HOW A SOLAR CELL PRODUCES ELECTRICITY · solar cell connected to an electrical circuit. Refer to How a Solar Cell Produces Electricity for further background discussion. SETTINGUPTHE

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HOW A SOLAR CELL PRODUCESELECTRICITY

INTRODUCTIONLook at the solar cell your teacher hasgiven you. Hold it in your hand. It doesnot appear to have much substance; it’sjust a thin wafer of solid material, withone side colored dark blue or black andthe other colored a silvery gray. On manycells, the dark blue-black side may havethin wires on it. The cell weighs very little,has no moving parts and does not feelwarm. In fact, the solar cell does not look

like it could do anything, yet it is capableof producing electricity. How does itwork?

HOW SOLAR CELLS ARE MADELook again at your solar cell. Even thoughit is very thin, it is made up of two layersof semiconductor material. The semicon-ductor layers in most solar cells are madeof silicon, although solar cells can be madefrom other materials as well.

Silicon is used to make solar cellsbecause it can be mixed with other sub-stances to change its electrical behavior in

PARTS OF A SOLAR CELL

Light

Wires

Metal contact

n-layer

p-layer

Metal or conducting material Stiff backing material(optional)

Reprinted with permission by KEEP and the Union of Conerned Scientists�

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a certain way. Look at the side thatappears dark blue or black. This side iscalled the “n-layer.” The “n” stands for“negative” because it is made by mixingsilicon with substances that have moreelectrons than it has. Underneath the n-layer is the “p-layer.” The “p” stands for“positive” because it is made by mixingsilicon with substances that have fewerelectrons than it has. You cannot see the p-layer by looking at the back of the solarcell because it is covered by a silvery-graymaterial, and may also have a stiff-backingmaterial attached to it. The silvery-graymaterial helps the cell conduct electricitywhen it is connected to an electrical cir-cuit. The stiff backing supports the celland keeps it from breaking.

When the two layers are placed togeth-er, a few of the electrons from the n-layermove into the p-layer. When this happens,an electrical barrier is automatically creat-ed, keeping the rest of the electrons in then-layer separated from the p-layer. Thisbarrier, which is only a few millionths ofan inch thick, is formed at the boundary ofthe n- and p-layers when the solar cell ismanufactured.

USING LIGHT TO PRODUCEELECTRICITY

What happens when light shines on thesolar cell? The light knocks electrons loosein the p-layer and sends them into the n-layer. The barrier, which acts like a one-way door, lets the electrons cross into then-layer but stops them from going back tothe p-layer. This gives the n-layer a nega-tive charge and the p-layer a positivecharge. It is as if light were turning thesolar cell into a kind of battery.

When the solar cell is connected in acircuit with, for example, a small lightbulb, the electrons flow as electric currentfrom the n-layer, through the wire and the

bulb, to the p-layer. As electrons arrive atthe p-layer of the cell, light sends themback into the n-layer, where they againflow through the circuit. Electric currentwill continue to flow as long as light isshining on the cell. The current can be usednot only to light a small bulb, but also torun a small electric motor or a radio.Groups of solar cells can also be connectedinto circuits to produce larger amounts ofcurrent than can be produced by a singlecell.

When the n-layer and the p-layer areplaced together, a few of the electrons fromthe n-layer move into the p-layer. When thismovement happens, an electrical barrier isautomatically created, keeping the rest ofthe electrons in the n-layer separated fromthe p-layer. This barrier, which is only afew millionths of an inch thick, is formedat the boundary of the n- and p-layerswhen the solar cell is manufactured.

Look once more at the solar cell, adevice that can produce electricity simplyby shining light on it. Miraculous, isn’t it?

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SOLAR CELL IN A CIRCUIT

Light

Electric current (flow of electrons)

Electric flow

Electrons leave n-layer to flow as an

electriccurrent

Electrons return to p-layer

Barrier

Light knocks elec-trons through thebarrier into the n-

layer

(-)n- layer

becomes neg-atively

charged

(+)p-layer

becomes positivelycharged

Light bulb

ELECTRICAL BARRIER FORMED AT THE BOUNDARY OF THE N- AND P-LAYERS OF A SOLAR CELL

Electronsn-layer

p-layer

Electricalbarrier

Boundary of n- and p-layers

Metal or ConductingMaterialElectrons in the n-layer are kept out of the p-layer by the electrical barrier.

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DEMONSTRATING HOW A SOLAR CELLPRODUCES ELECTRICITY

INTRODUCTION

Another way to help students understandhow a solar cell produces electricity is tohave them play the role of electrons in asolar cell connected to an electrical circuit.Refer to How a Solar Cell Produces Electricityfor further background discussion.

SETTING UP THE DEMONSTRATION

(DIAGRAM 1)

Mark off a portion of the classroomfloor with two areas connected together.These areas represent the n-layer and thep-layer of the solar cell, with the boundarybetween them being the p-n junction. Markthe p-layer area with a “+” sign and the

n-layer area with a “-” sign. Mark off apath around the classroom from the top ofthe n-layer to the bottom of the p-layer.This path represents the wire of an electriccircuit connected to the solar cell (connect-ing an electrical device to the circuit is dis-cussed later on).

