AlllDfc. EbDIVT

materials Research Activities at the

National Bureau of Standards

I (1975-1982) Pertaining to Active

Solar Heating and Cooling Systems

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Materials Division

Washington, DC 20234

November 1983

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

~QC“ Vctive Solar Heating and Cooling Division

]_Q0 Vashington, DC 20585

.U55

33-2732

NBSIR 83-2782

MATERIALS RESEARCH ACTIVITIES AT THE

NATIONAL BUREAU OF STANDARDS

(1975-1982) PERTAINING TO ACTIVE

SOLAR HEATING AND COOLING SYSTEMS

NATIONAL BUREAU OF STANDARDS

LIBRARY "Rr-T.

ac

i o o

3

-722. \°\

Charles W. C. Yancey

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Materials Division

Washington, DC 20234

November 1983

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

Active Solar Heating and Cooling Division

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

A summary of the solar energy materials-related research projects conducted by

the National Bureau of Standards, for the U.S. Department of Energy, since 1975

is presented. Research studies concerned with materials that are utilized in

the collector, transport and storage subsystems are summarized. Materials

research areas covered by the documentation include: cover plates, absorber

coatings , thermal insulation, sealants, containment materials, heat transfer

fluids, hoses and storage media materials. The primary objectives, scope and

principal results of the various studies are presented. The relationship

between test results and subsequent consensus standard adoption or revision is

drawn where applicable.

Key words: absorber coatings; containment materials; cover plates; durability;

heat transfer fluids; hoses; performance tests; sealants; solar

collectors; solar energy storage subsystem; storage media; thermal

insulation; transport subsystem.

iii
TABLE OF CONTENTS

Pgge

CHAPTER 1. INTRODUCTION I

1.1 BACKGROUND 1

1.2 ORGANIZATION OF THE REPORT 2

CHAPTER 2. COLLECTORS 4

2.1 COVER PLATES 4

2.1.1 Introduction 4

2.1.2 Optical and Mechanical Property Measurement

and Durability Assessment 4

2.1.3 Parametric Study of Optical Properties 7 2.1.4 Weathering Performance Measurement 13

2.1.5 Hail Impact Testing 18

2.2 ABSORBER PLATES 21

2.2.1 Introduction 21

2.2.2 Laboratory and Field Experimentation 22

2.2.3 Parametric Study of Optical Properties 26

2.2.4 Analytical Modeling 28

2.2.5 Laboratory Evaluation Devices 32

2.2.6 Evaluation of Selective Coating Candidates 33

2.3 INSULATION 35

2.3.1 Introduction 35

2.3.2 Research Activities 35

2.4 SEALANTS AND GASKETS 39

2.4.1 Introduction 39

2.4.2 Research Activities 39

CHAPTER 3. TRANSPORT AND STORAGE SYSTEM COMPONENTS 43

3.1 CONTAINMENT MATERIALS AND HEAT TRANSFER FLUIDS 43

3.1.1 Introduction 43

3.1.2 Screening Test for Metal/Heat Transfer Fluid

Combinations 43

3.1.3 Screening Test for Plastic Materials/Heat

Transfer Fluid Combinations 44

3.1.4 Simulated Service Testing for Corrosion 46

3.2 HOSES AND FLEXIBLE COUPLINGS 53

3.2.1 Introduction * 53

3.2.2 Research on Rubber Hoses 53

iv
TABLE OF CONTENTS (continued)

Page

3.3 STORAGE MEDIA 55

3.3.1 Introduction 55

3.3.2 Background 55

3.3.3 Problem Identification 58

3.3.4 Performance Test Method Needs 58

CHAPTER 4. SUMMARY OF ACCOMPLISHMENTS AND CURRENT STUDIES 59

4.1 RESEARCH ACCOMPLISHMENTS 59

4.2 FISCAL YEAR 1982 RESEARCH ACTIVITIES AT NATIONAL

LABORATORIES 59

CHAPTER 5. ACKNOWLEDGEMENTS 65

CHAPTER 6. INDEX OF REFERENCES 66

v

1. INTRODUCTION

1 .1 BACKGROUND

In 1974, the U.S. Department of Energy (DoE) initiated the Active Solar Heating

and Cooling (ASHC) Program to foster the advancement of solar energy technology

and the emergence of the solar energy industry. This report summarizes the

solar energy-materials-related research projects that were conducted since 1975

by the National Bureau of Standards (NBS) under the auspices of the U.S. Department of Energy's (DoE) Active Solar Heating and Cooling (ASHC) Program. Some

of the research conducted by other national laboratories (i.e., Los Alamos

National Laboratory (LANL) and Argonne National Laboratory (ANL)) under the

ASHC Program is also documented here. The reader is referred to companion

documentation reports by those laboratories for a complete listing of their

activities. Research projects and field studies conducted by universities and

private organizations for the DoE ASHC Program are not documented herein.

The primary objective of most of the research described herein was to obtain

initial performance and long-term durability data which are needed for the

development of performance criteria and evaluation standards for materials

used in solar collectors or in transport and storage system components. Much

of the background covering the development of performance criteria and evaluation-type test methods can be found in a NBS report entitled "Plan for the

Development and Implementation of Standards for Solar Heating and Cooling

Applications" [1].* The report contains the results of a DoE-sponsored study

which sought to identify high priority standards needs and, through their

development, to stimulate the use of solar energy heating and cooling systems

in buildings. In reference [1], the NBS, working in conjunction with the

American National Standards Institute (ANSI) Solar Energy Steering Committee,

tabulated some 210 standards which needed development. These standards were

ranked in descending order of priority.

The ranking of the materials test methods and specifications listed in

reference [1] was largely corroborated by the findings of a survey and field

study conducted by the NBS [2]. By identifying and documenting materialsperformance problems in operating solar energy systems, the investigators

sought to: 1) compile a data base on materials performance, 2) identify materials which need performance tests, 3) identify and assess existing standards

that may be applicable to solar materials evaluation and 4) provide recommendations for the development of needed performance standards. The materials

standards needs and their relative priority rankings are listed in table 1.1 as they were presented in reference [2]. The "key" properties and tests

associated with each of the standards areas were also identified.

* Figures in brackets pertain to the references listed in the index located

at the end of the report.

1
1.2 ORGANIZATION OF THE REPORT

The summaries that follow are Intended to present the primary objectives, the

scope and the principal results of the various studies conducted by NBS. When

a consensus standard has been adopted or an existing standard has been subsequently revised, that action is so indicated. The details of the affected

standard are not presented here as they are discussed in another NBS report,

"Test Method Development and Research for Evaluating Active Solar Energy

Systems, Materials and Components." A description of the individual materials and material combinations are presented and test variables are noted. The

information has been organized according to the major solar energy subsystems:

Collector, Transport, and Storage.

Research studies concerned with the materials used in components of the

collector subsystem are summarized in chapter 2. Included in that chapter are sections on cover plates, absorber coatings, insulation, and sealants and

gaskets. Chapter 3 contains sections on research activities relating to

materials utilized in either the transport or storage subsystems (containment

materials, heat transfer fluids, hoses, and fluid/storage media). Remaining

high-priority research needs, and some of the current activities sponsored by

DoE aimed at fulfilling those needs, are described in chapter 4.

Units of measurement used in this summary are the same as those provided in

the reports summarized.

2
Table 1.1 Recommended Priorities for the Development of

Materials-Related Standards (from Reference [1])

Materials Standards Area Priority

Absorptive coatings High

Collector insulation High

Cover plates High

Heat transport liquids High

Metallic containment High

Nonmetallic containment and absorber

substrates High

Seals High

Stagnation testing of collector subsystems High

Surface temperature measurements High

Flexible couplings Medium

Reflective surfaces Medium

Thermal storage media Medium

Transport and storage insulation Medium

Coatings/liners for transport or storage subsystems Low

Desiccants Low

Dielectric insulators Low

Filters/getters Low

i

i

I

3
2.1 2 . COLLECTORS

COVER PLATES

2.1.1 Introduction

In a flat-plate solar collector, the cover plate has at least four functions

which affect collector performance: 1) the cover plate should permit the

maximum possible transmission of solar energy to the absorber plates; 2) the cover should minimize the reflection of infrared solar radiation from the

absorber plate back into the atmosphere; 3) the cover plate should minimize

convective heat losses from the absorber plate by maintaining a confined air

space between the surface and the absorber; and 4) the cover plate should

protect the interior of the collector from potentially harmful external weather

conditions. Cover plate materials should be selected on the basis of their

optical, thermal and physical performance as well as their durability and

cost. Several research studies have sought to establish a technical basis for

the development of standard test methods for cover plate materials, that can

be used for the evaluation of their optical and mechanical properties and in

assessing the long-term durability of candidate materials. In the paragraphs

that follow, the details of those studies conducted by NBS are summarized.

2.1.2 Optical and Mechanical Property Measurement and Durability Assessment

During 1976 and 1977, the National Bureau of Standards performed laboratory

and field studies [3] on 10 cover plate materials to measure critical optical

and mechanical properties and to assess material durability under exposure to

natural and simulated weathering. The materials used in the study were typical of those commercially available in 1976 and are listed in table 2.1.

The thicknesses of the specimens were representative of those used in solar

collectors. The two glass sheet materials differed in iron oxide content

which directly affects the solar energy transmittance. The two fiber reinforced

plastics were obtained from different manufacturers and contained different

resins. The environmental degradation factors studied were heat aging and

solar radiation. Three procedures which simulated these factors were developed.

The three aging procedures were: 1) heat stability exposure in a mechanical

convection oven, 2) artificial solar radiation exposure in a Xenon arc weathering

machine, and 3) natural weathering exposure on mini collector boxes. The

latter two procedures were used to permit comparison of the exposures and to

determine whether the effects on the materials were independent or synergistic.

The material properties measured were: 1) solar energy transmittance, 2)

linear dimensional changes, 3) warpage, and 4) the effects of dirt retention.

The first two properties were measured by ASTM standard procedures while the

latter two were measured by procedures developed during the study. Solar

energy transmittance was measured by both methods A and B of ASTM Standard

E424-71 [4]. Method A is performed in the laboratory utilizing an integrating

sphere spectrophotometer. Method B is conducted outdoors using a pyranometer

in an enclosure and the sun as the energy source.

4
Table 2.1 Cover Plate Materials Tested for Physical Properties

and Durability (from reference [3])

Material

Nominal Thickness

mm (in)

Glass (0.01% iron oxide) 3.2 (1/8)

Glass (0.10% iron oxide) 3.2 (1/8)

Fluorinated (ethylene

propylene) copolymer 0.025 (0.001)

Poly (vinyl fluoride) 0.10 (0.004)

Poly(ethylene terephthalate) 0.13 (0.005)

Acrylic 0.076 (0.003)

Poly (methyl methacrylate) 1.5 (0.06)

Polycarbonate 1.02 (0.04)

Glass fiber reinforced plastic 1.02 (0.04)

Glass fiber reinforced plastic 1.5 (0.06)

• 5
Temperatures for the heat aging tests were based upon maximum cover plate

stagnation temperature measurements obtained during a preliminary study. Test

temperatures were set at 75°C (167°F), 100°C (212°F), 125°C (257°F), and 150°C

(302°F). The test specimens were suspended in an oven described in ASTM Standard D794, and exposed to the test temperatures for time intervals ranging up

to 2,000 hours. Once the specimens were removed from the oven, measurements

were made of solar transmittance, warpage and linear dimensions, and they were

then discarded. In the artificial solar exposure tests, a xenon arc light

apparatus was used to simulate the spectral distribution of sunlight. Test

specimens were exposed to the artificial radiation according to the procedure

described in ASTM Standard D2565-79. The specimens were removed for evaluation

at time intervals from 250 to 4,000 hours. For the natural weathering exposure

tests, special "mini collectors" containing the cover plate specimens were assembled to obtain the elevated temperature levels associated with stagnation

conditions. The minicollectors were weathered at three sites: New River,

Arizona; Miami, Florida; and Gaithersburg, Maryland. The weathering racks were exposed according to ASTM Standard D1435-75. Sets of weathering specimens were removed for property testing at 3, 6, 12, 18, and 24 month intervals.

(In a continuation of the weathering exposure tests, some specimens were left

at the sites for 36 or 48 months before being removed for evaluation. A discussion of the results of the extended exposure testing is presented in

section 2.1.4.)

Most of the data were presented in terms of either percent of solar energy

transmittance versus time of exposure or spectral transmittance (percent

transmittance versus wavelength). In addition, data indicating the percent

change in linear dimensions and the amount of warpage after heat aging exposure

were presented. There were a number of conclusions drawn as a result of the

laboratory and field studies; some of them are enumerated below:

1. Method A (spectrophotometer) of ASTM E424 has better precision and

repeatability than Method B (outdoor) and is therefore preferrable to

Method B for determining solar energy transmittance of cover plate

materials.

2. Accelerated laboratory aging procedures, such as those performed in the

study offer an effective means of screening candidate cover plate

materials

.

3. Heat aging at elevated temperature (150°C (302°F)) for a relatively

short time (500 hours) is an effective means of identifying materials

which will develop problems in outdoor weathering.

4. Measurement of linear dimensions after 10 hours exposure at a specific

temperature is sufficient to establish trends and determine approximate

dimensional changes due to that specific temperature.

5. Artificial weathering with a xenon arc light is helpful in

distinguishing materials that are affected by solar radiation.

6
6. The minicollectors adequately simulated operating and In some cases stagnation temperatures obtained by flat plate collectors.

7. Dirt and dust retention is a major cause of solar energy transmittance

decrease in all cover plate materials. Solar transmittance losses

due to dirt and dust- (which can be washed off) are generally from

three to five percent and are occasionally higher.

Based on the results of the NBS laboratory and field studies, two draft

standards for evaluating cover plate materials for flat plate solar collectors

were prepared and submitted to ASTM Subcommittee E44.04 on Materials Perfor-

- mance. The titles of the two draft standards are: 1) Standard Practice for

Exposure of Cover Materials for Solar Collectors to Natural Weathering Under

Conditions Simulating Stagnation Mode; and 2) Standard Practice for Evaluation

of Cover Materials for Flat Plate Solar Collectors. The draft standards, with

some modifications, have been adopted by ASTM as standard E881-82 and E765-80,

respectively.

2.1.3 Parametric Study of Optical Properties

The NBS subsequently conducted a study [5] in which the aforementioned

transmittance data were analyzed to determine the influence of air mass and

integration technique on Method A of ASTM Standard E424 [4] for measuring solar

energy transmittance. As mentioned above. Method A utilizes an integrating

sphere spectrophotometer for measuring the transmittance of solar collector

cover plates (and also the reflectance of absorber materials). The spectral

data so obtained are integrated by arbitrarily selecting one of two methods of

integration—weighted ordinates or selected ordinates. One of the objectives

of the NBS study was to determine whether the two methods of integration yield

the same optical property values for typical collector materials at an air mass

equal to 2.0. The second objective was to determine whether a variation in the

solar energy spectral distribution (air mass) affects the transmittance and

reflectance values of typical materials. The third objective was to develop

recommendations for the revision of E424, Method A, if necessary. Three air mass values were used in the calculations: 2.0 (which is specified in Method A

of E424), 1.5 and 1.0.

The available transmittance data had been obtained during the field and laboratory

study [3] of cover plates described above. The transmittance data used in this

study were obtained from: 1) control specimens (no natural or artificial

weathering) and 2) specimens exposed to oven aging at 150°C (302°F). The spectral transmittance curves for the control specimens are presented in figures

2.1 and 2.2. Similar spectral transmittance curves are available for the

specimens exposed to heat aging.

