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IEC NP 6XXXX TS  IEC 2014
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82/869/NP
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CONTENTS
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FOREWORD ........................................................................................................................... 2
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Scope .............................................................................................................................. 4
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Normative references ...................................................................................................... 4
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Terms and definitions ...................................................................................................... 4
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Imaging ........................................................................................................................... 5
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Thermography image processing to obtain quantitative metrics of damage in PV
modules......................................................................................................................... 11
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Qualitative interpretation of thermographic images of PV modules ................................. 12
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82/869/NP
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IEC NP 6XXXX TS  IEC 2014
INTERNATIONAL ELECTROTECHNICAL COMMISSION
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FOREWORD
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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 nongovernmental 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.
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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.
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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
misinterpretation by any end user.
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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.
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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
services carried out by independent certification bodies.
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6) All users should ensure that they have the latest edition of this publication.
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7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
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expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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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.
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
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International Standard IEC XXXXX has been prepared by subcommittee XX: TITLE, of IEC
technical committee XX:XXX.
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The text of this standard is based on the following documents:
FDIS
Report on voting
XX/XX/FDIS
XX/XX/RVD
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Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
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This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
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The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
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reconfirmed,
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withdrawn,
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replaced by a revised edition, or
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amended.
IEC NP 6XXXX TS  IEC 2014
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The National Committees are requested to note that for this publication the stability date
is 20XX.
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THIS TEXT IS INCLUDED FOR THE INFORMATION OF THE NATIONAL COMMITTEES AND WILL BE DELETED
AT THE PUBLICATION STAGE .
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82/869/NP
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IEC NP 6XXXX TS  IEC 2014
INFRARED THERMOGRAPHY OF PHOTOVOLTAIC MODULES
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1
Scope
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This International Technical Specification specifies methods to (1) capture thermal images of
photovoltaic modules by infrared thermography, (2) process images to obtain metrics about
the image taken in quantitative terms, and (3) provide guidance to qualitatively interpret the
images for features in the image that are observed. This document is applicable for PV
modules measured indoors with a power supply that places the cells in the modules in forward
bias, or outdoors where current in the module is being driven by the module itself or
neighboring modules in a series string of cells or modules.
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The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
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IEC 60904-1:1987, Photovoltaic devices – Part 1: Measurements of photovoltaic currentvoltage characteristics
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For the purposes of this document, the following terms and definitions apply.
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3.1
Infrared radiation (IR): Infrared radiation is a form of light emitted by objects according
to their material and temperature not visible to the human eye. An IR camera image
shows warm areas in a different tone that the cooler areas, so that the temperature of
objects in the camera’s field of view can be observed.
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3.2
Irradiance (G) (Unit:W/m ): electromagnetic radiated power per unit of area.
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3.3
Short-circuit current (Isc) (Unit: Amperes), measured by STC performance
measurement or at low irradiance levels, for example by a flash test according to IEC
60904-1.
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3.4
Maximum power current (Imp) (Unit: Amperes): measured by STC performance
measurement or at low irradiance levels, for example by a flash test according to IEC
60904-1.
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3.5
Open circuit: An electrical circuit that has a break, or “open”, somewhere in the
conductive path. A module or laminate that is “open-circuited” is defective or damaged
so that no current can flow through it. A module itself is in open circuit condition if the
module leads are not connected to anything or current is not flowing.
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3.6
Forward bias: Forcing current flow with a power supply where the leads are connected
to those of the same polarity (+ and -) on the sample.
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3.7
Emissivity (ε): the relative power of a surface to emit heat by radiation. It is the ratio of
the radiant energy emitted by a surface to that emitted by an ideal blackbody at the
same temperature.
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3.8
Thermal sensitivity: the ability to distinguish small temperature differences between
features in an image.
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3.9
Noise Equivalent Temperature Difference (NETD): the amount of infrared radiation
required to produce an output signal equal to the systems own noise. Lower NETD
characteristics of the thermal camera provide better quality images.
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3.10
Barrel distortion. Distortion in the image whereby rectangular features in an image
appear expanded.
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3.11
Thermal resistance (Θ) (Unit: °C/W): The thermal resistance tells us how hot an
electrical component gets when the device is dissipating a given amount of power in
Normative references
Terms and definitions
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open air at a given ambient temperature. Different components in the module will have
differing thermal resistance values.
