Page 175 - A Comprehensive Guide to Solar Energy Systems

176 A CoMPrehensIVe GuIde To soLAr enerGy sysTeMs

• resistance of emitter semiconductor material;
• resistance of contact metal-semiconductor on the front surface; and
• resistance of contact on the front surface.
These individual resistances depend on the PV cell material, contact materials, possible
technological tools, and contact pattern, and they should be minimized to decrease elec-
tric losses in the cell structure. Therefore, details about minimizing electric losses will be
discussed in connection with particular materials and technologies used in Chapters 9–13
and 18.
some electrical loss is also caused by the parallel resistance R p . The existence of fi-
nite parallel resistance is connected with microshunts in the junction, the fabrication
technology imperfections, and with defects created during aging and other degradation
processes.

8.5 Photovoltaic Modules—Principles and Construction

As the voltage of a single PV cell is low (usually less than 1 V), several cells must be connect-
ed in series to make a practical generator. The PV generator should operate under open
air conditions, should be easy and safe to manipulate. And for practical applications, PV
cells should be interconnected in a PV module—complete and environmentally protected
assembly of interconnected PV cells. The module construction and technology is closely
connected with the technology of PV cells. In the following sections, only general features
of PV modules will be given, whereas details will be given in later chapters describing PV
cells and modules fabricated from particular materials.

8.5.1 PV Modules and Their Characteristics

For applications, a number of cells connected in series are usually encapsulated in mod-
ules. As already discussed in section 8.3.3, the current of series connected cells is limited
by the cell that delivers the lowest generated current. hence, the total current in a string
of series connected cells is equal to that of the lowest current. Therefore, to obtain a high
efficiency for series connected cells, it is very important that all series connected cells have
the same I mp . In this case, the resulting voltage V mp will be the sum of individual cell volt-
ages and resulting power will be the sum of individual cell power.
An example of an analytical description of the module I–V characteristics, an equiva-
lent circuit, shown in Fig. 8.27, can be used.
If m cells of identical parameters are connected in series, the module I–V characteristic
can be approximated as

  V + R I     V + R I   V + R I
I = A J − I 01  exp  q s  −  − I1 02  exp  q s  −  −1 s . (8.29)
illPV
ζ 1
ζ 2
I=AillJPV−I01expqV+R   mkT     mkT   R p
sImζ 1 kT−1−I02expqV+-
RsImζ 2 kT−1−V+RsIRp. The parameters of the equivalent circuit suited for eq. (8.29) can be extracted from the
real measured I–V characteristics.