In the weighted ordinate method of integration, the spectral distribution of

sunlight (i.e., solar spectrum) is divided into a finite number of equal intervals, e.g., 50 nm. The relative energy for each interval is calculated by

dividing the energy calculated for that interval by the total energy under the

solar spectrum curve. The wavelength at the midpoint of the interval is then

used as the wavelength for the weighted ordinate. The total solar energy

7
Figure 2.1 Spectral Transmittance of Three Cover Plate Materials, Fiber

Reinforced Plastic, Fluorinated (ethylene propylene) Copolymer

and Glass (from reference [5])

8
Figure 2.2 Spectral Transmittance of Two Cover Plate Materials, Polycarbonate

and Poly (vinyl fluoride) (from reference [5])

9
transmittance for a given material is then determined by summing the products

of transmittances at specific wavelengths times the fraction of the total

energy in the interval times the transmittances at a specific wavelength. The

36 specific wavelengths (at 50 nm intervals) and their corresponding relative

energy values were taken directly from table A.l of Standard E424 and used in

computing the total solar energy transmittances for each material specimen at

air mass 2.0.

In the selected ordinate method, the solar spectrum is divided into a finite

number of intervals with equal increments of solar energy. The wavelength at

the midpoint of each interval is the selected ordinate. The spectral

transmittance of a given cover plate material is thus weighted equally in

each interval and no further weighting is necessary. The total solar energy

transmittance is determined by summing the materials transmittance value (as

obtained from the spectral transmittance curve) for each of the selected ordinates and dividing the sum by the number of selected ordinates. Standard E424

uses 20 selected ordinates for air mass 2.0. These 20 ordinates were used

in fulfilling the first objective of the study. In the comparative study of

the effects of air mass variation, 50 selected ordinates were used for air masses 1.0 and 1.5 based on data obtained from other studies. Transmittance

values that were obtained for the weathered and the control specimens using

both methods of integration are presented in table 2.2. Qualitative results

are summarized in table 2.3. Upon evaluating the results of the parametric

study, two conclusions were drawn:

1 . The selected and weighted ordinate methods do not produce identical

solar transmittance values. Transmittance can vary by up to one percent for some materials depending on which method is used in the

calculations. The differences occur with cover plate materials which

have absorption bands in the near infrared (NIR) region of their

spectra. Where differences occur, the weighted ordinate method gives

higher results.

2. Air mass values used in calculations have a considerable effect on the

transmittance values obtained. Changing the air mass values used in

calculating transmittances can lead to differences of 3 to 4 percent

for some materials. The differences may increase as degradation of

the material increases. The materials most affected by air mass change are those which transmit the least energy in the ultraviolet

region (below 390 nm)

.

As a result of this analytical investigation two recommendations were made

with respect to ASTM Standard E424-71:

1 . Because the two methods of integration do not yield identical results

for material having absorption bands in the near infrared region of

the spectra, Method A should specify only one method of calculating

transmittance.

10
Table 2.2 Tabulation of Transmittance Data for Cover Plate Materials (from reference [5])

Material

Exposure

Test

Time of

Exposure

Hours

Weighted Ordinate

at Air Mass 2.0

Selected

Ordinate at Air Mass 2.0

Selected

Ordinate at Air Mass 1.5

Selected

Ordinate at Air Mass 1.0

Glass Control 0 87.4 87.6 87.4 87.1

Fluorinated

(ethylene

propylene)

copolymer Control 0 96.4 96.5 96.3 96.4

Poly(vinyl

fluoride) Control 0 92.2 92.4 91.7 91.3

Polycarbonate Control 0 88.2 87.3 84.5 82.7

Fiber reinforced

plastic Control 0 78.2 76.8 76.7 72.8

Glass Heat Age 150°C 10 87.1 87.1 87.2 86.7

100 87.4 87.6 87.3 87.0

225 87.1 87.2 87.1 86.5

500 87.1 87.3 86.9 86.6

750 87.2 87.3 87.1 86.8

1000 86.9 86.1 86.9 86.5

1250 86.6 86.7 86.6 86.1

1500 87.5 87.6 87.3 86.0

1825 87.3 87.4 87.2 86.9

Fluorinated Heat Age 150°C 10 95.4 95.3 95.3 95.2

(ethylene 100 95.4 95.4 95.3 95.4

propylene) 225 95.5 95.5 95.4 95.3

copolymer 500 95.6 95.6 95.6 95.4

750 95.3 95.4 95.3 95.3

1000 95.4 95.5 95.5 95.5

1250 95.6 95.5 95.3 95.4

1500 95.6 95.5 95.3 95.4

1825 95.2 95.2 95.0 95.1

Poly(vinyl Heat Age 150°C 10 91.2 91.1 90.5 90.2

fluoride) 100 87.5 87.3 85.4 84.2

225 88.3 88.0 86.6 85.4

500 86.7 86.4 84.6 83.1

750 82.3 81.8 79.4 77.3

1000 80.4 80.0 77.2 75.0

1250 74.9 74.5 71.2 68.7

1500 71.6 71.2 67.7 65.1

1825 72.1 71.7 68.2 65.8

Polycarbonate Heat Age 150°C 10 86.9 86.1 83.4 81.5

225 87.1 86.2 83.4 81.7

500 86.7 85.6 82.9 81.0

750 86.3 85.4 82.6 80.8

1000 86.5 85.6 82.7 80.9

1250 86.1 85.3 82.4 80.5

1500 86.3 85.3 82.5 80.6

1825 85.6 84.8 81.9 80.0

Fiber reinforced Heat Age 150°C 10 70.9 69.7 67.2 65.2

plastic 100 60.6 59.8 56.6 54.4

225 54.3 53.5 50.2 48.0

500 52.4 51.7 48.4 46.3

750 43.9 43.3 40.4 38.4

1000 40.0 39.5 36.7 35.0

1250 40.0 39.4 36.7 34.9

1500 44.8 44.2 41.0 39.1

1825 39.7 39.1 36.5 34.7

11
Table 2.3 Qualitative Results of Parametric Study of Cover Plate Materials

(from reference [5])

Effect on 1 Ol transmittance

Integration Methods Air Mass

Fiberreinforced

plastic

Selected ordinate method

yielded lower values of

transmittance*

.

Transmittance decreased

with decreasing air mass*.

00 *H«

t-lM

0>

Polycarbonate Selected ordinate method

yielded lower values of

transmittance*

.

Transmittance decreased

with decreasing air mass*.

4J

cd X

0) u

<0 rH

tuu

<u>

o

CJ

Poly(vinyl

fluoride)

1) No difference for

unweathered

2) Selected ordinate

method yielded lower

values for weathered

specimens

.

Transmittance decreased

with decreasing air mass*.

Fluorinated Ethylene

propylene No difference* No difference for weathered

specimens.**

Glass No difference* No difference for weathered

specimens .**

* Applies to both weathered and unweathered specimens.

** For unweathered cover plate materials, the transmittance

decreased as the air mass decreased.

12
2. Since most collectors in service are exposed to more hours of radi

at air mass 1.5 than at air mass 2.0, the solar energy distributive!

used in Method A should be changed to air mass 1.5.

These proposed revisions are still under consideration by ASTM Committee E44

on Solar Energy Conversion.

2.1.4 Weathering Performance of Cover Materials

In order to obtain somewhat longer term natural weathering data on cover

plate materials, the NBS extended the exposure periods of the minicollectors

mentioned in section 2.1.2 for up to 24 months [6]. In addition to providing

a measure of the relative durabilities of the ten cover plate materials (see

table 2.1), the natural weathering data served as a basis of comparison for

data obtained from the artificial weathering with a xenon arc light. The

comparative analysis of the two methods of weathering was done with a view

toward the development of uniform methods of measuring long-term performance

of cover plate materials.

To facilitate the comparison of the effects of natural weathering with those

of artificial weathering, the energy deposited on materials during xenon arc exposure was calculated. The hours of xenon arc exposure within the artificial

weathering device and the corresponding calculated energy values are shown in

table 2.4. The exposure schedule and the solar radiation accumulated during

exposure at the three natural weathering sites are summarized in table 2.5.

The solar energy transmittance of the material specimens was determined using

method A of ASTM E424-71. For the natural weathering exposure tests, the

transmittance was obtained for both single and double cover minicollectors.

The transmittance of the field-weathered specimens was measured both before

and after a cleaning procedure to determine the effect of dirt accumulation on

the optical properties. The results of the field study were presented in

tabular and graphical forms, showing the percent of solar energy transmittance

as a function of exposure time. A typical set of test curves is shown in

figure 2.3. The solar energy transmittances of the inner and outer covers of

the minicollectors were graphically compared with that from the specimens

exposed to the xenon arc light. One such comparative curve is shown in figure

2.4.

In addition to measuring the effect of weathering on optical properties, the

effect on the tensile properties of three plastic films was measured using

ASTM D882, Tests for Tensile Properties of Thin Plastic Sheeting. The tensile

properties which were reported were: 1) breaking factor, 2) tensile strength,

3) elongation at break, 4) yield strength, and 5) elongation at yield. It was concluded that the evaluation process for cover plate materials should include

measurement of mechanical as well as optical properties. Further, it was concluded that tensile strength and elongation at break are useful parameters

for observing changes in the mechanical properties of plastic film materials

caused by weathering.

The results of the additional two years of natural weathering strengthened the

previous conclusion that minicollectors adequately simulated the conditions

13
Table 2.4 Calculated Value* for Energy Deposited on Cover

Material* During Artificial Weathering with Xenon

Arc Light (from rafaranca [6])

Hour* of

Exposure

Energy Deposited

(Calculated) GJ/m?

250 1.030

499 2.055

758 3.122

1000 4.118

2012 8.286

3000 12.355

4010 16.515

14
Table 2.5 Natural Weathering Exposure Data for Minicollectors

(from reference [6])

Solar Radiation (GJ/m^)

Months Gaithersburg, Miami

,

New River,

Exposure Dates Maryland Florida Arizona

9/1/77-12/1/77 1.176 1.455 2.012

3

12/20/77-3/20/78 1.422

9/1/77-3/1/78 2.251 2.864 3.487

6

3/24/78-9/24/78 5.123

9/1/77-9/1/78 5.477 6.344 8.006

12

3/1/78-3/1/79 5.429

4/1/78-4/1/79 6.523

7/29/78-7/29/79 8.259

18 9/1/77-3/1/79 7.680 9.333 11.642

24 9/1/77-9/1/79 10.720 13.125 16.370

9/1/77-9/1/80 16.402 19.398 24.414

36

4/1/78-4/1/81 19.859

4/17/78-4/17/81 16.693

48 9/1/77-9/1/81 22.076 26.333 33.038

15
s0

LA

R

eN

eR

6

V

T

R

A

N

S

A

ITT

A

N

C

E

Figure 2.3 Solar Energy Transmittance of Poly(methyl methacrylate)

After Weathering on Minicollectors in New River, Arizona

16
$

0

LA

ftE

N

E

ft6

V

T

ftA

ft$

ftIT

T

A

ftC

E

Figure .4 Comparison of Solar Transmittance of Poly(methyl methacrylate)

After Weathering on a Minicollector as an Outer Cover and In an Artificial Weathering Device with Xenon Arc Light

(from reference [6])

17
encountered by cover plates on some flat plate collectors. Furthermore, the

mechanical and optical property data associated with the artificial weathering

with xenon arc light strengthened the case for this laboratory method of

distinguishing materials sensitive to natural sunlight.

2.1.5 Hail Impact Testing

As implied in section 2.1.1, the physical deterioration of cover plates can substantially degrade the efficiency of the solar collector subsystem. In order

to effectively perform their intended optical and thermal functions, cover plates must be able to resist or be protected from forces which can cause rupture, cracking, extensive crazing or surface distortion. One of the main

environmental forces that can cause failure of a cover plate is hail impact.

Although some solar building construction standards required hail resistance

testing, there was no standard testing procedure for glazing materials. The

NBS conducted a laboratory study [7] aimed at developing such a standard test

method. Thus, the primary purpose of the research was to evaluate and refine

previously-used procedures toward the development of a standard test method

for simulating hail impact on solar collector covers.

In this study, ice balls of various diameters and velocities were propelled by

a compressed air launcher against targets consisting of typical cover plate

materials. The ice ball parameters used in the laboratory testing are given

in table 2.6. The materials selected for testing were considered to be representative of the three classes of cover plates normally encountered, namely,

rigid, semirigid and flexible. Tempered glass was used to represent a rigid

cover. Fiber reinforced plastic represented a semirigid cover and polymeric

thin film represent a flexible cover. The principal factors considered in

evaluating the impact test procedures were: 1) velocities of ice balls used

to simulate hail, 2) impact loads caused by falling hail versus propelled ice

balls, 3) impact locations on the collector cover, 4) support conditions for

the covers, and 5) assessment of collector cover failure or damage caused by

impacting ice balls. Some experimental effort was devoted to developing: 1) a procedure for preparing the ice balls, 2) a method of aligning the launcher on

the target area, 3) a method of measuring the velocity of the ice ball, and

4) a means of measuring kinetic forces.

The glass and plastic sheets were mounted in cover support frames. One type of

frame was designed to support both rigid and semirigid specimens, while a slightly modified design was used for flexible, thin film materials. The

frames were sized to accomodate 1.93 m by 0.86 m (76 in by 34 in) sheets; this

frame size was representative of flat plate collectors. To provide a basis

for comparing results obtained with the standardized support frame, several

specimens of tempered glass were mounted in a solar collector and subjected to

ice-ball impact. The test procedure involved the successive propulsion of ice

balls of various diameters and velocities against selected points on the

mounted plate material. Seventeen points of impact were marked on the exposed

face of the sheet. For a given specimen, an initial iceball size and corre- sponding velocity were selected which were not expected to cause failure. If

there was no visible damage for the initial set of parameters, the next larger

size ice ball and corresponding velocity were used for impact. On the other

18
Table 2.6 Ice Ball Parameters for Laboratory Testing (from reference [7])

Diameter

(in.

)

Mass

(lbm)

Velocity^/ Kinetic Energy^/ Momentum^/

Terminal

(ft/s)

Resultant z/

(ft/s)

Terminal

(ft-lbf)

Resultant

(ft-lbf)

Terminal

(lbf-s)

Resultant

(lbf-s)

3/4 0.0073 62 91 0.44 0.94 0.014 0.021

1 0.0173 73 98 1.43 2.58 0.039 0.053

1-1/4 0.0337 82 105 3.52 5.77 0.086 0.110

1-1/2 0.0584 90 112 7.35 11.38 0.163 0.203

1-3/4 0.0928 97 117 13.56 19.73 0.280 0.337

2 0.1384 105 124 23.69 33.04 0.451 0.533

2-1/4 0.1971 111 129 37.71 50.93 0.679 0.790

2-1/2 0.2705 117 134 57.50 75.42 0.983 1.126

JJ For laboratory testing, ice balls were propelled in a direction perpendicular

to the plane of impact.

]J Resultant velocity is the vector sum of the terminal velocity and a 66 ft/s

horizontal wind component.

3/ Corresponding to the terminal and resultant velocity.

19
Table 2.7 Summary of Ice Ball Impact Test Results (from reference [7))

Material

Ice Ball Causing Failure

Kinetic

Specimen Appendix Diameter Velocity Energy

Number Table (in.

)

(ft/s) (ft-lbf)

Impact Location^/

and Type of Failure

Prior

Impact Tests (lbm/ft)

Tempered glass. #1 A-3 1-3/4 119 R Exposed side 02 A-5 1-3/4 115 R textured, other 03 A-6 1-3/4 122 R side smooth. #4 A-7 1-3/4 101 R (1/8 in. thick) 0

5

A-8 1-1/2 95 T

06 A- 11 1-3/4 98 T

Tempered glass. 01 A- 13 2 129 R Both sides smooth. 02 A- 17 2-1/4 131 R (1/8 in. thick) 03 A- 18 2-1/4 127 R

04 A-20 2-1/4 114 T

05 A-21 2 130 R

06 A-22 2 128 R

Tempered glass. A A-38 2-1/2 135 R Both sides smooth. F A-39 1-3/4 119 R (1/8 in. thick) B A-40 2-1/2 136 R Mounted in Solar E A-41 2-1/2 136 R Collector C A-42 2-1/2 137 R

D A-43 1-3/4 115 R

Reinforced plastic. 01 A-23 1-1/2 90 T Polyester resin 01 A-23 1-1/4 82 T with glass fiber 02 A-24 3/4 62 T

mat. (0.040 in. 03 A-25 1-1/4 82 T thick)

04 A-26 1-1/4 105 R

05 A-27 1-1/2 98 R

06 A-28 1-1/4 82 T

07 A-29 1-3/4 97 T

Poly(vinyl 01 A-30 3/4 62 T fluoride) film.

