Renewable Energy Resources for Irrigation - Practices of Irrigation and On-farm Water Management - page 485

the least sunny month should be greater than 3.5 kW/m 2 on a horizontal surface
(UNESCAP, 1991 ) .
If measured data of solar irradiance is not available, it can be estimated from other
easily measurable weather data such as temperature. Procedure for such indirect
estimation and direct measurement by instrument has been described in detail in
Chapter 3 ( Weather )ofVolume1.
13.4.3 Solar or Photovoltaic Cells - Theoretical Perspectives
Photovoltaic system uses solar energy for generation of electricity. Photovoltaic is
a technology that converts radiant light energy (photo) to electricity (voltaic) due
to energy transfer that occurs at the subatomic level. Silicon is one of the elements
used as base material for the production of photovoltaic cells. A silicon atom has
four valences of electrons that are shared with adjacent silicon atoms in covalent
bonding. The boron atoms occupy a lattice position within the silicon structure,
and a positive-charged hole forms in place of the missing fourth electron. Silicon
material with boron impurities is called a p-type semiconductor. The phosphorous
atoms occupy a lattice position within the silicon structure and form a negative or
n-type semiconductor. Putting a layer of p-type and a layer of n-type semiconduc-
tor material together makes photovoltaic cell (solar cells). Solar energy comes in
small packages called photons. These photons hit the outer-level electrons in the
photovoltaic cells. 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 ) . The
power produced by photovoltaic cell is the product of the current and voltage being
produced at any given time. Photovoltaic power is directly proportional to the level
of irradiance available at any given time.
13.4.4 Solar Photovoltaic Pump
Photovoltaic array is composed of two or more photovoltaic modules. Photovoltaic
module is a combination of photovoltaic cells wired together in series and parallel
for the purpose of generating specific current and voltage at a given level of irra-
diance. To increase voltage output, photovoltaic cells are wired in series, and to
increase amperage output photovoltaic cells are wired in parallel.
13.4.4.1 Photovoltaic System Types
Photovoltaic system types in pumping water for irrigation may be grouped into three
categories:
i. Stand-alone photovoltaic system
ii. Stand-alone hybrid system
iii. Grid connected system