AlllDO TTSESS

NBS

PUBLICATIONS

NBSIR 81-2448

Effects of Air Mass and Integration

Methods on Results for Optical

Property Measurements of Solar

Cover Plate and Absorber

Materials

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Materials Division

Washington, DC 20234

December 1 981

Issued January 1982

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

lActive Heating and Cooling Division

Washington, DC 20585

81 - 2m

1981

c. 2

NBSIR 81-2448

» • # EFFECTS OF AIR MASS AND INTEGRATION

METHODS ON RESULTS FOR OPTICAL

PROPERTY MEASUREMENTS OF SOLAR

COVER PLATE AND ABSORBER

MATERIALS

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UBRART

F£B 3 1982

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Willard E. Roberts

Larry W. Masters

Elizabeth J. Clark

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Materials Division

Washington, DC 20234

December 1 981

Issued January 1982

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

This study was undertaken to compare methods of calculating the transmittance

of cover plate materials and the reflectance of absorber materials. The study

was limited to evaluation of factors which influence the results with method A

of ASTM E 424, Optical data were obtained for both aged and unaged test

specimens using an integrating sphere spectrophotometer. The data were integrated using: (1) the weighted and selected ordinate methods in ASTM E 424,

Method A, at air mass 2.0, and (2) the selected ordinate method at air mass

1.5 and 1.0. The solar reflectance and solar transmittance values calculated

using the various methods are presented in this report along with discussions

of the impact of the data in terms of possible revisions to ASTM E 424.

Keywords: air mass; ASTM E 424; integrating sphere spectrophotometer;

reflectance; selected ordinate; solar absorber materials; solar

cover plates; transmittance; weighted ordinate.

iii
TABLE OF CONTENTS

Page

ABSTRACT iii

LIST OF TABLES vi

LIST OF FIGURES vii

ACKNOWLEDGMENT lx

1. INTRODUCTION 1

1.1 Background 1

1.2 Objectives 2 1.3 Approach 2

2. FACTORS INFLUENCING METHOD A CALCULATIONS 3

2.1 Relationship of Solar Energy Distribution to Air Mass 3

2.2 Spectral Properties of the Material 3

2.3 Integration Methods 4

2.3.1 Weighted Ordinate 4

2.3.2 Selected Ordinate 4

3. DESCRIPTION OF DATA AND CALCULATIONS 5

3.1 Measurement of Optical Properties 5 3.2 Description of Materials 5 3.3 Calculation of Optical Properties 5

4. RESULTS AND DISCUSSION 7

4.1 Reflectance of Absorber Specimens 7

4.1.1 Control (Unweathered) Specimens 7 4.1.2 Weathered Specimens 7 4.1.3 Effect of Method of Calculation at Air Mass 2.0 8

4.1.4 Effect of Air Mass on Reflectance 8

4.2 Transmittance of Cover Plate Materials 8

4.2.1 Effect of Method of Calculation at Air Mass 2.0 8

4. 2. 1.1 Control (Unweathered) Specimens 8

4. 2. 1.2 Weathered Specimens 9

4. 2. 1.3 Summary 9

4.2.2 Effect of Air Mass 9

4. 2.

2.1

Control Specimens at Air Mass 2.0, 1.5,

and 1.0 9

Iv
TABLE OF CONTENTS

Page

4. 2. 2. 2 Weathered Specimens at Air Mass 2.0, 1.5,

and 1.0 10

4. 2. 2. 3 Summary 10

5. CONCLUSIONS 11

5.1 Absorber Coating Materials 11

5.2 Cover Plate Materials 11

6. RECOMMENDATIONS 12

7. REFERENCES 13

V
LIST OF TABLES

Page

Table 1. Absorber and Cover Plate Materials Used in This Study 14

Table 2. Relative Energies of Weighted Ordinates for Calculation of

Solar Energy Transmittance or Reflectance at Air Mass 2,0 15

Table 3. Twenty Selected Ordinates for Calculation of Solar

Transmittance or Reflectance at Air Mass 2,0 15

Table 4, Fifty Selected Ordinates for Calculation of Solar Transmittance

or Reflectance at Air Mass 1,5 16

Table 5, Fifty Selected Ordinates for Calculation of Solar Transmittance

or Reflectance at Air Mass 1,0 17

Table 6, Tabulation of Reflectance Data for Absorber Materials 18

Table 7, Tabulation of Transmittance for Cover Plate Materials 19

vi
LIST OF FIGURES

Page

Figure 1. Spectral distribution of sunlight at air masses 1, 1.5

and 2.0 20

Figure 2. Spectral reflectance of nonselective absorber materials .... 21

Figure 3. Spectral reflectance of selective absorber materials 22

Figure 4. Spectral transmittance of three cover plate materials 23

Figure 5. Spectral transmittance of two cover plate materials 24

Figure 6. Comparison of reflectance of weathered black chrome on steel

using the weighted and selected ordinate calculation methods

at air mass 2.0; weathering environment was 92 ®C and

97 percent relative humidity 25

Figure 7. Effect of method of calculation and air mass on reflectance

of weathered and unweathered black chrome on steel 26

Figure 8. Effect of method of calculation and air mass on reflectance

of weathered and unweathered black chrome on copper 27

Figure 9. Effect of air mass on the reflectance of black chrome on

steel following exposure at 92°C and 97 percent relative

humidity 28

Figure 10. Effect of air mass on the reflectance of urethane on steel

following exposure at 92 °C and 97 percent relative

humidity 29

Figure 11. Effect of method of calculation at air mass 2.0 on transmittance of poly(vinyl fluoride) weathered by heat

aging at 150°C 30

Figure 12. Effect of method of calculation at air mass 2.0 on transmittance of polycarbonate weathered by heat aging at