(0.004 in.

thick) 01 A-31 3/4 91 R

02 A-32 1 73 T

02 A-33 1 98 R

03 A-34 1-1/4 82 T

1-1/4 105 R

03 A-34 1-1/2 90 T

04 A-35

1-1/2

2

112 R

105 T

05 A-36 1-3/4 117 R

06 A-37 1-3/4 97 T

20 01 Fracture 1 R, 1 -1/2 R

19 02 Fracture 1-3/4 T - 20 02 Fracture None

14 02 Fracture None

7.5 02 Fracture None

13 02 Fracture 1-1/2 T, 1-1/2 R

36 014 Fracture 2 T

49 05 Fracture 2 R, 2 -1/4 T

47 03 Fracture None

38 09 Fracture 2 T

34 04 Fracture None

33 01 Fracture None

78 01 Fracture 1-3/4 R, 2 R

20 01 Fracture None

80 010 Fracture 2 R

79 010 Fracture 2 R

80 014 Fracture 1-3/4 R, 2 R

19 014 Fracture None

5. 2-9.2 01 to 011, Leak None

3. 1-5.3 012 to 017, Leak None 0.38-0.53 01 to 017, Cracks None

3. 0-3.

9

01, 03 to 6, 08 None

to 11, 013 to 16,

Leak

4. 4-5.

9

02 to 012, 014, 015, None

leak, 016, 017,

Break

8.3-11.7 01 to 017, Break None

2. 9-3.

03 to 09, Leak None

12-18 01 to 09, Break None

0.36-0.49 01 to 017, Slight None 3.3 side

damage , small

indentations

4.2 end

0.76-1.01 01 to 017, Slight 3/4 T damage , small

indentations

1. 1-1.6 01 to 017, Slight None 3.6 side

damage, indentations

5.3 end

2. 2-2.

01 to 017, Slight 1 T damage , indentatations

2. 9-4.0 01 to 04, 07 to 09, None 5.2 side

Indentations 5.5 end

5. 0-5.

05, 06, 010 to 012,

016, 017, Indentations

5. 8-6.

6

013, 014, Indenta- None

tions

9.3 015 Indentation

19-24 02, 06 to 09, None 5.2 side

Break 5.5 end 18-20 01, 03 to 05,

Indentations

16 01, 09, Indentations None 5.2 side

17-18 02, 03, 06 to 08, 5.5 end Break

11-15 01 to 09, 011 to 013, 5.2 side

Indentations 5.5 end 12-15 010, 014 to 017, None Break

R = resultant velocity, T = terminal velocity.

Locations are denoted by a number as shown in figure 20 in reference (7).

Edge loading of plastic film during mounting in the cover support frame.

20
hand, a damaged specimen was immediately discarded. This procedure was repeated with ice balls of increasing size until failure occurred.

A summary of results from tests on four solar collector cover materials is

presented in table 2.7. The table reports: 1) diameter, velocity, and kinetic

energy of the ice ball causing failure; 2) location of impact and type of

failure; 3) prior impact tests on the tempered glass specimens; and 4) the

amount of tensile edge loading of plastic film specimens during mounting in the cover support frame.

The laboratory experiments confirmed the feasibility of simulating the impact

of hailstones by propelling ice balls from a compressed air launcher with

interchangeable plastic barrels. It was concluded that the ice balls should

impact a specimen along a path perpendicular to the specimen surface at a resultant velocity which is the vector sum of the hailstone terminal velocity

and a horizontal wind velocity component equal to 72 km/h (45 mi/h). The

investigators emphasized the need for a system for accurate velocity measurement of ice balls in flight. Information from this study was used to prepare

a draft standard for consideration by ASTM Subcommittee E44.04 on Materials

Performance. Subsequently, the ASTM Task Group on Cover Plates, E44.04.02,

prepared a draft standard practice which was adopted as E822-81, "Practice for

Determining Resistance of Solar Collector Covers to Hail by Impact with

Propelled Ice Balls."

2 . 2 ABSORBER PLATES

2.2.1 Introduction

The purpose of absorber plates in solar energy systems is to convert solar

energy into useful thermal energy. For optimum efficiency, the solar absorber

should possess maximum possible absorptance (a) in the solar spectrum while

exhibiting minimum infrared emittance (e). Generally, high absorptance in the

ultraviolet, visible, and near infrared portions of the spectrum is obtained

by a black surface coating. Low infrared emittance is obtained by using a highly reflective metal substrate beneath the coating. A coating material

which captures solar energy in the high intensity visible and near infrared

spectral regions while maintaining poor infrared radiating properties is called

a selective coating.

Much of the research conducted since 1974 on absorber components has been

directed at: 1) understanding and modeling the microstructure of selective

coating candidate materials, 2) improving optical properties (i.e., absorptance

and emittance) of absorbers by studying the performance of various combinations

of coatings and substrate material, and 3) establishing standard test procedures

for evaluating the performance of absorbers under conditions of high temperature and humidity (for both short- and long-term operation). Several national

laboratories have engaged in studies to evaluate the performance of specific

absorber coating/substrate combinations under simulated normal operating and

stagnation conditions. Black chrome has been extensively studied as an absorptive coating because of its desirable properties.

21
2.2.2 Laboratory and Field Experimentation

The lack of short-term and long-term performance data on absorber materials

in operating collectors and the lack of standardized test methods for the

prediction of long-term performance have hampered efforts to derive solutions

to some documented collector problems. Of particular concern are the effects

of elevated temperature due to stagnation, thermal cycling, ultraviolet radiation, and moisture. The National Bureau of Standards conducted studies [8,9] on

the performance and durability of several absorber materials to aid in the

development of standardized, accelerated-aging test methods. It was intended

that the test methods be used to predict the long-term performance of

absorptive coatings for solar collectors.

Laboratory and field aging tests were conducted on twelve absorber materials

used primarily in building heating, cooling, and domestic hot water applications. Table 2.8 tabulates all of the absorber materials and the substrates

on which they were applied. Absorptance and infrared emittance were determined

using ASTM E424-71, Method A and ASTM E408-71 respectively. The optical property

measurements were made before and after the aging tests to determine the deleterious effects of the aging processes. In addition to optical measurements, the

specimens were visually inspected before and after aging for signs of loss of

adhesion properties and changes in visual appearance. The aging tests included

accelerated laboratory exposure and outdoor exposure in simulated solar collectors. The two-pronged test program has as one of its objectives to determine

whether or not the data obtained by the two methods of tests lead to the same level of degradation of absorptive coatings.

In the accelerated aging tests, the materials were exposed to elevated

temperatures, thermal cycling, ultraviolet radiation (UV) and moisture. Test

specimens were exposed in an oven to elevated temperatures of 150, 200 and

250°C and removed at various time increments for measurement of properties.

The maximum time of exposure was 14 weeks for the 150°C specimens, while the

remaining specimens were exposed for a maximum of 12 weeks. In the thermal

cycling tests, specimens were exposed to 84 cycles with each cycle consisting

of 7.5 hours at 200°C followed by 15.5 hours at -10°C. Ultraviolet radiation

exposure was simulated through the use of ASTM G26-77, in which a xenon arc accelerated weathering machine of measured solar intensity is specified.

Specimens were exposed for time increments ranging from 125 to 2100 hours

before measurement was performed. Test specimens were exposed in a humidity

cabinet at 92°C and 97 percent relative humidity and removed at time increments

of 1,2, 6 and 21 weeks for property measurement. In addition to the single

aging tests, a cumulative exposure test was performed. In the cumulative

exposure test, specimens were sequentially exposed to elevated temperature,

thermal cycling, UV radiation, and moisture. Optical properties were measured

after each exposure condition.

Absorptance and infrared emittance values were tabulated for the above-mentioned

accelerated exposure tests, thereby facilitating comparison with optical property measurements obtained for unaged specimens. The average initial absorptance and emittance values obtained from ten unaged specimens of each absorber

material are shown in table 2.9. As an example of the accelerated aging test

22
Table 2.8 Absorber Materials and Substrates Used in the Study

(from reference [8])

Code Number^/ Absorber Material Substrate

A8 Nickel coated foil Aluminum, alodine

B6 Anodized aluminum Aluminum, 6061, am

B7 Anodized aluminum Aluminum, 1100, an< C5 Copper oxide Copper

D2 Black chrome Steel, cold-rolled:

D4 Black chrome Copper, DHP 1221/

G9 Polyvinylidene fluoride 20 gage, G90, hot < galvanized steel

HI Polyvinylidene fluoride Steel, bonderized

H8 Polyvinylidene fluoride Aluminum, alodine

11 Urethane Steel, bonderized

14 Urethane Copper, DHP 122

18 Urethane Aluminum, alodine

J1 Alkyd Steel, bonderized

J4 Alkyd Copper, DHP 122

J8 Alkyd Aluminum, alodine

Kl Epoxide Steel, bonderized

K4 Epoxide Copper, DHP 122

K8 Epoxide Aluminum, alodine

LI Silicone Steel, bonderized

L4 Silicone Copper, DHP 122

L8 Silicone Aluminum, alodine

Ml Modified polyester Steel, bonderized

M4 Modified polyester Copper, DHP 122

M8 Modified polyester Aluminum, alodine

N2 Porcelain enamel Steel, cold-rolled

N8 Porcelain enamel Aluminum, alodine

U The letter refers to the absorber material and the number to the substrate.

U Includes 0.5 mil nickel flash.

3/ Includes 0.1 - 0.2 mil nickel flash.

23
Table 2.9 Absorptance and Infrared Emittance of Absorbers

Prior to Exposure (from reference [8])

Code Number Absorber Material Absorptance (q) Emittance (r.)

A8 Nickel foil 0.98 0.09

B6 Anodized aluminum 0.92 0.81

B7 Anodized aluminum 0.90 0.82

C5 Copper oxide 0.95 0.741/

D2 Black chrome 0.95 0.07

D4 Black chrome 0.96 0.06

G9 Polyvinylidene fluoride 0.95 0.85

HI Polyvinylidene flouride 0.96 0.85

H8 Polyvinylidene fluoride 0.96 0.88

11 Urethane 0.97 0.92

14 Urethane 0.97 0.92

18 Urethane 0.97 0.92

J1 Alkyd 0.97 0.92

J4 Alkyd 0.97 0.91

J8 Alkyd 0.97 0.92

K1 Epoxide 0.97 0.91

K4 Epoxide 0.97 0.91

K8 Epoxide 0.97 0.91

LI Silicone 0.97 0.88

L4 Silicone 0.97 0.87

L8 Silicone 0.97 0.88

Ml Modified polyester 0.93 0.86

M4 Modified polyester 0.89 0.84

M8 Modified polyester 0.90 0.81

N2 Porcelain 0.93 0.85

N8 Porcelain 0.93 0.84

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

24
NUMBER

OF

CYCLES

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25

designates

removal

of

absorber

material

during

exposure
results, table 2.10 presents the results of the thermal cycling exposures.

The draft standard that was developed as a result of the laboratory aging

tests was subsequently adopted by ASTM as standard E744-80 [10].

In the field aging study, specimens were exposed in test boxes to simulate

stagnation conditions at three sites: 1) Gaithersburg, Maryland; 2) Phoenix,

Arizona; and 3) Miami, Florida, for periods ranging from twenty to twenty-four

months. During these exposure intervals specimens were retrieved for laboratory

evaluation of their optical properties and then returned to the field for

continuing exposure. The solar absorptance and infrared emittance were measured after approximately 4-month exposure intervals. The field exposure

study extended over a period of about three calendar years, beginning in

the summer of 1978 and terminating in the fall of 1981.

Triplicate specimens of each coating/substrate combination were exposed in

accordance with a method that, with some modifications, was adopted by ASTM as E781-81 [11]. The exposure boxes simulated solar collectors of single cover

plate construction. The orientation (tilt angle) of the exposure boxes was adjusted four times each year to provide increased solar flux. Table 2.11 shows

the results of the outdoor exposures at Miami, Florida. The total direct solar

irradiation received during the 24 months of exposure was 13.62 x 10^ J/m^.

The comparison of data obtained from the laboratory tests with those from

the field exposure tests led to the conclusion that: "Degradation of absorptive

coatings, such as change of optical properties or appearance that was observed

during outdoor exposures, was also observed in one or more of the accelerated

laboratory exposures included in ASTM E744-80. This indicates that the degradation processes induced by stagnation exposure are adequately addressed by

the accelerated laboratory exposures." The significance of this conclusion

lies in the fact that ASTM E781-81 is now one of the testing options mentioned

by E744-80.

2.2.3 Parametric Study of Optical Properties

In the above mentioned parametric study of cover plates (section 2.1.3) [5],

NBS also analyzed laboratory test data on the performance of absorber materials.

The purpose of the study was to assess the effect of several variables on an ASTM standard method for measuring the reflectance of absorber materials

(Method A of ASTM E424). In Method A of E424 [4], an integrating sphere

spectrophotometer is used to measure the spectral reflectance of materials.

The spectral data so obtained is integrated by arbitrarily selecting one of

two methods of integration: weighted ordinates or selected ordinates. One

of the objectives of the NBS study was to determine whether the two methods of

integration yield identical optical property values for typical absorber

materials at air mass equal to 2.0. The second objective was to determine

whether the variation in the solar energy spectral distribution (i.e., air

mass) affects the reflectance values of typical absorber materials. The third

objective was to develop recommendations for the revision of E424, Method A,

if necessary.

