The challenge: to harness energy from the wind to power a simple windmill.
The teams must harness energy from a renewable source – the wind – to power a simple machine. The challenge is to test who can make the most efficient windmill to drive a simple grinding wheel to grind some coffee beans at Beachy Head. The teams have a huge number of different windmill designs to choose from, including one with multiple vanes that turn in the vertical axis and another with only a few vanes turning in the horizontal plane. The winner will be the team whose machine grinds the most coffee (by weight) in an hour.
Tony Glavey, Wayne Turner and Christopher Wright are three cheeky young geezers who all work at a Suffolk BMW garage. Tony is the team captain and is building his own computer, Chris constructs go-carts in his spare time and Wayne has a prize goatee. They're hoping that their day-to-day engineering experience can help them scoop the Scrapheap prize.
Tim Harris, Andrew Samuels and Juliet Grant are a team of technology teachers all working at the Heath Park School in Wolverhampton. The first two are university friends. When they started teaching at the same school, they met Juliet working as the school technician. Larger-than-life Tim is team captain, Andrew is head of the department and usually Tim’s boss, and Juliet is a demon with a welding torch. Can a lifetime of learning be their trump card?
Giles Pearson (Techno Teachers) is a Zimbabwean who has been building windmills all over the world for 20 years – in Germany, he helped with the largest horizontal-axis machine ever constructed. He now calls Cornwall home, and makes his living building small vertical-axis windmills to generate electricity on yachts.
Jim Barr (Manic Mechanics) is a lecturer in computing who designs water features – water pumps made from scrap – in his spare time. He has built many horizontal-axis machines from odds and ends and has written books on the subject.
For the past 15 years, Gordon Proven has run his own engineering company that devises alternative energy solutions, including hydro and solar energy systems. He has patented his own windmill blade design that prevents the destruction of the rotor by high winds. He claims that his mills have survived wind speeds of 150mph. Check out his website at www.provenenergy.com.
Grindstone cowboys.
This week the teams decide to follow their leaders. With the help of their Zimbabwean-born expert Giles Pearson, the Techno Teachers plan to construct a rather unorthodox vertical-axis machine with only two blades, called a Darrieus rotor (see Scrapheap science for more information). This will be competing against a rather more traditional design if all goes well for the orange-clad Manic Mechanics. Their expert, Jim Barr, has a familiar farm-type horizontal-axis design in mind, with many blades.
Of course, being BMW mechanics, the Manics' first inclination is to mount a car axle with its differential as a bearing between the shaft and the sails. However, a differential reverses the rotation when used like this, and the coffee grinder that must be powered only works one way. This confusion, along with the exact direction the blades will turn, comes back to haunt the Mechanics throughout the day.
Back to basics.
Both teams scavenge a large amount of scaffolding bars and angle iron. It looks as if this will be a 'back to basics', welding-and-structure-building day, similar to the Beach Boys' effort in the 'Demolition' challenge. Then Tony of the Manic Mechanics finds a ready-made motorway lighting gantry with wheels. Expert Jim quickly realises that this could form the basis of the windmill with a little modification.
Tony then gets the idea that they should be looking for all sorts of other prefabricated components, rather than custom-making parts for themselves. He presents an off-the-shelf hub casing to Jim, but Jim can't see how this plan can possibly work. Eventually, Tony is persuaded that some hard work will be required to compete successfully in Scrapheap Challenge - it isn't simply an Easter egg hunt. The Manics settle on a wheel hub (what else?) on which to mount their blades, but soon get a bit snippy with expert Jim as he seems to keep changing his mind.
The feeling in the Robert and Cathy camp is that both teams appear to be building 'wibbly wobbly windmills'. This is borne out as the Manics yet again change the plan for the spokes on which will rest their sails: they dismiss thin sheet in favour of strong 2.5cm (1in) tube. All this donkey work takes up much of the day.
Bad omen?
The Teachers, on the other side of the wall, have their own troubles. They are deep in a measurement crisis as Tim has cut the poles for their structure too short. However, unlike at the barber's, where more can be taken off but none put back on, super-welder Juliet gets to work rectifying his mistake. With three hours to go, the Technos seem to have sorted out their measurement troubles. But by now the weather has turned atrocious. Whether or not this is a bad omen for the test day remains to be seen.
The Teachers must incorporate some kind of initialising mechanism in their design as their windmill will not be self-starting. So they devise a system where they can wrap a rope around the base of the windmill and then all pull together to give it a jump start - like a giant spinning top. This design works omni-directionally, and the grinder, as mentioned before, only goes one way. Their windmill's real operational strength, however, is that, once in motion, it can make use of wind coming from any direction. The Manics, on the other hand, will have to point theirs so that it always faces the wind, thus leaving it open to potential weakness if the wind changes direction.
