IEC 60904-5:2011/AMD1:2022

IEC 60904-5 ®
Edition 2.0 2022-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
A MENDMENT 1
AM ENDEMENT 1
Photovoltaic devices –
Part 5: Determination of the equivalent cell temperature (ECT) of photovoltaic
(PV) devices by the open-circuit voltage method

Dispositifs photovoltaïques –
Partie 5: Détermination de la température de cellule équivalente (ECT) des
dispositifs photovoltaïques (PV) par la méthode de la tension en circuit ouvert

IEC 60904-5:2011-02/AMD1:2022-11(en-fr)

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IEC 60904-5 ®
Edition 2.0 2022-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
A MENDMENT 1
AM ENDEMENT 1
Photovoltaic devices –
Part 5: Determination of the equivalent cell temperature (ECT) of photovoltaic

(PV) devices by the open-circuit voltage method

Dispositifs photovoltaïques –
Partie 5: Détermination de la température de cellule équivalente (ECT) des

dispositifs photovoltaïques (PV) par la méthode de la tension en circuit ouvert

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.160 ISBN 978-2-8322-5728-9

– 2 – IEC 60904-5:2011/AMD1:2022
© IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOVOLTAIC DEVICES –
Part 5: Determination of the equivalent cell temperature (ECT) of
photovoltaic (PV) devices by the open-circuit voltage method

AMENDMENT 1
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
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6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
Amendment 1 to IEC 60904-5:2011 has been prepared by IEC technical committee 82: Solar
photovoltaic energy systems.
The text of this Amendment is based on the following documents:
Draft Report on voting
82/2069/FDIS 82/2082/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Amendment is English.

© IEC 2022
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications/.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
___________
INTRODUCTION
Add the following new text:
For modules with large thermal inertia such as glass-glass construction for BIPV applications,
measurements become even more challenging with increased temperature difference between
the cell and module external temperatures during transient conditions. In addition, for bifacial
PV modules the temperature sensors may shade an active cell, potentially even creating local
hotspots where sensors are located on effective cell areas.
NOTE 1 NMOT is defined as the equilibrium mean solar cell junction temperature within an open-rack mounted
module operating near peak power, in the following standard reference environment:
– Tilt angle: (37 ± 5)°.
– Total irradiance: 800 W/m .
– Ambient temperature: 20 °C.
– Wind speed: 1 m/s.
– Electrical load: A resistive load sized such that the module will operate near its maximum power point at STC or
an electronic maximum power point tracker (MPPT).
NOTE 2 NMOT is similar to the former NOCT except that it is measured with the module under maximum power
rather than in open circuit. Under maximum power conditions (electric) energy is withdrawn from the module,
therefore less thermal energy is dissipated throughout the module than under open-circuit conditions. Therefore
NMOT is typically a few degrees lower than the former NOCT.

1 Scope and object
Replace the first paragraph with the following text:
This part of IEC 60904 describes the preferred method for determining the equivalent cell
temperature (ECT) of PV devices (cells, modules and arrays of one type of module), for the
purposes of comparing their thermal characteristics, determining NOCT (nominal operating cell
temperature) or alternatively NMOT (nominal module operating temperature), and translating
measured I-V characteristics to other temperatures.

– 4 – IEC 60904-5:2011/AMD1:2022
© IEC 2022
2 Normative references
Add the following standards to the list of normative references:
IEC TS 60904-1-2:2019, Photovoltaic devices – Part 1-2: Measurement of current-voltage
characteristics of bifacial photovoltaic (PV) devices
IEC 60904-3, Photovoltaic devices – Part 3: Measurement principles for terrestrial photovoltaic
(PV) solar devices with reference spectral irradiance data
Delete the following standard from the list of normative references:
ISO/IEC 17025, General requirements for competence of testing and calibration laboratories

