ASTM E881-92(2003)
(Practice)Standard Practice for Exposure of Solar Collector Cover Materials to Natural Weathering Under Conditions Simulating Stagnation Mode
Standard Practice for Exposure of Solar Collector Cover Materials to Natural Weathering Under Conditions Simulating Stagnation Mode
SIGNIFICANCE AND USE
This practice describes a weathering box test fixture and establishes limits for the heat loss coefficients. Uniform exposure guidelines are provided to minimize the variables encountered during outdoor exposure testing.
Since the combination of elevated temperature and solar radiation may cause some solar collector cover materials to degrade more rapidly than either exposure alone, a weathering box that elevates the temperature of the cover materials is used.
This practice may be used to assist in the evaluation of solar collector cover materials in the stagnation mode. No single temperature or procedure can duplicate the range of temperatures and environmental conditions to which cover materials may be exposed during stagnation conditions. To assist in evaluation of solar collector cover materials in the operational mode, Practice E 782 should be used. Insufficient data exist to obtain exact correlation between the behavior of materials exposed in accordance with this practice and actual in-service performance.
This practice may also be useful in comparing the performance of different materials at one site or the performance of the same material at different sites, or both.
Means of evaluating the effects of weathering are provided in Practice E 765, and in other ASTM test methods that evaluate material properties.
Exposures of the type described in this practice may be used to evaluate the stability of solar collector cover materials when exposed outdoors to the varied influences that comprise weather. Exposure conditions are complex and changeable. Important factors are material temperature, climate, time of year, presence of industrial pollution, etc. Generally, because it is difficult to define or measure precisely the factors influencing degradation due to weathering, results of outdoor exposure tests must be taken as indicative only. Repeated exposure testing at different seasons over a period of more than one year is required to confirm exposure t...
SCOPE
1.1 This practice covers a procedure for the exposure of solar collector cover materials to the natural weather environment at elevated temperatures that approximate stagnation conditions in solar collectors having a combined back and edge loss coefficient of less than 1.5 W/(m2 C).
1.2 This practice is suitable for exposure of both glass and plastic solar collector cover materials. Provisions are made for exposure of single and double cover assemblies to accommodate the need for exposure of both inner and outer solar collector cover materials.
1.3 This practice does not apply to cover materials for evacuated collectors, photovoltaic cells, flat-plate collectors having a combined back and edge loss coefficient greater than 1.5 W/(m2 C), or flat-plate collectors whose design incorporates means for limiting temperatures during stagnation.
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.
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Designation: E 881 – 92 (Reapproved 2003)
Standard Practice for
Exposure of Solar Collector Cover Materials to Natural
Weathering Under Conditions Simulating Stagnation Mode
This standard is issued under the fixed designation E881; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope FederalSpecificationHH-I-558B, Amendment3,Insulation
Blocks,Boards,Felts,Sleeving(PipeandTubeCovering),
1.1 This practice covers a procedure for the exposure of
and Pipe Fitting CoveringThermal (Mineral Fiber, Indus-
solar collector cover materials to the natural weather environ-
trial Type) August 1976
ment at elevated temperatures that approximate stagnation
conditionsinsolarcollectorshavingacombinedbackandedge
3. Terminology
loss coefficient of less than 1.5 W/(m · °C).
3.1 Definitions—For definitions of terms used in this
1.2 This practice is suitable for exposure of both glass and
practice, refer to Terminology E772.
plastic solar collector cover materials. Provisions are made for
exposure of single and double cover assemblies to accommo-
4. Significance and Use
date the need for exposure of both inner and outer solar
4.1 Thispracticedescribesaweatheringboxtestfixtureand
collector cover materials.
establishes limits for the heat loss coefficients. Uniform expo-
1.3 This practice does not apply to cover materials for
sureguidelinesareprovidedtominimizethevariablesencoun-
evacuated collectors, photovoltaic cells, flat-plate collectors
tered during outdoor exposure testing.
having a combined back and edge loss coefficient greater than
4.2 Sincethecombinationofelevatedtemperatureandsolar
1.5 W/(m ·° C), or flat-plate collectors whose design incorpo-
radiation may cause some solar collector cover materials to
rates means for limiting temperatures during stagnation.
degrade more rapidly than either exposure alone, a weathering
1.4 This standard does not purport to address all of the
boxthatelevatesthetemperatureofthecovermaterialsisused.
