted a Specification for Rubber Hose

Used in Solar Energy Systems. This draft was adopted as ASTM Standard D 3952-80

[82].

The standard provides performance requirements for evaluating rubber hose used

to transport liquids in solar energy systems. It specifically excludes plastic

hoses, as well as general fabrication and installation requirements from its

scope. It establishes classes for different fluids (aqueous and nonaqueous),

fluid temperatures (above or below 100°C, 212°F), and types based on climate

(C, below -10°C in winter; and W, above -10°C in winter). It also specifies

eight standard sizes, and establishes tolerances for both internal diameter

and length. It further specifies that the hose shall consist of an inner

tube, reinforcement, and an outer cover, and provides requirements for each

of the three components.

The test methods for the tube/cover assembly given in table 28 require

assessment of ultimate elongation, tensile strength, and resistance to heating,

heat transfer fluid, ozone, and low temperature. Test methods for the composite hose, given in table 29, require determination of compression set,

bursting pressure, vapor transmission rate, corrosion of metals, effect on fluid, and stability of reinforcement. These two tables also specify the

level of performance to be met by the test specimens.

In table 28, general reference is made to ASTM D 380 for testing. In table 29,

D 395 (modified) is to be used for compression set, and D 380 is specified for

bursting pressure. The procedures used in the NBS laboratory work are outlined

and required for determining vapor transmission rate and its effect on metals

and fluids. These procedures require heating Class A or AT specimens in an oven to 100° ± 1°C and Class N at a standard temperature for 1 day, determining the mass, heating for 6 more days, and determining the mass again. A

new procedure is given for determining stability of reinforcement. This consists basically of heating (for 6 months) filled-hose segments at 100° ± 2°C

(for Class A and AT), or at the temperature specified in ASTM D 1349 (for Class

N), and then conducting bursting pressure tests on the heated and on non-heated

specimens. An annex to the standard provides general provisions and guidelines

for the connection of rubber hose to solar energy systems.

5. 5. 3. 2 Proposed Revision to ASTM Standard D 3952-80

In response to a negative committee ballot in the vote to approve D 3952-80

[82], NBS studied the need to test hose linings immersed in aqueous liquids

above their boiling point. This research [83] included the design of the

testing apparatus, the conduct of interlaboratory tests on typical hose linings

immersed at liquids at 150°C, comparative tests at 100°C, in accordance with

ASTM D 471, and preparation of a proposed revision to the existing standard.

Although ASTM D 454 and D 572 require pressure vessels that can be used for

102
Table 28. Requirements for Tube/Cover Assembly [from Ref. 82]

Property Requirement

Ultimate elongation, min. % 250

Tensile strength, min. MPa 6.2

psi 900

Resistance to heating3

Change in ultimate elongation, max. % -40

Change in tensile strength, max. % -20

Resistance to heat transfer fluid^

Change in ultimate elongation, max. % +25

Change in tensile strength, max. % +25

Change in hardness, max. +10

Change in volume, max. % +15

Resistance to ozone of outer cover, 100 no cracking

mPac

, for 166 h at 40°C

Resistance to low temperature. Type C no cracking

hose only, -40°C

a Class A shall be heated at 125 + 2°C (for 166 + 2 h. Class AT

and N hose shall be heated for 166 + 2 h at a standard temperature in accordance with Recommended Practice D 1349, that is,

between 25 and 49°C above the maximum service temperature, but

not less than 125°C. These test temperatures are: 125, 150,

175, 200, 225, and 250°C.

b The inner tube of Class A and AT hose shall be immersed in a mixture containing equal volumes of ethylene glycol and water

for 166 + 2 h at 100 + 2°C. The inner tube of Class N hose

shall be immersed in heat transfer fluid used in the solar

energy system for 166 + 2 h at a standard test temperature in

accordance with Recommended Practice D 1349 next above the maximum service temperature, but not less than 100°C. If the

vapor pressure of the fluid is above atmospheric pressure at

the test temperature, the next lower standard test temperature

shall be used. (A method is being considered in Subcommittee

Dll. 31 that would subject the inner tube of the hose to volatile fluids above their boiling point in pressure vessels so that the inner tube can be tested above the maximum service

temperature regardless of fluids used.)

c 100 MPa ozone partial pressure is equivalent to 100 pphm at

standard atmospheric pressure of 100 kPa.

103
Table 29. Requirements for Composite Hose [from Ref. 82]

Property Requirement

Compression set, max. %

Bursting pressure, min.