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Imaging
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4.1
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4.1.1
Thermal imaging Camera
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4.1.1.1
Thermal imaging camera detector
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Specialized thermal imaging cameras use focal plane arrays (FPAs) that respond to longer
wavelengths (mid- and long-wavelength infrared). The most common sensor types are InSb,
InGaAs, HgCdTe and quantum-well infrared photodetectors. The newer cameras use low-cost,
uncooled microbolometers as FPA sensors. Thermal imaging camera resolutions is
considerably lower than that of optical cameras, for example, 160x120 or 320x240, and
640x512 pixels.
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Better cameras will have NETD values in the range of 0.065 K or lower; whereas lesser
quality cameras will have NETD values of 0.1 or greater.
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4.1.1.2
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Lenses for thermographic imaging will be constructed of materials and coatings for the
infrared region to be imaged. Lenses vary from telephoto to wide-angle in focal length.
Choice will depend on the specific application and geometric considerations when capturing
the image. Wide-angle lenses, providing wider fields of view (45°, for example), are frequently
well suited for imaging the back side of modules in the field. Wide-angle lenses used in
conjunction with the higher resolution cameras require fewer images be taken, which makes
image taking quicker. Some wide-angle lens optics however cause undesirable barrel
distortion in the images which may need to be corrected by post-processing of the image.
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Table 1 shows the relationship between camera resolution, resolution when imaging an area
about the size of a 1.2 m x 1.6 m object such as a PV module, and some typical NETF values
of the cameras on the market.
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Table 1. Camera resolutions and example noise equivalent temperature difference
Apparatus
Thermal imaging camera lens
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Camera resolution (pixels)
Resolution of 1,2 m x 1,6 m wide
field of view (mm)
Thermal sensitivity (NETD)/K
640 x 480
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0,03-0,065
320 x 240
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0,05-0,07
240 x 180
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0,07-0,08
200 x 150
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0,07-0,08
140 x 140
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0.08-0,09
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4.1.2
Image processing and displaying software
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The image is transferred electronically from the camera to a computer for display and image
post-processing. Computer software should load thermography image files, assign colors to
each temperature measured within the PV module and any regions of interest, and provide a
legend to indicate the meaning of the colors. The lowest temperature should be represented
by black and the highest temperature in the image should be represented by white. The
colors in the scale between these extremes are not defined here, but there should be no
possibility of misinterpretation by re-use of colors to represent multiple temperatures.
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Software should produce histograms in counts versus temperature bin to qualitatively interpret
the images for features that are observed.
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Software features that may be helpful, depending on the nature of the original image, for postprocessing of images while applying this technical specification include:
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Temperature range adjustment in the legend and image;
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Cropping the image to the region of interest,
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Subtraction of a baseline thermal signature of an image taken prior to energizing the PV
module(s) so that an accurate temperature image void of artifacts is achieve, and,
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Determination of temperature at any given point on the image.
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4.1.3
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Tripods or camera mounts to position the camera in a specific position is usually desirable.
Holding the camera in hand while standing on the ground, a ramp, stepladder, a lift, and aerial
vehicles are additional possibilities to put the camera in position to frame the images of the
modules.
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4.1.4
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An electric DC power supply capable of applying Imp of the module and, optionally, two times
Imp of the module or modules of interest. Cabling from the module leads should be of
sufficinet guage to maintain less than 2% voltage drop over the leads, or alternatively, a four
wire configuration should be used to measure the voltage and current being supplied to the
module(s) under test.
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4.1.5
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The pyranometer and a suitable electrometer preferably with data logging capabilities to read
its signal to measure broadband solar irradiance on a planar surface and measure the solar
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radiation flux density (W/m ) with a field of view of 180 degrees (semi-hemispherical view). A
mounting bracket or other means to fix the pyranometer to the plane of the PV module array
may be used.
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4.1.6
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Infrared imaging of photovoltaic modules with connectors does not generally risk exposure to
live electrical wiring hazards, however any electrical safety protocols should be taken
according to the specific circumstances. Outdoor imaging will frequently require sun
protection (hat, sunscreen), and white mats or tarps to kneel, sit, and lay on if imaging the
underside of the module and to safely de-energize modules under sun during connection and
reconnection. The possibility of insects (tick, wasp, bee), poisonous plants and venomous
snakes should also be assessed at a site and suitable precautions should be taken.
