IEC TS 62862-1-1:2018

Solar thermal electric plants - Part 1-1: Terminology

IEC TS 62862-1-1:2018(E) contains the main terms and definitions used by the solar thermal electric (STE) industry and intends to be a reference for users of industry documents. Since the components and configurations of STE plants depend on the concentrating solar thermal technology used (i.e., central receiver, parabolic-trough collector, parabolic-dish or linear Fresnel concentrator), some terms are not applicable to all types of STE plants and notes have been introduced in their definitions for clarification.

General Information

• Status: Published
• Publication Date: 19-Feb-2018
• ICS: 27.160 - Solar energy engineering
• Technical Committee: TC 117 - Solar thermal electric plants
• Drafting Committee: MT 1 - TC 117/MT 1
• Current Stage: PPUB - Publication issued
• Start Date: 20-Feb-2018
• Completion Date: 06-Mar-2018

Standards Content (Sample)

IEC TS 62862-1-1 ®
Edition 1.0 2018-02
TECHNICAL
SPECIFICATION
colour
inside
Solar thermal electric plants –
Part 1-1: Terminology
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IEC TS 62862-1-1 ®
Edition 1.0 2018-02
TECHNICAL
SPECIFICATION
colour
inside
Solar thermal electric plants –

Part 1-1: Terminology
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-5352-6

– 2 – IEC TS 62862-1-1:2018 © IEC 2018
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
3.1 SECTION 01: DESCRIPTION OF SYSTEMS, SUB-SYSTEMS AND
COMPONENTS . 5
3.2 SECTION 02: ANGLE DEFINITIONS . 9
3.3 SECTION 03: AREA DEFINITIONS . 16
3.4 SECTION 04: OPTICAL PROPERTIES . 18
3.5 SECTION 05: SOLAR IRRADIANCE . 24
3.6 SECTION 06: ENERGIES DEFINITION (SOLAR FIELD PART) . 25
3.7 SECTION 07: ENERGIES DEFINITION (POWER BLOCK PART) . 26
3.8 SECTION 08: EFFICIENCY NUMBERS . 28
3.9 SECTION 09: THERMAL STORAGE SYSTEM . 30
3.10 SECTION 10: FINANCIAL FIGURES . 32
3.11 SECTION 11: MISCELLANEOUS . 32

Figure 1 – Angle of acceptance of specular reflectance, ψ . 10
Figure 2 – Angles of incidence in linear Fresnel collectors . 11
Figure 3 – Collector axis azimuth angle and collector normal azimuth angle (example
for northern hemisphere) . 12
Figure 4 – Rim angle of a parabolic-trough collector . 14
Figure 5 – Illustration of solar azimuth angle definition in the northern hemisphere . 15
Figure 6 – Illustration of solar azimuth angle definition in the southern hemisphere . 15
Figure 7 – Typical interconnection of the power generation (G), the auxiliary power
transformer and the main power transformer in a solar thermal electricity plant . 27

Table 1 – Optical terms and symbols. . 18

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SOLAR THERMAL ELECTRIC PLANTS –

Part 1-1: Terminology
FOREWORD
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Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 62862-1-1, which is a technical specification, has been prepared by IEC technical
committee 117: Solar thermal electric plants.

– 4 – IEC TS 62862-1-1:2018 © IEC 2018
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
117/75/DTS 117/85/RVDTS
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
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SOLAR THERMAL ELECTRIC PLANTS –

Part 1-1: Terminology
1 Scope
This part of IEC 62862 contains the main terms and definitions used by the solar thermal
electric (STE) industry and intends to be a reference for users of industry documents.
Since the components and configurations of STE plants depend on the concentrating solar
thermal technology used (i.e., central receiver, parabolic-trough collector, parabolic-dish or
linear Fresnel concentrator), some terms are not applicable to all types of STE plants and
notes have been introduced in their definitions for clarification.
The reference STE plant configuration assumed is composed of three main subsystems: solar
field, power block and (eventually) thermal storage system.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
ISO 9488:1999, Solar energy – Vocabulary
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1 SECTION 01: DESCRIPTION OF SYSTEMS, SUB-SYSTEMS AND COMPONENTS
3.1.1
absorber
element of the receiver absorbing radiant solar energy and transferring it to a fluid in the form
of heat
3.1.2
absorber cover
transparent element that covers the absorber to reduce heat losses and provide weather
protection
Note 1 to entry: When this element is made of glass it is usually referred to as a "glass cover".
3.1.3
active length of a linear receiver
length of the absorber exposed to concentrated solar radiation, at a reference temperature