Choose eight students to be electrons inthe solar cell. Have three electrons stand inthe area representing the p-layer, and fiveelectrons stand in the area representing then-layer.

Choose four or five other students torepresent the barrier that forms at the p-njunction. Have them stand along theboundary between the two areas to form abarrier. The barrier students’ job is to actlike a one-way door by allowing electronsto pass from the p-layer into the n-layerbut not from the n-layer into the p-layer.

SETTING UP THE DEMONSTRATION IN THE CLASSROOM(Diagram )

Flashlight

Path of electrical circuit

Electricaldevice

Solar cell

n-layerarea

p-layerarea

p-n junction

Electron students

Barrier students

Sun student

Electrical device student

⇒

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Give an additional student a flashlightto represent the sun. Have the sun standfacing the n-layer so the barrier and the p-layer are directly behind it. The flash-light is not to be turned on until everyoneis ready. Select another student to be anelectrical device. Have this student holdan electrical device that turns on and offeasily. Examples of electrical devicesinclude radios, cassette tape decks, smallfans or toys. A flashlight or small desklamp may be acceptable as long as stu-dents do not confuse it with the flashlightthat represents the sun. Have the electricaldevice stand somewhere alongside the cir-cuit, but not too close to the solar cell. Heor she is not to turn on the electricaldevice until electrons start movingthrough the circuit. Have the remainingstudents be electrons in the circuit wire

and position them evenly along the paththat represents the circuit.

DEMONSTRATING HOW THE SOLAR CELL

WORKS (DIAGRAM 2)When everyone is ready, have the sunshine the flashlight onto the solar cell sothe light passes through the n-layer andbarrier into the p-layer. When light shineson the electrons in both layers, they shouldbegin moving around. Light shining on theelectrons in the p-layer causes them tomove one by one toward the n-layer. Thebarriers allow these electrons to pass intothe n-layer momentarily. However, the bar-riers prevent or block excited electrons inthe n-layer and any electrons that justcrossed over from the p-layer into the n-layer from moving into the p-layer.

Flashlight turnedon by sun student Path of electrical circuit

Electricaldevice turnedon by device

Solar cell

n-layer

p-layerarea

p-n junction

Electron students

Barrier students

Sun student

Electrical device student

Sunlight shines

Electron students in n-layer cannot gothrough barrier

Electron students in

Electron students

flow through

Electron students in circuit enter p-layer

Electron students in

p-layer move

SETTING UP THE DEMONSTRATION IN THE CLASSROOM(Diagram )

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By now the n-layer is getting crowdedwith electrons who are repelling eachother because they all have the same neg-ative charge. Since they can’t get past thebarriers, the only place for them to go isinto the circuit. Have the first electronenter the circuit. The instant this happens,the electron in the circuit who is nearest tothe p-layer should step into the p-layerarea. Doing so signals all the electrons inthe circuit to begin walking single file (notin clumps) along the circuit path towardthe p-layer. After electrons enter the p-layer from the circuit, they should staythere momentarily until it is their turn toenter the n-layer.

At the same time the electrons start tomove, the electrical device turns on thedevice being held. The electrical device

should be turned on as long as the sun isshining and electrons are flowing as elec-trical current in the circuit.

With practice, the electrons shouldmove smoothly (flow as current) from thep-layer, to the n-layer, through the circuit,and back to the p-layer. When electronshave learned to flow as current throughthe solar cell and the circuit, have themalternately flow and stand still as theflashlight is turned on and off. This corre-sponds to exposing the solar cell to sun-light and then suddenly covering it, rein-forcing the idea that light causes the solarcell to produce electricity. The electricaldevice should also turn the device on andoff accordingly.

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EXPERIMENTS WITH SOLAR CELLS

INTRODUCTION

How much electricity can a solar cell pro-duce under different lighting conditions?These four experiments will help youanswer this question.

MATERIALS NEEDED FOR ALL FOUR

EXPERIMENTS

• Small solar photovoltaic cells withat least a 0.4 volt output

• DC ammeter with a range ofapproximately 0 to 10 amps

• DC voltmeter with a low rating (1 or 5volts DC minimum rating is fine)

• Graph paper

CONNECTING THE SOLAR CELL TO AN

AMMETER AND A VOLTMETER

In each experiment, you will be measuringthe current and voltage produced by thesolar cell. The cell’s current is measuredusing an ammeter, while the voltage ismeasured using a voltmeter. The diagramsbelow show how to connect your solar cellto each meter. Your teacher will help youlearn how to read the meters.

EXPERIMENT 1: SOLAR CELLSAND LIGHT INTENSITY

This experiment investigates how changesin light intensity affect the amount of cur-rent and voltage a solar cell can produce.

