Wind Power

Discover the unique power of the wind

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Wind through the ages

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Discover the the unique unique power Discover power of of the the wind wind

Wind through the ages

The entrancing power of the wind has always had a captivating effect on man. For thousands of years, people have been particularly fascinated by the possibility of capturing the wind and harnessing its power. This section explains how they have been utilising the power of the wind down through the ages.

Wind in the sails The technique of using a sail to capture the wind and utilising its power for propulsion is, in principle, the same today as it was 6,000 years ago, when the first sailing vessels appeared. Sailing vessels are propelled by the differential forces created on each side of a sail when the wind blows across it. The underpressure on the rear side of the sail interacts with the overpressure on the front side to drive the vessel forwards. Today, it is believed that people had learned to tame the wind as early as in 4000 BC. At around that time, the Chinese became the first people to attach sails to their primitive rafts. Approximately 600 years later, the Egyptians launched their first sailing vessels, initially to sail the waters of the Nile. Later on, they used sailing vessels to trade along the coasts of the Mediterranean. In addition, the Viking conquests were largely attributable to their ability to build and sail their fast ships more-or-less all over the world. Since the invention of the steamship around 150 years ago, the sailing ship has largely been replaced by more efficient, machine powered vessels, particularly in the industrialised countries. Today, sailing vessels are primarily used as a popular leisure pursuit, for races and for schooling. However, in less developed countries, sailing ships still play an important role in trade, fishing and transport. Here, the wind remains a crucial resource.

Conquering the skies It seems that man has always dreamed of using the power of the wind to fly. Indeed, ancient Greek mythology features stories of people who attempted to fly like birds. In the fifteenth century, the genius Leonardo da Vinci devoted much time and energy to studying the same field. Through a series of impressive sketches and complex wing designs, he attempted to copy the wing movements of the birds. His wing designs would never have helped man to fly, but today da Vinci’s work is consid-

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ered the first scientific attempt to create a flying machine.

Up and away in a balloon For centuries, it was considered an almost irrefutable fact that in order to fly, man would have to imitate the wings of the birds. However, it was actually a bubble of air that first helped man to break the hold of gravity and ascend into the clouds. The first passenger-carrying balloon lifted off in 1783. The primitive balloon was made of canvas and “powered” by the smoke from a bonfire. Since this early experiment, balloon design has been developed and refined. Both the technology and the materials involved have developed appreciably and today, ballooning is a hobby enjoyed by people over much of the world. From the very first balloon flight, it became clear that ballooning was linked to some element of risk. Not long after the first balloon flights were completed, the parachute was invented. Quite simply, parachutes were designed to save balloon pilots who found themselves in difficulty. However, they were also used for entertainment in connection with balloon displays. Today, parachutes are still used to save lives, but parachuting has also become a popular highadrenaline hobby.

The ships of the air Although ballooning had been popular for a couple of centuries, even the most enthusiastic balloon pilots could become a little frustrated at having the wind decide the direction they were to follow. Henri Giffard took a good look at this problem, and in 1852 introduced the first airship in the world. The airship was shaped like a cigar and fitted with a small steam engine that made actual navigation possible. Airships soon became popular “air liners”, and in the 1920s they flew people back and forth across the Atlantic. However, a number of fatal crashes – including the Hindenburg disaster, in which the airship exploded, killing 35 passengers – heralded the end of the age of airships.

Gliding planes It was not until the end of the 1800s that da Vinci’s ideas about using wings to fly were made real. It was at that time that George Cayley, the British engineer, drew inspiration from a simple toy: the

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kite. His observations of kites in flight convinced him that wings could carry a human being to the skies. He made his dream come true by building the first simple glider in the world. Since then, gliders have become more and more advanced, and it is now possible to complete controlled flights. Gliding is a popular hobby today, but there can be no doubt that motorised aircraft dominate air traffic.

A flying machine The first motorised flight in the world took place in the United States in 1903. Two brothers, Orville and Wilbur Wright had spent years working to develop both their aircraft and, in particular, their skills as pilots. Their aircraft – “Flyer” – was powered by a petrol engine and on its virgin flight managed to cover just 40 metres before landing safely on the ground. The years that followed the flight of “Flyer” saw new types of aircraft being developed at a dizzying pace, and just a few years later, longer flights had already become common. For example, the first flight from France to Britain across the English Channel was completed by an elegant little plane built in 1909. At the same time, experiments were carried out with new, more creative aircraft designs involving two sets of wings (biplanes) or even three sets of wings (triplanes). As early as the end of the 1920s, aircrafts had become appreciably more streamlined. The machines were already being made of metal and flew at higher speeds, which naturally opened up a host of new opportunities. Just a few years later, the Boeing 247 was introduced; the first “modern” passenger aircraft in the world. The development of new and improved types of aircraft has fascinated flying enthusiasts ever since the first plane took to the skies. And everything suggests that people will carry on developing the aeroplane to create bigger, faster models.

The helicopter The most advanced and versatile form of aircraft is the helicopter. The first primitive version of a helicopter was developed by Juan de la Cierva, the Spanish aircraft engineer, at the start of the 1920s.

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He discovered that a rotating wing could cut through the air just like a propeller, thus pulling the helicopter upwards. The advantages of the helicopter are that it can rise vertically through the air and hover in the same place for long periods. In addition, it requires very little space to land.

Wind becomes electricity The word “windmill” makes it plain that wind power was used to mill grain. The word “mill” itself stems from the Latin word for a machine that grinds grain: molina. Many European languages contain closely related words that all have the same meaning: French moulin; English: mill; German: Mühle; and Danish: mølle. The interpretation of the word is thus closely linked to the primary task of the mill for centuries. Persian inventors drew inspiration for the windmill from looking at the water mill. They took the mill wheel as their starting point and attached 6–12 “sails” made of hide or reeds to an axle. They then attached a millstone to the other end of the axle and erected the mill on a hill, surrounding it with funnel-shaped walls to ensure that the wind was channelled towards the mill sails. This primitive yet efficient windmill model spread to other countries including China, where it is still used today.

The first windmills in Europe The first European windmills were built around 1100 and were used both to grind grain and to pump water. For the agricultural community, windmills provided invaluable assistance in grinding grain, and in the low-lying farmland of the Netherlands, mills were used to pump water away from the fields. The first windmills were erected in Denmark around the middle of the 1200s. These mills were what are known as “post mills”. They typically featured four sails consisting of a wooden frame covered with canvas. The mill house itself was placed on a rotating base, which made it possible for a group of strong men to turn the entire mill construction into the wind.

The Dutch mill Later on, people discovered that a better approach was to build mills in which only the top of the mill tower (the mill cap) could

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be turned. This type of mill – known as a Dutch mill – reached Denmark at the start of the 1700s, and in 1870 there were more than 6,000 Dutch mills operating in Denmark. The advantage of this type of mill was that it allowed the construction of much bigger mills than the old post mills, and they could also provide more power. The popularity of the Dutch mills was largely attributable to Andrew Meikle, the Scottish inventor, who developed a range of technical improvements including one that ensured that the mill cap automatically turned to face into the wind. In addition, his development of moveable wooden slats to replace the fixed construction sails made it much easier to operate these mills. Today, one of the best-preserved examples of a Dutch mill in Denmark is to be found at Dybbøl Mølle in Southern Jutland. Damgård Mølle, which is also in Southern Jutland, is another fine example of a Dutch mill.

