Testo Solar Panel - Practical Guide

Practical guide
Thermography for
photovoltaic plants.

Introduction.
Photovoltaic systems are an important
contribution to the the energy transi-
tion, and to a sustainable handling of
resources – in recent years, driven by
state subsidies, numerous smaller and
larger photovoltaic systems have been
installed in many countries in the
world.
After the boom phase, the mainte-
nance of existing plants in particular is
gaining in significance.
This practical guide explains how
thermography can support you during
commissioning, documentation and
maintenance, and provides helpful tips
for using a thermal imager.

Contents:Contents:
Motivation and reasons for using thermography 4
Fault images and causes 7
Tips tricks on measurement and avoiding errors 11
What does the ideal thermal imager look like? 14
Testo thermal imagers for solar thermography 17
Thermal imagers – ideal tools for the inspection
of photovoltaic plants 18
3

Thermography
Motivation and reasons for using
thermography.
Detecting poor quality on the mar-
kets. In the boom years of photovolta-
ics, the order books were full and solar
engineers were hardly able to keep up.
This meant that it was not just highly
trained trade experts that were filling
orders. Large numbers of cross-trade
and inadequately qualified contractors
helped to meet the huge demand.
The consequences are still being felt
today: Construction errors, inade-
quate solar electricity yields from the
plants, up to security and fire risks. It
is primarily the plant operator who suf-
fers. However, a qualitatively inferior
implementation also reflects on the the
company doing the work, potentially
allowing damage claims to be allowed
based on a thermographic analysis.
Quality assurance and warranty.
Using thermography, it is possible
to check whether the quality of the
module cells fulfils the requirements.
The right combination of individual
modules avoids so-called mismatches,
where high-performance modules are
impeded by “inferior” modules. With
an inspection before the end of the
warranty period, any warranty claims
towards suppliers can be asserted in
good time.
Avoiding customer yield losses. A
new PV system is based on an exten-
sive and detailed yield and investment
analysis. Yield calculations are drawn
up for up to 20 years. However, these
calculations do not take into account
any performance losses due to poorly
installed systems. With the use of
thermography, it is possible, as early
on as the commissioning stage, to
produce acceptance documentation
Thermal irregularities point to a possible loss of
electricity yield.
4

and provide proof of proper installa-
tion. This means there are no surprises
for the end consumer and quality is
assured. To guarantee the yield on a
long-term basis, further regular checks
are important, as the efficiency of
a solar thermal system depends on
the temperature. If modules become
heated due to shade, faulty cells or
substrings, i.e. consume power with-
out generating, the efficiency drops by
0.5 % per Kelvin. An average temper-
ature rise of 10 °C compared with the
mean standard temperature means a
5 % lower electricity yield.
Recommendations are an important
marketing instrument, especially in
view of the current situation in the sec-
tor, for gaining further custom after an
order has been carried out. Because
only a satisfied customer will recom-
mend a professional and trustworthy
company.
Efficient additional and follow-up
business. After the boom years'
emphasis on installing a PV system as
quickly as possible, the focus today is
on testing and maintaining PV systems
regularly. Service contracts can form a
further source of revenue in classical
after-sales business. The use of
thermography allows a customer to
offer valuable after-sales-services
which secure the value of a photovol-
taic plant in the long term.
Fire protection. Fire protection is
of increasing importance. Modern
inverters and electrical components
are becoming more and more powerful
(high efficiency), and so the resulting
high level of heat emitted must also be
taken into consideration. Incorrectly
fitted or inadequately cooled electrical
components can quickly pose a fire
risk, particularly if the mounting base
is made of combustible material. Elec-
trical components installed outdoors
age especially quickly as a result of
weathering and UV radiation. Corroded
or loose electrical cables indicate ther-
mal irregularities, which would show
up on a thermal imager.
Benefit: time saving. Thermography
is a non-contact, visual measurement
method. Large-surface solar modules
can be scanned in a very short time.
Thermal irregularities or temperature
differences affecting modules are
immediately evident and are an initial
indication of possible faults. Whereas
previously all module strings were
measured individually, using thermog-
5