26
Table 2.11 Results of Outdoor Exposure at Miami, Florida

Cumulative Time of Exposure (Months) (from reference [9])

Absorptance (a) Emittance (e)

Code

Number

Absorptive

Coating 0 4y si/

Months

121/ 16i/ 2ai/ 24£/ 0 41! ai/

Months

121/ 16^/ 20i/ 24£/

A8 Nickel Foil 0.98 0.99 0.98 0.98 0.98 0.98 0.97 0.09 0.05 0.08 0.08 0.08 0.09 0.10

B6 Anodized Aluminum 0.92 0.92 0.92 0.91 0.91 0.91 0.91 0.81 0.81 0.81 0.81 0.81 0.81 0.81 B7 Anodized Aluminum 0.90 0.90 0.90 0.89 0.89 0.89 0.90 0.82 0.81 0.80 0.81 0.81 0.80 0.81 C5 Copper Oxide 0.95 0.90 0.89 0.88 0.88 0.87 0.86 0.741/ 0.50 0.53 0.52 0.52 0.53 0.54 D2 Black Chrome 0.95 0.97 0.96 0.95 0.95 0.95 0.94 0.07 0.04 0.07 0.06 0.06 0.07 0.08

D4 Black Chrome 0.96 0.94 0.93 0.92 0.92 0.92 0.91 0.06 0.06 0.06 0.06 0.05 0.06 0.07

G9 Polyvinylidene

Fluoride

0.95 0.95 0.95 0.95 0.95 0.95 0.94 0.85 0.85 0.86 0.85 0.85 0.84 0.84

HI Polyvinylidene

Fluoride

0.96 0.95 0.95 0.95 0.95 0.95 0.95 0.85 0.82 0.85 0.83 0.84 0.83 0.83

H8 Polyvinylidene

Fluoride

0.96 0.95 0.96 0.95 0.95 0.95 0.95 0.88 0.87 0.88 0.87 0.87 0.86 0.86

11 Urethane 0.97 0.97 0.97 0.97 0.97 0.97 0.96 0.92 0.92 0.92 0.93 0.93 0.93 0.93

14 Urethane 0.97 0.97 0.97 0.97 0.97 0.97 0.96 0.92 0.92 0.93 0.93 0.93 0.93 0.93

18 Urethane 0.97 0.97 0.97 0.97 0.97 0.97 0.96 0.92 0.92 0.92 0.93 0.93 0.93 0.93

J1 Alkyd 0.97 0.98 0.98 0.98 0.98 0.98 0.97 0.92 0.91 0.91 0.91 0.90 0.90 0.90

J4 Alkyd 0.97 0.98 0.98 0.98 0.98 0.98 0.97 0.91 0.91 0.91 0.91 0.91 0.91 0.91 J8 Alkyd 0.97 0.97 0.98 0.98 0.98 0.98 0.97 0.92 0.91 0.91 0.91 0.91 0.91 0.91

K1 Epoxide 0.97 0.98 0.98 0.98 0.98 0.98 0.97 0.91 0.91 0.91 0.90 0.90 0.90 0.90

K4 Epoxide 0.97 0.98 0.98 0.98 0.98 0.98 0.97 0.91 0.91 0.91 0.90 0.90 0.90 0.90

K8 Epoxide 0.97 0.98 0.98 0.98 0.98 0.98 0.97 0.91 0.90 0.91 0.90 0.90 0.90 0.90

LI Silicone 0.97 0.97 0.97 0.96 0.96 0.96 0.96 0.88 0.87 0.87 0.87 0.87 0.87 0.87

L4 Silicone 0.97 0.97 0.97 0.96 0.96 0.96 0.96 0.87 0.87 0.87 0.87 0.87 0.87 0.87

L8 Silicone 0.97 0.97 0.97 0.97 0.97 0.96 0.96 0.88 0.88 0.85 0.87 0.87 0.88 0.88

Ml Mod. Polyester 0.93 0.94 0.95 0.95 0.94 0.94 0.94 0.86 0.89 0.92 0.91 0.91 0.92 0.92

M4 Mod. Polyester 0.89 0.91 0.93 0.93 0.93 0.93 0.92 0.84 0.85 0.90 0.90 0.90 0.90 0.91 M8 Mod. Polyester 0.90 0.92 0.94 0.94 0.94 0.93 0.93 0.81 0.88 0.92 0.91 0.91 0.93 0.93

N2 Porcelain 0.93 0.94 ’ 0.94 0.93 0.94 0.91 0.93 0.85 0.85 0.85 0.85 0.84 0.85 0.85

N8 Porcelain 0.93 0.93 0.92 0.92 0.92 0.93 0.92 0.84 0.84 0.85 0.84 0.84 0.84 0.84

— Exposed from 6/78 to 10/78.

Exposed from 11/78 to 5/79 (with a 2 mo. non-exposure period)

Exposed from 7/79 to 11/79.

Exposed from 5/80 to 9/80.

Exposed from 12/80 to 4/80.

-

J

Exposed from 5/81 to 9/81.

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

27
The available reflectance data had been obtained during the laboratory/f ield

tests on absorber plates described above [5]. The reflectance data were obtained from: 1) control specimens (no natural or artificial weathering),

2) specimens exposed to oven aging, and 3) specimens exposed to humidity at

elevated temperatures. Four absorber materials, two selective and two nonselective, were chosen: 1) black chrome on steel (selective), 2) black chrome

on copper (selective), 3) urethane paint on steel (nonselective) , and 4) porcelain enamel on steel (nonselective). The spectral reflectance curves for the

control specimens are presented in figures 2.5 and 2.6. Similar reflectance

curves are available for specimens exposed to aging. For calculating the

reflectance values, three air masses were used: 2.0 (which is specified in

Method A of E424), 1.5 and 1.0. The reader is referred to section 2.1.3 in

which brief descriptions of the weighted ordinate and the selected ordinate

methods of integration, as applied to transmittance calculations, are presented.

The methods were used in an identical manner to obtain reflectance values.

Reflectance values that were obtained for the weathered and the control

specimens, using both methods of integration, are presented in table 2.12.

Qualitative results are summarized in table 2.13. Upon evaluating the results

of the parametric study, three conclusions were drawn:

1. At air mass 2 both the weighted and selected ordinate calculation

methods yield comparable reflectance values.

2. A change in air mass has no observable effect on the reflectance

value calculated for nonselective absorbers.

3. Air mass values used in calculating reflectance can lead to

discrepancies for selective absorbers. Black chrome reflectance

values increased by up to 1.2 percent when the air mass changed from

2.0 to 1.0.

As a result of this analytical study, two recommendations were made with respect

to the ASTM E424-71. Those two recommendations are discussed in section 2.1.3.

2.2.4 Analytical Modeling

The National Bureau of Standards has conducted studies [8, 5] on the

performance and long-term durability of various solar absorber materials to

aid in the development of consensus evaluation standards. The studies have

shown that, even though the total integrated values of absorptance and emittance of the coating systems may exhibit little change following exposure to

environmental conditions (such as elevated temperature), changes in optical

properties can often be identified by comparing the reflectance spectra for

aged and unaged specimens. In the absence of accurate predictive models,

NBS initiated a study [12] to develop a model for determining the effect of

exposure to elevated temperatures on optical properties of black chrome. First,

an experimental program was conducted to find how the reflectance of the specimens varies with time of exposure. Samples consisting of three specimens

of black chrome on steel substrate were exposed to elevated temperatures of

150°, 200°, and 250°C for increasing time durations. Reflectance was measured

28
i *~t i 1 —i -r -i 1 t

35 Urettiane on steel

.

Porcelain enamel on steel £ 30 - 1 25

D. • u

£ 20

§

15

UJ

’

at

10 . —— — _ __ 5 -

0 1 1—— 1 1- 1——i 1 i i1 L“ 1 L- 1 1 1 I I 400 600 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH |NM]

Figure 2.5 Spectral Reflectance of Nonselective Absorber Materials, Urethane

on Steel and Porcelain Enamel on Steel (from reference [5])

—i 1 i -

__i— i i i i 400 60 0 800 1000 1200 1400 1600 1800 2000 2200

WAVELENGTH (NM]

Figure 2.6 Spectral Reflectance of Selective Absorber Materials, Black

Chrome on Steel and Black Chrome on Copper (from reference [5])

29
Table 2.12 Tabulation of Reflectance Data for Absorber Materials (from reference (5])

Percent Reflectance

Material

Exposure^/

Test; Length

Weighted Ordinate

at Air Mass 2.0

Selected Ord.

at Air Mass 2.0

Selected Ord.

at Air Mass 1.5

Selected Ord.

at Air Mass 1.0

Black chrome on steel Moisture Test^/ A. 09 6.0 6.0 6.0

Black chrome on steel Control 4.9 4.9 6.2 6.0

Black chrome on copper Control 4.8 4.9 6.2 6.0

Urethane paint on steel Control 2.9 2.8 2.8 2.8

Porcelain on steel Control 6.6 6.5 6.6 6.6

Black chrome on steel Moisture Test^/

2 weeks

4.5 4.5 5.5 5.4

Black chrome on steel Mositure Test^^

6 weeks

4.2 4.2 5.3 5.2

Black chrome on steel Moisture Test^/

12 weeks

3.7 3.8 4.8 4.7

Black chrome on steel Heat Age 200°C

12 weeks

3.7 3.7 4.7 4.6

Black chrome on copper Heat Age 200°C

12 weeks

7.0 7.4 8.6 8.2

Urethane on steel Moisture Test^/

2 weeks

3.0 2.8 3.0 2.9

Urethane on steel Moisture Test^/

6 weeks

4.5 4.3 4.2 4.2

Urethane on steel Moisture Test^/

12 weeks

4.8 4.6 4.6 4.6

Porcelain on steel Heat Age 250°C

6 weeks

4.3 4.3 4.2 4.2

! Denotes that the weathering tests were carried out prior to obtaining optical measurements.

2/ Moisture test was conducted at 92°C and 97 percent relative humidity.

30
Table 2.13 Qualitative Results of Parametric Study of Absorber Materials

(from reference [5])

d)>

•H

4-1 O

cn 0) rH 1 1 co cu •H CDc

cu o

•u 53

co £

M

<D rOM <U O > w •H

4-1 < CJ

QJ rH

0)

Crt

Effect on Reflectance

of

Integration Methods Air Mass

No difference*

No difference*

No difference*

No difference*

No difference*

Higher values of

reflectance with

lower air mass*

* Applies to both weathered and unweathered specimens.

31
after 1, 10, and 22 days using an integrating sphere ref lectometer as described

in ASTM E424, Method A [4], before and after the exposure to the various

temperature conditions. Some tests were also conducted on black chrome with

copper substrate. The results of the latter test series were similar to those

for the black chrome on steel specimens.

The test data indicated: 1) that the change in reflectance, at the

above-mentioned temperature levels, increased with time up to 10 days, with a negligible change thereafter; and 2) that the reflectance curve (reflectance vs wavelength) undergoes a permanent horizontal shift with increasing time of

exposure to the elevated temperatures. An analytical model was developed

based on this horizontal shift, and a mathematical expression was derived for

computing the reflectance at a specific wavelength for black chrome on metal

exposed to temperatures below 250°C. Due to the limited application of the

model, additional research is needed to extend the model to other absorber

materials

.

2.2.5 Laboratory Evaluation Devices

Two researchers at the NBS, M.A. Post and D. Ballard [13], studied the

degradation of absorptive coatings by use of scanning electron microscopy (SEM)

and ASTM standard test methods. The objective of the study was two-fold:

1) to assess the applicability of scanning electron microscopy for discriminating the stages of the degradation process; and 2) to obtain test data on the

durability of selected absorptive coatings that could aid the development of

durability-related test methods.

The three coating systems included in the study were: 1) urethane topcoat over epoxy primer on zinc-phosphate-treated steel, 2) polyvinylidene fluoride over epoxy primer on galvanized steel, and 3) epoxy topcoat over phenolic butyrate

primer on anodized aluminum. The first system was exposed to condensing moisture in accordance with ASTM Standard D2247 for 14 days and was also subjected

to adhesion tests before and after exposure. The other two systems were exposed

to high relative humidity O 95 percent) and elevated temperature (92°C) for

21 weeks. Adhesion tests were not performed on coating systems 2 and 3. SEM

photographs were taken of all unexposed and exposed surfaces for comparative

purposes and to assist in the interpretation of microstructure.

Based on the SEM photographs obtained in the study, as compared with the

standard ASTM E424 test results, it was concluded that SEM is a valuable tool

for studying degradation of absorptive coatings. Some specific conclusions

were drawn with respect to the degradation of the three coating systems:

1. Coating system 1 (urethane topcoat over epoxy primer on zinc-phosphatetreated steel):

• Prolonged exposure to moisture contributes to degradation of the coating.

o Loss of adhesion occurs in the epoxy primer with some failure in the

zinc phosphate.

32
• The adhesion test caused only partial removal of zinc phosphate and of

the urethane coating.

• Exposure to moisture and elevated temperature alters the topography of

the urethane topcoat.

2) Coating System 2 (polyvinylidene fluoride over epoxy primer on galvanized

steel)

:

• Moisture degraded the polyvinylidene fluoride coating.

• Failure of the coating may start with the absorption of moisture through

holes in the mound-like areas that formed on the coating (identified by

the SEM)

.

• The moisture penetrated to the galvanized surface, resulting in failure

of the interface between the galvanized surface and the coating and the

galvanized surface and the steel substrate.

3) Coating System 3 (epoxy topcoat over phenolic butyrate primer on anodized

aluminum)

:

• Moisture degraded the epoxy topcoat.

• This degradation caused debonding between the epoxy binder and the

carbon-black spheres which comprised the top coating.

• Moisture penetrated through the cracks that formed in the coating to

the aluminum substrate surface, and the alumimum substrate corroded.

Most of the conclusions presented above were drawn as a result of analyzing the

SEM photographs and correlating them with the physical test results.

2.2.6 Evaluation of Selective Coating Candidates

The following noteworthy studies were conducted under the auspices of the DoE,

but were not included in the DoE Active Heating and Cooling Program. Because

the studies are pertinent to the subject of solar absorber coatings, brief

synopses are presented here.

In 1975 G.E. MacDonald of the NASA-Lewis Research Center reported [14] that

black chrome had been found to have high solar absorptance in the visible

region of the solar spectrum and low emissitivity in the infrared region, thus

making it a promising candidate for solar absorber coating applications.

In 1976 R.B. Pettit and R.R. Sowell of Sandia Laboratories reported [15] on a study in which solar absorptance and total hemispherical emittance properties

were obtained for three coatings: 1) a semiconductor-pigmented paint, 2) electroplated black-nickel, and 3) electrodeposited black-chrome. In addition to

measuring the as-plated optical properties, the investigators obtained results

from temperature aging and ultraviolet (UV) radiation exposure to determine the

33
stability of the coatings after being subjected to aging environments. Of the

three coating candidates, the black-chrome was found to have the best solar

selectivity properties and to be stable in high temperature and UV radiation

environments

.

Later, Pettit and Sowell further investigated the thermal stability of

electrodeposited black-chrome as a function of various plating parameters [16].

The investigators sought to determine the composition of the plating bath so that the effect of the bath constituents on the optical properties and thermal

stability of the coatings could be systematically investigated. In addition,

the effects of such variables as type of substrate, current density and bath

temperature on the thermal stability of the coating were evaluated. Thermal

stability was determined by measuring spectral hemispherical reflectance and

normal total emittance before and after thermal aging tests.

In 1979, C.M. Lampert and J. Washburn of Lawrence Berkeley Laboratory reported

[17] on a study of the microstructure of a proprietary black-chrome coating to

gain a better understanding of the mechanism of wavelength selectivity. The

black-chrome coating was deposited on copper, steel and nickel-plated substrates. The study was conducted in three parts: 1) measuring spectral reflectance of the various coating/substrate combinations, 2) obtaining

surface morphologies by scanning electron microscopy (SEM), and 3) obtaining

surface morphologies by transmission electron microscopy (TEM).

In a study [18] closely related to the aforementioned one, Lampert and Washburn

investigated the optical and microstructural stability of the same black-chrome

coating at different temperatures and atmospheres. One-hour heat treatments

were employed for temperatures ranging from the as-plated temperature to 600°C.

Samples were annealed in dry air at one atmosphere and in a vacuum. To evaluate the effects of heat treatment and the annealing environment, the samples

were analyzed by three procedures: 1) hemispherical spectral reflectance

measurements, 2) surface and cross sectional morphologies as produced by SEM,

and 3) morphologies as produced by TEM.

Although most of the research on selective coatings for flat-plate collectors

has been on the deposition of black oxide coatings on low-emissivity metal substrates, some researchers have investigated alternative coating/substrate

combinations. One such study was reported on in 1976 by D.P. Grimmer, K.C.

Herr and W.J.Mc Creary [19] of the Los Alamos Scientific Laboratory. The

objective of the study was to assess the feasibiity of producing a selective

absorber by the chemical vapor deposition (CVD) of nickel dendrites on an aluminum substrate. The CVD of nickel from nickel carbonyl was thought to

have the possibility of satisfying the optical requirements (i.e., high absorptance and low infrared emittance) while being cost-competitive with oxide

coatings. While the test results did suggest that additional developmental

research is necessary, it was concluded that the nickel dendrite coating

offers a promising, low-cost technique for a selective photothermal absorber.

34
2.3 INSULATION

2.3.1 Introduction

Insulation, as it applies to flat plate solar collectors, serves to minimize

heat loss from the back and sides of the collector enclosure. Therefore, the

effectiveness of the insulation is of great importance in determining the overall

thermal efficiency of the collector. Insulation should have: low thermal con- ductivity, dimensional stability, thermal stability under normal operating temperatures, and chemical compatibility with its environment for the life of the

collector.