Up effluent inlet.
With one hour to go, the Manics have almost all of their blades on, but the Technos are back to their old measuring problems: their blades are half a metre too short! As there are only two of them, and their combined surface area is all the Teachers have for catching the wind, this is a critical mistake. But as time is getting very short, they will have to live with it. Meanwhile, the Manics have just realised that the coffee grinder works only in one direction. Their rallying cry - 'Come on!' - is heard for miles around and they set to work trying to avoid disaster.
The car axle they have chosen as their bearing works extremely well as it has very low friction, but they have put the blades on back to front! The Manics are not the only ones up effluent inlet, however - the Teachers have just realised that, with ten minutes to go, their whole assembly has locked solid. A last-second fix will have to be augmented by some tinkering at Beachy Head in the morning. All this and the judge proclaims that he doesn't fancy either team's chances!
Bodged-up car scrap versus high-tech flimsiness.
The next day the teams arrive on the south coast to news of a gale warning. Despite this, the Technos - in a boom-or-bust frame of mind - decide to add to their short aerofoil sails. The Manics heed the meteorological advice and cut their sails down a bit, though. Their worry is that, if the wind is too strong, there will be nothing to stop their machine shaking itself to bits - it has no in-built device to add artificial friction or to tilt the head away from fatally strong gusts.
And then it begins to rain.
Sacks of coffee beans and collection bowls at the ready, the teams start the grind that could lead them to the next round. The Teachers wind up their machine and pull the rope with all their combined might. The rotor does a passable impression of a helicopter and all but takes off. The team quickly apply their grinder and things get off to a roaring start.
The Manics, too, have a good run to begin with, but soon the wind drops and their emasculated sails find it hard to cope with this eventuality. Although they do not come to a grinding halt, the tempo slows for the embattled garage guys. The Met Office warning seems to have been for naught as, with 30 minutes to go, the wind not only drops further still, but changes direction. This does not affect the Teachers, who soldier on regardless, but the Mechanics must swivel their entire machine if it is to catch the draught.
Ten minutes to go and the wind gets up again, which allows the Manics to catch up. Apparently oblivious to their pupils' reactions when the programme is aired, Teachers Andy and Tim sing and dance like lunatics as they go about the business of winding up their rotor and grinding more coffee.
At the very end, it seems to be neck and neck as the wind at last shows some strength. Only when the two very damp teams hang their collected coffee on the official scales does it transpire that the Techno Teachers have breezed through to the next round.
The challenge: tto harness energy from the wind to power a simple windmill.
Traditional windmill.
Pros.
Cons.
Darrieus windmill.
Pros.
Cons.
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:
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%.
The beginnings.
The origins of the windmill are lost in the uncertainty of ancient time. However, there is evidence of their use at least 4,000 years ago. In the 17th century BC, the Babylonian king Hammurabi appears to have planned to use them to irrigate Mesopotamia, and there are ruins of similar machines in Iran. These mills seem to be vertical-axis machines (see Scrapheap science) with multiple vertical sails, perhaps like the pumping machines still in use in China.
In the 1st century AD, Hero of Alexandria - an Egyptian scientist (or Greek engineer - his origins are unclear) better known for designing the very first steam turbine - produced a horizontal-axis windmill-driven pneumatic organ.
Wind power in Europe.
It is said that, in the Middle Ages, the Crusaders brought windmill technology to Western Europe. This eventually led to the widespread use of wind power for grain milling and water pumping typified by the wood and canvas-sailed windmills still seen in England and Holland.
The early machines - post mills - were constructed on top of a supporting structure that rotated the entire machine so that it was in the correct direction to catch the wind. Later machines only rotated the top section. The rotation mechanisms varied from human 'push' and 'wind a handle' to mini-windmills -'fantails' driving the top around through large gear ratios.
During this period, scientific and engineering principles were being developed rationally for the first time. John Smeaton (1724-92) applied science to the design of windmills in 1759. Using models, he determined the best parameters for blade size, shape, twist, revolutions per minute (rpm), ratio of tip speed to wind speed and so on - all of which are still fundamental to wind turbine design today. He presented his results to the Royal Society in a paper entitled 'An experimental enquiry concerning the natural power of water and wind to turn mills and other machines depending on a circular motion'.
Over the centuries, windpower technology became highly developed. Large, efficient, controllable windmills were in use even after the Industrial Revolution.
