ASTM E1980-11
(Practice)Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces
Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces
SIGNIFICANCE AND USE
Solar reflectance and thermal emittance are important factors affecting surface and near-surface ambient air temperature. Surfaces with low solar reflectance, absorb a high fraction of the incoming solar energy. A fraction of this absorbed energy is conducted into ground and buildings, a fraction is convected to air (leading to higher air temperatures), and a fraction is radiated to the sky. For equivalent conditions, the lower the emissivity of a surface the higher its steady-state temperature. Surfaces with low emissivity cannot effectively radiate to the sky and, therefore, get hot. Determination of solar reflectance and thermal emittance, and subsequent calculation of the relative temperature of the surfaces with respect to black and white reference temperature (defined as Solar Reflectance Index, SRI), may help designers and consumers to choose the proper materials to make their buildings and communities energy efficient. The method described here gives the SRI of surfaces based on measured solar reflectances and thermal emissivities of the surfaces.
SCOPE
1.1 This practice covers the calculation of the Solar Reflectance Index (SRI) of horizontal and low-sloped opaque surfaces at standard conditions. The method is intended to calculate SRI for surfaces with emissivity greater than 0.1.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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:E1980 −11
Standard Practice for
Calculating Solar Reflectance Index of Horizontal and Low-
1
Sloped Opaque Surfaces
This standard is issued under the fixed designation E1980; 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 (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The steady-state surface temperature (T ) under the sun is strongly correlated to solar reflectivity
s
andthermalemissivityofthesurface.Forequivalentconditions,the T ofdarksurfaces(withlowsolar
s
reflectance) is higher than light-colored surfaces (with high solar reflectance); and surfaces with low
thermal emissivity have higher T ’s than surfaces with high thermal emissivity. The procedure
s
recommended in this standard will allow a direct comparison of T of surfaces under the sun. The
s
procedure defines a Solar Reflectance Index (SRI) that measures the relative T of a surface with
s
respect to the standard white (SRI = 100) and standard black (SRI =0) under the standard solar and
ambient conditions.
1. Scope 3.1.1 convective coeffıcient (h )—the rate of heat transfer
c
fromthesurfacetoairinducedbytheairmovement,expressed
1.1 This practice covers the calculation of the Solar Reflec-
–2 –1
in watts per square metre per degree Kelvin, W·m ·K .
tance Index (SRI) of horizontal and low-sloped opaque sur-
faces at standard conditions. The method is intended to 3.1.2 low-sloped surfaces—surfaces with a slope smaller
calculate SRI for surfaces with emissivity greater than 0.1. than 9.5° from the horizontal.
3.1.3 reference black surface temperature (T)—is the
1.2 The values stated in SI units are to be regarded as
b
standard. No other units of measurement are included in this steady-state temperature of a black surface with solar reflec-
tance of 0.05 and thermal emissivity of 0.9, under the standard
standard.
solar and ambient conditions.
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the 3.1.4 reference white surface temperature (T)—is the
w
responsibility of the user of this standard to establish appro-
steady-state temperature of a white surface with solar reflec-
priate safety and health practices and determine the applica- tance of 0.80 and thermal emissivity of 0.9, under the standard
bility of regulatory limitations prior to use.
solar and ambient conditions.
3.1.5 sky temperature (T )—is the temperature of a black
sky
2. Referenced Documents
body that would radiate the same power toward the earth as
2
2.1 ASTM Standards:
does the sky.
G173TablesforReferenceSolarSpectralIrradiances:Direct
3.1.6 solar absorptance (α)—the fraction of solar flux ab-
Normal and Hemispherical on 37° Tilted Surface
sorbed by a surface. For an opaque surface α=1−a.
3. Terminology
3.1.7 solar flux (I)—is the direct and diffuse radiant power
from the sun received at ground level over the solar spectrum,
3.1 Definitions:
–2
expressed in watts per square metre, W·m .
3.1.8 solar reflectance (a)—the fraction of solar flux re-
1
ThistestmethodisunderthejurisdictionofASTMCommitteeD08onRoofing
flected by a surface.
and Waterproofing and is the direct responsibility of Subcommittee D08.18 on
Nonbituminous Organic Roof Coverings.
3.1.9 solar reflectance index (SRI)—is the relative T of a
s
Current edition approved Jan. 1, 2011. Published January 2011. Originally
surface with respect to the standard white (SRI = 100) and
publishedin1998.Lastpreviouseditionapprovedin2001asE1980–01,whichwas
standard black (SRI = 0) under the standard solar and ambient
withdrawninJanuary2010andreinstatedinJanuary2011.DOI:10.1520/E1980-11.