– 6 – IEC TS 62862-1-1:2018 © IEC 2018
Note 1 to entry: This temperature is 25 °C if not otherwise stated. The absorber length considered to be exposed
is that where the radiation impinging normal to the absorber's surface is not shadowed.
Note 2 to entry: Unit: the SI unit is m.
3.1.4
auxiliary heater
equipment in which thermal energy is transferred to the heat transfer fluid by means of
non-solar fuel consumption
3.1.5
collector aperture normal
vector perpendicular to the collector aperture plane
3.1.6
collector aperture plane
plane, perpendicular to the collector transversal plane, that contains the solar thermal
collector aperture area
3.1.7
collector axis
straight line resulting from the intersection of the
collector aperture plane and a plane containing the linear receiver and perpendicular to the
collector aperture plane
SEE: Figure 3.
3.1.8
collector longitudinal plane
plane defined by the collector axis and the collector aperture normal
SEE: Figure 2.
3.1.9
collector loop
set of line-focus solar thermal collectors assembled in series, in such a way that the same
mass flow is circulating through the absorber tube of each
heat transfer fluid
Note 1 to entry: A loop is composed of one or more collector rows connected in series.
3.1.10
collector row
set of line-focus solar thermal collectors assembled in series with the same heat transfer
fluid mass flow and direction
3.1.11
collector transversal plane
plane perpendicular to the collector axis
SEE: Figure 2.
3.1.12
concentrator
reflecting or refracting elements that concentrate and redirect the beam solar radiation onto
the receiver
3.1.13
dispatchability
capability of the STE plant to respond to the grid operator on demand, regardless of weather
conditions
Note 1 to entry: The design of the plant and the availability of backup energy determine the degree on which grid
operator demands can be fulfilled.
3.1.14
dispatchable STE plant
STE plant able to decouple the electricity production periods from the solar resource
availability periods in order to satify the grid operator dispatch demands
3.1.15
effective length factor of a linear receiver
ratio of the active length of a linear receiver to its total length, at the specific temperature of
the receiver tube
Note 1 to entry: Although the effective length factor can be given for any absorber temperature, its nominal
reference temperature is 25 °C if not specified otherwise.
3.1.16
facet
smallest reflecting or refracting element composing a solar concentrator
3.1.17
heat transfer fluid
HTF
fluid used to carry heat from one system component to another in the STE plant
3.1.18
heliostat
system that reflects the beam solar radiation towards a predetermined fixed target by
means of a single or a set of reflecting elements (facets) controlled by a 2-axis solar
tracking system
3.1.19
linear collector incident plane
plane defined by the solar vector and the collector axis.
3.1.20
linear Fresnel collector
line-focus solar thermal collector that uses reflectors composed of at least two longitudinal
segments with parallel axes to concentrate the solar radiation onto a fixed receiver
3.1.21
line-focus solar system
solar system using line-focus solar thermal collectors.
3.1.22
line-focus solar thermal collector
concentrating solar thermal collector that concentrates solar radiation in one plane only,
producing a linear focus
[SOURCE: ISO 9488:1999, 7.7, modified – The term has been changed from "line-focus
collector".]
3.1.23
line-focus solar thermal collector module
minimum subdivision of a line-focus solar thermal collector for which the concentrator, in its
whole transversal extent, can be actuated independently

