Page 178 - A Comprehensive Guide to Solar Energy Systems

Chapter 8 • Photovoltaics: The Basics 179

on the module surface). Partial shading can significantly influence the PV module output.
If there is a shading of one of cells, the current generated in the shaded cells is reduced. As
cells are connected in series, the module output current is limited to the current generated
by the shaded cell and the output power is decreased. If differences are significant, the
nonshaded cells can act like a reverse bias source on the shaded cell. Then, the shaded cell
does not generate energy, but it dissipates energy and heats up and that can potentially
result in module damage. similar situation can occur, if part of a PV cell has been damaged
by external impact or degradation processes.
To minimize losses due to local shading, a bypass diode is put in parallel, but in op-
posite polarity, with a cell (or with a group of cells). When the current forced through the
shaded substring is such that the reverse bias equals the diode threshold voltage, the by-
pass diode sinks all necessary current to keep the string at this biasing point thus pre-
venting the increase of the power dissipated in the shaded cell. The bypass diode allows
the module to keep delivering the power generated by the unaffected cells. The ability of
withstanding local shadings is also part of approval tests of every module type.
It is clear then that the minimum efficiency loss for a shading condition will be if a by-
pass diode is at each cell, but this has limits in both fabrication technology and cost of
processing. usually, bypass diodes are placed over an internal substring, in some cases over
a whole module. The number of bypass diodes and the junction box type together with
mechanical characteristics of the module are typically given in information data sheets.

References
[1] sze sM: Physics of semiconductor devices, new york, ny, 1981, John Wiley & sons.
[2] Fonash sJ: Solar cell device physics, new york, 1981, Academic Press.
[3] Handbook of photovoltaic science and engineering. In Luque A, hegedus s, editors: Chichester, 2003,
John Wiley & sons.
[4] Benda V, Gowar J, Grant dA: Power semiconductor devices—theory and applications, Chichester, 1999,
John Wiley & sons.
[5] Goetzberger A, Knobloch J, Voss B: Crystalline silicon solar cells, Chichester, 1998, John Wiley & sons.
[6] Würfel P: Physics of solar cells, Weinheim, 2005, Wiley-VCh Verlag.
[7] solar cells—materials, manufacture and operation. In Markvart T, Castaner L, editors: oxford, 2005,
elsevier.
Further reading

[8] Photovoltaic and photoactive materials—properties, technology and applications (NATO Science Series).
In Marshall JM, dimova-Malinovska d, editors: dortrecht, 2002, Kluwer Academic Publishers.