Download 16 – Power Electronics in Solar Energy Systems 2

Power Electronics in Solar Energy Systems
Part II
I - How does a solar / photovoltaic cell work?
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Photovoltaic cells are made of various semiconductor materials which become
electrically conductive when supplied by heat or light. The majority of solar cells
produced are made out of Silicon (Si) which exist in sufficient quantities and do not add
any burden on the environment.
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Doping technique is used to obtain a surplus of positive charge carriers (p-type) or a
surplus of negative carriers (n-type). When two layers of different doping are in contact,
then a p-n junction js formed on the boundary.
Figure 1 – Doping Process
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An electric field is built up which then causes the separation of charge carriers released
by light. Light is composed of small packets called photons. When these photons
bombard our cell, many electrons are freed within the electric field proximity, which then
pull the electrons from the p-side to n-side. Through metal contacts, an electric charge
can be tapped. If the outer circuit is closed, then direct current flows
Figure 2 – P-N juntion
Figure 3 – Solar cell Cross section
Figure 4 – Solar cell with connected load
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A solar cell is approximately 10 x 10 cm, protected by transparent antireflection film.
Each of these cells produces around 0.5 V (for Silicon). The voltage across a solar cell is
primarily dependent on the design and materials of the cell, while the electrical current
depends primarily on the incident solar irradiance and the cell area. (Solar Irradiation is
the process by which an object is exposed to the sun’s radiation.)
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The efficiency of operation of a solar cell is determined by the electrical power output
divided by the power provided by the light source. That is
η = [Po(electrical) / Po(light)] x 100
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Over 95% of solar cells have efficiencies of about 17%. However solar cells, having
power conversion efficiencies as high as 31% have been developed over the last decade
in laboratory environment.
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In order to obtain the appropriate voltages and outputs for different applications, single
solar cells are interconnected in series (for larger voltage) and in parallel (for larger
current) to form the photovoltaic module. Then several of these modules are connected to
each other to form the photovoltaic array.
Figure 5 – Solar Cell, module, Panel and Array
II – PV cell characteristics
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The I-V (current-voltage) characteristics curves of solar cells are nonlinear and follow the
general shape and equation shown below.
Where:
I – PV cell current
Isc – PV cell short circuit current (V = 0)
Voc – open circuit voltage
A and B – constants
e – Euler’s constant (2.7128…)
Im - maximum PV cell current
Vm - maximum PV cell voltage
IMPP – current at maximum power point
VMPP – voltage at maximum power point
Solar Insolation – is the solar irradiance measured at a given location on earth with a
surface element perpendicular to the sun’s rays, excluding diffuse insolation (the solar
radiation that is scattered or reflected by atmospheric components in the sky). Insolation
has the unit Watts per square meter (W/m2) or Kilowatt per square meter (kW/m2).
Figure 6 – PV cell I-V Characteristics
Figure 7 – PV cell P-V Characteristics
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Photovoltaic arrays are usually mounted in a fixed position and tilted toward the south to
optimize the noontime and daily energy production. The orientation of fixed panels
should be carefully chosen to capture the maximum energy for the season or year.
Photovoltaic arrays have an optimum operating point called the MAXIMUM POWER
POINT (MPP) as shown on the graphs of figures 6 and 7.
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Figure 7 shows that power increases as the voltage is increased, reaching a peak value
before decreasing as the resistance increases to the point where current drops off.
According to the maximum power transfer theory, this point is where the load is matched
to the solar panel’s resistance at a certain level of temperature and insolation. As shown
in figure 8, the I-V curve changes as the temperature and insolation levels change, thus
the MPP will vary accordingly.
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It has been shown that the open circuit voltage increases logarithmically while the short
circuit current increases linearly as the insolation level increases. Moreover, increasing
the cell’s temperature decreases the open circuit voltage and increases slightly the short
circuit current. This then makes the cell less efficient.
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Since solar power is relatively expensive, it is important to operate panels at their
maximum power conditions. Thus, PV system will operate more efficiently with systems
that adjust automatically their loads to match the PV resistance. In addition, panels would
change orientation to track the sun. We need then to control either the operating voltage
or current to get maximum power from the PV panel at the prevailing temperature and
insolation conditions.
Figure 8 – PV panel insolation and temperature characteristics