ISO/FDIS 24194

ISO/TC 180/SC 4
Secretariat: SAC
Date: 2025-12-142026-02-20
Solar energy — Collector fields — Check of performance
Energie solaire — Champs de capteurs — Vérification de la performance
FDIS stage
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ii
Contents
Foreword . iv
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols . 2
5 General . 5
6 Procedure for checking the power performance of solar thermal collector fields – Power
Check . 7
7 Procedure for checking the daily yield of solar thermal collector fields – Daily Yield
Check . 20
8 Procedure for checking the annual yield of solar thermal collector fields – Annual Yield
Check . 30
9 Measurements needed . 43
Annex A (informative) Recommended reporting format — Power Check . 54
Annex B (informative) Recommended reporting format — Daily Yield Check . 57
Annex C (informative) Recommended reporting format — Annual Yield Check . 59
Bibliography . 61

iii
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
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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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
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Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.htmlwww.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.,, in collaboration with the European Committee for
Standardization (CEN) Technical Committee CEN/TC 312, Thermal solar systems and components, in
accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 24194:2022), which has been technically
revised. It also incorporates the Amendment ISO 24194:2022/Amd 1:2024.
The main changes are as follows:
— — Introduction revised;
— — Scope specifies liquid heating collectors;
— — Clause 55 added summarizing validity for all 3 procedures, methodology and operating conditions
explained, source of input information described and possible applications mentioned;
— — Clause 6:6: Incorrect formula for direct irradiance deleted and only one general formula according to
ISO 9806 for liquid heating collectors introduced using direct and diffuse irradiance; example with
calculation and result included;
— — Clause 7:7: Daily yield method generalized so that it is applicable for all collectors in the scope;
example with calculation and result included;
iv
— — Clause 8:8: New method for annual yield added that is applicable for all collectors in the scope;
example with calculation and result included;
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
Introduction
0.1 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),
U
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. fU = 0,90) - but
possibility to use less expensive measurement equipment.
0.2 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 document applies especially to large-scale applications that can generate large
quantities of heat. Solar thermal collector fields 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 are 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
document are available for a temperature range from 25 °C to 450 °C. The heat transfer fluid used in solar
thermal collectors is usually water, water-based antifreeze or silicon oil.
vi
0.3 0.3  Application range of this document
This document is currently 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 document 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.
0.4 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. By checking
the performance, this document directly demonstrates 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 > 80 %, depending on the
temperature required and the technology chosen. The annual yield depends very muchsignificantly 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
[1]
aggregating the reduction potential in carbon dioxide equivalents (CO2eq) .[1].
0.5 0.5  Supporting the UN Sustainable Development Goals (SDGs)
This document is aligned with the following United Nations Sustainable Development Goals by promoting and
supporting the following.
— — 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).
vii
— — 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
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
viii
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).
— — 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 wind and infrared sensitive collectors (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.
— NOTE 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.
TheThis document specifies procedures for performance check can be done onof the thermal power output of
the collector field as well as on the daily yield and annual yield of the collector field. For the three procedures,
this document specifies how to compare a measured output with the calculated one.
The document is applicable to all sizes of collector fields of all sizes.
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
ISO/TR 9901, Solar energy — Pyranometers — Recommended practice for use
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9488 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/obphttps://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/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
a2 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
a8 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 Performance factor for cleanliness for the operating time between two -
C
cleanings
f Performance factor taking into account heat losses from pipes etc. in -
P
the collector loop.
f Safety factor taking into account measurement uncertainty. -
U
f Performance factor for other uncertainties e.g. related to non-ideal -
O
conditions such as non-ideal flow distribution and unforeseen heat
losses - and uncertainties in the model/procedure itself.
