pv modules design and cell mismatch effects

PV Modules Design and Cell Mismatch Effects

National Institute of Technology, Surathkal


Outline

• Introduction• Module Design• Module Circuit Design• Cell Mismatch Effects• Shading• Hot Spot Heating• Conclusion

Introduction

• Individual solar cells electrically connected together to increase their power output.

• Several aspects of PV module design may reduce either the power output or its lifetime.

• Most important effects are…– Losses due to the interconnection of

mismatched solar cells.– The temperature of the module.– Failure modes of PV modules.



Module Design

• A typical PV module consists of a number of interconnected solar cells encapsulated into a single unit.

• This is to prevent– Mechanical damage and– Corrosion

• Module lifetimes and warranties on bulk silicon PV modules are over 20 years.

A typical bulk silicon PV module used in outdoor remote power applications.

Module Materials• Front Surface Materials• Encapsulant• Rear Surface• Frame


Typical bulk silicon module materials.

Module Materials• Front Surface Materials

– Must have high transmission of light in the range of 350 nm to 1200 nm wavelength.

– Reflection should be low.– Different choices including acrylic,

polymers and glass.– Tempered, low iron-content glass is

commonly used– It is low cost, strong, stable, highly

transparent, impervious to water and gases and has good self-cleaning properties.

• Encapsulant• Rear Surface• Frame



Module Materials• Front Surface Materials• Encapsulant

– Provide adhesion between the solar cells, the top surface and the rear surface.

– It should be optically transparent and should have a low thermal resistance. And should be stable at high temperatures and high UV exposure.

– EVA (ethyl vinyl acetate) is the most commonly used.

• Rear Surface• Frame



Module Materials• Front Surface Materials• Encapsulant• Rear Surface– It must have low thermal

resistance and that it must prevent the ingress of water or water vapour.

– Commonly Tedlar, is used as the rear surface

• Frame



Module Materials• Front Surface Materials• Encapsulant• Rear Surface• Frame– It is the edging or framing of the

module.– A conventional PV module frame

is typically made of aluminium.– It should be free of projections

which could result in the lodgement of water, dust or other matter.


Several types of silicon PV modules.


Module Circuit Design

• An individual silicon solar cell has a voltage of just under 0.6V under 25 °C and AM1.5 illumination.

• The voltage of a PV module is usually chosen to be compatible with a 12V battery.

• Taking this to account most of the modules contain 36 solar cells in series.

• Output = 36 x 0.6 = 21.6 ≈ 21 V (Under test conditions.)



In a typical module, 36 cells are connected in series to produce a voltage sufficient to charge a 12V battery.



• If all cells have identical electrical characteristics, and experience the same insolation and temperature, then all the cells will be operating at exactly the same current and voltage. I-V curve for N cells in series x M cells in parallel.


Mismatch Effects• It happens when the interconnected solar cells or

modules have non identical properties• Output of the entire PV module under worst case

conditions is determined by the solar cell with the lowest output.

• The impact and power loss due to mismatch depend on– The operating point of the PV module– The circuit configuration; and– The parameters which are different from the remainder

of the solar cells.


Mismatch for Cells Connected in series

• The current must be the same, a mismatch in current means that the total current from the configuration is equal to the lowest current.


Open Circuit Voltage Mismatch

• The overall current from the PV module is unaffected

• At the MPP, the overall power is reduced as the poor cell is generating less power

• Current through the two solar cells is the same

• Overall voltage is found by adding the two voltages at a particular current


Short-Circuit Current Mismatch

• A mismatch in the SC current has a drastic impact on the PV module.

• At OC voltage, the impact of a reduced short-circuit current is relatively minor.

• As the current through the two cells must be the same, the overall current from the combination cannot exceed that of the poor cell.

• The high power dissipation in the poor cell can cause irreversible damage to the module.


Short-Circuit Current Mismatch


Shading

• The output declines proportionally to the amount of shading.

• For completely opaque objects such as a leaf, the decline in current output of the cell is proportional to the amount of the cell that is obscured.


Shading

The current is reduced by the amount of shading.


Hot Spot Heating

• This happens when a large number of series connected cells cause a large reverse bias across the shaded cell, leading to large dissipation of power in the poor cell.

• The enormous power dissipation occurring in a small area results in local overheating, or "hot-spots", which in turn leads to destructive effects, such as cell or glass cracking, melting of solder or degradation of the solar cell.

Hot Spot Heating

Heat dissipated in a shaded cell caused the module to crack.


Conclusion

• Series and parallel combination of PV cells gives required output voltage and current

• Cell mismatches have adverse effect on the module performance which reduce its power output and its lifetime

• Alternate circuit configuration has to be employed to avoid these problems.

References


• http://pveducation.org/pvcdrom/modules

Thank You


pv modules design and cell mismatch effects

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