safety concerns, if any, associated with its use. It is the
4.3 This practice may be used to assist in the evaluation of
responsibility of the user of this standard to establish appro-
solar collector cover materials in the stagnation mode. No
priate safety and health practices and determine the applica-
single temperature or procedure can duplicate the range of
bility of regulatory limitations prior to use.
temperatures and environmental conditions to which cover
materials may be exposed during stagnation conditions. To
2. Referenced Documents
assist in evaluation of solar collector cover materials in the
2.1 ASTM Standards:
2 operational mode, Practice E782 should be used. Insufficient
D1435 Practice for Outdoor Weathering of Plastics
data exist to obtain exact correlation between the behavior of
E765 Practice for Evaluation of Cover Materials for Flat
3 materials exposed in accordance with this practice and actual
Plate Solar Collectors
3 in-service performance.
E772 Terminology Relating to Solar Energy Conversion
4.4 This practice may also be useful in comparing the
E782 Practice for Exposure of Cover Materials for Solar
performance of different materials at one site or the perfor-
Collectors to Natural Weathering Under Conditions Simu-
3 mance of the same material at different sites, or both.
lating Operational Mode
4.5 Means of evaluating the effects of weathering are
G7 Practice for Atmospheric Environmental Exposure
4 provided in Practice E765, and in other ASTM test methods
Testing of Nonmetallic Materials
that evaluate material properties.
2.2 Other Documents:
4.6 Exposures of the type described in this practice may be
used to evaluate the stability of solar collector cover materials
when exposed outdoors to the varied influences that comprise
These test methods are under the jurisdiction of ASTM Committee E44 on
weather. Exposure conditions are complex and changeable.
Solar, Geothermal, and Other Alternative Energy Sources and is the direct
responsibility of Subcommittee E44.05 on Solar Heating and Cooling Subsystems Important factors are material temperature, climate, time of
and Systems.
year,presenceofindustrialpollution,etc.Generally,becauseit
Current edition approved Sept. 15, 1992. Published March 1993. Originally
approved in 1982. Last previous edition approved in 1996 as E881-92(1996).
Annual Book of ASTM Standards, Vol 08.01.
3 5
Annual Book of ASTM Standards, Vol 12.02. Available from the General Services Administration (3 FRI), Building 197,
Annual Book of ASTM Standards, Vol 14.02. Navy Yard Annex, Washington, DC 20407.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 881 – 92 (2003)
NOTE 2—It is desirable that the box and box top be made of a material
isdifficulttodefineormeasurepreciselythefactorsinfluencing
that will be unaffected by the exposure environment.Ametal resistant to
degradation due to weathering, results of outdoor exposure
corrosion encountered in the environment would be suitable. If wood is
tests must be taken as indicative only. Repeated exposure
used, it should be painted or treated on the exterior to make it resistant to
testingatdifferentseasonsoveraperiodofmorethanoneyear
moisture. In certain climates only rot-resistant wood should be used to
is required to confirm exposure tests at any one location.
minimize deterioration during exposure.
Control samples must always be used in weathering tests for
5.2.2 The insulation shall be a material suitable for use at a
comparative analysis.
high temperature (for example, 150°C (302°F)).
5. Weathering Box Test Fixture
NOTE 3—Insulation materials having resins or binders should not be
5.1 Test Fixture Requirements:
used because elevated temperatures may cause the resin or binder to
5.1.1 The weathering box test fixture shall be constructed deteriorate and outgas. Outgassing products condense on the cover
material causing changes in the solar transmittance of the solar collector
such that the combined back and edge loss coefficient is less
2 2
cover material.
than 1.5 W/(m · °C) (0.264 Btu/(ft · h ·° F)) (Note 1). (The
methodfordeterminingthiscoefficientisoutlinedinAppendix
5.2.3 The absorber shall be of an adequate size to cover the
X1ofthispractice.)Thedistancebetweentheabsorberandthe
interior surface of the weathering box aperture. The absorber
closestcoverplateshallbebetween13and38mm(0.5and1.5
shall have a flat black nonselective coating having an absorp-
in.). For a double-cover exposure the separation between the
tance not less than 0.90 after exposure.
inner and outer cover shall be between 13 and 38 mm (0.5 and
5.2.4 Theboxtopshallbeofanadequatesizetofitoverthe
1.5in.).Notmorethan10%oftheabsorberplateareashallbe
box.
shadedwhenthesunisata30°anglewiththeplaneofthefront
NOTE 4—The box top is intended to protect the edges of the test
surface of the exposure box.
specimen in contact with the box from reaching excessively high
NOTE 1—A good flat-plate solar collector has a combined back and
temperatures, to minimize exposure of the adhesive tape to sunlight, and
2 2
edge loss coefficient of less than about 1.5 W/(m · °C) (0.264 Btu/(ft ·
to minimize moisture penetration into the exposure test fixture.
h·°F).
