The challenge: to harness energy from the wind to power a simple windmill.

Traditional windmill

Pros

• Tried and tested
• Easy to control

Cons

• Blades need to be quite light
• Blades must be able to turn to face the wind direction
• Power can be lost on its journey from the rotor to the ground
  
  

Darrieus windmill

Pros

• Works no matter which direction the wind is coming from

Cons

• Higher risk - not tried and tested
• Can't start by itself
• Team will need extra time to build a brake for it

How does a windmill work?
The power of the wind acts on oblique blades or sails that radiate from a shaft. The blades/sails make the shaft turn. The power of the turning shaft is transmitted through a system of gears and shifts to machinery that mills grain, pumps water or generates electricity - converting wind energy into mechanical energy.

A short history of windmills
The first known windmills appeared in the Middle East in the 18th century BC. The sails of these early mills turned on a vertical shaft, which drove their millstones directly. European post mills, which had sails turning on a horizontal shaft, appeared in Portugal in the 12th century. About 300 years later, windmills were being used for land drainage, flour-milling, oil pressing, paper pulping and sawing timber.

Although steam power largely replaced wind power in the 19th century, there were increasing numbers of small multi-bladed machines, producing about half a kilowatt of power to pump water. Similar windmills are still used today for water pumping in the Third World.
 
Key innovations in windmill technology:

• Fantail rudder that automatically rotates the sails into the wind.
• Spring sail, using either manually or automatically controlled wood shutter openings, to maintain a constant sail speed in winds of varying speed.
• Improvements in blade size, shape and twist, 'rpm' (revolutions per minute) and the rate of tip speed to wind speed.
• Airbrakes to stop sails from rotating.
• Propeller-like airfoils in place of sails, increasing the usefulness of mills in light winds.

Horizontal- and vertical-axis machines
The horizontal axis refers to the rotating shaft of the windmill, not the plane in which the blades rotate. The horizontal-axis machine - with its main shaft parallel to the ground - is the traditional design.

Horizontal-axis windmills designed to produce mechanical energy are generally multi-blade machines. A typical example is the pumping windmill seen on farms on the American Midwest. Each blade has an aerofoil shape and so the rotor literally flies through the air rather than being pushed by drag. This kind of windmill has a relatively slow turning speed but does produce high torque - that is, twisting force - which is useful in producing mechanical energy. However, a horizontal-axis windmill is not very efficient, using only about 20% of the energy produced by the wind.

The rotor itself is mounted on a bearing that allows it to turn up- and downwind. It does this with the aid of a tail vane mounted off centre to the rotor. This allows the rotor to turn out of the wind if the wind speed gets too strong.

The simplest vertical-axis machine is a Savonious rotor, which consists of two oil-drum halves facing in opposite directions - they are typically seen rotating at petrol stations with labels such as 'Drinks' and 'Snacks' written on each side. These machines are extremely easy to construct and work by drag - the wind pushes the rotor round. This actually makes them quite inefficient. Although, theoretically, they can use about 57% of the available wind energy, the efficiency of most of them is below that for a variety of reasons, including friction of the bearings. In fact, a Savonious can manage to utilise only 10% of the wind energy - half of that achieved by a horizontal-axis machine.

A more efficient vertical-axis machine is the Darrieus rotor. Its two blades are aerofoil in shape and so are more efficient than the Savonious, and the rotor can turn quite fast. The only problem with the Darrieus is that it is not self-starting and needs a small drag rotor on top.

However, the advantage of all vertical-axis machines is that they can turn on wind coming from any direction. So, unlike the horizontal-axis machines, they don’t have to face up- or downwind in order to rotate.

Wind turbines
When is a windmill not a windmill? When it's used to generate electricity - then it's called a wind turbine generator.

The 20th century saw the development of wind power for electricity. The first wind generator was built in Denmark in 1890, but interest in larger machines only revived following the oil crisis of the 1970s.

Modern wind turbines have two types of propulsion. The first is drag - the process by which the wind pushes the blades. The second is lift - the process by which the blades are moved in the same way a plane's wing rises on an air current.

Windfarms
It's estimated that wind generators could provide up to a fifth of the demand for electric power in many countries. Today, 1% of California's electric power - enough to service 300,000 homes - is produced by over 16,000 spinning windmills gathered into 'windfarms' and linked to the public power grid. California, where winds averaging 70 miles per hour whip through mountain passes, has more than 80% of the world's windmills. Another 13% spin in Denmark, while Hawaii has 2.3%.


Find out more from Scrapheap Challenge judge Gordon Proven about windmill history and drag and lift machines.

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