ASTM E781-1986(2009) 431 Standard Practice for Evaluating Absorptive Solar Receiver Materials When Exposed to Conditions Simulating Stagnation in Solar Collectors With Cover Plates.pdf

1、Designation: E 781 86 (Reapproved 2009)Standard Practice forEvaluating Absorptive Solar Receiver Materials WhenExposed to Conditions Simulating Stagnation in SolarCollectors With Cover Plates1This standard is issued under the fixed designation E 781; the number immediately following the designation

2、indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers a test procedure

3、for evaluatingabsorptive solar receiver materials and coatings when exposedto sunlight under cover plate(s) for long durations. Thispractice is intended to evaluate the exposure resistance ofabsorber materials and coatings used in flat-plate collectorswhere maximum nonoperational stagnation temperat

4、ures willbe approximately 200C (392F).1.2 This practice shall not apply to receiver materials usedin solar collectors without covers (unglazed) or in evacuatedcollectors, that is, those that use a vacuum to suppressconvective and conductive thermal losses.1.3 The values stated in SI units are to be

5、regarded as thestandard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prio

6、r to use.2. Referenced Documents2.1 ASTM Standards:2B 537 Practice for Rating of Electroplated Panels Subjectedto Atmospheric ExposureD 1898 Standard Practice for Sampling of Plastics3E 408 Test Methods for Total Normal Emittance of SurfacesUsing Inspection-Meter TechniquesE 434 Test Method for Calo

7、rimetric Determination ofHemispherical Emittance and the Ratio of Solar Absorp-tance to Hemispherical Emittance Using Solar SimulationE 772 Terminology Relating to Solar Energy ConversionE 903 Test Method for Solar Absorptance, Reflectance, andTransmittance of Materials Using Integrating Spheres3E 9

8、62 Practice for Cleaning Cover Materials for Flat PlateSolar Collectors33. Terminology3.1 Definitions:3.1.1 See Terminology E 772 for definitions.4. Significance and Use4.1 Although this practice is intended for evaluating solarabsorber materials and coatings used in flat-plate collectors, nosingle

9、procedure can duplicate the wide range of temperaturesand environmental conditions to which these materials may beexposed during in-service conditions.4.2 This practice is intended as a screening test for absorbermaterials and coatings. All conditions are chosen to be repre-sentative of those encoun

10、tered in solar collectors with singlecover plates and with no added means of limiting the tempera-ture during stagnation conditions.4.3 This practice uses exposure in a simulated collector witha single cover plate. Although collectors with additional coverplates will produce higher temperatures at s

11、tagnation, thisprocedure is considered to provide adequate thermal testing formost applications.NOTE 1Mathematical modelling has shown that a selective absorber,single glazed flat-plate solar collector can attain absorber plate stagnationtemperatures as high as 226C (437F) with an ambient temperatur

12、e of37.8C (100F) and zero wind velocity; and a double glazed one as highas 245C (482F) under these conditions. The same configuration solarcollector with a nonselective absorber can attain absorber stagnationtemperatures as high as 146C (284F), if single glazed, and 185C(360F), if double glazed, wit

13、h the same environmental conditions, (see“Performance Criteria for Solar Heating and Cooling Systems in Com-mercial Buildings,” NBS Technical Note 11874).4.4 This practice evaluates the thermal stability of absorbermaterials. It does not evaluate the moisture stability of absorber1This practice is u

14、nder the jurisdiction of ASTM Committee E44 on Solar,Geothermal and OtherAlternative Energy Sources and is the direct responsibility ofSubcommittee E44.05 on Solar Heating and Cooling Systems and Materials.Current edition approved April 1, 2009. Published June 2009. Originallyapproved in 1981. Last

15、previous edition approved in 2003 as E 78186(2003).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Withdrawn

16、. The last approved version of this historical standard is referencedon www.astm.org.4Available from U.S. Government Printing Office Superintendent of Documents,732 N. Capitol St., NW, Mail Stop: SDE, Washington, DC 20401, http:/www.access.gpo.gov.1Copyright ASTM International, 100 Barr Harbor Drive

17、, PO Box C700, West Conshohocken, PA 19428-2959, United States.materials used in actual solar collectors exposed outdoors.Moisture intrusion into solar collectors is a frequent occurrencein addition to condensation caused by diurnal breathing.4.5 This practice differentiates between the testing of s

18、pec-trally selective absorbers and nonselective absorbers.4.5.1 Testing Spectrally Selective Absorber Coatings andMaterialsSpectrally selective solar absorptive coatings andmaterials require testing in a covered enclosure that contains aselectively coated sample mounting plate, such that the enclo-s