The greatest performance problems of collector insulation, obtained from field

surveys, are: 1) moisture, 2) swelling as a result of elevated temperatures,

3) degradation due to ultraviolet radiation, and 4) condensation of volatile

components on the underside of the cover plate thereby reducing transmittance

and collector efficiencies.

2.3.2 Insulation Materials Research

In 1978, the National Bureau of Standards conducted a laboratory-based study

[20] to develop test methods for evaluating solar collector insulation. The

objectives of the study were: 1) to identify performance requirements for

insulations used in solar collectors; 2) to identify or develop test methods

to measure insulation performance according to these requirements; 3) to evaluate and then recommend those test methods which are suitable for the insulation materials that are used in solar collectors; and 4) to draft a set of

proposed test methods for solar collector insulation materials. These methods

were submitted for the consideration of the American Society for Testing and

Materials (ASTM) , and served as a basis for ASTM E861-82, Standard Practice for

Evaluating Thermal Insulation Materials for Use in Solar Collectors.

During the study 21 types of insulation were tested. They are generally

classified under four generic types: (1) mineral fibrous materials such as glass, rock or slag wool; (2) mineral cellular materials such as calcium

silicate, foamed glass, perlite and vermiculite; (3) organic fibrous materials

such as wood, paper or synthetic fibers; (4) organic cellular materials such as urea-formaldehyde, polystyrene or polyurethane foams. Also, insulation is

generally available in four basic shapes or forms: (1) loose-fill insulation

(fibers, granules or nodules) which are usually poured or blown into place;

(2) flexible insulation (blanket, batt and felt) having varying degrees of

compressibility and flexibility; (3) rigid insulation (preformed block, board

or sheet) manufactured to standard lengths, widths and thicknesses; and

(4) formed-in-place insulation (liquid components) which may be poured, frothed

or sprayed in place to form rigid or semirigid foam insulation.

The following performance conditions were considered during the study:

1) sustained high or low temperatures (as high as 260°C and as low as -40°C);

2) diurnal temperature-humidity cycling; 3) condensation or accumulation of

water within the collector; 4) continuous contact between the insulation and

the components of the collector system; 5) mold; and 6) vibration.

35
The key physical properties were identified to be: mass, size, density,

water absorption, moisture adsorption, thermal conductivity, and friability.

ASTM standard test methods were used in the investigation where applicable.

These ASTM standards included:

C 167 Test for Thickness and Density of Blanket- or Batt-Type Thermal Insulating Materials.

C 209 Testing Insulation Board (Cellulosic Fiber),

Structural and Decorative [Water Absorption].

C 553 Specification for Mineral Fiber Blanket and Felt

Insulation (Industrial Type) [Moisture Adsorption]

.

C 518 Test for Steady-State Thermal Transmission Properties

by Means of Heat Flow Meter.

C 411 Test for Hot-Surface Performance of High-Temperature

Thermal Insulation.

C 356 Test for Linear Shrinkage of Preformed

High-Temperature Thermal Insulation Subjected to

Soaking Heat.

D 3273 Test for Resistance to Growth of Mold on the Surface

of Interior Coatings in an Environmental Chamber.

Because of the absence of suitable standard test procedures, tests were

developed: 1) to observe the effects of thermal cycling and outgassing;

2) to measure friability of the insulation; and 3) to observe the extent of

corrosion sustained by metal parts typically in contact with insulation.

The test results, which were reported in tabular form, showed the effect of

operating conditions on the insulation and on the collector component (e.g. cover plate). Tables 2.14 and 2.15 are typical of the tabulated test results.

Based on the laboratory test results a methodology was derived for screening

solar collector insulation materials with respect to performance and stability

under service conditions. The proposed procedures are either identical with

existing ASTM tests or were developed during the study, as indicated:

Water Absorption - ASTM C 209

Thermal Conductivity - ASTM C 518

Linear Shrinkage - ASTM C 356

Corrosion - Newly developed

Hot Surface Performance - ASTM C 411

Outgassing - Newly developed

Thermal Cycling - Newly developed

36
Table 2.14 Effect of Hot Surface Contact on the Appearance and

Linear Shrinkage of Insulations (from reference 120])

Generic Sample Surface Temperature at Which Linear Shrinkage 0

Type Numbers First Observable Change at 150 and/or

Takes Place3 260°C

Mineral 1-FG, 2-FG 209°C - Discolors and chars None

Fibrous 3-FG, 5-FG

8-SW, 10-RW 209°C - Discolors None

4-FG 150°C - Discolors and chars None

6-FG ,7-FG ,9-SW None

Mineral 12-GF None None

Cellular

11-CS 209°C - Evolves water None

Organic

Fibrous

13-CL 209°C - Evolves water c

14-CL 94°C - Evolves water c

15-WF 94°C - Discolors 3% at 150°C

>95% at 260°C

Organic

Cellular

20-PU 150°C - Melts 4* and chars 18% at 260°C

17-PS ,18-PS

19-PS

150°C - Melts e >95% at 150°C

21-RB 94°C - Discolors 18% at 150°C

16-UF 94°C - Bows 14% at 150°C

>95% at 260°C

a All observations made after 96 hours of exposure to the hot surface; samples

were exposed successively to 94, 150, 209, 260°C to determine the lowest

temperature required to induce an observable change.

b Linear shrinkage calculated as the percent reduction in the original length

of the sample.

c Shrinkage not applicable to cellulosic loose-fill insulations.

^ Sample melts on side exposed to hot surface.

e Sample melts completely.

Key

FG - Fiber Glass

SW - Slag Wool

RW - Rock Wool

CS - Calcium Silicate

GF - Foam Glass

CL - Cellulose

WF - Wood Fiber

UF - Urea Formaldehyde

PS - Polystyrene

PU - Polyurethane

RB - Foamed Rubber

37
Table 2.15 Corrosion of Metals by Insulation Materials (from reference [20])

Degree of Pitting3

Generic

Type

Sample

No.

Stainless

Steel

Brass Copper Aluminum

Mineral 7-FG Severe Severe Severe V. Mild

Fibrous

1-FG ,4-FG ,9-SW — Mild Mild —

3-FG Mild — — —

2-FG,6-FG,7-FG

8-FG , 10-RW — — — —

Mineral 11-CS — — Mild —

Cellular

12-GF — — — —

Organic 13-CL Severe — Mild Mild

Fibrous

14-CL Severe Mild Mild Mild

15-WF — — Mild —

Organic 20-PU Mild Mild Mild —

Cellular

16-UF — Mild Mild Mild

17-PS , 18-PS

,

19-PS ,21-RB — Mild Mild —

a Results reported as severe or mild pitting of the metal surface after 30-day

contact with insulation; where no pitting occurred, no result is given.

38
The proposed new procedures are applicable to flexible, semi-rigid, rigid,

loose-fill, and poured-in-place insulation materials. Also, the procedures are suitable for testing all types of insulation: mineral (fibrous and cellular),

and organic (fibrous and cellular). The proposed procedures were submitted to

ASTM Subcommittee E 44.04 on Materials Performance for its consideration, and,

as mentioned above, served as a basis for ASTM E861-82.

2.4 SEALANTS AND GASKETS

2.4.1 Introduction

Various types and forms of rubber seals are used in solar energy systems to

seal or weatherproof joints between adjoining materials or to separate

dissimilar metallic materials. Although many standard test methods have been

developed for rubber seals used in other building construction applications,

none of these are entirely adequate for the high temperatures normally

encountered in solar energy systems and subsystems. For example, the maximum

service temperature of interior coverplates used in flat plate solar collectors

where rubber seals are used, is estimated to be 150°C (302°F), whereas existing

standards seldom consider temperatures exceeding 70°C (158°F). A need was identified to develop standards for rubber seals, for solar energy systems,

which would perform at high temperatures for extended periods of time or withstand the degradation resulting from being in contact with liquids.

2.4.2 Sealant Materials Research

Research conducted at the NBS resulted in a report [21] on the topic of rubber

seal performance and standards for their evaluation.

The objectives of this study included: (1) identification of performance

requirements for rubber seals used in solar energy systems; (2) identification

and assessment of existing test methods for rubber seals and modification of

the existing methods or development of new methods as needed; (3) evaluation

of commercially available rubber materials in the laboratory and acquisition

of data heeded to recommend specific test methods; and (4) preparation of

draft standards for rubber seals for consideration as a consensus standard by

the American Society for Testing and Materials (ASTM).

The key properties of rubber seals which were measured in this study included:

1) ultimate elongation; 2) compression set; 3) hardness; 4) tensile strength;

5) low temperature flexibility; 6) volatile content; 7) condensible volatiles;

and 8) adhesion (for liquid seals). The key degradation factors considered

were: 1) elevated temperatures; 2) low temperatures, 3) contact with liquids;

4) ozone; 5) ultraviolet radiation; and 6) stresses resulting from cyclic

joint movement.

The types of rubber seal materials investigated (and their ASTM D1418*

designations) included: 1) acrylate copolymer (ACM), 2) ethylene-vinyl acetate

* ASTM D1418-79a, Recommended Practice for Rubber and Rubber Latices

Nomenclature.

39
copolymer (EAM) , 3) chloro-sulfony 1-polyethylene (CSM), 4) fluoro rubber (FKM),

5) terpolymer of ethylene, propylene and a diene (EPDM), 6) copolymer of

ethylene oxide and chloromethyl oxirane (epichlorohydrin) (ECO), 7) polychloromethyl oxirane (CO), 8) silicone copolymer containing methyl and vinyl groups

(VMQ), 9) polysulfide (EOT), and 10) polyurethane (EU). Thirteen of the 31

samples were supplied and tested as vulcanized sheets. Two samples which

were supplied as liquid-applied seals were prepared as sheets before testing.

The following tests for rubber were conducted on the vulcanized sheets:

ASTM D395

ASTM D412

ASTM D471

ASTM D865

ASTM D1149

ASTM D1229

ASTM D1415

ASTM D2137

Test for Rubber Property - Compression Set

Test for Rubber Properties in Tension

Test for Rubber Property - Effect of Liquids

Test for Rubber Deterioration by Heating in air (Test Tube

Enclosure)

Test for Rubber Deterioration - Surface Ozone Cracking in

a Chamber (Flat Specimen)

Test for Rubber Property - Compression Set at Low

Temperatures

Test for Rubber Property - International Hardness

Test for Rubber Property - Brittleness Point of Flexible

Polymers and Coated Fabrics

In addition, the total volatiles at 150°C and volatiles condensible at 23°C were determined by a modification of ASTM D865. Also, liquid immersion tests

were performed for a period of seven days, using a modification of ASTM D471.

Sheet samples were immersed in the following liquids: 1) ethylene glycol/

water (1:1); 2) propylene glycol/water (1:1); 3) polyalkylene glycol ether;

4) modified ester; 5) polyaromatic; 6) modified terphenyl; 7) silicone A; 8)

silicone B; 9) silicone C; and 10) alkyated aromatic.

The test methods used for evaluating the remaining 16 liquid applied sealants

included:

ASTM C792

ASTM C661

ASTM C719

ASTM C793

Test for Effects of Heat Aging on Weight Loss, Cracking,

and Chalking of Elastomeric Sealants

Test for Indentation Hardness of Elastomeric - Type

Sealants by Means of Durometer

Test for Adhesion and Cohesion of Elastomeric Joint

Sealants Under Cyclic Movement

Test for Effects of Accelerated Weathering on Elastomeric

Joint Sealants

Test results were presented in tabular form, showing the effect of specific

test conditions on the samples. Table 2.16 shows a typical set of results for

nine of the 11 sealant materials mentioned above. Based upon the results of

the laboratory test program, five proposed standards on rubber seals in solar

systems were prepared for consideration by ASTM Committee E44. The proposed

standards were entitled:

40
Table

2.16

Results

of

Tests

of

Samples 1

through

15i/

(from

reference

[21])

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cm inx

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ui pH pH pH o o o © 5a 1 XXX O' X y in in O x o O X X *y © *p- £ r- • • • — H o *y cm cn ro o © m o H G CO CD X H • • i < o o o o \ w

4)a § p* cn §H u t- o a « H t-i • w Z L> « a 0 fl a. u &

faa 1i ua 0 fa i ac H fa B

u4

aH

4-* s z £ H

u

Q s Cu

x « 1 o k • pH c • fa 1 UJ PO C *p- 5 M s H u N w •* c X •pH o U H u < U H u ac u u fad u • X O • b o • O u • UJ • u ui u • H u • QC 35 i•> fl- 4 4; 3 • o X • o z • o CO O 0 CL • o < • o P fl< • a H O ** O *n u © 3 o in ^ o o in o in H © 4

CL kT\ pH fa in pH in *-H z 1 kOl | m pH UI in pH < to fl- fl X i «-h ph H H hJ fH ^H o pM .J pH O pfa 4 p 4 <x* CO 9 mj 3

<1/

— </) «/ 3 <3

V U] • to

U

to to to

C

C <SL UI c O' CO

u

M Qb fad < to •pH X UI •H X H X CO X , 1 X CO X s 2 c* .c 4

UJ 00

X -J oc

X < Ofi x: bJ

X r—i X z X 8 z — X fa — X £ •H X ac X t- X fad X H -H UJ

X c

c. fc*

OX CO

u

O

X

u o X c~

X o < O

X 9 o X 4> ;z. o z : 5 o ^ ^4 z

UI c ^ —

h s r

h

n c zc

r>. pH

X c —

CM

X t- Ur 5 > u • _} <3 —

41
1. Standard Specification for Rubber Seals Used in Flat-Solar Collectors.

2. Standard Specification for Rubber Seals Contacting Liquids in Solar

Energy Systems.

3. Standard Specification for Rubber Seals Used in Solar Collectors

(other than Flat-Plate Collectors)

4. Standard Test Method for Determining the Effect of Outgassing on the Transmittance of Solar Collector Covers.

5. Recommendations for Sealing Joints between Solar Collectors and

Building Components with Rubber Seals.

The first three proposed standards have been adopted by ASTM as D3667-78,

D3832-79 and D3771-79, respectively.

42
3. TRANSPORT AND STORAGE SYSTEM COMPONENTS

3.1 CONTAINMENT MATERIALS AND HEAT TRANSFER FLUIDS

3.1.1 Introduction

Among the high priority standard test method needs identified in reference [1] was a method for evaluating the durability and reliability of containment

materials and heat transfer fluids in operational solar energy systems. Interaction between the heat transfer fluid and the surrounding materials may alter

the properties of the fluid and have a deleterious effect on the physical

properties of the containment materials. Thus, a reliable laboratory procedure

for the screening of metallic and nonmetallic containment materials for possible

use with heat transfer fluids was also identified as a standards need. Durability factors to be evaluated by the screening procedure included material

compatibility and chemical and thermal stability of the fluids.

In addition to the need for a screening procedure, a need for comparing the

corrosion resistance of various metals in operational solar heating and cooling

systems during both flow and stagnation (nonflow) conditions was identified.

The report noted that these simulated service tests should include such variables as flow rate, heat transfer rate, alternating wetting and drying, the

degree of aeration of the transfer fluid, and the presence of crevices.

Since 1977, the objectives of several DoE-sponsored research programs have

included the development or assessment of draft standards to be used in evaluating containment material/heat transfer liquid combinations for their durability and reliability when incorporated in solar energy systems. Summaries

of these research programs are presented in the next paragraph. They describe

the primary objectives, scope, key results, and principal conclusions obtained

from the referenced reports.

3.1.2 Screening Tests for Metal/Heat Transfer Fluid Combinations

During 1978 and 1979, several national laboratories, including NBS, collaborated

in a round-robin test program to assess the applicability of a proposed ASTM

Standard Guide for screening metallic containment materials using metal/fluid

combinations likely to be used in solar energy heating and cooling systems.

The proposed standard was duly considered by ASTM Subcommittee E44.04 on Performance of Materials and served as a basis for ASTM E7 12-80.