ADDITIONAL MATERIALS NEEDED

• A bright, directional light source, such asa shaded desk lamp or a clip-on reading lamp, with a 100-watt bulb

• Ruler, 12inches (30.5cm) orlonger

PROCEDURE

1. Place the solarcell 3 inches (7.6cm) from a bright,directional light source other than thesun. The solar cell should directly facethe light source.

2. Measure the current and the voltage ofthe cell at a distance of 3 inches (7.6cm). Record results on the LightIntensity Table on next page.

3. Repeat Steps 1 and 2 for distances of 6inches (15.2 cm), 9 inches (22.9 cm), and12 inches (30.5 cm).

4. Sketch a graph that shows the relation-ship between current and the solarcell’s distance from the light source.Label the horizontal (x) axis of thegraph “distance from light source” andmark it with the distances shown in theLight Intensity Table. Label the vertical

SOLAR CELL ATTACHED TO AMMETER OR VOLTMETER

Solar cell

Ammeteror

Voltmeter

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(y) axis “current” and mark it based onthe readings from the Light IntensityTable. Plot the values of current corre-sponding to each distance on thegraph.

5. Sketch another graph that shows therelationship between voltage and thesolar cell’s distance from the lightsource. Label the horizontal (x) axis ofthe graph “distance from light source”and mark it with the distances shownin the Light Intensity Table. Label thevertical (y) axis “voltage” and mark itbased on the readings from the LightIntensity Table. Plot the values of volt-age corresponding to each distance onthe graph.

QUESTIONS

1. Using the Light Intensity Table andgraphs, state the relationship betweenthe current produced by the solar celland the cell’s distance from the lightsource. Then state the relationshipbetween the voltage produced by thesolar cell and the cell’s distance fromthe light source.

2. Based on your results, would the solarcells produce more electricity on asunny day or a cloudy day? Why?

3. Predict the time of day when the solarcells will produce the most electricity.

EXPERIMENT 2: SOLAR CELLS AND THE ANGLEOF THE LIGHT SOURCE

This experiment investigates how theangle between the sun and the solar cellaffects the amount of current and voltage asolar cell can produce.

ADDITIONAL MATERIAL NEEDED FOR THIS

EXPERIMENT

• Protractor

PROCEDURE

1. Point the solar cell directly at the sun orat the light source. Slant the cell so itsshadow is directly behind it, with thecell’s face perpendicular to the sun’srays. (One way to discover the sun’sdirection is to insert a stick in theground and tilt it until it has no shad-ow.) Measure the current and voltage,and record them under the column ofthe Angle Table that reads “90°.”

LIGHT INTENSITY TABLE

Solar cell

Lightrays

90º

SOLAR CELL ALIGNED PERPENDICULAR TO LIGHT RAYS

Distance fromLight Source 3 inches 6 inches 9 inches 12 inches

Current (Amps)

Voltage (Volts)

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2. Tilt the solar cell back at an angle 15°from the perpendicular or 75° to thesun’s rays. Use the protractor to deter-mine this angle. Measure the currentand voltage of the solar cell, and recordthem under the column of the AngleTable that reads 75°. Continue decreas-ing the angle of the cell at 15° intervals(60°, 45°, 30°, 15°) and record currentand voltage measurements under theappropriate columns of the AngleTable.

3. Sketch a graph that shows the relation-ship between current and the angle ofthe solar cell to the sun’s rays. Label thehorizontal (x) axis of the graph “angleof solar cell” and mark it with theangles shown in the Angle Table. Labelthe vertical (y) axis “current” and markit based on the readings from the AngleTable. Plot the values of current corre-sponding to each angle on the graph.

QUESTIONS

1. Describe the relationship between thecurrent produced by the solar cell andthe cell’s angle to the light source. Thenstate the relationship between the volt-age produced by the solar cell and thecell’s angle to the light source.

2. The electricity produced by a solar cellheld in one position will vary through-out the day due to the change in theposition of the sun. What would youneed to do to keep the solar cell pro-ducing the same amount of electricitythroughout most of the day?

Challenge Question (optional)3. The graph showing the current with

respect to the angle of the solar cell tothe sun is called a sinusoidal curvebecause it is based on a trigonometricrelationship called the sine function.Show how the graph you sketched isrelated to this function.

SOLAR CELL TILTED WITH RESPECT TO LIGHT RAYS (60º)

Angle of Solar Cellto Light Source 90° 75° 60° 45° 30° 15°

Current (Amps)

Voltage (Volts)

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EXPERIMENT 3: SOLAR CELLS ANDCONCENTRATED LIGHT

This experiment considers how concen-trating the light on a solar cell affects theamount of electricity it produces.

ADDITIONAL MATERIALS NEEDED

FOR THIS EXPERIMENT

• Cardboard• Aluminum foil• Glue• Magnifying glass (optional)

PROCEDURE

1. Measure the current and voltage of thesolar cell under the sun or a stronglight source, and record the results inthe Concentrated Light Table. Make

sure the angle and distance between thecell and the light source are the samethroughout the experiment.

