prEN ISO 24194

SLOVENSKI STANDARD
01-julij-2025
Sončna energija - Polja sprejemnikov sončne energije - Preverjanje zmogljivosti
(ISO/DIS 24194:2025)
Solar energy - Collector fields - Check of performance (ISO/DIS 24194:2025)
Sonnenenergie - Kollektorfelder - Überprüfung der Leistungsfähigkeit (ISO/DIS
24194:2025)
Energie solaire - Champs de capteurs - Vérification de la performance (ISO/DIS
24194:2025)
Ta slovenski standard je istoveten z: prEN ISO 24194
ICS:
27.160 Sončna energija Solar energy engineering
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 24194
ISO/TC 180/SC 4
Solar energy — Collector fields —
Secretariat: SAC
Check of performance
Voting begins on:
Energie solaire — Champs de capteurs — Vérification de la
2025-05-14
performance
Voting terminates on:
ICS: 27.160 2025-08-06
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
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Reference number
ISO/DIS 24194:2025(en)
DRAFT
ISO/DIS 24194:2025(en)
International
Standard
ISO/DIS 24194
ISO/TC 180/SC 4
Solar energy — Collector fields —
Secretariat: SAC
Check of performance
Voting begins on:
Energie solaire — Champs de capteurs — Vérification de la
performance
Voting terminates on:
ICS: 27.160
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2025
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
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Published in Switzerland Reference number
ISO/DIS 24194:2025(en)
ii
ISO/DIS 24194:2025(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 2
5 General . 5
5.1 Validity .5
5.2 Methodology and operating conditions .6
5.3 Source of input information .6
5.4 Application .6
6 Procedure for checking the power performance of solar thermal collector fields –
Power Check . 7
6.1 Stating an estimate for the thermal power output of a collector field .7
6.2 Calculating estimated power output .7
6.2.1 General .7
6.2.2 General Formula (1) for power estimate – using beam and diffuse irradiance .8
6.2.3 Simplified Formula (2) – using hemispherical irradiance.8
6.3 Restrictions on operating conditions .8
6.4 Shadows .9
6.4.1 Shading from the surroundings .9
6.4.2 General shadow impact of collectors on each other .9
6.4.3 Shadows on fixed collectors in rows .10
6.4.4 Shadows on one-axis tracking collectors in row .10
6.5 Collector incidence angle . 13
6.6 Stagnation and periods of deliberately reduced performance . 13
6.7 Unmodeled Effects . 13
6.8 Example of setting up an equation for calculating performance estimate.14
6.9 Determination of potential valid periods . 15
6.10 Checking collector field power performance . 15
7 Procedure for checking the daily yield of solar thermal collector fields – Daily Yield
Check . 17
7.1 Stating an estimate for the daily yield of a collector field .17
7.2 Calculating daily energy yield . .17
7.2.1 General .17
7.2.2 Formula (19) for daily yield estimate .17
7.3 Restrictions on operating conditions .18
7.4 Shadows .19
7.5 Collector incidence angle .19
7.6 Example of setting up an equation for calculating performance estimate.19
7.7 Determination of potential valid periods .21
7.8 Checking collector field daily yield performance .21
8 Procedure for checking the annual yield of solar thermal collector fields – Annual Yield
Check .23
8.1 Stating an estimate for the annual yield of a collector field . 23
8.2 Calculating annual energy yield . 23
8.2.1 General . 23
8.2.2 General Formula (27) for liquid heating collectors .27
8.3 Restrictions on operating conditions .31
8.4 Shadows .32
8.4.1 Shading from the surroundings .32
8.4.2 General shadow impact of collectors on each other .32

iii
ISO/DIS 24194:2025(en)
8.5 Switch-off times and power reducing control strategies – planned and unplanned .32
8.6 Example of setting up an equation for calculating performance estimate. 33
8.7 Determination of potential valid periods . 33
8.8 Checking collector field annual yield performance . 33
9 Measurements needed.35
9.1 General . 35
9.2 Requirements on measurements, sensors and data .37
9.2.1 Accuracy .37
9.2.2 Time .37
9.2.3 Solar radiation measurement and satellite data . 38
9.2.4 Temperature measurements . 40
9.2.5 Flow rate measurement .41
9.2.6 Power measurement/calculation .41
9.2.7 Measurement of wind speed .42
9.3 Valid data records .42
Annex A (informative) Recommended reporting format — Power Check .43
Annex B (informative) Recommended reporting format — Daily Yield Check .45
Annex C (informative) Recommended reporting format — Annual Yield Check.46
Bibliography .48