5.2.5 The glazing frame is intended to hold the cover plate
5.1.2 Boxes that meet the requirements of 5.1.1 are de-
material. The glazing frame shall have dimensions similar to
scribed in Table 1. Fig. 1 and Fig. 2 illustrate the weathering
the perimeter of the box. For a double-cover exposure the
box test fixtures. Although Fig. 1 shows a square box, any
frame shall provide a separation between the two cover plates
shape is permitted if the requirements in 5.1.1 are met.
ofnotlessthan13mm(0.5in.)orgreaterthan38mm(1.5in.).
Appendix X1 of this practice describes the method for deter-
Exact dimensions of the frame are related to the requirements
mining the combined back and edge loss coefficient.
in 5.1.1. A vent hole may be drilled at one end of the glazing
5.2 Contents of the Weathering Box Test Fixture: (1) a box,
frame to provide drainage and to minimize moisture accumu-
( 2) insulation, (3) absorber, ( 4) box top, (5) spacer, ( 6)
lation.
glazing frame, and (7) adhesive tapes.
5.2.1 Theboxmayhaveanydimensionsandbemadeofany
material as long as the requirements in 5.1.1 are met. A weep
hole shall be drilled at the lower end of the bottom of the box
Federal Specification HH-I-558B has several classes of insulation material
to provide drainage and to minimize moisture accumulation. intended for high-temperature use.
TABLE 1 Examples of Weathering Box Test Fixtures with Combined Heat Loss Coefficient for Back and Edge Losses Less than
2 2
1.5 W/(m ·°C) (0.264 Btu/(ft ·h·°F))
Example 1 Example 2
Box material steel aluminum
Insulation material glass fiber glass fiber
l, length of aperture inside edge insulation 0.25 m (9.8 in.) 0.61 m (24 in.)
w, width of aperture inside edge insulation 0.13 m (5.2 in.) 0.61 m (24 in.)
h, distance from top of absorber to bottom of cover 0.013 m (0.5 in.) 0.038 m (1.5 in.)
plate
2 2 2 2
A , area of aperture of test fixture A =(l 3 w) 0.033 m (51 in. ) 0.372 m (576 in. )
a a
2 2 2 2
A , area of back insulation A =(l 3 w) 0.033 m (51 in. ) 0.372 m (576 in. )
b b
2 2 2 2
A , area of edge insulation A =2(l + w)h 0.01 m (15 in. ) 0.093 m (144 in. )
e e
d , thickness of back insulation 0.077 m (3 in.) 0.05 m (2 in.)
b
d , thickness of box 0.001 m (0.04 in.) 0.002 m (0.08 in.)
c
d , thickness of edge insulation 0.013 m (0.5 in.) 0.025 m (1 in.)
e
2 2
K , conductivity of back insulation 0.038 W/(m·°C) (0.22 Btu/(ft ·h·°F)) 0.038 W/(m·°C) (0.022 Btu/(ft ·h·°F))
b
2 2
K , conductivity of box 43 W/(m·°C) (24.9 Btu/(ft ·h·°F) 204 W/(m·°C) (118 Btu/(ft ·h·°F))
c
2 2
K , conductivity of edge insulation 0.038 W/(m·°C) (0.022 Btu/(ft ·h·°F)) 0.038 W/(m·°C) (0.022 Btu/(ft ·h·°F))
e
A /A 11
b a
A /A 0.305 0.25
e a
2 2 2 2
d /K 2.03 m ·°C/W (11.4 (ft ·h·°F)/Btu) 1.32 m ·°C/W (7.5 (ft ·h·°F)/Btu)
b b
−5 2 −4 2 −6 2 −5 2
d /K 2.33 3 10 m ·°C/W (1.32 3 10 (ft ·h·°F)/Btu) 9.8 3 10 m ·°C/W (5.6 3 10 (ft ·h·°F)/Btu)
c c
2 2 2 2
d /K 0.342 m ·°C/W (1.94 (ft ·h·°F)/Btu) 0.658 m ·°C/W (3.74 (ft ·h·°F)/Btu)
e e
2 2 2 2
U , back + U , edge 1.38 W/(m ·°C) (0.243 Btu/(ft ·h·°F)) 1.14 W/(m ·°C) (0.201 Btu/(ft ·h·°F))
L L
E 881 – 92 (2003)
FIG. 1 Top View of Weathering Box Test Fixture
FIG. 2 Assembled Weathering Box Test Fixture
5.2.6 The spacer shall provide a separation of 13 to 38 mm
(0.5to1.5in.)betweentheabsorberandtheclosestcoverplate.