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or conditions.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
3.1.10 solar spectrum—spectral distribution of typical ter-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. restrial sunlight at air mass 1.5 as defined in Tables G173.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
1
---------------------- Page: 1 ----------------------
E1980−11
3.1.11 standard solar and ambient conditions— for the where:
purpose of this calculation, is defined as a solar flux of 1000
~α 2 0.029ε! 8.7971h
~ !
c
–2
W·m , ambient air temperature of 310 Kelvin (K), and sky χ 5 (5)
9.5205ε1h
c
temperatureof300K.Threeconvectivecoefficientof5,12,30
–2 –1 –1
For α greater than 0.1, and excluding collector surfaces
W·m ·K , corresponding to low- (0 to 2 ms ), medium- (2
–1 –1
(surface with high solar absorptance and low thermal
to6ms ), and hig
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
An American National Standard Designation: E1980 – 11
Designation:E 1980–01
Standard Practice for
Calculating Solar Reflectance Index of Horizontal and Low-
1
Sloped Opaque Surfaces
This standard is issued under the fixed designation E1980; 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 (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The steady-state surface temperature (T ) under the sun is strongly correlated to solar reflectivity
s
andthermalemissivityofthesurface.Forequivalentconditions,the T ofdarksurfaces(withlowsolar
s
reflectance) is higher than light-colored surfaces (with high solar reflectance); and surfaces with low
thermal emissivity have higher T ’s than surfaces with high thermal emissivity. The procedure
s
recommended in this standard will allow a direct comparison of T of surfaces under the sun. The
s
procedure defines a Solar Reflectance Index (SRI) that measures the relative T of a surface with
s
respect to the standard white (SRI = 100) and standard black (SRI =0) under the standard solar and
ambient conditions.
1. Scope
1.1 This practice covers the calculation of the Solar Reflectance Index (SRI) of horizontal and low-sloped opaque surfaces at
standard conditions. The method is intended to calculate SRI for surfaces with emissivity greater than 0.1.
1.2
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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
2.1 ASTM Standards:
E408Test Methods for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques
2
E772Terminology Relating to Solar Energy Conversion
3
E891Tables for Terrestrial Direct Normal Solar Irradiance for Air Mass 1.5
3
E903Test Method for Solar Absorption, Reflectance, and Transmittance of Materials Using Integrating Spheres
E1918Test Method for Measuring Solar Reflectance of Horizontal and Low-Sloped Surfaces in the Field G173 Tables for
Reference Solar
Spectral Irradi-
ances: Direct
Normal and
Hemispherical on
37 Tilted Surface
3. Terminology
3.1 Definitions:
1
This test method is under the jurisdiction ofASTM Committee E06 on Performance of Building Construction and is the direct responsibility of Subcommittee E06.21
on Serviceability.
´1
CurrenteditionapprovedApril10,2001.PublishedApril2001.OriginallypublishedasE1980–98.LastpreviouseditionE1980–98 .D08onRoofingandWaterproofing
and is the direct responsibility of Subcommittee D08.18 on Nonbituminous Organic Roof Coverings.
Current edition approved Jan. 1, 2011. Published January 2011. Originally published in 1998. Last previous edition approved in 2001 as E1980–01, which was withdrawn
in January 2010 and reinstated in January 2011. DOI: 10.1520/E1980-11.
2
Annual ASTM Book of Standards, Vol 15.03.
2
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
1
---------------------- Page: 1 ----------------------
E1980 – 11
3.1.1 convective coeffıcient (h )—the rate of heat transfer from the surface to air induced by the air movement, expressed in
c
–2 –1
watts per square metre per degree Kelvin, W·m ·K .
3.1.2 low-sloped surfaces—surfaces with a slope smaller than 9.5° from the horizontal.
3.1.3 reference black surface temperature (T )—isthesteady-statetemperatureofablacksurfacewithsolarreflectanceof0.05
b
and thermal emissivity of 0.9, under the standard solar and ambient conditions.
3.1.4 reference white surface temperature (T )—isthesteady-statetemperatureofawhitesurfacewithsolarreflectanceof0.80
w
and thermal emissivity of 0.9, under the standard solar and ambient conditions.
3.1.5 sky temperature (T )—is the temperature of a black body that would radiate the same power toward the earth as does
sky
the sky.
3.1.6 solar absorptance (a)—the fraction of solar flux absorbed by a surface. For an opaque surface a=1-−a.
3
...
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