3.1 Principle
Replace the text of this subclause with the following new text:
Experience shows that the equivalent cell temperature can be determined more precisely by
. However, increased
the method described herein than by any alternative technique [1]
variability and errors have been observed at irradiances below 400 W/m , so this method should
only be used at irradiances above this threshold.
3.2 General measurement requirements
Add the following new text:
a) Use of the ECT method requires calibration of the device to be measured.
NOTE It is not sufficient to use calibration of another device of the same type, because even small differences
in parameters between a calibrated device and a similar one can lead to significant errors (e.g. 0,3 % variation
in module V leads to 1 °C impact on ECT temperature).
OC
Renumber existing item a)1) as item b)1) as follows:
1) The variation of V needs to be linear as defined in IEC 60904-10 with respect to
OC
temperature.
Replace item a)2) as item b)2) with the following new text:
2) The variation of V with respect to irradiance needs to have a quadratic dependence on
OC
the logarithm of irradiance.
Delete items a)3) and a)4).
___________
Numbers in square brackets refer to the Bibliography.

© IEC 2022
Replace the first two paragraphs of item b) with the following new text:
c) The irradiance measurements shall be made using a PV reference device packaged and
calibrated in conformance with IEC 60904-2 or a pyranometer. Either use a PV reference
device that is spectrally matched to the device under test (DUT), or perform a spectral
mismatch correction and report in conformance with IEC 60904-7. The reference device
shall be linear in short-circuit current as defined in IEC 60904-10 over the irradiance range
of interest.
In accordance with IEC 60904-2, to be considered spectrally matched, a reference device
shall be constructed using the same cell technology and encapsulation package as the
device under test.
Add the following new item d):
d) Some devices, in particular multi-junction, might have a spectral dependency of the open-
circuit voltage [2]. For these devices, the spectral irradiance shall be determined with a
spectroradiometer.
Replace existing item c) as item e) with the following new text:
e) The active surface of the device under test shall be coplanar within ±2° of the active surface
of the reference device.
Replace existing item d) as item f) with the following new text:
f) For appropriate connection method and measurement of voltages refer to IEC 60904-1.

4 Apparatus
Replace items a), b) and c) as follows:
a) A PV reference device that meets the conditions stated in 3 c).
b) Equipment to measure the open-circuit voltage to an instrumental measurement uncertainty
better than ±0,2 %.
c) Equipment to measure temperature to an instrumental measurement uncertainty of ±1 K.

5 Determination of required input parameters
Replace the bulleted list, given after the introductory text "The procedure requires a number of
input parameters. These are:" with the following:
• Relative temperature coefficient of open circuit voltage, β . This shall be determined from
rel
cell or module measurements of representative samples in accordance with IEC 60891.
For bifacial modules, the temperature coefficient only needs to be determined from front
side measurements.
• Open-circuit voltage (V ) at a reference condition (G , T ) in accordance with
OC1 1 1
IEC 60904-1 or IEC TS 60904-1-2 for a cell or module or in accordance with IEC 61829 for
a PV array. The reference condition is often chosen to be the standard test conditions, i.e.
G = 1 000 W/m and T = 25 °C with a reference spectral irradiance distribution as
STC STC
defined in IEC 60904-3.
• When outdoor measurement (G , T ) is carried out, it is recommended to apply insulating
1 1
thermal tape, e.g. polyethylene foam, 1 mm thickness, with mass density less than
0,03 g/cm , to cover the temperature sensor which is fixed by either aluminium or polyimide

– 6 – IEC 60904-5:2011/AMD1:2022
© IEC 2022
tape. If the temperature around the module is subjected to spatial and temporal variability,
use of insulating thermal tape shields the temperature sensor from influence of
environmental factors such as wind, allowing more accurate measurements.
NOTE A method to determine the mass density can be found in ISO 7214[4].
• The procedure requires the irradiance correction factors, B and B . B is linked to the
1 2 1
thermal diode voltage and B accounts for non-linearity of V with irradiance scaling. The
2 OC
determination of these constants requires the measurement of the module I-V characteristic
in accordance with IEC 60891 under at least five different irradiance levels.