Mills on Danish farms Originally, the large commercial mills had the exclusive right to grind grain for the farmers of the region, but in 1862, this monopoly was revoked. Farm owners were subsequently allowed to set up independent mills on their own farms. The mill quickly became a popular “tool” on Danish farms. Farm mills were not only used to grind grain; a simple system involving a perpendicular drive linked to a horizontal axle with a drive belt made it possible to use the mill to power other farm machinery such as threshing machines. On some farms, the mill was even used to pump water from the well to a container, which ensured a supply of running water to the taps. Mills thus took over a lot of the hard work of the farm, so it is no surprise that they became so popular. It is not known precisely how many mills were built in Denmark, but it is likely that in 1920, there were between 20,000 and 30,000 mills on Danish farms.

A mill to generate electricity In the winter of 1887–88, the visionary American inventor Charles F. Brush built the first windmill intended to generate electricity. It was erected in Cleveland, Ohio. This windmill was not just the first automatically operating mill that generated electricity – it was also of a truly impressive size for the time. The rotor had a diameter of 17 metres and featured 144 cedar rotor blades. The mill was located in the garden behind the Brush family man-

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sion, and, via a dynamo, generated power for the 12 batteries that supplied current to no fewer than 350 incandescent lamps, two arc lamps and three motors. This giant windmill was a peculiarity of its age and remained in operation for 20 years. However, “slow” windmills of this kind were gradually overtaken by the “fast” mills with rotor blades, which the Danish inventor Poul la Cour discovered were better for generating energy than the slow models.

Denmark’s windmill inventor Towards the end of the 1800s, Denmark joined the leading countries in windmill development for the first time. It was at this time that Poul la Cour, the greatest figure in the history of the Danish windmill industry, started to develop a range of inventions that attracted considerable international attention. Before turning his attention to windmills, Poul la Cour had already proved his skill as an inventor by developing patented solutions in the fields of telegraphy and radio. In 1878, he was given a position at Askov College of Further Education, and in 1891, he was awarded a grant to build his first experimental mill in the school grounds. With the construction of this mill, he aimed to prove his theory that wind energy could be stored by using it to separate water into hydrogen and oxygen – and then using the resultant gases to power lights and motors. In the windmill itself, the motion of the sails was to be used to power a dynamo to generate electricity. This electricity was to be led into a tank of water, which it would then separate into hydrogen and oxygen. Each of these gases was to be stored in a separate gas tank, from where the two gases were led in separate lead pipes from the mill to the college lamps.

Poul la Cour’s workshop Within a year of completing the first windmill, Poul la Cour had a new invention ready for patenting. He had developed an intricate system of weights and pulleys that could be used to “even out” the gusts of the wind to provide an even, uniform level of pressure for transferring to the dynamo. In 1896, Poul la Cour was awarded a grant to build an even bigger windmill in which he could continue with his experiments and inventions. In the spacious machine room of the giant mill, he constructed two wind tunnels. Here, he carried out experiments on as-

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pects such as the number of sails, speed of revolution and capacity. He then collated the results of his wind tunnel research in a book entitled Forsøgsmøllen (The Experimental Mill). It was this book that cemented his international reputation as a windmill inventor. Poul la Cour’s experimental mill at Askov, Denmark, still exists and is a building with a long and fascinating history. In 1902, it was made the power station for the entire town of Askov, and in 1904 it was converted into a research centre for the use of electricity in rural areas. In this capacity, it was used as a venue for courses for rural electricians. One of the teachers was, of course, Poul la Cour himself. The centre was used to teach everything from practical installation work, geometry and physics, to bookkeeping, Danish and German. Today, the experimental mill at Askov College houses the Poul la Cour Museum and stands as a manifestation of Denmark’s trail-blazing inventions in the field of windmills.

Windmill renaissance during the war During the first half of the 1900s, windmills were gradually meeting greater and greater competition from coal-fired power stations and the nationwide high-voltage grid, and many people predicted the complete disappearance of the windmill. However, the two World Wars resulted in shortages of coal and oil, so wind power found itself back on the agenda. Danish pioneering spirit and inventiveness helped the windmill to develop into an even more efficient source of energy.

The Agricco turbine In 1918, inspired by developments within the aeronautical industry, two Danish engineers – Poul Vinding and Johannes Jensen – developed a completely new type of windmill, or turbine, with blades designed on the basis of aerodynamic principles. The new turbine, the “Agricco”, which was immediately patented, featured rotating blades that resembled an aircraft propeller. In addition, the blades could be regulated to suit the wind speed and the turbine itself featured an automatic yaw system, which meant that it automatically faced into the wind.

The aeromotor During World War II, the cement group F.L. Smidth joined forces with the aircraft company Kramme & Zeuthen to develop another

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remarkable, direct current-generating wind turbine: the Aeromotor. This turbine greatly resembled the turbines we know today and was one of the models developed as a result of the increase in interest in turbines attributable to the war. The turbine tower was made of solid concrete, while the turbine blades were slim and aerodynamic.

Yet another Danish wind turbine genius When, at the start of the 1900s, Poul la Cour was teaching windmill/turbine technology at Askov College, one of his students was a young man named Johannes Juul. Around 50 years later, his interest in wind turbines and exciting inventions resulted in a turbine that would prove to be the blueprint for the wind turbines of today. Its introduction was preceded by a painstaking research project completed by the talented inventor. Johannes Juul did not limit himself to taking systematic measurements of the wind; he also built his own wind tunnel, which he used to test his theories and no fewer than around 25 different blade designs. His ambition was to produce a turbine that generated alternating current – and he wanted to connect an asynchronous generator. However, he was well aware that this would make completely new demands on generator size, blade dimensions and the speed of revolution. In return, the turbine would be auto-regulating and would stop automatically in high winds. Johannes Juul’s remarkably thorough preliminary work paid off, and when the first turbine was erected in South Zealand in 1950, it lived up to all his expectations. For financial reasons, the turbine had only two blades, but a year later, Johannes Juul added an extra blade to a similar turbine to stabilise the construction. This new turbine made it possible to utilise a much higher proportion of the energy of the wind than had been possible previously.

The blueprint for the turbines of today Finally, in 1957, Johannes Juul – aged 70 – could unveil the Gedser turbine, which became the blueprint for the turbines of today. Just north of Gedser, Denmark, a 200 kW trial turbine was installed on top of a 25-metre-high concrete tower. The turbine featured a generator and three fixed blades – stabilised with bracing wire – with rotating tips. These three principles used on the visionary Gedser turbine became the cornerstones for Danish turbines from the

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middle of the 1970s onwards. In 1962, long after Johannes Juul had retired, he presented a range of visionary ideas about the wind turbine of the future. He was convinced that the turbine of the future would be based on the Gedser turbine, but that it would be improved by the use of new materials such as plastic and fibreglass. In addition, he was sure that the Danish wind turbine sector would take a dominant position on the global market.