Exact dimensions of the spacer are related to the requirements
in 5.1.1.
NOTE 5—Certain designs of weathering boxes may eliminate the need
for the spacer.
E 881 – 92 (2003)
5.2.7 The adhesive tapes shall be stable when exposed to such as those described in Practice D1435. The racks shall be
moisture and elevated temperatures. They shall be compatible capable of having the angles adjusted and have their axis of
with the specific materials from which the box, glazing frame, rotation on an east-west line.
box top, and cover plate are made. 6.2 Use a variable angle exposure to maximize solar radia-
5.2.8 Organic materials are potential sources of outgassing tion incident upon the weathering box. Adjust the racks
andshallbeeliminatedfromtheinterioroftheweatheringbox accordingtotheschedulegiveninTable2.Positiverackangles
where possible. For example, metallic parts shall be cleaned to face south. Choose the angles so that the weathering boxes are
remove traces of grease or other foreign matter. Other possible never closer to the horizontal than by 5°. Other variable
sources of outgassing include coatings and sealants. Test exposure schedules requiring more than four adjustments per
fixture components containing organic materials (for example, year may be used. The method for determining the variable
absorber coatings or insulation) shall be heated in an oven at angle exposure schedule is described in Appendix X2 of this
150°C (302°F) for 24 h before the test fixture is assembled. practice.
Thisshouldminimizeoutgassingthatresultsfromdeterioration 6.3 When a number of weathering boxes are exposed
of the organic components exposed to elevated temperatures. simultaneously,mounttheboxessidebysidewiththesidesnot
5.3 Test Specimen: touching.
5.3.1 Thetestspecimenshallbeofanadequatesizetocover 6.4 Do not clean the solar collector cover materials during
the aperture of the box or glazing frame and to permit suitable exposure.
attachment. 6.5 Visually inspect the test specimens at intervals of not
more than one month. Record all changes in appearance.
NOTE 6—Adequate allowances should be made for materials that will
undergo dimensional changes due to temperature.
7. Report
5.3.2 The test specimen identification marks shall not inter-
7.1 The report shall include the following:
fere with either the exposure or the subsequent testing.
7.1.1 Description of the weathering box test fixture and its
5.4 Sample Mounting:
calculated combined back and edge loss coefficient,
5.4.1 Rigid and Semirigid Glazings:
7.1.2 Whether the solar collector cover materials are ex-
5.4.1.1 Lay the test specimen for single cover exposure
posed as a single- or double-cover configuration and whether
directly on either the spacer or the glazing frames. If used, the
the test specimen was the inner or outer cover,
frame is then placed on the spacer in the weathering box (see
7.1.3 Complete identification of the solar collector outer
Fig. 2).
cover material(s),
5.4.1.2 Lay the test specimen for inner cover exposure of a
7.1.4 Complete identification of the solar collector inner
double cover assembly on the spacer or attach it to the glazing
cover material(s) (if any),
framebeforetheglazingframeisplacedinthebox(seeFig.2).
7.1.5 A description of the test specimen attachment and
5.4.1.3 Lay the test specimen for outer cover exposure of a
mounting procedures,
doublecoverassemblyonthetopoftheglazingframe(seeFig.
7.1.6 Latitude,longitude,altitude,andaddressofthetesting
2).
site including a description of the type of climate,
5.4.2 Films—Placefilmtestspecimensontheglazingframe
NOTE 7—Types of climate are described in Practice G7.
usingadhesivetransfertapetoholdthetestspecimentaut.Itis
essential that uniform tensioning be obtained prior to applying 7.1.7 Exposure data:
7.1.7.1 Calendar dates of exposure and
the tape. Then place the frame in the box similar to 5.4.1.1,
5.4.1.2, and 5.4.1.
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