2. Make a cardboard reflector to concen-trate light onto the solar cell. Cut outthe cardboard shape in the diagrambelow and glue aluminum foil on thefour flaps. Place the solar cell in thebase and fold up the four sides toreflect light on the cell. Make sure theangle of the cell and its distance fromthe light source are the same as theywere in Step 1. Measure the current andvoltage produced by the cell, andrecord the results in the ConcentratedLight Table.

3. As an option, use a magnifying glass toconcentrate light onto the solar cell. Todo this, move the magnifying glassaround until a bright area appears onthe cell. Make sure the angle of the celland its distance from the light sourceare the same as in Step 1. Measure thecurrent and voltage produced, andrecord the results in the ConcentratedLight Table.

CONCENTRATED LIGHT TABLE

Nature of Light Source Not ConcentratedConcentrated Using

Aluminum Foil ReflectorConcentrated Using

Magnifying Glass (optional)

Current (Amps)

Voltage (Volts)

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QUESTIONS

1. Does concentrating the light on thesolar cell increase the current it pro-duces? If so, by how much?

2. Does concentrating the light on thesolar cell increase the voltage it pro-duces? If so, by how much?

3. Compare the amount of current andvoltage produced by the solar cellsusing the aluminum foil reflector andthe magnifying glass. Which methodof concentrating light produces morecurrent and voltage? (Optional)

EXPERIMENT 4: SOLAR CELLS IN SERIES AND PARALLEL CIRCUITS

This experiment investigates how muchcurrent and voltage is produced whensolar cells are connected in series andparallel circuits.

ADDITIONAL MATERIALS NEEDED

FOR THIS EXPERIMENT

• Two to 4 short wire leads with alliga-tor clips on each end

• One or two additional solar cells

NOTE: You may need to share solar cellswith other groups if your teacher has notgiven you additional ones.

PROCEDURE

1. Connect two or three solar cells in aseries circuit as shown in the diagram.Record how many cells you have con-nected in the Series and ParallelCircuit Table. Place the series circuitunder the sun or a strong light source.Measure the current and voltage pro-duced, and record the results in theSeries and Parallel Circuit Table.

2. Connect two or three solar cells in aparallel circuit as shown in the dia-gram. Record how many cells you haveconnected in the Series and ParallelCircuit Table. Place the parallel circuitunder the sun or a strong light source.Measure the current and voltage pro-duced, and record the results in theSeries and Parallel Circuit Table.

QUESTIONS

1. Which circuit produced the most cur-rent? State a general relationshipbetween the number of cells in thiskind of circuit and the amount of cur-rent it produces.

2. Which circuit produced the most volt-age? State a general relationshipbetween the number of cells in thiskind of circuit and the amount of volt-age it produces.

CONNECTING CELLS IN A SERIES CIRCUIT

Ammeteror

Voltmeter

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3. Suppose you wanted to pro-duce increased amounts ofcurrent and voltage usingseveral solar cells connectedtogether. How would youconnect them? Draw a circuitdiagram below showing theway you would connect thecells.

CONNECTING SOLAR CELLS IN A PARALLEL CIRCUIT

Number of Solar Cells inCircuit

Voltage(Volts)

Current(Amps)

Series

Parallel Circuit

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B. FROM ENERGYTO ELECTRICITY

INSOLATION GRAPH EXERCISE

BACKGROUND:

When you do an experiment, there areusually two ideas you are working with,such as time of year and the amount ofsolar energy. The question is, how do youknow which side of the graph is used forthe time of year versus the amount of solarenergy?

The two variables shown on a graphare termed INDEPENDENT and DEPENDENT.When designing an investigation, one ofthe values will always depend on the valueof the other. For example, we could placea cup of room temperature water onto ahot burner and record temperatures andtimes as it heated up. In this example,time marches on no matter what we dowith the experimental setup. It is inde-pendent of the experimental design. Theindependent variable usually has the sameinterval between each data point. On theother hand, temperature changes produceddepend on when we take the data, obvi-ously the water will get hotter the longer itis on the burner. Thus, temperature is thedependent variable as its value is deter-mined by when we take the data. Theindependent variable is always on the hori-zontal, or x-axis of the graph. The depend-ent variable is always placed on the verti-cal, or y-axis of the graph.

The next step is to determine the SCALEto be used for each variable. The scale isthe numbering system used for labelingthe axis of a graph so data can easily bedetermined by looking at the graph. The

basic rule is to choose an interval betweenthe labeled lines that will make the dataline large enough to fill about two-thirdsof the available graph paper.

A graph is a common and useful wayto show the results of an experimentbecause it makes visual the findings of anexperiment.

EXTENSIONS

Have students graph data of the dailyhour by hour kilowatts collected. Graphwith the hours of the day in military time.Second, third and fourth lines can beadded to this graph at three-month inter-vals (March 21, June 21, September 21 andDecember 21). Lead a discussion on daylength and incoming solar radiation andthe power output by the panels.

Another graphing assignment is withthe same day but with data from differentangle panels such as Memorial HighSchool at 45 degrees and West High Schoolat 25 degrees. The students again need touse military time. Ask them to interpolateand extrapolate their graphs. This graphleads to discussion on sun angle.

