For thousands of years, mankind has been fascinated by the challenge of mastering the wind. The dream of defying Aeolus (Greek god of the wind) and taming the might of the storm held generations of inventors under ist spell. To attain limitless mobility by using the forces of Nature, thereby expanding the horizons of the then known world, was a challenge even in antiquity. Thus, sailing and shipbuilding were constantly pursued and developed despite doldrum, hurricane, tornado and shipwreck. Progress could only be achieved by employing innovative technologies. These, together with an unbridled lust for voyages of discovery, built up in the minds of sovereigns and scholars a mosaic of the world, the contours of which became ever more enclosed as time went by.
With wind-harnessing technology on land and on the sea, potentials could be realized and works undertaken that far outpaced any previously imagined bounds. For example, only the power of animals and of the human arm, it would never have been possible for the Netherlands to achieve the drainage that it has through wind-powered pumping and land reclamation.
Archaeological discoveries relating to the use of wind energy predate the beginning of the modern era. Their origins lay in the Near and Middle East. Definite indications of windmills and their use, however, date only from the tenth century in Persia. The constructional techniques of the time made use of vertical axes to apply the drag principle of wind energy capture. Such mills were mostly found in the Arab countries. Presumably, news of these machines reached Europe as a result of the Crusades. Here, however, horizontal-axis mills with tilted wings or sails, made their appearance in the early Middle Ages.
The use of wind energy in Western Europe on a large scale began predominantly in England and Holland in the Middle Ages. Technically mature post mills and Dutch windmills were used mostly for pumping water and for grinding.
Historical Dutch windmill
© Bundesverband WindEnergie e.V.
More than 200 000 (two hundred thousand) of these wooden machines were built throughout North-West Europe, representing by far the greatest proportion of energy capture by technical means in this region. At the beginning of the twentieth century, some 20 000 (twenty thousand) were still in use in Germany.
From the nineteenth century onwards, mostly in the USA, the so-called „western wheel“ type of turbine became widespread. These multibladed fans were built of sheet steel, with around 20 blades, and were used mostly for irrigation. By the end of the 1930s, some 8 million units had been built and installed, representing an enormous economic potential.
American windmill used for water pumping
© Bundesverband WindEnergie e.V.
The first attempts to use a wind turbine with aerodynamically formed rotor blades to generate electricity was made over half a century ago. Since then, besides the design and construction of large projects in the 1940s by the German engineers Kleinhenz [1] und Honnef [2], the pilot projects of the American Smith-Putnam (1250nominal output, 53rotor diameter, 1941), the Gedser wind turbine in Denmark (200nominal output, 24rotor diameter, 1957) and the technically trail-blazing Hütter W 34 turbine (100nominal output, 34rotor diameter, 1958) are worthy of mention.
The German constructor Allgaier started the first mass production of wind power plants in the early 1950s. They were designed to supply electricity to farmsteads lying far from the public grid. In coastal areas these turbines drove 10 kW generators; inland they were fitted with 6units. Their aerodynamically formed blades of 10 diameter could be pitched about the longitunal axis so as to regulate the power taken from the wind. Even today, some of these turbines are in operation with full functionality, after more than 50 years of service.
After the 60s, cheaper fossil fuels made wind energy technology economically uninteresting, and it was only in the 1970s that it returned to the spotlight due to rising fuel prices. Some states then developed experimental plants in various output classes.
In particular in the USA, Sweden and the Federal Republic of Germany, turbines with outputs in the megawatt class have attracted most attention. Here, with the exceptions of the American MOD-2, with five units and the Swedish-American WTS-4 with five or two units, large converters such as the German GROWIAN, the Swedish WTS-75 AEOLUS model, the Danish Tvind turbine and the US MOD-5B variants in Hawaii were all one-offs. Despite many and varied teething troubles with the pilot installations, it was clear even then that technical solutions could be expected in the foreseeable future that would permit the reliable operation of large-scale wind turbines. Second-generation megawatt-class systems such as the WKA 60 and the Aeolus II have confirmed this expectation.
Mainly in the US state of California, but also in Denmark, Holland and the Federal Republic of Germany, considerable efforts were being made, independently of the development of large turbines, to use wind power to supply energy to the grid on a large scale. In the 1980s, wind turbines with total capacity of around 1500 MW were installed in California alone. In the initial phases, turbines of the 50categories were used. Scaling-up the systems that were successful through the 100, 150 and 250 kW classes and the 500/600 kW order of magnitude has led to wind farms with turbines in the megawatt range.
This development has made the mass production of wind turbines possible. A considerable improvement of performance can thus be achieved. Progressively increasing turbine size using designs of widely differing types and costs has led to the development of machines in the 500and megawatt classes that are remarkable for their high availability and good return-on-investment potential.
