energy transition

Once upon a time... Gedser's windmill...

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Together with the ecological reasons that have favoured the rebirth of wind energy, the different directions taken by man to build his future have been done without really respecting the environmental balance. The recent surge in wind power reveals the whims of an energy, very much in its own image, which has always obsessed man, who, in the course of the various economic and industrial revolutions, has been able to abandon its use in order to resume it later. Paradoxically, it is at a time when man seems to have definitively severed his link with nature by cloning it, and can no longer part with it, that he must think globally about embracing its elementary principles if he wants to preserve his chances of survival. Example with the wind turbine of Gedser.
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L’he proposed artifact is an electricity generating wind turbine that can be considered the mother of modern wind turbines. It was designed by Johannes Juul, a Danish electrical engineer, in the 1950s, in the previous century. Within the ARIAD/R project, J.Juul's innovation meets various criteria. It has undergone several cycles of resurgence and today it has regained a certain legitimacy.
In addition to a historical refocus on wind energy and the Danish specificity in this field, this article presents the originality of the prototype in various contexts - economic, industrial, cultural and social. It also allows the recognition of its creator, the rehabilitation of his invention and the description of the Danish model of alternative energy at a time when questions about environmental protection are raising many questions at the global level, particularly following the COP21 in Paris in the fall of 2015.
 
 
In Greek mythology, Aeolus, King of the Winds, viceroy of Zeus, lives on land in the Aeolian Islands, an archipelago of seven islands northeast of Sicily, Italy. Eole has four main winds: Boreas, the north wind (in Latin Aquillon); Zephira, the west wind (in Latin Favonius); Notos, the south wind (in Latin Auster); and Euros, the east wind (in Latin as in Greek). In the Odyssey Homer tells that Aeolus, appointed guardian of the Winds by Zeus, as he left the island of Cyclops with his armada of ships, presented Odysseus with a leather bag in which he enclosed all the storm winds. Although the bag was completely sealed, the curious crew thought they discovered gold in it and opened it. The Winds escaped from it. They unleashed a furious storm that forced them, after perilous days, to land in the land of the Lestrygons populated by giant cannibals who destroyed all but one ship, set back from the land, where Odysseus was. During the Homeric epic, much later, Juno, Queen of the Gods, again resorted to Aeolus to drown his Trojan rival Aeneas.
 
We will understand, through these stories, that Aeolus, king of the winds, also knows, despite all predictions, how to be capricious, and sometimes decide the worst, to be in the end strategist source of many trials. This is, in this case, one of the reproaches that the detractors of wind energy today, called "wind of wrath", make to him by claiming that one cannot trust Eole and, consequently, count on his energy. When the wind is too weak or when it is too strong, Eole imposes the rest of the operation of the machines, which make use of its almighty power, and which cannot be trusted. Today, ever more distant from our distant roots, victims of a certain carelessness, we will not object, with Eole, to this harmony of the universe, this respect for nature, until now the guarantor of our environment. This debate is crucial and, in this sense, the Gedser wind turbine, since its erection in the 1950s, with its successive rebirths, is a strong symbol of this.
 
First Egyptian sailing boats around 2000 BC
 
If the Gedser wind turbine is the result of a historical tradition that dates back to the 19th century, we have to go much further to appreciate the different uses men have made of wind energy. The boats with sails are probably the first use of this energy. Historians date the first sailing boats to 3000 years before Christ. Windmills are also very old. Some wind turbine historians imagine their existence more than 3000 years ago. The oldest documented windmills are found in the Sistan region, an arid region of Iran and Afghanistan. The mill tower of Sistan is characterized by its vertical axis of rotation taking the wind in a vertical slot in the wall of the tower. The windmill with a horizontal axis of rotation is that of the canvassed wings, inspired from the Arab dhows that went up the rivers. Clearly, there is a relationship between the principle of sailing and the energy action of windmills. The wind steering mechanism of the sensor is applied to the dhows' sails and is obtained by rotating the balsam mast around the vertical axis of the mainmast. This principle was adopted in the first windmills.
 