Large and small successes In the years after the age of wind turbine geniuses such as Poul la Cour and Johannes Juul, Danish pioneering spirit and interest in wind energy have found expression in a range of wind turbine inventions of various types. For example, in 1975 a group of teachers and pupils at the Tvind schools in West Jutland started work on the “biggest wind turbine in the world” – a project they took on without having any professional knowledge of the area. Through working relationships with engineers centred on areas such as the blade profile, this ambitious wind turbine project was completed three years later. The turbine had a blade diameter of 54 metres and the concrete tower was 53 metres high. The turbine blades were replaced in 1993, and the Tvind turbine is still operating in windswept West Jutland. It was also in the middle of the 1970s that master carpenter Christian Riisager built a very efficient three-blade turbine with a blade diameter of just 6.5 metres and a tower height of 12 metres. The design was inspired by the old Gedser turbine, and Christian Riisager applied for – and received – permission to connect his turbine to the municipal grid. A few years later, Christian Riisager started to sell his turbines, and a number of manufacturers started to design turbines inspired by the Riisager model. Towards the end of the 1970s, the working relationship involving Henrik Stiesdal, the engineer, and Karl Erik Jørgensen, the smith, resulted in the development of the HVK pioneering turbine. This efficient turbine featured a generator output of 22 kW as well as a number of the properties that were subsequently to distinguish Danish wind turbines: three “free-standing” fibreglass-reinforced blades with rotating tip brakes, an electric yaw system and two generators connected to the grid – one each for high and low wind

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speeds. The turbines developed by the two mid-Jutland pioneers – Karl Erik Jørgensen and Christian Riisager – on the basis of research carried out earlier by the sector visionaries later became known as “The Danish Concept”. The distinguishing features of turbines built according to “The Danish Concept” were a high, slim design with three fast-turning blades facing into the wind. The qualities of this model were soon recognised, and this type of turbine was then exported to most parts of the world, where it outperformed wind turbines made by some of the largest and most advanced industrial companies in the world. “The Danish Concept” became the cornerstone of Denmark’s international wind turbine success.

The Darrieus turbine Around the end of the 1970s, a West Jutland machine factory – Vestas – which, among other things, had previously concentrated on the production of agricultural trailers and machinery, started to investigate the potential of the wind turbine as an alternative source of energy. Among the early experiments at Vestas was Leon Bjernvig’s vertical wind turbine. This was a variation on the Darrieus turbine and resembled an upright egg whisk. However, the Darrieus turbine never became a success.

The strong Danish foursome At the end of the 1970s, after only limited success with the Darrieus turbine (the “egg whisk turbine”), Vestas was on the lookout for a new, efficient turbine. They found it in Henrik Stiesdal’s and Karl Erik Jørgensen’s HVK turbine, and after months of thorough testing, Vestas purchased the rights to this turbine model in 1979. The HVK turbine thus became the “forefather” of the Vestas turbine range. In the meantime, the oil crisis was also having an effect on the East Jutland company Nordtank, which had previously built tankers for the oil industry. As a result, this company started looking for a new business area and decided to start developing wind turbines. At the end of 1980, Nordtank’s first wind turbine was ready, and it was unveiled at the Ungskuet fair in Herning, Denmark, that same year. It was a robust, well-functioning turbine and featured an innovation in that it was designed with a closed tubular tower that made it possible to position the control and electrical installations

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Wind through the ages

in the tower itself. This simultaneously improved safety conditions for the service technicians, who no longer had to climb up a latticework tower to work on the turbine. Instead, they could use ladders inside the tower. In 1980, another new wind turbine manufacturer appeared on the scene. This company, Danregn, was actually a specialist in the area of irrigation systems, but the agricultural crisis had forced it to seek out new business areas. At the Ungskuet fair in Herning in 1980, the Danregn management dropped by the Nordtank stand and were inspired by what they saw. Later that same year, Danregn Vindkraft launched its first wind turbine. Danregn Vindkraft, which was based in Brande, Denmark, later changed its name to Bonus Energy A/S. Finally, in 1983, Micon – the fourth company in the foursome of successful Danish wind turbine manufacturers – was founded in Randers, Denmark. Behind this company were two brothers, Erling and Peder Mørup, who drew inspiration for their first turbine at the Agromek trade fair in Herning. In the period leading up to the new millennium, these four wind turbine manufacturers – Vestas, Nordtank, Bonus and Micon – were recognised internationally as the Danish foursome of successful wind turbine companies.

Danish turbines in California In 1980, the state of California decreed that 10 per cent of its energy in 2000 was to stem from wind power. At the same time, the power stations offered to purchase electricity for a fixed price for ten years. These initiatives triggered “wind-turbine fever” in the sunshine state. However, the Californian wind power fairytale did not get off to a good start. The first years were distinguished by damaged turbines, dubious investments and fast money. The American turbines were of suspect quality, and the wind power fairytale soon spun out of control as investors proved more interested in tax deductions than alternative energy. In 1982, three Danish wind turbine manufacturers – Vestas, Nordtank and Bonus – noticed the great potential of the American market for wind turbines, so they all sent teams to California to sell turbines. On account of the increase in demand from the United

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Wind through the ages

States, Danregn changed its name to Bonus in 1983 out of consideration the Americans, who had difficulty pronouncing Danregn Vindkraft. During the wind turbine boom in California, Vestas made contact with Zond Systems, a Californian wind power company. This resulted in a working relationship between the American and Danish companies along with contracts for Vestas to deliver turbines. Lots of turbines.

Ups and downs In the middle of the 1980s, Danish wind turbine exports to the United States received a severe blow, the result of a combination of aspects including falling oil prices, an unfavourable exchange rate and a decline in energy policy interest in wind turbines. The collapse of the American wind power boom had serious effects on the Danish wind turbine manufacturers which all, with the exception of Bonus Energy, either had to suspend payments or file for bankruptcy. Towards the end of the 1980s, the Danish wind turbine companies began to see light in the darkness again. New people joined the Boards and management teams, and this was one of the reasons why Vestas, Nordtank and Micon all got back on their feet again. At the same time, it transpired that the American market was not quite as dead as it seemed, and new markets also started showing interest in Danish turbines. In fact, the United States turned out still to be an exciting and attractive market for wind turbines. Around 1990, Vestas re-established its working relationship with Zond Systems and began to export large numbers of wind turbines to the United States once more. In 1988, Micon managed to find its feet again after several years on the verge of total collapse. This turnaround was largely attributable to two orders for turbines for a Danida project in India. Nordtank managed to recover, too. In 1987 and 1988, this company succeeded in ramping up turbine production for both domestic and overseas markets.

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Wind through the ages

The golden decade of wind turbines The 1990s turned out to be a golden decade for the wind turbine industry. The big four Danish wind turbine companies enjoyed almost explosive growth in both turnover and employment, and wind turbines became one of the leading Danish exports. In fact, the Danish companies held an impressive 45 per cent share of the global market. The success of the Danish wind turbine sector also resulted in increasing professionalisation of the companies themselves. This took the form of stock exchange flotations and numerous mergers within the sector. Nordtank became the first Danish wind turbine company to float its operations in autumn 1995. Vestas preferred to wait a little longer, before following suit in May 1998. In July 1997, Nordtank merged with Micon to create the wind turbine specialist NEG Micon.

A new millennium At the start of the new millennium, the lines were drawn for another exciting period for the Danish wind turbine industry. In 2002, the American giant GE Enron purchased Wind Corp. to create a new company, GE Wind Energy. In spring 2004, the undisputed world leader of the wind power industry was formed when Vestas joined forces with NEG Micon. Later that year, the German company Siemens purchased the Danish company Bonus to become a major player on the growing market for wind turbines under the name of Siemens Wind Power.