150'*C 31

Figure 13. Effect of air mass on transmittance of fluorinated ethylene

propylene) copolymer weathered by heat aging at 150®C 32

Figure 14. Effect of air mass on transmittance of glass weathered by

heat aging at 150°C 33

Figure 15. Effect of air mass on the transmittance of polycarbonate

weathered by heat aging at 150°C 34

vii
LIST OF FIGURES (Continued)

Page

Figure 16. Effect of air mass on the transmittance of fiber reinforced

plastic weathered by heat aging at 150°C 35

Figure 17. Effect of air mass and Increased time of heat age on the

transmittance of poly(vinyl fluoride) 36

Figure 18. Spectral transmittance of poly(vinyl fluoride) exposed to

150°C for four time periods 37

viii
ACKNOWLEDGMENT

The authors gratefully acknowledge 1) the many suggestions provided by members

of ASTM E44.04 on Materials Performance in planning and carrying out this

study, 2) the interest demonstrated by the Department of Energy in supporting

this research and 3) the technical assistance of Mr. Lawrence Kaetzel and

Mrs. Catherine Kelly in obtaining reflectance and transmittance data.

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1 . INTRODUCTION

1.1 BACKGROUND

Solar transmittance and reflectance are the key attributes required,

respectively, of cover plate and absorber coating materials for solar collectors. Transmittance and reflectance values are used by collector manufacturers,

architects, and researchers in choosing materials, in calculating expected

thermal efficiency of collectors and in measuring short-term and long-term

performance of collectors and materials. ASTM E 424-71, Standard Methods of

Test for Solar Energy Transmittance and Reflectance (Terrestrial) of Sheet

Materials [1]!./ is currently being revised by the American Society for Testing

and Materials Committee E44 on Solar Energy Conversion. This study was undertaken to develop data to assess the effect of several variables in ASTM

E 424-71 to determine if revision of these parts is necessary. ASTM E 424 is

the most commonly used standard method for determining solar transmittance and

reflectance

ASTM E 424 contains two methods of measurement. Method A is performed using

an integrating sphere spectrophotometer in a laboratory and is applicable for

both transmittance and reflectance. Method B is performed outdoors using a pyranometer in an enclosure as the detector and the sun as the energy source.

It is applicable only for transmittance. For method A, the value of the solar

energy transmitted or reflected is obtained by measuring the spectral transmittance or reflectance with a spectrophotometer and then integrating the spectral data over a standard solar energy distribution curve for air mass 2.0.

Two methods of integration, weighted ordinates and selected ordinates, are provided in ASTM E 424. Since the user of method A may arbitrarily choose

either method of integration, it is essential to determine if the two methods

of integration provide the same results for all materials.

It is also important to determine the effect of air mass on the solar optical

properties of solar collector materials. ASTM E 424 states that both method A

and method B produce essentially equivalent results. However, method A requires

that the solar energy spectral distribution for air mass 2.0 be used, while no similar limitation is placed on measurements made using method B. Transmittance

values determined outdoors with method B are seldom measured under conditions

which relate to air mass 2.0. They are more likely to be measured when the

air mass is lower .than 2.0.

Most materials used in solar energy systems have distinctly different spectral

transmittance and reflectance curves. For some materials the curve is essentially flat over the entire spectral range of solar energy, while for other

materials the curve is not flat. As a consequence, no single intervals spectral characteristics can be considered typical of all materials used in solar

collectors

1 / Numbers in brackets refer to references in section 7.

1
Clearly, two factors have a potential to influence the transmittance and

reflectance values obtained using method A of E 424, These factors are method

of integration, and solar energy spectral distribution (air mass). The importance of each factor depends on the material. The effect of each variable was examined in this study.

1.2 OBJECTIVES

The objectives of this study were: (1) to determine if the selected and weighted

ordinate methods of integration produce equal transmittance and reflectance

values for typical materials; (2) to determine if the solar energy spectral

distribution (air mass) used in the calculations affects the transmittance and

reflectance values for typical materials; and (3) to develop recommendations

for revisions to E 424, method A, if necessary.

