History from windmill to wind turbine

The historic development of using wind as a source of power shows an evolution from simple drag-type vertical-axis windmills generating mechanical power for local use, via stand-alone wind turbines designed for battery charging and single grid-connected wind turbines producing AC power using aerodynamic lift, to wind farms supplying electricity to the utility grid for distribution to the consumers. In this subsection we shall briefly review this transition from windmills to wind turbines. The next subsection presents an outlook on the future of wind power. Finally, the required improvements in both wind turbine design and operation to achieve and maintain cost-effective wind turbines are discussed.

The first windmills were developed to automate the tasks of grain-grinding and water-pumping. Although the Chinese reportedly invented the windmill, the earliest-documented design is the vertical-axis windmill used in the region Slstan in eastern Persia for grinding grain and hulling rice in the tenth century A.D. [279]. One of the most important climatic features of this extensive border region of present day Afghanistan and Iran is a northerly wind that blows unceasingly during the summer months of June to September at velocities ranging between 27 and 47 meters per second. This wind is locally referred to as "the wind of 120 days".

The Persian windmills were usually laid out in a single line that was built at the top of a mountain, hill or tower with high walls separating them as illustrated in Fig. 1.1 [321]. The famous example near the town of Neh had one line of 75 windmills. The lines were oriented perpendicular to the prevailing wind direction. Each individual windmill consisted of a two-storey structure made of sun-dried bricks. The upper part of the structure contained the millstones (about 2 m in diameter), while the lower part contained a vertical spindle (or wind-wheel) which was fitted with between six and twelve radial arms as illustrated in Fig. 1.2. Each of these arms was covered with fabric that is allowed to bulge in order to catch the wind. In the walls of the lower part containing the wind-wheel were apertures being aligned with the primary wind direction. As a consequence, this kind of windmill can only work in a region where there is a steady prevailing wind. The apertures were wider on the outside than on the inside, forcing the wind to increase its velocity as it enters the wheel-house and rotate the wind-wheel, which then directly drives the millstones. In addition, a series of shutters were used (presumably on the outside of the structure) to admit or shut out the wind, and thereby regulate the rotational speed.

Persian Vertical Axis Windmill
Figure 1.1: Left photograph: Downwind view of a vertical-axis windmill of the Persian type in the town of Neh. Right photograph: Close-up view of the working surface made of bundles of reed [321]. Reprinted by permission of The MIT Press.

Incoming wind

Incoming wind

Persian Windmill
Figure 1.2: Cross-section of the wind-wheel of a Persian windmill showing the apertures being aligned with the primary wind direction.

Vertical-axis windmills of this basic design were still operating in Iran in 1977 (and may be still used today) [105]. This means that the basic design has lasted at least 1000 years, although a major change has taken place: the millstones have been placed below the rotor as already shown in Fig. 1.1. The advantage of having the sails above the millstones was that the working surface could be substantially enlarged. Another noticeable change is the use of bundles of reeds instead of fabric to provide the working surface. It must be noted that the Persian windmill never came into use in Northwest Europe.

The history of the Western windmill begins with the first documented appearance of the European or "Dutch" windmill in Normandy, France in the year 1180 [80]. The "Dutch" windmill had four sails and was of the horizontal-axis configuration. Wooden cog-and-ring gears were used to translate the motion of the horizontal-axis to a vertical movement to turn a grindstone. The reason for the sudden evolution from the vertical-axis Persian design is unknown, but the fact that European water wheels also had a horizontal-axis configuration - and apparently served as the technological model for the early windmills - may provide part of the answer. Another reason may have been the higher structural efficiency of drag-type horizontal machines over drag-type vertical machines. In addition, the omnidirectional wind, as opposed to the Slstan environment, may have called for an adaptation to suit the conditions.

Windmills spread rapidly throughout Europe in the thirteenth century. In a relatively short time, tens of thousands were in use for a variety of duties. The applications ranged from grinding grain, shredding tobacco, sawing timber, processing spices and paint pigments, milling flax, pressing oil or pumping water for polder drainage. The performance increased greatly between the twelfth and nineteenth century with the introduction of metal parts. A primary improvement of the European windmills was their designer's use of sails that generated aerodynamic lift. This feature provided improved rotor efficiency compared with the Persian mills by allowing an increase in rotor speed, which also allowed for superior grinding as well as pumping action.

The lower cost of wind power to water power and the fact that more sites were available for windmills than there were for water mills caused an increase in the use of windmills. In The Netherlands, this growth contributed to the country's golden age (from 1590 till about 1670). As late as 1850, 90% of the power used in Dutch industry came from the wind. Steam supplied the rest. Industrialization, first in Europe and later in America, led to a gradual decline in the use of windmills. The steam engine took over the tasks previously performed by windmills.