f Overall performance factor given by the supplier and/or mathematical -
perf
product based on the individual factors f , f , f , f
C P U O
fsh Shading factor -
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
ISO/DISFDIS 24194:20252026(en)
G Direct normal solar irradiance W/m
bn
G Diffuse solar irradiance on the plane of collector W/m
d
Hglobal,a Annual hemispherical irradiation on a horizontal plane kWh/m
H Annual hemispherical irradiation on collector plane kWh/m
hem,a
H Daily hemispherical irradiation on collector plane kWh/m
hem,d
H Effective annual irradiation on collector plane during operating hours kWh/m
op,a
Hop,d Effective daily irradiation on collector plane during operating hours kWh/m
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
KθT Incidence angle modifier in the transversal plane —
K Incidence angle modifier for diffuse solar radiation —
d
K Daily average incidence angle modifier for hemispherical solar —
hem,av
radiation
L Length of a collector (from bottom to top) m
L Overall Length of the pipe system without collectors m
pipe
Lsh Length of the shaded area m
kg/s
 Mass flow rate of heat transfer fluid
m𝑚𝑚˙
Nc Number of collectors in a row -
Coordinate of the point C on the X-axis (C is the point that would reach -
PX the shadow formed by the top of the sun facing side of a collector row
if it were unobstructed)
P Coordinate of the point C on the y-axis -
Y
QGTY Annual gross thermal yield kWh/m
 ˙
W
Q𝑄𝑄estimate Estimated power output
 ˙
Q𝑄𝑄 Measured power output W
meas
2 2
q̇ Specific measured power output per m collector gross area W/m
meas
Qcap,d Daily capacity heat losses of solar thermal system J
Q Annual yield estimation of solar thermal system J
estimate,a
Q Daily yield estimation of solar thermal system J
estimate,d
Q� estimate-col,d Daily average gross power output collector field W
Q Annual yield measurement of the heat meter J
HM,a
Q Daily yield measurement of the heat meter J
HM,d
Q� Daily average heat losses of piping W
pipe,d
·
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
Vf Fluid capacity of the collector m
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 (m K)/kWh
x
i
difference 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
Surface azimuth angle, the deviation of the projection on horizontal
γ °
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 °
s
projection of direct 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
ηa Annual collector efficiency based on QGTY and Ha —
η Collector efficiency based on direct 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 direct irradiance —
0,b b m a
G
b
η Peak collector efficiency (η at ϑ − ϑ = 0 K) based on —
0,hem 0,hem m a
hemispherical irradiance G
hem
𝜂𝜂  —
η ˙ Collector efficiency, with reference to mass flow m𝑚𝑚˙
 ℎ𝑒𝑒𝑒𝑒,𝑒𝑒 𝑖𝑖
hem ,m 𝑖𝑖 i
i
ISO/DISFDIS 24194:20252026(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 °
L
of the collector and incident sunbeam projected into the longitudinal
plane
θ Transversal angle of incidence: angle between the normal to the plane °
T
of the collector and incident sunbeam projected into the transversal
plane
θ Zenith angle, the angle between the vertical and the line to the sun, °
Z
that is, the angle of incidence of direct radiation on a horizontal
surface. cos θ = sin h
Z
ϑ Ambient air temperature °C
a
ϑe Collector outlet temperature °C
ϑ Collector inlet temperature °C
i
ϑ Mean temperature of heat transfer fluid in collector loop °C
m
pprim Density of heat transfer fluid in primary loop at flow meter kg/m
position/temperature
ρ Density of heat transfer fluid in secondary loop at flow meter kg/m
sec
position/temperature
2· 4
σ Stefan-Boltzmann constant W/(m K )
5 General
5.1 Validity
The procedures described in this document can only be applied if the thermal performance of the collector
has been tested in accordance with ISO 9806 and modelled with the corresponding collector parameters. The
typical operating temperature of the system for the check shall be within the valid range. This means that the
operating temperature shall be in a range above measured ambient air temperature –10 K and below the
resulting temperature of measured ambient air temperature and the maximum temperature difference
between the collector and the ambient air during the collector test +30 K.