If you have access to multi-meters andsolar panels, build on the question of thesun’s angle. Have students use a multi-meter to measure voltage charged by asolar energy panel. Attach alligatorclamps on the panel to the wires on themulti-meter. Connections are color coded.Attach black to black and red to red. Have

CREDITS:Activity adapted from lesson plans designed by:

Claudia Johnson, Memorial High SchoolDon Vincent, Madison West High School

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PROCEDURE:

Orientation:

Discuss with students the terms INDE-PENDENT, DEPENDENT and SCALE.Familiarize students with MGE’s Website.

Steps:

1. Discuss elements of graph procedure:

Rule #1 Graphs must be done neatly ifthey are to explain experiments clearly.

Rule #2 Always label graph. Label thetwo variables with the name and units.

Rule #3 If the graph has more than onedata line plotted on it, use different colors orsymbols in plotting the information andmake a key indicating what each color orsymbol signifies.

Grade �– curriculum guide designed for Madison area schools by Madison Gas and Electric

SUMMARY:

By graphing data studentswill learn how the time ofday and the time of yearaffect solar energy.

OBJECTIVES:

Students will be able to:

* Make a graph using correct graphing techniques.

* Interpolate a graph.

* Draw conclusions from a graph.

TIME:

Preparation: 15 minutes

Activity: 50 minutes

MATERIALS:

Computer with graphing pro-gram and internet connectionor graph paper and coloredpencils.

GETTING READY:

Retrieve the monthly kilowatt-hoursproduced by one of the MGE solarschools solar panels from MGE’s Website. Make a few copies of MGE’s Website in case you experience technicalproblems during the class periodwhen retrieving data.

Rule #4 When putting numbers on thegraph, always use at least two-thirds ofthe graph paper.

2. Use the provided data collectedover the past year of the averagemonthly kilowatts of energy absorbedat one of the Solar Schools sites solarenergy panels.

3. Have students utilize this informa-tion for their graphs. Ask them topredict the next week according to theprevious weeks and the same week ayear before. Have students discusstheir predictions.

4. A week later, have students sharethe actual results and compare theresults.

ASSESSMENT:

Formative:• Can students accurately describe the

elements of a graph?• Can students interpolate and extrap-

olate a graph?

• Can students, given data, make agraph using all the elements of agraph?

SUMMATIVE:

Challenge students to find other fac-tors related to the kilowatts producedwith the solar panels and graph them.

PROJECT DETAIL: YEARLY INSOLATION GRAPH EXERCISE

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Based on this experiment, have studentsmake a drawing illustrating the maximumsolar input. Include the following: the sun,PV panel, angle, etc.

A final graphing assignment could beto graph the average daily temperatureand the amount of energy output in kilowatts produced. Lead a discussionaround incoming solar radiation and tem-peratures.

PREDICTING PV OUTPUT EXERCISEBACKGROUND

Daily photovoltaic output is directly relat-ed to local cloudiness. In other words,local weather impacts the ability of photo-voltaic panels to produce electricity.Therefore, students can utilize weatherforecasts to predict the photovoltaic out-put of solar panels.

Students will observe the daily surfacemaps for Sunday, Monday, Tuesday, andWednesday. Following Wednesday’sobservation, the students will predict thesolar PV output for their location forThursday, Friday and Saturday based onthe forecasted weather conditions.Students will then calculate their percenterrors for each day predicted.

PROCEDURE

ORIENTATION:1. Discuss with the class the characteris-

tics of the best location for a solar ener-gy array. Familiarize students withMGE’s Web site comparing the solaroutput of various panel arrays.Discuss the impact of location andangle arrangement on photovoltaicoutput.

2. After deciding on a location, ask whatconditions will affect the solar energy output throughout the year.

3. Students should note the seasonalangle of the sun, length of daylighthours and cloudiness.

4. Focus on cloudiness. Discuss exactlyhow cloud cover affects solar energyoutput and ask the students how theycould know what the cloud covermight be like in the short-term future.

5. Students should identify satelliteimagery and weather maps as key pre-dicting tools of cloudiness.

6. Stress the short-term predictability ofsatellites and maps and ask the stu-

CREDITS:Activity adapted from lesson plans designed by:Tyler Spence, Memorial High SchoolTom Palmer, Abundant Life Christian School

students place their PV cell in sunlight andread the meter. Assign them to move theirpanel to various angles (0 degrees, 45degrees, and 90 degrees). Then have themplace aluminum foil around their PV celland repeat the tests.

Follow this up with a discussion.Which angle gave the best results? Whichcombination gave the best results? Why?

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dents if they can think of any way ofpredicting the cloud cover in the longterm.

STEPS1. Prior to beginning the activity, collect

"baseline data" for energy output foryour location. Take the average energyoutput for two sunny days, two partlysunny days, and two overcast days.Fill these in on the prediction work-sheet for the use of the students.