Detail of an illumination showing a windmill in the 13th century
 

But it was at the time of the mechanical revolution, in the 13th century, when great technical changes were made - the birth of the camshaft (which allows the synchronisation of the movement of a part), of the connecting rod (which allows a force to be transmitted to the joints), that we can mention the exploitation of wind energy. The old wind turbine is presented as a mill-cabin with a mobile body pivoting on its axis. The mill consists of four blades that rotate around a horizontal axis. Historians point out that, from the time they first appeared in the West, windmills were highly appreciated by peasants, because the lords imposed taxes on the use of the watercourses that flowed through their lands. But Eole exempts them from these taxes by substituting its energy with that of the oceanids, because during feudalism, no legislative status applied to wind power. By its legal independence and its low cost, the windmill thus embodies this anti-seigneurial offensive. Even if the establishment of a windmill is judged as an encroachment and condemned as such, the exploitation of wind energy is therefore started and nothing will stop it.
Two centuries later, according to studies made on cartularies, building deeds, visit reports and tax documents, there are seventy-two thousand windmills in France, and probably as many, in all proportion, in other European countries. Proof that wind energy is not of today, even if it is experiencing an unprecedented golden age at the beginning of the 21st century.
 
If the first windmills were mainly used to grind grain, they were then used to pump water, water men and animals and irrigate the land. Also used for various agricultural and artisanal activities - making oil, paper, pumping sea water - building up the Dutch territory by polding an extra quarter of its surface, windmills can be used to saw wood, or to sharpen all kinds of materials with grinding wheels.
 
18th Century Canada Windmill - © Parks Canada Agency / Agence Parcs Canada, 2004.
 
Towards the end of the 18th century, windmills were the equivalent of the electric motor in pre-industrial Europe. They provided about 1,500 megawatts, a level that was only later reached in the late 1980s. Also in the 18th century, settlers from Europe imported their technical know-how to Canada and erected the first windmills in North America. The modern mechanical windmill, also known as a "pre-wind turbine" or pumping windmill, was developed in the United States by its creator, Daniel Halladay. It consists of a mast or support pillar, blades, a rotor and operates in total autonomy thanks to its rudder. D.Halladay patented his autonomous mill in 1854. It consists of six sections with eight wooden blades, a forty-eight-bladed wheel, similar to a propeller turbine. He sells his resistant and efficient wind turbines to industrialists and farmers taking advantage of a fast-growing market. Although these models are used to drive water pumps for irrigation, they already represent a high-performance kinetic mechanical prototype that will be used on a large scale during the industrial revolution.
 
Wind turbines of Poul La Cour
 
The first industrial wind turbine was built in Denmark in 1891, thanks to its inventor, Poul La Cour(1846-1908), meteorologist, engineer and electrician and future teacher of Johannes Juul. By combining the mechanical energy generated by the wind with the production of electricity, we enter the era of the aeromotors or wind turbines (called windchargers in the United States) and the wind industry. Although these power-generating machines are roughly identical to pumped-storage wind turbines in terms of operation - the principle of slow multi-blade rotation - certain characteristics related to electricity, such as energy storage, make them distinct artefacts. It should be noted that the evolution of the wind industry goes hand in hand with that of electricity. One of the obsessions of the Danish pioneers was to transform mechanical energy into electrical energy, and to store it for transport.
 
One cannot mention the invention of the Danish engineer Johannes Juul, (1887-1969), without the invention(s) of his professor Poul La Cour. Not only because La Cour's pedagogical and scientific skills pushed his future apprentices towards innovation and guaranteed notable developments in the field of modern wind turbines. But also because La Cour reflects the particular philosophy of the Danish spirit, its interest in education and the local economy based on the experience of the rural environment and a virtuous stubbornness to find solutions adapted to the local community. In this respect the Askov school is a model of alternative education.
At the beginning of the 20th century, the country was short of electricians. The Court offered its services to Askov Folk High School to meet this need and trained the first country electricians or "wind electricians". Juul, whose parents were against traditional schooling, enrolled at the Folk High School in Askov and received his certificate at the age of seventeen in 1904 on the occasion of the first graduation. All the local trades are represented there, farmers, blacksmiths, carpenters, and thus contribute to making it possible to manufacture ingenious, inexpensive, efficient and robust wind turbines.
 
Poul La Cour, as early as 1895, built his own electrolysis generator. The wind energy will be converted into hydrogen and oxygen and then stored in tanks. The mixture of the two components will then be used to power the gas light of the Askov school. In order to improve the efficiency of the wind turbine and to better cover the useful wind surface, La Cour then works on the aerodynamic efficiency of the rotor, its speed and lift, i.e. the rotating part of the turbine, the propeller. He built two wind tunnels in his laboratory as experiments and concluded that the number of blades should be reduced and the rotation speed should be fast. In 1897 he designed a six-bladed wind turbine with a conical rotor to light the town of Askov. Although efficient, the rotor was too heavy and did not meet rural needs, he preferred to opt for the manufacture of smaller turbines in line with local demand. Under his initiative, the Danish Wind Electricity Association (DVES) was created, bringing together farmers, politicians and industrialists to participate in the development of wind turbines in the country.
 
