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