2. Begin printing off each day, startingSunday, the surface analysis map forthe noontime hour.

3. Each day, Sunday through Tuesday,have a student record the total solarenergy output for the day on the sur-face analysis map so students can ana-lyze the map and note its relation to thesolar output.

4. On Wednesday, pass out the predictionworksheet. The students should fill intheir solar output predictions forThursday, Friday and Saturday in pen.

5. Each day, have the students go toMGE’s Web site (www�mge�com) andrecord the daily solar energy output forthe previous day. The students can thencompare their predicted value to theactual value and calculate their percenterror for the day. Values should berecorded on the prediction worksheet.

6. The following Monday, students canfinish their evaluations and calculatetheir weekly percent error value.

7. On the second Wednesday, the activitystarts again with the personal goal toachieve a lower-percent error during thesecond week.

8. Individual competition may suffice,however class competition may bedesirable.

CLOSURE1. Have students investigate the climatic

data for the region, focusing on thecloudiness. Have them hypothesizewhy certain times of the year are typically more cloudy or clear.

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SUMMARY

Students will learn to predict pho-tovoltaic output by analyzing sur-face weather maps.

OBJECTIVES:

Students will be able to:· Predict the movement of

weather systems across the UnitedStates using meteorology skills andsimple mathematical computations.

· Make educated guesses based onpatterns within collected data.

· Improve forecasting skills based onanalysis of past predictions.

TIME:

Preparation: 20 minutes

Activity: Four 50-minuteclass periods

MATERIALS:

Internet access

A week of daily maps

Prediction worksheet

GETTING READY:1. One week before the activity is run in

class, have the students collect “base-line data” on the effects of cloudinesson solar PV output of one of the MGEHost Schools. Pick an overcast day, apartly sunny day and a clear day andrecord the solar PV output for theseconditions. These values will aid inthe PV output predictions the follow-ing week.

2. Print daily surface analysis mapsstarting on Sunday of the week youwill begin this activity. Print theanalysis at the same time everyday.On Wednesday, give these four maps(Sunday through Wednesday) to thestudents.

3. Make copies of the PV Predictionworksheet.

ASSESSMENT:FORMATIVE:1. Will the baseline data collected at the

beginning of the activity be able to beused year round? Why or why not?

2. Did your estimations become more accurate from the first week to thesecond week? Try to explain why orwhy not.

3. What type of pressure system will typically bring sunny skies? Cloudyskies? Use what you know aboutmeteorology to explain this phenom-enon.

SUMMATIVE: 1. Have students investigate where in

the United States would be the bestplace to have a photovoltaic energysupply. The students should be ableto rationalize their answer based onmeteorology.Finally, have students do the sameexercise as in number two above butfor the world. Where in the worldwould be the best place for acquiringsolar energy through a photovoltaicarray?

PROJECT DETAIL: PREDICTING PV OUTPUT EXERCISE

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EXTENSIONS:

CLASSROOM SIMULATION: SOLAR DAY TRADING

As a twist on the prediction activity,students may like to compete inmore of a simulation that uses morestrategy. In this simulation, each stu-dent is told they have the ability touse 100 solar panels the size of theschool's PV array. The catch is thateach array costs a set amount ofmoney to operate for a day of use.

Therefore, students must look at thesurface analysis maps and try to predictthe amount of sunshine that will be avail-able for the next day. After making thisdecision, the student will “activate” a cer-tain number of their 100 arrays for the nextday. The amount of energy generated bytheir activated panels will be their school'svalue times the number they activated. Ifthey thought it would be a clear day, thenall 100 would be put into use (for a size-able fee). If they thought it to be overcast,then it would be cost efficient to only run afew. The students will need to do acost/benefit analysis to determine whenthe generation of electricity through theirPV system will be profitable and when itwill not. As an added factor in the compe-tition, the teacher may control the pricepaid for PV electricity over the time periodthe simulation is run. This will add intothe discussion the fact that the price ofelectricity fluctuates over time and canhave an effect on the profitability of solargenerated electricity.GUESS WATT?

If classroom time is limited, or as a meansto initiate interest before beginning the les-son, facilitate a class or school-wide com-petition to Guess Watt the solar panels areproducing. Help students by explainingphotovoltaic cells, watts, kilowatts, and

Sample Monthly Energy Output Graph

kilowatt-hours; demonstrate how to accessthe MGE’s Web site; and develop a basic'entry form' for students to input their pre-dictions. Pick a central location to post PVdata about a selected Solar Host School,and post information about output andweather conditions for a week. Then chal-lenge students to predict the next week ofdaily output. Post winners each day andhave an overall Guess Watt scholar select-ed for the overall winner.

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ASSESSMENTFORMATIVE:1. Will the baseline data collected at the

beginning of the activity be able to beused year round? Why or why not?

2. Did your estimations become more accurate from the first week to the sec-ond week? Try to explain why or whynot.

3. What type of pressure system will typically bring sunny skies? Cloudyskies? Use what you know about mete-orology to explain this phenomenon.