Sources The text is based on the following sources: *

Christopher Chant, Sejlskibe (Sailing Ships), 1992.

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www.experimentarium.dk

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Andrew Nahum, Flyvemaskiner (Flying Machines), 1991.

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Bjarne Chr. Jensen, Ballonflyvning, historie og historier, (Ballooning, history and stories) 1994.

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Per Dannemand Andersen, Risø Publications: Review of Historical and Modern Utilization of Wind Power.

*

www.dkvind.dk/fakta/fakta_pdf/M5.pdf

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Wind through the ages

*

Benny Christensen (Danmarks Vindkrafthistoriske Samling – The Danish Wind Power History Collection): Mindre danske vindmøller 1860–1980, (Small Danish Wind Turbines, 1860–1980) 2001.

*

www.windpower.org

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www.lafavre.us/brush/mansion.htm

*

Jytte Thorndahl, Fra stemmegafler til knaldgas (From tuning forks to oxyhydrogen), from Elektrikeren

*

www.poullacour.dk

*

Ib Konrad Jensen, Mænd i modvind (Men facing a headwind), 2003.

Pictures: *

Pictures of RA II by kind permission of the Kon-tiki Museum, Oslo, Norway.

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Picture of glider by kind permission of K. Krøjgaard.

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Pictures of biplanes and the Ciervo C.30 (autogiro) by kind permission of the Danish Air Force History Collection, Karup Airfield, Denmark.

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Picture of the Dutch mill (Damgård mill) by kind permis sion of Christen Poder.

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Picture of Brundby Post Mill on Samsø by kind permission of www.moellearkivet.dk

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Picture of Charles Brush with the kind permission of Westers Reserve Historical Society, Cleveland, Ohio, USA.

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Pictures of Poul la Cour and the experimental turbines by kind permission of the La Cour Museum.

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Pictures of Johannes Juul and the experimental turbine at Gedser by kind permission of the Electricity Museum, Bjerringbro, Denmark.

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Pictures of the farm mill (Heeager), the Agricco turbine, the Aeromotor and the Darrieus turbine by kind permis sion of the Danish Wind History Collection.

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Picture of the Tirstrup turbine by kind permission of the Tistrup-Hodde Parish Archives.

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Pictures of the Tvind turbine and the HVK turbine by kind permission of Benny Christensen, the Danish Wind History Collection. > 14

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How does wind arise?

This section explains how the wind arises and describes the weather conditions suited to the erection of wind turbines.

Meteorological rules In order to understand how the wind arises, it is important to know some rules of physics that apply to the field of meteorology: 1. Cold air is heavier than warm air 2. The wind blows from areas of high pressure to areas of low pressure 3. High pressure is formed when the air is cooled and sinks down through the atmosphere (cf. rule 1) 4. Low pressure is formed when the air is heated and rises up through the atmosphere (cf. rule 1) 5. The rotation of the Earth deflects the wind to the right in the northern hemisphere and to the left in the southern hemisphere (known as the Coriolis effect).

The sun generates wind Imagine that an area of the Earth is heated by the sun. On account of the non-uniform nature of the Earth, this area will be heated more than the areas surrounding it, so the air immediately above it will start to rise. When air rises in this manner, a vacuum-like state is created close to the surface of the Earth, because the pressure in this area starts to fall. The surrounding area will, however, try to balance out the difference in pressure between the heated and non-heated areas by moving cooler air into the vacuum. If the sun is strong enough to maintain its heating effect – and thus to continue the rising of the air – wind will be generated. The ability of wind turbines to generate energy is naturally dependent on wind. The following sections explain which weather conditions are favourable for wind turbines, and describe how these weather conditions arise.

Weather conditions for wind turbines When erecting wind turbines, it is important to be fully familiar with the local weather conditions to ensure that the turbines installed generate as much energy as possible. Normally, wind turbines are installed:

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How does wind arise?

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in places where a local wind blows frequently,

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in zones where extratropical lows often pass, or

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in zones where trade or monsoon winds blow.

Local winds Wind is created through pressure differences in the atmosphere. The greater the difference in pressure, the stronger the wind can become. Local weather systems are often caused by differences in the heating of the Earth’s surface by the sun. One example of this is sea breezes which, in the summer months, can arise over land close to the sea or a large lake when the weather is clear and calm. When the sun heats the Earth’s surface, the air close to the surface is heated and rises – and the wind starts to blow in from the sea or the lake. If the air rises high enough, it will be cooled to such an extent that it may form clouds or even rain showers. Towards the end of the afternoon, when the heating by the sun decreases, the wind stops blowing and the clouds disappear. At night, the wind can turn so that it flows from the land towards the sea (land breeze). This often occurs on still, clear nights when the heat radiated by the Earth can pass almost unhindered through the atmosphere to space. When the Earth radiates heat, the surface cools down, rather like a patch of exposed skin in a cold room, or a wood-burning stove when the fire has gone out. The air closest to the surface is also cooled, as it transfers some of its heat to the soil. If the process continues long enough, the air above the land will finally become colder than the air above the sea – and a land breeze will set in. (The sea also radiates heat into the atmosphere, but here, the mixing of the waters almost completely negates the fall in temperature near the surface). Mountain and valley winds are other examples of local wind systems created by solar heating. These winds arise in mountainous regions in clear weather. When the sun heats up the slopes of the mountains during the day, the wind begins to flow up the slopes and up through the valleys as hot air naturally rises. At night, when the mountains are cooled by the radiation of heat into the atmosphere, the wind changes direction and flows down the slopes and down through the valleys.

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How does wind arise?

Winds that arise locally because of solar heating are known as thermal winds. Local winds attributable to the shape of the landscape (orography) are known as orographic winds. Mountain and valley winds are both thermal and orographic. Areas subject to local wind systems make good sites for erecting turbines. When planning wind farms, a lot of work is done to find precisely the places where the wind blows most strongly – on mountain peaks and crests, for example. However, places where the wind gusts can be so strong that they can actually damage the turbines are naturally to be avoided. For more information about where it is most profitable to install turbines, see the section entitled “Where are wind turbines erected?”, which you can access via the main menu.

Extratropical low pressure systems The generation of wind energy is not exclusively limited to areas with local wind systems. Most of the wind turbines in the world are sited in what are known as the westerlies – the broad zones north and south of the tropics where the wind typically blows from the west and large passing lows and storms (also called extratropical cyclones) determine wind and weather conditions. Around such low pressure systems there is plenty of energy for wind turbines to exploit. In the southern hemisphere, the zone of the westerlies has been named “The roaring forties” on account of the very strong winds that blow here. Westerly winds and extratropical lows occur because the sun heats the Earth differently at different latitudes. In the low latitudes, solar heating is generally stronger than the cooling attributable to the radiation of heat to space. In higher latitudes, the reverse applies. Extratropical lows occur as waves in the zone – known as the polar front – that separates hot and cold air. Due to the rotation of the Earth, winds do not blow directly towards areas of low pressure, but are deflected so that they blow around these areas – anticlockwise in the northern hemisphere and clockwise in the southern hemisphere. This is known as the Coriolis effect (cf. rule no. 5).