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Handle solar modules and laminates with care if moving them into position for imaging.
Watch for sharp edges and burrs in the metal frame. Wear cut-resistant (Kevlar, or similar)
gloves when handling laminates to prevent cuts and/or wear gloves providing insulation to
prevent thermal transfer of hand heat to module surfaces. Solar modules and laminates can
be heavy or awkward to handle. Get help to move large or heavy modules or laminates. Make
sure modules are well secured if moving and staging them for imaging.
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4.2
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4.2.1
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4.2.1.1
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The thermal camera should be in calibration according to any procedures specified by the
manufacturer. Time and date should be properly entered in the camera if the functionality
exists so that images may be later related to recorded environmental conditions for
performing analyses.
Tripod or camera mount
Power supply
Pyranometer (outdoor only)
Safety and handling
Procedure
Camera settings and positioning
Calibration
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4.2.1.2
Emissivity
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Typical emissivity values that should be set into the camera in the absence of specific data
measured or provided my the material manufacturer are 0.85 for the glass and 0.95 for the
polymer back sheet.
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4.2.1.3
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Angle of view relative to surface normal shall be maintained between 0° and 40° and between
0°C and 45°C for polymer back sheets. When possible, angle of view is optimally made close
to normal with respect to the module surface.
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Note: Imaging with angles exceeding these specifications will lead to the module appearing colder than actual
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4.2.1.4
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Two resolutions for intercomparison of thermal images are defined as follows, reference Table
1.
Angle from normal of module plane
Resolution
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A) Less than or equal to 5 mm
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B) Between 10 and 25 mm
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Comparisons for quality of module(s) shall be made between images of similar resolution
category.
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4.2.1.5
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For routine measurements, a temperature range for the camera is chosen after a survey of a
sampling of modules to be measured and then kept consistent.
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Minimum temperature of the module usually depends on the environmental conditions and the
nature of the modules, whereas maximum temperature of the modules depends on these
same factors plus the important factor of specific current concentrations and joule heating in
the module, which can vary greatly from module to module. The temperature range of the
camera will need to be reset and the image may need to be recaptured for particularly hot
regions of a module. Consistency of multiple images taken from multiple samples may need
to be achieved through post-processing in software.
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4.2.1.6
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Reflections from heat sources including the sun, electronic equipment, people (including the
photographer) and other reflected sources should be minimized. This may be achieved in
some cases by shielding the heat source from the optical path to the module; for example, a
blanket may be suspended between the photographer and the module with a portal for the
camera lens to prevent reflection of the photographer’s own body heat.
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In the case of measurements on fielded systems, care should be taken to not shadow the sun
on the module significantly before or during image capture, either by the photographer, clouds,
or other buildings.
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Instances where reflections and effects of shadowing are inevitable, notation of such
circumstances and conditions shall accompany the recorded thermal image along with
indication of the resulting artifact on the image.
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4.2.1.7
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Laboratory measurements, for consistency, shall be made in an isothermic environment, void
of localized effects of air conditioning and ventilation. The imaging laboratory temperature
shall be between 20°C and 25°C. In uncontrolled environments, reference checks to
variations in the non-module surfaces shall be made and the variations in the temperature
subtracted. Wind may cause local temperature gradients over a module surface and over an
array. Such gradients must be subtracted from the image if relative temperatures across a
module array will be compared. Preferably, imaging should be carried out when effects of
wind are not present. Ambient temperature shall be measured and recorded during the image
taking. Modules shall be free of condensed water from rain, dew, spray, etc.
Temperature range setting of the camera
Reflections and shadowing
Ambient temperature, wind, and condensed water
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4.2.2
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4.2.2.1
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If the module may be moved, place it securly in position for imaging. Modules may be imaged
on the front (normally sun-facing side) with consideration to the following guidelines.
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4.2.2.1.1
Preparations and setup
Glass/Polymeric modules
Heat transfer through the front glass will generally be a longer, slower, and less
precise route than through thin polymeric backsheet, therefore imaging heat
from the module on the rear, especially in the case of transient heat generated
when turning power on to a module with an external power supply, will provide
better pinpointing of the heat source. Because the backsheet of modules are
generally featureless, it may be necessary to perform image manipulation and
overlaying with an optical image or physically measure the location of features
found by thermography to pinpoint heat sources. In the case of thermography
with use of external power supply applied to the PV module with polymeric
backsheet, measurement from the module from the rear is recommended for
clarity of image.
