ASTM E781-86(1996)

Standard Practice for Evaluating Absorptive Solar Receiver Materials When Exposed to Conditions Simulating Stagnation in Solar Collectors With Cover Plates

SCOPE
1.1 This practice covers a test procedure for evaluating absorptive solar receiver materials and coatings when exposed to sunlight under cover plate(s) for long durations. This practice is intended to evaluate the exposure resistance of absorber materials and coatings used in flat-plate collectors where maximum nonoperational stagnation temperatures will be approximately 200°C (392°F).  
1.2 This practice shall not apply to receiver materials used in solar collectors without covers (unglazed) or in evacuated collectors, that is, those that use a vacuum to suppress convective and conductive thermal losses.  
1.3 The values stated in SI units are to be regarded as the standard.  
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Historical
• Publication Date: 31-Dec-1995
• Technical Committee: E44 - Solar, Geothermal and Other Alternative Energy Sources

Relations

• Replaced By: ASTM E781-86(2003) - Standard Practice for Evaluating Absorptive Solar Receiver Materials When Exposed to Conditions Simulating Stagnation in Solar Collectors With Cover Plates
• Effective Date: 09-Jun-1986
• Referred By: ASTM E744-07(2022) - Standard Practice for Evaluating Solar Absorptive Materials for Thermal Applications
• Effective Date: 01-Jan-1996
• Referred By: ASTM E782-95(2022) - Standard Practice for Exposure of Cover Materials for Solar Collectors to Natural Weathering Under Conditions Simulating Operational Mode
• Effective Date: 01-Jan-1996

Standards Content (Sample)