Trade winds and monsoons Closer to the equator, tropical and subtropical wind systems – the trades and the monsoons – dominate. The trade winds blow across the sea from the subtropical areas of high pressure to be found

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How does wind arise?

around latitudes 30º north and south of the equator, and in towards the area of low pressure in what is known as the intertropical convergence zone close to the equator. The rotation of the Earth deflects the wind to the right in the northern hemisphere (the north-east trades) and to the left in the southern hemisphere (the southeast trades), cf. rule no. 5. The monsoons are thermal winds on a large scale. They blow in from the sea across the subtropical continents in the summer, and in the other direction in the winter. The countries of South-east Asia and those around the Indian Ocean are particularly affected by the monsoons – the south-west monsoon in the summer and the north-east monsoon in the winter.

The shape of the landscape The shape of the landscape has a significant effect on the strength and stability of the wind. The more uneven the landscape, the more unstable the wind. In this context, we are referring not only to the large-scale formation of the landscape with mountains and valleys (the orography), but also to the small-scale unevenness of the surface (the roughness). An area of woodland or a builtup area will be rougher than an open field, which, in turn, will be rougher than the surface of the sea or a lake. The rougher a surface, the more it will hinder the wind by creating more friction. Therefore, the wind blows more strongly over the sea than across the land; and more strongly over open land than in wooded or built-up areas. For additional information about orography and roughness, see the section entitled “Where are wind turbines erected?”, which you can access from the main menu. When erecting wind turbines, it is best to choose a site where the wind can blow freely over the turbines from all directions. That is why turbines are typically erected away from towns. To generate the most energy, it is best to erect the turbines at offshore sites – but this is a more complicated and costly process.

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How do windturbines work?

Wind turbines use the energy in the wind to generate electricity. This section traces the route energy follows from the wind itself, through the turbine and out into the grid – and then on to households in the form of electrical current. It also describes how turbines regulate their output to prevent overloading in high winds.

The main components of a wind turbine Wind turbines consist of four large main components: a foundation unit, a tower, a nacelle (turbine housing) and a rotor. In principle, the foundation unit takes the form of a giant concrete block buried in the earth. The nacelle is positioned at the top of the tower, and the rotor is attached to the front of the nacelle. Click the picture to the right to build your own wind turbine. Click the turbine foundation to call up the various components. The principal task of the tower is to raise the nacelle high into the air because the wind speed – and thus the power of the wind – is much greater 50–100 metres above the ground. The tower is also used to guide the cables from the nacelle down to the electrical grid in the ground. The nacelle contains the large primary components such as the main axle, gearbox, generator, transformer, control system and electrical cabinet. The rotor consists of a hub to which three blades are attached.

From wind to current Wind turbines use the power of the wind to generate energy. This happens when the blades on the rotor capture the wind, which makes them turn. When no wind is blowing, the turbine will adjust the blades to an angle of 45º, which is the position in which the turbine can draw as much energy as possible from gentle winds. The blades begin to turn very slowly, without generating any energy. This is known as “idling”. When there is sufficient wind for the turbine to start generating energy – normally at wind speeds of around 4 metres per second, the blades will gradually start to rotate longitudinally towards an angle of 0º, which means that the broad surface of the blade is facing into the wind. When the wind then strikes the blade, it generates overpressure on the front surface of the blade and underpressure on the reverse. In other words, the wind pushes onto the front surface and simultaneously generates a suction effect across the rear surface – and it is this difference in pressure that makes the rotor turn. Wind turbines typically gener-

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ate energy at wind speeds of 4–25 metres per second. When turbines are generating electricity, the rotor speed will be 9–19 revolutions per minute, depending on the wind speed and the turbine type. At the maximum speed of revolution, the blade tips reach a speed of 250 km/h.

Blades and wind speed Click the picture to the right to see the relationship between wind speed and the position of the blades.

The nacelle components The wind thus causes the rotor to turn, converting the energy in the wind into rotating, mechanical energy. This rotating, mechanical energy is channelled to a gearbox in the nacelle. From there, the energy flows to a generator, where it is converted into electrical energy. The purpose of the gearbox is thus to convert the slow speed of rotation of the blades into the high speed of revolution of the generator. This conversion is performed at a ratio of 1:100, which means that if the blades are rotating at a speed of 15 revolutions per minute, the generator will rotate at 1500 revolutions per minute (depending on the type of turbine). Through this process, the generator converts mechanical energy into electrical energy.

Connection to the grid The electrical control system in the turbine links up the generator, leading the electrical output generated through a high voltage transformer to the grid, which supplies current to households. In just 2–3 hours, a V90-3.0 MW turbine can generate enough electricity to cover the annual consumption of an average Danish household. This means that in a year, a turbine of this type can cover the electricity requirements of around 3,400 Danish households.

Yaw Wind turbines are designed to ensure that their rotors always face into the wind. This process is controlled by a wind vane positioned on the top of the nacelle. This instrument determines the direction of the wind – just like a weather vane. When the wind changes direction, a contact is activated in the wind vane, initiating the motors that turn the turbine into the wind. This is known as yaw. Turbine blades can also “pitch” – i.e. turn on their longitudinal

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How do windturbines work?

axes so as to adjust to the wind speed. This ensures that the blades always capture as much of the power of the wind as possible, thus generating as much energy as possible. Wind turbines are designed to function optimally in wind speeds of 4–25 metres per second. In other words, turbines will always reap the maximum amount of energy from the wind at wind speeds within this range. The volume of energy a wind turbine can generate depends on factors such as the size of the generator, the dimensions of the rotor and the strength of the wind. For example, a V903.0 MW turbine, which has a rotor diameter of 90 metres, starts to generate power in wind speeds as low as 4 metres per second, and achieves its maximum power output (3 MW) at 15 metres per second. When the wind speed reaches 4 metres per second, the angle of the blades will be 0º so as to ensure that the turbine draws as much energy as possible from the wind. When the wind speed reaches 10–12 metres per second, the blades will rotate longitudinally away from the wind slightly to prevent the turbine generating more energy than its components are dimensioned for. This is known as output regulation.

Output regulation There are three ways to regulate output: 1) Passive stall: The turbine operates with a constant speed of revolution and has non-adjustable blades. In this case, aerodynamics will force the blade profile to stall, i.e. to generate turbulence which limits uplift and thus stops the turbine drawing energy from the wind. This will occur at wind speeds in excess of 12–15 m/s, depending on the turbine type. 2) Active stall: The turbine operates with a constant speed of revolution but has adjustable blades. In this case, the turbine regulates output by turning the rear edge of the blades into the wind to produce a stall effect at wind speeds in excess of 12–15 m/s. 3) Pitch: There are two types of pitch-based output regulation: - Pitch: The turbine operates with a constant speed of revolution and has adjustable blades. In this case, the leading edge of the blade is turned into the wind to reduce uplift. -

Variable speed pitch: The turbine operates with a variable

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How do windturbines work?

speed of revolution and has adjustable blades. In this case, the leading edge of the blade is turned into the wind to reduce uplift. The turbines in the Vestas range use only variable speed pitch and active stall to regulate output.

Shut-down in high winds If the wind reaches speeds in excess of 25 metres per second, the turbine stops because such speeds place too much strain on turbine components. At the same time, wind speeds only rarely exceed the stop limit, so there is little need to generate energy from winds blowing at higher speeds. It would therefore be prohibitively expensive to design a model that could handle such high wind speeds. When wind speeds exceed 25 metres per second, the blades pitch to 90º, which means that the leading or rear edges of the blades (depending on the output regulation principle applied) point directly into the wind. This makes the blades function as giant air brakes, slowing the turbine down until it comes to a complete stop.