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Thermography with use of external power supply applied to the PV module
4.2.2.1.2
Glass/Glass modules
Measuring modules with both glass superstrates and substrates where no
thermal imaging clarity is gained on either side, imaging the sun-facing side of
the module is recommended.
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Make all necessary preparations according to clause 4.2.1 above including preparations for
camera settings, focusing, angle, image framing and control of extraneous signals and image
artifacts. Check that the DC power supply is available, but in the off position.
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4.2.2.2
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Connect the (+) and (-) electrical leads from the DC power supply to the sample, matching the
(+) lead of the power supply with the (+) of the sample. Use ties or tape to drape the leads so
that they minimize interference with the camera’s view of the sample.
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4.2.2.3
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Imaging is taken at intervals for maximum information retrieval. At time zero, a recording of
the initial temperature and any artifacts and reflections will be seen. At the first image taken
soon after application of Imp electrical bias, the source of any extreme heating can best be
pinpointed. At the end of the Imp dwell, heat spreads and nears equilibrium temperature.
Temperatures at this point are used for indexing the severity of the heating. The current is
then increased to two times Imp, after which features associated with less severe heating can
be better imaged. Increased frequency of image capture may optionally be collected for
better time resolution of the heat generation.
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a) Capture image at time zero, before biasing of the module
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b) Apply Imp forward bias to the module; voltage should be applied as necessary (unlimited)
to load the module within the limits of safety. If there is no current through the circuit,
their may be an open circuit within the module in which case alternate procedures or
characterization techniques may be necessary.
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c) Capture an image at 20 seconds of bias, record the actual current and voltage appied to
the module
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d) Capture an image at 2 minutes of bias from time zero.
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e) Apply 2 times Imp bias to the module; voltage should be applied as necessary (unlimited)
to load the module within the limits of safety.
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f)
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g) Tabulate the module model and serial number, image number, currents and voltages, and
any comments based on onbersvations about the image during image taking.
Electrical connection
Biasing and imaging
Capture an image at 4 minutes from time zero, record the actual current and voltage
appied to the module, record the actual current and voltage appied to the module
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4.2.3
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4.2.3.1
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This document focuses primarily on imaging in the modules’ nominal maximum power
condition in the case of performing thermography with power from solar irradiation.
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4.2.3.1.1
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Modules in normally functioning series strings connected to the grid via an inverter, battery, or
other DC load may be used if optimized by a controller or inverter to operate the modules at
nominal Imp.
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4.2.3.1.2
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Modules in stand alone configuration or string may be connected to a DC load or a power
supply to control current through the cell circuitry to achieve nominal Imp.
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4.2.3.2
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If the module may be moved, place it securly in position for imaging. Modules may be imaged
on the front (normally sun-facing side) or the rear side according with consideration to the
following guidelines.
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4.2.3.2.1
Configurations for performing thermography with power from solar
irradiation
Modules in series strings – inverter and/or load connected
Modules – stand alone DC load or power supply-connected
Preparations and setup
Glass/Polymeric modules
Heat transfer through the front glass will generally be a longer, slower, and less
precise route than through thin polymeric backsheet, therefore imaging heat
from the module on the rear, especially in the case of transient heat generated
when turning power on to a module with exposure to solar irradiance, will
provide better pinpointing of the heat source. However, much of this advantage
is lost when performing steady state measurements under constant irradiance
and load, where the modules are in an equilibrium condition. Further,
obstructions to direct views on module rear in arrays due to racks, wiring, and
posts are a general disadvantage when imaging on rear. Because the backsheet
of modules are generally featureless, it may be necessary to perform image
manipulation and overlaying with an optical image or physically measure the
location of features found by thermography to pinpoint heat sources. For PV
modules with polymeric backsheets, only in the limited case of thermography
under transient application of solar irradiance with use of external power supply
or load, with consistent, direct visual site to the rear compared to the module
front, is rear imaging recommended for clarity of image. Imaging the module
front glass is otherwise normally recommended.
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Thermography with power applied to the module by solar irradiation
4.2.3.2.2
Glass/Glass modules
Measuring modules with both glass superstates and substrates where no
thermal imaging clarity is gained on either side, imaging the sun-facing side of
the module is recommended.