19、ure and mounting plate simulate the temperature conditions ofa selective flat-plate collector exposed under stagnation condi-tions.4.5.2 Testing Nonselective Coatings and MaterialsSpectrally nonselective solar absorptive coatings and materialsrequire testing in a covered enclosure that contains a no

20、nse-lective coated sample mounting plate, such that the enclosureand mounting plate simulate the temperature conditions of acovered, nonselective flat-plate collector exposed under stag-nation conditions.5. Test Apparatus5.1 Test Enclosure(Fig. 1), consisting of a box thatapproximates the dimensions

21、 of a typical flat-plate solarcollector and shall have minimum dimensions of 0.75 by 1.5 by0.1 m (29 by 60 by 4 in.) deep. The box should be constructedof materials that are impervious to moisture. Wood should notbe used for construction of the box. Care shall be taken toprevent water leakage at joi

22、nts, seams, and seals.5.1.1 Pre-Exposure of Test BoxPrior to use, the testapparatus shall be placed in an operational environment whereall components are allowed to equilibrate at the stagnationtemperature for a sufficient length of time to allow foroutgassing of the components. This procedure may a

23、id ineliminating contamination of the cover plate and the samplesduring actual testing periods and is especially important wherecoatings employing organic components are used. If the coverplate is in place during this outgassing procedure, it shall becleaned before the box is put into service in ord

24、er to restore itsoriginal transmittance.5.2 Cover PlateThe box shall have a single cover platethat is glazed and hinged to provide access.5.2.1 Two types of cover plate materials may be used:5.2.1.1 Type ITempered low-iron glass with spectral char-acteristics approximating those shown in Fig. 2.5.2.

25、1.2 Type IIAlternative types of solar transmittingglass or plastic materials might be used for the cover plate ifthe absorber is to be used under that material.5.2.2 The solar-weighted transmittance values of the coverplate test patches (5.2.3 and 5.2.4) shall remain above theindicated percentage of

26、 their initial values in the followingwavelength regions:300 to 400 nm 90 %400 to 2100 nm 95 %5.2.3 An easily removable test patch of the cover materialmeasuring 50 by 50 mm (2 by 2 in.) shall be fastened onto theinner surface of the transparent cover plate in or near one lowercorner. By periodicall

27、y measuring the transmittance of this testpatch, an indication of the effect of any condensable effluentson the cover material can be monitored.5.2.4 An easily removable specimen of the cover platematerial measuring 50 by 50 mm (2 by 2 in.) should also bemounted directly on an exterior upper corner

28、of the cover plateto monitor the effects of atmospheric contamination andultraviolet degradation. These effects are generally more severefor plastic materials than for glass.5.3 SealsA seal that does not outgas at the stagnationtemperature should be used to make the box weather-resistant.5.4 Insulat

29、ionThe bottom and sides of the enclosure shallbe insulated to have a thermal conductance of less than 0.515W/(m2K)(0.091 Btu/(hft2F), that is, an R value of 11 orgreater with materials that do not outgas at the stagnationtemperature.5.5 Sample Mounting PlateA metallic mounting platewith lateral dime

30、nsions approximately the same as the internalenclosure dimensions (less the thickness of the insulation onthe sides of the box) shall be mounted approximately 10 mm(0.4 in.) above the bottom insulation by a thermally insulatingmaterial.5.5.1 The mounting plate used to support selective speci-mens sh

31、all have a selective surface. The solar absorptance (a)of the selective coating shall be greater than 0.90, and the roomtemperature emittance () shall be less than 0.15 at all times.5.5.2 The mounting plate used to support nonselectivespecimens shall be coated with any nonselective black coatingFIG.

32、 1 Typical Cross Section of Exposure Test ApparatusE 781 86 (2009)2that is thermally resistant to temperatures up to approximately200C (392F). The solar absorptance (a) of the coatings(solar-weighted average from 350 to 2500 nm) shall be greaterthan 0.90 at all times.5.5.3 A specimen of the mounting

33、 plate material (50 by 50mm (2 by 2 in.) shall be securely fastened to and in contactwith the mounting plate. The purpose of this specimen is tomonitor the optical characteristics of the mounting plate.5.6 Condensation ControlIt is desirable to minimize oreliminate the presence of condensate on the

34、inside of the coverplate and on the specimens during periods of solar irradiationto ensure uniform results. The use of a desiccant may aid inminimizing condensation. In some geographical locations,weep holes drilled at the lower corners of the test enclosuremay be required to drain condensate and th