1.3 APPROACH

To accomplish these objectives, spectral transmittance and reflectance data

generated in two previous studies [2, 3] were used. The data Included both

weathered and unweathered cover plate and absorber materials. The approach

was: a) to develop computer programs to integrate spectral data using the air mass 2, weighted and selected ordinate methods of E 424, b) to calculate and

compare the transmittance and reflectance using the weighted and selected

ordinate integration methods for selected materials, c) to develop programs to

integrate the spectral data using the solar energy distributions for air mass

1.0 and 1.5, and d) to calculate and compare transmittance and reflectance for

the selected materials at air masses of 2.0, 1.5 and 1.0,

2
2. FACTORS INFLUENCING METHOD A CALCULATIONS

2.1 RELATIONSHIP OF SOLAR ENERGY DISTRIBUTION TO AIR MASS

The solar energy radiation received at the earth's surface is dependent upon

geographic location, day of year, and time of day. The spectral distribution

of the emitted solar energy is fixed and corresponds closely to the spectrum

of a 5762°K black body source. The spectral distribution of the energy

received at the earth's surface differs from that of the emitted energy because

of scattering and absorption processes in the atmosphere. The degree to which

these processes modify the spectral distribution depends on the path length

traversed through the atmosphere. For all but very high solar zenith angles

(i.e., 02 ^

70°) this path length is directly proportional to the air mass parameter, m, defined as

1 m = cos 02

where m = air mass , and

02 = angle subtended by the zenith and the line of sight to the sun.

The air mass is equal to one when the sun is directly overhead and is greater

than one as the line of sight to the sun deviates from the zenith. For example,

when the sun reaches an angle of 60° between the zenith and the line of sight,

the distance through which the beam travels before it reaches the earth has

doubled, and the air mass is equal to 2.

By convention, the air mass is taken to be equal to zero outside the earth's

atmosphere

The solar energy spectral distributions for air masses 1.0, 1.5, and 2.0 are illustrated in figure 1. These air mass values correspond to solar zenith

angles of 0°, 48°, and 60°, respectively. Conditions corresponding to air mass 1.0 never occur in the continental United States. The authors of

ASTM E 424 chose air mass 2.0, zenith angle 60°, as representative, but most

solar collectors that are in service are exposed to more hours of radiation

that approximates air mass 1.5 (02 = 48.19°). The most important differences

between the distributions occur in the ultraviolet and visible regions.

2.2 SPECTRAL PROPERTIES OF THE MATERIAL

The spectral reflectance and transmittance properties of absorber and cover

plate materials vary depending upon the type of material. Absorber materials

can be categorized as either selective or nonselective . The spectral reflectance of a nonselective absorber is a relatively flat line over the range of

solar energy (figure 2), while the spectral reflectance of a selective absorber

changes over the range of solar energy (figure 3), with its reflectance

generally increasing above 1000 nm.

3
Other than vertical shifts, the differences in the spectral transmittance curves

of cover plate materials occur primarily in two regions; (1) the ultraviolet

region (below 400 nm), and (2) the near infrared region above 1100 nm (figures 4

and 5). The absorption bands in these regions are dependent upon the material

composition. Vertical shifts in the flat parts of the transmittance curves are dependent upon both material composition and thickness.

2.3 INTEGRATION METHODS

2.3.1 Weighted Ordinate

In the weighted ordinate method of integration the solar spectrum is divided

into a finite number of equal intervals, e.g., 50 nm. Each interval is integrated to determine the energy, then these energies are summed to obtain the

total energy under the curve. The relative energy for each interval is then

calculated and is represented as a percentage of the total energy. The wavelength at the midpoint of the interval (e.g., 450 nm represents 425 to 475 nm)

is then used as the wavelength for the weighted ordinate. The total solar

energy transmittance is obtained by integrating over the standard solar energy

distribution as follows,

Tse = I

• Ex

^1

where Tgg = total solar energy transmittance in percent

X = wavelength of weighted ordinate

T;^ = transmittance at wavelength X

E;^^ = fraction of the total energy in the interval centered at the

wavelength X

Reflectance is obtained using a similar equation.

ASTM E 424 contains the weighted ordinates for air mass 2 at 50 mm intervals.

2.3.2 Selected Ordinates

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

number of intervals. However, Instead of equal intervals, as used in the

weighted ordinate method, the spectrum is divided into intervals that represent

equal increments of solar energy. The spectral transmittance or reflectance

of the material is thus weighted equally in each such interval and no further

weighting is necessary [5]

The wavelength at the midpoint of the Interval is

the selected ordinate. The total solar energy transmittance or reflectance is

determined by summing the transmittance values for each of the selected ordinates and dividing this value by the number of selected ordinates. E 424 uses twenty selected ordinates for air mass 2. Other numbers (50 and 100) of

selected ordinates have also been used when greater accuracy was desired.

4
3.1 DESCRIPTION OF DATA AND CALCULATIONS

MEASUREMENT OF OPTICAL PROPERTIES

The spectral transmittance and reflectance of materials were measured as described in ASTM E 424, method A over the spectral range from 300 to 2150 nm

utilizing a spectrophotometer with a 76 mm (3 in) diameter integrating sphere.

The illumination and viewing mode was normal diffuse for transmittance and

diffuse for reflectance. The transmittance and reflectance measurements

were made by placing the test specimens in direct contact with the sphere aperture so that the incident monochromatic radiation was normal to the plane of

the specimen for transmittance specimens and within 7 degrees of perpendicularity to the plane of the reflectance specimens. The spectral reflectance and

transmittance data were digitized and sent directly to a computer where they were stored for future manipulation.

