Week 7 - Wind Energy

Energy basics

Energy basics

  • First law of thermodynamics:

    • You can neither create or destroy energy

    • But it can change forms

Energy conversion processes

Energy units

  • Energy is measured in Joules (J)

  • A joule is the energy exerted when applying 1 Newton over one meter

  • A watt is a joule per second (W = Js-1)

  • Watts are also amps*volts

  • A kilowatt hour (kWh) is a measure of energy

Why does the wind blow?

Why does the wind blow?

  • More solar radiation received at the equator

  • Nature “dislikes” gradients

    • will try to smooth it out

  • Moves warm air aloft and towards the poles

  • Moves cool air away from the poles

  • Results in a three cell set up in each hemisphere

    • Hadley circulation

  • Rotation of the Earth results in circular movements of air masses

    • Cyclones and anti-cyclones

Prevailing Winds - large scale

Wind profile and surface roughness

The rougher the surface the more friction there is and the slower the wind speeds.

Global wind resources

  • Best resources

    • Mid and high latitudes

    • Coastal or elevated areas

    • Drops rapidly inland

  • Australia has excellent resources

  • Tropics not so good

  • Continental interiors generally not much good either

Parts of a turbine


  • Rotor

    • Most important part

    • Blades and the hub

    • At the connection points there are drives which allow the turbine to change the pitch of the blades

  • Main shaft

    • Takes rotational energy from the rotor converting it from the revolutions from the gear box

If wind direction changes the whole nacelle will turn using the yaw drive to ensure that the rotor is facing into the wind to maximise the energy efficiency.

Global wind deployment

Current state of the art

  • The largest offshore wind farm opened

    • Uses the world’s biggest turbines

    • Doggerbank, UK - 3.6GW

  • Golden Plains Wind Farm

    • Located in Western Victoria

    • Will be the largest wind farm in Australia when complete

    • 1.3GW

Maths of wind energy

Wind energy maths

  • Flow through the disk turns the rotor

  • Lift and drag slows down the wind

  • Kinetic energy is transferred to rotor

How much power in the wind?

  • The mass of air passing through the swept area of a turbine is the velocity times the Area times the density

    • m = vAD

  • Area is the radius time pi squared. Radius of a large turbine can exceed 50m

    • A = πr2

  • So the energy that a turbine “sees” is

    • K = ½ πr2Dv3

BUT:

  • We can’t extract all the energy from the wind or it would stop, preventing more wind to flow through

  • The limit can be calculated by calculating by calculating the derivative of the power equation (Betz’ law)

  • This shows that the maximum extractable energy is 16/27 (59%) of the total available

How much power?

  • K = ½ πr2Dv3

Assume 50m radius turbine, air density of 1.23kg/m3 and a wind speed of 12m/s

  • What’s the total power avaliable?

    • 8.35MW

  • What’s the maximum available once you take into account the Betz limit?

    • 4.95MW

Power Curve, Weibull and Environment

Power curve

  • Wind turbines output is limited by turbine size

  • Cut-in speed about 3m/s

  • Cut-out speed at 25 m/s


No matter how high the wind speed is if the turbine isn’t big enough it won’t be able to collect that energy.

Above 25m/s wind speed will cause the turbine to automatically cut-out, the brakes will be applied and the blades with feather which is designed to protect the wind turbine from damage.

Weibull Curve for Wind Speed

A common way to describe variable wind speeds is to use histogram data and fit a Weibull distribution to it.

Things to consider when planning to build a wind farm

  • Wind resource

    • Is there enough wind?

  • Transmission capacity

    • If a line is there, is it already congested?

  • Road access

  • Heritage overlay/native claim

  • Flora and fauna (esp.birds) at risk in the area

  • Neighbours

    • Noise

    • Flicker effect

    • Visual amenity

  • Geology

    • Will the ground support the turbine towers