In 1918 wind energy accounted for one third of electricity production in rural areas in Denmark. This was the first golden age of wind power, which was also due to the shortage of fuel and other expensive fossil fuels (coal) during the First World War. After these years, the price of oil fell and the growing popularity of the combustion engine greatly reduced the general interest in wind energy. However, important scientific and technical developments in the fields of mechanics, meteorology and aeronautics were taking place at that time.
Two factors thus encourage the Danes to pursue fundamental research in the wind sector: know-how in the field on the one hand, and very favourable wind conditions on the other.
 
Agricco Wind Turbine
 
Johannes Jensen and Poul Vinding's Agricco aerodynamic turbine model, designed by Johannes Jensen and Poul Vinding, already in 1920, showed productivity gains far superior to La Cour's Lykegaard - 43 % of energy for Agricco against 23 % for Lykegaard. Not only did they adopt the latest aerodynamic innovations of the aeronautics industry with a certain flexibility, but they also responded to the new challenge, alternating current and connection to the electricity grid. However, during the inter-war period, these wind turbines were probably too far ahead of their time, hence the aptly named "Lykegaard Wind Turbines". "The right wind mill - the wrong time" - which could be translated as "the perfect windmill - adverse circumstances". They are thus disappearing from the market in favour of smaller turbines that meet lesser ambitions such as local electricity production, water pumping, lighting or other agricultural tasks.
 
However, the wind power industry was given a second wind during the Second World War for the same reasons as the first one, shortage and high cost of fossil fuels, coal and oil in particular.
 
Wind turbine F.L. Smith built in 1942 on Bogoe Island - Denmark
 
In May 1940, the industrial company F.L. Smith & Co (FLS), specialized in cement and other machinery, built a wind engine with a sixty kW generator coupled to a two-bladed rotor with a gearbox. It is mounted on a twenty-four-metre high concrete tower which, under the effect of the vibration frequency of the tower, then identical to that of the lower blade, begins to crack. To avoid this problem, four lateral edges are added to the tower to reinforce the foundation. The Danes built twelve to two wooden blades and seven to three wooden blades for direct current power generation. Depending on the site, the annual production varies from 30,000 kWh to more than 80,000 kWh for the two blades and 90,000 kWh to 135,000 kWh for the three blades. Two units last more than 15 years without any problems, among them the mill on Bogoe Island where Juul works for the South East Zealand Electricity Company (SEAS). It is this prototype that will inspire the design of the Gedser wind turbine.
 
Although Juul started early in wind energy and electricity by studying with Poul La Cour, he worked first as a self-employed electrician and then at the SEAS, where he was head of installation equipment from 1926. A member of the Danish Engineers Association in 1940, he returned to wind energy after the Second World War, almost fifty years after his debut with La Cour, when he realised that wind energy would have an important role to play in the future, based on the lessons learned from the Second World War - fragility and energy dependency, unstable cost of electricity tariffs.
 
He then began research on alternating current (less energy is lost than with direct current, which optimizes the transmission of electricity), where Agricco engineers Vinding and Jensen had left them in the 1920s. In 1947, he designed an asynchronous motor controlled by an electrodynamic system. It should be noted that the asynchronous generator makes it possible, in principle, to withstand slight variations in speed and therefore to better adapt to irregularities in the wind, which can weaken or increase at any time. In addition, these speed variations generate significant mechanical stresses on the system. Their control also ensures greater robustness. The asynchronous generator also allows better management of electricity transmission by adapting the intensity of the energy received with that of the network.
 
Juul's dilemma will therefore be first to design a self-regulating industrial wind turbine. Juul's experiments over the next two years involved studying the winds he made with modern turbines in makeshift wind tunnels that he built himself, like his mentor Poul La Cour, on three-blade, two-metre models. In all, twenty-five profiles were completed. For the regulation and braking of the rotor, Juul adopts the stall principle. Above a certain wind speed the blades, slightly twisted and cut into three sections, root, middle and tip, gradually stall, each in turn, to reduce the efficiency of the rotor. The generator starts like an ordinary engine, when the blades have reached a sufficient speed, the engine works like a generator, and when the wind speed increases, it reaches its maximum power (limited today to ninety km/h). The combination of the braking effect of the generator and the aerodynamic stall of the blades regulates the turbine at high wind speeds. In order to ensure the limitation of the rotor bearing, Juul invented an additional means of braking, the rotation of the blade tips (tip brake), which he used for the first time on the Gedser wind turbine.
 