In the part of the test program that was conducted by NBS [22], alloys of

copper, aluminum, stainless steel, and carbon steel, in combination with ASTM

D1384 tap water were subjected to five of the six tests described in the Standard Guide. The five tests which were all conducted at atmospheric pressure,

were as follows: 1) basic immersion test; 2) heat rejecting surface test;

3) repeated drip dry test; 4) crevice test; and 5) tube loop test. It was felt

that these five tests were adequate for simulating the effects of the following

four principal system-operating conditions: 1) flowing liquid in a full system;

2) stagnant (no flow), full system; 3) stagnant, partially full system; and

4) stagnant, empty system. The metallic specimens were exposed to the test

43
conditions for thirty days. At the conclusion of each test, the specimens were removed, cleaned according to ASTM Gl-72 Standard procedure, and weighed

to determine the loss of material due to corrosion.

Upon completing the test series, it was concluded that the basic immersion test

(Test // 1) was effective in separating the corrosion rates of the aluminum and

copper alloys. It was also determined that the corrosion rates of stainless

steel alloy (AISI 409, 439 and 444) were negligible, and the variability in

corrosion rates for the SAE 1014 mild steel was low. It was further concluded

that Test //I was adequate for prescreening. The results of Tests 2, 3, and 4

indicated that the variation in corrosion rate as a function of alloy composition may be determined through the use of these tests. However, Test 4 resulted

in an undesirable variability in the pitting of the aluminum specimens. The

test procedure was modified to reduce the variability. Test 5, while qualitative in nature, resulted in light pitting in the sample of 3003 aluminum tubing.

3.1.3 Screening Tests for Plastic Materials/Heat Transfer Fluid Combinations

Plastics can be used in place of metals as containment materials in active

solar heating and cooling systems. The fact that the mechanical properties of

most plastic materials are time- and temperature-dependent suggests that

durability/reliability test methods are needed to assist in evaluating the

long-term performance of such materials. Under operating conditions, the con- tainment materials will be in contact with transfer fluids of varying temperatures and pressures. During stagnation conditions, components such as absorbers, transport piping and storage tanks may be subjected to elevated

temperatures for sustained periods. Thus, it was necessary to develop standard

test methods for screening plastic materials for their compatibility with

transfer fluids such as water and glycol solutions at high temperatures.

As a consequence, the NBS engaged in a laboratory-based program [23] to obtain

test data which could provide the technical basis for a screening methodology

for plastic containment materials. The performance requirements and degradation factors that may affect plastic containment materials were first identified. The three general containment applications identified for plastics were:

absorbers, transport piping, and storage facilities. These components should

resist deterioration resulting from elevated temperatures, thermal cycling,

moisture, air pollutants, and heat transfer fluids. Also, absorbers unprotected by insulation must be UV stable when subjected to direct solar radiation.

Furthermore, the plastic materials must be compatible with the heat transfer

fluids to the extent that they do not degrade the fluids. In the NBS study,

tests were performed on 11 plastic containment materials in order to assess:

1) the effects of heat aging, and 2) their chemical compatibility with heat

transfer fluids.

The containment materials included in the laboratory study are listed in

table 3.1.

In the heat stability aging tests, small coupon test specimens were exposed to

temperatures of 100°C (212°F) and 125°C (232°F) in a mechanical convection

oven. Separate sets of specimens were used for each temperature. The materials

44
Table 3.1 Plastic Containment Materials Included in the Laboratory Study

(from reference [23])

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 mm wall

Polypropylene copolymer extrusion, with 0.64 mm wall

Piping

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) 29 mm OD, 2.2 mm wall

Storage

Poly(vinyl chloride) liner 0.8 mm sheet

Chlorosulfonated polyethylene liner - woven fabric

0.8 mm sheet

Fiber reinforced plastic tank 1.3 mm sheet

were evaluated for linear dimensions, hardness, weight change, appearance, and

reflectance (absorbers only) by ASTM standard procedures, after 100, 250, 500,

and 1,000 hours exposure. In the chemical compatibility tests, the plastic

coupons were evaluated using the procedure described in ASTM D543, Test for

Resistance of Plastics to Chemical Reagents. The specimens were immersed in

the heat transfer fluid which was maintained at either room temperature or 70°C (158°F). The heat transfer fluids were: 1) water, 2) 100 percent ethylene

glycol, 3) 50 percent ethylene glycol in water, 4) 100 percent propylene glycol,

5) 50 percent propylene glycol in water, and 6) 100 percent silicone oil.

After one week of immersion the specimens were evaluated for linear dimensions,

hardness, weight change, appearance, and reflectance.

45
As a result of this test program, it was concluded that accelerated laboratory

screening tests (i.e., elevated temperature and contact with heat transfer

fluids) can be effectively used to determine whether plastic containment candidates are likely to be degraded in the range of in-service temperatures of solar

systems. Exposure to the six heat transfer fluids demonstrated that there are interactions between the plastic and fluids in the test temperature range.

Silicone oil generally caused the plastics to swell, soften, and gain weight.

The effects of the other fluids ranged from essentially no change to an increase

in hardness or softness, weight gains or dimensional changes.

Based upon the NBS laboratory study, a draft standard was prepared and submitted

to ASTM Committee E44 on Solar Energy Conversion. Subsequently, ASTM adopted

a new national standard, E 862-82 [24] which was based on the draft standard

resulting from the above mentioned study.

3.1.4 Simulated Service Testing for Corrosion

Because the interaction between the heat transfer fluid and its containment

system can result in deleterious changes in the properties of the fluid and/or

corrosion of metallic components of the containment system, it is often advissable to evaluate the responses of containment systems under actual or simulated operating conditions. The NBS participated in a study [25] to assess

the validity of a proposed ASTM test method in which accelerated metallic

corrosion is induced during simulated flow and stagnation conditions in a solar collector system. The corrosion responses of aluminum, copper and stainless steel tubing, and the chemical stabilities of ethylene and propylene

glycol solutions were evaluated during the study.

In the heat transfer liquid stability test series, coupons of 1100 aluminum and

110 copper alloys were immersed in solutions of ethylene glycol and propylene

glycol at 100°C (212°F) for 16 weeks. The solutions were either aerated with

compressed air or deaerated with high purity nitrogen. Based on the assumption

that the degradation of ethylene and propylene glycol results in the generation

of an acid solution, pH was measured against time to determine the degree of

degradation. The results of this test series were presented as graphic plots

of pH vs time for three sets of conditions: 1) aeration with no metal present,

2) deaeration in the presence of aluminum and copper coupons at 100°C, and

3) aeration in the presence of aluminum and copper coupons. It was concluded

that the presence of copper metal and the availability of oxygen accelerated

the degradation of ethylene and propylene glycol, while the presence of aluminum

had a minimal effect. It was also observed that while ethylene glycol slightly

more thermally stable than propylene glycol, it was significantly less oxidatively stable. Because of the latter observation, ethylene glycol was generally

used in the series of seven simulated service tests. Distilled water only,

containing 1,000 ppm of NaCl was used in one test.

Four of the simulated service tests were conducted using aluminum tubing, two

tests used copper tubing and one test involved stainless steel tubing. The

systems were operated under conditions of either continuous flow at 90°C, or cycles of flow followed by stagnation at 90°, 120°, and 135°C. A flow rate of

0.64 m/s was used in all systems. Test durations were 16 weeks and 24 weeks.

46
In the continuous circulation tests, the transfer liquid was pumped

from a heated reservoir through a system of tube loops simulating a typical

collector. A schematic of the various components of the loop system is shown in

figure 3.1. In the stagnation phase, the liquid was allowed to stand in one

of the vertical legs for 4 hours, while the other vertical leg remained empty.

The corrosion rate of the metallic tube specimen was determined by measuring

mass loss for each of the 7 tests. The test conditions and test results are summarized in tables 3.2 and 3.3.

It was observed that the corrosion rates in both copper systems were low,

although the corrosion rate on the stagnant empty side of Copper 2 was higher

than on the stagnant full side. The stainless steel test was inconclusive

although some mass loss and pitting were observed. Because the four aluminum

systems contained an uninhibited, unbuffered glycol solution, the corrosion

rates were higher. The corrosion rates in the stagnant empty legs for the

three aluminum systems increased with increasing stagnation temperature. Since

the methodology used in this study was found to be adequate for the metals and

heat transfer fluids commonly used in collector systems, it was submitted for

consideration by ASTM Committee E 44 (on Solar Energy Conversion), and served

as the basis for ASTM E 745-80 [26].

In another laboratory-based study [27], Argonne National Laboratory evaluated

the corrosion behavior of representative metals (6061 aluminum, CA 122 copper,

black iron, 1008 low-carbon steel, galvanized black iron and galvanized 1008

steel) in ethylene glycol-water solutions under temperature and flow conditions

common to solar collector systems. The corrosion experiments were conducted

under the heat-flux conditions encountered in normal operations. Several

experiments were performed in a static autoclave at elevated temperatures,

which simulated stagnation conditions in the collector. The thermal stability

and corrosivity of the fluids were evaluated. The feasibility of using an electrochemical sensor to instantaneously measure corrosion rates instead of

using metal weight loss measurements was also investigated.

The corrosion behavior of the metal specimens was determined by using a heat-flux loop facility (HFLF) which consisted of a heat-rejection loop and a heat-absorption loop. During operation, the two loops were maintained at

different temperatures while a mixing flow was maintained between the loops to

simulate thermal cycling of the fluid in a solar collector system. The corro- sion behavior of a metal at a low temperature was thus evaluated while in

contact with fluids subjected to high temperatures in the same flow system.

The heat-transfer fluids used in the corrosion study were aqueous solutions of

ethylene glycol. Most of the experiments involved an inhibited ethylene glycol

(IEG) solution, manufactured for use as an automotive coolant. The coolant

was diluted with corrosive water (CW) prepared according to ASTM D2570. For

comparative purposes, tests were also conducted with uninhibited (i.e., antifreeze-grade) ethylene glycol (UEG) , diluted with the same corrosive water

solution. The metal specimens were 25.4-mm lengths of ring sections of 3/8 in

Schedule 40 pipe. Several lengths formed the upper half of each specimen

which served as a heat flux ring, while other lengths comprised the lower half

serving as an isothermal ring. In the heat-rejection loop, the heat-flux

ring was electrically heated, while in the heat-absorption loop the ring was

47
Figure 3.1 Schematic Showing the Various Components of the Loop Systems.

Systems are initially charged with 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 solenoid 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 the 3-way solenoid valve [6]. (From reference [25].)

48
Table

3.2

Summary

of

Test

Conditions

for

Tubing

Specimens

(from

reference

[25]) 3 (0 CO 3 o M -X

4-4 •H cu CU 3

CO 44 CU : : <U s 3 3 CO Ds 5 H >43 v£> n£> Q r-H CM r-H o CJ o

CU cu CU 3

44 CD CO 3

CO -•^ **s*. M 0 _ 0 0

-<r 'd- mT

O nO vO

r-l • • . pH o o o 0o

•H 3 44 CJ cj CJ CJ CJ

>4 CO I 0 0 0 1 o 0 0 0 1 o o UO 1 O o 4-1 bO CT\ CM CO CM CM

CO CO rH iH rH i-H U 44

<U CO a0

<U H D* uou CJ o CJ CJ o o o o 0 0 0 0

rH o o o o o O

O

pH ON ON ON on ON ON ON

cu Pi rH a 3 CJ

<44 to * oo 44 3 O CU CU EH CU CO 3 2 3 S'? 3 &-? N r-H 5

CO CO cu o <r CU UO * CU

a 0 s 0 rH m oo rH CM 2 CU X) a o O to co iO 1 Pi 2 3 Cu

•rl •rl 43 * r4 -3 #4 00 <44 Eh rH

44 44 44 rH

Q 44 pH CO •H CO rH

O

•H 0 cu o M (U

O o 44 <3 Pi •rl o

CO rH o 2 cu

O v© 3 3 44 o o O toEn 44 S'? 3 S'? 44 3 * a co O rH CO 3 140 pH 2 o 3 •H r-H 0 m bo<j CM bO O <J UO S* (=> + oo * 44

CO CO rH cu 0 1 co O 0

•H •rl •rH 140 >4 4= 44 r CO w% 3 o 0 r 44 m to

—

•U cO rH

X CU co ON Pi

X 3

CO

w

O

O 3 <U o 3 3 2 CO acivo rH co XJ CU 3 3 o CU rH 1-4 CU 3 i—1 3 rH 3

(4 44 Pi >1 oo CU rH

O to rH 3 CU r 4C 44 Cu co * 4C bO

J3 X) MH 0 44 - CD 3 44

4J cu CO

O <U co CU 43

O 3 X o CO 0 rH >-> CO CO > CO rH to 3 O co 144 rH CO CO o \ <4H rH Pi rH a >4 CU O rH <U 43 3 3 O to

i—1 X

H EH CU bO rH < co EH CU cu

H

w 0 H

N CO r-H CM rH

<U =tfc =Sfc =4te =tj= =Jt= 44

CO co tO •—H r-H r-H r-H 3 3 '*

CO < < < <ti CJ CJ CO

CO 3

bO <3 CJ CO 3 to

•H o co

rO rH o CM n£> 0 rH o CM *-H H < CO r-H CO •o

j-i 3*

W)s

•H0a§

a.

CO3>

t—I 3>

UHo

CO 44c

a)3o

i*oo

CO

cOuo

COPio

4Jo

<u30ooMo

l»H X)3

co 2* I 49
Table 3.3 Summary of Test Results (from reference [25])

Tubing

System

T,

°C

Flow/ Stagnation

pH

Initial Final

Corrosion Rate,

mpy*

Empty Full

Side Side

Deepest

Empty

Side

Pit

Full

Side

ai in 90 — 8.35 5.56 0.49 1.06 mm

ai in 90 90 — 5.08 0.38 0.40 negl. negl.

A1 #3 90 120 — 5.33 0.54 0.28 0.85 0.55

AI it 4 90 135 — 5.64 1.34 0.67 0.12 negl.

Reserve Aik. cu in 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

*microns per year

(Reserve alkalinity refers to the amount of buffering remaining.)
air cooled. Only the outside surface of the specimens was in contact with the

flowing ethylene glycol solutions.

In the first study conducted with the HFLF, the ring specimens were exposed to a 50 percent solution of UEG in corrosive water to establish baseline data for

comparison with results from specimens exposed to the IEG solutions. The specimens were exposed to a flowing UEG solution for about 48 hours at temperatures

of 140 and 105°C (413 and 378°K) in the heat-rejection and heat-absorption

loops, respectively. The heat fluxes in the two loops were ~1.0 and 0.4 kW/m^

respectively. Material weight-loss measurement was made and corrosion rates

were determined. Then, two 1000-hour tests were performed in the HFLF with a 50 percent solution of IEG and corrosive water. The temperature of the fluid

in the heat rejection loop was 120°C (393°K) in the first 1000-h test and

140°C in the second run; the temperature in the heat absorption loop was

105°C in both runs. The specimens were weighed to determine metal weight-loss

after about 500 hours of exposure and at the end of the 1000-hour period. The

specimens were observed for visible corrosion and were also photographed. The

measured metal weight loss values and the corresponding corrosion rates were

presented in tabular form for the three test runs. Representative results are presented in table 3.4.

In addition to the weight-loss measurement method of determining corrosion

rates, two other trial methods of making direct corrosion rate measurements were

employed: 1) a commercial electrochemical corrosivity sensor, and 2) potent iodynamic corrosion measurements. A commercial corrosivity sensor developed for

use in automotive coolant systems was redesigned for application in solar energy

systems. The sensor continuously measured the electrical potential of the

solution. As the ethylene glycol degraded, its pH changed due to the formation

of corrosive acids and there was a corresponding change in the potential of

the sensor's probe in the solution. As the solution became more acidic, its

corrosive ability increased. Thus, it was possible to correlate the sensor

output (i.e., voltage) with the corrosive behavior of the metal specimens.

Although the sensor could readily distinguish changes in pH, it was concluded

that additional data are required before a final assessment of the sensor

energy system can be made.