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 781 – 86 (Reapproved 1996)
Standard Practice for
Evaluating Absorptive Solar Receiver Materials When
Exposed to Conditions Simulating Stagnation in Solar
Collectors With Cover Plates
This standard is issued under the fixed designation E 781; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 962 Practice for Cleaning Cover Materials for Flat Plate
Solar Collectors
1.1 This practice covers a test procedure for evaluating
absorptive solar receiver materials and coatings when exposed
3. Terminology
to sunlight under cover plate(s) for long durations. This
3.1 Definitions—See Terminology E 772 for definitions.
practice is intended to evaluate the exposure resistance of
absorber materials and coatings used in flat-plate collectors
4. Significance and Use
where maximum nonoperational stagnation temperatures will
4.1 Although this practice is intended for evaluating solar
be approximately 200°C (392°F).
absorber materials and coatings used in flat-plate collectors, no
1.2 This practice shall not apply to receiver materials used
single procedure can duplicate the wide range of temperatures
in solar collectors without covers (unglazed) or in evacuated
and environmental conditions to which these materials may be
collectors, that is, those that use a vacuum to suppress
exposed during in-service conditions.
convective and conductive thermal losses.
4.2 This practice is intended as a screening test for absorber
1.3 The values stated in SI units are to be regarded as the
materials and coatings. All conditions are chosen to be repre-
standard.
sentative of those encountered in solar collectors with single
1.4 This standard does not purport to address all of the
cover plates and with no added means of limiting the tempera-
safety concerns, if any, associated with its use. It is the
ture during stagnation conditions.
responsibility of the user of this standard to establish appro-
4.3 This practice uses exposure in a simulated collector with
priate safety and health practices and determine the applica-
a single cover plate. Although collectors with additional cover
bility of regulatory limitations prior to use.
plates will produce higher temperatures at stagnation, this
procedure is considered to provide adequate thermal testing for
2. Referenced Documents
most applications.
2.1 ASTM Standards:
B 537 Practice for Rating of Electroplated Panels Subjected
NOTE 1—Mathematical modelling has shown that a selective absorber,
to Atmospheric Exposure single glazed flat-plate solar collector can attain absorber plate stagnation
temperatures as high as 226°C (437°F) with an ambient temperature of
D 1898 Practice for Sampling of Plastics
37.8°C (100°F) and zero wind velocity; and a double glazed one as high
E 408 Test Methods for Total Normal Emittance of Surfaces
as 245°C (482°F) under these conditions. The same configuration solar
Using Inspection-Meter Techniques
collector with a nonselective absorber can attain absorber stagnation
E 434 Test Method for Calorimetric Determination of
temperatures as high as 146°C (284°F), if single glazed, and 185°C
Hemispherical Emittance and the Ratio of Solar Absorp-
(360°F), if double glazed, with the same environmental conditions, (see
tance to Hemispherical Emittance Using Solar Simulation
“Performance Criteria for Solar Heating and Cooling Systems in Com-
5 6
E 772 Terminology Relating to Solar Energy Conversion mercial Buildings,” NBS Technical Note 1187 ).
E 903 Test Method for Solar Absorptance, Reflectance, and
4.4 This practice evaluates the thermal stability of absorber
Transmittance of Materials Using Integrating Spheres
materials. It does not evaluate the moisture stability of absorber
materials used in actual solar collectors exposed outdoors.
These test methods are under the jurisdiction of ASTM Committee E44 on
Moisture intrusion into solar collectors is a frequent occurrence
Solar, Geothermal, and Other Alternative Energy Sources and is the direct
in addition to condensation caused by diurnal breathing.
responsibility of Subcommittee E44.05 on Solar Heating and Cooling Subsystems
4.5 This practice differentiates between the testing of spec-
and Systems.
Current edition approved June 9, 1986. Published August 1986. Originally
trally selective absorbers and nonselective absorbers.
published as E 781 – 81. Last previous edition E 781 – 81.
Annual Book of ASTM Standards, Vol 02.05.
Annual Book of ASTM Standards, Vol 08.01.
4 6
Annual Book of ASTM Standards, Vol 15.03. Available from Superintendent of Documents, U.S. Government Printing
Annual Book of ASTM Standards, Vol 12.02. Office, Washington, DC 20402.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 781
4.5.1 Testing Spectrally Selective Absorber Coatings and indicated percentage of their initial values in the following
Materials—Spectrally selective solar absorptive coatings and wavelength regions:
materials require testing in a covered enclosure that contains a 300 to 400 nm − 90 %
selectively coated sample mounting plate, such that the enclo- 400 to 2100 nm − 95 %
sure and mounting plate simulate the temperature conditions of
5.2.3 An easily removable test patch of the cover material
a selective flat-plate collector exposed under stagnation condi-
measuring 50 by 50 mm (2 by 2 in.) shall be fastened onto the
tions.
inner surface of the transparent cover plate in or near one lower
4.5.2 Testing Nonselective Coatings and Materials—
corner. By periodically measuring the transmittance of this test
Spectrally nonselective solar absorptive coatings and materials
patch, an indication of the effect of any condensable effluents
require testing in a covered enclosure that contains a nonse-
on the cover material can be monitored.
lective coated sample mounting plate, such that the enclosure
5.2.4 An easily removable specimen of the cover plate
and mounting plate simulate the temperature conditions of a
material measuring 50 by 50 mm (2 by 2 in.) should also be
covered, nonselective flat-plate collector exposed under stag-
mounted directly on an exterior upper corner of the cover plate
nation conditions.
to monitor the effects of atmospheric contamination and
ultraviolet degradation. These effects are generally more severe
5. Test Apparatus
for plastic materials than for glass.
5.1 Test Enclosure (Fig. 1), consisting of a box that
5.3 Seals—A seal that does not outgas at the stagnation
approximates the dimensions of a typical flat-plate solar
temperature should be used to make the box weather-resistant.
collector and shall have minimum dimensions of 0.75 by 1.5 by
5.4 Insulation—The bottom and sides of the enclosure shall
0.1 m (29 by 60 by 4 in.) deep. The box should be constructed
be insulated to have a thermal conductance of less than 0.515
of materials that are impervious to moisture. Wood should not
2 2
W/(m ·K)(0.091 Btu/(h·ft ·°F)), that is, an R value of 11 or
be used for construction of the box. Care shall be taken to
greater with materials that do not outgas at the stagnation
prevent water leakage at joints, seams, and seals.
temperature.
5.1.1 Pre-Exposure of Test Box—Prior to use, the test
5.5 Sample Mounting Plate—A metallic mounting plate
apparatus shall be placed in an operational environment where
with lateral dimensions approximately the same as the internal
all components are allowed to equilibrate at the stagnation
enclosure dimensions (less the thickness of the insulation on
temperature for a sufficient length of time to allow for
the sides of the box) shall be mounted approximately 10 mm
outgassing of the components. This procedure may aid in
(0.4 in.) above the bottom insulation by a thermally insulating
eliminating contamination of the cover plate and the samples
material.
during actual testing periods and is especially important where
5.5.1 The mounting plate used to support selective speci-
coatings employing organic components are used. If the cover
mens shall have a selective surface. The solar absorptance (a)
plate is in place during this outgassing procedure, it shall be
of the selective coating shall be greater than 0.90, and the room
cleaned before the box is put into service in order to restore its
temperature emittance (e) shall be less than 0.15 at all times.
original transmittance.
5.5.2 The mounting plate used to support nonselective
5.2 Cover Plate— The box shall have a single cover plate
specimens shall be coated with any nonselective black coating
that is glazed and hinged to provide access.
that is thermally resistant to temperatures up to approximately
5.2.1 Two types of cover plate materials may be used:
200°C (392°F). The solar absorptance (a) of the coatings
5.2.1.1 Type I—Tempered low-iron glass with spectral char-
(solar-weighted average from 350 to 2500 nm) shall be greater
acteristics approximating those shown in Fig. 2.
than 0.90 at all times.
5.2.1.2 Type II—Alternative types of solar transmitting
glass or plastic materials might be used for the cover plate if 5.5.3 A specimen of the mounting plate material (50 by 50
the absorber is to be used under that material. mm (2 by 2 in.)) shall be securely fastened to and in contact
5.2.2 The solar-weighted transmittance values of
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