– 8 – IEC TS 62862-1-1:2018 © IEC 2018
3.1.24
parabolic-dish collector
point-focus solar thermal collector using a parabolic-dish reflector
[SOURCE: ISO 9488:1999, 7.10.]
3.1.25
parabolic-trough collector
solar collector assembly
SCA
line-focus solar thermal collector that concentrates the solar radiation by means of a
reflector with a parabolic cross section
Note 1 to entry: It is composed of a set of elements that altogether can track the sun as a single unit.
[ SOURCE: ISO 9488:1999, 7.8, modified – Note 1 to entry added.]
3.1.26
parabolic-trough solar field circuit
linear Fresnel solar field circuit
parabolic-trough heat transfer fluid system
linear Fresnel heat transfer fluid sytem
system made up of the component parts through which the solar field heat transfer fluid flows
from/to other sub-system of the plant (e.g. power block, thermal storage system, auxiliary
heater)
3.1.27
point-focus solar system
solar system using point-focus solar thermal collectors or a central receiver
3.1.28
point-focus solar thermal collector
solar thermal collector that concentrates the solar radiation on a single point or non-linear
focus
3.1.29
positive collector axis
defines the orientation of the solar thermal collector
Note 1 to entry: The alignment in space is described by the collector axis azimuth angle.
Note 2 to entry: The axis orientation is positive or there is a positive collector axis when the projection of the
collector axis into the horizontal plane points towards the south in the northern hemisphere, and towards the north
in the southern hemisphere. In the case of east-west aligned solar thermal collectors, the positive collector axis is
when the projection points towards the west.
3.1.30
power block
STE plant equipment or components in which thermal-to-electric conversion takes place
Note 1 to entry: In those STE plants provided with steam generators fed by the heat transfer fluid used in the
solar field, the steam-generating system is included in the power block. In STE plants with direct steam generation,
the solar receivers are not included in the power block.
3.1.31
receiver
set of components (absorbers, glass cover, bellows, getters, etc.) that converts the
concentrated solar radiation into thermal energy
Note 1 to entry: For solar tower plants, other plant components, required for the receiver to work, are included.

3.1.32
central receiver
single receiver used with solar fields composed of heliostats
3.1.33
linear receiver
receiver used in line-focus solar thermal collectors
3.1.34
solar field
part of the STE plant that collects and concentrates the beam solar radiation
Note 1 to entry: In STE plants with parabolic-trough collector or Fresnel linear collectors, the solar field is
composed of a set of solar thermal collectors and their piping interconnections and headers. The solar field inlet is
the last connecting point in the direction from the pumping equipment to the solar thermal collectors at which either
the storage system, auxiliary heater or pumps are connected, while the solar field outlet is the first connecting point
in the direction from the solar thermal collectors to the power block at which either the thermal storage system or
auxiliary heater is connected. In a central receiver plant, the solar field is composed of the heliostats. In STE plants
with parabolic dishes, the solar field is composed of the parabolic dishes.
3.1.35
solar thermal collector
device designed to absorb the solar radiation (concentrated or non-concentrated) and transfer
the thermal energy thus generated to a heat transfer fluid
Note 1 to entry: For concentrating solar thermal collectors, the main components are: the concentrator, the
receiver and the supporting structure.
3.1.36
solar thermal electricity plant
STE plant
solar thermal power plant
STP plant
facility, which applies solar concentration and thermodynamic processes, to convert direct
solar radiation into electricity suitable for its distribution and consumption
Note 1 to entry: The facility can include further sources of thermal energy, such as fossil fuel or biomass, in
parallel to solar radiation.
Note 2 to entry: Historically, "CSP" (concentrated solar power) universally referred only to, and was used in place
of, "STE". Only in recent years has the term "STE" (solar thermal electricity) become widespread and have some
organizations changed the definition of CSP to include both STE and concentrating photovoltaics (CPV). However,
some organizations still use "CSP" to refer to, and in place of, "STE", and in these cases CSP does not include
CPV. Therefore, the meaning of CSP varies between organizations without a clear definition and is not used herein.
The term "CST" (concentrating solar thermal) is used to globally or individually refer to the technologies used to
concentrate and convert solar radiation into thermal energy (i.e. CST technology or technologies).
3.1.37
supporting structure
structure that serves to support the components of the solar thermal collector with the
required mechanical stiffness.
3.2 SECTION 02: ANGLE DEFINITIONS
3.2.1
acceptance angle of a concentrating solar thermal collector
2·θ
c
angular range (2·θ ) over which all parallel rays intercepted by the solar thermal collector hit
c
the absorber without moving all or part of the collector
Note 1 to entry: For nominal values, a perfect shape of the concentrator is assumed.
Note 2 to entry: Unit: the non-SI unit is °.

