Power Generation - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 195

and finding more applications. Many small and large nations are increas-
ing their use of solar energy.
A typical 100-cm silicon cell produces a maximum current of just un-
der 3 amps at a voltage of around 0.5 volts. Since many PV applications
involve charging lead-acid batteries, which have a typical nominal voltage
of 12 volts, the solar modules often consist of around 36 individual cells
wired in series to ensure that the voltage is usually above 13 volts which
is enough to charge a 12 volt battery even on overcast days.
In a typical monocrystalline module, the open circuit voltage is 21
volts and the short circuit current is about 5 amps. The peak power output
of the module is 73 watts, achieved when the module is delivering a cur-
rent of some 4.3 amps at a voltage of 17 volts.
When cells are delivering power to electrical loads under real-world
conditions, the intensity of solar radiation often varies over time. Many
systems use a maximum power point circuit that automatically varies the
load seen by the cell in such a way that it is always operating around the
maximum power point and so delivering maximum power to the load.
PV MATERIALS
Silicon is the most popular material for photovoltaic (PV) power. An-
other material is gallium arsenide (GaAs), which is a compound semicon-
ductor. GaAs has a crystal structure similar to that of silicon, but it consists
of alternating gallium and arsenic atoms. It is well suited for PV applica-
tions since it has a high light absorption coefficient and only a thin layer of
material is required, which reduces the cost.
GaAs cells can also operate at relatively high temperatures without
the performance degradation that silicon and many other semiconductors
experience. This allows GaAs cells to be suitable for concentrating PV sys-
tems.
Cells made from GaAs are more costly than silicon cells, because the
production process is not as well developed, and gallium and arsenic are
not abundant materials. GaAs cells have been used when very high effi-
ciency is needed regardless of cost such as required in space applications.
They were also used in the Sunraycer, a photovoltaic-powered electric car,
which won the Pentax World Solar Challenge race for solar-powered ve-
hicles in 1987. It ran the 3000-km from Darwin to Adelaide, Australia at
an average day time speed of 66-km per hour. The 1990 race was won by a