Vapor transmission, rate, max.

yg/(m*s)

Effect on aluminum, brass, copper,

and steel

Effect on fluid

Stability of reinforcement, maximum

decrease in bursting pressure, %

85a

5 times

working

pressure

3 .5 b » c

no corrosion4^

no sediment or dark discoloration6

50

a Compression set is measured after conditioning under 25 percent

compression for 166 + 2 h at 100 + 2°C for Class A hose and

at the standard test temperature in accordance with Recommended

Practice D1349, that is, between 25 and 49°C above the maximum

service temperature but not less than 125°C. These test temperatures

are: 125, 150, 175, 200, 225, and 250°C.

b Equivalent to 0.9 g/day for a specimen having an internal

length of 300 mm between plugs.

c The requirement for Class N hose with specified fluids should

be agreed upon between the manufacturer and the purchaser of the

hose.

^ Condition of the plugs at the end of the test for vapor

transmission rate. Discoloration of the plugs without corrosion

is permitted.

6 Condition of the fluid at the end of the test for vapor

transmission rate. Slight cloudiness or slight discoloration is

permitted.

104
testing rubber hose, these vessels are expensive, and not generally available.

As a result, NBS designed the pressure vessel shown in figure 39. This vessel,

which holds seven test tubes, each 38 mm in outside diameter and about 300 mm

long, also contains a pressure relief valve that opens at about 1000 kPa.

In the NBS research, 15 specimens were cut from each of six commercially

available hose linings, and allocated to participating laboratories. These

specimens were randomly selected for each treatment.

The mass of each specimen was determined before and after immersion and

heating. The tensile strength and ultimate elongation of the untreated and

treated specimens were also determined. In all tests, the change in mass was between 1.1 and 7.2 percent. This change was not considered sufficient to

affect the use of hose in solar energy systems. Change in tensile strength was within the 20 percent range (considered acceptable), except for one vulcanizate, in which the change after immersion at 150°C (302°F) was in the 50 to

70 percent range. The change in ultimate elongation was generally within a 20 percent range, except for one vulcanizate, for which the change at 150°C

was in the 20 to 60 percent range. The large changes in tensile strength and

elongation were observed for the same vulcanizate.

The results of the test series indicated that the temperature and time of

immersion was not critical for good vulcanizates , but that for some vulcanizates, the test results obtained at 100°C (212°F) could be misleading if those

specimens are to be used at 150°C (302°F). As a result, NBS recommended a revision to ASTM D 3952-80. This revision would be in the form of a footnote

to table 28. Footnote B requires that the inner tube be immersed in the

liquid contained in a pressure vessel at least 20°C above the maximum service

temperature. ASTM D 3952-80 currently allows a lower test temperature if the

vapor pressure is above atmospheric pressure. As of this writing, the proposed

revision is still being considered by ASTM Subcommittee Dll. 31.

5.6 CONTAINMENT MATERIALS AND HEAT TRANSFER FLUIDS

5.6.1 Introduction

Because containment materials and heat transfer fluids interact in pairs,

neither of the two materials can be assessed separately. Accordingly, the

evaluation of test methods and the development of screening methods for these

materials were carried out together. Since interactions between the heat

transfer fluid and surrounding materials may increase the possibility of corrosion of the containment materials, there is a need for laboratory procedures

for screening material compatibility, as well as fluid chemical and thermal

stability.

5.6.2 NBS Research

Consequently, NBS conducted research to support the development of testing

methodologies in several areas. In the first study [84] laboratory methods for

assessing the compatibility of metallic containment materials with heat

transfer liquids were evaluated. In the second study [85], test methods wer<'

105
A.

B.C.

D.

Blank fleeing drilled and capped for praaaure relief device. Casket

Pipe flange fitting welded to pipe. Ncalnal 125 no (5 in) pipe with plate welded to hot ton end. ® A

Figure 39. Pressure vessel for immersion tests at 150°C

(from Ref. 83)

106
developed for assessing the corrosion of various metallic containment materials

as a function of different heat transfer fluids under a variety of simulated

service conditions, both operational and stagnation. In the third study [86],

laboratory procedures were developed for screening plastic containment materials for thermal stability under heat aging and for chemical compatibility

with heat transfer fluids.

The liquid heat transfer fluids most commonly used in thermal solar energy

systems are ethylene glycol, propylene glycol, or aqueous solutions therof,

and silicone oil. Containment materials include various alloys of metals such

as aluminum, copper, and stainless steel, as well as plastics such as thermoplastic, thermoset, and elastomers.

In the course of the evaluation of containment materials and heat transfer

fluids [84], NBS participated in a round-robin evaluation of a proposed

ASTM practice for screening of metallic materials. The tests were conducted

solely to evaluate the procedural aspects of the proposed practice, and were

not intended to be an assessment of the metals evaluated [84].