3.2

DESCRIPTION OF MATERIALS

The spectral transmittance and reflectance data analyzed in this study were obtained from previous studies on absorber materials [3] and cover plate materials [2] . These studies investigated the performance of materials before and

after artificial weathering and natural weathering. Data sets representing

typical materials and exposures were selected for analysis in this study. Four

absorbers, two selective and two nonselective, were chosen. The materials are listed in table 1. The reflectance data (from absorber materials) used in this

study were for: (1) control specimens (no natural or artificial weathering),

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

elevated temperatures. The spectra reflectance curves for the control speci- mens are shown in figures 2 and 3. The five cover plate materials used in

this study illustrate the range of cover materials used in solar collectors.

They include glass, plastic film and plastic sheet, and are also listed in

(table 1). The transmittance data (for cover plate materials) used in this

study were from: (1) control specimens, and (2) specimens exposed to oven

aging. The spectral transmittance curves for the control specimens are given

in figures 4 and 5.

3.3

CALCULATION OF OPTICAL PROPERTIES

The two integration methods were compared using air mass 2 data from E 424-71.

The selected and weighted ordinate values used in the calculations are given

in tables 2 and 3.

Three air masses, i.e., 2.0, 1.5, and 1.0, were chosen to determine the effect

of different air masses on the calculation of reflectance and transmittance.

These represent zenith angles of 60°, 48.19°, and 0°. The selected ordinate

integration method was used for these comparisons. For air mass 2 the twenty

selected ordinates in E 424 were used, while the data for air masses 1.5 and

1.0 were taken from "the sensitivity of solar transmittance, reflectance and

absorptance of selected averaging procedures and solar irradiance," [6] and

"solar spectral irradiance at ground level" [7]

Fifty selected ordinates

were produced for air masses 1.5 and 1.0. These are given in (tables 4

5
and 5), respectively. Note that for both air mass 1.5 and air mass 1.0, the

last ordinate is beyond the range of the spectrophotometer (300-2150 nm). The

approach used in this study to calculate the out-of-limit ordinate was:

(1) to sum each of the first 49 ordinates; (2) to the above value, add the

value of the 49th ordinate a second time to give a total of 50 ordinates; and

(3) to divide the total by 50 to give the solar transmittance or reflectance

value for air mass 1.5 and air mass 1.0. This procedure was considered to

be less biased than the alternatives of (1) taking the 50th ordinate to be

either zero or 1, or (2) averaging only the first 49 ordinates.

6
4. RESULTS AND DISCUSSION

In comparing the reflectance and transmittance data calculated using different

integration methods and air masses, the authors considered a difference of

0.5 percent and greater to be significant.

4.1

REFLECTANCE OF ABSORBER SPECIMENS

4.1.1 Control (Unweathered) Specimens

Reflectance data obtained for the control (unweathered) specimens of the four

absorber materials are presented at the top of table 6. Data are presented

for: 1) air mass 2.0 using both the weighted and selected ordinate calculation

methods; and 2) air mass 1.5 and 1.0 using the selected ordinate method.

For the control specimens, comparison of reflectance obtained by both the

weighted and selected ordinate methods using ASTM E 424 at air mass 2.0 show

the two methods of calculation to yield comparable results. Differences of

0.1 percent occurred for each material, but this is considered insignificant.

For black chrome on steel, the reflectance was 4.9 percent using both weighted

and selected ordinates calculation methods; for black chrome on copper, the

reflectance was 4.8 and 4.9 percent, respectively.

Comparisons of reflectance for air masses 2.0, 1.5 and 1.0 for control black

chrome absorber materials show that higher reflectance values are obtained at

air masses 1.5 and 1.0 than at 2.0. At air mass 1.5, both black chrome speci- mens had a reflectance of 6.2 percent while, at air mass 1.0, the reflectance

was 6.0 percent for both specimens.

For the control specimens of nonselective absorber materials (urethane and

porcelain enamel), little or no effect was observed on reflectance by the

method of calculation or by the air mass.

4.1.2 Weathered Specimens

Table 6 also contains reflectance data for weathered absorber materials

including data for: 1) air mass 2.0 using both the weighted and selected ordinate calculation methods; and 2) air mass 1.5 and 1.0 using the selected ordinate

method

When calculated at air mass 2 using selected and weighted ordinates the

reflectance of black chrome on steel after exposure to moisture is essentially

identical. This is illustrated in figure 6. After weathering reflectance

values of black chrome on steel and copper were higher at air masses 1.5 and

1.0 than those at air mass 2.0 by up to 1.6 percent. The effect of air mass

and method of calculation on reflectance of black chrome are further illustrated

in (figures 7, 8 and 9).

For weathered nonselective absorber materials (urethane and porcelain enamel),

little or no effect was observed on reflectance by the method of calculation

7
or by the air mass used. The effect of air mass is illustrated in figure 10

for urethane on steel.

4.1.3 Effect of Method of Calculation at Air Mass 2.0

The data in table 6 show that there was little if any effect on reflectance of

absorber materials from the method of calculation at air mass 2.0; hence, both

the weighted or selected ordinate methods from ASTM E 424 would be expected to

yield comparable reflectance values.

4.1.4 Effect of Air Mass on Reflectance

For the selected ordinate method, reflectance values for unweathered black

chrome absorbers (selective) when calculated using mass 1.5 and air mass 1.0 were from 1.1 to 1.2 percent higher than those for at air mass 2.0. For

weathered black chrome, the reflectance values at air mass 1.5 and 1.0 were from 0.8 to 1.2 percent higher than those at air mass 2.0. The higher solar

reflectance values at air mass 1.5 and 1.0 are due to the shape of the

reflectance curves in the near-infrared region of the spectrum (see figure 3).