Vestas technologies The technologies Vestas uses for output and generator regulation are: Active Stall®: a hydraulic active stall technology that ensures that the rotor captures the maximum amount of energy from the wind while simultaneously minimising load on the turbine design and controlling turbine production. This technology is used in the V821.65 MW turbine. OptiTip®: a microprocessor-controlled pitch regulation system that constantly adjusts the angle of the blades to the optimal position in relation to the prevalent wind. This technology is used in all the turbines from the Vestas range other than the V82-1.65 MW model. OptiSlip®: a generator system that makes possible a variation of up to 10 per cent between the speeds of revolution of the blades and generator in the event of powerful gusts of wind. In addition to minimising load on the turbine components, OptiSlip® also contributes to a significant improvement in power quality. OptiSlip® turbines are also fitted with OptiTip®. The V80-1.8 MW turbine is the only model in the current Vestas range to use OptiSlip®. OptiSpeed®: a development of the OptiSlip® technology. OptiS> 22

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How do windturbines work?

peed® allows the rotation speed of turbine blades to vary by up to 60 per cent, thus optimising energy generation – especially at modest wind speeds. In addition, OptiSpeed® makes it possible to adjust noise levels to match local requirements. As the variable speed of revolution reduces load, the OptiSpeed® system minimises strain on the gearbox, blades and tower. OptiSpeed® turbines are also fitted with OptiTip®. OptiSpeed® technology is used in all turbines in the Vestas range except the V82-1.65 MW and V80-1.8 MW models.

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Windturbine projects

From start to finish, wind turbine projects can be divided into three main phases: a sales phase, a project phase and a service phase. This section presents an overview of some of the activities that take place in each of these three phases.

The sales phase In the context of the sale of a wind turbine project, the initial contact between the customer and Vestas may be established in different ways. For example, the customer may contact Vestas directly, or the project may be put out to tender. However, before any wind turbine project can be implemented, the authorities must grant approval.

Direct contact Some customers prefer to work with specific manufacturers and therefore contact them directly. This preference may be based on factors such as the manufacturer operating local production, being a leading player within the sector, or simply because the customer enjoyed a good working relationship with the manufacturer during previous wind turbine projects.

Tenders When a wind turbine project is put out to tender, the customer wishes to receive tenders for the execution of the project from several different manufacturers. There are two types of tendering: open and closed. In open tendering – also known as public tendering – manufacturers contact the customer who has put the wind turbine project out to tender. In contrast, the closed tendering approach involves the customer inviting selected manufacturers to bid for the wind turbine project. This approach may, for example, be chosen because only the selected manufacturers have the necessary technological competence, or because the turbines must be of a given size due to the conditions at the site in question. It may also be chosen because there is no direct requirement for open tendering, which generally costs more than a closed process. When putting a project out to tender, the customer prepares a set of material, which contains the information the manufacturer needs to prepare a tender for the project. This information may,

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Windturbine projects

for example, comprise technological requirements, a description of the conditions at the site where the turbines are to be installed, delivery schedules and the like. Tendering processes are run according to fixed procedures. For example, all the tenders from the various manufacturers have to be delivered to a specific address by a specific time on a specific day. Once all the tenders have been submitted, the customer chooses a supplier. In the same way as the fixed procedure for submitting tenders, there are rules governing the deadline by which the customer is to make the choice and inform the preferred supplier. Once the supplier who has won the tender – and thus the contract – has been informed, the actual contract negotiations can begin.

Negotiating the contract When negotiating a contract, Vestas and the customer lay down the conditions that are to be included in it. For example, these may include the project price, terms of payment and delivery, as well as a range of technological conditions such as tower type, tower height, monitoring system and so on. Contact negotiations are often protracted and can sometimes take several years to conclude. The duration of the negotiations depends on factors such as the size of the project, whether the customer is a new or existing customer (who will already be familiar with the process), and the options for financing the project.

Choosing the type of turbine It is crucial to the profitability of the wind turbine project that the customer choose the type of turbine best suited to installation at the site in question. In order to establish what type of turbine is best suited to the site, it is necessary to study information about the wind conditions and the features of the landscape at the site. For additional information about choosing the type of turbine, see the section entitled “Where are wind turbines erected?”, which you can access from the main menu.

Permission from the authorities National authorities have a lot of influence on the implementation of wind turbine projects. Before the project can be implemented, the customer bears the responsibility for applying for – and receiv-

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Windturbine projects

ing – all the necessary permits from the authorities. The authorities lay down requirements concerning aspects such as the height of the turbines and their positioning in the landscape. The authorities also issue construction permits to the customer, define the criteria for connecting the wind turbines to the national grid, and lay down the safety requirements. For example, all the foundations for turbines erected in Taiwan and Japan have to be designed according to the legislation pertaining to earthquakes.

Close collaboration with the customer Throughout the sales process, Vestas works closely with the customer – so closely, in fact, that the working relationship can best be described as a partnership. Vestas has a network of agents, sales companies and offices that covers the entire globe. This helps ensure that Vestas has in-depth knowledge of local conditions on the various markets and, in particular, is fully familiar with the culture in the customer’s country. For this reason, sales staff are always firmly linked to a limited number of markets to help them build up the best possible knowledge of the local conditions and culture. In the sales phase, the sales staff naturally play a central role. Their task is to help the customer with, for example, information about the product and the site, and to inform the customer about financing options. The intention here is to boost confidence in Vestas and Vestas’ products and, at the same time, to assist the customer in collecting the required approvals and finding the necessary financing. The overall aim is to ensure that the customer’s project can be implemented as efficiently as possible.

The project phase Once the contract has been negotiated and signed, the sales phase draws to a close. The project then moves into the actual project phase, which comprises everything from logistics and transport to the erection and commissioning of the turbines. The project phase is distinguished by stringent requirements for planning and flexibility.

Transfer to the project manager The transfer from the sales phase to the project phase is marked by a hand-over meeting and a kick-off meeting. The sales team and

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Windturbine projects

the project manager participate in the hand-over meeting. Very often, the project manager will have been involved in the final stages of the sales phase, and at the hand-over meeting, the project is formally transferred to the project manager. The purpose of this meeting is to examine all the significant, practical details concerning the project. These typically include: •

finances

•

schedule

•

division of responsibility (for what areas are the customer and Vestas each responsible?)

•

subcontractors

•

logistics

•

transport

•

special agreements, if any

After the hand-over meeting, the project manager is “equipped” to take control of the project and can start to collect information and initiate assignments. The kick-off meeting involves the sales team, the project manager and the customer. At this meeting, the customer and project manager are formally introduced. The meeting also marks the start of the planning for the subsequent stages of the project phase, and is used to define the practical facilities that need to be established for the service technicians who are to work at the site. This involves, for example, setting up the site office, telephone lines, ADSL connection, toilet facilities and, possibly, a canteen. After the kick-off meeting, the work on the project phase begins in earnest.

Transport The first period of the project phase focuses largely on aspects such as logistics and transport. Vestas’ logistic department is responsible for ensuring that all the turbine components (nacelles, blades, hubs and towers) are ready for transportation to the site on time. Vestas’ transport department has ultimate responsibility for organising and co-ordinating the transportation of the turbine components from Vestas’ production facilities to the site.