• Test A — Basic Immersion Test at Atmospheric Pressure

• Test B — Heat-Rejecting Surface Test at Atmospheric Pressure

• Test C — High-Pressure Test

• Test D — Repeated Drip-Dry at Atmospheric Pressure

• Test E — Crevice Test at Atmospheric Pressure

• Test F — Tube-Loop Test at Atmospheric Pressure

These various test were evaluated during the round-robin testing. The results

of the laboratory work indicated that the tests, described in the proposed

standard (now ASTM E 712-80), could be carried out within reasonable time, at

moderate expense, and, with the exception of Test D, with minimal equipment

fabrication. It was found that the tests allow for the screening of fluid/metal

pairs. Specifically, Test A allowed the separation of corrosion rates of

aluminum and copper-based alloys, and Tests B, D, and E indicated that variation

in corrosion rate as a function of alloy composition may be observed. However,

Test E indicated a variability in crevice imposition. Results for this test

series are shown in table 30, while the different types of apparatus used are shown in figures 40, 41, and 42.

NBS also conducted two series of simulated service tests to evaluate the

corrosion of metallic materials with heat-transfer fluids [85], The two series

of tests were designed to simulate field conditions. One set of tests was designed to determine the heat transfer liquid stability, (see figure 43),

while the other set assessed the use of a simulated solar loop system (see

figure 44). Table 31 summarizes the simulated service conditions.

The loop systems were designed to operate in three modes — continuous flow;

flow stagnation cycles below the boiling temperature of the liquid; and flowstagnation cycles at a flow-temperature below the boiling temperature of the

liquid and at a stagnation temperature above the boiling temperature of the

liquid. Because a series of initial tests had revealed ethylene glycol to be

less stable than propylene glycol under the desired operating conditions, it

107
Table 30. Summary of Test Results (from Ref. 84)

Test A Test B Test D Test E

Metal

Alloy

Average

Corrosion

Rate

Maximum

Pit

Depth

Corrosion

Rate

Maximum

Pit

Depth

Average

Corrosion

Rate

Average

Corrosion

Rate

Maximum

Pit

Depth

1100 A1 0.14 0.18 1.42 0.04 0.46 0.22 0.03 to 0.29

3003 A1 0.23 0.12 2.02 0.04 0.31 0.37 0.03 to 0.67

122 Cu 1.52 0.09 8.32 0.05 0.97 1.47 0.03 to 0.06

706 Cu/Ni 1.15 0.04 4.03 * 0.51 1.20 0.02 to 0.05

409 S/S * * - - - - -

439 S/S * * * ** * * *

444 S/S * * * ** * * *

1105 Steel 46.2 Perforation - - - - -

Negligible rate or too shallow pits to measure.

No pits observed.

Corrosion rate in mdd (mg/dm^/day)

Pit depth in mm.

Figure 40. Schematic of apparatus used in test B (from Ref. 84)

108
Figure 41. Isometric drawing of apparatus used in test D (from Ref. 84)

Figure 42. Schematic drawing of apparatus for test F (from Ref. 84)

109
Figure A3. Schematic of the apparatus used to study the degradation of

ethylene and proplyene glycol (from Ref. 85)

Table 31. Summary of Simulated Service Test Conditions (from Ref. 85)

Other Materials

Exposed to the Heat Transfer Temperature Test Alloy System Liquids Heat Transfer Liquids Flow Stagnation Flow Rate Duration

3003 A1 A1 #1 Teflon, epoxy. Ethylene glycol 50%

90°C —

0.6A m/sec 16 weeks

A1 #2 polyethylene

ASTM D138A 90° C 90° C 0.6A m/sec 16 weeks

A1 #3 glass, 316 Water 50% 90° C 120°C 0.6A m/sec 16 weeks

A1 # A stainless steel 90° C 135°C 0.6A m/sec 16 weeks

122 Cu Cu #1 Above and 95/5 Ethylene glycol, 25% 90°C 0.6A m/sec 2A weeks

Cu #2 Sn/Sb solder GM 6038-M, Type A 90° C 120°C 0.6A m/sec 2A weeks

antifreeze 25%

ASTM D138A water 50%

316 S/S s/s in Teflon, epoxy. Distilled water 90° C 120°C 0.6A m/sec 16 weeks

polyethylene,

pyrex glass

+1000 ppm NaCl

110
Schematic shoving Che various components of the loop systems, the systems are initially charged vlth liquid through the priming port [1] by opening the manual two-way valve [2]. the common flow path [3] is from the reservoir [4], through the pump [5], to the 3-way solenoid valve [6]. The primary flow path [7] is across the samples. The flow

path is changed from primary to secondary by activating the 3-way sole- noid valve [6] and the secondary path [8] during loop stagnation is through the flowmeter [9]. The hot plate [10] maintains the liquid

temperature during stagnation. In systems which stagnate above the boiling point of the liquid, a 2-way solenoid valve [11] placed at the top of the loop is activated during stagnation. In continuous flow

systems, a manual 3-way valve replaces Che 3-way solenoid valve [6].