Juul designed his first aero-engine in 1950 in Vester Egesborg with his company SEAS. For economic reasons he uses two leeward blades, experiments with different generator powers - ten and fifteen kW. The tests show that his wind turbine uses nearly 60 % of wind effect, which is the maximum possible with reference to the Betz's Law on the shaping of the wings (1920), and according to which the kinetic energy of the wind can be exploited to a maximum of 59.3 percent. It integrates into this concept the principles of self-regulation developed a year earlier.
Its design, which takes into account the aerodynamic characteristics of the blades, the electrodynamic principles of the generator and the variations in wind speed, is completely new in 1952. Juul continued his research on Bogoe Island. In 1952 he replaced the two-bladed FLS aero-engine of 1942, which produced the island's electricity in direct current, with a three-bladed turbine equipped with an alternating current generator connected to the network. The result was convincing. Despite a smaller rotor diameter, the aeromotor produces 80,000 kWh annually, three times the output of the two-bladed FLS, partly due to the three blades and the high capacity of the grid. Juul filed the patent for its control system the same year and participated in international conferences to present its tests and what would become known worldwide as the Danish concept.
 
In 1957, the last wind turbine of Juul, named "the venerable one", was erected in Gedser at the southern tip of the island of Falster in the south of the country particularly famous for its winds. It was built with the help of the "Wind Power Committee" under the auspices of the Association of Danish Power Plants (DEF). The Danish Ministry of Labour is providing a Marshall Plan fund for the implementation of the project with a total of half a million Danish kroner, about seventy thousand euros today. This budget also includes the installation of three test meteorological stations to measure wind speed at three Danish sites, including Gedser.
 
In many aspects, the Gedser wind turbine takes over and improves on the characteristics of the Bogoe wind turbine - foundation, structure, nacelle (rotor, brake, AC asynchronous generator). It is designed by Professor Ramboell and built by Larsen & Nielsen Constructor A/S of Copenhagen. The tower of the wind turbine, made of cement and with four folds flared downwards, gives it a distinguished silhouette in haute couture style, hence its nickname today of "the tower of the wind turbine". "Old lady". It is twenty-five meters high, its rotor is three-bladed, self-regulated, its asynchronous generator of two hundred kW in alternating current connected to the electrical network. Its three blades look more like airplane wings, moreover the locals more readily use the term "wing" to evoke them than "blade". They are made of metal and wood and each has ailerons that pivot at the ends as a brake.
 
Gedser Wind Turbine
 
In order to respond to the high wind speed and to avoid an accident, the fate of many wind turbines of that time that flew away in the first storm, Juul used a system of wiring and ropes to stabilize the "wings". The design of the Gedser wind turbine undoubtedly suffers from these improvised adjustments. Nevertheless, this "do-it-yourself" aspect has to be recognized as a Danish claim. It represents what characterises the country's success in this energy sector. It is thus, thanks to an applied learning process, combining talent with ingenuity, but also strong experimental requirements, that the Danish model owes its current leadership status in wind energy. In this sense, the Gedser wind turbine symbolises the properties of the Danish model: reliability, safety, robustness, efficiency and profitability.
 
The Gedser wind turbine produced electricity for ten years, from 1957 to 1967, without any operating problems. It is used as a test to measure the wind electricity production for the first five years. Then, for the next five years, these energy production tests are unfairly compared to those of large thermal power plants in order to justify energy choices in their favour. Successive governments during this period, then in the midst of productivist euphoria, focused primarily on the most profitable, cheapest energy sources, in this case also those that pollute the most and in a carefree manner. At that time, the price per barrel was exceptionally low. In spite of the very promising performance of the Juul prototype, the country abandoned wind energy production, as it did not consider it sufficiently economical in view of the results. Juul protests by defending prophecy-like convictions: wind power would allow the country to store electricity reserves; job creation; economic profits in exchange with neighbouring Scandinavian countries (Sweden and Norway) producing hydropower; and the export of its wind power know-how to other European countries. But at the time of his statements, no one could predict the economic crises or the need for electricity, and it was far too early to anticipate the serious damage caused by the indiscriminate exploitation of fossil fuels or other nuclear energy. The Gedser wind turbine stopped working in 1967, following a gearbox failure. Juul died that same year, without any further recognition during his lifetime.
 
