Experiment

Note: All of these activities need a sunny day with little or no cloud cover.

Each day, the sun rises in the east, moves across the sky, and sets in the west. Whenever the sun is shining on us, it is sending energy in our direction.

We can feel the heat from the sun, and we can see objects that are illuminated by the light from the sun as it moves across the sky. We should not look directly at the sun because of the risk of injuring our eyes. However, if we could get a solar cell to turn and look at the sun all day, then it would be receiving the maximum amount of sunlight possible and converting it into a more useful energy form—electricity.

In this activity, you will explore how to maximize the performance of a solar cell for the part of the world where you live.

Tools and Materials

You will need the following from sources such as those listed in our Materials List.

• Solar cell backpack with two attached solar cells connected to junction box and Power Out cable
• Additional solar cell
• Voltmeter (Make sure it is turned off.)
• Two electric motors
• Two wheels to fit the electric motors
• One pair of alligator clip leads
• Coaxial adapter
• Coaxial DC power jack
• Stopwatch
• Protractor
• Transparent tape
• Three sheets of basswood or other lightweight wood each 30 cm by 10 cm by 5 mm (12 inches by 4 inches by one-fourth inch) with 1.25 cm (one-half inch) diameter hole
• Wooden block with 1.25 cm (one-half inch) hole and wooden dowel 1.25 cm (one-half inch) diameter 30 cm (12 inches) long for constructing sundial
• Tape measure
• Magnetic compass

What To Do

If you place a solar cell flat on the ground and leave it there all day, what will its electricity production look like over time? Will its output change as the sun travels across the sky from east to west?

Set up a solar cell by doing the following:

1.Unzip the back side of the solar cell flap of the backpack. Unplug the backpack junction box and the Power Out cable. Remove them from the backpack.

2. Collect the following items.

• Solar cell
• Junction box
• Power Out cable
• Coaxial adapter
• Coaxial DC power jack
• Two alligator clip leads

3. Connect the Power Out cable to the Power Out port of the junction box. Insert the coaxial adapter into the other end of the Power Out cable, and then attach the coaxial DC power jack.

Attach the ends of two alligator clip leads to the power jack, one to the central post and one to the side post.

4. Place the solar cell assembly on a flat place out of doors in the early morning hours of what looks like a relatively clear day. It is best to use an area that is not shaded during the day, so that you will not have to disturb or move the assembly. However, if you must move the cell assembly, be sure it is facing the same direction throughout the activity.

5. Assemble a simple sundial by inserting the dowel into the hole in the block of wood. Place this near your solar cell, but make sure that the dowel shadow does not fall on the solar cell.

6. Tape the magnetic compass over the central point of the protractor. You will use this to determine the direction of the dowel shadow at different times of the day.

At hourly time intervals, you will need to do three things:

A. Connect the solar cell output alligator clips to the probes of a voltmeter to find the output voltage of the solar cell. Disconnect the voltmeter and turn it off between readings.

Note: Set the voltmeter scale so that the needle is not pushed past the end of the scale on either side. If the needle is pushed below zero, you will need to reverse the alligator clips on the voltmeter probes. If the needle is pushed past the far end of the scale, you will need to turn the red knob to change the scale. Most likely, the 50-volt scale will be the best to use.

B. To the solar cell output alligator clips, connect a small DC motor with a wheel on its axle. Place a piece of tape on the wheel and make a mark on the tape for an indicator. Use the stopwatch to determine how long it takes for the motor axle to make ten rotations. In general, the faster the motor is turning, the higher the performance of the solar cell. (We recommend that you make at least three such measurements for each angle and then calculate the average time for each angle. This technique helps to reduce measuring errors.)

C. Measure the length of the shadow of the dowel, as well as the angle between the shadow and a line running north and south. In the photo below, the protractor has been turned so that its long side is aligned with the north-south line of the compass. The center of the compass is on one edge of the dowel shadow. The compass reading is made where that same shadow edge crosses the rounded part of the compass scale.

Record your measurements in the following table.

Based on the motor performance or the voltmeter reading, how would you describe the performance of your solar cell over the period of the day? Is there a period of time, maybe four or five hours long, where the solar cell performance is relatively and consistently high?

Graph the motor performance or voltmeter (vertical axis) versus the time of day.

Graph the length of the shadow versus the time of day.

Graph the angle of the shadow versus the time of day.

Do you see any relationships among the graphs?

Adjusting for the Tilt of the Earth

How can you position your solar cell to take maximum advantage of the sun’s energy if it does not go directly overhead during the day? Let's see…