iv
ISO/DIS 24194:2025(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent
rights identified during the development of the document will be in the Introduction and/or on the ISO list of
patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 180, Solar energy, Subcommittee SC 4, Systems
- Thermal performance, reliability and durability.
This second edition cancels and replaces the first edition ISO 24194:2022 and the amendment
ISO 24194:2022/Amd 1:2024 as it has been revised and extended.
This edition includes the following significant changes compared with the previous edition:
— Introduction: Information and explanations on how large-scale solar installations behave with regard to
climate change and other environmental aspects.
— Scope: The scope has been specified to liquid heating collectors
— Chapter 5: General chapter added with validity of all 3 procedures was summarized in one general
chapter instead of being repeated for every procedure separately, Methodology and operating conditions
are explained, source of input information is described and possible applications are mentioned
— Chapter 6: Wrong formula for beam irradiance has been deleted and only one general formula according
to ISO 9806 for liquid heating collectors introduced using beam and diffuse irradiance, which can
be simplified in special cases using only hemispherical irradiance. No general distinction between
concentrating and non concentrating collectors. Example is now given with calculation and result.
— Chapter 7: Daily yield method has been generalized so that is applicable for all collectors in the scope.
Example is now given with calculation and result.
— Chapter 8: New method for annual yield has been added applicable for all collectors in the scope. Example
is given with calculation and result.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
ISO/DIS 24194:2025(en)
Introduction
0.1  General
This document specifies procedures for checking the performance of solar thermal collector fields. Measured
performance is compared with calculated performance - and conditions for conformity are given.
Three levels for accuracy in the checking can be chosen:
— Level I - giving possibility for giving a very accurate estimate (with low safety retention, e.g. f = 0,95)
safe
- but with requirements for use of expensive measurement equipment and maintenance.
— Level II/III - allowing for a less accurate estimate (with higher safety retention, e.g. f = 0,90) - but
safe
possibility to use less expensive measurement equipment.
0.2  Relevance of solar thermal collector fields in the context of the energy transition and heat supply
Heat accounts for half of the world's total energy consumption - significantly more than electricity (20 %)
and transport (30 %), and heat accounts for more than 40 % of the global energy-related carbon dioxide
emissions. Solar thermal technologies use the sun's energy directly to produce heat that can be used in a
wide range of applications. Solar thermal technologies are therefore key technologies for the reduction of
global carbon dioxide emissions. The core element of any solar thermal system is the solar thermal collector
and is dealt within ISO 9806. This standard applies especially to large-scale applications that can generate
huge quantities of heat. They have, compared to all other heat generators, the highest efficiency in converting
solar energy into heat and thus they require the least amount of space. Large solar thermal systems typically
used in the following areas of application
— District heating with solar fractions from a few percent to 100 % in combination with seasonal storages
— Industrial processes for food production such as dairies, breweries etc.
— Industrial cleaning processes
— Chemical processes
— Desalination
— Swimming pool heating
— Solar thermal process heat systems provide heat for industrial processes such as drying, food processing,
pasteurisation, washing, cleaning and all other manufacturing processes at medium temperatures
— Electricity generation based on heat
— All other application where heat is needed up to a temperature level of 450 °C
The achievable temperature level depends on the type of collector used. Today, collectors covered by this
standard are available for a temperature range from 25 to 450 °C. The heat transfer fluid used in solar
thermal collectors is usually water, water-based antifreeze or silicon oil.
0.3  Application range of this international standard
The current status of this standard is limited to liquid heating collectors.
This document sets out procedures for checking the thermal power output, daily and annual yield of solar
thermal fields to verify performance expectations.
This standard is intended to provide a solid basis for monitoring these fields during the commissioning and
operating phase and over the course of their lifetime, which is typically more than 25 years. It is therefore a
suitable tool to check the influence of soiling and degradation, which can also be linked to increased stresses
caused by climate change, for example.

