ASTM E772-87(1993)e1
(Terminology)Standard Terminology Relating to Solar Energy Conversion
Standard Terminology Relating to Solar Energy Conversion
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NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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Designation: E 772 – 87 (Reapproved 1993) An American National Standard
Standard Terminology Relating to
Solar Energy Conversion
This standard is issued under the fixed designation E 772; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
absorber—that part of a solar collector whose primary func- angle of reflection—the angle between the direction of propa-
tion is to absorb radiant energy and transform it into another gation of a reflected ray and the normal to the surface at the
form of energy. point of reflection.
angle of refraction—the angle between the direction of
NOTE 1—A thermal absorber usually possesses a solid surface through
propagation of a refracted ray and the normal to the interface
which energy is transmitted by thermal conduction to the transfer fluid;
at the point of refraction.
however, the transfer fluid itself can be the absorber in the case of an
optically transparent container and a “black liquid”. A photovoltaic aperture area—see area, aperture.
absorber converts part of the incident solar flux into electrical energy, and
apparent solar time, apt—the hours of the day as computed
part to thermal energy.
from the position of the sun using the equation of time. (See
ASHRAE Handbook of Applications, 1982, Chapter 57.)
absorptance, a— the ratio of the absorbed radiant or luminous
area, absorber—the total uninsulated heat transfer surface
flux to the incident flux. (Practice E 491, for Solar Simula-
area of the absorber, including unirradiated as well as
tion for Thermal Balance Testing of Spacecraft ). See
irradiated portions.
radiometric properties and quantities.
area, aperture— of a flat plate collector, (1) the maximum
absorption—the process by which incident radiant energy is
projected area of a solar collector through which the uncon-
transformed into another form of energy by interaction with
centrated solar radiant energy may be admitted to the
matter.
absorber. (2) effective aperture area—the area as defined
air handling unit—a device used for distributing conditioned
above projected normal to the sun’s rays and corrected for
air supply to a room, space, or area.
2 2
any shading. Units: square metres (m ) [square feet (ft )].
air mass, AM—the ratio of the mass of atmosphere in the
area, collector panel—the total area of the panel assembly
actual observer-sun path to the mass that would exist if the
(with its containing box, if present), projected on the
observer was at sea level, at standard barometric pressure,
aperture plane.
and the sun was directly overhead.
area, gross aperture— of a concentrating collector, the
NOTE 2—(Sometimes called air mass ratio.) Air mass varies with the
maximum projected area through which the unconcentrated
zenith angle of the sun and the local barometric pressure, which changes
solar radiant energy is admitted, including any area of the
with altitude. For sun zenith angle, Z, of 62° or less and local atmospheric
reflector or refractor shaded by the receiver and its supports,
pressure, P, where P is standard atmospheric pressure, AM . sec Z
o
and including gaps between reflector segments within a
(P/P ).
o
collector module.
albedo—the use of the term albedo is discouraged in favor of
area, gross collector—the maximum area of the complete
the preferred term, reflectance.
collector module, including integral mounting means, pro-
altazimuthal mount—a supporting device that facilitates
jected on the aperture plane.
tracking of the sun and allows rotation about horizontal and
area, net aperture—of a concentrating collector, the maxi-
vertical axes. It can be used to aim equipment such as
mum projected area through which the unconcentrated solar
heliostats, concentrating collectors, exposure specimens, or
radiant energy is admitted, excluding any area of the
radiometers.
reflector or refractor shaded by the receiver and its supports,
angle of incidence—the angle between a ray and the normal to
and excluding gaps between reflector segments within a
the plane on which it is incident. (The plane of incidence
collector module.
may be the aperture plane, the collector, or any other plane
auxiliary energy subsystem—in solar energy applications,
of interest.)
equipment using nonsolar energy sources to supplement or
backup the output provided by a solar energy system.
beam, radiant energy—a collection of rays confined to a
These definitions are under the jurisdiction of ASTM Committee E44 on Solar,
specific path.
