Solar
Energy Techonologies - Crystalline Solar Cells
Introduction
Most solar cells are made of a single
crystal or multi-crystalline silicon material. Silicon
ingots are made by the process of crystal growth, or
by casting in specially designed furnaces. The ingots
are then sliced into thin wafers. Single crystal wafers
are usually of 125x125 mm or larger sizes with 'pseudo-square'
shape; multi-crystalline wafers are typically square-shaped
with a dimension of 100 X 100 mm or larger. Using high-temperature
diffusion furnaces, 'impurities' like boron or phosphorous
are introduced into the silicon wafers to form a p-n
junction. The silicon wafers are thus converted into
solar cells. When exposed to sunlight, a current is
generated in each cell. Contacts are attached to the
top and bottom of each solar cell to enable inter-connections
and drawing of the current.
Thin-Film solar cells
Thin-Film solar cells are made from
amorphous silicon (a-Si), cooper indium selenide/cadmium
sulphide (CuInSe2/CdS) or cadmium telluride/cadmium
sulphide (CdTe/CdS), by using thin-film deposition techniques.
These technologies are at various stages of development
and have not yet reached the maturity of crystalline
silicon. Production of thin-film PV modules is also
limited.
PV module
PV modules are usually made from strings
of crystalline silicon solar cells. These cells are
made of extremely thin silicon wafers (about 300 Um)
and hence are extremely fragile. To protect the cells
from damage, a string of cells is hermetically sealed
between a layer of toughened glass and layers of ethyl
vinyl acetate (EVA). An insulating tedlar sheet is placed
beneath the EVA layers to give further protection to
the cell string. An outer frame is attached to give
strength to the module and to enable easy mounting on
structures. A terminal box is attached to the back of
a module; here, the two ends (positive and negative)
of the solar string are welded or soldered to the terminals.
This entire assembly constitutes a PV module. When the
PV module is in use, the terminals are connected either
directly to a load, or to another module to form an
array. Single PV modules of capacities ranging from
10 Wp to 120 Wp can provide power for different loads.
For large power applications, a PV array consisting
of a number of modules connected in parallel and/or
series is used.
Standard Capacity/ Ratings and Specifications
The wattage output of a PV module
is rated in terms of peak watt (Wp) units. The peak
watt output power from a module is defined as the maximum
power output that the module could deliver under standard
test conditions (STC). The STC conditions used in a
laboratory are
1000 watts per square metre solar
radiation intensity
Air-mass 1.5 reference spectral
distribution
25oC ambient temperature.
SPV modules of various capacities
are available, and are being used for a variety of applications.
Theoretically, a PV module of any capacity (voltage
and current) rating can be fabricated. However, the
standard capacities available in the country range from
5 Wp to 120 Wp. The voltage output of a PV module depends
on the number of solar cells connected in series inside
the module. In India, a crystalline silicon module generally
contains 36 solar cells connected in series. The module
provides a usable direct current (DC) voltage of about
16.5 V, which is normally used to charge a 12-V battery.
In an SPV system, the components other
than the PV module are collectively known as 'balance
of system' (BoS), which includes batteries for storage
of electricity, electronic charge controller, inverter,
etc. These batteries are charged during the daytime
using the DC power generated by the SPV module. The
battery/battery bank supplies power to loads during
the night or non-sunny hours. An inverter is required
to convert the DC power from the PV module or battery
to AC power for operating the load. Some loads such
as DC pumps do not require an inverter or even a battery
bank.
Main Application:Solar Energy
Contact:
Ministry of Non-Conventonal Energy Sources,
Govt. of India
Block-14, CGO Complex, Lodi Road
New Delhi – 110 003
Tel: 91 11 2436 0707/ 2436 0404
Fax: 91 11 2436 1298