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4.2.3.3
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If the plane of array (POA) global irradiance is not already provided in the system monitoring
equipment for the module(s) to be imaged, place the pyrometer in the plane of the module
array. Images to be subsequently analyzed shall be taken with POA equal to or greater than
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700 W/m . The POA global irradiance at image shall be recorded either manually or
electronically such that the irradiance at the time of each image can be later assigned.
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4.2.3.4
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The string current and optionally voltage should be monitored and recorded either manually or
with a data logger such that the electrical state of the module at the time of each image can
be assigned.
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4.2.3.4.1
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Irradiance measurements
Electrical connection
Inverter and/or load connected
In the case of an electrical connection to a load in a grid-tied system, the string
current and voltage may be monitored if the inverter or charge controller
provides this information. Alternatives include a DC clamp meter with 2%
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tolerance at the current level anticipated to be measured may be used instead to
monitor the current.
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4.2.3.4.2
Stand-alone electronic load or DC power supply
In the case of loading the module with an electronic load or DC power supply,
the current and voltage may be controlled and monitored with this equipment.
With the module(s) shaded under a blanket or tarp, connect the (+) and (-)
electrical leads from the DC power supply to the sample, matching the (+) lead
of the power supply with the (+) of the sample. Use ties or tape to drape any
potentially interfering leads or wires so that they minimize interference with the
camera’s view of the sample.
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4.2.3.5
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4.2.3.5.1
Biasing and imaging
Imaging the module(s) in steady thermal state
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This method is generally (not exclusively) applicable to modules in systems
under continuous electrical load.
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a) Make all necessary preparations according to clause 4.2.1 above including preparations
for camera settings, angle, image framing, focus, and control of extraneous signals and
image artifacts.
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b) Verify that the modules are energized and tracked to their nominal maximum power point,
within 5% of the G·Imp/1000 calculation, Imp is preferably determined according to the
methods in IEC 60904-1; however, the module nameplate Imp would be used alternatively.
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c) Focus camera and capture image.
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d) Tabulate the module(s) model and serial number, image number, currents and voltages,
ambient temperature, irradiance, and any comments observed about the image during
image taking.
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4.2.3.5.2
Imaging the module(s) in transient thermal state
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This method is generally (not exclusively) applicable to modules in stand-alone
configurations connected to power supplies or electronic loads.
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Imaging is performed at intervals for maximum information retrieval in the case of
transient measurements. Recording of the initial temperature at the instant of application
of solar irradiance and bias will elucidate artifacts and reflections. At the first image taken
soon after energizing the module(s), the source of any extreme heating can be pinpointed.
At the end of the Isc dwell, heat spreads and nears equilibrium temperature. Increased
frequency of image capture may optionally be collected for better time resolution of the
heat generation.
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a) Make all necessary preparations according to clause 4.2.1 above including preparations
for camera settings, angle, image framing, focus, and control of extraneous signals and
image artifacts.
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b) At time zero, the module(s) to be imaged should have been covered from illumination with
a white tarp and unloaded (electrically) so that they are in equilibrium, representative of
their de-energized state. Additionally, the power supply or electronic load should be
connected with the positive terminal of the power supply connected to the positive
terminal of the module or module string, powered, with sufficient voltage to bias the
module strings to their maximum power point, but the current control to zero such that the
module(s) are in open circuit condition.
399
400
401
402
403
c) Remove the tarp cover the module(s) and set the current of the module with the power
supply or electronic load according to the measured irradiance at that instant, calculated
by G·Imp/1000. Imp is preferably determined according to the methods for STC power
determination in IEC 60904-1; however, the module nameplate Imp would be used
alternatively.
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405
Note: The intent is to achieve string current throught the module as it would exist in a series string of
otherwise normally functioning modules when optimized to their peak power by a controller or inverter
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d) Capture an image at 20 seconds after energizing the module; record the actual current in
the module circuit.
408
e) Capture an image at 2 minutes of bias from time zero.
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f)
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g) De-energize the module. De-energize the electronic load or power supply and cover the
module(s) with a white tarp and before electrically disconnecting the module.
412
413
414
h) Tabulate the module model and serial number, image number, currents, ambient
temperature, irradiance, and any comments observed about the image during image
taking.
415
5
416
Capture an image at 5 minutes of bias from time zero.