Geothermal, and Other Alternative Energy Sources and are the direct responsibility
of Subcommittee E44.09 on Photovoltaic Electric Power Conversion.
blackbody—a hypothetical “body” that completely absorbs all
Current edition approved Feb. 27, 1987. Published April 1987. Originally
incident radiant energy, independent of wavelength and
published as E 772 – 80. Last previous edition E 772 – 84.
2 direction; that is, one which neither reflects nor transmits any
Annual Book of ASTM Standards, Vol 15.03.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 772
of the incident radiant energy. It is the emitter of electro- concentration ratio, geometric—the ratio of the collector
magnetic radiant energy which, at a given temperature, aperture area to the absorber area.
presents the maximum spectral density of radiant exitance or concentrator—an optical device (lenses or mirrors) that, as
radiance at all wavelengths. part of a solar collector, receives the unconcentrated solar
irradiance and redirects (concentrates) it to a smaller area
NOTE 3—No real material is a blackbody. A completely enclosed cavity
(the receiver).
with opaque walls at a uniform temperature contains blackbody radiation.
conical, v— over a solid angle larger than an infinitesimal
A blackbody radiator can be approximated in the laboratory to any desired
element of solid angle and less than a hemisphere. The
degree of approximation by a furnace containing a cavity with opaque
walls at a uniform temperature, that contains an aperture through which
geometry of the solid angle must be described in the text. For
the blackbody radiation is observed. The degree of approximation to a true
incident beams it is assumed that the radiance is constant
blackbody radiator is inversely related to the ratio of the area of the
over the entire solid angle. (See Radiometric properties
aperture to the area of the interior wall of the cavity.
and quantities)
blackbody radiant energy— see radiant energy, blackbody. containment material—in a solar energy system, a material
building heat loss factor—a measure of the heat loss rate of that encloses the heat-transfer fluid or is in contact with the
a building expressed in joules per degree day (or Btu per heat transfer or heat storage material, or both.
degree day). This factor is multiplied by the number of convection—the transport of heat by fluid flow.
degree days in a given period to estimate the energy required convection, forced—convection caused by mechanical forces
to heat the building during that period.
such as fans and injectors.
charge capacity—see thermal capacity. convection, natural—convection within a fluid, due to density
cloud cover—that portion of the sky which is covered by
differences caused by temperature differences.
clouds, usually expressed in tenths of sky covered.
cover plate, collector—a sheet of transparent (or translucent)
collector, concentrating—a solar collector that uses reflectors,
glazing placed above the absorber in a solar collector, to
lenses, or other optical elements to redirect and concentrate
provide thermal and environmental protection.
the solar irradiance on the collector aperture onto an ab-
degree day—see degree day, heating and degree day, cool-
sorber of which the surface area is smaller than the collector
ing.
aperture area.
degree-day, cooling, (DDC or DDF, Celsius or Fahrenheit
collector effıciency— see efficiency, collector.
respectively)—one cooling degree-day is counted for each
collector, evacuated tube—a solar collector made from trans-
degree that the daily mean temperature is higher than a base
parent tubing (usually glass) with an evacuated space be-
temperature; used to estimate energy requirements for air
tween the tube and the absorber. The absorber may consist of
conditioning or refrigeration.
an inner tube or another shape, with means for removal of
degree-day, heating, (DDC or DDF, Celsius or Fahrenheit
thermal energy and may be specially coated.
respectively)—one heating degree-day is counted for each
collector, flat plate—a nonconcentrating solar collector in
degree that the daily mean temperature is lower than a base
which the absorbing surface is essentially planar.
temperature; used to estimate energy requirements for heat-
collector, line-focus—a concentrating solar collector that con-
ing.
centrates the solar flux in one dimension only.
design life—the period of time during which a system or
collector, point focus—a concentrating collector that focuses
component is expected to perform its intended function,
the solar flux to a point.
without significant degradation of performance and without
collector, solar thermal—a device designed to absorb solar
requiring major maintenance or replacement.
irradiance and to transfer the thermal energy to a fluid
diffuse, adj—referring to radiometric quantities, indicates that
passing through it.
the flux propagates in many directions, as opposed to direct
collector subsystem—that portion of the solar system which
beam which refers to collimated flux. When referring to
includes the solar collectors and related piping or ducts.