With regard to the potentiodynamic polarization measurements mentioned above,

data were obtained on the corrosion potential, corrosion current and the

instantaneous corrosion rate for iron, copper, and aluminum in ethylene

glycol-corrosive water in the as-formulated condition and after exposure to a temperature of ~125°C (400°K) for 1000-hours in the HFLF.

Among the conclusions resulting from the three tests in the heat-flux loop

facility were the following:

1. A heat flux of 0.4 to 1.0 kW/m^ did not have a significant effect on the corrosion behavior of iron, copper, and aluminum in a 50

volume percent inhibited ethylene glycol-corrosive water solution

at temperatures between 105 and 140°C (378 and 413°K). The corrosion

rate of copper was considerably higher than that of iron or aluminum

in the 1000-h tests at low flow velocities.

51
Table 3.4 Corrosion Results for Aluminum, Copper, and Iron After Exposure to 50 Volume Percent IEG*

(GM-6038M)-CW* Solution for 1000-h in the HFLF* (Run 3) (from reference (27))

Specimen Location

Heat Rejection Loop (140° C) Heat Absorption (105° C)

Isothermal Heat Flux (1 .0 kW/m2

) Isothermal Heat Flux (0.4 kW/m2

)

Def ilmed Corrosion Def ilmed Corrosion Def ilmed Corrosion Def ilmed Corrosion

Time

,

Metal Rate, 3 Metal Rate, 3 Metal Rate, 3 Metal Rate, 3 Material h Loss, mg mm/y Loss, mg mm/y Loss, mg mm/y Loss , mg mm/y

Aluminum 500 b 2 0.010 2 0.010 c c c c (6061) 1000 3 0.007 3 0.007 c c c c

Copper 500b 184 0.26 194 0.28 15 0.02 11 0.02

(CA 122) 500d 63 0.09 - - 15 0.02 - - 1000 292 0.21 257 0.18 28 0.02 25 0.02

Black Iron 500b 11 - 17 - 12 - 21 - (Asreceived)

1000 24 22 29 40

(Polished) 500b 6 0.010 5 0.008 7 0.011 12 0.020

1000 6 0.005 8 0.007 11 0.009 14 0.011

a Surface area of the 3/8-in Schedule 40 pipe specimens was 13.8 cm2

. Value based upon linear rate law.

15 Time corresponds to the first 500-h of the 1000-h experiment.

c Specimens exhibited a slight net weight gain after defilming and correcting for metal loss due to the

chemical cleaning operation.

d Time corresponds to the last 500-h of the 1000-h experiment.

* IEG - Inhibited ethylene glycol solution

CW - ASTM D2570 corrosive water

HFLF - Heat-flux loop facility

52
2. In a short-term (48 hours) corrosion test at 140°C with a 50 volume

percent solution of uninhibited ethylene glycol in corrosive water,

aluminum exhibited localized pitting, and 70 percent of the zinc

coating on galvanized iron was removed. The corrosion rate of iron

was significant, whereas copper experienced minimal attack in the

fluid at a low flow rate.

3.2 HOSES AND FLEXIBLE COUPLINGS

3.2.1 Introduction

Rubber hose is used extensively in solar energy systems to connect solar

collectors to manifolds and to connect other components in these systems. Hose

has several advantages for making these connections; for example, rubber hose

does not place high stresses on the components to be connected. On the negative side, the disadvantages of rubber hose include the frequent occurrence of

leakage at the connections and uncertain life expectancy when used in the severe temperature conditions common to solar energy systems. The need for developing

standards that account for the advantages and disadvantages of using rubber

hose was the basis for two studies [28,30] conducted by the NBS during 1979

and 1980. Although some standards exist for rubber hose used in specific

industries (e.g., automotive), they are generally not suitable for hose used

in solar energy systems. This inapplicability is due primarily to durability

considerations under the severe environmental conditions encountered in solar

systems.

3.2.2 Research on Rubber Hoses for Solar Energy Systems

The objectives of the first study [28] were: 1) to identify the performance

requirements for rubber hose used in solar systems; 2) to assess existing standard test methods for rubber hose and develop new ones where needed; 3) to

evaluate commercially available hose as a basis for making recommendations on standards requirements; and 4) to prepare draft standards for rubber hose for

consideration by ASTM as consensus standards.

Eleven commercial rubber hoses and one polytetrafluorethylene (PTFE) hose with

stainless steel fibrous braid were included in the study. These hoses were not

designed for use in solar energy systems. There were three categories of hoses

according to rubber type: 1) chloropene rubber (CR) (neoprene); 2) terpoly- mer of ethylene, propylene, and a diene (EPDM); and 3) silicone rubber having

both methyl and vinyl groups on the polymer chain (VMQ).

A total of five tests were performed, the first one involving the continuous

circulation of ethylene-glycol/water mixture through the twelve hoses for a period of more than seven months. The circulation tests were conducted under a predetermined temperature cycle as follows: 1) at 100°C for 82 percent of the

time; 2) between -25°C and -40°C for 2 percent of the time; and 3) between 10°C

and 25°C for 16 percent of the time. The remaining four tests covered:

1) ozone resistance in a chamber at 40°C; 2) compression set resulting from the

pressure exerted by a prescribed compression clamp; 3) water vapor transmission

loss through the hose wall; and 4) bursting pressure using a hand-operated

53
hydraulic pump. For determining the ozone resistance of the hoses, the procedures in ASTM D380 and D1149 were used. ASTM D395 was modified for use in mea- suring compression set. A new test method was devised for measuring water

vapor transmission through hoses. The procedure in ASTM D380 was followed in

measuring the bursting pressure, except for a minor modification.

According to the investigators, the study achieved the objective of determining

the important requirements of hoses used in solar energy systems. Based on the

results of the study, a proposed standard specification for rubber hose was prepared. The standard has requirements for evaluating the following characteristics: 1) physical properties of inner tube and outer cover before and after

accelerated aging; 2) resistance to ozone of the outer rubber cover; 3) compatibility of the inner tube with the heat transfer fluid; 4) flexibility at low

temperature; 5) compression set of the composite hose; 6) bursting pressure of

the composite hose; 7) vapor transmission through the hose walls; 8) effect of

the hose on heat transfer fluid and metal; and 9) durability of reinforcement

material. Laboratory-based tests are included for assessing all of these

characteristics, except for the deterioration of hose reinforcement material.

A short-term laboratory test is not adequate to assess this characteristic.

However, the behavior of fibers at various temperatures in many environments

is known or could be established.

A proposed standard specification was submitted for consideration by ASTM

Subcommittee Dll. 3 on Rubber Hose. The draft standard, with minor

modifications, was adopted by ASTM as D3952-80 [29].

Because ASTM D3952-80 [29] did not cover the testing of rubber hoses used in

liquids above their boiling point, a need for specifying the performance of

hoses used with aqueous liquids at temperatures above the boiling point of

water was identified. Thus, a second study [30] was initiated in 1980 to

determine what revisions of the standard, if any, were necessary. The study

consisted of conducting interlaboratory tests on commercial hose linings.

Specimens of six hose lining vulcanizates , in 2-mm-thick sheet forms were

immersed in an aqueous glycol solution at both 100 and 150°C for periods of

166 h and 332 h, respectively. Since some of the tests were conducted at

temperatures above the fluid’s boiling point, a pressure vessel was used. The

pressure vessel consisted of commercial pipe welded shut at one end and equipped

with a large flange at the other to accommodate a pressure relief valve opening

at 1000 kPa. In addition to the immersed specimens, there was a set of

untreated control specimens whose properties served as a basis of comparison.

Mass measurements were made before and after immersion to calculate the percent

change. Also, the tensile strength and ultimate elongation of the immersed

and the control specimens were measured to determine the change in these properties due to the immersion. Finally, the physical appearance of the specimens

and the test liquid were observed before and after immersion for resulting

changes.

It was concluded from the study that temperature and time of immersion are not

critical for good vulcanizates. However, one of the silicone rubber vulcanizates, although complying with ASTM D3952-80, did not withstand temperatures

54
above 100°C, and would not be satisfactory where temperatures frequently reach

150°C. It was decided, therefore, that ASTM D3952-80 was not adequate to assure satisfactory performance of rubber hose for solar energy system use at

temperatures above coolant boiling points. Consequently, proposed revisions

to ASTM D3952-80 were prepared. The proposed revisions have not, as of this

writing, been adopted.

3.3 STORAGE MEDIA

3.3.1 Introduction

Storage of solar energy is one of the more important aspects of the efficient

utilization of solar energy. Storage of sensible heat in rock bins or water

tanks has long been used, but these storage methods require considerable space.

In contrast, phase change storage systems may require as little as one- seventeenth of the volume of equivalent rock storage or one-eighth of the

volume of water. Another advantage in using phase change materials (PCM's) is

that thermal stratification does not occur in phase change storage systems

because temperatures remain nearly constant throughout the charge-discharge

cycle.

In a phase change system the solar energy is converted into latent heat of

fusion within the PCM. Only after the phase change is completed and all of

the latent heat is absorbed, will the temperature of the PCM increase again.

When the PCM is allowed to cool, the reverse process takes place; latent heat

is released as sensible heat before the temperature of the PCM decreases again.

During 1980 the NBS conducted a review of the literature to identify PCM

systems currently in use or with potential for widespread use, and to identify

problems associated with the PCM's and their containment systems. The results

of the literature review were reported in February 1982 [31]. The literature

study was a forerunner to experimental work needed for developing test methods

by which the performance of inorganic PCM's can be evaluated.

3.3.2 Background

PCM's used in solar applications are typically either organic materials (such

as paraffin wax) or inorganic salts (such as sodium sulfate decahydrate, which

is commonly called Glauber's salt). The latent heat exchange in paraffin occurs during melting and solidification of the wax at a constant temperature. For

Glauber's salt the phase change occurs during formation of a saturated aqueous

solution of sodium sulfate and crystalline sodium sulfate precipitate. In

addition to being divided into organic and inorganic groups, PCM's are further

divided into four types of encapsulation or containment: 1 & 2) macroencapsulation in metal and in plastic (e.g. trays, pouches, cans, tubes, and boxes);

3) microencapsulation (small granules or pellets of PCM packaged in individual

envelopes), and 4) bulk storage in tanks. Phase change systems presently in use or with future potential are summarized in tables 3.5 and 3.6.

55
Table 3.5 Typical Comblnatlona of Inorganic PCM '

8

and Encapsulating

Materials for Thermal Storage (from reference [31])

Type of Encapsulant Inorganic System

Glauber's salt in rigid plastic Celling tiles filled with Salt hydrate mixture in Dlsodium phosphate dodecahycontainers. Problems with Glauber's salt in plastic flexible plastic chubs. drate in rigid plastic.

performance. foil pouches. Water permeability a concern.

[C] [C] [A] [R] Plastic

CaCl

2

* 6H20 eutectic mixture in Salt hydrates in rigid CaCl 2

* 6H20 is high den- Glauber *8 salt in rigid

high density polyethylene plastic trays. Problems sity polyethylene tubes polyethylene trays. blackened tubes. with salt segregation. 50-75 mm diameter.

[C] [A] [R] ic]

Metal Glauber's salt in stainless

steel tubes.

[C]

Glauber's salt with immiscible Salt hydrate in tanks Sodium acetate trihy- Rotating drum with Glauber's

heat transfer liquid in tanks. with heat exchanger. drate with Intermediate salt inside with heat immiscible heat transfer fluid. No solid

heat transfer surface.

exchangers.

[C] [A] [A] 1 R 1 Bulk Sodium thiosulfate pentahydrate Various salt hydrates in Dlsodium phosphate

in tanks with plastic heat tanks with high density decahydrate and misclexchange tubes. polyethylene heat ble heat transfer

exchangers. Tested 1000 fluid. Problems with

cycles

.

surface.

[C] [A] [R]

Microencap- Glauber *8 salt crystals coated sulation with 3% of encapsulating material, packaging in trays.

[C]

C - commercially available A - advanced demonstration stage R - research stage
Table 3.6 Typical Combinations of Organic PCM's and Encapsulating Materials for

Thermal Storage (from reference [31])

Type of

Encapsulant Organic System

Plastic Poroplastic sheets impregnated

with stearic acid-paraffin

combination. [R]

Metal Paraffin in 1 gal. steel cans. Tested 3 years, few problems

identified.

Paraffin in beer cans. Paraffin stored in

modular A1 panels to

supply process water.

[c] [R] 1C]

MacroencapsulationForm-stable crystalline poly- mer pellets (Melting Point - 120-140°C) in ethylene glycol.

[R] 1

Form-stable, electron

cross linked polyethylene pellets.

[R]

50-100 ym paraffin

spheres encapsulated in

polymeric materials.

Used in a 40%

solids-water slurry.

[C]

C = commercially available A = advanced demonstration stage R » research stage

57
3.3.3 Problem Identification

Upon studying the literature on PCM's, the NBS investigators were able to

identify degradation characteristics of inorganic and organic PCM's, and of

various encapsulating materials. Three major problems associated with inorganic

salt hydrate PCM's are: 1) crystal segregation due to incongruent melting of

the salt hydrates, 2) supercooling to the point below which useful heat can be

extracted, and 3) crystal growth rate and growth habits which can adversely

affect the rate of heat transfer. The third problem becomes particularly

critical when thermal cycling occurs in systems to which thickening agents have

been added.

The investigators concluded that organic PCM's may be slightly less desirable

than inorganic salt hydrates due to lower heats of fusion, higher cost, and the

danger associated with storing large quantities of flammable substances. However, the organic PCM's do allow repeated thermal cycling with no apparent loss

in thermal performance. The most promising organic systems identified consist

of paraffin waxes encapsulated in either steel or plastic.

In addition to the problems associated with PCM's, there are factors which may

contribute to the degradation of the encapsulating materials. Metals, except

stainless steel, have not been used with salt hydrates due to the likelihood

of corrosion. Instead, salt hydrates are macroencapsulated in plastic materials. On the other hand, paraffin waxes are not compatible with most plastics

since the waxes can cause stress cracking in these materials. The potential

problem of water migration through the plastic container walls was also con- sidered. Water lost through the walls can not be conveniently replaced, and

would in time reduce the efficiency of the system since insufficient water

would be available to completely rehydrate the PCM salts.

3.3.4 Performance Test Method Needs

It was concluded from this study that PCM technology is tending toward the use

of inorganic salt hydrate phase change systems. Thus, there is a need for the

development of test methods by which the long-term performance of these phase

change systems can be evaluated. The methods should address the phenomena of

material degradation and loss of energy storage capacity. Two major obstacles

to the development of performance tests are: 1) the diversity of possible

materials and system designs requires that the test methods be sufficiently

flexible to accomodate unanticipated materials and systems, and 2) test methods

must consider the effects of both acceleration (of tests) and simulation of

actual operating conditions. Acceleration is usually attainable, but simulation

depends on an understanding of failure mechanisms (such as settling of salts), some of which may be unknown or poorly understood. However, test parameters

for a generalized thermal cycle of phase change can be identified, including:

1) maximum temperature during the cycle, 2) transition temperature in heating,

3) transition temperature in cooling, 4) minimum temperature during the cycle,

and 5) rate of change from one temperature to another.

58
4. SUMMARY OF ACCOMPLISHMENTS AND CURRENT STUDIES

4.1 RESEARCH ACCOMPLISHMENTS

Consistent with the priority rankings listed in table 1.1, the solar energy

materials research since 1974 has largely focused on absorptive coatings and

cover plates. The other high-priority areas have also received some research

attention, as indicated on table 4.1. The intent of table 4.1 is to show the

distribution of the studies summarized herein with respect to the various

high- and medium-priority materials categories. Listed in the first column

are the key properties associated with the identified materials. The bracketed

numerical entries correspond to the reference numbers listed in the index of

references which follows this chapter. Thus, by referring to table 4.1 and to

the index of references, one can identify studies that concentrated on the

evaluation of one or more key properties of a given material.

