Windmills are here to stay!
By Doug & Dilys Griffiths - 31/01/2011
Mention the word ‘windmill’ and we, of the older generation, probably hold a mental picture of Dutch windmills; while our younger generation would immediately visualise a present day wind farm.
Love them or hate them, windmills are here to stay. It may surprise you to know they’ve been around for about 5000 years in one form or another and records show of their existence for helping in the manual effort for grinding cereals or raising water levels.
There were mills in 2000 BC in Syria & Lebanon, which were powered by wind-filled cups supported on a vertical shaft. These curved cups rotated well, irrespective of the wind direction and replaced the work of men or animals
The Chinese record a similar sort of machine at an even earlier time in or near to 3000 BC. Ship sails were used which billowed out with the wind for one half of the rotation then collapsed flat on their return. These early mills were for raising water to irrigate the land. Small curved cups can still be seen in use today whizzing around, attached to arms on top of boats or on towers, for measuring wind speed.
A change in design was noted in about 700AD. Drawings of windmills in Crete show canvas sails being carried on a fixed horizontal axis. The vertical axis didn’t return until the mid 20th century
Nowadays, provided we are interested, we are able visit historic ‘post mills’ or ‘tower mills’. The first post mill appeared around the 13th Century, in Germany. It was designed for the complete body of the structure to be turned, by hand, into the wind.
The ‘tower mill’ design probably came from the Netherlands, a century later. This design was very different, in that the top of the mill carried the sails and only the top of the structure rotated originally, by hand, into the wind. Typical of a seafaring nation, this moveable top closely resembled an upside-down boat.
Doubts about the origins of the tower mill continue to be held today, because of the presence of a stone tower mill at Hauville in France. This is recorded as being erected circa 1220 and was still in use there in the 19th century.
In the middle ages, the mills were the property of the Lord or Squire of the manor. He employed a miller and villagers would take their grain to the mill to be ground. Payments were made in grain. The miller was a much distrusted person and the poet Chaucer wrote: “His was the master hand at stealing grain. He felt it with his thumb and thus he knew its quality-and took three times his due.”
The big turning point toward today’s Windfarm really began in the eighteenth century, when the Royal Society considered wind energy seriously. They published work done by John Smeaton on the angling and shape of sails This was the forerunner of modern aircraft and windmill propellers.
Smeaton, together with mathematician Maclaurin, worked out that a sail radius of 30 feet and a 7 degree twist could give a power output equal to 18.3 men or 3.7 horses! This strange calculation, natural in those times of men and horse labour, is recognised today as 44horsepower or 30kW.
These 18th century calculations are considered to be amazingly accurate even today. The demise of the traditional windmill was eventually brought about by the advent of steam motive power for grinding and raising water. Most old windmills became redundant by the early 20th century.
We think it’s an amazing fact that in 1891, in a town called Askov, in the small country of Denmark, a university college should be inspired to build an experimental power station, driven by wind. They called it a Wind Turbine, because it generated electricity. This remarkable achievement went unnoticed. Other countries lacked their initiative, resulting in no real growth until 1950’s – 60 years later!
No great developments took place with electricity producing mills until the 1960’s Experiments were made to produce larger outputs from wind generation and powers of 100kilowatts were developed by using experimental styles of blade shapes.
New wind-turbine ‘sails’, developed by international countries, changed into shapes similar to aeroplane propellers. Power output could be altered, by varying the pitch of the blade. Blade tips now reached speeds of over 200m.p.h. and required stronger, lighter materials.
Demands to increase power output resulted in the establishment of Wind Farms, which have to conform with local planning requirements, ecology and their closeness to human habitation. It was discovered that the total wind energy over the British Isles was sufficient for only about 20% of the demand. Wind farms can also be noisy, so location, relative to habitation, was critical. In order to achieve the most of available wind, account has to be taken of possible ground surfaces increasing wind drag. Items such as rocks, shrubs, small hills, trees, buildings, will all increase wind friction and reduce power. Hence the reason for tall towers to carry the blades and keep them away from the earth’s influences. When towers are sited in remote areas, the required cable connections can be very costly - as much as two million pounds per mile! Maintenance, also, is very expensive for isolated locations
At the turn of the 20th century, the wind turbine had blades with a small 1kW generator situated behind them on top of the tower. By 1960 the towers had grown and the generator, housed in a little cubicle at the top, produced powers of 100kW or more. Currently, the blades can span 50 metres with a portacabin sized cubicle, holding the 4/5Megawatts generator and control gear, – all swivelling around with the wind direction. Compare this output with that of an old fashioned 19th century windmill of 25kW and muse over the fact that a modern, large wind farm, can generate about 1000Megawatts or more.
If you’re interested in statistics :-
- Globally, wind generation is increasing by 10Gigawatts every year.
(in case you don’t know, 10Gigawatts= 10,000Megawatts =10,000,000,000watts or 200.000 kettles kept on the boil) - Britain’s present power is approximately 3000Megawatts and aims to have around 34,000Megawatts by 2020. We consider this to be a very optimistic target.
Since there is insufficient land space to accommodate all the necessary mills, suitable coastal sites are being developed for large wind farms on the sea. The problem with wind generation is that the power depends on Mother Nature. This problem can be overcome by linking all the wind farms (including all countries in Europe as well as Britain), to a giant Grid. The possible development of this linking between all countries from Russia to Ireland is already on the planning boards.