Thermal imagers-Thermography on Photovoltaic Modules

Committing to the futureCommitting to the futureCommitting to the future

Training forThermography on

Photovoltaic Modules

INNOTEC LLC (INNOTEK MMC), tel:+99455 6610664, +994124087047 email: [email protected], www.aztesto.com

2/30testo AG,

Committing to the future

Div TI, vl, Photovoltaics training, 29.4.2011

Contents

Design of a Photovoltaic module

Why should measurements be carried out?

At which intervals should measurements be made?

Optimum measurement enviromment / conditions

Defect images

Observation alternatives

The "ideal" imager

Benefits


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Solar cell / solar system design (I)


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Solar cell / solar system design (II)

A photovoltaic plant consists of solar modules which are strung together into arrays (e. g. mono/polycrystalline silicon, thin film material )

Solar modules consist of light-sensitive cells which generate direct current

Alternating current converters transform the direct current generated by the solar modules into alternating current

Solar modules are fixed on appropriate structures, mostly on aluminum for heat transfer

A solar cell has an efficiency factor of approximately 5 to 20%


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Terms and effectivity

83 %

17 % 17 % direct current

83 % heat

Solar energy

Solar module

Alternating current

converter

94 % alternating current

6 % heat

Module

Cell

StringThe weakest cell in a serial string dictates the maximum power current!

Strings are serially switched!

Modules are usually also serially switched!


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Serial / parallel connection of the modules

Parallel connection

Less sensitive to malfunction, modules work independently of

each other

Serial connection

The "weakest module" dictates the maximum current flow

Parallel connection less sensitive

Alternating current

converter

Alternating current

converter


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Which are the main reasons for maintenance?

Defective cells lead to considerable loss of performance from the system

A temperature increase of 10 °C above the operating temperature recommended by the manufacturer can lead to a 50 % shortening of its lifetime

Defective cells can lead to an enormous heat development, and thus to the danger of fire.

80% of all systems contain defects

In order to operate a plant efficiently, error sources must be localized and repaired!


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Optimum measurement (I)

• Dry, cloud-free days

• Intense sunshine

• Irradiation value of 500-600 W/m²

• If the sun irradiation changes to below the minimum necessary irradiation value, the thermographic image is useless.

• Ideal measurement at low outside temperatures

Pto

t[%

ma

x]


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Optimum measurement (II)

PV module should be positioned vertically to the sun

Postioning of the imager to the module surface 60°-120° ideal at 90°

Distance of 2-3m between imager and PV module

Take emissivity of glass into account: however negligable, since ony temperature differences are evaluated.


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Optimum measurement (III)

Ima

ge

r h

eig

ht

1

,7 m

Ima

ge

r h

eig

ht

2

.2 m

60°

32°

23°


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Where can defects be determined?

Photovoltaic modules

Surface

Connectors

Bypass diodes

Corroded contact points

increased resistance heat develops

Plug-in connections

Cable contacts

Alternating current converter

Generators


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Which defects can be determined?

Module failure (also partial failure)

Defective wiring

Damp penetration

Delamination

Cracks in module

Defective bypass diodes

Cracks in wafers

Loose contacts

Wear and tear

Alternating current converters, junction boxes etc.


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Example images: Hot-Spot effect

Defective cell:

Converts power from the adjacent cells into heat!

Converts all incoming solar energy into heat

Known as the Hot-Spot effect

Defective solar modules, identifiable in the thermal imager as a Hot-Spot (approx. 15K above mean temperature)

Loss of performance of the system -danger of fire from overheating!


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Example images: Overshadowing – dirt (I)

Overshadowed cell, or cell covered by dirt:

Converts power from the adjacent cells into heat!

Performance of all cells switched serially is affected (string)

Performance of all modules switched serially is affected (string)

Known as overshadowing

Loss of performance of the system -ideally comparable using a reference

image!INNOTEC LLC (INNOTEK MMC), tel:+99455 6610664, +994124087047 email: [email protected], www.aztesto.com

15/30testo AG,



Example images: Overshadowing – dirt (II)

1. OVershadowed or dirty cell, heats up more than the

others!

2. Entire string suffers peroformance loss, heats

up!

3. Heated string radiates, adjacent strings suffer

performance loss!

Dirt / overshadowing leads to considerable loss of performance!

There are module bypass diodes and string (bypass) diodes

Bypass diode avoids the effect string level

The bypass diode prevents power from flowing back into the module


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Example images: defective bypass diodes

Defective bypass diode disables entire string / module

A "Hot-Spot effect" or overshadowing can no longer be countered

Power flows back into the module, it heats up

Intact cells can be damaged by the increased heat input

The warmer a solar cell becomes, the lower its degree of effectivity!

Loss of performance and danger of cell damage!


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Example images: Cracks in the cell

Broken glass and cell rupture can be distinguished with the help of a thermal imager!

Broken glass does not lead to loss of performance

Cell rupture leads to a reduction in performance

Cracks in wafer

Ruptured cell


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Example images: Delamination

Contact errors between the conductive ribbon and the solar cell

Power cannot be transferred away

increased heat development

no thermal image in the module grid

loss of performanceDelamination is due to production errors

loss of performance


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Example images: Mismatch / patchwork pattern

Different performance data from solar cells within a module can lead to a loss of performance due to mismatch.