vi
ISO/DIS 24194:2025(en)
WARNING – This International Standard does not purport to address all the safety or environmental
problems associated with its use. It is the responsibility of the user of this International Standard
to establish appropriate safety, health, and environmental practices, including climate change
mitigation efforts. Such efforts could include, but are not limited to, reducing greenhouse gas
emissions, minimizing waste, using renewable resources where possible, and conducting regular
environmental impact assessments. Users should also determine the applicability of regulatory
limitations prior to use.
0.4  Environmental impact of solar thermal systems
As these systems consist mainly of solar collectors more specific details can be found in ISO 9806. This
standard checking the performance does directly proof energy savings and by replacing fossil fuels also the
accompanying greenhouse gas reductions.
The efficiency of converting incoming solar radiation into useful heat can reach more than 80 %, depending
on the temperature required and the technology chosen. The annual yield depends very much on the
geographical location and the use of the collector. Certainly, the annual energy yield per area is higher with
solar collectors than with any other renewable technology. The yield is considered to be around three times
higher than photovoltaic power generation and 50-100 times higher than for biomass. Given the range of
different products and applications, it is not possible to quantify the greenhouse gas reduction or climate
change mitigation potential of this technology in general terms. The greenhouse gas reduction potential of a
solar thermal collector depends on the application in which it is used and what kind of conventional fuel and
system is replaced.
For the determination of the greenhouse gas reduction of a system in which a solar thermal system is used,
it is recommended to make this quantification using the global warming potential (GWP) for each GHG as
per Table 7.15 of the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) by
aggregating the reduction potential in carbon dioxide equivalents (CO2eq):
https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf
0.5  Supporting the UN Sustainable Development Goals (SDGs)
Achieving the Sustainable Development Goals (SDGs) established by the United Nations in 2015 has
become a high priority for society. This International Standard is aligned with the following United Nations
Sustainable Development Goals by promoting and supporting:
— Solar thermal collectors and solar thermal technologies to reduce dependence on energy prices to
increase the resilience of humanity to climate-related extreme events and other economic, social and
environmental shocks and disasters (SDG 1.5)
— Solar thermal collectors and solar thermal technologies for water treatment and disinfection to reduce
water-borne diseases (SDG 3.3)
— Solar thermal collectors and solar thermal technologies as a clean and safe energy source to replace other
energy sources, to significantly reduce the number of deaths and illnesses from hazardous chemicals
and polluted air caused by combustion of fossil fuels (SDG 3.9)
— Solar thermal collectors and solar thermal technologies in water purification and desalination facilities
to achieve universal and equitable access to safe and affordable drinking water for all (SDG 6.1)
— Solar thermal collectors and technologies as a key technology in capacity building programmes for water
treatment activities in developing countries (SDG 6.A).
— Solar thermal collectors to ensure access to affordable, reliable and modern energy for all (SDG 7.1).
— Solar thermal collectors to significantly increase the share of renewable energy for all (SDG 7.2).
— Solar thermal collectors in various applications to increase the overall global energy efficiency (SDG 7.3)
— International cooperation in solar thermal technologies to facilitate access to clean energy research
and technology, especially in the fields of renewable energy, energy efficiency and fossil fuel-free

