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, particularly the Soltice Organization's PV Tutorial. More general details can be found at Soltice Foundation site, through the US Dept of Energy and through the many sources listed at the NREL education Programs page. A simple net search will yield many more sources as well.

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 held at a fixed distance near the panel, rotate the panel, 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 and the current flowing through the load. The load will consist of the ammeter and various series combinations of 1-ohm resistors. The value of the resistance for each load will be measured directly using the multimeter set to the ohmmeter 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 throughout this section of the experiment. (If either are inadvertently bumped during the experiment, reposition them until the open voltage is the same as the value initially recorded.)

   • The ammeter by itself will be the first load you will test. First, measure the resistance of the ammeter with multimeter set to the ohmmeter function. Place the ammeter in the 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. (Note: Don't forget to disconnect the panel while measuring the resistance. 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.

   • Optional: 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. However, if there is inclement weather, a large wattage point light source will be used in the lab. Use the load which provided the maximum power output in the first experiment.

   •   First you will create a "positioning template" to make it easier to control the angle that solar panel makes with the sun or light source.
     1. On a standard 8 x 11 piece of paper, draw a vertical line near the midpoint of the page, extending the full 8 inches. Draw a horizontal line through the middle. These two lines will be used to align the panel at 0^o and the 90^o to the sun.
     2. With a protractor, create lines in 10^o increments as shown. Be sure each line is at least 4 inches long. Label the angles clearly as shown.
     3. Tape the template to a notebook or other portable flat surface that will make it easy to hold the template.

   • Take the multimeter, ammeter, solar panel, load resistor(s), and your positioning template to a spot outside the lab where you have direct sunlight.^*    Connect the circuit as was done inside the laboratory. Take care with the instruments! They are both delicate and expensive.

     ^* In case of inclement weather, there are high intensity lamps available to peform this part of the experiment inside.

   • Place the solar panel such that the front edge is along the horizontal, "0^o, line. The panel will cast a shadow. Position the template/surface until the panel faces the sun directly, such that the shadow of the left edge of the panel lines up perfectly with the vertical line.

   • Create a table with the following column titles: Voltage, Current, Power, Angle, Width, and Area. Record the voltage and current output at this 0^o angle of incidence. In addition, make a small pen mark at the front right edge of the panel.

   • Keeping the template stationary, move the panel until it is aligned with the 10^o line. Be sure that the left edge of the panel remains fixed. This easy to see since the shadow will be along the vertical line. Again, record the voltage and current output and make a small pen mark at the front right edge of the panel. (Note the red mark in the diagram for the case where the panel is at 20^o.)
   • Repeat this process for the remaining angles of 20^o through 90^o. After you have done this, return to the lab.

   • Calculate the power output of the solar panel for each angle. Place the results in your table. 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.

   • Extend a line from each of the pen marks (the ones you made at the right edge of the panel for each angle) down to the 0^o base line. Be sure that the extended line is perpendicular to the baseline. Measure the distance from the extend line to the origin where the left edge of panel was located. Record this value in your table.

     This extended line represents the "shadow" of the right edge. The effective area of the solar panel (that's the area that intercepts the sun's rays) is the product of this length times the height of the panel. At 0^o, this will be the actual area of the panel. At 90^o, the effective area should be zero. Calculate the effective area for each angle and record it in your table. Plot the power output vs the effective area.

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 in the second section and the angle of incidence in the third section. 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 Effective Area plot. Can you explain these shapes qualitatively?