The reflectance values of the nonselective absorbers (urethane and porcelain

enamel) were comparable at all three air masses. The reason for this is that the

spectral curves of these materials are almost flat line over the entire spectrum

from 300 to 2150 nm (figure 2).

The data indicate that the use of different air mass values could lead to

discrepancies in values of reflectance for selective absorber materials.

4.2 TRANSMITTANCE OF COVER PLATE MATERIALS

Transmittance data for weathered and control (unweathered) cover plate materials

are tabulated in table 7. Data are presented for: 1) air mass 2.0 using both

the weighted and selected ordinate calculation methods; and 2) air mass 1.5 and

1.0 using the selected ordinate method.

4.2.1 Effect of Method of Calculation at Air Mass 2.0

4. 2. 1.1 Control (Unweathered) Specimens

Comparison of transmittance data at air mass 2.0 for unweathered materials

using the weighted and selected ordinate methods from ASTM E 424 shows that:

(1) for glass, fluorinated (ethylene propylene) copolymer, and poly(vinyl

fluoride), the transmittance differed by no more than 0.2 percent; and (2) for

polycarbonate and fiber reinforced plastic the transmittance was less using the

selected ordinate method than that obtained using the weighted ordinate method.

For example, the transmittance of the fiber reinforced plastic was 78.2 and

76.8 percent for the weighted and selected ordinate methods, respectively.

8
4. 2. 1.2 Weathered Specimens

Transmittance values, obtained by the weighted and selected ordinate methods, were comparable for glass and fluorinated (ethylene propylene) copolymer in

that the maximum differences were 0.2 percent. For poly(vlnyl fluoride), polycarbonate and fiber reinforced plastic, however, the transmittance values

obtained with the weighted ordinate method were consistently higher than those

obtained with the selected ordinate method. For example, polycarbonate transmittance values were from 0.8 to 1.1 percent higher using the weighted ordinate

method. This effect is illustrated in figures 11 and 12 for poly(vlnyl

fluoride) and polycarbonate, respectively.

4. 2. 1.3 Summary

The data show that comparable transmittance values are obtained using the

weighted and selected ordinate methods for glass and fluorinated (ethylene

propylene) copolymer. The data show that discrepancies arise between the two

methods of calculation for poly(vinyl fluoride), polycarbonate and fiber reinforced plastic. The discrepancies result from absorption bands for these materials in the near-infrared region (e.g., polycarbonate and fiber reinforced

plastic) or from a shift in the absorption on the ultraviolet region of aged

materials of the spectra. For poly(vinyl fluoride), polycarbonate and fiber

reinforced plastic, the weighted ordinate method yields consistently higher

transmittance values than the selected ordinate method. The data indicate that

the method of calculation (weighted versus selected ordinates) can lead to

significant differences in transmittance for some cover plate materials.

4.2.2 Effect of Air Mass

4. 2. 2.1 Control Specimens at Air Mass 2.0, 1.5 and 1.0

Transmittance data of unweathered materials, calculated using the selected

ordinate method at air mass 2.0, 1.5, and 1.0, show that for fluorinated (ethylene propylene) copolymer the difference in transmittance values is small,

i.e., 0.3 percent or less, between air mass 2.0 and air mass 1.0. For glass

the difference between air mass 2.0 and air mass 1.0 is 0.5 percent, although

it is only 0.2 percent between air mass 2.0 and air mass 1.5. For poly(vinyl

fluoride), polycarbonate and fiber reinforced plastic, however, the difference

in transmittance values between air mass 2.0 and air mass 1.0 is greater. For

example, the percent transmittance of polycarbonate varies from 87.3 at air mass 2.0 to 82.7 at air mass 1.0. For fiber reinforced plastic, the difference

in transmittance was 2.1 percent between air mass 2.0 and 1.5 and 1.9 percent

between air mass 1.5 and 1.0. For unweathered cover plate materials, the

transmittance usually decreased as the air mass decreased.

For unweathered materials, the decreasing values in solar transmittance with

decreasing air mass values are primarily caused by two factors; (1) absorption bands in the Infrared region of the spectra; and (2) increasing energy at

the ends of the solar spectral band as the air mass decreases. The increasing

energy mentioned above causes a slight shift in the equal energy distribution

points to the ends of the spectra. The spectral transmittance curves of many

9
of the cover plate materials fall off rapidly in the ultraviolet region, thus

causing losses in the integrated value as the air mass decreases. As shown in

the data, this change can be as much as 4.6 percent for some unaged materials

between air mass 2.0 and air mass 1.0.

4. 2. 2. 2 Weathered Specimens at Air Mass 2.0, 1.5 and 1.0

Transmittance data of heat aged cover plate materials, calculated using the

selected ordinate method at air masses 2.0, 1.5 and 1.0 show that air mass has

little effect on the calculated transmittance for fluorinated( ethylene propylene) copolymer. For glass the difference between air mass 2.0 and air mass

1.0 was about 0.6 percent while it was less comparing air mass 2.0 and 1.5.