35、ereby preclude thecondensation of moisture on the cover plate. Weep holes shallbe limited in size and number so as not to disturb theequilibrium temperature of the test enclosure interior.6. Test Specimens6.1 Test specimens shall be defined to be either the coatingapplied to a specific substrate or

36、the absorber material itself(for materials other than coatings).6.2 The specimens shall be prepared in accordance with theprocedures and conditions recommended by the coating ormaterial supplier.7. Procedure7.1 Number of Test Specimens (see Note 2)The number oftest specimens shall be defined and sel

37、ected based on the needfor replication and the test plan option used as described in7.1.1 and 7.1.2.NOTE 2Practice D 1898 provides guidance on statistical proceduresfor sampling.NOTE 3While replication is desirable whenever available and re-sources permit, a high level of experience with the weather

38、ing character-istics of any given product may permit single specimen testing with goodreliability.7.1.1 Test Plan Option ARemove test specimens at pre-selected deadlines (or remove sections by cutting, stamping, orotherwise segmenting), measure nondestructively, and return tothe test for continued e

39、xposure.7.1.2 Test Plan Option BReplicate specimens to permitwithdrawals at preselected intervals without replacement.7.1.3 Exercise caution with test specimens that may outgasor are subjected to rapid initial decomposition at elevatedtemperatures. Such outgassing may significantly alter thetransmit

40、tance of the cover plate. If this occurs, clean the coverplate (see 5.1.1) prior to resuming testing. The decomposingtest specimens may be removed from the test apparatus. Noteobservations and actions taken in the test report.7.2 Mounting Test SpecimensMount the test specimens onthe sample mounting

41、plate. Perform mounting and handlingwhile wearing gloves so as not to damage or contaminate thesurface of either the test material or the backing plate (Note 4).The location and number of test specimens shall be consistentwith thermal uniformity. Shading of specimens shall not occur,except just afte

42、r sunrise and just before sunset.NOTE 4The potential for galvanic corrosion should be consideredwhen mounting the specimens.7.3 Orientation of Test Fixture and Duration of Exposure:7.3.1 Mount the test fixture facing the equator with its majoraxis oriented east-west such that the angle between the n

43、ormalto the plane of the test apparatus and the zenith is equal to thelatitude of the testing site.7.3.2 Expose the specimens to a minimum total solarirradiation of 6.2 3 109J/m2(5.6 3 105Btu/ft2) (see Note 5).In order to develop kinetic or rate information, it is recom-mended that the parameters ch

44、osen for studying these effectsbe measured at progressively longer exposure increments.NOTE 5This exposure value is based on the solar radiation received ina 12-month period assuming an average value of 1.7 3 107J/m2per day(1500 Btu/ft2per day).FIG. 2 Transmittance of Low-Iron GlassE 781 86 (2009)37

45、.4 Measurements of Test Specimen PropertiesPrior toexposure and at preselected exposure intervals, measure theproperties of the test specimens as described in 7.4.1, 7.4.2, and7.4.3.7.4.1 Solar AbsorptanceMeasure the solar absorptance inaccordance with Test Method E 903, unless otherwise speci-fied.

46、NOTE 6The spectral reflectance curves from which solar absorptanceis calculated are often a more sensitive indicator of the onset ofdegradation of absorber materials than integrated solar absorptancevalues. This is especially true for changes occurring in spectral regionswhere there is a limited amo

47、unt of energy in the solar spectrum, that is, inthe near infrared region.7.4.2 EmittanceMeasure the total hemispherical or nor-mal emittance, or both, in accordance with Test Methods E 434or E 408, unless otherwise specified.7.4.3 General AppearanceAssess the general appearanceof the specimens in ac

48、cordance with Practice B 537, whereapplicable.7.5 Temperature MeasurementsDaily measure and recordthe temperature of the absorber sample mounting plate nearsolar noon. Measure the temperature at two locations, onelocated 150 mm (6 in.) from the upper edge and the other at thesame distance from the l

49、ower edge along the minor axis of theplate (see 7.3.1). Thermal sensors shall be in good thermalcontact with the plate and shielded from direct solar radiation.7.6 Solar Radiation MeasurementsMeasure total incidentradiation on the plane containing the test fixture during theexposure period using a sensor meeting the requirements of aWorld Meteorological Organization second-class pyranometer.7.7 Other Measurements:7.7.1 Spectral TransmittanceMeasure the spectral trans-mittance of the cover plate test patches periodically in thewa