The use of wind as an energy source begins in antiquity. Vertical-axis windmills for grinding grain were reported in Persia in the tenth century and in China in the thirteenth century . At one time wind was a major source of energy for transportation (sailboats), grinding grain, and pumping water. Windmills, along with water mills, were the largest power sources before the invention of the steam engine. Windmills, numbering in the thousands, for grinding grain and pumping drainage water were common across Europe, and some windmills were even used for industrial purposes, such as sawing wood. As the Europeans set off colonizing the world, windmills were built across the world .
Except for sailing, the main long-term use of wind has been for pumping water. Besides the Dutch windmills, another famous example was the sail wing blades for pumping water for irrigation on the island of Crete. One of the blades had a whistle on it to notify the operator to change the sail area when the winds were too high.
At one time there were over 9,000 windmills in the Netherlands. Of course there were different designs, from the earlier post mill to the taller mills where only the top rotated to keep the blades perpendicular to the wind. Today, the Dutch windmills are a famous attraction in the Netherlands (Figure 1.1). The machines for pumping large volumes of water from a low head were as large as 25 m in diameter and were almost all wood. Even the helical pump, an Archimedean screw, was made of wood (Figure 1.2). They were quite sophisticated in terms of the aerodynamics of the blades. The miller would rotate (yaw) the top of the windmill from the ground with a rope attached to a wooden beam on the cap,
FIGURE 1.1 Dutch windmills, World Heritage Site, Kinderdijk, The Netherlands.
so the rotor would be perpendicular to the wind. Others would have a small fan rotor to yaw the big rotor. The rotational speed and power were regulated by the amount of sail that was on the blades. The miller and his family lived in the bottom of the windmill, and the smoke from the fireplace was vented to the upper floors to control insects. For the thatched windmill, fire was a major hazard.
Farm windmills were one of the primary factors in the settlement of the Great Plains of the United States . From 1850 on water pumping windmills were manufactured in the tens of thousands. The early wood machines (Figure 1.3) have largely disappeared from the landscape, except for an isolated farmhouse or in museums.
By 1900, almost all windmills were made of metal, still with multiblade vanes, and the fan or blades were 3-5 m in diameter (Figure 1.4). Although the peak use of farm windmills was in the 1930s and 1940s, when over 6 million were in operation, these windmills are still being
manufactured and are being used to pump water for livestock and residences. The American Wind Power Center in Lubbock, Texas, has an outstanding collection of farm windmills.
Most of the farm windmills are in Africa, Argentina, Australia, Canada, and the United States. As the farm windmill is fairly expensive, there has been a resurgence of design changes to create a less expensive system. Another major change is the development and commercialization of standalone, electric-electric systems for pumping enough water for villages or irrigation, or both .
The farm windmill proves that wind energy is a valuable commodity, even though the size is small. For example, there are an estimated 30,000 operating farm windmills in the Southern High Plains of the United States. Even though the power output is low, 0.2-0.5 kilowatts (kW), they collectively provide an estimated output of 6 megawatts (MW). If these windmills for pumping water were converted to electricity from the electric grid, it would require around 15 MW of thermal power at the generating station and over $1,000 million for the transmission lines, electric pumps, etc. This does not count the dollars saved in fossil fuel with an energy equivalent of 130 million kilowatt-hours (kWh) per year (equivalent to 80,000 barrels of oil per year). Because many of these windmills are 30 years old or older and maintenance costs are $250-400 per year, farmers and ranchers are looking at alternatives such as solar water pumping rather than purchasing new farm windmills.
In 1888, Brush built a windmill to generate electricity, which was based on the rotor (large number of slats) and tail vane of a large farm windmill. The wooden rotor (17 m diameter) was connected to a direct current generator through a 50:1 step-up gearbox to produce around 12 kW in good winds. The unit operated for 20 years; however, the low rotational speed was too inefficient for the production of electricity. For example, a wind turbine with the same-diameter rotor would produce around 100 kW.
As electricity became practical, isolated locations were too far from generating plants and transmission lines were too costly. Therefore, a number of manufacturers built stand-alone wind systems for generating electricity (Figures 1.5 and 1.6), based on a propeller type rotor with two or three blades. Most of the wind chargers had a direct current generator, 6 to 32 volts (V), and some of the later
models were 110 V. The electricity was stored in batteries, and these wet-cell, lead-acid batteries required careful maintenance for long life.
These systems with two or three propeller blades are quite different from the farm windmill, which had a large number of blades covering most of the rotor swept area. The farm windmill is well engineered for pumping low volumes of water; however, it is too inefficient for generating electricity because the blade design and large number of blades means slow rotational speed of the rotor.
The wind chargers became obsolete in the United States when inexpensive electricity (subsidized) became available from rural electric cooperatives in the 1940s and 1950s. After the energy crisis of 1973, a number of these units were repaired for personal use or to sell. Small companies also imported wind machines from Australia and Europe to sell in the United States during the 1970s.
There were a number of attempts to design and construct large wind turbines for utility use [6-11]. These designs centered on different concepts for capturing wind energy (Figure 1.7): airfoil-shaped blades with the axis of the rotor being horizontal or vertical, Savonius, and Magnus effect. With a vertical axis there are no orientation problems of the rotor due to different wind directions.
