Pumping Water for Irrigation Using Solar Energy 1

Photovoltaic cells are able to turn the energy in solar radiation into electricity due to an energy transfer that occurs at the sub-atomic level. Solar energy comes in small packages called photons. These photons hit the outer level electrons in the photovoltaic cells like the flappers hit the metal ball in the pin ball machine. The dislocated electrons form the electrical current. Silicon is one of the elements used as a base material for the production of photovoltaic cells. A silicon atom has four valence electrons which are shared with adjacent silicon atoms in covalent bonding (Figure 1a). To produce the positive-charged side of a photovoltaic cell, boron atoms which have only three valence electrons are introduced into the lattice structure of pure silicon. The boron atoms occupy a lattice position within the silicon structure, and a positive-charged hole forms in place of the missing fourth electron (Figure 1b). Silicon material with boron impurities is called a positive or p-type semiconductor. To produce the negative-charged side of a photovoltaic cell, phosphorus atoms which have five valence electrons are introduced into the pure silicon structure. The phosphorus atoms occupy a lattice position within the silicon structure and form a negative or n-type semiconductor (Figure 1c). Photovoltaic cells are made by putting a layer of ntype and a layer of p-type semiconductor material together. When the photons in solar radiation strike a photovoltaic cell, the kinetic energy of the photons is transferred to the valence level of electrons. The freed electrons and positive-charged holes attract each other and create positive-negative pairs. The formation of these pairs creates electricity (Garg, 1987).