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Teacher Activity Sheet
Introduction
The International Space Station’s (ISS) only source of power is sunlight. The ISS has many solar panels, arranged in arrays that provide all the energy needed to perform vital maneuvers, complete scientific experiments, and maintain life support systems for the crew. Each of the solar arrays is 112 ft. long by 39 ft. wide. The solar arrays are made up of photovoltaic cells. There are quite a few types of solar cells that vary according to type of material used and materials added. The ISS solar cells are made from purified silicon, which directly converts light to electricity. The large numbers of cells arranged in the panels and arrays produce high power levels.
A spacecraft orbiting Earth is not always in direct sunlight. As it revolves, it goes into Earth’s shadow and is out of direct sunlight. The ISS then relies on nickel-hydrogen rechargeable batteries to provide the continuous amounts of power needed for life support systems.
Solar cells vary greatly in their efficiency. The most efficient cells are made from single giant crystals of silicon. Cheaper cells are usually polycrystalline cells made up of masses of smaller silicon crystals. Cells also vary in their response to different wavelengths of light. The broader the spectrum of light that hits it, the more electricity it produces. Scientists are trying to increase the efficiency of solar cells to allow for production of greater amounts of electricity. The most efficient cells now range from 10-20 percent.
Photovoltaic systems are set up to maximize the sun’s light by aligning the system at the most efficient angle with the sun. The angle must change for the time of day and for the season.
Skills
You practice measurement, laboratory, and design skills and techniques through the construction of the PV cell apparatus and the measurement of the voltage produced by manipulating the variable to be tested. You review the scientific method and practice graphing the data to draw conclusions about the effects of wavelength and angle on voltage produced.
Objectives
You will:
- Use scientific methods to identify manipulated and responding variables in an experiment.
- Demonstrate skill in the measurement of small voltages using a voltmeter.
- Graph the quantitative data.
- Draw conclusions about the efficiency of PV cells and the color of wavelength.
- Draw conclusions about the efficiency of PV cells and the angle of the cell to the light source.
- Relate the PV experiment to your Space Station Alpha mission and realize the importance of solar energy to ISS functions.
Activity Overview
In this experiment you construct a simple photovoltaic system using a PV cell and a voltmeter to learn how the wavelength of light affects the amount of electricity produced from the cell. You also experiment to find out how the solar cell angle to the light source affects energy produced.
Key Questions
- How does a solar cell work?
- How does the color of light hitting a solar cell affect its efficiency?
- How does the angle of the cell to the light source affect energy production?
- How can you measure the efficiency of a solar cell?
Key Concepts
- The space station needs a constant source of power in order for the astronauts to stay alive and for the scientific work of the station to continue.
- The source of power for the station is energy from the sun.
- Photovoltaic cells convert the sun’s energy into electricity.
Materials and Preparation
- Voltmeter or ammeter
- PV cell (with attached leads)
- Several sheets of colored cellophane sheets (as many colors as you want to test, but minimally use a clear sheet, a red sheet, and a blue sheet.)
- Protractor
Vocabulary
- ammeter: instrument for measuring electric current (in amps).
- array: solar panels wired together to work together to produce greater amounts of electricity.
- electricity: a flow of electrons over a wire.
- electrons: the negatively charged particles of an atom.
- photovoltaic cell (PV cell): (comes from “photo,” meaning light, and “voltaic,” meaning producing electricity) converts sunlight into electricity; made from at least two layers of semiconductor material, such as silicon.
- voltmeter: an instrument that measures voltage, or the potential difference between two points in a circuit.
- wavelengths: the distance between two points of a light wave.
Procedure
Part A
- Construct your photovoltaic system:
- Attach the red wire from the PV cell to the red lead of the voltmeter. If your PV wires do not already have an alligator clip attached, you can just wrap the two wires together.)
- Attach the black wires for the PV cell to the black lead wires of the voltmeter.
- Be sure that your voltmeter is set to measure direct current. Set the meter to its lowest range to be sure that you are recording your lowest currents produced.
- Use the sun, or shine a light source on the PV cell and check to see if you are getting a reading. If you do not get a reading, check the wire connections.
- Keeping the sunlight (or light source) constant, cover the PV cell with a piece of colored cellophane. Record the number of volts on your data table.
- Repeat the procedure using all the colors of cellophane, including the clear cellophane. Why are you using a clear piece of cellophane? (Hint: Think about the parts of a scientific experiment.)
- Take a magnifying glass and concentrate the light on the PV cell. Measure the new current produced and record it in your data table.
- Take a piece of aluminum foil and design a light reflector for your PV cell to concentrate the light shining on it. Measure the new current with the reflector attached and record in your data table.
- Graph your data. Be careful to label each axis.
- Answer the questions in the Reflection section of this lab.
Part B
- Place the PV cell directly pointing to the sun (or light source) at a 90-degree angle.
- Record the number of volts in your data table.
- Using a protractor, slant the PV cell 15 degrees away from the direct position and record the volts generated.
- Repeat the procedure, changing the angle another 15 degrees each time and recording the volts.
- Graph your data.
- Answer the questions in the Reflection section of this lab.
Management
The PV cells are inexpensive and can be obtained through a science catalog or a local electronics store. If you get ones that already have the attached leads and/or alligator clips, you save time troubleshooting and get more accurate readings than if you are not careful about having a secure connection.
There are also inexpensive voltmeters available through the same catalogs or at Radio Shack. Make sure you get ones capable of detecting a low amount of current and set the ammeter on the lowest setting.
Review the background material with the students before they begin. Students generally enjoy this activity because they see solar power being produced and measured with the ammeter and can see how the variables affect the power produced.
Reflection and Discussion
- Which color produced the greatest voltage? Why?
Answers may vary according to student result and the colors of cellophane used.
- Which color produced the smallest voltage? Why?
Answers may vary according to student result and the colors of cellophane used.
- Why did you use the clear cellophane?
The clear cellophane is used to act as a control. When using the colored cellophane as a variable to test the effect of different wavelengths on the amount of current produced, a clear piece of cellophane must be used to control any effect the cellophane itself might have in affecting the solar cell.
- How would you answer someone who asked you what time of day would produce the most electricity from a PV cell?
The answer would have to include a discussion of cloud cover and geographical location. There may be more power produced at 5 p.m. with no cloud cover than at noon with heavy cloud cover. Similarly, if the geographical location is closer to the equator, then time of day would have less of an effect. The angle of the PV cell would also have to be considered.
- How would you increase the output of a PV cell during the day, when the angle of the sun’s rays is constantly changing?
A system or design that would allow for the changing of the angle would provide optimal output.
- Why do you think that more electricity from solar power could be produced in the winter in northern latitudes even though the Earth is farther away from the sun?
Answers will vary.
Transfer and Extension
You can extend this lesson by testing with different kinds of solar cells, if available. You can also review the difference between amps and volts and have the students test with both an ammeter and a voltmeter. Students can also shade varying portions of the solar cell and record the reduction in amps or volts. Having the students learn how to construct a circuit in series or parallel and insert a power draw into the circuit with their solar cell is another way to test the PV system.
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