Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 257

In laboratories, dye solar cells have reached efficiencies above 10 %, while
first small modules have reached efficiencies of up to 5 %. Currently, this solar
cell type is being investigated as to its long-term stability /6-28/. Major emphasis
is put on the replacement of a liquid electrolyte by gel or a solid electrolyte to
avoid the risk of leakage of the modules. Further more it is necessary to under-
stand, that these types of solar cells include electrochemical processes which are
much more affected by ageing mechanisms than processes within a solid state.
Solar module. Individual photovoltaic cells are combined to a photovoltaic mod-
ule forming the basic unit of a solar generator. Usually, such a module consists of
electrically interconnected photovoltaic cells, embedding materials including a
front pane of glass and a rear side cover, electrical connecting cables or a connec-
tion box and partly also a frame, usually made of plastic or aluminium. However,
also frameless modules are increasingly applied which require special edge seal-
ing measures. The embedding of the individual cells into a module helps to pro-
tect the single cells against atmospheric impacts, ensure a defined upper voltage
level and maximum amperage respectively, and thus enables the installation of
photovoltaic generators with user-defined current-voltage characteristics.
Embedding of the cells, as well as edge sealing, is subject to very high re-
quirements. For instance, the cell surface which is subject to temperature fluctua-
tions ranging from about -40 °C to approximately +80 °C within the course of one
year must be protected against any kind of humidity (such as rain and condensa-
tion) throughout the overall technical lifetime of 20 to 30 years and even longer.
Furthermore, mechanical deterioration due to hailstones of up to several centime-
ters in diameter or due to the wind loads with gusts of 50 m/s and more need to be
prevented. Moreover, high insulation strength needs to be ensured and the used
materials must not be afflicted by bacteria nor be eroded by animals (e.g. birds).
The commercially available photovoltaic modules meet all these requirements and
ensure a safe operation throughout the overall technical lifetime.
Due to great variety of possible applications, solar modules of different power
ratings are available on the market. The nominal open-circuit voltage of the mod-
ule is determined by the number of serial connected cells and by the nominal
open-circuit voltage of the individual cells (Fig. 6.15). The nominal short-circuit
current of the module is determined by the number of cell strings connected in
parallel and by the nominal short-circuit current of the individual cell. This nomi-
nal short-circuit current depends on cell technology, the quality of the used mate-
rial, the manufacturing process, as well as mainly the size of the cell. As the short-
circuit current is linearly proportional to the solar irradiance it is also linearly
proportional to the effective surface of the cell. Accordingly, the characteristic
current-voltage curve of the entire module changes - compared to that of a single
cell - depending on the interconnection of the cells.
The rated power of the photovoltaic module depends on the total number of
cells. Power ratings of common photovoltaic applications range from approxi-
mately 50 to 75 W for 36 serial connected silicon cells with a cell surface of ap-