Abstract

Objective

Charge a supercapacitor with a solar panel, then measure how the charging time affects how long the supercapacitor can power a car's electric motor.

Introduction

The amount of energy that the sun produces is truly amazing. Did you know that the solar energy hitting Earth exceeds the total energy consumed by humanity by a factor of over 20,000 times? Tapping into this clean and renewable energy source is one way to meet society's growing energy needs. Solar cells convert energy from the Sun into electrical energy. Energy from a solar cell can be used to power almost anything that needs energy to work, including cars, boats, airplanes, and household appliances. One down side of solar panels is that they stop producing energy when the sun goes down, but of course our need for energy continues past sunset.

One way to harness the energy of the Sun and save it for later is to attach a capacitor to a solar cell. When the solar cell is exposed to the Sun, the electrical energy can be used to pump electrons into the capacitor. If you then disconnect the capacitor from the solar cell, the energy will stay in the capacitor in the form of an electric field. When the capacitor is hooked up to a motor or a lightbulb, the electrons can flow back out, making the motor spin or the lightbulb shine.

A capacitor storing energy as an electric field can be compared to a water tower that stores water at some distance above the ground. To store the water, it is pumped "uphill" into water towers, where it is ready to be used as the need arises. When the outflow pipe is opened, water flows "downhill," out of the tower, under the force of gravity. In a similar way, a capacitor stores excess electrons that have been pumped into it by the solar cell. Then, when the capacitor is attached to a motor or other electronic device, the electrons flow out, under the influence of the electric field in the capacitor.

In this electronics science project, you will assemble a motor from a special car kit, a solar cell, and a supercapacitor. It is called a supercapacitor because it is big and can store lots of electrons. The unit for measuring the storing power of a capacitor is the farad. The supercapacitor you will use is rated at 6 farads, which is very large compared to ordinary capacitors (capacitance is usually measured in microfarads or picofarads). Because the capacitor is so big, it can store enough energy to make the motor spin even when there is no light hitting the solar cell. When the car is exposed to sunlight, the solar panel produces energy, some of which makes the motor work, and some of which is stored in the capacitor. You will measure how long the motor keeps spinning after the device is put in the shade. The more energy that was stored, the longer the motor will turn!

Terms, Concepts, and Questions to Start Background Research

• Energy
• Renewable
• Solar cell
• Electrical energy
• Capacitor
• Electron
• Electric field
• Supercapacitor
• Voltage

Questions

• Based on your research, how does a capacitor store energy?
• What is the unit for capacitance?
• Based on your research, what are some real-world examples of how capacitors are used as energy storage devices?

Bibliography

• Windows to the Universe. (2007). Solar Radiation. Retrieved March 4, 2010, from http://www.windows.ucar.edu/tour/link=/earth/climate/sun_radiation_at_earth.html&edu=elem
• How Stuff Works (2010). How Capacitors Work. Retrieved March 3, 2010, from http://www.howstuffworks.com/capacitor.htm

Materials and Equipment

• Energy Kit, from Science Wiz; available at www.amazon.com
• Motor includes: Supercapacitor, car frame, solar cell
• Double-sided tape
• Stopwatch
• Cardboard piece, 12 x 12 inches (in.)
• Helper (1)
• Lab notebook
• Graph paper

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Experimental Procedure

Preparing the Experimental Setup

1. Open the "Energy" box and look at the materials included.
1. You will not be making the full car for this project, just the part that has the car frame, the solar cell, and the motor.
2. Attach the motor to the frame using double-sided tape.
3. Attach the solar cell to the frame with double-sided tape.
4. Pass the bare end of the red wire from the solar cell through one of the tabs on the motor. Twist the wire onto the tab.
5. Pass the bare end of the black wire from the solar cell through the other tab on the motor. Twist the wire onto the tab.

Figure 1. The frame (red) holds the solar cell and the motor. The red wire from the solar cell is positive and the black wire is negative. The capacitor has a positive and a negative side, too, indicated by a "+" and a "-" sign.