solar irradiance, it is the global irradiance less the direct
collector, tracking—a solar collector that moves so as to
beam irradiance. When referring to reflectance, it is the
follow the apparent motion of the sun during the day,
directional hemispherical reflectance less the specular reflec-
rotating about one axis or two orthogonal axes.
tance.
collector, trickle—a flat plate solar collector in which unpres-
NOTE 4—Diffuse has been used in the past to refer to hemispherical
surized liquid flows or “trickles” over the absorber.
collection (including the specular component) or irradiation, with equal
collector cover (glazings)— see cover plate, collector.
radiance for all directions over a hemisphere. This use is deprecated in
combustible liquid—a liquid having a flash point at or above
favor of the more precise term hemispherical.
37.8°C (100°F). The flash point of a liquid having a viscosity
directional—over an infinitesimal element of solid angle in a
less than 45 SUS at 37.8°C (100°F) and a flash point below
given direction. For properties, a solid angle small enough
93.4°C (200°F) shall be determined in accordance with the
that the property does not vary within the solid angle may be
Test Methods D 93, for Flash Point by the Pensky-Martens
considered an element of solid angle. Indicated by the
Closed Tester.
symbols u, F, where u is the angle between the given
concentrating collector— see collector, concentrating.
direction and the normal to the sample surface, and;uf is the
azimuth angle of the direction measured counter-clockwise
Annual Book of ASTM Standards, Vol 05.01. from a reference mark on the sample. See radiometric
E 772
properties and quantities. ignitable mixture with air near the surface of the liquid
discharge capacity, thermal—the amount of heat that can be within the vessel as specified by appropriate test procedure
removed from a storage device during a period of time and and apparatus.
for a specific set of values for the initial and final tempera-
flux, radiant—see radiant flux.
tures of the storage device, the temperature of the entering
forced convection— see convection, forced.
fluid, and the mass flow rate of fluid through the storage
free convection—see convection, natural.
system.
Fresnel lens, circular—a sheet of transparent material into
discharge test time—the duration of a single transient test in
which concentric grooves have been formed in such a pattern
which energy is removed from the storage device.
that light will be focused as with a lens. (Focusing mirrors of
distribution subsystem—that portion of the solar system from
similar design are also available.)
the storage device to the point of ultimate use.
Fresnel lens, linear—a sheet of transparent material into
drainback solar energy system—see solar energy system,
which parallel grooves have been formed in such a pattern
drainback.
that light will be focused as by a cylindrical lens. (Focusing
draindown solar energy system—see solar energy system,
mirrors of similar design are also available.)
draindown.
Fresnel-reflector system—flat mirrors arranged in an array
efficiency, collector—of a solar thermal collector, the ratio of
such that they reflect onto a target, the illuminated area of
the amount of energy removed by the heat transfer fluid to
which simulates the shape and size of the flat mirror. (Such
the solar energy incident on the collector.
an array simulates the ray-tracing of a parabolic trough of the
same aperture angle.)
NOTE 5—For flat-plate collectors, the value of the incident solar energy
used is usually based on gross collector area; for concentrating collectors full radiator—see blackbody.
the value is usually based on the aperture area.
gross collector area— see area, gross collector.
heat-actuated cooling—the use of thermal energy to initiate a
efficiency, instantaneous collector—ratio of the amount of
thermodynamic cycle which results in a local decrease in
energy removed by the heat transfer fluid of a solar collector
temperature.
over a specified time period (usually 5 or 15 min) to the solar
heat capacity—see thermal capacity.
energy incident on the collector area in the same period,
heat loss rate—the rate at which heat is lost from a system or
under steady-state or quasi-steady state.
component of a system, per degree temperature difference
NOTE 6—For flat plate collectors, the area used is usually the gross
between its average temperature and the average ambient air
collector area; for concentrating collectors the area used is usually the
temperature.
gross aperture area.
heat transfer fluid—( 1) in solar energy systems, a liquid or
efficiency, period system—ratio of the useful energy supplied
gas that passes through the solar collector and carries the
by the solar energy system over a period of time to the solar
absorbed thermal energy away from the collector. (2) any
energy incident on the collector ar
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