Experiment No 1
Title: Measurement of V-I characteristics of a solar panel at various levels of
insolation, and the identification of the equivalent circuit parameters
Procedure:
1. Choose the ammeter, voltmeter and rheostat ratings so that you get 20
uniformly spaced points on the V-I characteristics. Note that you cannot
connect a single rheostat for this purpose. You will need a low resistance to
obtain points near the short-circuit condition, a high resistance to obtain
points near the open circuit condition, and an intermediate value to obtain
the maximum power point. This generally requires two or three rheostats of
different ratings, with shorting switch connected across the high-resistance
rheostat.
2. Vary the resistance in steps and obtain the V-I characteristics. Do NOT
write down the readings to be plotted later. Plot directly while you are taking
the readings. Otherwise you will not be able to get equally spaced points on
the curve. Obtain the open circuit and short circuit points by actually
opening and shorting the terminals (not by bringing the rheostat jockeys to
zero position). Be very careful about getting the correct slopes at the short
circuit and the open circuit points.
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3. Vary the insolation in three steps. If you are using a solar simulator, this can
be done by switching off some lamps. If you are using sunlight, this can be
done by changing the inclination.
4. Measure the dark characteristics by giving an external dc power supply in
forward bias mode. Keep the solar panel in dark condition (simply put it
upside down on the table, cover it with cloth). Increase the supply voltage in
steps and measure the current. Again, make sure you get equally spaced
points on the curve.
Report:
1. The V-I graph at various insolations on the same graph paper.
2. Show the approximate trajectory of the maximum power point as the
insolation is varied.
3. Report your conclusion about the variation of fill factor, VOC, etc. with
insolaton.
4. Plot the dark characteristics.
5. Take one representative V-I curve and obtain the equivalent circuit
parameters. Tabulate the equivalent circuit parameters clearly. Describe
the algorithm used to obtain the parameters.
6. Draw on the same graph, the V-I characteristics obtained from experiment,
and the one calculated from the obtained equivalent circuit.
Equivalent circuit for a photovoltaic cell:
A material or device that is capable of converting the energy contained in
photons of light into an electrical voltage and current is said to be photovoltaic (PV).
A simple equivalent circuit model for a PV cell consists of a real diode in parallel
with an ideal current source. The ideal current source delivers current in proportion
to the solar flux to which it is exposed. A more accurate model of a PV cell
considers the effect of series and parallel resistance as shown in Fig.1. In a
practical PV cell, there is a series resistance in a current path through the
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semiconductor material, the metal grid, contacts and current collecting bus. These
resistive losses are lumped together as a series resistor (Rs). Similarly a certain
loss is associated with a small leakage of current through a resistive path in
parallel with the intrinsic device. This can be represented by a parallel resistor
(Rp). Its effect is much less conspicuous in a PV module compared to the series
resistance, and it will only become noticeable when a number of PV modules are
connected in parallel for a larger system.
Fig :1 Equivalent circuit of a photovoltaic cell
The voltage and current equation for the equivalent circuit of the PV cell
considering both the series and shunt resistances is expressed as in (1).
(1)
where ISC is the short-circuit current (equal to Iph), q is the electron charge
(1.602×10-19 C), V is the terminal voltage , k is the Boltzmann’s constant
(1.381×10-23 J/K), RS is the equivalent series resistance, RP is the equivalent
parallel resistance, T is the junction temperature in Kelvin (K).
I-V characteristics of a PV cell:
Fig.2 shows the I-V characteristics of a PV cell under light as well as dark
conditions. The short-circuit current ISC is directly proportional to the amount solar
insolation received. It is also clearly evident from the figure that the dark
characteristics is similar to the diode characteristics turned upside down.
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Fig:2 Photovoltaic current-voltage relationship for dark (no sunlight) and light (an
illuminated cell) conditions.
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