Figure 44. Schematic of simulated service loop system (from Ref. 85)

111
was used as the heat transfer fluid to allow some liquid degradation for

assessing the effects of the test conditions. The results indicated that the

responses of both the copper and aluminum specimens varied depending on the

test conditions, while those for stainless steel were inconclusive. Operation

in the flow stagnation cycle was more conducive to copper corrosion, while

corrosion occurred for the aluminum samples in a less consistent fashion.

Table 32 describes some of the test results, while figures 45, 46, and 47

present the results graphically.

Since plastic materials are becomming more common in solar energy systems, NBS

also conducted laboratory studies to obtain data needed for developing standards for screening plastic containment materials for heat effects and for

compatibility with heat transfer fluids [86].

The first task was to identify potential uses, service conditions, and

degradation factors for plastic containment materials. It was determined that

plastics are used in absorbers, transport plumbing, storage tanks, and containers. Service conditions included elevated temperatures, temperature

cycling, solar radiation, moisture, air pollutants, internal pressure, contact

with heat transfer fluids, and contact with adjacent materials. For the purposes of screening tests, it was decided that performance under elevated

temperatures and contact with heat transfer fluids would be most appropriate.

During the screening tests, the plastic specimen was subjected to aging

procedures including heat stability aging and immersion in heat transfer fluid

(chemical compatibility). Figure 48 presents a flow chart of the testing

sequence. Table 33 presents materials studied by NBS. Key material properties such as linear dimensions, hardness, weight change, appearance, and

reflectance (absorbers only) were measured before and after aging. Heat aging

was conducted at temperatures of 100°C (212°F) and 125°C (257°F), for 100, 250,

500, and 1000 hours of exposures. Heat transfer fluids included water, 100

percent ethylene glycol, 50 percent ethylene glycol in water, 100 percent

propylene glycol, 50 percent propylene glycol in water, and 100 percent silicone oil. Immersion tests were conducted at room temperature and at 70°C

(158°F) for 7 days.

Heat aging resulted in weight loss for the PVC storage tank liner, along with

uneven changes in length and width. Heat aging generally increased hardness

slightly, although a significant increase was found for the PVC tank liner.

EPDM, CPVC pipe, and PVC pipes had significant weight losses after exposure to

125°C (257°F) for 1000 hours. The CPVC pipe also developed bubble-like bumps

on the interior and exterior surfaces, while the cross-linked polyethylene

specimen melted.

The tests for chemical compatibility demonstrated generally small changes in

weight for room temperature tests. The polypropylene absorber gained 0.1 percent weight in concentrated propylene glycol, but lost 23 percent in a dilute

solution. Silicone oil at 70°C (158°F) caused significant weight gains,

dimensional changes, and softening, in all materials.

112
Table 32. Summary of Simulated Service Test Results (from Ref. 85)

System

T,

°C

Flow/ Stag.

pH

Initial Final

Corrosion Rate

Empty

Side

mpy

Full

Side

Deepest

Empty

Side

Pit

Full

Side

A1 #1 90 - 8.35 5.56 0.49 1.06 mm

A1 #2 90 90 8.35 5.08 0.38 0.40 negl

negl.

A1 //3 90 120 8.35 5.33 0.54 0.28 0.85 0.55

A1 #4 90 135 8.35 5.64 1.34 0.67 0.12 negl.

Reserve Aik.

Cu #1 90 - 3.89 3.56 0.03

Cu #2 90 120 3.89 3.05 0.12 0.08

pH

S/S 90 120 7.72 8.57 negligible

Figure 45. Plots of solution pH vs. time for aerated solutions of

ethylene and propylene glycol at 100°C (from Ref. 85)

113
Figure 46. Plots of solution pH vs. time for deaerated solutions of

ethylene and propylene glycol in contact with aluminum

and copper at 100° C (from Ref. 85)

Figure 47. Plots of solutions pH vs. time for aerated solutions of

ethylene and propylene glycol in contact with aluminum

and copper at 100°C (from Ref. 85)

114
Figure 48. Outline of testing sequence for screening plastic containment

materials (from Ref. 85)

115
Table 33. Plastic Containment Materials Studied by NBS (from Ref. 86)

Materials Description

Collectors Absorbers

Ethylene propylene diene monomer (EPDM) extrusion with 1 .9 mm wall

Silicone coated fiberglass fabric 0.36 mm sheet

Crosslinked polyethylene (XLPE) 17 mm diameter tubing with

1.5 mm wall

Polypropylene extrusion, with 0.5 ram wail

Polypropylene copolymer

Piping

extrusion, with 0.64 mm wall

Chlorinated poly(vinyl chloride) (CPVC) 34 mm OD, 4.5 mm wall

Poly(vinyl chloride) (PVC) 34 mm OD, 4.5 mm wall

Acrylonitrile-butadiene-styrene (ABS)

Storage

29 mm OD, 2.2 ram wall

Poly(vinyl chloride) liner 0.8 mm sheet

Chlorosulf onated polyethylene liner - over woven fabric

0.8 mm sheet

Fiber reinforced plastic tank 1.3 mm sheet

116
In summary, the data for plastic materials showed that the environment

experienced in solar energy systems can degrade some materials. They also

indicated that accelerated laboratory screening tests can be effectively used

to determine if a plastic material is likely to be affected by specific

degrading factors. Additional tests were recommended to evaluate candidate

plastic materials for specific applications in solar energy systems.