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Windturbine projects

The entire process is monitored by the project manager, who coordinates input on the basis of reports from the logistic and transport departments. The project manager will often be present at the dispatch of a large number of wind turbines to make sure that the components are loaded properly to prevent damage in transit. As a general rule, Vestas is responsible for the transportation of the wind turbines from the production facilities all the way to the site. Depending on the location of the site, the components are typically transported by lorry, ship or train. Visit the Vestas Cinema, which you can access from the main menu, to see a film about how wind turbines are transported.

Preparing the site Before the turbines arrive, the site is prepared to receive the various components so that the work to erect the turbines can start as quickly as possible. The cranes and lifting equipment must be in position, and the foundations – which were laid at the site in advance – must have hardened. If the project is what is known as a “turnkey project” – i.e. a project in which Vestas is also responsible for establishing the infrastructure at the site – Vestas’ subcontractors will have started work at the site many months before the turbines arrive. The reason for this is that it is often necessary to make comprehensive changes at the site. For example, transformer stations have to be installed, new roads have to be laid, and cable connections have to be established. It is essential that the large, heavy lorries that transport the turbine components can access the site via a solid road network. In addition, the electricity cables have to be buried and led up through the foundations. The site itself is not the only thing that has to be prepared for the arrival of the turbines. The site personnel have to get ready, too. They must be thoroughly prepared for the task and informed of all the significant details of the project. Preparation of the site personnel is quite simply crucial to the success of the project. Erecting the turbines When the turbine components arrive at the site, the erection work itself can begin. It is not a good idea to wait until all the turbine components have arrived before starting the erection work. On the contrary, the various turbines are erected as soon as the rel-

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Windturbine projects

evant components arrive. It is essential to ensure that the work is performed as efficiently as possible, as there are appreciable costs linked to having cranes and other lifting equipment at the site. Firstly, the tower, which consists of several sections, is installed on top of the foundations. Then the nacelle is hoisted to the top of the tower, and finally, the hub and blades are fitted to the nacelle. The rotor (the hub and blades) can be lifted into position as a complete unit, or the hub can first be fitted to the nacelle, with the blades subsequently being hoisted one at a time and connected to the hub. When a turbine has been erected, the work is far from finished, as the cable work still has to be completed. Visit the Vestas Cinema, which you can access from the main menu, to see a film about how wind turbines are erected.

Connection to the grid As the turbines are erected, they are consecutively commissioned and made ready for connection to the grid. Before the turbines are handed over to the customer and officially connected to the grid, they are subjected to a thorough test phase which involves operating without error for a set number of hours and then being connected to the grid for a specific period (the number of hours may vary according to the terms of the contract). Once all the turbine tests have been completed successfully, what is known as a Take Over Certificate (TOC) is issued and the project can be handed over to the customer. The TOC serves as documentation that the customer has received the project as agreed in the contract.

Challenges Various challenges can arise during the project phase. That is why the key concepts of any wind turbine project are planning and flexibility. A great many factors are involved, and they must all combine to create a coherent whole. Unfortunately, not everything can be controlled. For example, the weather plays a significant role. If the wind turbines are to be transported by ship, the weather largely determines whether the turbines arrive on time. The weather also exerts its influence during the erection phase: if the wind is too strong, it is not possible to raise the components, and work has to be stopped

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Windturbine projects

for a while. This means that it is essential to remain flexible and to “go the extra mile” while the weather is good. Other factors that can delay projects include applying for driving permits for the heavy transport vehicles, and clearing customs at the border – which sometimes takes a long time. Therefore it is essential always to think ahead and to be ready to change plans to avoid major delays in the work.

The service phase The service phase begins when the wind turbine project has been handed over to the customer. During this phase, the service department makes sure to monitor the wind turbines and maintain contact with the customer. Before the service department formally takes over responsibility for the wind turbine project, a meeting is held between the project manager and a service manager. At this meeting, the wind turbine project is examined in detail so that the service manager is thoroughly briefed on the progress of the project to date and on all the significant details of the project.

Service visits A number of service visits are performed at regular intervals during the 2-year warranty period that applies to Vestas turbine models. During each visit, the service technicians follow a set procedure. In fact, they have a checklist to follow. During service visits, the technicians tighten all the bolts, lubricate the rotating parts (the generator and blade bearings, for example) and check to see whether any parts need replacing. As a part of all service visits, the technicians check the wind turbine blades and take an oil sample from the gearbox for subsequent analysis. Technicians conclude all their service visits by cleaning the wind turbines internally. During final service visit, the service technician performs a complete examination of all the wind turbines in the project to make sure that none of the turbines contains any defects when the warranty period expires.

Training service technicians Vestas trains its own service technicians, all whom have to complete a comprehensive training course before being allowed to work on installed turbines. All technicians receive the same training to

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Windturbine projects

ensure that the service visits are always performed in the same way and always meet Vestas’ standards – no matter where in the world the turbines may be located. During the training course, technicians acquire in-depth knowledge not only of wind turbine technology, but also of safety regulations. The technicians have to pass a course which involves, for example, learning how to climb around in turbines in a safe and responsible manner. Safety is extremely important – particularly when a service technician is working on top of an installed nacelle. As all Vestas service technicians have completed the same training course, they can all work on any site. Very large sites will often have a group of technicians permanently attached. The number of associated service technicians depends on the size of the site, and some sites have teams of up to 20 technicians. The advantage of having technicians permanently linked to large sites is that it allows these technicians to build up in-depth knowledge of both the turbines at the site and the site itself. In addition to the technicians who live and work on individual markets, Vestas employs a number of travelling service technicians. These technicians travel around to work on sites all over the world. Travelling service technicians spend much of their time working away from home. Some can spend up to 250 days a year abroad. The advantage of employing travelling service technicians is that it improves flexibility. As a general rule, travelling service technicians work on small sites that do not have a permanent team of technicians. They can also travel to sites where extra manpower is required for a set period of time. Even though travelling service technicians are always on the move, great emphasis is placed on close contact with the Vestas service department.

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Where are wind turbines erected?

The implementation of a wind turbine project is always preceded by months of measurement of factors such as wind speed and wind direction at the intended site. This section explains the factors used to determine whether a site is suitable for wind turbines, and those that define the type of turbine used.

Measuring wind resources When establishing whether an area is suitable for installing wind turbines, it is naturally essential to make sure that there is sufficient wind. The first step is to find out whether there are any data available from previous studies of the wind in the area, or whether there are any existing wind reports including maps of the wind resources at the site. It is also a good idea to speak to local residents, who often have a good sense of the wind conditions in the region. Once it has been established that an area has reasonable wind potential, one or more measuring masts are erected, depending on the size of the planned project. These masts are typically 40–80 metres high, with measuring equipment installed at 3–5 different heights. As a general rule, to obtain the best results the measuring masts should be as high as the turbines are expected to be. The actual measurement of the wind resources is carried out by several wind meters (cup anemometers), which are attached to the measuring masts at different heights. The primary intention here is to measure the wind shear at the site. “Wind shear” is an expression for the ratio between the increase in wind speed and the increase in height. It is important to measure wind speed at various heights to make it possible to calculate how much it increases. This measurement is used not only to calculate how much energy the turbines will generate, but also to establish the loads to which the turbines will be subjected.

