Active Standard ASTM E781 | Developed by Subcommittee: E44.05
Book of Standards Volume: 12.02
Historical (view previous versions of standard)
Significance and Use
Although this practice is intended for evaluating solar absorber materials and coatings used in flat-plate collectors, no single procedure can duplicate the wide range of temperatures and environmental conditions to which these materials may be exposed during in-service conditions.
This practice is intended as a screening test for absorber materials and coatings. All conditions are chosen to be representative of those encountered in solar collectors with single cover plates and with no added means of limiting the temperature during stagnation conditions.
This practice uses exposure in a simulated collector with a single cover plate. Although collectors with additional cover plates will produce higher temperatures at stagnation, this procedure is considered to provide adequate thermal testing for most applications.
Note 1—Mathematical modelling has shown that a selective absorber, single glazed flat-plate solar collector can attain absorber plate stagnation temperatures as high as 226°C (437°F) with an ambient temperature of 37.8°C (100°F) and zero wind velocity; and a double glazed one as high as 245°C (482°F) under these conditions. The same configuration solar collector with a nonselective absorber can attain absorber stagnation temperatures as high as 146°C (284°F), if single glazed, and 185°C (360°F), if double glazed, with the same environmental conditions, (see “Performance Criteria for Solar Heating and Cooling Systems in Commercial Buildings,” NBS Technical Note 1187 ).
This practice evaluates the thermal stability of absorber materials. It does not evaluate the moisture stability of absorber materials used in actual solar collectors exposed outdoors. Moisture intrusion into solar collectors is a frequent occurrence in addition to condensation caused by diurnal breathing.
This practice differentiates between the testing of spectrally selective absorbers and nonselective absorbers.
Testing Spectrally Selective Absorber Coatings and Materials—Spectrally selective solar absorptive coatings and materials require testing in a covered enclosure that contains a selectively coated sample mounting plate, such that the enclosure and mounting plate simulate the temperature conditions of a selective flat-plate collector exposed under stagnation conditions.
Testing Nonselective Coatings and Materials—Spectrally nonselective solar absorptive coatings and materials require testing in a covered enclosure that contains a nonselective coated sample mounting plate, such that the enclosure and mounting plate simulate the temperature conditions of a covered, nonselective flat-plate collector exposed under stagnation conditions.
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.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
B537 Practice for Rating of Electroplated Panels Subjected to Atmospheric Exposure
D1898 Practice for Sampling of Plastics
E408 Test Methods for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques
E434 Test Method for Calorimetric Determination of Hemispherical Emittance and the Ratio of Solar Absorptance to Hemispherical Emittance Using Solar Simulation
E772 Terminology of Solar Energy Conversion
E903 Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres
ICS Number Code 27.160 (Solar energy engineering)