– 10 – IEC TS 62862-1-1:2018 © IEC 2018
3.2.2
angle of acceptance of specular reflectance
ψ
polar angle defined by the direction of the ideal specular reflected beam and the direction of
the admissible maximum dispersion of reflection on the surface
SEE: Figure 1.
θ = β
θ β
ψ
IEC
Figure 1 – Angle of acceptance of specular reflectance, ψ
Key:
𝜃 Incidence angle
β Reflection angle
ψ Angle of acceptance of specular reflectance
1 Reflecting surface
Note 1 to entry: Unit: the non-SI unit is °.
3.2.3
angle of incidence of the beam solar radiation
incidence angle of the beam solar radiation
incident angle of the beam solar radiation
θ
angle between the straight line joining the centre of the solar disk to a point on an irradiated
surface and the outward normal to the irradiated surface at that point
SEE: Figure 2.
→
*
V
s
θ
T
θ
L
Longitudinal
θ
θ
LS
solar
plane
Longitudinal
Transversal
collector
collector
plane
plane
North
South
γ
A
γ
S
Collector
longitudinal axis
Collector
IEC
transversal axis
Figure 2 – Angles of incidence in linear Fresnel collectors
Note 1 to entry: For parabolic-trough collectors and parabolic dishes, the irradiated surface is the solar thermal
collector aperture area. For linear Fresnel collectors, the irradiated surface is fixed in space and usually horizontal.
Note 2 to entry: Unit: the non-SI unit is °.
[SOURCE: ISO 9488:1999, 2.11, modified – Term changed to "angle of incidence of the beam
solar radiation" and reference to Figure 2 added.]
3.2.4
transversal angle of incidence
θ
τ
angle between the collector aperture normal and the projection of the sun beam into the
transversal plane (plane perpendicular to the collector axis)
SEE: Figure 2.
Note 1 to entry: The transversal angle of incidence gets positive if the projection of the solar beam into the
transversal plane rotates in clockwise direction from the vertical for an observer placed at the northern end of the
solar thermal collector. For a collector exactly aligned east-west, the angle gets positive if the projection of the
solar beam into the transversal plane rotates counter-clockwise from the vertical for an observer placed at the
eastern end of the solar thermal collector.
Note 2 to entry: Unit: the non-SI unit is °.
3.2.5
longitudinal angle of incidence
θ
L
angle between the collector aperture normal and the projection of the sun beam into the
longitudinal plane (plane defined by the collector axis and the collector aperture normal)

– 12 – IEC TS 62862-1-1:2018 © IEC 2018
SEE: Figure 2.
Note 1 to entry: Usually, for symmetric solar thermal collector systems, it is sufficient to use the magnitude of the
angle independent of the direction (0 . 90°). For non-symmetric solar thermal collectors, the longitudinal angle is
defined positive when the Sun is located in the positive collector axis direction; otherwise it is negative.
Note 2 to entry: The term "longitudinal" is more historical, while "axial" is more recent.
Note 3 to entry: Unit: the non-SI unit is °.
3.2.6
longitudinal solar angle
θ
LS
angle between the sun beam vector and the projection of the sun beam into the transversal
plane
SEE: Figure 2.
Note 1 to entry: Only for ideally tracked parabolic-trough collectors, the longitudinal solar angle equals the
incidence angle.
Note 2 to entry: Unit: the non-SI unit is °.
3.2.7
collector axis azimuth angle
ϒ
A
angle between the positive collector axis and due south (in the northern hemisphere) or due
north (in the southern hemisphere), measured clockwise in the northern hemisphere and
anticlockwise in the southern hemisphere, using the projections on the local horizontal plane
SEE: Figure 3.
Collector axis
Collector normal
Valid in northern
hemisphere
Projection of collector normal
onto the horizontal
E
Collector normal
azimuth angle
Collector axis
ϒ = –150°
N
azimuth angle
S ϒ = –45°
A
Projection of collector axis
onto the horizontal IEC
Figure 3 – Collector axis azimuth angle and collector normal azimuth angle
(example for northern hemisphere)
Note 1 to entry: The same sign criteria used for solar azimuth angle is applied for collector axis azimuth angle.
Note 2 to entry: Unit: the non-SI unit is °.