Measuring wind direction In addition to measuring the wind speed, it is also essential to establish the direction from which the wind typically blows. What are known as “wind vanes” are used for this purpose. These are instruments that function according to the same principles as a weather vane. The anemometers and wind vanes are connected to a battery-powered data logger that processes the data and stores them on a mini-

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Where are wind turbines erected?

hard disk. Wind data are measured at regular intervals, typically of 10 minutes. It is recommended to take measurements for at least a year in order to collate sufficient information to make it possible to calculate the mean annual wind speed. The wind speed measured is presented as a function of the wind direction in a circular graph (a “pie chart”) divided into twelve sections of equal size. This diagram is also called a “wind rose” and illustrates the wind speed, the directions from which the wind blows, and the dominant direction. The wind direction is principally used to determine how the turbines are to be positioned in relation to each other. The necessary distance between the turbines and rows is, in fact, heavily dependent on not only the wind speed, but also the wind direction as it is important to ensure that the turbines do not generate turbulence or block the wind for each other.

Ensuring profitability Measuring the wind for a year thus helps define the annual mean wind. This is the value that primarily provides the basis for calculating how much power a wind turbine will be able to generate. However, the annual mean wind can vary greatly from year to year – by up to ± 20 per cent – which translates, as a rule of thumb, into variation in energy generation of approximately double that figure, i.e. ± 40 per cent. Such a large margin of uncertainty would result in serious problems in calculating the profitability of the project. This, in turn, would make it very difficult to find the financing required. Therefore, it is common to use long-term data from a reference mast, which measures wind conditions over a 20-year period. Using data from such a reference mast, it is possible to calculate the average wind speed for the entire 20-year period. When data from the measuring masts overlap those from the reference mast, the annual mean wind for the specific year is corrected to the average wind speed so as to make it possible to forecast the mean wind for the coming 20 years. This is known as long-term correlation. In Denmark, a reference mast could be one of the DMI (Danish Meteorological Institute) weather stations that have been set up all over the country. In other countries it could be one of the measurement masts set up by the public authorities or installed in connection with an airport. Wherever such masts are available,

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Where are wind turbines erected?

attempts are made to access the relevant data so as to minimise the risk of incorrectly forecasting the production of a future wind farm or, as a worst-case scenario, installing the wrong type of turbine for the site resources. Data from a reference mast often reduce the level of uncertainty linked to calculations based on information from just one year of measurements.

Wind conditions for each turbine It is important to establish the wind conditions not only around the measuring mast itself, but also in every single place where a wind turbine is to be erected. In order to determine the areas richest in energy at the projected site, it is necessary to draw up a “windstream field”, otherwise known as a “wind resource map”. To do this, the WAsP calculation tool can be used. This tool not only uses information from the measuring mast, but also takes into account factors that affect the wind – such as the roughness of the terrain (plants, trees, buildings, etc.) and the orography of the region (the contours of the landscape in the form of hills and/or mountains). The difference between “orography” and “roughness” is that “orography” focuses on the contours of the landscape, whereas “roughness” is centred on everything that is built on or grows on the landscape. Together, roughness and orography produce what is known as the topography (the surface shape) of the area.

Roughness As from a height of around 1,000 metres above ground level, the wind is not affected by the conditions on the ground, but the closer to the earth the wind comes, the more it is affected and slowed by uneven features of the landscape such as buildings and trees. Roughness is defined according to what are known as “roughness categories” that run from Class 0 (sea surface) to Class 4 (high, dense woodland or large cities with skyscrapers). For additional information about roughness, click the following link: www.windpower.org/en/tour/wres/shear.htm

Orography The wind is also affected by the orography (contours) of the landscape. Generally speaking, the wind blows more strongly at higher altitudes, so it is often best to position wind turbines on the peak or crest of a mountain. When the wind blows over a mountain crest,

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Where are wind turbines erected?

the wind is often subject to compression. This compression triggers a “speed-up” effect – also called an acceleration – which means that even a wind that is gentle at the foot of a mountain can become very strong at the peak. However, it is important to remember that the energy in the wind is also dependent on the density of the air – and that density decreases as altitude increases. When the density drops, so does the amount of energy in the wind, which is a factor that is mainly a problem in high mountains. In Denmark, the National Survey and Cadastre publishes a contour map of the landscape which the WAsP calculation program converts into a 3D flow domain. The WAsP calculation program thus uses information from three sources to predict wind speeds at a given site – data from the measuring mast, information about the roughness of the area and a contour map converted into a 3D model. Using this information, the program can simulate wind conditions at every single spot on the site, making it possible to calculate the energy production of each turbine. In parallel with the measurement of wind conditions at the site, work is also done to answer the following questions: 1. What are the options for grid connection? Will it actually be possible to deliver the power? Are there any connection options relatively nearby? (As a rough rule of thumb, a project can accommodate the construction of one kilometre of grid connection cable per installed MW. However, this depends to a great extent on the area, the project itself, etc.) 2. Is the project profitable, and are there enough investors? 3. Will the local and national authorities issue the permits necessary to install turbines in the area?

Choosing the type of turbine Numerous parameters come into play when choosing the optimal type of turbine for a specific site. The most common are listed below: 1. Are there any local restrictions regarding turbine height, noise levels, nature conservation and the like? 2. Does the turbine meet requirements from the authorities (IEC, for example)?

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Discover the the unique unique power Discover power of of the the wind wind

Where are wind turbines erected?

3. Is there any risk of earthquakes, typhoons or other extreme, external influences? 4. Is it physically possible to transport the turbine components to the site, or does the infrastructure place limits on turbine size? 5. Is it possible to access the required crane capacity locally, or will cranes also have to be transported long distances to the site? The turbines have been designed in accordance with international wind turbine standards to ensure that they are products of international quality. In fact, the turbines have been designed according to standards laid down by GL (Germanischer Lloyd) and IEC (International Energy Center) – two almost identical approval authorities. The IEC standard comprises four categories that divide the turbines into different levels which reflect the design loads of the turbines – or, in other words, how much the turbines can withstand during their 20-year service lifetimes. The table below lists some of the principal parameters that apply to turbine design. Please note that there are many other parameters of significance to the overall load calculation.

IEC Category

Expected mean wind (20 years) (m/s)

Expected 50-year wind gusts (m/s)

IEC S

According to manufacturer’s design parameters

According to manufacturer’s design parameters

IEC1 IEC2 IEC3

<10 <8.5 <7.5

<70 <59.5 <52.4

Expected 50-year wind 10-minute average (m/s) According to manufacturer’s design parameters

<50 <42.5 <37.5

IEC S: stands for “site specific”, which means that the turbine is approved for a specific project, taking into account the factors applicable here, and not according to the other fixed categories. This category requires new approval for each project, in contrast to the other approvals which apply to all sites that meet the requirements for the other categories.

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Discover the the unique unique power Discover power of of the the wind wind

Where are wind turbines erected?

The principal reason why IEC 1 turbines are not simply used for all sites is that they are over-dimensioned for many sites, which, for example, means more expensive components. Using IEC 1 turbines for projects involving sites with less strong winds would, for example, result in the projects being less profitable or even unprofitable. Using the right type of turbine for each site makes it possible not only to reduce costs for the project but also to cut noise levels and improve production from the turbines.

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