Impact of Partial Shading on Solar PV Module connecting cells in series

CONNECTING SOLAR CELLS

• SOLAR CELLS CONNECTED IN- series gives greater voltage-parallel gives greater current

Connected in series

• when we have two identical solar cells connected in series.

• Since the cells are in series, the currents will be matched voltages will add.

Cont..• Solar Cells in Series Since the voltages add when in

series, if the mismatch is in voltage there is no problem.

• When the mismatch is in current then we have a much bigger problem since in series we want current constant through all of the solar cells.

• We can get a situation where the worst performing solar cell is reverse biased and is dissipating power Major cause of cracking and all-around destruction of solar cells in modules

Cont..• Consider 3 modules ,each module containing

3 cells

Shading of cell

Shading of cells• Problems become more serious when shaded cells get

reverse biased. • A group of cells under full illumination is connected in

series with another group of cells under shaded illumination.

• The photon current of fully illuminated cells ILi is high compared with that of the shaded illuminated cells ILs.

• If the module current I < ILs, diode Ds is forward biased and there is no risk for the shaded cells.

• But if I > ILs, then the diode current IDs = ILs-I flow through the diode in the reverse direction.

• Reverse biased diode Ds offers high resistance will consume power and will significantly reduce the load current I itself.

Hot spot heating

• If we have current mismatch for series connected solar cells then power can be dissipated in bad cell with a maximum occurring

• when the chain is short circuited – good cells bias the bad cell so large amount of power dumped into bad cell

• This is called hot spot heating• Can severely damage the module

BYPASS DIODE

• Put ‘bypass’ diode in parallel to cell with opposite polarity

conclusion

• Thus during partial shading conditions the presence of the bypass diode limits the voltage across the cell in reverse bias to pass a certain current and hence less power is dissipated

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