ELEC3205 ELECTRICAL ENERGY SYSTEMS & MANAGEMENT
Experiment 1. Solar Power and I-V Characteristics
Aims
ƒ
ƒ
ƒ
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Learn the properties of a photovoltaic cell including its equivalent circuit;
Test I-V and P-V characteristics for a photovoltaic module;
Determine the optimal conditions for operating a PV panel in a circuit with a known load
and understand MPPT (maximum power point tracking);
Investigate the effects of solar insolation, shading, and tilting angle of a solar panel on an
I-V characteristic.
Overview
Incident sunlight can be converted into electricity by photovoltaic conversion using a solar
panel. A solar panel consists of individual cells that are large-area semiconductor diodes,
constructed so that light can penetrate into the region of the p-n junction. The junction formed
between the n-type silicon wafer and the p-type surface layer governs the diode characteristics
as well as the photovoltaic effect. Light is absorbed in the silicon, generating both excess holes
and electrons. These excess charges can flow through an external circuit to produce power.
Photons
Electrons
−
n-type
V
p-type
I
External load
(e.g. battery,
lights, motor)
+
Id
−
ISC
Id
V
I
External load
(e.g. battery,
lights, motor)
+
Id = I0 (eAVd-1)
I0
Vd
Figure 1. Equivalent Circuit of a Solar Cell
Figure 1 shows the equivalent circuit to describe a solar cell. The diode current
I d = I 0 (e AVd − 1) comes from the standard I-V equation for a diode, and is also illustrated in the
Figure 1. It is clear that the current I that flows to the external circuit is
I = I SC − I 0 (e AVd − 1)
(1)
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ELEC3205 ELECTRICAL ENERGY SYSTEMS & MANAGEMENT
where ISC is short circuit current, I0 is the reverse saturation current of the diode, and A is
temperature-dependent constant, A=q/kT [1]. If the solar cell is open circuited, then all of the ISC
flows through the diode and produces an open circuit voltage Voc of about 0.5-0.6V. If the solar
cell is short circuited, then no current flows through the diode, and all of the short-circuit current
ISC flows through the short circuit.
Since the Voc for one solar cell is approximately 0.5-0.6V, then individual cells are connected
in series as a “solar panel” to produce more usable voltage and power output levels. Most solar
panels are made to charge 12V batteries and consist of 36 individual cells (or units) in series to
yield panel Voc ≈ 18-20V. The voltage for maximum panel power output is usually about 1617V. Each 0.5-0.6V series unit can contain a number of individual cells in parallel, thereby
increasing the total panel surface area and power generating capability.
Pm
Power (W)
Current (A)
Power
Current
ISC
Im
Maximum power
point (MPP)
0
0
0
Vm
VOC
Voltage (V)
Figure 2. I-V Characteristics of Solar Panel
Figure 2 illustrates the I-V curve and power output of a solar panel. If no load is connected with
solar panel which is sitting in the sun, an open circuit voltage Voc will be produced but no
current follows. If the terminals of the solar panel are shorted together, the short-circuit current
ISC will flow but the output voltage will be zero. In both cases, no power is delivered by the
solar panel. When a load is connected, we need to consider the I-V curve of the panel and the IV curve of the load to figure out how much power can be delivered to the load. The maximum
power point (MPP) is the spot near the knee of the I-V curve, and the voltage and current at the
MPP are designated as Vm and Im. For a particular load, the maximum point is changing as the
I-V curve is varying with the temperature, insolation, and shading. Because solar power is
relatively expensive, it is important to operate panels at their maximum power conditions. In
fact DC-DC converters are often used to “match” the load resistance to the Thevenin equivalent
resistance of the panel to maximize the power drawn from the panel. These “smart” converters
are often referred to as “tracking converters”.
Safety and Warning - Electrical Shocks and Burn Hazards
Photovoltaic (PV) modules generate electricity when exposed to light, even when they are not
connected in a circuit. Shocks and burns can result from contacts with output wiring and module
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ELEC3205 ELECTRICAL ENERGY SYSTEMS & MANAGEMENT
itself. These hazards are increased when multiple modules are interconnected to increase array
output current or voltage.
Cover module front surfaces completely with an opaque cloth or other opaque material before
performing any operation involving the module or electrical connections. Use appropriate safety
equipment (insulated tools, insulating gloves, etc.) and procedures.
Put sun cream to protect your skin from exposing to the sunlight when testing PV panels.
Experimental Setup
V
Variable
resistive load
Solar panel
I
Material
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A photovoltaic panel (BP Solar 10W, 12V)
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A solar panel rack (support and change positions of a PV panel)
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2 multimeters (or an ammeter and a voltmeter) or a AVO meter
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Insulated wires with alligator clips or other cables
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A resistor panel (1Ω, 4.7Ω, 10Ω, 22Ω, 32Ω, 47Ω, 51Ω, 82Ω, 100Ω, 220Ω, 10W)
Experimental Work
1. Measure V-I characteristic of the PV panel at a certain condition (such as weather, date,
and angle of the panel):
ƒ Begin with short-circuit: Short the output terminals of the PV panel with a wire.
Measure the short circuit current and panel output voltage. The panel voltage will be
small for this case. Record both the voltage and current in a table.
ƒ Connect the heavy-duty variable resistors to the panel, starting from lower resistance
to higher one so that the panel voltage increases from zero toward open circuit in
steps of approximately 2~3V. Measure the voltage and current for each resistor and
record them in the table.
ƒ Measure the voltage and current when the panel is open-circuit. Record the panel
voltage and current in the table.
ƒ Use the data recorded in the table to plot I versus V manually.
2. Repeat step 1 in two very different sunlights, i.e. different weathers or different time of a
day.
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ELEC3205 ELECTRICAL ENERGY SYSTEMS & MANAGEMENT
3.
4.
5.
6.
Repeat the step 1, and measure V-I curves at different tilting angles of the solar panel.
What is your best tilting angle to track the maximum power?
Repeat the step 1, measure V-I curves when shading 1-2 cells of the panel.
Using Excel to plot V-I and P-V (P = V • I) curves of the solar panel. Visually estimate
Vm, Im, and Pmax (i.e., peak power conditions) from your plots. At what load the solar
panel will delivery the maximum power?
Work out the I0 and A in equation (1) from the measured V-I curve for one case only.
Given q = 1.602 × 10-19C, and k = 1.381 × 10-23J/K. [Hint: The evaluation of these
variables may involve several iterative calculations. You may need to use mathematic
program such as MATLAB to help you with the calculation process.]
Experimental Report
The results of this lab might be used for the term project later on. Therefore it is worth
summarizing and analyzing them in a report. In general, you are required to write a technical
report for this experiment, which should cover:
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Yourself and your group members
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Experiment date, time, weather and other experimental conditions
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Experimental approach, procedure, devices, setup and actual tasks
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Experiment results, including data, graphs, and calculations
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Analysis and explanations of experiment results, comparisons, observations, and
comments
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Some photos of the experimental setup
The original technical diary and the plagiarism coversheet from the school of EIE should be
attached to a report. The due date of the technical report to be handed in to Rui at Lab522 is 22
Aug. (Fri) at 5pm latest.
Assessment
This lab will be assessed based on a group report and accounts for 2% of the UoS.
Contact for Help
Assistance is provided on Wednesday and Thursday between 9~11am during an experiment.
You also can get the help by emailing ruihong@ee.usyd.edu.au.
Reference
[1] Gilbert M. Masters, “Renewable and Efficient Electric Power System,” Wiley, 2004
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