Page 177 - A Comprehensive Guide to Solar Energy Systems

178 A CoMPrehensIVe GuIde To soLAr enerGy sysTeMs

circuit. For reliable outdoor operation over a long period (20–30 years), the cells must be
properly encapsulated to ensure protection from the weather, including humidity; the
withstanding of mechanical loads; protection from impacts; and electrical isolation from
supporting constructions. Most modules are constructed in a form of a flat plate creating
a solid substrate to support the PV cells. such flat modules are usually fitted in a frame to
enhance the mechanical stability. on the back surface of flat modules a junction box is
fitted, in which the electrical connections to PV cells are connected to the output wires
which are used to connect the module to the PV system. In thin film technologies, flexible
modules can be made for special applications.
optical losses can occur as a result of refraction at the interface between air and the
transparent encapsulating material; absorption in the encapsulating material; and refrac-
tion on the interface between the encapsulating material and PV cells (refraction index of
the cell antireflection coating should be accommodated to refraction index of the encapsu-
lating material). optical losses can also occur as a result of the angular distribution of light.
The operating temperature T c of cell encapsulated in a module is higher than the am-
bient temperature. The input radiation power absorbed in PV cells is partially converted
in the output electrical power and the rest is converted to heat that is dissipated into the
surroundings. Considering a relatively low temperature difference between the PV cells
and the ambient, convection is assumed as the main mechanism for heat dissipation in
terrestrial, flat plate applications. on this basis, a simplifying assumption is made that the
cell-ambient temperature increases linearly with irradiance G r and that the cell tempera-
ture T c in a module of efficiency η can be expressed as

Tc=Ta+rthcaGr(1−η), T c =T a +r thca G (1 − η), (8.30)
r
where r thca is the thermal resistivity between the cell and ambient.
The thickness and thermal conductivity of individual layers of the module structure are
important for reaching a low thermal resistivity r thca of the module that depends also on
convection conditions (e.g., speed of wind).
To estimate the average cell temperature in the module, manufacturers provide (in the
parameter list) the noCT parameter, which is defined as the cell temperature under con-
−2
−1
ditions: irradiance 800 W m , ambient temperature 25°C, and wind speed 1 m s . The
noCT parameter can help evaluate loses connected with enhanced cell temperature un-
der real operating conditions.
under external conditions, modules are exposed to various external stresses such as
temperature changes, mechanical stress (from wind, snow, hail, etc.), moisture (originat-
ing from rain, dew, and frost), agents transported via atmosphere (dust, sand, and other
agents), and solar radiance (including the highly energetic uV radiation).

8.5.3 Local Shading and Hot Spot Formation

under real conditions, PV modules can be partially shaded from a neighboring object (a
tree, a part of building, etc.) or by a local front surface obstructions (e.g., by leaves fallen