NOTE The wind related parameters a3, a6 and the infrared related parameter a4 are included in the procedure for
checking the power performance. It is assumed that the procedure can also be applied to these types of collectors and
the aim is to provide an opportunity to gain experience. However, this requires additional measurements not described
in this document.
The supplier may impose other conditions that shall be met in order to achieve valid results, e.g. regular
cleaning or maintenance procedures.
The procedures can also be applied to fields of combined collector types - e.g. single glazed and double-glazed:
— — if size, inlet and outlet temperatures are available for each field of collectors of same type, estimates
can be given for each of these fields;
— — an overall estimate for a combination of fields with two or more similar collector types can be given
choosing a representative set of collector parameters and, if applicable, an area-weighted average
orientation for the whole field. Collector types are similar if they have the same design (flat plate,
evacuated tube, parabolic trough etc.) and the difference between their QGTY in the reference climate zone
or at the site under typical operating conditions shall be less than 20 %. For a combination of fields with
similar orientation, the maximum deviation from the average orientation shall be ±10° in azimuth and tilt.
5.2 Methodology and operating conditions
The power and daily yield checks refer to typical blue sky operating conditions using the modelling of the ISO
9806 multiplied by a performance factor and additionally limited by the given restrictions.
The annual yield check is a correlation method based on the QGTY and its derived curve of the annual efficiency
of the collector and the nominal yield, which then results in a nominal efficiency curve. Multiplied by a
performance factor, the estimated annual efficiency curve is created for different weather and operating
conditions. To ensure that the correlation is valid for different conditions, only the effective irradiation in
collector plane that can be used by the collector shall be taken into account. Generally, this means that only
the share of diffuse irradiation multiplied with the incidence angle modifier for diffuse radiation Kd shall be
taken into account. With highly concentrating tracking collectors, for example, the effective irradiation is only
the direct radiation. The annual yield check includes all regular operating conditions including planned
switch-off and stagnation times as well as times of reduced power by control strategies. To take this into
account, the effective irradiation shall be reduced so that this part of the irradiation which is not used has no
influence on the efficiency. However, the effective irradiation shall not be reduced if the reason for reduced
yields or system outages is due to faulty behaviour of the solar system. The range of application is limited by
additional given restrictions.
5.3 Orientation of the collector plane
The orientation of the collectors – azimuth ϒ and tilt β – influences the irradiation in the collector plane and
thus the collector power. While the influence of small deviations is minor in the case of non-concentrating
collectors and fixed orientation, the accuracy of the orientation towards the sun plays a decisive role in the
case of tracking collectors and with increasing concentration. The estimated value for the performance check
shall always be calculated using the actual measured azimuth of the tracking axis to true north and the
corresponding optimal tilt. This applies in particular to the tracking axis of linear concentrating collectors such
as parabolic troughs. It is strongly recommended to compare the measured azimuth of the axis with the
azimuth used to track the collector field. If deviations > ± 0,5° occur, e.g. caused by differences between true
and grid north, incorrect settings in the controller can lead to power reductions, even if the tracker itself is
working accurately. An indication of this error is when the measured power over a day shows a significantly
different behaviour than the estimated curve but meets the curve once or twice a day.
5.4 Performance factor
The performance factor combines multiple sources of uncertainty, including optical losses and additional heat
losses beyond those taken into account, non-ideal conditions, measurement uncertainty etc. The effect of
measurement uncertainty on the estimated power output can, in principle, be modelled in detail, by applying
[2]
GUM [2] uncertainty quantification methods, using documented uncertainty information for each sensor and
the measurement chain. Specific comments on documenting relevant measurement chain information are
given in 9.2.1.9.2.1. The higher the performance factor, the closer the estimate is to the optimal conditions.
The following approach can be helpful in determining the performance factor.
