ELEC5206 SUSTAINABLE ENERGY SYSTEMS Semester 2, 2011 Lab 1 – Electrical Characteristics of Photovoltaic Cells Aims Learn the properties of a photovoltaic (PV) panel 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 on a solar panel through the I-V characteristic measurement. 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 (1) I I SC I 0 (e AVd 1) 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 School of Electrical and Information Engineering, University of Sydney Page 1/4 ELEC5206 SUSTAINABLE ENERGY SYSTEMS Semester 2, 2011 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 16-17V. 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 I-V 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 itself. These hazards are increased when multiple modules are interconnected to increase array output current or voltage. School of Electrical and Information Engineering, University of Sydney Page 2/4 ELEC5206 SUSTAINABLE ENERGY SYSTEMS Semester 2, 2011 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 l I Material A photovoltaic panel (BP Solar 10W, 12V) A solar panel rack (support and change positions of a PV panel) 2 multimeters (or an ammeter and a voltmeter) or a AVO meter Insulated wires with alligator clips or other cables A resistor panel (1, 4.7, 10, 22, 32, 47, 51, 82, 100, 220, 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): 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 the table below. 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. Measure the voltage and current for each resistor and record them in the table below. Measure the voltage and current when the panel is open-circuit. Record the panel voltage and current in the table. 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. 3. 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? 4. Repeat the step 1, measure V-I curves when shading 1-2 cells of the panel. 5. 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? 6. Work out the I0 and A in equation (1) from the measured V-I curve for one of measuring conditions (note: do not use temperature to calculate A. I0 and A have to be worked out School of Electrical and Information Engineering, University of Sydney Page 3/4 ELEC5206 SUSTAINABLE ENERGY SYSTEMS Semester 2, 2011 from measured V and I data). Given q = 1.602 10-19C, and k = 1.381 10-23J/K. [Hint: Due to the complex calculation you may need to use mathematic program such as MATLAB to help you with the calculation]. Table sample for recording of measurement Weather: Temperature: Tilt Angle: Time: Voltage (V) Current (A) Load Resistance () 0 (S.C) 1 4.7 10 22 32 47 51 82 100 220 Inf. (O.C) Shading: Power (W) Experimental Report In a technical report the results should be summarized and analyzed in which covers the following: Yourself and your group members Experiment date, time, weather and other experimental conditions Experimental approach, procedure, devices, setup and actual tasks Experiment results, including data, graphs, and calculations Analysis and explanations of experiment results, comparisons, observations, and comments 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. This lab will be assessed based on a group report and accounts for 2% of the UoS. Reference [1] Gilbert M. Masters, “Renewable and Efficient Electric Power System,” Wiley, 2004 School of Electrical and Information Engineering, University of Sydney Page 4/4