The science idea
Wind power is one of the fastest-growing sources of renewable energy, and India has some of the largest wind farms in the world. A wind turbine does not burn anything: it uses the kinetic energy of moving air to turn a generator. In this project you build a small turbine that is simple enough to make at home and good enough to light an LED, and you learn how the energy is changed from one form to another.
The generator is a small DC motor used backwards. Inside the motor there is a coil of wire that can rotate between permanent magnets. When you spin the shaft, the coil turns in the magnetic field, and the magnetic flux through it changes. By Faraday’s law of electromagnetic induction, a changing flux induces a voltage (an emf) in the coil: emf = N × dΦ/dt. The faster the shaft turns, the faster the flux changes and the larger the voltage is, so the voltage is roughly proportional to the speed of rotation. The same machine works as a motor when you push a current through it, and as a generator when you turn it by force: that is why generators and motors are so alike.
The wind pushes the blades because they are tilted. A blade set at a small angle (the pitch) to the plane of rotation is like a sail: as the wind meets it, the air is deflected, and the force that results has a component in the direction in which the blade can move, so the rotor turns. A twist along the blade, with a bigger angle at the hub and a smaller one at the tip, is found in large turbines, but a simple tilted blade works for a model.
The power in the wind passing through the swept area A is P = ½ ρ A v³, where ρ is the density of air (about 1.2 kg per cubic metre) and v is the wind speed. Notice that it depends on the cube of the speed: doubling the wind speed gives eight times the power. A rotor of radius 12 cm has A = 0.045 m², and in a 4 m/s breeze the wind carries about 1.7 W through it. A turbine can never take it all: the best possible fraction is about 59 percent (the Betz limit), and a home-made model gets far less, perhaps a few percent. A few milliwatts are enough to light an LED.
The electrical output is dim for a reason: the model is small, the motor is not designed as a generator and the wind is slow. You will find that the LED lights only when the shaft spins fast enough that the voltage rises above about 1.8 V, the voltage a red LED needs. That is a nice measurement: the speed of the wind at which the LED starts to light, called the cut-in speed of your turbine.
Materials and tools
Cost: Under Rs 100
The bottle, cardboard and tape are free. A small DC motor (from an old toy, printer or CD drive), a red LED pair and a 220 Ω resistor cost at most Rs 60 to Rs 100 if you buy them.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Small DC motor (3 V to 12 V type, such as from a toy car, a printer, a CD or DVD drive or a hair trimmer) | 1 | Take one from broken equipment (an adult does this). The motor with a small shaft and two terminals is best. A motor with a gearbox is not suitable. |
| Plastic bottle (1 to 2 litres) for the blades | 1 | A clean cold-drink or water bottle. |
| Bottle cap (for the hub) | 1 | From the same bottle. |
| Red LEDs (5 mm) | 2 | Buy two (a few rupees) or take them from old toys. |
| Resistor, 220 Ω (red, red, brown) | 1 | Buy one (about Re 1) or take it from an old circuit. |
| Insulated wire | About 1.5 metres | Thin wire from an old charger or earphones (an adult strips the ends). |
| Card tube (for the tower), a heavy box or tin (for the base), cardboard (for the tail vane) | 1 each | A kitchen foil roll or a thick roll of card, a shoe box with stones, a cereal box. |
| Hot glue or strong glue, tape, rubber bands | As needed | Ask an adult to help with hot glue. |
| Table fan (as a steady wind source), multimeter (optional) | 1 fan, 1 multimeter | A table fan with its guard in place. Borrow a multimeter from school. |
Tools
- Scissors and a craft knife (adult for the first cut)
- Ruler, pencil and a marker
- Pliers and a small screwdriver
- A tape measure to measure the distance from the fan
- A stopwatch (to count turns), and a piece of coloured tape for a mark on a blade
Safety first
Read this before you start
- The generator and the LED circuit are low voltage and are safe, but never connect any part of this project to the mains supply, a cell or a battery. The turbine makes its own very small voltage.
- Use a table fan only as a wind source, with its guard on and placed on a stable table. Never put your fingers or objects into a fan. Keep long hair and loose clothes away from the fan and from the turbine blades.
- Never connect an LED directly to the generator. The resistor must always be in the circuit, or a fast spin can burn out the LED.
- An adult should cut the bottle, remove the motor from old equipment and use the hot glue. Do not take a motor out of a device that is plugged in or that has a battery connected.
- Test outdoors only on the ground, in a safe open space away from roads, power lines and trees. Never climb on a roof or a wall to test a turbine.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
13 steps. Do them in order, and test each part before you go on to the next.
