NAME:



[pic]PHOTOVOLTAIC (SOLAR)

CELLS

UNIT WORKBOOK

NAME:_____________________________

Introduction

Solar cells are used in many electrical devices through converting part of the energy of visible light into electrical energy. Although a single cell produces low voltage and current, many connected together can produce a larger current and voltage. Solar cells connected together are known as the photovoltaic module. Batteries, such as those in a torch, also produce electricity but are known as dry cells.

(projectsol.)

Voltage and current can be determined using a Digital Multimeter (DMM). Voltage is a measure of energy (in Volts) and current is a measure of the rate that electrons pass through the cell (in Amps).

A digital multimeter (media/DMR-1000.jpg)

PART A – SERIES AND PARALLEL CONNECTIONS

You will need: DMM with leads, 2 “torch” batteries, aluminium foil.

1) Set the DMM to 20 on the Volt (V) scale.

2) Connect the DMM leads to the terminals of a single cell (as shown in figure 1) and note the voltage.

Voltage = ___________________

3) Reverse the connections and note the reading.

Voltage = ___________________

4) Connect 2 cells together (positive to negative) and measure the voltage across their ends. This is called a series connection.

Voltage = ___________________

5) Based on your answer for step 4, what would you expect if 3 cells were connected in series?

Answer = ___________________

In series connections, each cell gives energy to the electrons moving through it and so has an increased voltage.

6) Place 2 cells side by side and connected their ends using aluminium foil as shown in figure 2.

7) Connect the DMM leads to each piece of foil and measure the voltage. This is called a parallel connection.

Voltage = ___________________

In parallel connections, more electrons can move around the circuit as they can move through both cells at the same time. Therefore, the current can be larger but each electron just passes through one cell and gets one lot of energy, so the voltage is the same as with a single cell.

By making combinations of cells in series and parallel, we can obtain any combination of voltage and current that we need.

This combination

can supply the same voltage as this

but the combination with 4 cells will be able to supply twice as much current as the series connection with 2 cells.

Part B – Photovoltaic Modules

One photovoltaic module is made up of many single photovoltaic cells. A single cell can produce a maximum of about 2 volts, depending on how it is made. Some types may produce a lower voltage. High quality cells, used in satellites, produce higher voltages and are more efficient. Polycrystalline cells are the cheapest and can only produce about half a volt per cell.

You will need: DMM with leads, 2 photovoltaic modules, black paper.

1) Set the DMM to 20 on the volt scale.

2) Connect the leads to the terminals of a photovoltaic module, expose to bright light and measure the voltage.

Voltage = _______________

3) Assuming the module is made of polycrystalline cells, how many cells are connected in series to make the module?

Answer = _______________

4) Cover the module completely with black paper and measure the voltage.

Voltage = _______________

5) Cover half of the module and measure the voltage.

Voltage = _______________

6) Is there any difference between covering half of the module top to bottom or side to side? If so, what is the difference?

Answer = _______________________________

7) Using the DMM, measure the voltage when 2 modules are connected in series and then in parallel.

Voltage – In series = _____________ In Parallel = ______________

Part C – Power output of PV module

Although high voltage is generally connected with high power, it can sometimes be connected with low power. For example, cases where there is static electricity, such as a clothes dryer or feet rubbing on carpet can get enough voltage to produce a spark sometimes centimetres long through the air. This requires several thousand volts. However, although each charge has high energy (volts) the total energy available is small due to the small amount of electric charges collected on the clothes from the dryer or on us from rubbing our feet on the carpet. In fact, you cannot even run a light globe with this charge. When an electric charge is moving, the rate of movement of charge (the current) determines the amount of electric charge that delivers energy.

Power is therefore found by multiplying the voltage by the current – P = V x I

The unit for power is the Watt (W).

To measure the power supplied by the PV module, the “solar cell” must be connected to something that needs an electric current flowing through it to make it work. From this, we can measure the voltage and current and multiply both to find the power.

You will need: 2 DMMs with leads, PV module, small electric motor, resistor.

1) Connect the leads from the PV module to the terminals of the electric motor and explain what happens.

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2) What happens when you tilt the module or partially cover it?

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3) What happens when you reverse the connections to the motor terminals?

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4) Connect the PV module, resistor and a DMM as shown in the diagram below, set the DMM dial to 200m on the Amp range and measure the current.

Current = __________________

RESISTOR

5) With the second DMM, measure the voltage across the resistor by connecting the leads to either end of the resistor.

Voltage = __________________

6) How does this compare to the voltage you measured across the panel alone before?

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7) Calculate the power being produced by the module.

Power = ___________________

8) Connect the PV module, motor and one DMM as shown in the diagram below, set the DMM dial to 200m on the Amp range and measure the current.

Current = __________________

[pic]

9) Set another DMM to 20 on the volt scale and connect its leads to the terminals of the motor to measure the voltage across the motor.

