ASTM E781-1986(2015) 0346 Standard Practice for Evaluating Absorptive Solar Receiver Materials When Exposed to Conditions Simulating Stagnation in Solar Collectors With Cover Plate.pdf

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1、Designation: E781 86 (Reapproved 2015)Standard Practice forEvaluating Absorptive Solar Receiver Materials WhenExposed to Conditions Simulating Stagnation in SolarCollectors With Cover Plates1This standard is issued under the fixed designation E781; the number immediately following the designation in

2、dicates 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 fo

3、r 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 temperatur

4、es 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 re

5、garded 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 prior

6、to use.2. Referenced Documents2.1 ASTM Standards:2B537 Practice for Rating of Electroplated Panels Subjectedto Atmospheric ExposureD1898 Practice for Sampling of Plastics (Withdrawn 1998)3E408 Test Methods for Total Normal Emittance of SurfacesUsing Inspection-Meter TechniquesE434 Test Method for Ca

7、lorimetric Determination of Hemi-spherical Emittance and the Ratio of SolarAbsorptance toHemispherical Emittance Using Solar SimulationE772 Terminology of Solar Energy ConversionE903 Test Method for Solar Absorptance, Reflectance, andTransmittance of Materials Using Integrating SpheresE9623. Termino

8、logy3.1 Definitions:3.1.1 See Terminology E772 for definitions.4. Significance and Use4.1 Although this practice is intended for evaluating solarabsorber materials and coatings used in flat-plate collectors, nosingle procedure can duplicate the wide range of temperaturesand environmental conditions

9、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 encountered in solar collectors with singlecover plates and with no added means of limiti

10、ng 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 stagnation, thisprocedure is considered to provide adequate thermal testing formost

11、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 temperature of37.8C (100F) and zero wind velocity; and a double glazed one as highas 245C (48

12、2F) 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, with the same environmental conditions, (see“Performance Criteria for Solar Heating an

13、d 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 under the jurisdiction of ASTM Committee E44 on Solar,Geothermal and OtherAlternativ

14、e Energy Sources and is the direct responsibility ofSubcommittee E44.05 on Solar Heating and Cooling Systems and Materials.Current edition approved March 1, 2015. Published April 2015. Originallyapproved in 1981. Last previous edition approved in 2009 as E78186(2009). DOI:10.1520/E0781-86R15.2For re

15、ferenced 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.3The last approved version of this historical standard is referenced

16、 onwww.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.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1materia

17、ls 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 spec-trally selective absorbers and nonselective absorbers.4.5.1 Testin

18、g 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-sure and mounting plate simulate the temperature conditions ofa selecti

19、ve 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 nonse-lective coated sample mounting plate, such that the enclosureand m

20、ounting 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 that ap-proximates the dimensions of a typical flat-plate solar collectorand shall have minimum dimen

21、sions of 0.75 by 1.5 by 0.1 m(29 by 60 by 4 in.) deep. The box should be constructed ofmaterials that are impervious to moisture. Wood should not beused for construction of the box. Care shall be taken to preventwater leakage at joints, seams, and seals.5.1.1 Pre-Exposure of Test BoxPrior to use, th

22、e 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 aid ineliminating contamination of the cover plate and the samplesdur

23、ing 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 order to restore itsoriginal transmittance.5.2 Cover PlateThe box shall

24、 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. 2.5.2.1.2 Type IIAlternative types of solar transmitting glassor plastic m

25、aterials might be used for the cover plate if theabsorber 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 their initial values in the followingwavelength regions:300 to 400

26、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 periodically measuring the transmittance of this testpatch, an indication of th

27、e 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 of the cover plateto monitor the effects of atmospheric contaminatio

28、n 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 InsulationThe bottom and sides of the enclosure shallbe insulated to have a

29、 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 dimensions approximately the same as the internalenclosure dimensions (l

30、ess 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 shall have a selective surface. The solar absorptance ()of the selecti

31、ve 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. 1 Typical Cross Section of Exposure Test ApparatusE781 86 (2015)2tha

32、t is thermally resistant to temperatures up to approximately200C (392F). The solar absorptance () 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 plate material (50 by 50mm (2 by 2 in.) shall be securely fastened to

33、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 inside of the coverplate and on the specimens during periods of solar i

34、rradiationto 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 thereby preclude thecondensation of moisture on the cover plate. Weep hol

35、es 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 the absorber material itself(for materials other than coatings).6.2 The

36、 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 selected based on the needfor replication and the test plan option used as

37、 described in7.1.1 and 7.1.2.NOTE 2Practice D1898 provides guidance on statistical proceduresfor sampling.NOTE 3While replication is desirable whenever available and re-sources permit, a high level of experience with the weathering character-istics of any given product may permit single specimen tes

38、ting with goodreliability.7.1.1 Test Plan Option ARemove test specimens at prese-lected deadlines (or remove sections by cutting, stamping, orotherwise segmenting), measure nondestructively, and return tothe test for continued exposure.7.1.2 Test Plan Option BReplicate specimens to permitwithdrawals

39、 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 thetransmittance of the cover plate. If this occurs, clean the coverplate (see 5.1.

40、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 plate. Perform mounting and handlingwhile wearing gloves so as not to da

41、mage 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 after sunrise and just before sunset.NOTE 4The potential for galvanic corros

42、ion 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 normalto the plane of the test apparatus and the zenith is equal to thela

43、titude of the testing site.7.3.2 Expose the specimens to a minimum total solarirradiation of 6.2 109J/m2(5.6 105Btu/ft2) (see Note 5). Inorder to develop kinetic or rate information, it is recommendedthat the parameters chosen for studying these effects bemeasured at progressively longer exposure in

44、crements.NOTE 5This exposure value is based on the solar radiation received ina 12-month period assuming an average value of 1.7 107J/m2per day(1500 Btu/ft2per day).FIG. 2 Transmittance of Low-Iron GlassE781 86 (2015)37.4 Measurements of Test Specimen PropertiesPrior toexposure and at preselected ex

45、posure 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 E903, unless otherwise specified.NOTE 6The spectral reflectance curves from which solar absorptanceis calculated are

46、 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 amount of energy in the solar spectrum, that is, inthe near infrared region.7.4.2 Emit

47、tanceMeasure the total hemispherical or nor-mal emittance, or both, in accordance with Test Methods E434or E408, unless otherwise specified.7.4.3 General AppearanceAssess the general appearanceof the specimens in accordance with Practice B537, whereapplicable.7.5 Temperature MeasurementsDaily measur

48、e 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 lower edge along the minor axis of theplate (see 7.3.1). Thermal sensors shall be in go

49、od 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 thewavelength region 300 to 2100 nm in accordance with TestMethod E903 (see 5.2.2 a