vii
ISO/DIS 24194:2025(en)
technologies as well as to facilitate investments in solar thermal based energy infrastructure and clean
energy technologies (SDG 7.A)
— Solar thermal collectors to develop infrastructure and technologies for modern and sustainable energy
that is accessible to all in developing countries, in particular least developed countries, small island
developing states and landlocked developing countries, and to provide a sound basis for the development
of appropriate support programmes. (SDG 7.B)
— Solar thermal collectors and technologies as a technology well suited for local production, thereby
achieving higher levels of economic productivity through diversification, technological upgrading and
innovation, including focusing on high value-added and labour-intensive sectors (SDG 8.2).
— Solar thermal collectors and solar thermal technologies as locally manufacturable devices with
low requirements for production technologies and financial investments, to promote and support
productive activities, decent job creation, entrepreneurship, creativity and innovation, and to promote
the formalisation and growth of micro, small and medium-sized enterprises, including through access to
financial services (SDG 8.3).
— Solar thermal collectors and technologies to progressively improve the global resource efficiency of
consumption and production, and to endeavour the decoupling of economic growth from environmental
degradation. (SDG 8.4)
— Solar thermal collectors as part of high quality, reliable, sustainable and resilient infrastructure thereby
supporting the economic development and human well-being, with a focus on affordable and equitable
access for all (SDG 9.1)
— Solar thermal collectors to contribute to the inclusive and sustainable industrialisation and to
significantly increasing the industry's share of employment and gross domestic product, particularly in
least developed countries, in accordance with national circumstances. (SDG 9.2)
— solar thermal collectors as a cost-effective means to increase the availability of energy and the
predictability of energy costs by reducing dependence on fossil energy, and thereby to facilitate the access
of small industrial and other enterprises, particularly in developing countries, to financial services,
including affordable credit, and to integrate these industries into value chains and markets (SDG 9.3).
— Solar thermal collectors to upgrade infrastructure and retrofit industries to make them sustainable,
with increased efficiency in resource use and greater adoption of clean and environmentally sound
technologies and industrial processes (SDG 9.4).
— Scientific research and technological capacity building in the field of solar thermal technologies in
all countries, particularly developing countries, including the promotion of innovation, research and
development activities (SDG 9.5).
— Solar thermal collectors and solar thermal technologies to facilitate sustainable and resilient
infrastructure development in developing countries and to facilitate financial, technological and
technical assistance to African countries, least developed countries, landlocked developing countries
and small island developing states (SDG 9.A).
— Solar thermal collectors as a way to enhance domestic technology development, research and innovation
in developing countries. In addition, the standard supports the development of an enabling policy
environment for solar thermal technologies for industrial diversification and commodity value addition
(SDG 9.B).
— Solar thermal collectors for fossil and biomass free production of heat and hot water to reduce the
adverse per capita environmental impact of cities and in particular by contributing to better air quality.
(SDG 11.6)
— Solar thermal collectors as an element of sustainable and resilient buildings, using local materials and
local production capacity (SDG 11.C)
— Solar thermal collectors for local and affordable thermal food processing to reduce global food waste
and food losses along production and supply chains, including post-harvest losses (SDG 12.3).

viii
ISO/DIS 24194:2025(en)
— Solar thermal collectors as a product that significantly reduces waste generation through its repairable
designs and through material reduction, the possibility of using recycled materials, and the reuse of
components (SDG 12.5).
— Solar thermal collectors and solar thermal technologies to encourage companies to adopt sustainable
practices and integrate sustainability information into their reporting cycles (SDG 12.6).
— Solar thermal collectors and solar thermal technologies as a locally manufactured technology, using
mainly recycled materials, as part of sustainable public procurement practices. (SDG 12.7)
— Solar thermal collectors and solar thermal technologies to strengthen the technological capacity to move
towards more sustainable patterns of consumption and production, not only in developing countries
(SDG 12.A)
— Subsidy schemes for solar thermal collectors, and thus also the reduction and phasing out of inefficient
fossil fuel subsidies, especially in developing countries, and minimising adverse impacts on their
development in a manner that protects the poor and affected communities (SDG 12.C).
— Solar thermal collectors and solar thermal technologies to reduce the overuse of biomass for heat
generation, thereby also supporting the implementation of sustainable management of all types of
forests, halting deforestation and even enabling afforestation and reforestation worldwide (SDG 15.2).
— The transfer of know-how, dissemination and diffusion of environmentally sound technologies to
developing countries on favourable terms (SDG 17.7)