Much of the materials-related research conducted by national laboratories has

been aimed at the development of standard test methods with which to evaluate:

initial properties, performance under extreme operating conditions, and longterm performance. A number of new or modified standards have resulted, at

least in part, from the studies summarized herein. Table 4.2 lists 18 new ASTM solar energy materials standards, all of which were adopted since 1978.

Where appropriate, these standards take into account some of the conditions

that occur in solar energy heating and cooling systems such as stagnation, and

the corrosivity of heat-transfer fluids in contact with metallic containment

materials.

4.2 FISCAL YEAR 1982 RESEARCH ACTIVITIES AT NATIONAL LABORATORIES

Several national laboratories are currently engaged in a wide range of research

activities related to solar energy materials. As indicated by the projects

listed in table 4.3, the primary focus is still on absorptive coatings and

cover plate materials. Table 4.3 lists the current project titles, together

with a statement of the primary objectives for each project. In addition to

seeking to develop standard test methods, the table indicates that some projects

are also aimed at developing mathematical models of various physical or chemical

processes that affect solar energy materials. By using such models, key mechanisms of degradation can be mathematically modeled. It should then be possible,

for example, to screen and select materials without engaging in extensive

long-term testing.

59
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60

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in [

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refer

to

references

listed

in

Chapter
Table 4.2 List of Standards on Solar Energy Materials Adopted By

the American Society for Testing and Materials

Absorptive Coatings and Substrates

E744-80 - Standard Practice for Evaluating Solar Absorptive Materials for

Thermal Applications

E781-81 - Standard Practice for Evaluating Absorptive Solar Receiver Materials

When Exposed to Conditions Simulating Stagnation in Solar Collectors

with Cover Plates

B638-81 - Standard Specification for Copper and Copper-Alloy Solar Heat

Absorber Panels

Cover Plates

E765-80 - Standard Practice for Evaluation of Cover Materials for Flat Plate

Solar Collectors

E782-81 - Standard Practice for Exposure of Cover Materials for Solar Collectors

to Natural Weathering Under Conditions Simulating Operational Mode

E822-81 - Standard Practice for Determining Resistance of Solar Collector

Covers to Hail by Impact with Propelled Ice Balls

E881-82 - Standard Practice for Exposure of Solar Collector Cover Plate

Materials to Natural Weathering Under Conditions Simulating Stagnation

Mode

Collector Insulation

E861-82 - Standard Practice for Evaluating Thermal Insulation Materials for

Use in Solar Collectors

Containment Materials and Heat Transfer Fluids

E7 12-80 - Standard Practice for Laboratory Screening of Metallic Containment

Materials for Use with Liquids in Solar Heating and Cooling Systems

E745-80 - Standard Practices for Simulated Service Testing for Corrosion of

Metallic Containment Materials for Use with Heat-Transfer Fluids in

Solar Heating and Cooling Systems

E862-82 - Standard Practice for Screening Polymeric Containment Materials for

the Effects of Heat-Transfer Fluids in Solar Heating and Cooling

Systems

61
Table 4.2 List of Standards on Solar Energy Materials Adopted by

the American Society for Testing and Materials (continued)

Sealants

D3667-78 - Standard Specification for Rubber Seals Used in Flat Plate Solar

Collectors

D3771-79 - Standard Specification for Rubber Seals Used in Concentrating Solar

Collectors

D3832-79 - Standard Specification for Rubber Seals Contacting Liquids in Solar

Energy Systems

D3903-80 - Standard Specification for Rubber Seals Used in Air-Heat Transport

of Solar Energy Systems

Hoses/Flexible Couplings

D3952-80 - Standard Specification for Rubber Hose Used in Solar Energy Systems

General Optical Property Measurements

E434-80 - Standard Test Method for Calorimetric Determination of Hemispherical

Emittance and the Ratio of Solar Absorptance to Hemispherical

Eraittance Using Solar Simulation

E903-82 - Standard Test Method for Solar Absorptance, Reflectance, and

Transmittance of Materials Using Integrating Spheres

62
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64

Phase

Change

Test

Methods

for

Phase

To

provide

assessment

of

the

long-term

National

Bureau

of

Standards

Materials

Change

Materials

(PCM's)

performance

and

durability

and

to

Identify

failure

mechanisms

of

PCM's.
5 . 0 ACKNOWLEDGMENTS

The author gratefully acknowledges the considerable contribution of

Scott Heider, P.E. in reviewing and summarizing the reference documents,

organizing this report and performing editorial functions. The continuing

guidance on report organization and content, as well as the review comments

provided by Mr. Robert Dikkers is greatly appreciated. The author thanks

Messrs. Larry Masters and Thomas Faison for reviewing several drafts of this

report. This work was supported by the Active Heating and Cooling Materials

and Components Branch of the U.S. Department of Energy's Office of Solar Heat

Technologies. The skillful and patient typing of the drafts of this report

by Ms. Mary Rams burg is also gratefully acknowledged.

65
6. INDEX OF REFERENCES

[1] Waksman, D., Pielert, J. H. , Dikkers, R. D., Streed, E. R. , and

Niessing, W. J., "Plan for the Development and Implementation of Standards for Solar Heating and Cooling Applications," NBS Report NBSIR

78-1143A, National Bureau of Standards, Washington, DC, June 1978, 58 p.

[2] Skoda, L. F. and Masters, L. W. , “Solar Energy Systems - Survey of

Materials Performance,” NBS Report NBSIR 77-1314, National Bureau of

Standards, Washington, DC, October 1977, 113 p.

[3] Clark, E. J., Roberts, W. E., Grimes, J. W. , and Embree, E. J.,

"Solar Energy Systems—Standards for Cover Plates for Flat Plate Solar

Collectors,” NBS Technical Note 1132, National Bureau of Standards,

Washington, DC, December 1980, 157 p.

[4] ASTM E424-71, Standard Method of Test for Solar Energy Transmittance and

Reflectance (Terrestrial) of Sheet Materials, Annual Book of Standards,

Part 41, American Society of Testing and Materials, Philadelphia, PA.

[5] Roberts, W. E., Masters, L. W. , and Clark, E. J., "Effects of Air Mass

and Integration Methods on Results for Optical Property Measurements of

Solar Cover Plate and Absorber Materials," NBS Report NBSIR 81-2448,

National Bureau of Standards, Washington, DC, January 1982, 46 p.

[6] Clark, E. J. and Roberts, W. E., "Weathering Performance of Cover

Materials for Flat-Plate Solar Collectors," NBS Technical Note 1170,

National Bureau of Standards, Washington, DC, November 1982, 70 p.

[7] Jenkins, D. R. and Mathey, R. G., "Hail Impact Testing Procedure for

Solar Collector Covers," NBSIR 82-2487, National Bureau of Standards,

Washington, DC, April 1982, 86 p.

[8] Masters, L. W. , Seiler, J. F., Embree, E. J., and Roberts, W. E.,

"Solar Energy Systems - Standards for Absorber Materials," NBSIR 81-2232,

National Bureau of Standards, Washington, DC, January 1981, 60 p.

[9] Masters, L. W. , Seiler, J. F., and Roberts, W. E., "Outdoor Exposure

Tests of Solar Absorptive Coatings," NBSIR 82-2582, National Bureau of

Standards, Washington, DC, October 1982, 16 p.

[10] ASTM E744-80, Standard Practice for Evaluating Solar Absorptive Materials

for Thermal Applications, Annual Book of Standards, Part 41, American

Society for Testing and Materials, Philadelphia, PA.

[11] ASTM E781-81, Standard Practice for Evaluating Absorptive Solar Receiver

Materials When Exposed to Conditions Simulating Stagnation in Solar

Collectors With Cover Plates, Annual Book of Standards, Part 41,

American Society for Testing and Materials, Phildelphia, PA.

66
[12] Wu, S. and Masters, L. W. , "A Model for Predicting the Effect of Exposure

to Elevated Temperature on the Optical Properties of Black Chrome,"

Journal of Coatings Technology , Vol. 54, No. 69, August 1982, pp. 41-45.

[13] Post, M. A. and Ballard, D. , "Degradation of Solar Coatings Observed

by Scanning Electron Microscopy," Journal of Coatings Technology , Vol. 52,

No. 664, May 1980, pp. 55-62.

[14] McDonald, G. E., "Spectral Reflectance Properties of Black Chrome for Use

as a Solar Selective Coating," Solar Energy , 17, 1975, p. 119.

[15] Pettit, R. B. and Sowell, R. R. , "Solar Absorptance and Emittance

Properties of Several Solar Coatings," Journal of Vacuum Science and

Technology , Vol. 13, No. 2, March/April 1976, pp. 596-602.

[16] Pettit, R. B. and Sowell, R. R. , "Recent Developments Regarding

Electrodeposited Black Chrome Solar Coatings," SAND-78-2423C , Sandia

Laboratories, Albuquerque, New Mexico, 1978, 7 p.

[17] Lampert, C. M. and Washburn, J., "Microstructure of a Black Chrome Solar

Selective Absorber," Solar Energy Materials," Vol. 1, No. 1, February

1979, pp. 81-92.

[18] Lampert, C. M. and Washburn, J., "Microstructure and Optical Properties

of Black Chrome Before and After Exposure to High Temperatures,"

Proceedings Second Annual Conference on Absorber Surfaces for Solar

Receivers, sponsored by the U.S. Department of Energy, January 1979,

SERI/TP-49-182.

[19] Grimmer, D. P., Herr, K. C. and McCreary, W. J., "A Possible Selective

Solar Photothermal Absorber: Ni Dendrites Formed on A1 Surfaces by the

CVD of Ni(C04)," LA-UR-76-2020 , Los Alamos Scientific Laboratory of the

University of California, 1976, 16 p.

[20] Godette, M. , Lee, J., and Fearn, J., "Test Method for Collector

Insulation," NBSIR 79-1908, National Bureau of Standards, Washington, DC,

August 1979, 18 p.

[21] Stiehler, R. D., Hockman, A., Embree, E. J., and Masters, L. W.

,

"Solar Energy Systems - Standards for Rubber Seals," NBSIR 77-1437,

National Bureau of Standards, Washington, DC, March 1978, 63 p.

[22] Brown, P. W. and Grimes, J. W. , "Evaluation of a Proposed ASTM Standard

Guide to Assess the Compatibility of Metal-Transfer Heat Liquid Pairs

in Solar Heating and Cooling Systems," NBS Report NBSIR 79-1919, National

Bureau of Standards, Washington, DC, November 1979, 44 p.

[23] Clark, E. J., Kelly, C. D., and Roberts, W. E., "Solar Energy Systems - Standards for Screening Plastic Containment Materials," NBS Report

NBSIR 82-2533, National Bureau of Standards, Washington, DC, June 1982,

46 p.

67
[24] ASTM Standard E862-82, Standard Practice for Screening Polymeric

Containment Materials for the Effects of Heat-Transfer Fluids in

Solar Heating and Cooling Systems, Annual Book of Standards, Part 41,

American Society for Testing and Materials, Philadelphia, PA.

[25] Brown, P. W. and Grimes, J. W. , "Simulated Service Testing for Corrosion

in Solar Heating and Cooling Systems," NBS Report NBSIR 81-2339, National

Bureau of Standards, Washington, DC, September 1981, 34 p.

[26] ASTM Standard E745-80, Standard Practices for Simulated Service Testing

for Corrosion of Metallic Containment Materials for Use with Heat-Transfer

Fluids in Solar Heating and Cooling Systems, Annual Book of Standards,

Part 41, American Society for Testing and Materials, Philadelphia, PA.

[27] Zeman, G. J., "Corrosion Behavior of Several Metals in Ethylene GlycolBase Heat-Transfer Fluids Under Conditions Encountered in Solar Energy

Systems," ANL-79-96, Argonne National Laboratory, Argonne, IL,

January 1980, 45 p.

[28] Stiehler, R. D. and Michalak, J. L., "Solar Energy Systems - Standards

for Rubber Hose," NBSIR 79-1917, National Bureau of Standards,

Washington, DC, October 1979, 30 p.

[29] ASTM D3952-80, Standard Specification for Rubber Hose Used in Solar

Energy Systems, Annual Book of Standards, Part 38, American Society

for Testing and Materials, Philadelphia, PA.

[30] Stiehler, R. D., "Solar Energy Systems - Standards for Rubber Hose

Used with Liquids Above Their Boiling Points," NBSIR 81-2352, National

Bureau of Standards, Washington, DC, September 1981, 28 p.

[31] Ings, J. B. and Brown, P. W. , "Factors Affecting the Service Lives

of Phase Change Storage Systems," NBSIR 81-2422, National Bureau of

Standards, Washington, DC, February 1982, 19 p.

68
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 83-2782

2. Performing Organ. Report No. 3. Publication Date December 1983

4. TITLE AND SUBTITLE

MATERIALS RESEARCH ACTIVITIES AT THE NATIONAL BUREAU OF STANDARDS (1975-1982)

PERTAINING TO ACTIVE SOLAR HEATING AND COOLING SYSTEMS

5. AUTHOR(S)

Charles W. C. Yancey

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions)

national bureau of standards

7. Contract/Grant No.

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP) U. S. Department of Energy

Office of Solar Heat Technologies

Active Solar Heating and Cooling Division

Washington, D.C. 20585

10. SUPPLEMENTARY NOTES

| | 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

bibliography or literature survey, mention it here)

A summary of the solar energy material s-related research projects conducted by the

National Bureau of Standards, for the U.S. Department of Energy, since 1975 is presented. Research studies concerned with materials that are utilized in the collector,

transport and storage subsystems are summarized. Materials research areas covered

by the documentation include: cover plates, absorber coatings, thermal insulation,

sealants, containment materials, heat transfer fluids, hoses and storage media

materials. The primary objectives, scope and principal results of the various

studies are presented. The relationship between test results and subsequent

consensus standard adoption or revision is drawn where applicable.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Absorber coatings; containment materials; cover plates; durability; heat transfer

fluids; hoses; performance tests; sealants; solar collectors; solar energy storage

subsystem; storage media; thermal insulation; transport subsystem

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

Isk ,| Unlimited

| | For Official Distribution. Do Not Release to NTIS

| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

73

15. Price

m Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $ 10.00

U S COMM* DC 6043-P80
J«

to"

••

:.i

• •

?
HBS-114A (rev. 2 -ac) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 83-2782

2. Performing Organ. Report No 3. Publication Date December 1983

4. TITLE AND SUBTITLE

MATERIALS RESEARCH ACTIVITIES AT THE NATIONAL BUREAU OF STANDARDS (1975-1982)

PERTAINING TO ACTIVE SOLAR HEATING AND COOLING SYSTEMS

5. AUTHOR(S)

Charles W. C. Yancey

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

U. S. Department of Energy

Office of Solar Heat Technologies

Active Solar Heating and Cooling Division

Washington, D.C. 20585

10. SUPPLEMENTARY NOTES

! ~| 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

bibliography or literature survey, mention it here)

A summary of the solar energy materials- related research projects conducted by the

National Bureau of Standards, for the U.S. Department of Energy, since 1975 is presented. Research studies concerned with materials that are utilized in the collector,

transport and storage subsystems are summarized. Materials research areas covered

by the documentation include: cover plates, absorber coatings, thermal insulation,

sealants, containment materials, heat transfer fluids, hoses and storage media

materials. The primary objectives, scope and principal results of the various

studies are presented. The relationship between test results and subsequent

consensus standard adoption or revision is drawn where applicable.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Absorber coatings; containment materials; cover plates; durability; heat transfer

fluids; hoses; performance tests; sealants; solar collectors; solar energy storage

subsystem; storage media; thermal insulation; transport subsystem

13. AVAILABILITY

|yl Unlimited

| | For Official Distribution. Do Not Release to NTIS

| | Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

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

14. NO. OF

PRINTED PAGES

73

15. Price

$ 10.00

U S COMM*DC 804S-P80
I • A . 1

1

,

.

.

4 '

•

,

•

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