raphy one can now concentrate on the
thermally conspicuous modules and
cells for all further measurements (e.g.
with a characteristic curve measuring
instrument).
Benefit: insurance cover.
Up to now, faulty bypass diodes were
extremely difficult to localise after
storms. Thermography is an easy and
quick tool for identifying this type of
damage.
The costs for rectifying the fault are
generally borne by the insurance
company.
Security in inspections.
PV systems are live during daylight
hours. In the case of modern module
strings, voltages are often up to 1000
V. This poses a considerable risk of
electric shock to personnel. for this
reason, thermography is a very safe in-
spection method, as recording thermal
images is always done with the nec-
essary distance to the measurement
object. This means that safe distance
requirements are easily met.
Thermography
Faulty modules after a lightning storm.
6

Fault images and causes.
Looking for the hotspot. Shadowed
or defective module cells form an
internal electrical resistance which can
lead to undesired warming hotspot.
The cell can heat up so much that not
only is it damaged itself, but it also
damages the casing material (EVA) and
the backing film (TPT).
Bypass diodes are intended to prevent
this effect. However, faulty or unsuita-
ble bypass diodes (where shade is
minimal) continue to lead to uncontrol-
lable hotspots. If shade is not taken
into account in the planning phase (e.g.
caused by HV plants or trees), the
module cells and bypass diodes are
subject to permanent loading all year
round.
Hotspots and their consequences.
There are generally two consequences
of hotspots:
• The electricity yield decreases,
as individual cells or the entire
module are consuming electricity
instead of generating it.
• Unwanted electricity consumption
heats up the cells and modules.
Aside from the damage to individ-
ual cells and a further reduction in
the electricity yield, this can also
lead to a real fire risk.
Identifying hotspots with thermog-
raphy.
In general, faults in the operation of
PV systems as of solar radiation of
approx. 600 W/m² can be quickly
diagnosed from changes in the thermal
properties showing up on a thermal
imager. These kind of changes occur,
for example, due to:
• Defective bypass diodes
• Contact faults and short circuits in
solar cells
• Moisture penetration, dirt
• Cracks in cells or in the module's
glass
• Non-functioning or disconnected
modules
• So-called mismatches, i.e. loss
of performance due to different
capacities of individual modules
• Faulty wiring and loose contacts
• Wear and tear
7

Thermography
Fault images in cells and modules
The infrared image shows typical fault
images for defective individual cells
and substrings. The connection sock-
ets visible in the image show visible
warming. This does not necessarily
indicate a fault. However, connection
sockets can overheat, so testing the
temperature development as required
is necessary.
Modules at open circuit. it is not
uncommon for modules to run at
open circuit. This may be caused by
incorrectly connected modules or
cables that have worn through or been
chewed through. This is conspicuous
in the thermal image by a consistently
warmer infrared image in comparison
to the other modules.
Delamination. Due to external
influences or poor quality, the EVA
protective layer may come away. Any
moisture getting in may lead to cell
corrosion and to a performance loss.
With a thermal imager, this can be
detected before the layers become
visibly “milky”.
Typical fault images in solar cells and modules.
Faulty
substring
Faulty indi-
vidual cell
Connection
points
8

Cell rupture. Micro-cracking and
cell ruptures can already occur during
transport and installation. External
mechanical influences can also be the
cause of this. While micro-cracking
is not critical, cell rupture can reduce
performance.
Checking electrical and mechani-
cal components.
Aside from the individual cells and
modules, electrical components can
also be checked using thermography.
Corrosion on electrical conductors
and connectors or loose cables can
lead to electrical transfer resistances,
indicated by a considerable rise in
temperature. This means that, in
addition to the generating modules,
electrical components can also be
checked:
• Corroded contacts or connectors
• Inverter
• Loose contacts
• Overheated connection points
Left inverter is significantly hotter.
DC cable without critical heating.
Significant heating at electrical connections.
9