Voltage = __________________

10) Attach a small fan to the shaft of the motor, measure the current and voltage when the motor is running. Circle the value or values that changed.

Current = __________________

Voltage = __________________

11) Now hold on to the fan with the motor running (be careful not to cut yourself!), measure the current and voltage and circle the value or values that changed.

Current = _________________

Voltage = _________________

12) From questions 10 & 11, explain what values changed and why you think they changed.

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Note: The “m” setting on the DMM indicates “milli-amp” (mA). 1000mA = 1 amp. Therefore if the dial is set to 200m and the reading is 170, this means that 170mA of current is flowing. Using a calculator, divide the mA by 1000 to get amps, multiply this by the volts and the answer is the power in Watts.

13) Point the module in 3 different directions and determine the power output from the module for each direction.

Direction 1 –

Voltage = __________________

Current = __________________

Power = _________________

Direction 2

Voltage = __________________

Current = __________________

Power = _________________

Direction 3

Voltage = __________________

Current = __________________

Power = _________________

14) Knowing the power output from one of the PV modules, how many modules would it take to run a 34cm colour TV, which uses about 60W of electric power?

Answer = ___________________

Photovoltaic modules are often compared by the power they produce per square metre of area.

15) Write, in words, the formula to find area

Area = ___________________

16) Find the area of the PV module and express your answer in square metres. (To convert square centimetres to square metres, divide by 10 000).

Answer = _____________________

17) Calculate the power produced by a square metre of modules similar to the one you are using.

Answer = _____________________

Part D – How does a motor work?

Inside an Electric Motor

Let's start by looking at the overall plan of a simple two-pole DC electric motor. A simple motor has six parts, as shown in the diagram:

Armature or rotor

Commutator

Brushes

Axle

Field magnet

DC power supply of some sort, such as a battery

An electric motor is all about magnets and magnetism: A motor uses magnets to create motion. If you have ever played with magnets you know about the fundamental law of all magnets: Opposites attract and likes repel. So if you have two bar magnets with their ends marked "north" and "south," then the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south). Inside an electric motor, these attracting and repelling forces create rotational motion.

In the above diagram, you can see two magnets in the motor: The armature (or rotor) is an electromagnet, while the field magnet is a permanent magnet (the field magnet could be an electromagnet as well, but in most small motors it isn't in order to save power).

Electromagnets and Motors

An electromagnet is the basis of an electric motor. You can understand how things work in the motor by imagining the following scenario. Say that you created a simple electromagnet by wrapping 100 loops of wire around a nail and connecting it to a battery. The nail would become a magnet and have a north and south pole while the battery is connected.

Now say that you take your nail electromagnet, run an axle through the middle of it, and suspend it in the middle of a horseshoe magnet as shown in the figure below. If you were to attach a battery to the electromagnet so that the north end of the nail appeared as shown, the basic law of magnetism tells you what would happen: The north end of the electromagnet would be repelled from the north end of the horseshoe magnet and attracted to the south end of the horseshoe magnet. The south end of the electromagnet would be repelled in a similar way. The nail would move about half a turn and then stop in the position shown.

You can see that this half-turn of motion is simply due to the way magnets naturally attract and repel one another. The key to an electric motor is to then go one step further so that, at the moment that this half-turn of motion completes, the field of the electromagnet flips. The flip causes the electromagnet to complete another half-turn of motion. You flip the magnetic field just by changing the direction of the electrons flowing in the wire (you do that by flipping the battery over). If the field of the electromagnet were flipped at precisely the right moment at the end of each half-turn of motion, the electric motor would spin freely.

Armature, Commutator and Brushes

Consider the image on the previous page. The armature takes the place of the nail in an electric motor. The armature is an electromagnet made by coiling thin wire around two or more poles of a metal core.

The armature has an axle, and the commutator is attached to the axle. In the diagram to the right, you can see three different views of the same armature: front, side, and end-on. In the end-on view, the winding is eliminated to make the commutator more obvious. You can see that the commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet.

The "flipping the electric field" part of an electric motor is accomplished by two parts: the commutator and the brushes.

The diagram at the right shows how the commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment. The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator.

© , 2001

Questions

Describe the components of the motor you made in terms of the motor components in the first section of the reading.

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What is the “fundamental law of all magnets” and how is this law used to drive a motor?

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Do you think you could drive your motor with a solar cell? Explain why or why not.

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Bonus:

Explain how you can increase the speed of the motor.

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ELECTRIC MOTOR CONSTRUCTION:

PARTS

1. Enamelled copper wire, about 60 cm long.

2. 35mm film canister

3. Sandpaper.

4. 2 small disk or rectangular ceramic magnets (available at Jaycar.)

5. 2 large paper clips.