6.2 Operating in a controlled environment
Add the following as new item c):
c) For bifacial modules, a non-irradiated background is required as described in
IEC TS 60904‑1‑2.
Replace existing item c) as item d) with the following:
d) Take simultaneous readings of the open-circuit voltage of the device under test V and
OC2
the incident irradiance (G ). Should there be any variation in the irradiance, treat as a
measurement in arbitrary irradiance conditions as given in 6.3 and carry out the appropriate
correction. An irradiance correction should be carried out if the scatter in the determined
ECT is more than 1 K.
Renumber existing item d) as item e):
e) Calculate the ECT as described in Clause 7.
6.3 Taking measurements under arbitrary irradiance conditions
Replace existing item a) with the following:
a) Mount the radiation sensor coplanar with the device under test to an agreement better than
±2 °.
Replace existing items b) and c) as follows:
b) For bifacial modules, two different setups are recommended for the measurement:
Method 1: use a low reflectivity black cover material to reduce back-to-front irradiance ratio
to < 1 %, in order to minimize the rear irradiance contribution. The cover should be mounted
behind the module in a way to limit interference with the module natural convective heat
dissipation as much as possible.
Method 2: measure the plane-of-array irradiance on front side G and the average
f
i
irradiance on the rear side G using PV reference devices compliant to IEC 60904-2. G
r r
i i
is the average of at least 5 measurement points located per the requirements of
IEC TS 60904‑1‑2:2019, 6.3.2. The equivalent irradiance GE on the bifacial module is then
determined by:
GG+×φG
(1)
Ef r
i i i
where φ is the module bifaciality coefficient as determined in accordance with
IEC TS 60904-1-2.
=
© IEC 2022
NOTE 1 Decision on which method to use is left to the user, on consideration of the targeted measurement
uncertainty budget. Method 1 is expected to enable lower uncertainty when applied to NOCT or NMOT
measurements, and for translating field measured I-V characteristics to standard test conditions.
NOTE 2 When applying method 2, particularly for bifacial systems, proper selection of the modules to be tested
has to consider thermal and irradiance non-uniformities at the system level. IEC 61829 provides some guidance
on the selection of typical modules within a PV array, recommending in particular to avoid selecting modules at
ends of rows.
c) Take simultaneous readings of the open-circuit voltage of the device under test V and
OC2
(method 1), or alternatively of the irradiance on
the incident plane-of-array irradiance G
front side G and average irradiance on the rear side G (method 2).
f r
2 2
7 Calculation of equivalent cell temperature
Replace the existing text with the following new text:
The equivalent cell temperature ECT is derived from the single diode equations describing the
current voltage characteristic.
Solving the equation for V = V , with V = V and I = I = 0 results in the following
2 OC2 1 OC1 2 1
dependence of the open circuit voltage:
GG 
f(GG, )=1+×B ln + B×ln
 (2)
12 1 2
GG
22
 
V ×+1,β × T −T ×f GG
( ) ( )
oc1 rel 2 1 1 2
 
(3)
V =
oc2
f GG,
( )
where
V is the open-circuit voltage measured in Clause 5 at the chosen reference conditions,
OC1
irradiance G and module temperature T ;
1 1
V is the open-circuit voltage measured in Clause 6 at irradiance G and module
OC2 2
temperature T ;
the relative temperature coefficient of the open-circuit voltage β and the irradiance
rel
correction factors B and B are determined in Clause 5.
1 2
NOTE These formulae are derived from the IEC 60891 correction procedure 2 [3].
For measurement of bifacial modules using method 2, the irradiance G has to be replaced by
the equivalent irradiance G .
E

GG
f GG, =1+×B ln + B×ln 
(4)
( )
1 E 1 2

GG
EE
22
– 8 – IEC 60904-5:2011/AMD1:2022
© IEC 2022

V ×+1,β × T −T ×f GG
( )
( )
oc1 rel 2 1 1 E

V = (5)
oc2
f GG,
( )
1 E
The relation between the different values of V can then be rewritten to calculate the
OC
equivalent ECT per the formulas given below, for monofacial (6) and bifacial (7) devices:
V
OC2
ECT=T=T+ × × f GG, −1
( )
 (6)
2 1 12
V
β ×f (GG, )
OC1
rel 1 2
V 
OC2
ECT=T=T+ × × f GG, −1
 ( ) 
21 1 E (7)
2 2
V
β ×f GG,
 OC1 
( )
rel 1 E
In the case of base measurements described in Clause 5 being taken at standard test conditions,
the ECT for monofacial devices can be determined as:

1 V
OC2
ECT=T=25+ × × f 1000,G−1
( )
 (8)
...