ix
DRAFT International Standard ISO/DIS 24194:2025(en)
Solar energy — Collector fields — Check of performance
1 Scope
This document specifies three procedures to check the performance of solar thermal collector fields. This
document is applicable to most collector types addressed by ISO 9806. This includes glazed liquid heating
flat plate collectors, evacuated tube collectors, and tracking, concentrating collectors. However, certain
limitations apply in comparison to ISO 9806, specifically concerning WISC, hybrid collectors, and air
collectors:
— WISC collectors are included only when both wind speed data measured on the collector plane and
longwave radiation data are available. Note that such data is rarely available in practical applications.
— Hybrid collectors (“collectors co-generating thermal and electrical power” in ISO 9806, commonly PVT)
are excluded. The reason is that their thermal performance in real-world operation depends on the
operation of the electrical part: If the electrical part is switched off, limited, or curtailed for any reason,
while the thermal part remains active, this would lead to an overestimation of the thermal power output,
based on performance parameters determined under MPP conditions (maximum electrical power
generation), as specified in ISO 9806.
— Solar air heating collectors (SAHC) are excluded due to the complexity of accurately assessing their
performance: Testing and evaluating SAHC performance is complex, because it requires accounting for
the enthalpy difference in the primary loop. Additionally, their thermal efficiency is highly dependent
on mass flow rate, and their performance is tested at three different air mass flow rates, resulting in
three separate parameter sets. For the ISO 24194 Power Check, this makes it challenging to select the
appropriate parameters for estimating thermal power output.
The check can be done on the thermal power output of the collector field as well as on the daily yield and
annual yield of the collector field. The document specifies for the three procedures how to compare a
measured output with a calculated one.
The document applies for all sizes of collector fields.
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 9060, Solar energy — Specification and classification of instruments for measuring hemispherical solar and
direct solar radiation
ISO 9488, Solar energy — Vocabulary
ISO 9806, Solar energy — Solar thermal collectors — Test methods
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9488 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp

ISO/DIS 24194:2025(en)
— IEC Electropedia: available at https:// www .electropedia .org/
4 Symbols
A Gross area of collector as defined in ISO 9488 m
G
A Gross area of collector field m
GF
2·
a Heat loss coefficient W/(m K)
2· 2
a Temperature dependence of the heat loss coefficient W/(m K )
3·
a Wind speed dependence of the heat loss coefficient J/(m K)
a Sky temperature dependence of the heat loss coefficient —
2·
a Effective thermal capacity. In some literature and data sheets denoted J/(m K)
C . Note that C unit is kJ/m K.
eff eff
a Wind speed dependence of the zero-loss efficiency s/m
2· 4
a Radiation losses dependence W/(m K )
C Effective thermal capacity of collector J/K
C Geometric concentration ratio —
R
·
c Specific heat capacity of heat transfer fluid J/(kg K)
f
D Gap in between adjacent collectors m
E Longwave irradiance (λ > 3 μm) W/m
L
f Safety factor for cleanliness for the operating time between two cleanings -
C
f Safety factor taking into account heat losses from pipes etc. in the col- -
P
lector loop.
f Safety factor taking into account measurement uncertainty. -
U
f Safety factor for other uncertainties e.g. related to non-ideal conditions -
O
such as non-ideal flow distribution and unforeseen heat losses - and
uncertainties in the model/procedure itself.
f Value given by the supplier and/or mathematical product based on the -
safe
individual safety factors f , f , f
P U O
f Shading factor -
sh
G Hemispherical solar irradiance on a horizontal plane W/m
global
G Hemispherical solar irradiance on the plane of collector W/m
hem
G Direct solar irradiance (beam irradiance) on the plane of collector W/m
b
G Solar radiation received per unit area by a surface that is always held W/m
bn
perpendicular (or normal) to the rays that come in a straight line from
the direction of the sun at its current position
G Diffuse solar irradiance on the plane of collector W/m
d
ISO/DIS 24194:2025(en)
GTY Annual gross thermal yield kWh/m
a
H Annual hemispherical irradiation on a horizontal plane kWh/m
global,a
H Annual hemispherical irradiation on collector plane kWh/m
hem,a
H Daily hemispherical irradiation on collector plane kWh/m
hem,d
H Height of the shaded area m
sh
h Solar altitude angle sin h = cos θ °
Z
h Minimum solar altitude angle °
min
K (θ ,θ ) Incidence angle modifier for hemispherical solar radiation —
hem L T
K (θ ,θ ) Incidence angle modifier for direct solar irradiance —
b L T
K Incidence angle modifier in the longitudinal plane —
θL
K Incidence angle modifier in the transversal plane —
θT
K Incidence angle modifier for diffuse solar radiation —
d
K Daily average incidence angle modifier for hemispherical solar radiation —
hem,av
L Length of a collector (from bottom to top) m
L Overall Length of the pipe system without collectors m
pipe
L Length of the shaded area m
sh

m
Mass flow rate of heat transfer fluid kg/s
N Number of collectors in a row -
c
Coordinate of the point C on the X-axis (C is the point that would reach -
P the shadow formed by the top of the sun facing side of a collector row
X
if it were unobstructed)
P Coordinate of the point C on the y-axis -
Y

