Page 85 - Electrician - TT (Volume 1)
P. 85

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.










                                                           72

                                    CITS : Power - Electrician & Wireman - Lesson 13-19
   80   81   82   83   84   85   86   87   88   89   90