Test the motor as a generator
Join the motor terminals to a multimeter set to the DC volts range (about 20 V). Spin the shaft quickly with your fingers: the meter should show a voltage. Note which terminal is + (it depends on the spin direction).
Tip: Spin it fast for a stronger reading. If you see no reading at all, the motor may have a gearbox or a broken wire, so choose another.
Cut the blades
Cut a long strip off the straight part of the bottle. On it draw four blades, each 10 cm long and 3 cm wide at the root, narrowing to 2 cm at the tip. Cut them out. The natural curve of the bottle gives each blade a slight curvature.
Common mistake: Cutting blades of different sizes. They then do not balance, and the turbine shakes.
Make the hub
Take a bottle cap. Ask an adult to make a hole in its centre, just big enough for the shaft of the motor to push in tightly. Cut four small slits around its edge, the same width as the base of a blade, and about 1 cm deep.
Tip: If the hole is a little big, wrap a bit of tape around the shaft to make it fit.
Fix the blades to the hub
Glue or tape each blade into one slit so that the blades are equally spaced at 90 degrees. Tilt each blade by about 20 degrees to the plane of the cap. All four must be tilted in the same direction, like the vanes of a fan.
Common mistake: Tilting some blades one way and some the other. The pushes cancel each other and the rotor does not turn.
Balance the rotor
Push the hub onto the motor shaft. Spin the rotor with your finger. If it stops with one blade always at the bottom, that blade is too heavy. Trim a little from it, or add a small piece of tape to the lightest blade.
Tip: Balance reduces vibration and friction, which makes the turbine start in a lighter wind.
Make the tower and the base
Put stones in a box to make a heavy base. Fix a card tube about 25 cm tall upright in the base with glue and braces. Use a second layer of card to stiffen the top.
Common mistake: A light base. The turbine tips over in the wind. Add weight and widen the base.
Mount the generator and tail vane
Fix the motor on top of the tower with the rotor facing the wind. Fix a card tail vane to a short stick at the back so that the turbine swings to face the wind. The vane should be about 8 cm × 8 cm.
Tip: Place the pivot of the tower slightly in front of the vane, so the rotor can turn freely.
Wire the circuit
Connect the motor terminals through the 220 Ω resistor to the two LEDs, joined in parallel but in opposite directions, as in the circuit diagram. Run the wires down the tower and tape them. The long leg (anode) of one LED joins the short leg (cathode) of the other.
Common mistake: Connecting the two LEDs the same way round. One then never lights. They must face opposite directions.
Test with a fan
Put the table fan about 30 cm from the rotor at its lowest speed and point it at the blades. Watch the LEDs and note whether either one lights. Move the fan closer or use a higher setting if needed.
Tip: Try the other speed settings, one at a time. A larger speed raises the voltage and the glow.
Measure the voltage
With the LEDs disconnected, connect a multimeter across the motor terminals. Record the open-circuit voltage for different fan settings or distances from the fan. Put a coloured mark on one blade and count the turns in 10 seconds.
Common mistake: Measuring with the multimeter set to the current range. Always use the DC volts range for this project.
Test a variable
Change one thing: the number of blades (2, 3, 4 and 6), the angle of the blades (10, 20, 30 degrees), or the length of the blades. Measure the voltage three times each, keeping the fan at the same distance and setting.
Tip: Make new blades rather than bending the old ones again and again, as they lose their shape.
Test outside
On a breezy day, take the turbine to an open place on the ground. Face it into the wind and see whether the LED lights. Do not leave the turbine unattended.
Common mistake: Testing in gusty wind near people or traffic. Choose a safe open space and keep a distance.
Calculate and present
Find the swept area A = πr², estimate the wind power P = ½ρAv³ for a wind speed v (use a nearby weather report, or a wind-speed app, for outdoor tests), and compare it with the electrical power from your measurement (for example V² ÷ R, with the LED load). Write down the efficiency.
Tip: Do not be disappointed by a small figure of a few percent. Real small turbines are not much better.
How it should look when it is finished
A small wind turbine about 35 cm tall: a card-tube tower on a heavy base, with a four-bladed rotor made from a plastic bottle and a tail vane at the back. Two thin wires run down the tower to a tiny circuit board with a resistor and two red LEDs. Held in front of a fan, the rotor spins, and one of the LEDs glows red.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
The wind strikes the tilted blades and is turned aside, and by Newton’s third law the blades are pushed in the opposite direction. The sideways part of the force turns the rotor, which spins the motor’s shaft.