Measured power output W
Q
meas

W
Q Estimated power output
estimate
2 2
q
Specific measured power output per m collector gross area W/m
Q Daily capacity heat losses of solar thermal system J
cap,d
Q Daily yield estimation of solar thermal system J
estimate,d
Q Annual yield estimation of solar thermal system J
estimate,a
Q̇Daily average gross power output collector field W
estimate-col,d
Q Annual yield measurement of the heat meter J
HM,a
Q Daily yield measurement of the heat meter J
HM,d
Q̇Daily average heat losses of piping W
pipe,d
ISO/DIS 24194:2025(en)
·
q Empirical specific heat loses per m pipe W/(m K)
l-pipe
S Spacing center to center in between adjacent rows m
T Absolute temperature K
U Fictitious constant heat loss coefficient of the system kWh/K
sys
t Time s
t Time start of measurement s
s
t Time end of measurement s
e
u Surrounding air speed (wind speed) m/s
V Fluid capacity of the collector m
f
 3
V Volumetric flow rate m /s
V Volume of the pipe system without collectors l
pipe
w Width of a collector (from left to right) m
x-axis value of collector operating points as ratio of temperature differ- (m K)/kWh
x
i
ence and irradiation
z Number of days with reduced heat output -
ΔT Temperature difference between fluid outlet and inlet (ϑ - ϑ ) K
e i
β Slope (or tilt), the angle between the plane of the collector and the °
horizontal.
Note: For collectors rotating around a North-South axis, β is positive in
the morning when facing eastwards - and negative in the afternoon when
facing westwards
γ Surface azimuth angle, the deviation of the projection on horizontal °
plane of the normal to the surface from the local meridian, with zero
due south, east negative and west positive
γ Solar azimuth angle, the angular displacement from south of the projection °
s
of beam radiation on the horizontal plan, east negative and west positive
δ Declination, the angular position of the sun at solar noon with respect °
to the plane of the equator, north positive.
ϕ Latitude, the angular location north or south of the equator, north positive °
η Annual collector efficiency based on GTY and H —
a a a
η Collector efficiency based on beam irradiance G —
b b
η Collector efficiency based on hemispherical irradiance G —
hem hem
η Peak collector efficiency (η at ϑ − ϑ = 0 K) based on beam irradiance G —
0,b b m a b
η Peak collector efficiency (η at ϑ − ϑ = 0 K) based on hemispherical —
0,hem 0,hem m a
irradiance G
hem
η
Collector efficiency, with reference to mass flow m —

hem,m
i
i
ISO/DIS 24194:2025(en)
ω Hour angle, the angular displacement of the sun east or west of the local °
meridian due to rotation of the earth on its axis at 15° per hour; morning
negative, afternoon positive
θ Angle of incidence °
θ Longitudinal angle of incidence: angle between the normal to the plane of °
L
the collector and incident sunbeam projected into the longitudinal plane
θ Transversal angle of incidence: angle between the normal to the plane of °
T
the collector and incident sunbeam projected into the transversal plane
θ Zenith angle, the angle between the vertical and the line to the sun, that °
Z
is, the angle of incidence of beam radiation on a horizontal surface. cos
θ = sin h
Z
ϑ Ambient air temperature °C
a
ϑ Collector outlet temperature °C
e
ϑ Collector inlet temperature °C
i
ϑ Mean temperature of heat transfer fluid in collector loop °C
m
ρ Density of heat transfer fluid at flow meter position/temperature kg/m
sec
2· 4
σ Stefan-Boltzmann constant W/(m K )
5 General
5.1 Validity
The procedures described in this document can only b
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