What it shows
A vertical-axis turbine with two curved scoops spins in a wind, and the turning axle winds up a thread to lift a weight. The useful power it delivers can be compared with the power in the wind, which is ½ ρ A v³.
The science
Moving air carries kinetic energy, and the power in the wind that crosses an area A at speed v is P = ½ ρ A v³, where ρ is the air density of about 1.2 kg/m³. Doubling the wind speed gives eight times the power. A Savonius rotor catches the push of the air more on the hollow side than on the curved back, so it turns, and it works with wind from any direction. The useful power output lifting a weight is P_out = m g h / t, and its ratio to the wind power is the power coefficient, which for a Savonius turbine is at most about 0.15 to 0.2.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two halves of a plastic drum or a large bottle cut lengthways, or a cylinder cut in half (about 20 cm tall)
- Two round cardboard or plastic discs as the end plates
- A 30 cm steel rod, a long bolt or a bicycle spoke for the axle, and two bearings made from bottle caps with holes
- A base board and two uprights
- Thread and small weights such as washers, with a paper clip hook
- A table fan at its three speeds, or a strong natural breeze
- A hand anemometer or an anemometer app, and a stopwatch
Safety
For this project: An adult must cut the plastic and plug in the fan; keep fingers and hair away from the fan and the spinning turbine.
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
7 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.
Fix the two half-cylinders between the end plates, offset like an S when seen from above, and thread the axle through the centre.
Mount the axle in the two bearings on the base so that it spins upright without much friction.
Tie a thread to the lower end of the axle and a paper-clip hook with a small weight to the other end, so turning winds the thread.
Place the fan 50 cm from the turbine and measure the wind speed at the turbine with the anemometer on each of the three fan speeds.
For each speed, time how long the turbine takes to lift a 20 g weight by 30 cm and calculate P_out = m g h / t.
Calculate the wind power through the turbine’s frontal area A (height × diameter) as ½ ρ A v³ and the ratio P_out / P_wind.
Plot P_out against wind speed and against v³.
What you should see
The turbine spins at all three speeds and lifts the weight faster at higher speeds. The power grows steeply with wind speed, roughly as v³, and the power coefficient comes out to a few percent.
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 formula for the power in the wind?
P = ½ ρ A v³.
2Why does a small increase in wind speed raise the power so much?
Power depends on the cube of the speed.
3What is the advantage of a vertical-axis turbine?
It does not need to be turned to face the wind and is simple to build.
4Why can no turbine capture all the wind’s power?
The air must still leave with some speed; the Betz limit says the maximum is about 59 percent.
Take it further
Add a small DC motor as a generator and measure the voltage and current of an LED load at each speed.
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
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it
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