What it shows
Small wind turbines for rooftops and villages must work with turbulent, low wind speeds. A Savonius rotor made from half-cut drums or pipes works without turning to face the wind. The project builds three rotor designs, tests them in a box fan wind stream and calculates their power coefficient.
The science
The wind pushes more on the concave side of an S-shaped rotor than on the convex side, so it rotates by drag difference. An overlap gap lets air pass to the back of the next blade and improves torque. Power in the wind is 0.5 ρ A v³, and the power coefficient Cp is the electrical output (or shaft output) divided by this. Savonius rotors typically have Cp of 0.15 to 0.30, well below the Betz limit of 0.59.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Three rotors made from half-cut PVC pipes or aluminium sheets with different overlap ratios (0, 15 and 30 percent), height 30 cm and diameter 25 cm
- A shaft, bearings and a stand
- A small DC motor or alternator as a generator
- A table fan or a household pedestal fan used as a wind source on a stand (not modified)
- An anemometer or a phone app with a sensor, and a tape measure
- A resistor load and two multimeters
Safety
For this project: Keep fingers and hair away from the fan and the rotor. Fix the rotor firmly so that blades do not fly off. Use the fan as supplied from a sealed plug and do not open or modify it. An adult must supervise.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
11 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Cut the half-pipes carefully and make end plates of plywood to hold the two blades with the chosen overlaps.
Fix each rotor on a shaft with ball bearings at both ends so it spins freely.
Couple the shaft to the motor-generator through a small gear or a belt.
Place the fan 1 m from the rotor, measure wind speed across the rotor area at several points and take the average.
Measure the swept area as height times diameter for each rotor.
Run each rotor with a load of 100 Ω and then 50 Ω and 20 Ω, and record voltage, current and rpm.
Calculate electrical power and the tip speed ratio.
Calculate Cp as electrical power divided by the wind power through the swept area, and include the generator efficiency when you estimate the shaft Cp.
Plot Cp against tip speed ratio for each rotor.
Repeat at a lower fan speed and note how the starting behaviour changes.
Write the report on the three designs, results, comparisons with a horizontal-axis turbine and applications.
What you should see
All three rotors self-start and the one with a modest overlap of about 15 percent usually gives the highest Cp, in the range 0.1 to 0.25 based on shaft power. Electrical Cp is lower because of generator losses.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the Betz limit?
The theoretical maximum of 59 percent of wind power that any turbine can extract.
2Why does a vertical-axis turbine not need to face the wind?
It responds to wind from any direction because its shape is the same on every side.
3What does tip speed ratio mean?
The speed of the blade tip divided by the wind speed.
4Why does wind power depend on speed cubed?
Kinetic energy depends on v squared and the mass flow rate on v, so power depends on v cubed.
Take it further
Twist the rotor by 90 degrees between two stacked stages to smooth the torque and compare its Cp with that of the single stage.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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