In the motor the coil spins between permanent magnets. As the coil turns, the magnetic flux through it changes, and an emf is induced in it (Faraday’s law). The faster it spins, the larger the voltage. This voltage drives a current through the resistor and the LED, which converts a little of the electrical energy into light.
The resistor takes the surplus voltage so that the current stays near 10 mA. Two LEDs in opposite directions work for either direction of spin, because a motor produces a voltage of the opposite sign when it turns the other way. The tail vane keeps the rotor facing the wind.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The rotor does not turn in a light wind | The motor shaft is stiff or the blades are not tilted enough. | Choose a free-running motor, make the blades lighter and tilt them to about 20 degrees. |
| The rotor spins but no LED lights | The voltage is too low, an LED is reversed or a joint is loose. | Use a stronger wind, check the voltage with a meter, make sure the LEDs face opposite ways and press the joints. |
| The meter shows a very small voltage | The motor has a high gear ratio or small magnets. | Try another motor. A motor from a CD or DVD drive often gives a higher voltage. |
| The tower wobbles or tips over | Unbalanced blades or a light base. | Balance the blades and add weight to the base. |
| The rotor turns the wrong way round for the LED | Only one LED is connected, or both face the same way. | Connect two LEDs in opposite directions. |
| The LED flashes and then stops working | The resistor is missing or too small and the LED burned out. | Always use a 220 Ω resistor and replace the LED. |
Test and record
Measure the open-circuit voltage with the multimeter on the DC volts range, with the LEDs disconnected. Count the turns of the rotor in 10 seconds by following a marked blade. Rotations per minute = turns × 6.
| Setting (blades, fan speed or distance) | Voltage (V) | Turns in 10 s | Rotations per minute | LED lit? (no, dim, bright) |
|---|---|---|---|---|
| 4 blades, fan low, 30 cm | ||||
| 4 blades, fan medium, 30 cm | ||||
| 4 blades, fan high, 30 cm | ||||
| 2 blades, fan medium, 30 cm | ||||
| 6 blades, fan medium, 30 cm |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- How does a wind turbine produce electricity?
The wind turns the blades and the shaft of a generator. In the generator, a coil turns in a magnetic field and electromagnetic induction produces a voltage.
- State Faraday’s law of induction.
The emf induced in a coil is equal to the rate of change of magnetic flux through it, multiplied by the number of turns.
- Why does the voltage increase as the shaft spins faster?
The flux through the coil changes faster, so the induced emf is larger. It is roughly proportional to the speed.
- What is the formula for the power in the wind, and what does it tell you?
P = ½ρAv³. The power is proportional to the cube of the wind speed, so doubling the speed gives eight times the power.
- Why is a resistor used with the LED?
It limits the current so that the LED does not burn out. R = (V − V LED) ÷ I.
- Why are two LEDs used in opposite directions?
An LED conducts only one way, and the direction of the generator’s current depends on which way it spins. Two LEDs ensure that one lights in either direction.
- What is the Betz limit?
The maximum fraction of the wind’s kinetic energy that any turbine can extract, about 59 percent.
- List the energy changes in your turbine.
Kinetic energy of wind → rotational kinetic energy of blades → electrical energy → light and heat.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a mini wind turbine and to investigate how the voltage depends on the wind speed and the number of blades.
- Apparatus
- A DC motor, plastic bottle blades, a bottle cap, card, wires, resistor, two LEDs, a multimeter, a table fan, a ruler and a stopwatch.
- Procedure
- Describe how you built the blades, hub, tower and tail vane, wired the LED circuit, and measured the voltage and the turns at different fan settings and with different numbers of blades. Include the diagrams.
- Observation
- The table of voltage, turns, rotations per minute and LED glow for each setting.
- Result
- The voltage rose from ... V at low speed to ... V at high speed. The LED started to glow at ... V. Four blades gave a higher voltage than two blades (state your values).
- Conclusion
- A wind turbine turns the kinetic energy of wind into electrical energy by electromagnetic induction. The voltage increases with the speed of rotation, and the power in the wind increases as the cube of its speed.
Take it further
- Add a bridge rectifier (four 1N4007 diodes) so that one LED lights whichever way the rotor turns, and add a capacitor to smooth the output.
- Plot a graph of voltage against rotations per minute and show that it is nearly a straight line.
- Compare blades made of bottle plastic, card and thin aluminium sheet.
- Measure the cut-in wind speed (the lowest speed at which the LED lights) with a hand-held anemometer or a phone app.
- Estimate the power output as V² ÷ R across a known load resistor and plot it against fan speed.
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