3.2.8
collector axis tilt angle
β
A
angle between the horizontal plane and the collector axis (defined when looking in positive
collector axis)
Note 1 to entry: Positive from horizon upwards and negative from horizon downwards.
Note 2 to entry: Unit: the non-SI unit is °.
3.2.9
collector normal azimuth angle
ϒ
N
direction that a solar thermal collector faces, expressed as the azimuth angle of the horizontal
projection of the collector aperture normal
SEE: Figure 3.
Note 1 to entry: The angle definition is the same as for the solar azimuth angle.
Note 2 to entry: Unit: the non-SI unit is °.
3.2.10
collector normal tilt angle
β
N
angle between the horizontal plane and the plane of the specified surface
Note 1 to entry: Unit: the non-SI unit is °.
[SOURCE: ISO 9488:1999, 11.2, modified – The term has been changed from "tilt angle".]
3.2.11
rim angle of a solar thermal collector
φ
half of the angle defined by the lines connecting the centre of the receiver with the edge of the
concentrator
Note 1 to entry: It is calculated as:
tan(φ/2) = w/4f
Where
w is the aperture width of solar thermal collector;
f is the focal distance (see Figure 4).
[SOURCE: Rabl A., 1985, Active solar thermal collectors and their applications, New York:
Oxford University Press.]
– 14 – IEC TS 62862-1-1:2018 © IEC 2018
d
ϕ
w
IEC
Figure 4 – Rim angle of a parabolic-trough collector
Note 1 to entry : Unit: the non-SI unit is °.
3.2.12
solar altitude angle
solar elevation angle
α
S
complement of the solar zenith angle
Note 1 to entry: Unit: the non-SI unit is °.
[SOURCE: ISO 9488:1999, 2.7, modified – A formula has been deleted and Note 1 to entry
has been added.]
3.2.13
solar azimuth angle
ϒ
s
projected angle between a straight line from the apparent position of the sun to the point of
observation and due south (in the northern hemisphere) or due north (in the southern
hemisphere), measured clockwise in the northern hemisphere and anticlockwise in the
southern hemisphere, using the projections on the local horizontal plane
SEE: Figure 5 and Figure 6.
f
Valid in northern
hemisphere
Incident solar beam
E
Solar azimuth angle
ϒ = –45°
A
S
Projection of the incident solar beam
onto the horizontal
IEC
Figure 5 – Illustration of solar azimuth angle definition in the northern hemisphere
Valid in southern
hemisphere
Incident solar beam
N
Solar azimuth angle
ϒ = –35°
N
Projection of the incident solar beam
onto the horizontal
E
S
IEC
Figure 6 – Illustration of solar azimuth angle definition in the southern hemisphere
Note 1 to entry: The solar azimuth is negative in the morning (eastern directions), 0° or 180° at noon (depending
on the relative values on solar declination and local latitude), and positive in the afternoon (western directions),
over the whole globe. It diverges from the geographical azimuth, which is measured clockwise from due north, over
the whole globe.
Note 2 to entry: Unit: the non-SI unit is °.
[SOURCE: ISO 9488:1999, 2.4, modified – Reference to figures and Note 2 to entry have
been added.]
3.2.14
solar zenith angle
θ
Z
angular distance of the sun beam from the vertical
Note 1 to entry: Unit: the non-SI unit is °.
[SOURCE: ISO 9488:1999, 2.6, modified – Note 1 to entry has been added.]