The individual manufacturers have chosen very different routes to market success in relation to this trend. NEG Micon has retained the classic Danish stall-regulated turbines with an asynchronous generator rigidly coupled to the grid in the power classes up to 1.5Bonus, Nordex and Vestas as well as GE/Tacke have altered their turbine configuration in the different size classes, particularly with regard to the turbine regulation (stall or pitch) and generator systems (fixed-speed or variable speed with a thyristor/IGBT frequency converter). Currently 3 to 5 MW systems from all well-known manufacturers are being operated as prototypes or are available on the market.
To some degrees, companies that have entered into the production of wind generators at a later stage have been able to draw upon existing developments and techniques, thus allowing their first efforts to overtake the system of established manufacturers. DeWind started ist development with a pitch-regulated 600turbine and a variable-speed generator system (double-fed asynchronous machine), which could not have been produced at an economical cost a few years previously and which is currently favoured by most manufacturers. Then 1 and 2 MW systems of the same design followed.
The development of wind power systems has largely been carried out by medium-sized companies. Smaller manufacturers, however, face financial limits in the development of MW systems. The 1.5 MW turbine MD 70/MD 77, again with the double-fed asynchronous generator design, which was developed by pro + pro for the manufacturers BWU, Fuhrländer, Jacobs Energie (now REpower Systems) and Südwind / Nordex is opening up new development and market opportunities for smaller companies in the field of large-scale plants.
One new development has been the trend towards gearless wind turbines. Several attempts have been made to introduce and establish in the market small, high-speed, horizontal-axis turbines with direct-drive generators. Up until now these attempts have met the limited success. Microturbines with a permanent-magnet synchronous generator driven directly from the turbine are usually used as battery chargers. The success of such systems is rooted in their attractive design and low price as well in the modern worldwide sales concept and the simple installation of the plants.
Vertical-axis rotors, so-called Darrieus turbines, are enchantingly simple in structure. In their basis form they have up until now mostly been built with gearing and generators at base level. Variants in the form of so-called H-Darrieus gearless turbines in the 300class were first designed with rotating towers and large multiple generators at ground level. Further development led to machines with fixed tripods and annular generators in the head. These variants have not, however, been successful in establishing themselves widely in the wind power market.
The Enercon E 40 horizontal-axis turbine was the first system in the 500class with a direct-drive generator to establish itself in the market with great success in a very short time. The generator, specially developed for this model, connects directly to the turbine and needs no independent bearings. In this way, wear on mechanical components running at high speed is reduced to a minimum (see the report „An Example for a Direct Drive System.“. Operational run times of 180 000 hours have been quoted for many years.
The gearless E 30, E 40, E 58, E 66 and E 112 models from Enercon were produced as a development of the stall-regulated geared models E 5/ E 6 and E 7/ E 8, by way of the E 32/ E 33 variable-pitch turbines. In parallel, but with a slightly delay, the conversion from thyristors to pulse inverters was accomplished. This configuration thus unites the advantages of variable speeds (and the associated reduction in drive-train loading) with those of a grid supply having substantially lower harmonic feedback.
In comparison to the gearless designs with electrically excited synchronous generators, permanently magnet machines permit the arrangement of higher number of poles around the rotor or stator. By using high-quality permanently magnetic materials, relatively favourable construction sizes can thus be achieved and very high efficiencies attained, particularly in the partial load range. Such a plant configuration of the 600 kW class (Genesys 600) has been able to achieve excellent returns over several years of fault-free operation. A 2unit with such a generator design (Harakosan Z 72) was designed with a medium-voltage generator of 4 kV system voltage.
A further possibility, which has been considered for large, slow-running turbines in particular, is the combination of a low-speed generator and a turbine-side gearbox (for example Multibrid turbine). The single-stage gearbox turns the generator shaft at around eight times the turbine speed of approximately 100 revolutions per minute. Thus, even for units in the 5 MW range, generators in compact and technically favourable construction sizes of approximately 3diameter can be used.
A further large-scale turbine in the 5class with a rotor diameter of over 125is the REpower 5M. A double-fed asynchronous generator with medium-voltage isolation in the low-voltage range (950 V stator-side or 690 V rotor-side) is used in this system (see also Offshore Class Prototypes).
Kleinhenz, F.: Projekt eines Großwindkraftwerkes. Der Bauingenieur, 23 (23/24), 1942.
Honnef, H.: Windkraftwerke. Vieweg, Braunschweig, 1932
Abridged version of the parts of the chapter 1 from: Heier, Siegfried: Grid Integration of Wind Energy Conversion Systems, London: John Wiley & Sons, 2006
http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0470868996.html
Electronic publication of english text with friendly permission of the author and of TEUBNER Verlag, Germany.