5.6.3 ASTM Standards

5. 6. 3.1 ASTM E 712-80

Standard Practice for Laboratory Screening of Metallic Containment Materials

for Use with Liquids in Solar Heating and Cooling Systems (ASTM E 712-80) [87],

provides laboratory test procedures for evaluating the corrosion performance of

metallic containment materials under conditions similar to those that may occur

in solar heating and cooling systems. Although the six tests were identified

in 5.6.2, they will be briefly summarized here.

® Practice A is concerned with the interaction of metal and fluid at the same temperature. It is useful for plumbing, pumps and tanking, but

less useful for collector panels.

• Practices B and F are concerned with the deterioration of the metal

when there is heat transfer from the metal to the fluid. These are especially applicable to collector panels.

• Practice C is useful in relation to systems experiencing high

temperatures or closed to the atmosphere.

• Practices D and E evaluate specific corrosion problems associated with

particular metal/fluid pairs.

Any one or more of the various tests may be used to evaluate the deterioration

of the metallic containment material in a metal/fluid pair. These tests require

proper consideration of the metal/fluid pairing, and eventual confirmation by

component and systems testing under actual or simulated service conditions.

The test must be long enough to allow corrosion to occur, as well as to allow

exhaustion of any chemical corrosion inhibitor. A minimum of 30 days is

recommended

This standard provides guidance in selecting materials and reagents, sampling,

configuration of test specimens including surface finish, and metallurgical

condition, and procedures for conventional, electrolytic, or chemical cleaning

after testing.

Deterioration of containment material is determined by measurement of the mass

loss and microscopic examination (at 10X magnification), for incidence of

localized attack. Formulas are given for calculating mass loss per unit area

per unit time, as well as the rate of loss in thickness of the specimen. Incidence of localized corrosion, and/or changes in the heat transfer fluid are to

be reported.

117
The six practices for evaluating metal/fluid interaction given in the standard

are described below.

Practice A is a basic immersion test at atmospheric pressure. It is a simple,

rapid exposure test, open to the atmosphere, and designed to evaluate metal/

fluid interaction. In this practice, a reaction vessel such as a 1000 ml

(.264 gal) beaker is used. The specimen is placed in this vessel which contains about 500 ml (.132 gal) of heat-transfer fluid. The vessel is then

heated to the desired test temperature for the duration of the test (at least

30 days). At the conclusion the specimen is weighed and examined visually.

Practice B is designed to simulate deterioration of containment material as a result of heat transfer through the containment material into the heat-transfer

fluid. As typically constructed, it is also open to the atmosphere. The test

apparatus was shown in figure 40. In this test a specimen is mounted at the

bottom of a reaction flask between a gasket and a heating mechanism. The temperature on the surface of the test specimen is controlled by means of an instrument such as a thermocouple. After the test specimen is cleaned and

weighed, it is then secured to the reaction flask, which is filled with 250 ml

of the heat transfer fluid. Although any schedule of testing temperatures and

durations may be selected, simulation of anticipated field operation through

a schedule of heating, holding and cooling is recommended. At the conclusion,

the specimen is weighed and examined for corrosion.

Practice C is designed to simulate conditions of high pressure and temperature

under full, partially full, and empty stagnant conditions. Unlike Practices

A and B, it can provide for test conditions which simulate systems closed to

the atmosphere. In this test, an autoclave or comparable device for producing

required temperatures and pressures serves as the test chamber. Specimens are mounted independently on a test rack which is filled with test fluid. Fluid

levels are selected so that the specimen is totally immersed, partially immersed,

or suspended in a vapor phase. Again, the test may be run for any schedule of

time, temperature, and pressure, but a minimum of 30 days is recommended. It

should be noted that this particular test was not included in the series of

tests which formed the round robin in which NBS and other laboratories

participated

Practice D is designed to simulate alternate wetting and drying conditions.

Like practices A and B, it is open to the atmosphere. In this test the

specimen is repeatedly lowered into the test fluid, then raised from it,

dried, and reimmersed. Selection of a drying device should consider the

possible degradation effects related to the method of drying such as degradation of fluid by infrared radiation from heat lamps or effects of atmospheric

exposure. Again specimens are cleaned and weighed before testing. While any

cycle of drip and dry times may be selected, a minimum duration of 30 days is

recommended

Practice E is used to evaluate general corrosion resistance and to detect

susceptibility to crevice corrosion attack for metallic containment materials.