– 16 – IEC TS 62862-1-1:2018 © IEC 2018
3.2.15
tracking angle of a linear collector
ρ
track
angle between the vertical and the normal to the collector aperture
plane
Note 1 to entry: For tilted solar thermal collectors, it is the angle between the projection of the vertical onto the
transversal plane and the normal to the collector aperture plane. For linear Fresnel collector, there is an individual
angle for each mirror line of the solar thermal collector.
Note 2 to entry: The angle gets positive if the collector aperture normal rotates clockwise from the vertical for an
observer placed at the northern end of the solar thermal collector (arbitrary aligned). For a solar thermal collector
exactly aligned east-west, the angle gets positive if the collector aperture normal rotates anticlockwise from the
vertical for an observer placed at the eastern end of the collector.
Note 3 to entry: Unit: the non-SI unit is °.
3.3 SECTION 03: AREA DEFINITIONS
3.3.1
absorber gross area
total area of the absorber
Note 1 to entry: For receivers without a secondary concentrator and composed of several parallel tubes, it is
given by the sum of the products of the total length and the perimeter of each tube. For receivers with or without a
secondary concentrator and composed of a single absorber tube, it is given by the products of the total length of
the absorber tube and its perimeter.
3.3.2
net area factor
ratio of the receiver net collection area to the receiver aperture area
3.3.3
net area factor
ratio of the solar thermal collectior net collection area to the solar
termal collector aperture area
3.3.4
receiver aperture area
maximum receiver flat area that accepts the concentrated solar radiation
Note 1 to entry: This is the area of the flat surface defined by the outer perimeter of the receiver, including non-
active zones (if any) between adjacent absorber elements composing the receiver. For receivers without a
secondary concentrator and composed of several parallel tubes, it is given by the product of the total length of
each tube and the total width of the receiver. For receivers without secondary concentrator and composed by a
single tube, it is given by the product of the total length and the diameter of the absorber tube (i.e. excluding the
glass cover (if any)). For receivers with a secondary concentrator, it is given by the product of the total length of
the receiver and the width of the aperture area of the secondary concentrator. For cavity receivers, it is the flat
surface associated to the aperture of the cavity.
Note 2 to entry: Unit: the SI unit is m .
3.3.5
receiver net collection area
flat surface that accepts the concentrated solar radiation
Note 1 to entry: For receivers without a secondary concentrator and composed by several parallel tubes, it is
given by the sum of the products of the active length and diameter of each tube. For receivers without a secondary
concentrator and composed of a single tube, it is given by the products of the active length of the receiver tube and
the arch length of the receiver tube that is defined by the double of the rim angle. For linear receivers with a
secondary concentrator, it is given by the product of the active length of the receiver and the width of the flat area
defined by the perimeter of the secondary concentrator. For cavity receivers, it is the flat surface associated with
the perimeter of the cavity.
Note 1 to entry: Unit: the SI unit is m .

3.3.6
solar thermal collector aperture area
A
maximum projected area that accepts the solar radiation
Note 1 to entry: Unit: the SI unit is m .
3.3.7
solar thermal collector gross aperture area
A
gross
area of the flat surface defined by the outer perimeter of the solar thermal collector, including
the gaps between adjacent reflectors
Note 1 to entry: This definition may be used for modules, heliostats, heliostat fields, parabolic dishes, linear
Fresnel reflectors, etc., as well as complete solar thermal collectors.
3.3.8
solar thermal collector net aperture area
A
net
area of the perpendicular projection over the collector aperture plane of the solar thermal
collector reflecting/refracting components
Note 1 to entry: In line-focus solar system, it is this surface plus the part of the perpendicular projection of the
steel receiver tube onto the collector aperture plane that does not overlap, provided that the sun-oriented side of
the receiver is absorbing radiation.
Note 2 to entry: The net aperture area of a Linear Fresnel collector or heliostat is defined as the sum of the net
aperture areas of its mirror segment. The net aperture area of a mirror segment is the perpendicular projection of
the reflective mirror area over its collector aperture plane when they are in the horizontal position.
3.3.9
solar thermal collector nominal aperture area
A
nom
flat aperture area, defined by product specifications, for example by the manufacturer
Note 1 to entry: The value should be between "net" and "gross". This term is necessary to have consistent
definitions with efficiency and performance figures, as "gross" and "net" may include ambiguities resulting in invalid
output characteristics. It may be used for modules, heliostats, heliostat fields, parabolic dishes, linear Fresnel
collector, reflectors, etc., as well as complete solar thermal collectors.
3.3.10
solar thermal collector net collection area
area of the perpendicular projection over the collector aperture plane of the solar thermal
collector reflecting/refracting components
Note 1 to entry: In a parabolic-trough collector, it is this surface plus the part of the perpendicular projection of
the steel receiver tube onto the collector aperture plane that does not overlap.
Note 2 to entry: Unit: the SI unit is m .