Sample Daily Energy Output Graph

SUMMATIVE: 1. Have students investigate where in the

United States would be the best placeto have a photovoltaic energy supply.The students should be able to rational-ize their answer based on meteorology.

Finally, have students do the same exerciseas in number two above, but for the world.Where in the world would be the bestplace for acquiring solar energy through aphotovoltaic array?

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EQUATIONS NEEDED:

Percent Error = [Predicted Output - Actual Output] ×× 100%Actual Output

PREDICTING PHOTOVOLTAIC OUTPUT WITH METEOROLOGY

STUDENT PREDICTION WORKSHEET

NAME ______________________________ PERIOD_____________

Baseline DataSunny Day Partly Sunny Day Overcast Day

Day 1Output (kWh)Day 2Output (kWh)

AverageOutput (kWh)

Week One Predictions

SUN MON TUES WED THUR FRI SAT TOTALS

PredictedOutput

ActualOutput

PercentError

Week Two Predictions

SUN MON TUES WED THUR FRI SAT TOTALS

PredictedOutput

ActualOutput

PercentError

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SUMMARY

Through a laboratory exercise, studentsutilize photovoltaic panels to challenge themisconception that Earth’s seasons are theresult of variations in the distance betweenthe Earth and the sun.

BACKGROUND

Earth is a nearly perfect sphere that orbitsthe sun over the period of a year. Duringthat time, Earth’s tilt points to the sameplace in space. Thus, at one place in Earth’sorbit the tilt of the North Pole is as much“toward” the sun as it will ever be. On thisday, the Earth is at its northern hemispheresummer solstice. For the next three months,the North Pole is less and less tiltedtoward the sun until the tilt is neithertoward nor away from the sun. On thisday the Earth is at the northern hemi-sphere autumnal equinox. For the nextthree months, the North Pole is tilted more

OBJECTIVES

Students will be able to…•Explain the cause of Earth’s

seasons.•Measure the electrical energy

produced by a solar cell.•Calibrate solar cell sensitivity•Define insolation.

SEASONS OF THE SUN EXERCISE

TIME

Preparation: Two hours Pre-Activity: 50 minutes Activity: Two 50-minute class periods

MATERIALS•14″ globes on ring stands•Velcro, self-adhesive•Four solar cells per globe•Multi-meters (one per solar

cell)•Bright light source: 300 watt

bulbs

and more away from the sun until it reach-es a point in its orbit at which it is as tiltedas much “away” from the sun as it willever be. On this day, the Earth is at itsnorthern hemispheric winter solstice.Over the next six months, the Earth com-pletes the second half of its orbit duringwhich time the tilt goes from away, to nei-ther toward nor away, to as toward as itwill ever be.

Over the course of the unit, studentswill come to recognize the patterndescribed above is the cause for the Earth’sseasons. In this lab, students gather datato demonstrate how latitudinal location onEarth impacts the average energy flow dueto changes in angle of insolation.

CREDITS:Activity adapted from lesson plans designed by:Benjamin Senson, Memorial High School

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GETTING READY:

DETAILED PREPARATION OF GLOBES AND SOLAR CELLS:

A fairly large globe must be obtained, 14′diameter or larger is preferable. For thesummer solstice, it is required that you useglobes that rest in a ring rather than thosemounted in a bracket that connects thenorth pole to the south pole. This bracketobstructs the mounting of the polar photo-cell and blocks rotation of the other photo-cells into the noontime position required tocollect accurate data.

On the globe, select the meridian(north/south line) that passes from pole topole through the location at which yourstudents live. On this line, you shouldplace the fuzzy side of self-adhesive Velcroso that it runs from pole to equator. Usethe fuzzy side so that dirt does not collecton the globe over time.

Three or four identical photocells arerequired for this lab. If possible, use photo-cells that are small enough to allow all ofthem to be placed lengthwise (their longestdimension), end-to-end from the equatorto the pole without any overlap of anykind. For each cell, place a square of thebristled side of self-adhesive Velcro to thebackside of each photocell. Use extra-strength Velcro to eliminate shifting of thecell during the lab procedure.

Using only Velcro®, there is a signifi-cant risk the photocell will shift enough,with changes in the globes position, toadversely impact the value of the data col-lected. Preventing this shift requires onemore preparation step. Place one of thephotocells flat against the globe and adjustthe photocell so that both of the ends areequally raised from the surface of theglobe. Place a pencil or small piece of

dowel under each raised end until it fillsthe gap between the photocell and theglobe. Mark the location of the dowels.Remove the photocell and attach the dow-els to the backside of the cell either perma-nently (hot melt glue, superglue, ducocement, etc.) or temporarily with tape.These wedges will prevent the photocellsfrom shifting during the lab procedures.

Photocells must also be calibratedagainst each other. This procedure isdescribed below under the “Procedures”section. However, prior to starting the lab,you should label one electrical lead fromeach photocell with a number so that youhave one cell per each number 1, 2, 3, etc.

To set up this lab, set up one verybright incandescent light source (200 to 300watts) in a location where a student canplace their globe fairly close to the lightsource, but far enough away that there isnot a significant change in the angle ofinsolation due to the distance from thebulb (>10′ or as far as is practical).

