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Energy From Wind

Air density, hub height and turbine siting

  • 3 things to know
  • AS 1

Overview

How air density, temperature and altitude affect output, hub height and rotor diameter, why wind speed rises with height, wind resource assessment, terrain and visual impact, and cost–benefit decisions on tower height.

What you need to know

Answer each one before you reveal it.

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Key term

How does altitude affect the output of a wind turbine?

Key term

Assess whether a developer should choose a taller tower for an inland turbine.

Key term

How does air density affect the power output of a wind turbine?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    How does air density affect the power output of a wind turbine?

    Model answer

    Power output is directly proportional to air density. Denser air carries more kinetic energy at the same speed.

  2. 2

    Why does a turbine produce more power on a cold day than a warm day at the same wind speed?

    Model answer

    Cold air is denser than warm air, because gas density rises as temperature falls. Air at 0°C is about 11% denser than air at 30°C.

  3. 3

    How does altitude affect the output of a wind turbine?

    Model answer

    Air pressure and density fall with altitude. A turbine on high ground therefore extracts less power at a given wind speed. At 1,000 m, air density is roughly 10% lower than at sea level.

  4. 4

    Why are hill tops still often good turbine sites despite lower air density?

    Model answer

    Wind speeds are usually higher on exposed hills, and power rises with the cube of speed. The gain from faster wind far outweighs the small loss from thinner air.

  5. 5

    Below its rated wind speed, a turbine produces 900 kW where air density is 1.2 kg/m³. Estimate its output at the same wind speed where air density is 1.1 kg/m³.

    Model answer

    The turbine would produce about 825 kW. Power is proportional to density, so 900 kW × 1.1 ÷ 1.2 = 825 kW.

  6. 6

    Define hub height.

    Model answer

    Hub height is the height from the ground to the centre of the rotor. The overall tip height equals hub height plus the rotor radius.

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