When the oil crisis hit in 1973-74 and shook the major powers, the Gedser turbine, which held a record of ten years of uninterrupted activity, experienced its first resurgence. At the time, it was the only large-scale prototype producing electricity from alternating current wind power. The Danes, already innovators, were once again interested in this type of energy. The productive output and robustness of the Gedser wind turbine also encouraged the Americans to financially assist the Danes in renovating the model. The Energy Research and Development Administration (ERDA), the Department of Energy (DOE) and the National Aeronautics and Space Administration (NASA) are involved. The Gedser wind turbine will finally be operational in 1977, with the financial support of these institutions. From 1978 to 1979, studies were undertaken under the supervision of the National Research Centre for Renewable Energy (RISOE), the Maritime Institute and the Technical University of Denmark (DTU). This rehabilitation project allows the establishment of tests to measure the speed, resistance, safety and reliability of the electrical system in order to develop the wind turbines of the future.
 
As a result of these tests, the report of the Technical University of Denmark (DTU) was published in 1980. It compares three more modern wind turbines - the "Nibe A" built by the Danish Ministry of Energy and the Danish Electricity Company; the "Mod-OA" built by the US Department of Energy (ERDA); the Kalkugnen wind turbine of the Swedish National Department for Energy Resources Development - with the Gedser turbine built more than twenty years earlier.
The results show equivalent performance for all four models, despite the older technology of the Gedser wind turbine, especially with regard to the efficiency curve of the wind system and the extreme robustness of the manufacturing. On the basis of these analyses, the report concludes by pointing out the fundamental principles necessary for the design of modern wind turbines: a three-bladed rotor facing the wind horizontally, blades with pivoting tips, stall control, and an asynchronous generator. It should be noted that by the end of the 1970s, the norm will also be to build fibreglass blades, something Juul, a visionary, had already envisaged as early as the 1950s, but was refused.
 
Compared to the very high-budget research and development programs in the United States and Germany, countries that are also innovative in wind energy, both the Danes and the Dutch have always proceeded differently: starting from experience on small machines in a favourable environment to move on to more ambitious projects. Thus, rather than soliciting the aeronautics community and the large electricity industry, the Danes have created teams dedicated to this sector with laboratories of worldwide expertise, notably the RISOE and the DTU mentioned above, which switched from nuclear to wind power as early as 1978 in the midst of the oil crisis and anti-nuclear protests.
It was at this time that RISOE, with the help of NASA, studied the Gedser wind turbine to build a six hundred and thirty kilo Watt turbine, the "Nibe A", according to its model, in 1979. It was also at this time that Christian Riisager built a small seven-kilowatt wind turbine, which he coupled to the domestic electricity grid without permission, and a group of students and anti-nuclear activists built the highly successful two-megawatt Tvind prototype. Finally, it was during this period that the North West Jutland Institute for Renewable Energies (NIVE) was established, bringing together engineers, blacksmiths and professors, actively collaborating with the two thousand small and medium sized blacksmith companies and other craftsmen who saw the huge economic potential of the (re)emerging wind industry.
 
From 1975 to 1979, of the twenty-three manufacturers specialising in wind turbine construction, only three remain in Denmark today, forty years later: Vestas Wind System, Siemens Wind Power (formerly Bonus) and LM Glasfiber. The first two are positioned at the forefront of the wind industry today. These companies are still using the concepts developed in Denmark from 1975 to 1979, having adapted them to recent technologies. As early as 1979 Vestas, then a small agricultural machinery workshop since 1898, a victim of the crisis in the agricultural sector, turned to the wind energy market. It became the economic leader for more than forty years. In 2015 it was joined by its German competitor Siemens and the American General Electric, which recently acquired the French group Alstom. But Vestas is regaining its leadership in offshore installation which, today, represents the key market of the wind energy sector, although it is being followed by a Chinese market which is now very dynamic, both partner and competitor with Goldwind which stole the first place from it last February.
 