Figures 13 and 14 Illustrate the effect of air mass on f luorinated(ethylene

propylene) copolymer and glass respectively, with increased time of heat aging.

As was observed with control specimens, however, the transmittance of weathered

poly (vinyl fluoride), polycarbonate plastic decreased as air mass decreased.

For poly(vinyl fluoride) and fiber reinforced plastic, the transmittance differences calculated using air mass 2.0 and 1.0 values were about 4.5-5 percent, while the differences between air mass 2 and 1.5 were 2.5-3 percent.

Figures 15 and 16 illustrate the effect of air mass on the transmittance of

polycarbonate and fiber reinforced plastic, respectively. Poly(vinyl fluoride)

transmittance values showed a further trend of interest in that, as the time

of exposure to heat aging increased, the difference between the transmittance

calculated using various air masses tended to increase. This is illustrated

in figure 17.

For weathered materials, decreases in transmittance with decreasing air mass values are primarily due to the ultraviolet region of the spectra. The transmittance spectra of many plastic cover plate materials falls off sharply at

350 nm or above (see figures 4 and 5). As these materials age, the wavelength

at which they stop transmitting energy becomes longer. This is illustrated in

figure 18, which contains the spectral transmittance of aged poly(vinyl

fluoride). With increased aging, the absorption band at the left of graph

moves toward the right to the longer wavelengths. When solar transmittance is

calculated, the resultant value is lower as the air mass decreases, due to

increasing solar energy at the lower wavelengths of the solar spectrum.

4. 2. 2. 3 Summary

The data show that comparable values of transmittances are obtained using the

selected ordinate method at air mass 2.0, 1.5 and 1.0 for f luorinated(ethylene

propylene) copolymer. For glass the differences are about 0.5 percent between

air mass 2.0 and 1.0 while for poly(vinyl fluoride), polycarbonate and fiber

reinforced plastic differences of several percent may occur. The data further

show (figure 12) that the discrepancies tend to increase for poly(vinyl

fluoride) with increased time of heat aging.

10
5. CONCLUSIONS

Since method A of E 424 is the preferred method, it is expected that the

reflectance and transmittance data produced by its use should be equivalent.

The variations permitted in the calculations should not result in different

values of reflectance and transmittance. The following conclusions relate to

calculations of solar energy transmittance and reflectance using ASTM E 424,

method A.

5.1 ABSORBER COATING MATERIALS

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

yield comparable reflectance values.

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

calculated for nonselective absorbers.

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

for selective absorbers. Black chrome reflectance values changed by up

to 1.2 percent when the air mass changed from 2.0 to 1.0.

5.2 COVER PLATE MATERIALS

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

their near infrared (NIR) region of their spectra. When differences occur

the weighted ordinates 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 transmittance 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 the air mass change are those

which transmit the least energy in the ultraviolet region (below 390 nm).

11
6 . RECOMMENDATIONS

1 . One integration method should be specified for the calculation of

transmittance and reflectance using ASTM E 424, method A. The selected

and weighted ordinate methods of integration do not produce identical

results for material having absorption hands in the near infrared region

of the spectra. Since optical properties are heavily used in technical

evaluation of competing products, it is necessary to assure that data for

various materials can be compared. Thus a single calculation method is

required.

2. It is suggested that the solar energy distribution used in ASTM E 424 , method A , be changed to air mass 1.5 . The reason for this recommendation

is that most collectors that are in service are exposed to more hours of

radiation that approximate air mass 1.5 or 48.19° than air mass 2.0 or 60°

zenith angle.

12
7. REFERENCES

[1] ASTM E 424-71, Standard Methods of Test for Solar Energy Transmittance

and Reflectance (Terrestrial) of Sheet Materials, Annual Book of Standards, Part 41, American Society for Testing and Materials, 1916 Race

Street, Philadelphia, PA 19103,

[2] 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," National Bureau of Standards Technical Note 1132, (1980),

Available from Superintendent of Documents, U,S, Government Printing

Office, Washington, D,C, 20402,

[3] Masters, L, W,, Seller, J, F,, Embree, E, J,, and Roberts, W, E,, "Solar

Energy Systems - Standards for Absorber Materials," National Bureau of

Standards Report, NBSIR 81-2232, (1981),

[4] Duffie, J, A,, and Beckman, W, A,, Solar Energy Thermal Processes, New

York, John Wiley and Sons, (1974), p, 40,

[51 Olson, 0, H,, "Selected Ordinates for Solar Absorptivity Calculations,"

Applied Optics, Vol, 2, No, 1, January 1963,

[61 Lind, M, A,, Pettit, R, B, , Masterson, K, D,, "The Sensitivity of Solar

Transmittance, Reflectance and Absorptance to Selected Averaging Procedures

and Solar Irradiance Distributions," Journal of Solar Energy Engineering,

Vol, 102, No, 35, February 1980,

[71 Mecherikunnel, A, T, , Richmond, J, C,, "Solar Spectral Irradiance at

Ground Level," Institute of Environmental Sciences, Seminar on Testing

Solar Energy Materials and Systems, May 1978,

13
Table 1. Absorber and Cover Plate Materials Used in This Study

Absorber Materials Substrate

Black chrome Steel

Black chrome Copper

Urethane paint Steel

Porcelain enamel Steel

Cover Plate Materials

Nominal

mm Thickness

(in)