The procedure differs from ASTM Method G 48 (a test for stainless steel corrosion using ferric chloride) in providing for the use of washers of inert

118
material rather than rubber bands to form the crevice contacts. Again, the

test material is placed in a test flask with the heat transfer fluid, and

heated to the desired test temperature. Test material is then examined for

crevice corrosion attack, at the completion of testing.

Practice F is designed to simulate the deterioration of containment materials

when heat is transferred through the containment material tubing to heat transfer fluid located inside the tube. In this test, a U-shaped tube of the

containment material is wrapped with heating tape. The tape is attached to a power source. One leg of the U-shaped tube enters a reservoir containing the

heat transfer fluid, while the other is attached to a variable flow pump (see

figure 42). Provision is made for introducing secondary samples of similar or dissimilar metal tubing as well. A cycling mechanism controls both

temperature and fluid flow in any desired cycle for the selected duration.

After the test is concluded, the specimens are weighed and examined for

deterioration. Appropriate test conditions include: specimen preparation,

time and temperature schedule, degree of atmospheric exposure of heat transfer

fluid, stirring, and flow rate. The method of temperature measurement and

control must be specified, along with any boiling of the heat transfer fluid.

At the conclusion of all testing, the specimens are cleaned, measured and

weighed. They are also examined for evidence of localized deterioration using

10 X magnification. Any changes in the heat transfer fluid such as odor or appearance must be reported, as well as the build up of any film.

5. 6. 3. 2 ASTM E 745-80

Standard 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) [88], contains three practices, A, B, and C,

which simulate field service conditions for evaluating the performance of

metallic containment materials under corrosive conditions. The material is

treated as an integral part of a metal/fluid pair. The practices provide

procedures for evaluating deterioration under several conditions, including:

operating full flow; stagnant empty vented; stagnant, closed to atmosphere,

non-draindown; and stagnant, closed to atmosphere, draindown.

Practice A is a laboratory simulation for evaluating coupon test specimens

under different operating conditions of solar heating and cooling systems.

Practice B is also a laboratory simulation which utilizes a component or simulated subcomponent. Unlike Practice A, it provides for heating of the heat

transfer fluid by the containment material. Practice C is a field simulation

of components under various operating conditions of solar heating and cooling

systems, using controlled schedules.

Because the various procedures do not restrict the selection of either the

containment materials or the fluid, the standard specifies that attention must

be paid to pairing material and fluid appropriately, knowing the probable

corrosion mechanisms, and avoiding hazardous fluids, materials or material/

fluid pairs. The duration of testing must be adequate for measuring the rat.'

119
of corrosion of the containment material. Because corrosion is both timedependent and non-linear, a minimum test period of not less than 6 months is

recommended. The ratio of metal surface area of fluid volume must be considered for fluids with additives designed to minimise corrosivity (due to

depletion kinetics.)

Following completion of the test procedures, the deterioration of the

containment material is determined by measurement of weight loss when possible,

measurement of metal thinning, and by examination at 10 X magnification for

localised corrosion. Formulas are given for calculating corrosion rates based

on weight loss or reduced density. Other factors to be identified include:

instances of localised corrosion, changes in the heat transfer fluid, and any

film formation or build up.

The report should include chemical identification of the containment material

and heat transfer fluid, as well as the dimensions and configuration of the

material specimen. It should also identify all procedures used, as well as average weight loss and penetration rate.

Practice A is designed to evaluate the resistance to deterioration of

(unheated) metallic containment materials in contact with the heat transfer

fluid. It provides guidelines for surface finish, metallurgical condition, and

cleaning (before and after testing), including general, electrolytic, and

chemical. The procedure calls for preparation and cleaning of test specimens

and cycling of temperature conditions on a daily schedule. Once every two

weeks, the daily cycling is to be replaced with an autoclave cycle. The test

should last for at least 6 months, although specimens may be withdrawn at

interim periods for checking.

Practice B is designed to evaluate the performance under corrosive conditions

of containment materials as components (or simulated subcomponents) under

laboratory procedures which simulate field conditions. Test specimens should

represent materials and components used in solar applications. The apparatus

consists of storage tank, pump, plumbing as needed, process controller, monitoring equipment, refrigeration unit, and means of heating the metallic containment

loop (See figures 40 and 41). It is designed to simulate a variety of service

conditions. Thus, the standard recommends that consideration be given to

selecting a schedule of full operation and stagnation cycles which simulate

anticipated field operations. The test should be conducted on a specified

cycle for at least 90 days.

