Page 85 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Solar cells

Objectives: At the end of this lesson you shall be able to:
• state the necessity of tapping natural resources for energy
• state about the solar cell /photo voltaic cell
• explain the basic principle, construction and characteristics of the solar cell.

Heat energy
Heat energy is the most sought energy for human being to cook the food as well as to keep warm in cold climate.
However the use of wood as the fuel for fire, has ended up in deforestation and resulted in drought.
Search of fuel led the man to use coal and then oil. However these commodities are fast dwindling and after
few hundred years both may completely vanish from earth. As such it is essential that human race should find
alternative source of energy from nature.
Hence the use of natural resources like heat from sun thought by several scientists and one of the solutions to
the energy crisis is the invention of solar cells.
Solar cell / Photovoltaic cell
A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by
the photovoltaic effect, which is a physical and chemical phenomenon.It is a form of photoelectric cell, defined
as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light.
Solar cells are the building blocks of photovoltaic modules, otherwise known as solar panels.
Solar cells are described as being photovoltaic irrespective of whether the source is sunlight or an artificial light.
They are used as a photo-detector (for example infrared detectors), detecting light or other electromagnetic
radiation near the visible range, or measuring light intensity.
The operation of a photovoltaic (PV) cell requires 3 basic attributes:
• The absorption of light, generating electron-hole pairs extraction.

• The separation of charge carriers of opposite types.
• The separate extraction of those carriers to an external circuit.
The solar cells is essentially a large photo diode designed to operate as photo voltaic device and to give as much
output power as possible. When these cells are under the influence of light rays from sun, they give out about
100 mw/cm2 power.
Fig 1 shows the construction, symbol and cross section of a typical power solar cell. The top surface consist of a
extremely thin layer of P-type material through which light can penetrate to the junction.
The nickel plated ring around the P-type material is the positive output terminal, and the bottom plating is
the negative output terminal. Commercially produced solar cells will be available in flat strip form for efficient
coverage of available surface areas.
According to different manufacturing standards, the output power varies from 50mw/cm2 to 125mw/cm2 . The
graph shows the characteristic of a solar cell which gives 100mw/cm2. Considering the characteristic curve it is
apparent that the cell will deliver an output current of 50mA when the output terminals are short circuited then the
output voltage will be zero.
On the other hand open circuited voltage of the cell will be 0.55mv but the output current is zero. Therefore
again the output power is zero. For maximum output power the device must be operated at the knee of the
characteristic. In solar cells the output power decreases at high temperature.
Several cells must be connected in series to produce the required output voltage, and number of parallel groups
to be provided as per the required output current.

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CITS : Power - Electrician & Wireman - Lesson 13-19