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

Fluorinated (ethylene propylene) copolymer 0.025 (0.001)

Poly(vinyl fluoride) 0.10 (0.004)

Polycarbonate 1.02 (0.04)

Fiber reinforced plastic 1.05 (0.06)

14
Table 2, Relative Energies of Weighted Ordinates for Calculation of

Solar Energy Transmittance or Reflectance at Air Mass 2.0

IWavelength

1 nm

r Relative I Energy I

1

Wavelength

nm

1 Relative I Energy I

1

Wavelength

nm Relative

Energy

1 350

I 1.27 1 9501 3.29 1 1550 1.49

1 400 3.18 1 1000 4.25 1 1600 1.36

1 450 6.79 1 1050 3.72 1 1650 1.17

1 500 8.20 1 1100 1.70 1 1700 0.89

1 550 8.03 I 1150 1.46 1 1750 0.54

1 600 7.88 1 1200 2.52 1 1800 0.01

1 650 7.92 1 1250 2.21 1 1850 0.00

1 700 7.48 1 1300 1.78 1 1900 0.00

1 750 5.85 1 1350 0.12 I 1950 0.12

I 800 5.79 1 1400 0.00 I 2000 0.02

1 850 5.66 1 1450 0.16 I 2050 0.26

1 900 3.24 1

1 1500 1.06 11 2100 0.58

1

1

1

1

1

1 TOTAL 100.00

Table 3. Twenty Selected Ordinates for Calculation of

Solar Transmittance or Reflectance at Air Mass 2.0

Ordinate

Number

Wavelength

nm

1

1 Ordinate

1 Number

1

Wavelength

nm

1 390

1

1 11 745

2 444 1 12 786

3 481 1 13 831

4 511 1 14 877

5 543 1 15 959

6 574 1 16 1026

7 606 1 17 1105

8 639 1 18 1228

9 669 1 19 1497

10 705 1 20

1

1722

15
Table 4. Fifty Selected Ordinates for Calculation of Solar Transmittance

or Reflectance at Air Mass 1.5

Ordinate

Number

Wavelength

nm OrdinateNumber

Wavelength

nm

1 342 26 717

2 375 27 736

3 400 28 753

4 418 29 773

5 434 30 792

6 449 31 812

7 461 32 834

8 472 33 855

9 482 34 876

10 494 35 901

11 505 36 933

12 517 37 975

13 529 38 1003

14 541 39 1029

15 553 40 1057

16 567 41 1091

17 581 42 1169

18 595 43 1215

19 610 44 1261

20 624 45 1317

21 638 46 1528

22 653 47 1606

23 668 48 1693

24 684 49 2023

25 700 50 2276

16
Table 5. Fifty Selected Ordinates for Calculation of Solar

Transmittance or Reflectance at Air Mass 1.0

Ordinate

Number

Wavelength

nm OrdinateNumber

Wavelength

nm

1 331 26 689

2 356 27 713

3 380 28 731

4 401 29 752

5 415 30 770

6 429 31 785

7 443 32 814

8 455 33 834

9 465 34 849

10 477 35 871

11 488 36 902

12 500 37 938

13 512 38 975

14 524 39 987

15 537 40 1025

16 545 41 1056

17 558 42 1117

18 572 43 1173

19 587 44 1221

20 601 45 1287

21 615 46 1495

22 629 47 1587

23 644 48 1703

24 659 49 2055

25 675 50 2298

17
Table 6. Tabulation of Reflectance Data for Absorber Materials

Percent Reflectance

Material

Exposure-^' Test; Length

Weighted Ordinate

at Air Mass 2.0

Selected

Air Mass

at2.0

Selected at Air Mass 1.5Selected at Air Mass 1.0

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 Moisture 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 the weathering test carried out prior to obtaining optical measurements.

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

18
Table 7. Tabulation of Transmittance Data for Cover Plate Materials

Material

Exposure

Test

Time of ExposureWeighted 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 Fluorlnated

(ethylene

propylene)

copolymer Control 0 96.4 96.5 96.3 96.4 Poly(vlnyl

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 74.7 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 87.1 86.9 86.5

1250 86.6 86.7 86.6 86.1

1500 87.5 87.6 87.3 87.0

1825 87.3 87.4 87.2 86.9

Fluorlnated 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(vlnyl 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

19
Air

mass

=

1.0

(Sun

directly

overhead

at

solar

noon)

Figure 1 Spectral distribution of sunlight at air masses 1, 1.5 and 2.0

WAVELENGTH

|NM|
m

CO

om

CO CM

omo

CN

^ ^ mo

(lN33d3d) 33NV1331J3d

Figure 2. Spectral reflectance of nonselective absorber materials,

urethane on steel and porcelain enamel on steel

21

WAVELENGTH

(NM)
Black

chrome

on

steel

Black

chrome

on

copper

(iN33d3d) 33NV1331J3B

22

are 3. Spectral reflectance of selective absorber materials, black chrome on steel and black chrome on copper