– 18 – IEC TS 62862-1-1:2018 © IEC 2018
3.4 SECTION 04: OPTICAL PROPERTIES
Table 1 – Optical terms and symbols
Absorptance, α Emittance, ε Reflectance, ρ Transmittance, τ
Hemispherical Hemispherical
Near specular Direct
Spectral Spectral Spectral: Spectral:
α (λ, θ , T ) ε (λ, θ , T ) • Hemispherical • Hemispherical
λ i s λ i s
ρ (λ,θ ,h,T ) τ (λ,θ ,h,T )
λ,h i s λ,h i s
• Near-specular • Direct
ρ (λ,θ , ψ,T ) τ (λ,θ ,d,T )
λ,φ i s λ,d i s
Solar Radiant Solar: Solar:
α ([λ , λ ], θ , T )
ε ([λ , λ ],θ ,T ) • Hemispherical • Hemispherical
S a b i s
T a b i s
ρ ([λ ,λ ],θ ,h,T ) τ ([λ ,λ ],θ ,h,T )
S,h a b i s S,h a b i s
• Near specular • Direct
τ ([λ ,λ ],θ ,d,T )
S,d a b i s
ρ ([λ ,λ ],θ , ψ,T )
S,φ a b i s
3.4.1
absorptance
α
ratio of the radiant flux absorbed by a surface element to the radiation incident on it
Note 1 to entry: The absorptance is a dimensionless variable ranging from 0 to 1, which depends on the
wavelength, 𝜆, the direction of the incident radiation, 𝜃 , and the nature, finish and temperature, T , of the
i s
surface which the incident radiation is impinging on.
Note 2 to entry: Unless otherwise specified, the temperature T taken into consideration is the ambient
s
temperature. In this case, the symbol T , can be omitted.
s
Note 3 to entry: The terms "absorptivity" and "absorptance" are generally used interchangeably, as if they were
synonymous. According to the National Bureau of Standards, the ending "-ivity" expresses the property of a
material in general, for a homogeneous and semi-infinite sample, i.e. an intrinsic material property, (resistivity,
thermal conductivity, etc.). The ending "-ance" expresses the property of a particular sample of material or a
particular surface. In this case, the value depends on the specific conditions of the material (dirt, oxidation, grated,
etc.) or even the thickness of the sample, i.e. strength or electrical conductance. Therefore, when talking about
optical properties of materials used in solar applications (reflectance, transmittance, absorptance and emittance)
the ending "-ance" shall be used, because such materials can be degraded by the thermal process, soiling,
mechanical damage, etc.
[SOURCE: ISO 9488:1999, 5.1, modified – Notes to entry have been added.]
3.4.2
direct transmittance
near-direct transmittance
ratio of the radiant flux passing through a material within the solid angle 2·π· ψ (ψ is the angle
of acceptance of the transmitted radiation) around the incident beam direction to the radiation
incident on it
SEE: Table 1.
3.4.3
emittance
ε
ratio of the radiant energy flux emitted by a body from its surface to the radiant energy flux
emitted by a perfect black body emitter at the same conditions
Note 1 to entry: The emittance is a dimensionless variable ranging from 0 to 1, which depends on wavelength, 𝜆,
direction of emission, 𝜃 , and nature, finish and temperature, T , of the surface emitting the radiant energy
i s
Note 2 to entry: The terms " emissivity" and " emittance" are generally used interchangeably, as if they were
synonymous. According to the National Bureau of Standards, the ending "-ivity" expresses the property of a
material in general, for a homogeneous and semi-infinite sample i.e. an intrinsic material property (resistivity, thermal