6. Insert the supercapacitor into the holes in the motor tabs.
1. The supercapacitor has a positive and a negative side, indicated by "+" and "-" signs on the side of the capacitor.
2. Attach the side of the capacitor with the positive lead (wire) to the tab of the motor with the red wire.
3. Attach the side of the capacitor with the negative lead (wire) to the tab of the motor with the black wire.

Exposing the Device to Sunlight

1. Place the device in direct sunlight.
2. The motor should start spinning after a few seconds. If the motor does not spin, check the connections.
1. Note: Keep the angle between the solar cell and the Sun constant.
3. Cover the solar cell with the piece of cardboard to block the sunlight.
4. The motor should stop running.
5. Place the solar cell in the direct sunlight.
1. You can spin the shaft with your finger to start it spinning if the shaft seems a little sticky.
6. As soon as the motor starts to spin due to the sun, have your helper start the timer.
7. After 5 seconds (sec.) in the sunlight with the motor spinning, stop the timer and block the sunlight from hitting the solar cell with a piece of cardboard. The motor should spin for a few seconds while in the shade, until it has drained the energy from the supercapacitor.
1. Time how long it takes for the motor to stop once it is moved out of the sun.
2. Your helper will need to stop the timer after 5 sec., then restart it immediately to time how long the motor spins in the shade. You might want to practice this step before you begin recording data.
8. Once you are ready to run the experiment and begin collecting data, record the data in a table, like the one below, in your lab notebook.
1. The time in the sunlight with the motor spinning is the time the capacitor is charging in the sunlight.
2. The time until the motor stops spinning after it has been moved to the shade is the time it takes for the supercapacitor to discharge.

9. Repeat steps 7–8 for a total of two data points per time, exposing the solar panel once more for 5 sec., then for longer periods of time, as shown in the data table—twice for 10 sec., twice for 15 sec., and twice for 30 sec.
10. Repeat steps 7–9 two more times to show that your results are repeatable. Create new data tables for each new trial.

Analyzing Your Data

1. Calculate and record the averages in a data table like the one below.

2. Graph Charging time (time in sunlight) on the x-axis and Discharge time (time until motor stopped working while in shade) on the y-axis. How does the amount of time for which the motor spins when it's out of the sunlight depend on the time spent collecting solar energy?

Variations

• Assemble the supercapacitor car with the solar panel, according to the instructions, adding wheels and the gear that connect the motor to the wheels. Repeat the experiments above, charging the motor for various times. As a measure of how much energy the capacitor is holding, measure how far the car goes on a flat floor out of direct sunlight.
• Try other time trials, in addition to the ones listed in the data table above.
• Remove the supercapacitor from the motor. Expose the solar cell to sunlight until the motor starts to spin. Now move it into the shade. How long does it take the motor to stop in the shade without the supercapacitor?
• Attach the supercapacitor to the solar panel without the motor. (The motor is draining energy as the capacitor charges.) To do this, remove the solar panel wires from the motor tabs. Remove the supercapacitor from the motor tabs. Wrap the bare end of the red wire around the positive lead from the capacitor and the black wire to the negative lead. Charge the supercapacitor for 5 sec. in full sunlight. Touch the leads (wires) from the capacitor to the motor leads. Does the motor spin for the same amount of time as when the motor was attached?
• Charge the capacitor as above, then wait various times (15 min., 30 min., 1 hour) before touching it to the motor. Did the supercapacitor still have enough energy to run the motor?
• Use a multimeter to measure the actual voltage of the capacitor. Repeat the procedures above, but collect data for the voltage of the capacitor. For example, how does the voltage of the capacitor change over time as it is charged by the solar cell? See the Science Buddies help page Electronics Primer: Using a Multimeter.

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

David B. Whyte, PhD, Science Buddies

Last edit date: 2010-04-09 12:00:00