Photovoltaic Cells

You will be provided with a solar cell module (somewhat smaller than that pictured above) that you will test for various properties such as voltage and power output. In addition, you will research photovoltaics either on the net (several links are provided below) or at the library.

NET RESEARCH

First, you should have read the photovoltaic section of the Electric Solar Energy section in the Energy module. In addition there is a very good tutorial by the American Solar Energy Society. More details can be found through the non-profit Soltice Foundation, through the US Dept of Energy and through the many sources listed at the Related Websites page provided by the ASE Society. These links include several Universities involved with photovoltaic research. Many of the sites have e-mail links to people who might be happy to answer questions.

Power Output Related to Incident Radiation and Load

1. Setting Up - Qualitative Observations

   You will need to place the solar panel into the jaw clamp on the lab stand such that the panel surface is vertical and can receive light with as little obstruction as possible. There are two screw post terminals on the back marked "+" and "-". When exposed to light, conventional positive current will flow out of the positive terminal, through the load, and into the negative terminal.

   First, you will make qualitative observations using the motor provided as the load. Connect the motor to the panel using lead wires with the alligator clips. (Be sure the leads are connected with the correct polarity.) Adjust the panel to the height of the 100 W light bulb. Turn on the light bulb and move it near panel. The motor should begin to spin. (You may have to help it get started.) Move the bulb towards and away from the panel and note the effect on the motor. With the bulb near the panel, move the bulb around the panel, maintaining the same distance but changing the angle of incident radiation. We will make quantitative measures these effects in the next section.

2. Power vs Load

   The "load" on the solar panel (or any device that provides energy) is the device(s) through which the current from the panel flows. For this section, you will place a variety of resistive loads on the panel and measure the voltage across the panel terminals and current flowing from the panel. The load will consist of the ammeter and various series and parallel combinations of 1-ohm resistors. The value of the resistance for each load will be measured directly using the multimeter set to the ohmeter function.

   • Remove the motor and connect the digital multimeter to the leads. Set the multimeter to the 2-Volt DC range. Place the bulb about 10 cm directly in front of the panel. The voltmeter has very high resistance and very little current will flow from the panel. Record the voltage. This is called the open or no-load voltage; it is the voltage produced when there is no load on the panel. Leave the bulb and panel in these positions. If either are inadvertently bumped during the experiment, reposition them until the open voltage is the same as the value initially recorded.

   • Begin with only the ammeter as the load on the circuit. Measure the resistance of the ammeter with multimeter set to the ohmeter function. Place the ammeter in circuit with the panel. Change the multimeter to the 2-volt DC setting and monitor the voltage supplied by the panel. In a table, record the load resistance, the voltage, and the current. Compute the power (P = IV) and place this value in the table as well. The current values will be in the mA range, so you may use units of mW for the power.

     (NOTE: If the current is not near the maximum reading for your ammeter, adjust the position of the bulb until it does so. You will then need to repeat the first procedure to determine the new open voltage.)

   • Place a 1-ohm resistor in series with the ammeter and determine the resistance of the new load. (Don't forget to disconnect the panel while making this measurement. You wish to determine the load resistance, and do not want to include the panel's internal resistance. However, the ammeter still contributes resistance and must be included. ) Connect the circuit and again record resistance, voltage, current and calculate the power for the panel.

   • Repeat this process 4 more times, placing additional 1-ohm resistors in series.

   • Plot power vs resistance and note the trend. You should notice that the power output of the panel is not constant, but is at a maximum for one of the values of the load resistance.

   • It is possible to find more accurately the value of the load resistance that maximizes the power output of the panel. To do this, use the load configuration that gave the maximum power output for the "starting" value. To this arrangement, add 1-ohm resistors in various series and/or parallel combinations to produce a new resistance value that is between the starting value and the values just above and below it in the original measurements. Try this on your own first. Your lab instructor will provide you with some ideas of how to do this if necessary.

3. Power Output and Angle of Incidence

   In this section, you will determine the effect that the angle of incidence of the light upon the panel has upon the power output. Ideally, this portion of the lab will take place outside, using sunlight. But if there is inclement weather, a large wattage point light source will be used in the lab.

   

   • For this experiment, you will use the load which provided the maximum power output in the first experiment. Take the multimeter, ammeter, solar panel, load resistor(s). leads, a ruler and a protracter to a spot outside the lab where you have direct sunlight. Take care with the instruments!

   • Connect the circuit as was done inside the laboratory. Place the protractor at one edge of the panel. The angle that the plane of the panel makes to the sun's rays can be fixed by placing a pen or pencil point at the desired angle along the circular arc of the protracter and rotating the panel until the shadow passes through the protracter's center point. (The lab instructor will demonstrate the method.)

   • Record the voltage and current output for angles of 0^o through 90^o in 10^o intervals. Calculate the power for each angle.

   • Disconnect the panel from the circuit. Place a blank sheet of paper with its surface directly facing the sun and behind the panel. The panel will cast a shadow. For each of the angles used in the previous procedure, measure the width and length of the shadow cast by panel. (One of the dimensions should remain the same.) Calculate the area of the shadow for each angle. This represents the area of sunlight that the panel intercepts at each angle.

   • Plot the power output as a function of the angle of incidence between the sun's rays and the normal to the plane of the solar panel. (Note: Use the angle between the sun's rays and the surface of the panel. That is, 90^o will be the angle when the panel faces the sun directly and 0^o will be when the sun's rays are parallel to the panel's surface.

   • Using the area calculated from the shadow of the panel, plot the power output vs the area intercepted by the panel.

REPORT

The lab report should begin with a one to two page introduction to photovoltaics based upon your internet or library research. The introduction should be broad in scope and may focus on any of the following aspects; 1) the basic science, 2) the technical specifications of a home system, or 3) the ecomomics of PV's.

The introduction should be followed with the standard format report of your experiments. In the report, present your results with appropriate tables, plots, and diagrams. Your conclusions should explain what you have measured and describe the significance of your results. for example, comment on the importance of the load factor and the angle of incidence. Specifically, is there a narrow or a broad range of loads or angles that provide optimum performance? Comment on the shape of the Power vs Load plot and the Power vs Area plot. Can you explain these shapes qualitatively?