In 1896, at the height of the industrial revolution, wind still pumped 41 % of the polders in The Netherlands. However, in 1904, wind provided only 11% of Dutch industrial energy. These windmills had a rotor diameter and hub height of 25 m and 30 m respectively, and were capable of producing the equivalent of 2550 kW in mechanical form. For comparison, modern wind turbines of the same size are capable of extracting ten times more power from the wind [80]. As steam power developed, the uncertain power of the wind became less and less economic (in particular after cheap coal came available), and we are left today with a tiny fraction of the elegant structures that once extracted power from the wind. These remaining windmills, scattered throughout the world, are a historic, and certainly very photogenic, reminder of a past technological age.

The first wind turbine to harness the wind for the generation of electricity was built by Charles F. Brush in Cleveland, Ohio, USA in 1888. The so-called "Brush" windmill was featured with a 17-m diameter multi-blade rotor mounted on an 18-m high rectangular tower as illustrated in Fig. 1.3. The upwind rotor consisted of 144 thin wooden blades, and a large fantail to turn the rotor out of the wind. The turbine was equipped with a 12 kW direct-current generator, and a belt-and-pulley transmission with a step-up ratio of (50:1). The DC generator was located on the basement of the tower. The power output was used for charging storage batteries. Despite its relative success in operating for 20 years, the Brush windmill demonstrated the limitations of the low-speed, high-solidity rotor for the generation of electricity [279].

The next important step in the transition from windmills to wind turbines was taken by P. la Cour in 1891 in Askow, Denmark. He developed the first variable speed wind turbine that incorporated the aerodynamic design principles (low-solidity, four-bladed rotors incorporating primitive airfoil shapes and blade twist) used in the best Dutch windmills. The resulting higher speed of the La Cour rotor made this type of wind turbine quite practical for electricity generation.

By the late 1930s, the pioneering machines of Brush and La Cour had evolved into two- or three bladed horizontal-axis wind turbines with the rotor upwind of the tower and low solidity, using a tail vane to position the rotor at right angles with

Charles Brush
Figure 1.3: "The windmill dynamo and electric light plant of Mr. Charles F. Brush", Scientific American, December 20, 1890. Copy of an original in the Department of Special Collections, Case Western Reserve University Library Cleveland, Ohio.

the wind direction. The majority of these direct-current producing turbines were operated at variable speed with fixed pitch angle rotor blades. The turbines were generally reliable and long-lived machines giving reasonable maintenance. They did not, however, have the cost-effectiveness and capacity to compete with conventional power systems.

The majority of the wind turbines built before 1970 were small machines designed for battery charging. The 1.25 MW Smith-Putnam wind turbine constituted a notable exception. This constant speed turbine, built in 1941, had a two-bladed rotor of 53.3-meter diameter mounted on a 33.5 m high truss tower. It featured full-span active control of the blade pitch angle using a fly-ball governor, active yaw control by means of a servomotor, and flapping hinges to reduce gyroscopic loads on the rotor shaft. The turbine was erected on the top of a hill called "Grandpa's Knob" near Rutland, Vermont, USA. It supplied AC power to the local grid for 695 hours from October 1941 till March 1945 when a blade failure due to fatigue disabled the turbine [225] (in 1943 a bearing failed which could not be replaced for two years due to the Second World War [80]).

During the period 1945-1970 new growth in wind turbine technology development took place mainly in western Europe, but at a very modest pace [279]. By 1970, there was little or no activity world-wide for producing electricity using wind turbines. The energy crisis of 1973 renewed interest in wind power from both govern mental and environmentalist sides. From an environmental point of view, generating electricity using wind turbines consumes no feedstock of fuel, emits no greenhouse gases (e.g. carbon dioxide, methane, nitrous oxide, or halocarbons), and creates almost no waste products. Although the aforementioned gases all contribute to global warming, carbon dioxide in itself accounts for 66-74 percent of the warming [317]. As a consequence of this, the market is highly dependent on the political situation and willingness to support wind power in return for a cleaner environment1.

During the years 1973-2002, the commercial wind turbine market evolved from small grid-connected machines in the 1 to 99 kilowatt size range for rural and remote use, via medium-scale turbines (100 to 999 kW) for remote community or industrial market use, to utility interconnected wind farms consisting of megawatt sized turbines. For the purpose of illustration, Fig. 1.4 shows the gradual increase in the average installed power size of cumulative installation in the period 1994-2001. Observe that the average installed power size of all wind turbines installed globally doubled in the period 1997-2001. The growth in installed power size is also reflected by the following figures: the average installed power size of all wind turbines installed globally by the end of 2001 is 445 kW, while the average installed power size of the turbines installed in 2001 is 915 kW [34].

1994 1995 1996 1997 1998 1999 2000 2001

Year

Figure 1.4: Development of the average wind turbine installed power size of cumulative installation in the period 1994-2001 [34].

