MICRO CRACKS IN PHOTOVOLATIC(PV)

MODULES

BYO.RENUKA

ABSTRACT

Degradation of power output of a photovoltaic( PV) module

Study of microcracks is essential to answer how these degrade the performance

content INTRODUCTION ORIGIN OF CRACKS CRACK DETECTION TECHNIQUES CLASSIFICATION OF CRACKS EFFECTS OF CRACKS CORRELATION OF CRACKS WITH

MODULE PARAMETER CONCULSION

INTRODUCTION• Micro cracks in a solar cell is an important issue for PV module• Cause power loss and effect the reliability• Cracks developed in young modules initially not effect power

output• Module experience heat, wind, humidity, mechanical loading • Cells having cracks above a limit are rejected before integration

of cell string• Done by ultrasonic methods, flux thermograph,

electroluminescence imaging• The exact effect of cracks are not well known because the growth

of cracks depends on the handling of module, location of module, climate and other environment conditions.

• Two modules with same amount of crack may give different power output at two different places.

• Impact of these cracks on power output to reduce the number of rejected cells and reduce loss of manufacturer.

ORIGIN OF CRACKS PV cells are made of silicon & brittle in nature Occurrences of micro cracks in a PV module are of three categories During Production Due to poor equipment & inexperienced operator During Transport Mitigated by a good packaging with more protection Installation Bad installation may develop cracks and also 0ther damage to modules

CRACK DETECTION TECHNIQUESNon destructive methods :

Optical Transmission:

cracks are detected by IR light

size of crack detection depend on CCD camera

Not good for detection in finished solar cell

Infrared ultrasound lock-in thermography:

Ultrasound energy in feed periodically into wafe

Transducer generates ultrasound energy at 20KHz

Energy is fed to silicon wafer by ultrasound coupler

Information is converted into an image by lock in

thermography

Electroluminescence imaging:

o It is also called as contact techniqueo Used for finished PV moduleso Usually done in dark environmento Cracks appears as dark gray line

Photoluminescence imaging:

It is non contact method for detection

of cracks

Acquition time of less than a second

Fluorescence:

In EL outdoor images taken are of poor quality and reuires change of circuit

Fluroscence detects cracks with aging

Method Advantage Disadvantage

Optical transmission

Detect small cracks up to 1um,

throughput 1 wafer per sec.

Used in production stage, inapplicable for finished cells

Ultrasound lock-in thermography

Can be used for both wafers and

solar cells

Long acquisition time

Electroluminescence

High throughput Interference with other defects, contact method used

only for finished cells

Photoluminescence

High throughput, contactless

Interference with other defects e.g. scratches

Fluorescence High throughput , also used for

decolourization

Interference with defects

CLASSIFICATION OF CRACKS Cracks are of various sizes and characteristic A classification of cracks according to orientation No crack: A cell which has no crack as reference Dendritic crack: present at any part of cell & any direction+45/-45 degree crack are oriented with respect to reference cell

Parallel to bus bar:

Perpendicular to bus bars:

Cross line crack:

Several direction

Relative occurrence of cracks

Mode A crack:

Not influence the current flow

No crack resistance and are still electrically connected to cell

Mode B crack

Affects the current through cell Crack resistance

Area more than mode A

Mode C Crack :Isolate the crack area from cellDegrade power outputCritical than two mode cracksCurrent from the cell is directly proportional toactive area

EFFECTS OF CRACKS

Impact of micro cracks on power output not

significant in initial stage of crack

Initially crack is electrically connected

with cell

Different cracks impact the power differently

depending on their orientation,size,location

Based on orientations into 3 categories

according to how much they degrade power

category 1:- low criticality

category 2:-Moderate criticality

category 3:- High criticality

Type of crack category

Dendritic III

+45 degree II

-45 degree II

Parallel to busbars III

Perpendicular to busbars

I

Cross line II

Several direction III

In 60 PV cell modules if half of the cells have

mode A crack

power loss about 1% and if all cell have A

crack then power loss 2.5%

Graph showing power loss with no. of cracks

after mechanical load test

How various crack modes impact the

power output of PV modules montages has

done experiment

Cell micro cracks impact power loss

very little extent if they do not generate

inactive area

Relating power loss with a number of cracks after humidity freeze cycle

Power loss is more for modules

having more no. of cracks

Maximum degration of around

10%

After humidity freeze test EL

image shows that many modes

A crack has changed to mode B

and C

CORRELATION OF CRACKS WITH MODULE PARAMETER

Correlate effects of cracks with

location, Pmax degradation

Location of cracks in module can

guess source of crack

Divide the module into 3 zones

central,intermidate and

periphery

All India survey, most cracks are

located at periphery Location of cracks

DRAK AREA THE DEGRATION

Micro cracks of solar cell effects the

short circuit current

Mode C and Mode B cracks mainly

effects the IC

Increasing dark area the degradation

in IC increases

Old modules shows high degradtion

than younger modules

CONCLUSION Si wafer cannot degrade the power output of a PV module by

more than 2.5%

if the crack does not harm the electrical connection from the

active cell area.

A PV module can tolerate up to 8% loss of active area of a cell

without impacting the power output of the module

A good way to avoid power loss due to micro cracks is to avoid

cell breakage and use more flexible cell metallization

The flexible metallization will prevent isolation of cell parts in a

cracked cell.