The performance factor f can be divided into factors considering specific influences. As an example, f
perf perf
could be calculated from f = f ·f · f · f , where
perf C P U O
fC is the performance factor for cleanliness for the operating time between two cleanings. If regular
cleaning is mandatory and is only carried out from time to time, a dynamic performance factor
can be used e.g.: f = max (1-r·d, f ) where r is the daily reduction
c C,min
(e.g. 1%/d ≙ r = 0,01/d), d the number of days after cleaning and fC,min is the expected minimum
performance factor for cleanliness without cleaning. If cleaning is a continuous process, a
ISO/DISFDIS 24194:20252026(en)
constant average performance factor can be calculated with the same formula inserting
d = 0,5 x number of days per cleaning cycle period. For the annual yield method a constant
average factor shall be included in f ;
perf
is the performance factor considering heat losses from pipes etc. in the collector loop. To be
fP
estimated based on an evaluation of the pipe losses (e.g. by Formula (23));by 0);
fU is the safety factor considering measurement uncertainty. To be estimated - with the
requirements given in 9.2,9.2, a factor range of 0,93–0,98 (level I), 0,90-0,95 (level II) and 0,85-
0,90 (III) can be used – or detailed documentation for the uncertainty calculation is required
according to ISO/IEC Guide 98-3;
f is the performance factor for other uncertainties e.g. related to non-ideal conditions such as:
O
— non-ideal flow distribution. To be estimated - should be close to 1;
— unforeseen heat losses. To be estimated - should be close to 1;
— uncertainties in the model/procedure itself. To be estimated - should be close to 1.
f is the combined performance factor: f = f · f · f · f .
perf perf C P U O.
5.5 Source of input information
The methods work with results from independent tests, e.g. the collector parameters from ISO 9806, which
are then linked with empirical formulae and supplier information. The procedures may use measurement data
from controls, logging systems, heat meters and preferably qualified radiation data from independent sources,
as radiation measurement has the greatest influence on the error rate. Basically, the aim is to minimize
possible sources of error and to obtain results that are as reliable as possible, e.g. when determining the annual
irradiation sum.
The contribution of suppliers is essential as they know the performance of their system best. Therefore, they
shall specify individual performance factors and provide a nominal annual yield taking into account all
boundary conditions, which may be confidential. If an operating schedule, planned switch-off and stagnation
times as well as control strategies to reduce the heat input into the connected system are part of the design
for the nominal yield, the effective annual reference irradiation shall be reduced. The supplier shall either
provide the information to calculate this reduction or provide the value himself. If the solar thermal system
causes a reduction in yield, e.g. due to a faulty control strategy, although the connected consumer would have
allowed a heat feed-in, this is considered to be faulty operation. Effective irradiation under faulty operation
shall not be deducted.
5.6 Application
The procedures are primarily intended for operators of solar energy systems to check the performance of the
system in practical operation. The performance and daily yield test can be used, for example, to define
acceptance criteria when commissioning a system. The Annual Yield Check can be used to estimate the annual
yield under varying weather conditions and system states. The methods also enable continuous monitoring in
order to detect problems at an early stage and also over the entire service life, such as increasing soiling of the
system or errors in the system supplied with solar heat.
6 Procedure for checking the power performance of solar thermal collector fields –
Power Check
6.1 Stating an estimate for the thermal power output of a collector field
The estimated power output of the collector field is given as a formula depending on collector parameters
according to ISO 9806, operating conditions and a performance factor f . The measured power shall match
perf
or exceed the corresponding calculated power according to this formula. Measured and calculated power are
only compared under some specific conditions to avoid too large uncertainties - see 6.3.6.3.
The performance factor f <1 is chosen by the supplier considering potential influences from soiling, pipe
perf
heat loss, measurement uncertainties, model uncertainties etc. which are reducing the performance and shall
be specified with an accuracy of 2 digits. In Annex A,In Annex A, a template for stating the performance
estimate is given. All the collector parameters and factors applied shall be indicated in the reporting sheet as
well as the level of accuracy (see Introduction and 9.2)9.2)
6.2 Calculating estimated power output
6.2.1 General
The basis for the calculation is the formula for the power output per collector as defined in the ISO 9806:2025,
Formula (A.1). The estimated thermal output is then determined by multiplying the calculated power with a
performance factor.