conductivity, etc.). The ending "-ance" expresses the property of a particular sample of material or a particular surface.
In this case, the value depends on the specific conditions of the material (dirt, oxidation, grated, etc.) or even the
thickness of the sample, i.e. strength or electrical conductance. Therefore, when talking about optical properties of
materials used in solar applications (reflectance, transmittance, absorptance and emittance) the ending
"-ance" shall be used, because such materials can be degraded by the thermal process, soiling,
mechanical damage, etc.
3.4.4
hemispherical reflectance
ratio of the radiant flux reflected by a surface within the complete hemisphere over that
surface, to the radiation incident on it
SEE: Table 1.
3.4.5
hemispherical transmittance
ratio of the radiant flux passing through a material within the complete hemisphere over its
exit surface, to the radiation incident on it
SEE: Table 1.
3.4.6
near-normal incidence
situation where the incidence angle is below 15°
3.4.7
radiant emittance
ratio of the radiant energy flux emitted in all spatial directions by a material from its surface at
a given temperature T to that emitted by a black body at the same temperature and the same
s
wavelength range
Note 1 to entry: The radiant emittance is indicated with the subscript "T" after the emittance symbol, ε . The
T
radiant emittance is represented as ε ([λ ,λ ],θ ,T ), being [λ ,λ ] the wavelength range, θ the incidence angle and
T a b i s a b i
T the temperature, which shall be indicated. It is expressed as in the next equation:
s
λ
b
ε (λ,θ ,T )i (λ,T )dλ
λ i S λ,bb S
∫
λ
a
ε ([λ ,λ ],θ ,T ) =
T a b i S
λ
b
i (λ,T )dλ
λ,bb S
∫
λ
a
As there is not a standard method for measuring a sample at different temperatures, an approximate value
measured at room temperature can be used. In this case, the expression is:
λ
b
ε (λ,θ ,T )i (λ,T )dλ
λ i S (room temperature) λ,bb S (operating temperature)
∫
λ
a
ε ([λ ,λ ],θ ,T ) =
T a b i S
λ
b
i (λ,T )dλ
λ,bb S (operating temperature)
∫
λ
a
Where:
ε (λ, θ T ) is the spectral emittance;
,
λ i s
– 20 – IEC TS 62862-1-1:2018 © IEC 2018
i (λ, T ) is the emission intensity of a black body for every wavelength at a given

λ,bb s
temperature, calculated according to Planck's law where h is Planck's constant (6,626 ×
−34 8
10 J s), c is the light velocity (3,0 × 10 m/s) and k is Boltzmann's constant
−23
(1,380 648 52 × 10 J/K).
8π h d 1
i (λ,T ) =
λ,bb S
5 (hc λ k T )
λ e −1
λ
b
i (λ,T )dλ is the Stefan-Boltzman law, giving the total power emitted per unit area at the surface of a
λ,bb S
∫
λ
a
black body.
The wavelength range within which the measuring and respective weighting is performed, [λ ,λ ], will be
a b
determined according to the temperature T , so that it covers the emission spectrum of a black body at the
s
temperature T , assessed in accordance with Planck's law.
s
SEE: Table 1.
3.4.8
reflectance
ρ
ratio of the radiant flux reflected from a surface to that of the incident radiation
Note 1 to entry: Reflectance is a dimensionless variable ranging from 0 to 1, which depends on wavelength, λ,
direction of the incident radiation, θ , direction of the reflected radiation, θ , the nature, the finish and
i r
temperature of the surface which the radiation is falling on, T .
s
Note 2 to entry: Unless oth
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