6. A plastic cup.

7. Masking tape.

8. 1.5 volt battery (AA-, C-, or D-size)

9. 2 electrical lead wires with alligator clips

10. battery holder

Photo shows one example. A single battery works fine.

1. Wind the enamelled copper wire into a coil around a 35 mm film canister (about 2.5 cm in diameter) leaving about 5 cm free at each end. Make four or five loops.

2. Wrap the freed ends of the wire around the coil a couple of times on opposite sides to hold the coil together as shown in diagram on right. Make sure the ends are truly opposite each other so that the coil will spin freely when supported on these two protruding ends. Leave 5 cm projecting from each side of the coil, and cut off any extra. (See diagram.)

[pic]

3. Use sandpaper to remove ALL the insulation from the wire projecting from ONE end of the coil. For the other end of the coil: - hold the coil vertical and use sandpaper to remove the insulation from just the upper side of the projecting wire (a thin strip of bare wire is sufficient). The orientation of the coil relative to the bared surface is important.

4. Turn the cup upside down and place a magnet on top in the centre Place the second magnet inside the cup, directly beneath the original magnet. This will create a stronger magnetic field as well as hold the top magnet in place.

5. Unfold one end of each paper clip and tape them to opposite sides of the cup, with their unfolded ends down. (See diagram.) Rest the ends of the coil in the cradles formed by the paper clips. Adjust the height of the paper clips so that when the coil spins, it clears the magnets by about 1.5 mm. Adjust the coil and the clips until the coil stays balanced and centred while spinning freely on the clips. Good balance is important in getting the motor to operate well.

6. Once you have determined how long the projecting ends of the coil must be to rest in the paper-clip cradles, you may trim off any excess wire.

7. Take the two lead wires and connect one to each paper clip: Wind the bare wire around the base of the clip and tape it in place. (if the lead wire have alligator clips on the end, just clip on to paper clip)

8. Connect the other ends of the wires to the terminals of the battery.

9. Give the coil a gentle spin to start it turning. If it doesn’t keep spinning on its own, check to make sure that:

▪ the coil assembly is well balanced when spinning,

▪ the enamel has been thoroughly scraped off the wire at one end of the coil,

▪ the projecting wire at the other end has enamel removed from just one side,

▪ the orientation of bared surface and coil is correct,

▪ good contact is made between paper clips and protruding ends of coils that rest upon them, and

▪ the coil and the magnet are close to each other but do not hit each other.

▪ You might also try adjusting the distance separating the cradles: this may affect the quality of the contact between the coil and the cradles.

Part E – What affects the Photovoltaic (PV) Cell’s performance?

The performance of a PhotoVoltaic cell or module can be affected by the following:

• The angle at which the Sun’s rays hit it.

• The temperature.

• The cleanliness of the surface.

• The colour of the light falling on the surface.

Effect of Direction of Light

The diagram below shows the Sun’s rays as perpendicular to the front surface of the PV module.

PV Module

You will need: PV module, DMM with leads, protractor.

1) What other rays of light will be reaching the front surface of the module?

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2) Using the DMM, measure the voltage of the module in this position.

Voltage = _________________

The module can be tilted so that Sun’s rays do not hit it perpendicularly. It can be tilted up towards the sky or down towards the ground. We can measure the amount of tilt using the angle of the module from the position where it is perpendicular to the Sun’s rays as shown below.

3) Use the DMM to measure the voltage from the module when it is tilted to 30°, 45° and 60° (use a protractor to measure these tilts) and write your answers in the table below:

|Tilted Up |Angle |Tilted Down |

| |0° | |

| |30° | |

| |45° | |

| |60° | |

The module can also be held so that its front surface is pointed away from the Sun.

4) In this position, what rays are reaching the module?

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5) Using the DMM, measure the voltage from the module in this position.

Voltage = __________________

6) Will the PV module produce any electric power on an overcast day? Explain.

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When the Sun’s rays reach the atmosphere, some will reach the ground, whereas some are absorbed or scattered by gas molecules or dust and water particles in the air. Some of the scattered light reaches the ground and some goes back into space. Blue light is scattered more as it has a shorter wavelength than, for example, red or yellow light, which is why the sky is blue. Light coming from the sky, but not directly from the Sun is called Skylight.

Effect of Colour

You will need: PV module, DMM with leads, Red cellophane, blue cellophane, yellow cellophane, green cellophane.

Cover the surface of the module with coloured cellophane. Without altering the angle of the module, use the DMM to measure the voltage and place your results in the table below:

|Direct Sunlight & Skylight |

|Colour |Voltage |

|Red | |

|Yellow | |

|Green | |

|Blue | |

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Figure 1

Figure 2

Figure 1: Parts of an electric motor

Figure 2: Electromagnet in a horseshoe magnet

Figure 3: Armature

Figure 4: Brushes and commutator

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