This formula can be simplified for collectors with an incidence angle modifier for diffuse solar radiation
Kd ≥ 0,8, e.g. collectors with a low concentration ratio CR ≤ 1,25. In these cases, it is not necessary to
differentiate between direct and diffuse irradiance. It is sufficient to measure the hemispherical irradiance.
The estimate is given by stating the formula to be used for calculating the power output, including specific
values for the parameters in the formula. All parameters which have been identified for a certain collector
shall be used. The two possible formulae – general Formula (1)0 and simplified Formula (2)0 - are given in
6.2.26.2.2 and 6.2.3.6.2.3.
When an estimate is given, it shall always be stated which formula shall be used for checking the performance.
Using Formula (2)0 will normally give bigger uncertainty than using Formula (1)0 because there is no
distinction between direct and diffuse radiation.
6.2.2 General Formula (1)0 for power estimate – using direct and diffuse irradiance
The estimated power output for liquid heating collectors is calculated according to Formula (1)0 using
Formula (A.1) of ISO 9806 multiplied by a performance factor:
𝑄𝑄̇=𝐴𝐴 ∙ 𝜂𝜂 𝐾𝐾 𝜃𝜃 ,𝜃𝜃 𝐺𝐺 +𝜂𝜂 𝐾𝐾𝐺𝐺 −𝑎𝑎 𝜗𝜗 −𝜗𝜗 −𝑎𝑎 𝜗𝜗 −𝜗𝜗 − 𝑎𝑎𝑢𝑢𝜗𝜗 −𝜗𝜗 +
� ( ) ( ) ( ) ( )
estimate GF 0,b b T L b 0,b d d 1 m a 2 m a 3 m a
𝑑𝑑𝜗𝜗
m
4 4
˙
𝑎𝑎 𝐸𝐸 −𝜎𝜎𝑇𝑇 −𝑎𝑎 −𝑎𝑎𝑢𝑢(𝐾𝐾 𝜃𝜃 ,𝜃𝜃 𝐺𝐺 +𝐾𝐾𝐺𝐺 )−𝑎𝑎 𝜗𝜗 −𝜗𝜗 ∙𝑓𝑓 𝑄𝑄 =𝐴𝐴 ⋅
( ) � � ( ) ( )�
4 L a 5 6 b T L b d d 8 m a perf estimate GF
𝑑𝑑𝑑𝑑
𝜂𝜂 𝐾𝐾 (𝜃𝜃 ,𝜃𝜃 )𝐺𝐺 +𝜂𝜂 𝐾𝐾𝐺𝐺 −𝑎𝑎 (𝜗𝜗 −𝜗𝜗 )−𝑎𝑎 (𝜗𝜗 −𝜗𝜗 ) − 𝑎𝑎𝑢𝑢(𝜗𝜗 −𝜗𝜗 )
0,b b T L b 0,b d d 1 m a 2 m a 3 m a
[ ]⋅𝑓𝑓 (1)
𝑑𝑑𝜗𝜗 perf
4 m 4
+𝑎𝑎 (𝐸𝐸 −𝜎𝜎𝑇𝑇 )−𝑎𝑎 ( )−𝑎𝑎𝑢𝑢(𝐾𝐾 (𝜃𝜃 ,𝜃𝜃 )𝐺𝐺 +𝐾𝐾𝐺𝐺 )−𝑎𝑎 (𝜗𝜗 −𝜗𝜗 )
4 L a 5 6 b T L b d d 8 m a
𝑑𝑑𝑑𝑑
For large-size one axis tracking concentrating solar collectors, such as the parabolic-through where the
incidence angle modifier have been determined for a single collector, the long
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