Practice C calls for a field test of the performance of containment materials

in components or simulated subcomponents in controlled operations which simulate various field conditions. It specifies that the test specimen be a metallic absorber panel, either full-size or a simulation of full-size. Both

panel and collector are to be of materials representative of functional

systems. The test apparatus includes: the collector, storage tank, pump,

suitable plumbing, process controller, and temperature monitoring device.

120
Practice C permits any schedule of operation, although it recommends use of a schedule similar to intended field operations. If it is desired to test stagnation condtions, procedures are given for determining the effects of accelerated stagnation over 1 year. The practice notes that leaky panels may be

patched with compatible sealers. Also, temperature of the fluid and panel

should be measured on a regular schedule during the operational and test cycles.

5. 6. 3. 3 ASTM E 862-82

Standard Practice for Screening Polymeric Containment Materials for the Effects

of Heat and and Heat Transfer Fluids in Solar Heating and Cooling Systems

(ASTM E 862-82) [89] provides procedures for evaluating selected mechanical

and dimensional properties of polymeric materials when exposed to heat and

heat transfer fluids. This practice is intended to provide short-term exposure

data for screening out unsuitable materials and a basis for a relative ranking

of the resistance of candidate materials to heat and heat transfer fluids.

The practice provides procedures for assessing the effects of elevated

temperatures in terms of physical, mechnical, and chemical changes in the

polymeric materials studied. It also provides procedures for determining the

chemical compatibility of polymeric materials with either aqueous or organicbased heat transfer fluids. It recommends minimum number of specimens as well

as geometry and edge characteristics for each property test. Conditioning

procedures are also specified.

The following property measurement tests are to be performed: dimensional

stability; mass and appearance; tensile properties; flexural properties and hardness. These tests establish initial properties and dimensions for unexposed

specimens

The second set of procedures is the determination of the specimen response to

continuous in-service temperature for 1000 hours, and response to stagnation

temperatues for 300 hours for polymeric materials used in components such as absorbers, filters, valves, etc. Following temperature exposure, physical

characteristics are again measured.

The third set of procedures involves assessment of chemical compatibility

using Method D 543 (Test for Resistance of Plastics to Chemical Reagents)

with a 7 day exposure to the appropriate heat transfer fluid, both concentrated

and diluted to in-service conditions. Again, physical properties and dimensional

stability are to be measured following chemical exposure.

The standard proposes no acceptance or failure criteria, as this practice is

intended to standardize the testing and screening procedures. It does note,

however, that failure criteria for materials are given in other ASTM standards,

and suggest some general definitions that are likely to be appropriate. The

standard also indicates that determination of the rate of change with time or temperature in a property may be far more important to determining likely

performance in a solar component or system than measurement of a single value

under one arbitrary condition.

121
6. Conclusions

6.1 SUMMARY

The preceding sections have briefly reviewed research and test method

development activities for active solar heating and cooling systems during

the period 1974 to 1982. The majority of related voluntary national consensus

standards which were developed and approved as a result of the various research

projects and recommendations were also discussed. Table 34 summarizes the research

activities and the test methods and other standards which have been developed

to aid in evaluating the thermal and durability/reliability performance of

solar domestic hot water systems, thermal storage devices, collectors, and

materials. Additional summary information on research and standards development activities pertaining to materials is contained in reference 52.

6.2 RESEARCH NEEDS

The preceding sections indicate that DOE/NBS research played a key role in determining

thermal and material properties and subsequent performance requirements for

various systems, components, and materials of active solar energy systems.

Despite the progress made toward the development of test methods and other

standards, additional research is needed to develop new standards, as well as to

revise existing standards.

A high priority need is the development of test methods and standards for

assessing the long-term performance of collector components and materials in a functioning active solar energy system, and for determining the thermal

efficiency, durability, and reliability of the total system.

Thus, high priority should be given to the development of improved test methods

for measuring the thermal performance of integrated collector storage domestic

hot water systems. There is also a need to expand rating and test procedures

to innovative systems and operating conditions which may differ from those for

which the procedures were originally developed.

Several area for further research have been proposed for components of solar

systems. Previous research studies [36, 37, 48] have identified a number of areas to be considered in revising test methods for thermal performance of

solar collectors. Additional durability/reliability test methods (e.g. rain

penetration) for solar collectors should also be developed. Section 3.3

discussed the need for improvements in the thermal performance testing of

storage devices [22]. Finally, a number of recent studies [90, 91] have

recommended the development of test methods for solar controllers and sensors.

For collector materials, a number of research needs have been identified.

Recent research data and information [92] contain recommendations for revising

existing standards for evaluating cover plate and absorber materials durability.

There is also a need to develop test methods and mathematical models for

predicting service life for cover plate and absorber materials, phase change

storage materials, heat transfer fluids, and reflective surfaces.