Thermography image processing to obtain quantitative metrics of damage in
PV modules
417
5.1
Principles of heated and cooled regions in PV modules
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419
Thermography detects regions of localized heating or cooling. Generalized causes of this
contrast are as follows, whereas clause 6) discusses the specific physical phenomena:
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421
422
423
424
425
Hot areas are generally associated with Joule heating according to I R, when current is
concentrated due to barriers in normally available paths, including areas of broken cells and
broken ribbons or regions of inhomogeneous series resistance due to factors such as
corrosion, resistive contacts or semiconductor regions, either built into the construction of the
module or associated with degradation. Alternate paths current will take will usually
2
significantly heat the module because of the I term.
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427
428
Heating of cells may also occur according to mismatch in current generating ability of
particular cells in series strings. These cells will dissipate power driven by the current from
normally current producing cells.
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430
Heating of cells may also occur according to excessively high recombination or shunt paths
through the semiconductor layer of the module.
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432
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434
435
436
Current may not flow or flow less than normal through a series string of cells in the module
because of shorted bypass diode, areas of broken cells disconnected from the circuit, or,
more generally, open circuits. These lead to cooler areas where current does not flow when
driving current with an external power supply, or hotter when the module is sun powered
because the incident sun power is not sent to the external load and is largely ejected as heat
from the effected area.
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440
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442
To a first approximation, the temperature rise for a given area will indicate the power
dissipation from joule heating according to the thermal resistance (°C/W) characteristics of a
device that is not generating electrical power. Considering the relative temperature rise
measured in thermography at each pixel multiplied by the number of pixels at the particular
elevated temperature, a relevant metric for the power loss by the severity of the heating
measured spatially by thermography is obtained.
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5.2
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445
446
447
448
449
450
This section describes quantitative metrics of module quality that can be extracted from
thermography images. These methods, except where noted, do not fundamentally inform
about the underlying mechanisms that may be affecting the module quality. Any or all of
these analyses may optionally be applied to extract quantitative metrics, which provide
indicators or the consistency or variability in the thermal signature of the module(s). A
consistent, less variable distribution is desirable; however, current must be able to pass
through the module
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5.2.1
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453
454
455
456
457
458
459
Creation of histograms and their derived statistic from the images of the module active area(s)
and image analysis software will be generally required for application of these techniques.
The histograms consist of counts of pixels at each temperature bin. Bin size must be
coordinated to be a multiple of the temperature binning in the thermography image produced
by the camera to prevent moiré or interference patterns in the distribution. These techniques
should be applied only to the active area of the module(s). Irrelevant areas of the background
must be eliminated graphically (cropping) or through analysis of the temperature distribution,
removing areas producing irrelevant temperature measurements.
2
Analyses
General
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5.2.2
Image information
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463
Complete details about module(s) model and serial number, image number, currents, ambient
temperature, irradiance, and any comments observed about the image during image taking
will be required.
464
5.2.3
465
466
467
468
Images taken with modules sun-powered will display contrasts that are stringly irradiancedependent. When compared, images must be performed at nominally simillar irradaiance
levels. [we do not yet have callibration factors to scale effects of irradiance on heating of
modules]
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5.2.4
470
471
472
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475
476
The temperatures of the highest and lowest areas of the module are collected and subtracted
from one another. For elimination of outliers, the median of the highest 0.5% of the
temperatures is taken and compared to the median of the lowest 0.5% of the temperatures
measured. The high temperature, low and difference between these are reported are the
relevant metrics. The high and low temperatures are influenced by the ambient temperature,
so the ambient temperature at which the image(s) were taken must be reported with the
image analyses as well.
477
5.2.5
478
479
480
481
In this analysis the mean temperature of all pixels of the module image is computed and the
variance is calculated as the average of the squared differences from the mean. A higher
variance means greater variation of temperatures across the module(s), an indicator of hot
and cold spots.
482
5.2.6
483
484
Kurtosis is an indicator of existence of tails to the distribution, a high kurtosis indicates there
are extremes of hot and cold regions of the module.
485
5.2.7
486
487
Skewness indicates a tail to the distribution that is predominantly either positive or negative.
A skew to the positive side is an indicator of regions of higher temperature.