Mismatch not all cells in the module match each other

The weakest cell within a string dictates the maximum current flow.

The same effect can occur at module level

The reason for this is the serial switching

Mismatch is only an error if the manufacturer's specifications are not reached

If a solar module does not match a string, it

reduces the total performance of the

string!


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Example images: Non-functioning modules - miscellaneous

Non-functioning modules appear with regular heating, and warmer than the adjacent modules

Miscellaneous defects

E.g. due to incorrect connection, animal bites, module failure

Damp penetration, danger of delamination

Loose contacts

Wear and tear

Defective wiring

These errors cannot be generalized using an exemplary thermal image


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Example images: Not every Hot-Spot is a defect

Aluminium support brackets lead to heat transfer

Connection sockets are sometimes warm, and visible in the thermogram

Continuous discrepancies do not indicate a defect

Not every Hot-Spot is automatically a defect!


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Summary – example images (I)

Pattern Description Possible error Possible cause Electrical measurement Comment

One module regularly

warmer than others

Non-functioning

moduleModule disconnected

Module usually fully

functional

Warming of a module

by line ( 1string)

One cell string short-

circuited

Bypass diode defective

Internal short circuit

Loss of performance of sub-

string

Hot-Spot detected by

IR measurement

Single random cells

considerably warmer

Module is short-

circuited

All bypass diodes

defective

Performance of module

almost nil

Voc reduced

"Patchwork pattern"


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Summary – example images (II)

Pattern Description Possible error Possible cause Electrical measurement Comment

One cell considerably

warmer

Overshadowing

Defective cells

Not necessarily loss of

performance

Visual inspection

required

Part of cell

considerably warmerCell rupture

Outside mechanical

influence

Drastic loss of performance

Reduction of Isc and FF

Artefact

Overshadowing (due to

bird droppings...)

Reduction of performance,

depending on size and

overshadowing of cells

Visual inspection

required

Cell crack Construction errorReduction of performance,

depending on size of crack

Crack is detected only

by Hor-Spot IR

measurement

Spot heating


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Examine front and back

No problems with reflections

Higher emissivity

Heat distribution can be evaluated almost as well from "below"!

As a thermographer, extend your field of view!


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Measurement with a telephoto lens

Distance measurement

High geometric resolution thanks to telephoto lens

Telephoto lens measurement, and you no longer need to climb on to the roof!


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The ideal thermal imager (I)

Testo, of course!

testo 876

Fulfils all the requirements of Photon magazine for an thermal imager for use in photovotaics

Offers additional features for make work easier,e.g.. Hot-Spot, speech recording, …

Characteristic Requirement according to Photon876

Temperature range -10 to +100° C -20° to 100°C / -20° to 280°C

NETD < 100 mK < 80 mK

Temperature accuracy < 5 % +/- 2°C

Operating temperature -10° to +40° C -15° to 40° C

Protection class IP 54 IP 54

Image frequency > 5 Hz 9 Hz

Battery life min. 2h approx. 4h

Shield for overshadowing display desirable Rotatable display

Auto focus desirable Motor focus

Orientation by laser or (better) digital image Real image

Weight < 1.5 kg 0.9 kg

Detector resolution min. 120 x 120 pixels 160 x 120 pixels

Lens < 35° 32° Standard + 9° Tele

Manufacturer's service. desirable present

Training by manufacturer desirable present

Price (net) < 10.000,- € 5.000,- €

Requirements for thermal imagers for use in solar installations


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The ideal thermal imager (II)

Motor focus offers real one-hand operation, ideal for ladders, etc.

Rotatable display can be read optimally even when using a tripod, no sun reflexions on the display

Exchangeable lenses allow two analysis versions:Close analysis(wide-angle, optimal with 32°)distant analysis (telephoto, optimal with 9°)

Speech recordingsimplifies analysis and supports one-hand operation


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Benefits for solar engineers / photovoltaic system fitters

System can remain in operation, does not need to be switched off for analysis

Identification of manufacturers errors after commissioning

Fast and meaningful defect detection

Well correlated with electrical measurements

Theuse of a telephoto lens allows an estimate to be made as to whether the system needs to be examined more closely

Invisible defects can be deteected

Comparison with reference images after commissioning

Ideal for evaluation when purchasing used PV modules

The use of a thermal imager is a MUST for all solar engineers!


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When should the solar engineer come to carry out the measurement?

Recommendation of a solar engineer to his customers regarding measurement frequency:

1. Testing directly after commissioning

Any defects can be eliminated right from the beginning

Good reference for later follow-on measurements

2. Testing shortly before the end of the guarantee period

Guarantee claims can be made if defects are discovered

• Regular tests in order to achieve the optimum effectivity of the PV system

• For evaluation purposes when purchasing used PV modules

The more regular, the more secure are the highest yields!


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Advantages for the manufacturer of the photovoltaic system

The use of a thermal imager is a MUST for every manufacturer!

Invisible defects can be detected

Fast and meaningful defect detection

Testing correct installation

Well correlated with electrical measurements


Technology

Thermal imagers-Thermography on Photovoltaic Modules