The globally installed wind power capacity reached 24.93 GW by the end of 2001 as shown in Fig. 1.5 [34]. This is an average increase of over 28% per year in the displayed period. Observe that the installed capacity has increased more than fourfold in the period 1996-2001, and that last year's growth was almost 36%. This strong growth eclipses that of all other fuel sources: oil, natural gas, and nuclear power are growing at a rate of 1.9 % or less each year, while the coal consumption

1It must be noted that, at present, wind is still an environmental driven market, although common market aspects are finally beginning to play a more important role.

had an average annual growth rate of -0.6% in the 1990s. In 1999, natural gas - the cleanest fossil fuel - has become the fuel of choice for power generation, replacing coal. Solar photovoltaics, which convert sunlight in electricity, had an annual average global growth of 17 % in the last decade, while hydropower, geothermal power, and biomass energy have experienced a steady growth over the same period ranging from 1 to 4 percent annually [317]. These figures not only indicate that wind energy is trending towards the preferred renewable electricity source, but also show that wind is the fastest growing energy source in the world.

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Figure 1.5: Global installed cumulative wind power [33, 34].

Most of the installed wind power capacity is located in Europe (i.e. 71.5%), followed by the United States of America (18.4%) and Asia (9.0%). The global wind-generated electricity production in 2001 was about 50.3 TWh. Even though this figure looks impressive from a wind power point of view, wind power still only accounted for approximately 0.32 % of total electricity generation (partly due to the (also) constantly rising worldwide demand for electricity). The development of this share is depicted in Fig. 1.6 for the period 1996-2001 [34].

1996 1997 1998 1999 2000 2001

Year

Figure 1.6: The development of the share of wind power in the global electricity mix in the period 1996-2001. A capacity factor (see Definitions) of 0.23 is assumed [34].

1996 1997 1998 1999 2000 2001

Year

Figure 1.6: The development of the share of wind power in the global electricity mix in the period 1996-2001. A capacity factor (see Definitions) of 0.23 is assumed [34].

Over the last 20 years the cost of electricity from onshore wind power has dropped substantially: from 23-38 euro cents per kilowatt-hour in the early 1980s to 3-8 euro cents today for a mean wind speed of respective 10 and 5 m/s at hub height [203, 260]. But the price for conventional power plant generated electricity also declined [84]. The costs of wind power came down largely because of improved reliability. Advances in technology and learning curve made turbines cheaper to produce and far more reliable. At present, about 70% of the cost per kWh comes from the capital cost of initial investment [172].

Despite the improved reliability and technical understanding even in the past few years a number of serious failures, such as broken blades, bearing damages and wear on gearbox teeth, occurred see e.g. [94, 146, 310, 311, 312]. The origin of these failures can be twofold: i) direct failures due to extreme loading, or ii) failures due to fatigue loads. It is now generally accepted that fatigue loads are the main cause of failure in the present onshore wind turbines [284]. In addition, it is also expected that fatigue will be the design driver when considering the combined wind and water wave loading acting on offshore wind turbines.

Obviously, premature field failures lead to a relatively high kilowatt-hour price due to increased maintenance cost, costly retrofits and, indirectly, increased design conservatism. At present, a realistic value for the operation and maintenance (O&M) cost lies between 0.44 and 0.87 euro cents per kilowatt-hour [34]. This implies that the O&M cost make up 6-29 % of the cost per kilowatt-hour. It should be noted that the O&M cost for offshore wind farms are even higher due to fact that the wind farms are exposed to a more aggressive and less known environment. In addition, safe access for maintenance is either very expensive or limited by a narrow weather window.

Nevertheless, onshore wind power is, at excellent wind sites, as competitive if not more competitive as the lowest cost traditional fuel, natural gas. In Fig. 1.7 the electricity generation cost of coal, natural gas, nuclear, and both onshore and offshore wind are compared. Observe that there is no single price that can be assigned to any source of generation. In particular, the kilowatt-hour price of nuclear power as well as onshore wind power span a wide range. The wide range of the latter can be easily explained by recognizing that the cost of wind power are critically dependent on site wind speeds since the power available in the wind is proportional to the cube of the mean wind velocity. The mean wind velocity, in turn, varies widely across a country because of obstacles (e.g. buildings, line of trees) to the wind, and varying surface roughness of the terrain. Therefore, it is expected that the move from onshore to offshore sites offers a very appealing opportunity for the future of wind power.

The aforementioned premature field failures have not only resulted in a relatively high price of electricity generated by wind turbines, but also in a public image of wind energy as being not very reliable. The public opinion is reflected in headlines like "Wind energy encounters head winds" [25], "Nobody wants a wind turbine" [31], "Benefit of wind turbines is negligible" [75], "Wind energy parasitizes on conventional power plants" [87] and "The wind war" [244].

From the preceding it can be concluded that in the past decades the wind industry has grown from a niche business serving the environmentally aware into one that has

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Coal Natural gas Nuclear Onshore wind Offshore wind

Figure 1.7: Cost comparison of producing electricity: traditional fuel sources versus wind power [49]. Grey column: minimum cost, and white column: maximum cost in euro cents per kilowatt-hour.

established itself as the most competitive form of renewable energy. Nonetheless, wind energy is not yet cost-effective, and consequently, the share of wind power in the global electricity mix is almost negligible. Furthermore, it should be stressed that establishing a reliable image is of paramount importance for successful penetration into the electricity market. This implies that development and deployment of new technology will be crucial to successful large-scale application of wind energy.

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