488
5.2.8
489
490
491
492
493
494
495
Regions of the image of the modules extending above the median temperature are identified
and their temperatures are subtracted from the median temperature, the power loss for each
of these regions is calculated according to the product of the temperature differnnce divided
by the appropriate thermal resistance value. (table to come; this technique follows that of
Buerhop and coworkers “Quality Control Of PV-Modules In The Field Using Infraredth
Thermography” 26 EPVSEC). The products of the power (W) dissipated in heat and area
fraction affected is then calculated as the relevant metric.
496
5.2.9
497
498
499
500
501
502
503
504
The histograms of counts (probability-type, where the sum of all counts is unity) vs.
temperature of module(s) to be tested are superimposed upon the equivalent histogram of
ideal module(s). To align different median temperatures if the images were captured at
differing ambient temperatures, their medians may be made to align by adding or subtracting,
as appropriate, the difference in the medians if test module(s) do not show greater than 25%
area damaged. The difference in probability fraction in each bin between the suspect and the
ideal modules is computed. The sign and magnitude of the elements in the resulting array
plotted as a function of temperature is the relevant metric.
505
6
506
507
508
509
510
Descriptions of observables, features, and known causes, along with thermographic images
captured indoors at Imp current, and outdoors at Imp current are given in Table 2. Much of
this is referenced and adapted from Koentges et al “Reviewing the practicality and utility of
electroluminescence and thermography images” 2014 PVMRW; and TESTO:Training for
Thermography on Photovoltaic Modules. These are’ concept place holders’. We will
Irradiance effects
Maximum and minimum temperatures
Variance
Kurtosis
Skewness
Pixel (or area)-weighted temperature above median temperature
Pixel (or area)-weighted temperature above that of an ideal module
Qualitative interpretation of thermographic images of PV modules
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gradually adapt these for this IEC document as it progresses.
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Table 2 – Descriptions of observables, features, and known causes, along with
thermographic images or their sketches, captured indoors at Imp current, and outdoors
at Imp current.
Observables, features, and
known causes
Module is open circuited,
disconnected, current does not
pass
Short circuited sub-string; either
from shorted bypass diode or in
the internal circuitry. Voltage
over module is significantly
below normal.
Whole module is short circuitedAll bypass diodes in short circuit
or module incorrectly connected.
Voltage, as would be measured
over the module leads, is
significantly below normal.
Massive shunts caused by
potential induced degradation
(PID) and/or polarization, or
other through-junction shunting
mechanisms, characterized
largely by reduced fill factor.
Indoor – power supply biased
Outdoors – sun powered
Module colder than typical because
flowing current is not available to
generate power
Module hotter than typical because all
sunlight incident on module is
converted to heat and no power is
conducted to load
Module substring or segment colder
than typical because flowing current
is not available to generate power
Module substring or segment hotter
than typical because all sunlight
incident on module is converted to heat
and no power is conducted to load
Module colder than typical because
short circuit prevents current flowing
through cell circuit. Area of shortcircuit may appear hot, such as at
the junction box
Single cells are warmer, lower parts
and close to frame may be hotter
than upper parts. Areas in cell that
have degraded junction properties
appear hottest.
Shadowing on cell or cells,
putting cell into reverse bias and
dissipating power (sun poweredconfiguration only);or,
Single cells are warmer in a random,
patchwork pattern
Single cells are warmer, lower parts
and close to frame hotter than upper
and middle parts.
One or several cells are warmer;
Bottom, Delamination example.
defective cell (mismatched in
current, or significantly shunted);
or,
delamination of encapsulant
Broken or dis-bonded
interconnect ribbon
Numerous broken cells due to
static load (snow load, etc.)
Regions of cells with broken or disbonded interconnect ribbon appear
cooler if disconnected from the cell
circuit, balance of cell with elevated
current density appear hotter
Regions of broken cells largely
disconnected from the cell circuit
appear cooler, balance of cell with
elevated current density appear
hotter
Cells with broken or dis-bonded
interconnect ribbon may appear
warmer when the smaller remaining
cell area in the circuit is dissipating
current from other cells in the string.
The balance of cell, if disconnected
from the cell circuit may also appear
warmer when power from these regions
cannot be electrically transmitted to the
circuit.
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Bibliography
516
517
518
519
Koentges et al “Reviewing the practicality
thermography images” 2014 PVMRW
520
TESTO:Training for Thermography on Photovoltaic Modules
521
and
Bibliographical reference 3
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523
_____________
utility
of electroluminescence and