Thermography
Overview off fault images and causes.
The following overview lists typical fault images and their possible causes.
Infrared image 4
Description: Only part of a cell is significantly hotter.
Possible faults: Cell rupture.
Possible cause: Transportation or installation damage
or other external mechanical influence.
Infrared image 3
Description: “Patchwork pattern” where individual cells
are randomly distributed and significantly hotter.
Possible faults: Complete module in short-circuit.
Possible cause: Incorrectly connected or all bypass
diodes faulty.
Infrared image 2
Description: The module has line-like heating of a
string.
Possible faults: Short circuit in a cell string.
Possible cause: Faulty bypass diode e.g. after a storm.
Infrared image 1
Description: Constant heating of module compared
with the others.
Possible faults: Module is at open circuit.
Possible cause: Module not connected, cable worn
through or broken.
Infrared image 5
Description: Heating at specific points or unevenly.
Possible faults: Crack in a cell or artefact formation.
Possible cause: Manufacturing fault with cell cracking.
Shade due, for example, to dirt (bird droppings, etc.).
Infrared image 6
Description: Heating of an individual cell.
Possible faults: Not necessarily a fault.
Possible cause: Shade or faulty cell.
10

Tips tricks on measurement and
avoiding errors.
Meteorological prerequisite.
Testing should take place on clear,
dry days, with intensive solar radia-
tion (approx. 600 W/m²). During direct
solar radiation, the solar panels work
at full capacity, and damaged solar
cells show up warmer than the other
cells on the infrared image because
they are overloaded or have stopped
working. Radiation of approx. 600 W/
m2
is a guide value. If the solar
radiation changes during the measure-
ment, for example, due to overcloud-
ing, the infrared image can no longer
be used.
To achieve the highest possible and
therefore easily detectable tempera-
ture gradients, we recommend
carrying out the measurement when
outdoor temperatures are low (e.g.
morning or evening). The cooling
effect on panels caused by wind may
also have to be taken into account.
Correct alignment. During thermo-
graphic measurement, the alignment
of the imager in relation to the PV
module is key. The energy radiated is
dependent on direction, i.e. during the
IR temperature measurement, the
alignment of the imager in relation to
the module surface should be 60 – 90
°C. The PV module should be aligned
so that it is as vertical as possible to
the direction of solar radiation.
Angle-related measurement errors
lead, for example, to possible temper-
ature differences and false reflections.
It should be ensured that the measure-
ment image is not affected by reflec-
tions, for example of the imager itself,
the measuring technology technician,
the sun or nearby buildings. Reflected
radiation is also detected by the
imager. Reflections can be detected
through changes in the angle of view,
as they also move.
Cloud reflections are visible.
11

With freestanding solar modules
supported by mounting systems,
thermal images can also be taken
from the back, as reflections can be
virtually excluded and a higher
emission level is achieved. The heat
transfer is sufficient to be able to
assess temperature distribution on the
back. This means that incorrect
measurements and misinterpretations
are avoided.
Interpretation and evaluation. If
temperature deviations occur during
the evaluation of the thermograms,
this does not necessarily mean that
the affected modules must be faulty.
For example, questionable thermal
images can indicate partial shade
caused by dirt. At the same time, an
individual damaged cell does not
necessarily lead to a loss of perfor-
mance of the entire panel. Only the
failure of entire sub-sections of the
panel will result in major performance
losses. Additional checks such as a
visual inspection, a characteristic
curve measurement or an electrolu-
minescence measurement are
therefore necessary in order to
localise suspected causes of faults.
Care should be taken when interpret-
ing the absolute temperatures shown
on the thermograms. Reflections of the
cold sky radiation can, for example,
lead to misinterpretations – clear blue
summer sky radiates at up to -25 °C.
We recommend working here with ΔT
values and paying particular attention
to extreme temperature differences
within a panel or compared with the
adjacent panel.
Thermography
Correct alignment for measurement of a mod-
ule.
Taking an image of a module from the back.
12