The 20th century.
During the 20th century, attention turned to generating electricity from the wind. Many designs were built and tried all over the world:
1931
Yalta, Ukraine
100 kilowatt
1933
Berlin, Germany
50 megawatt
1940
Smith-Putnam, Vermont (US)
1.25 megawatt
1955
St Albans, England
100 kilowatt
This last was a very unusual design. It had hollow blades through which air was drawn into tip vents and then sucked right through the tower to drive a small high-speed air turbine in the base. This wind turbine was moved to Algeria in 1956 and ran until 1965.
The oil crisis of the 1970s and rumours of global warming and environmental damage have spurred renewed interest in wind-generated electricity. Modern wind machines have been developed with a simplicity of design and great reliability. They are now used in windfarms for large power systems, and for individual houses, remote power supplies - for example, for mobile phone transmitters - or entire villages in underdeveloped countries.
There are two basic types of wind machine: those using drag to drive the machine and those using lift.
Wind machines tend to be pigeon-holed into horizontal- or vertical-axis machines, but the real distinction should be whether the shaft is in line with the wind flow or across it - in other words, there are axial and non-axial machines.
Lift machines can be axial or non-axial, but drag machines are essentially non-axial. The shaft can, of course, be horizontal or vertical or anywhere in between.
Drag machines.
In drag machines, the force providing the movement is basically downwind and the blade is always travelling at less than wind speed. Drag machines rely on the different drag characteristics (upwind and downwind) of certain shapes - in general, some form of cup, which has more drag when the wind is blowing into the open side than when blowing on to the closed side. Alternatively, the blade can be allowed, or forced, to furl as it travels the upwind side, or is shaded by a wind break. If the shafts of these machines are vertical, then - except for the screened and forced-furling machines - they need no orientation to the wind direction.
The distinction between drag and lift machines is not sharp. In many drag machines, there is scope for some lift action as the blade crosses the wind. A special type, the Savonious rotor, has such an effect and also tries to use the reaction of the diverted wind flow through the machine to produce drive.
Drag machine calculations
A=area (m2)
V=speed (m/second)
rpm=revolutions per minute
Normally the high drag shape is going downwind, so the relative wind speed (V=wr metres/second) is the wind speed (V=w metres/second) minus the blade speed (U=metres/second).
The blade speed (U) is the mean distance from the centre of rotation (r metres) times the rotational speed in radians per second. Radians per second = rpm x 2PI/60.
So …
U = r x RPM x 2PI/60
And so downwind …
Vwr = Vw - U
The relative wind speed on the upwind side acting on the low drag face is equal to the wind speed plus the blade speed:
Vwr = Vw + U
Since the force is proportional to Vwr2, as rpm increases, the forces on the upwind low drag side will rise to be the same as those on the downwind side. There will be no more driving torque, so the speed will increase no more. This gives a rough speed-limiting characteristic to these types. Drag machines that move or rotate the upwind blade out of the wind have a very low drag coefficient in this mode and will have little or no inherent speed limiting.
The total energy from such machines can be estimated from the overall swept area potential, with an efficiency from 5 to 20%.
An overriding characteristic of drag machines is their slow rotational speed, so some form of gearing up may be necessary to get the required drive speed for the load.
Lift machines.
Aerodynamic lift is generally the force at right angles to the relative wind direction produced when air flows over a shaped body. Lift is only produced in special cases
A Flettner rotor is a cylindrical rotor placed across an air flow and rotated about its own axis to produce lift similar to that of an airfoil. However, the lift is poor and drag high compared to normal airfoils, so this is not often used. Yet a ship once crossed the Atlantic using Flettner rotor sails.
Only the relatively small forward force - the forward component of the result of lift and drag - can provide power. This force multiplied by the blade speed gives the power output per section of blade.
The ratio of blade speed to wind speed determines both the speed of the rotor and its solidity - that is, the amount of the rotor swept area that is blocked by solid blade. As a rough rule, the ratio of the tip speed to wind speed is the inverse of solidity. Another way to visualise this is that a blade should be travelling fast enough to catch all the wind passing through the gaps without catching up with the previous blade's wake.
As the tip speed ratio (TSR) increases, the relative wind angle reduces - also reducing the lift/drag resultant angle - so a low-drag airfoil becomes critical; otherwise, the forward force reduces towards nil. If the tip speed ratio is low, drag is not so critical and flat or curved plates will suffice, but lots are needed to increase solidity - hence the old western-style wind pump design!
The lift machine calculations.
