Impact of Partial Shading

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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