Hotspots do not necessarily in-
dicate a defective cell.Not every
hotspot automatically indicates a fault
in a solar cell. For example, mounting
systems and connection points may
be visible as a result of the heat trans-
fer to the module surface.
Modules with significant deviations are
not necessarily faulty, they may just
be dirty and should be cleaned.
Level and span. Adjustment of the
so-called level and span is extremely
important for the identification of
faults. In automatic mode, thermal
imagers detect the hottest and coldest
point and adjust colour grading across
the entire range. The wide spread
therefore eliminates any relevant tem-
perature differences.
The image overlay shows dirt caused by bird
droppings at the hottest hotspot.
Connection points on the back are visible.
Manual adjustment. Automatic adjustment.
13

Thermography
Checking photovoltaic plants using
thermography places very high
requirements on the use of a thermal
imager. Several criteria must be taken
into account when choosing a thermal
imager suitable for this purpose:
• Infrared resolution of the detector
• Thermal resolution (NETD).
• Exchangeable lenses.
• Camera functions
• Software
IR resolution or geometric resolu-
tion. The geometric resolution (given
in mrad) describes a thermal imager's
capability of recognizing objects ( e.g.
individual faulty modules) from a
certain distance. As the geometric
resolution is dependent, among other
things, on the IR resolution of the
detector, IR resolutions of at least
320 × 240 pixels (76,800 measurement
points) are recommended in the case
of large PV systems and for measure-
ments from a long distance. When
checking small systems and carrying
out measurements from a short
distance, IR resolutions from 160 ×
120 pixels (19,200 measurement
values) may also be sufficient.
Thermal resolution (NETD). The
thermal resolution describes the
capability of a thermal imager to
detect temperature differences on an
object surface. A thermal resolution of
0.05 °C (or 50 mK), for example,
means that the thermal imager can
detect this difference and can carry
out different colour grading on the
display. The lower the thermal resolu-
tion, the better the IR image generat-
ed.
Exchangeable lenses. Aside from
the IR resolution of the detector, the
opening angle of the lens also affects
the geometric resolution. In order to be
able save time measuring large areas,
e.g. from a elevated platform, imagers
with exchangeable telephoto lenses
should be selected. The imagers testo
882, testo 885 and testo 890 enable
fast lens changes.
What does the ideal thermal imager
look like?
A high geometric resolution facilitates the
inspection of large plants.14

Rotating display. A rotating display,
such as featured in the testo 885 and
testo 890 imagers, for example, assists
you in the correct positioning of the
imager (see Tips Tricks) in order to
avoid measurement errors. This makes
it possible to take thermographic im-
ages overhead. Measurements on the
rear of modules are also made easier.
The thermal imager can be rotated into
the required position without someone
having to lie on the floor.
Solar mode. This recording mode is
one which is especially useful for solar
thermography. In solar mode, the solar
radiation in W/m² can be stored with
each image to document the relevant
ambient conditions.
Video sequences. A full radiometric
video measurement or logging function
enables video sequences to be re-
corded. In this image mode featured in
the imagers testo 885 and testo 890,
a vehicle moves away from individual
freestanding system arrays supported
by a mounting system while the ther-
mal imager records video sequences.
The images are then evaluated using
software on the PC to save time.
In large plants it is only the conspicu-
ous modules which need to be further
examined.
Image of a roof installation with a telephoto
lens from a long distance.
testo 885 with rotatable display
for overhead images.
Solar radiation in W/m² is also stored in each
image.
15