The challengers have the option to go either way, but achieving a good airfoil is difficult and not likely so a low TSR design is indicated. A high TSR machine also has low starting and running torque. The power output relies on high speed:
P = T x w (torque Nm x rotational speed radians/s = watts)
High-solidity machines produce similar power from high torque and low speed.
Blade speed is worked out as for drag machines.
Both relative wind speed and angle are important. So …
Vwr = (Vw2 + U2)1/2
The angle of attack of the blade depends on the apparent wind angle - Awr=ATAN (Vw/U) - and the pitch angle (Ap): the angle of the reference centre line of the airfoil to the direction of travel.
So the angle of attack is:
Aa = Awr - Ap
The lift and drag coefficients of the airfoil depend on the angle of attack: lift increases more or less linearly to about 12 to 15 degrees, when stall starts to reduce lift and dramatically increase drag.
Airfoil design strives to increase the lift coefficient while decreasing drag, while maintaining a practical shape and, in the case of modern wind turbines, low noise. Airfoils are now designed by computers and are tested for lift and drag in a wind tunnel.
In a propeller-type machine where the blade sections are at different radii and therefore different U values, the blade can be considered in sections to determine both pitch angle and width. A rule of thumb can be used: about 3 degrees pitch angle at the tip; tip width (chord) calculated to give the total blades' width equal to the tip circumference divided by the TSR; plus a bit for luck! The blades should ideally be twisted to about 10-15 degrees pitch near the hub and about double the tip chord.
In a cross-axis machine (normally vertical axis), the blades may be all at one radius (H-type arrangement) so the calculations can be done for the entire blade in one section.
In a vertical-axis machine, the airfoil is usually symmetrical - that is, there is no curve to the central axis of the airfoil since the wind will attack it from each side as it rotates round the shaft. The blades may be fixed and the angle of attack is dependent only on the apparent wind angle. Until the blade is moving forward at reasonable speed, it will not produce any forward drive, so the machine is not self-starting. In this case, an auxiliary starting device is needed - or a 'push' start.
If the blades are allowed, or forced by a cam or similar, to rotate to the wind as they rotate around the shaft (the pitch angle needs to oscillate from positive to negative at each half revolution), the machine can be self-starting and incorporate over-speed systems.
British Wind Energy Association
www.bwea.com
The main body for the wind energy industry in the UK. The site contains information on wind energy, wind turbine technology, renewable energy case studies and an events diary, as well as an online enquiry service.
Danish Wind Turbine Manufactures Association.
www.windpower.dk/core.htm
The Danes are world leaders in the field and this is one of the best sites for general information. It offers a guided tour through 100 animated pages on wind resources, wind turbine technology, economics and environmental aspects of wind energy. It also contains a mailing list, publications and many other useful references.
Energybook
www.energybook.co.uk
Jemmett Engineering website, containing information about wind power and other forms of energy.
National Wind Power
www.natwindpower.co.uk/homepage.htm
One of the largest manufacturers of wind turbines in the UK, with 14 windfarms producing electricity. The site contains detailed information and facts about wind power from an ecological perspective.
Proven: World Friendly Energy
www.provenenergy.com
The site of Scrapheap judge Gordon Proven, wind engineer extraordinaire. It contains details of some beautifully designed projects built to provide reliable, non-polluting electricity from renewable sources.
The Scoraig Community
www.scoraig.com/
Hugh Piggot is the man behind many of the windmills at Scoraig, on the north-west Scottish coast, and has written several books on DIY windmills. On this site, he gives some basic technical advice for home-built wind power, well as links and further reading.
The Wind Works
www.users.uswest.net/~jaybo/index.htm
A US-based site from a semi-retired windsmith, with a step-by-step guide to the basic requirements for building windmills, from conception to completion.
Centre for Alternative Technology
www.cat.org.uk
Educational charity striving to achieve the best cooperation between the natural, technological and human worlds. The site contains a virtual tour guide of the centre, as well as samples of publications, booklets and factsheets on anything to do with sustainable living. They also run an online information service: ask them questions about anything from garden pests to photovoltaic power.
Country Guardian
ourworld.compuserve.com/homepages/windfarms
This pressure group believes that windfarms are bad for the countryside. Through this site, you can join the debate, for or against.
Wind Energy Training Course
www.iesd.dmu.ac.uk/~slb/wetc1add.html
De Montford University's page of useful addresses and contacts for organisations concerned with wind energy.
Windmill History
www.geocities.com/Yosemite/1001/windhist.htm
A brief history of windmills from the 12th century, plus a comprehensive list of links.
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