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PROCEDURE

ORIENTATION:

Reinforce the persistence of the Earth's tiltas it moves through its orbit. Model ordemonstrate the equivalence of maintain-ing the tilt in the same direction as theEarth orbits. Describe the lab procedureswith emphasis that the rotation of theglobe in place, from tilt toward, to neithertoward nor away, to tilt away from thesun, is equivalent to gathering data at thesummer solstice, equinoxes and winter sol-stice.

Demonstrate how to set up the globeand photocells and demonstrate use of themulti-meters.

Attach the Photocells to the Globe:1. At the equator, place the center of pho-

tocell number one so it lies directlyover the point at which the equator andthe Velcro meridian meet. Orient thephotocell so that its longest dimensionruns north-south on the globe (parallelto the Velcro).

2. At 30 degrees north latitude, place pho-tocell number two. Match the orienta-tion of photocell number one for thisand all subsequent photocells.

3. At 60 degrees north latitude, place pho-tocell number three.

4. At 90 degrees north latitude, or thenorth pole, place photocell numberfour.

Attach the Multi-meters to the Photocells:

For each photocell, connect the multi-meters positive lead to the red wire com-ing from the photocell. Connect the blacklead to the ground, common or negativelead of the multi-meter. Adjust the multi-meter to read out current in amps.

STEPS:1. Have student set up their globes.2. Have students calibrate their photo-

cells.3. Have students attach the photocells to

their globe.4. Have students attach the multi-meters

to the photocells.

Summer Solstice:1. Rotate the globe in its stand so the

North Pole is tilted toward the brightlight source in the room. The ArcticCircle should cross over the top of theglobe.

2. Rotate the globe around its polar axisso the Velcro meridian with the mount-ed photocells is pointed directly at thebright light source in the room.

2. Turn off all room lights other than thesingle, bright light source.

3. For each photocell, read the currentproduced as measured on the multi-meter.

4. Multiply each value by its calibrationratio and record the result.

Equinoxes:

Repeat the Summer Solstice proceduresbut with the globe oriented with the polaraxis straight up and down.

Winter Solstice:

Repeat the Summer Solstice proceduresbut with the North Pole tilted away fromthe bright light source in the room. TheArctic Circle should cross over the top ofthe globe.

ANALYSIS:

1. Calculate the change in noontime cali-brated current produced by each photo-cell, summer to winter.

2. Students should graph the following:

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EXTENSIONS

PRE-LAB DISCUSSION:Conduct a pre-lab discussion to reinforcethe distance misconception. This approachis used to demonstrate the reasonablenessof the belief based on first-hand, casualobservations of the nature. Every studentknows that the closer one gets to a heatsource, the warmer one feels and viceversa.

THREE STRIKES DISCUSSION:The misconception of distance causing sea-sons is the result of three impossibilitieswhich would have to occur for this beliefto be true. This includes mathematicalanalysis of the percentage change in energydue to distance (too small of a variation torepresent seasonality), hemispheric separa-tion (north winter during south's summer)and mismatch to orbital position (Earth isclosest to the sun on Jan. 4th and farthestfrom the sun on July 4th). Discuss miscon-ceptions.

POST-LAB DISCUSSION:Conduct a post-lab discussion to compareand contrast the predictions made by boththe distance model and tilt-revolutionmodels of Earth's seasons. Form a classconsensus regarding the more appropriateexplanation for Earth’s seasons.

PLANETARIUM VISIT:Bring students to a local planetarium.Planetariums provide visual representationand temporal experience (albeit highlyaccelerated) of both the change in angle ofinsolation and length of day throughoutthe year.

COMPUTER LAB: Collect data for variations in sunrise/setpositions, noontime elevations of the sun,and Day length. Have students graph andanalyze this data to recognize the correla-tion between these different sets of infor-mation and one cause, Earth’s tilt and itsorbit around the sun.

ASSESSMENTFORMATIVE:Pre-conception quiz and/or discussion SUMMATIVE:Grading of lab questionsLAB QUESTIONS:1. At what location on the globe is the

greatest insolation absorbed during thesummer solstice?

2. At what location on the globe is thegreatest insolation absorbed during theequinoxes? Can you prove youranswer?

3. No matter what time of year it is, whatlocation absorbs the greatest insola-tion?

4. Average the current readings for eachphotocell across all four seasons. Whathappens to the average insolationabsorbed as you move from the equa-tor to the pole?

5. Based on your answer to question 4,what should happen to the averagetemperature as you move from theequator toward the pole?

6. Compare the change in insolation fromsummer to winter, what happens to therange in the current produced by thephotocell as you move from the equa-tor toward the pole?

7. Based on your answer to question 6,what should happen to the tempera-ture range (difference in the seasons)experienced as you move from theequator toward the pole?

Calibrated noontime current versus timeof year (Summer, Equinox, Winter), allphotocells on one.

Change in calibrated current versusLatitude.

3. Respond to lab questions below.