Thermography
Software. The analysis software (e.g. testo IRSoft) enables the optimization
and analysis of the thermal images, and ensures that the findings in the images
are clearly presented and documented. The software should be intuitive to use,
clearly set out and extremely user-friendly. In the testo IRSoft, meaningful, pro-
fessional reports can be created in minutes using pre-defined report templates.
Temperature histogram of a solar module.
Minimum: 38.7 °C Maximum: 77.9 °C Mean value: 53.4 °C
The image shows the temperature
histogram of a solar module. Various
aspects can be read off from this.
While the temperature mean value is
53.4 °C, there are maximum values
of up to 77.9 °C compared with the
minimum temperature value of 38.7
°C. The frequency as a percentage
enables a conclusion to be reached
as to how many cells are in critical
temperature ranges. The image used in
the example shows that approx. 55%
of all temperature values are higher
than 63 °C and therefore already 10 °C
more than the mean value of 53.4 °C.
16

Technical data in comparison.
Testo thermal imagers for
solar thermography.
testo 872 testo 882 testo 885 testo 890
Resolution 320 x 240 320 x 240 320 x 240 640 x 480
Resolution with
SuperResolution
640 x 480 640 x 480 640 x 480 1280 x 960
Thermal
sensitivity.
0.06 °C 0.05 °C 0.03 °C 0.04 °C
Focus Fixed focus Automatic/manual Automatic/manual Automatic/manual
Field of view 42° x 30° 32° x 23° 30° x 23° 42° x 32°
Image refresh rate 9 Hz 33 Hz 33 Hz 33 Hz
Accuracy ±2 °C, ±2 % of m.v. ±2 °C, ±2 % of m.v. ±2 °C, ±2 % of m.v. ±2 °C, ±2 % of m.v.
Exchangeable
lenses.
No No
Telephoto and
super-telephoto lens
(optional)
Telephoto and
super-telephoto lens
(optional)
Solar mode Yes Yes Yes Yes
Sequence
capturing and
fully radiometric
video
measurement
No No Yes Yes
17

Thermography
Even a small technical defect is
sufficient to have a considerably
negative effect on the solar yield – and
therefore the economic viability of a
photovoltaic plant. The causes are
various: Carelessness during installa-
tion, degeneration of the laminates or
slow damage due to years of UV
radiation and weathering. The use of a
thermal imager helps to determine the
causes of error quickly and reliably,
and to eliminate them.
At the forefront of a thermographic
analysis is the identification of
hotspots, which not only cause yield
losses, but also represent areas of
danger. This also plays an important
role when it comes to the issue of
warranty claims. Imager tests are
furthermore carried out on electrical
distributors, in order to detect the
location of bad wiring. Thermal images
can also ensure that live components
do not overheat and cooling systems
are working properly.
Thermal imagers – ideal tools for the
inspection of photovoltaic plants.
Thermal imagers from Testo are
specially designed for solar thermog-
raphy requirements. They allow solar
engineers to offer their customers a
valuable after-sales service, while
plant operators obtain a reliable
statement on the status of their solar
plants.
Solar thermography: Overview of
applications and benefits
• Early identification of faults,
avoidance of yield loss
• Increasing operational safety,
prevention of fire danger
• Fast, safe inspections
• Identification of hotspots,
modules at open circuit, short
circuits, delamination, cell rupture,
corroded and loose contacts,
overheated connection sockets
• Creation of added value for solar
engineers and plant operators
18

Practical application tips
• Measure in sunshine and at low
outdoor temperatures
• Point the thermal imager correctly,
bear reflections in mind
• If possible, measure on the rear
• Carefully analyse the causes of
temperature deviations
Selecting the right thermal imager
• Observe suitable geometric and
thermal resolution for the applica-
tion
• Imagers with exchangeable lenses
and rotating display provide more
flexibility
• Useful functions such as solar
mode and video sequence
recording, as well as a versatile
analysis software, simplify
measurement and analysis
19

www.testo.com
Subjecttochange,includingtechnicalmodifications,withoutnotice.29814035/msp/I/04.2017

Testo Solar Panel - Practical Guide

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