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Pumping Water for Irrigation Using Solar Energy
Fact Sheet EES-63
November 1991
Pumping Water for Irrigation Using Solar Energy
1
H.J. Helikson, D.Z. Haman and C.D. Baird
2
This publication discusses photovoltaic technology and
the cost of photovoltaic power for water pumping.
The information presented includes:
an overview of how electricity is generated from
solar radiation using photovoltaic cells,
a description of a demonstrational photovoltaic
powered water pumping system, and
a discussion of the present day price of such a
system and the potential future effects of current
trends which continue to decrease the cost of
photovoltaic power.
FROM SUNSHINE TO ELECTRICAL
CURRENT
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 n-
type 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).
1.
This document is Fact Sheet EES-63, a series of the Florida Energy Extension Service, Florida Cooperative Extension Service, Institute of Food
and Agricultural Sciences, University of Florida. Publication date: November 1991.
2.
H.J. Helikson, Former Agricultural Energy Specialist; D.Z. Haman, Assistant Professor, Agricultural Engineering Dept.; C.D. Baird, Professor,
Agricultural Engineering Dept., Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville
FL 32611.
The Florida Energy Extension Service receives funding from the Florida Energy Office, Department of Community Affairs and is operated
by the University of Floridas Institute of Food and Agricultural Sciences through the Cooperative Extension Service. The information
contained herein is the product of the Florida Energy Extension Service and does not necessarily reflect the views of the Florida Energy Office.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national
origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean Pumping Water for Irrigation Using Solar Energy
Page 2
A photovoltaic cell is analyzed by its open circuit
voltage and short circuit current capabilities (Figure
2a). Open circuit voltage is the voltage output from
Figure 1. Silicon lattice schematic with 4 valence electrons
(top); with boron impurities (center); with phosphorous
impurity, the extra electron gives a negative charge (bottom)
a photovoltaic cell when no current is flowing through
the circuit. It is the maximum possible voltage that a
photovoltaic cell can produce in sunlight.
Short
circuit current is the current flowing freely from a
photovoltaic cell through an external circuit that has
no load or resistance. It is the maximum possible
current that the photovoltaic cell can produce at a
given level of irradiance
2
(Florida Solar Energy
Center, 1988).
To increase voltage output, photovoltaic cells are
wired in series; to increase amperage output,
photovoltaic
cells
are
wired
in
parallel.
A
photovoltaic module is a combination of photovoltaic
cells wired together in series and parallel with the
purpose of generating a specific current and voltage
at a given level of irradiance. A photovoltaic array is
composed of two or more photovoltaic modules.
The maximum power point is the point on a given
photovoltaic I-V graph which gives the highest
amperage and voltage product at a given level of
irradiance (Figure 2b). This is the desired point of
operation for a photovoltaic array.
The direct current (DC) power received by an
electrical load from a photovoltaic array is mainly
controlled by two parameters
3
: the solar irradiance
available to the module, and the current-and-voltage
demand of the load. The voltage production of a
photovoltaic cell remains practically constant under all
levels of irradiance, but the current produced is
directly proportional to the level of irradiance
available at any given point in time. Since the power
produced by a 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 (Figure
2c).
The current-and-voltage components of a DC
electrical load form a straight line on an I-V graph,
after the initial start-up power surge, which rises at a
constant current-to-voltage ratio (Figure 3a).
In
comparison, the representative I-V graph of a
photovoltaic power supply shows a constant amperage
while the voltage increases until the amperage falls
sharply to zero at the open circuit voltage.
If a
photovoltaic array is designed to produce 24 volts but
the load only requires 12 volts, the load will only draw
from the photovoltaic array the power which
corresponds to 12 volts on the I-V curve even though
the photovoltaic array is able to produce more power.
Figure 3a shows an electric load line and a
photovoltaic power supply line which are not properly
matched.
In addition to the difference in I-V curve formation
between an electric load and a photovoltaic electric
supply, there is a continual variation in the amperage
level of the photovoltaic power supply due to changes
in the level of irradiance available throughout the day.
Amperage fluctuations continually change the location
of the maximum power point on the I-V graph and Pumping Water for Irrigation Using Solar Energy
Page 3
hinder the matching of the maximum power points of
Figure 2. Amperage-voltage (I-V) curve for photovoltaic
module (top); I-V curve showing maximum power point
(center); I-V curve showing various levels of solar irradiance
(bottom).
a photovoltaic module to points along the straight
load line of the DC electric load (Figure 3b). Care
must be taken when designing a photovoltaic system
to match the I-V load curve and maximum power
points over the widest possible range to create a
system with high overall efficiency.
SYSTEM DESCRIPTION
A photovoltaic array comprised of two units of three
modules each was used to power the water pumping
system used in this demonstration (Figure 3c). The
six photovoltaic modules had a photon responsive
surface area of 3.17 m
2
. The three modules of each
unit were connected end-to-end and reflectors,
constructed from sheet metal and aluminum foil tape,
were attached to the two long sides of each unit. The
reflectors doubled the area of the array structure
normal to the sun and increased the short circuit
amperage of the units up to 33 percent overall.
The photovoltaic array was attached to a one-axis
tracking mechanism. This system enabled the array
to remain essentially normal to the sun throughout
the day so that the photovoltaic modules were able to
utilize a larger portion of the available sunlight. The
tracking mechanism was powered and controlled by
two, small photovoltaic modules which functioned
independently from the six primary modules (Dinh,
1988).
Photovoltaic cells have minimal current resistance
when exposed to light, but when they are shaded, all
current flow through them is blocked. The tracking-
control photovoltaic modules on the photovoltaic
system used in this demonstration were placed on the
east and west sides of the array. When both tracking
modules were in equal sunlight, the electricity
produced by them flowed between the two modules
and the array remained stationary. When one of the
modules was shaded, the electricity produced by the
module remaining in sun