After a further period of inactivity and abandonment from 1980 to 1993, and an attempt to preserve the site with the cultural authorities, the nacelle and 'wings' of the Gedser wind turbine were dismantled. The different parts are transported to the Energy Museum in Bjerringbro in Jutland, some 400 kilometres from the site. They will be preserved until they are restored more than ten years later. Thanks to the efforts of the museum's management and the financial support of Power Company E2, the original Gedser turbine, although incomplete (two blades out of three and without a tower), can be seen again in 2005. The tower of the Gedser mill was stripped of its original wind turbine and its original "wings", was given a new nacelle and new blades, and was put back into service in 1993, after fourteen years of shutdown, on the initiative of the municipality and the owner of the land. The new wind turbine is installed on the old foundations and continues to produce electricity to this day.
 
It has therefore taken several energy crises, most often caused by global conflicts, for ecological awareness to emerge. Finally, we accept that if our destiny is to produce energies that contribute to the improvement of our living conditions and promote our comfort, we must act in harmony with nature, without destroying the planet. The Gedser wind turbine, designed before this revelation, but reborn thanks to it, thus recovers a form of resurgence that is quite symbolic. It cannot be denied that it feeds this burst of consciousness today. At a time of global warming, the world is wondering, not without a certain fear, about depleting energy resources and the reliability of nuclear energy. Mankind is questioning its responsibility for the climatic disasters that are occurring at an increasingly sustained rate: greenhouse effect, glacial melting, tsunami, exceptional flooding of rivers, etc.
 
According to our field of study, the Gedser wind turbine represents a particular type of object, which has not found its place in the system of objects because of the competition of the energy market it encountered at the time of its design. Denmark's leadership in wind energy is linked to the Danish spirit that gave birth to the Gedser wind turbine, its inventor, but also its predecessors and successors. A Danish historian concedes three Danish skills: learning through systematic research, practical experience and the interaction in innovation of all actors and institutions involved. The Gedser wind turbine embodies a memory, a technique and certainly a soul. It represents the transition from one type of energy to another. Today it is also the struggle to maintain a cultural symbol, the Danish response to the famous quote : think global, act localIt is important to "think globally, act locally". Classified by the Danish Ministry of Culture since 2006 as one of one hundred and eight works in the list of Denmark's cultural canons - which includes artistic and technical works - the Gedser wind turbine is not protected. The current owner of the wind turbine site is now actively campaigning for its recognition by UNESCO and its inclusion in the World Heritage List. This publication is fully committed to supporting this action which would show its ultimate resurgence.
 
Frank PecquetUniversity of Paris I Panthéon Sorbonne
 
Photo: The Gedser wind turbine, the reference model for modern wind turbines.
 
Bibliography :
"Analysis of data from the Gedser wind turbine 1977-1979" Lundsager, Per; Frandsen, Sten Tronæs  Christensen, Carl Jørgen, Editor Risø-M; No. 2242, Technical University of Denmark, 1980, 144 pages. In English. Translation of the title "Analysis of Gedser Wind Turbine Data 1977-1979.
"Gedsermøllen - den første modern vindmølle, Jytte Thorndahl. Editor Elmuseet, Bjerringbro, 2005 99 pages, in Danish. Translation of the title "Gedser wind turbine - the first modern wind turbine".
"Gedser Wind Turbine - Mother of all wind turbines" Editor Gitte Ahrenkiel, Publisher Books on Demand, Copenhagen, 2015. Translation of the title " Gedser Wind Turbine - Mother of all wind turbines "
"Hamilton, Edith, Edition Marabout Université, Verviers, 1978, 414 pages.
"L'énergie éolienne - du petit éolien à l'éolien offshore" Marc Rapin; Jean Marc Noel, Editions Dunod, 2nd edition, 2015, 340 pages.
"Les premiers moulins à vent" Robert Philippe, CNRS publication in Annales de Normandie, 32ᵉ year, n°2, June 1982, pp 99-120.
"Wind Power, The Danish way: from Poul La Cour to Modern Wind Turbines" Editor Benny Christensen, Publisher The Pour La Cour Foundation, Askov 2009, 88 pages. Translation of the title "Wind Power - the Danish Model".
 

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Carré Jean
5 months

Propellers have their establishment requirements. Let's look at the different aspects. Speed: The speed of a propeller cannot be zero, because being the product of a force by its displacement, the energy would be zero. A conversion would take place, however, the wind entering the propeller straight in, would exit downstream in rotation. The practical losses of 100% in this situation decrease as the propeller speed increases. Practice has shown that a speed at the periphery of the propeller in the range of seven to eighteen times the wind speed appears to be the most advantageous. Beyond this range, the mechanical stresses are too high, costly and… Read more "

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