122
Table 34. Summary of Active Solar System, Component, and

Materials Standards and Related Research

System/ Component /Materials Standard Research*

1. Solar Domestic Hot Water Systems

a. Thermal Performance

2. Thermal Storage Devices

ASHRAE 95-81

9-11, 14

a. Thermal Performance

3. Collectors

ASHRAE 94-77 15-17, 20-22

a. Thermal Performance ASHRAE 93-77

ASHRAE 96-80

16,25,29,30,

33,36,37

b. Durability/Reliability

4. Functional Materials

ASTM E 823-81 40, 47-49

a. Cover Plates ASTM E 765-80

ASTM E 782-81

ASTM E 822-81

ASTM E 881-82

53, 62-65

b. Absorptive Materials ASTM E 744-80

ASTM E 781-81

53,64,69,71

c. Insulation (Collector) ASTM E 861-82 75

d. Gaskets and Sealants ASTM D 3667-78

ASTM D 3771-79

ASTM D 3832-79

ASTM D 3930-80

74

e. Rubber Hose

f. Containment Materials/Heat

ASTM D 3952-80 82,83

Transfer Fluids ASTM E 712-80

ASTM E 745-80

ASTM E 862-82

84-86

L

The numbers pertain to the references listed in Section 7.

123
Another need is in the development of test methods, both optical and mechanical,

to increase the detectability of materials degradation. Current test methods are

not sensitive enough to measure degradation, even though this degradation can be

serious enough to impair collector performance. These methods are particularly

needed for glazing and absorbers.

Finally, there is need to identify degradation mechanisms of various materials.

Before test methods and mathematical models can be developed, the processes occuring

in and between materials in a total solar system environment must be identified and

understood. In particular, the effects of multiple degrading environments, such as heat and ultraviolet exposure, must be studied as a complete unit. At the same

time the compatibility of joined materials in dynamic operation is needed to develop

test methods which can predict operational effects observed during field use.

124
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85. Brown, P. W. and Grimes, J.W. Simulated Service Testing for Corrosion

in Solar Heating and Cooling Systems. NBSIR 81-2339, June 1981.

86. Clark, E.J., Kelly, C.D., and Roberts, W.E. Solar Energy Systems - Standards for Screening Plastic Containment Materials. NBSIR 82-2533,

June 1982.

87. Standard Practice for Laboratory Screening of Metallic Containment

Materials for Use with Liquids in Solar Heating and Cooling Systems.

ASTM E 712-80, ASTM Book of Standards , part 41, 1981.

130
88. Standard 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, ASTM Book of Standards, part 41,

1981.

89. Standard 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.

90. Farrington, R.B., Myers, D. Evaluation and Laboratory Testing of Solar

Domestic Hot Water Control Systems. SERI/TR-254-1805 , February 1983.

91. Dikkers, R.D., Kennish, W.J., Winn, C.B., Huston, W. Research Practices

for Improving the Effectiveness of Active Solar Hot Water and Space

Conditioning Systems. NBS Draft Report, February 1984.

92. Waksman, D., Thomas, W.C., Streed, E.R. NBS Solar Collector Durability/

Reliability Test Program: Final Report. NBS Technical Note (to be

published) .

93. Brown, P. Factors Affecting the Service Lives of Phase Change Storage

Systems. NBSIR 81-2422, February 1982.

131
NBS-114A (REV. 2-80) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1 1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) | NBSIR 84-2845 April 1984

4. TITLE AND SUBTITLE

DEVELOPMENT OF TEST METHODS AND STANDARDS FOR ACTIVE SOLAR HEATING AND COOLING

SYSTEMS

5. AUTHOR(S)

Heinz R. Trechsel and Belinda L. Collins

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. national bureau of standards DE-AI01 76PR06010

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20^85

10. SUPPLEMENTARY NOTES

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

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

Since test methods and standards for active solar heating and

cooling systems did not exist in 1976, the Department of Energy sponsored

research at the National Bureau of Standards (NBS) and other laboratories

to aid in the development of research-based standards. This research

was intended to facilitate a sound data base for the development of

national consensus standards and test methods. In the present report,

research by NBS and other laboratories is described for solar domestic

hot water systems, solar collectors, thermal storage devices and collector

materials. For collectors, the report describes research and test

methods for determining the performance of cover plates, absorber

materials, collector insulation, gaskets and sealants, rubber hose,

containment materials, and heat transfer fluids.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) absorbers; collectors; materials; consensus standards; domestic hot water;

durability; heat transfer fluids; reliability; solar energy; test methods;

thermal performance; thermal storage devices.

13.

AVAILABILITY

Uni imited

1 | For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

14. NO. OF

PRINTED PAGES

144

15. Price P3 Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $ 14.50

USCOMM-DC 6043-P 80