Technologies K–10 · Years 7–8

Wind turbine blades: measuring electrical power and the Betz limit

Engineering Technologies and Systems focus area (NSW Technology 7–8, 2023); Design and Technologies: Knowledge and understanding, Technologies context: Engineering principles and systems (ACARA v9)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Wind carries power that rises with the cube of wind speed, and no turbine can capture more than 16/27 of it, so blade design is judged by how much of that power reaches the generator.

Safety card

Low riskAn adult supervises

Hazards

Spinning blades hitting fingers or eyes
Fan blades

Controls

Safety glasses; hands away from the rotor while running; switch the fan off to adjust blades
Fan used only with its guard fitted

Note

No chemicals. Follow NSW Department of Education TAS compliance advice (mandated controls and Equipment Safety in Schools), education.nsw.gov.au/teaching-and-learning/curriculum/tas/tas-compliance; record the activity on RiskAssess (riskassess.com.au).

What you need

Small DC motor used as a generator, with a hub for blades on dowels
Card and balsa for blades; protractor to set the blade pitch
Desk fan with a guard; vane anemometer reading in m/s
Digital multimeter and a 10 Ω load resistor; ruler

How to do it

Measure the wind speed where the rotor will sit, 1 m from the fan.
Fit 3 blades at 30° pitch with a rotor radius of 0.15 m. Measure the voltage across the 10 Ω load and work out the power, P = V² ÷ R.
Change the pitch to 10°, 20°, 45° and 60°, measuring the power each time.
Change the number of blades (2, 3, 4, 6) at the best pitch.
Work out the wind's power through the rotor area and the Betz limit, and compare with the measured power.
Recommend a blade design using the data.

What you should see

Through a rotor of radius 0.15 m (0.0707 m²) at 3.0 m/s, the wind carries ½ × 1.204 kg/m³ × 0.0707 m² × (3.0 m/s)³ = 1.149 W, and the Betz limit allows at most 0.681 W to be captured. Measured electrical power is a small fraction of this, because the blades, bearings and small motor all lose energy; the learner finds it from the voltage across the load with P = V² ÷ R. Doubling the wind speed multiplies the wind's power by 8. Power rises and then falls as the pitch increases, so there is a best pitch between flat and edge-on. The learner knows it worked when the measured power is below the Betz limit at every setting and the pitch test shows a peak.

What changes

What you change: blade pitch, then number of blades
What you measure: electrical power into 10 Ω (mW)
What you keep the same: the same fan setting and distance
the same rotor radius
the same load resistor

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

More blades always give more power (extra blades add drag; there is a best number).
A turbine can capture all the wind's power (the Betz limit caps it at 16/27).
Twice the wind speed gives twice the power (it gives 8 times the wind power).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Technology 7–8 Syllabus (2023), NESA. Current: taught from 2026. Code read from the outcomes page on 2026-09-22.TE4-MSC-01 TE4-SDP-01
Technology Mandatory 7–8 Syllabus (2017), NESA. Outgoing: replaced by the Technology 7–8 Syllabus (2023) from 2026 and not available after December 2027. Code read from the official syllabus document (DOCX) on 2026-09-22.TE4-8EN
Australian Curriculum v9AC9TDE8K03AC9TDE8P02

Sources

The pages the author read to write this activity.

All Concept Studio activities