WAVELENGTH

|NM)
^ S ITS § CO CM

(lN33ti3dl BONVniNSNVdl

Lire 4 . Spectral transmittance of three cover plate materials, fiber reinforced

fluorinated (ethylene propylene) copolymer and glass

23
(lN33H3d)33NVILINSNVdl

PLgure 5. Spectral transmittance of two cover plate materials, polycarbonate and

poly( vinyl fluoride)

24

WAVELENGTH,

NM
PERCENT

REFLECTANCE

7.0

6.0 -

5.0 -

(I

4.0 -

3.0 L

0

Figure 6.

o Weighted ordinate at air mass 2.0

Seiected ordinate at air mass 2.0

[]

()

2 4 6 8 10 12

WEEKS EXPOSURE

Comparison of reflectance of weathered black chrome on steel

using the weighted and selected ordinate calculation methods at

air mass 2.0; weathering environment was 92°C and 97 percent

relative humidity

25
7

Unaged

0 —mm— mm — 2.0 2.0 1.5

WEIGHTED SELECTED SELECTED

AIR MASS

Figure 7. Effect of method of calculation and air mass on weathered and unweathered black chrome on steel

1.0

SELECTED

reflectance of

26
9

8

7

6

5

4

3

2

1

0

Unaged

Aged in 200° oven for 12 weeks

2.0

WEIGHTED

2.0

SELECTED

1.5

SELECTED

1.0

SELECTED

AIR MASS

igure 8. Effect of method of calculation and air mass on weathered and unweathered black chrome on copperreflectance

27
PERCENT

REFLECTANCE

7.0

[]

6. 0 ^^

5.0 -

()

3.0 L

0

Figure 9.

o Selected ordinate method at air mass 2.0

Selected ordinate method at air mass 1.5

A Selected ordinate method at air mass 1.0

A

A

O

O

<)

2 4 6 8 10 12

WEEKS

Effect of air mass on the reflectance of black chrome on steel

following exposure at 92°C and 97 percent relative humidity

28
5.0 T

A

O

2.0

o Selected ordinate at air mass 2.0

Selected ordinate at air mass 1.5

A Selected ordinate at air mass 1.0

0 2 4 6 8

WEEKS

10 12

Figure 10. Effect of air mass on the reflectance of urethane on steel following

exposure at 92°C and 97 percent relative humidity

29
PERCENT

TRANSMIHANCE

TME |HRS.|

Figure 11. Effect of method of calculation at air mass 2.0 on transmittance

of poly(vinyl fluoride) weathered by heat aging at 150°C

30
PERCENT

TRANSMITTANCE

Figure 12. Effect of method of calculation at air mass 2.0 on transmittance

of polycarbonate weathered by heat aging at 150°C

31
PERCENT

TRANSMIHANCE

Figure 13. Effect of air mass on transmittance of fluorinated ethylene

propylene) copolymer weathered by heat aging at 150 C

32
PERCENT

TRANSMIHANCE

0 333 667 1000 1333 1667 2000

TIME (HRS.)

r igure 14. Effect of air mass on transmittance of glass weathered by heat

aging at 150°C

33
PERCENT

TRANSMIHANCE

Figure 15. Effect of air mass on the transmittance of polycarbonate weathered

by heat aging at 150°C

34
PERCENT

TRANSMIHANCE

90.0 r

Figure 16. Effect of air mass on the transmittance of fiber reinforced plastic

weathered by heat aging at 150°C

35
PERCENT

TRANSMITTANCE

Figure 17. Effect of air mass and increased time of heat age on the

transmittance of poly(vinyl fluoride)

36
1200

1400

1600

1800

2000

2200

WAVElfNGTH

(nm]

PERCENT TRANSMIHANCE

Figure 18. Spectral transmittance of poly(vinyl fluoride) exposed to 150°C

fo ' four time periods

37

100.0 h

Control

- no

heat

exposure
U.S. Dt£PT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3« Publication Date

SHEET (See instructions) NBSIR 81-2448 January 1982

4. TITLE AND SUBTITLE

Effects of Air Mass and Integration Methods on Optical Property Measurements of

Solar Cover Plate and Absorber Materials.

5. AUTHOR(S)

Willard E. Roberts, Larry W. Masters, and Elizabeth J. Clark

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

Dept, of Energy

Office of Solar Applications for Buildings

Office of the Assistant Secretary

Conservation and Solar Applications, Washington, D.C. 20585

10. SUPPLEMENTARY NOTES

Q21 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 iiterature survey, mention it here)

This study was undertaken to compare methods of calculating the transmittance of cover plate materials and the reflectance of absorber materials. Optical data were obtained

for both aged and unaged test specimens using an integrating sphere spectrophotometer.

The data were integrated using: (1) the weighted and selected ordinate methods in

ASTM E 424, Method A, at air mass 2.0, and (2) the selected ordinate method at air mass 1.5 and 1.0. The Solar reflectance and solar transmittance values calculated

using the various methods are presented in this report along with discussions of the

impact of the data in terms of possible revisions to ASTM E 424.

12. KEY WORDS (Six to twelve entries; aiphabetical order; capitalize oniy proper names; and separate key words by semicolon s) air mass;

ordinate;

ASTM E 424; integrating sphere spectrophotometer; reflectance; selected

solar absorber materials; solar cover plates; transmittance; weighted ordinat(

13. AVAILABILITY 14. NO. OF

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