A rotating cylinder in an airstream will experience a force or thrust perpendicular to the wind, the Magnus effect. In 1926 Flettner built a horizontal-axis wind turbine with four blades, where each blade was a tapered cylinder driven by an electric motor. The cylinders (blades) were 5 m long and 0.8 m in diameter at the midpoint. The rotor was 20 m in diameter on a 33 m tower, with a rated power of 30 kW at a wind speed of 10 m/s.
Madaras proposed mounting vertical rotating cylinders on railroad cars, which would travel around a circular track propelled by the Magnus effect. The generators were to be connected to the
Vertical axis wind turbine
Vertical axis wind turbine
FIGURE 1.7 Diagram of different rotors.
FIGURE 1.7 Diagram of different rotors.
axles of the railroad cars. In 1933, a prototype installation, which consisted of a cylinder 29 m tall and 8.5 m in diameter mounted on a concrete base, was spun when the wind was blowing and the force was measured. Results were inconclusive and the prospect was abandoned.
The Magnus effect has been used for ships, called Flettner rotors [12, 13], and one ship operated using rotors for fuel savings from 1926 to 1933. In 1984 the Costeau Society had a sailing ship, Alcyone, built that used two fixed cylinders with an aspirated turbosail .
In Finland, Savonius built S-shaped rotors, which were similar to two halves of a cylinder separated by a distance smaller than the diameter. In 1927, Darrieus invented a wind machine where the shape of the blade was similar to a jumping rope. His patent also covered straight vertical blades, a giromill. Later the Darrieus design was reinvented by researchers in Canada .
In 1931 the Russians built a 100 kW wind turbine near Yalta on the Black Sea. The rotor was 30 m in diameter on a 30 m rotating tower. The rotor was kept facing into the wind by moving the inclined supporting strut that connected the back of the turbine to a carriage on a circular track. The blade covering was galvanized steel and the gears were of wood. The adjustable angle (pitch) of the blades to the rotor plane controlled the rotational speed and power. Annual output was around 280,000 kWh/year.
The Smith-Putnam wind turbine (Figure 1.8) was developed, fabricated, and erected in 2 years, 1939-1941 . The turbine, which was located on Grandpa's Knob, Vermont, was connected to the grid of Central Vermont Public Service. The rotor was 53 m in diameter on a 38 m tower. Blades were stainless steel with a 3.4 m chord, and each weighed 8,700 kg. The generator was synchronized with the line frequency by adjusting the pitch of the blades. At wind speeds above 35 m/s the blades were changed to the feathered position (parallel to the wind) to shut the unit down. Rated power output was 1,250 kW at 14 m/s. The rotor was on the downwind side of the tower and the blades were free to move independently (teeter, perpendicular to the wind) due to wind loading.
FIGURE 1.8 Smith—Putnam wind turbine, 1250 kW. (Photo from archive files of Carl Wilcox. With permission.)
Testing of the wind turbine started in October 1941, and in May 1942, after 360 h of operation, cracks were discovered in the blades near the root. The root sections were strengthened and the cracks were repaired by arc welding. A main bearing failed in February 1943, and it was not replaced until March 1945 because of a shortage of materials due to World War II. After the bearing was replaced, the unit was operated as a generating station for 3 weeks when a blade failed due to stress at the root. Total running time was only around 1,100 h. Even though the prototype project showed that a wind turbine could be connected to the utility grid, it was not further pursued because of economics. The industrial photos of the construction of the Smith-Putnam wind turbine are available online .
Percy Thomas, an engineer with the Federal Power Commission, pursued the feasibility of wind machines. He compiled the first map for wind power in the United States and published reports on design and feasibility of wind turbines .
After World War II, research and development efforts on wind turbines were centered in Europe. E. W. Golding summarized the efforts in Great Britain , and further efforts are reported in the conference proceedings of the United Nations . The British built two large wind turbines. One wind turbine was built by the John Brown Company on Costa Hill, Orkney, in 1955. The John Brown unit was rated at 100 kW at 16 m/s, with a rotor diameter of 15 m on a 24 m tower. The wind turbine was connected to a diesel-powered grid and only ran intermittently in 1955 due to operational problems.
The other unit was built by Enfield, based on a design by the Frenchman Andreau, and was erected at St. Albans in 1952. The Enfield-Andreau wind turbine rotor was 24 m in diameter on a 30 m tower, with a rated power of 100 kW at 13 m/s. This unit was quite different in that the blades were hollow, and when they rotated, the air flowed through an air turbine, connected to an alternator at ground level, and out of the tip of the blades (Figure 1.9). This unit was moved to Grand Vent, Algeria, for further testing in 1957. Frictional losses were too large for this unit to be successful.
The French built several prototype wind turbines from 1958 to 1966. A 800 kW wind turbine was located at Nogent Le Roi, which had a rotor diameter of 31 m and was operated at constant rotor speed connected to a synchronous generator. The top weighed 162 metric tons and was mounted on
FIGURE 1.9 Diagram of Enfield—Andreau wind turbine, 100 kW.
a 32 m tower. This unit fed electricity into the national grid from 1958 to 1963. Two other units were located at St. Remy-Des-Landes. The smaller Neyrpic machine had a rotor diameter of 21 m on a 17 m tower, and the asynchronous generator produced 130 kW at 12 m/s. The larger unit had a rated power of 1,000 kW at 17 m/s and operated for 7 months, until operation ceased in June 1964 due to a broken turbine shaft. Even though the prototypes clearly showed the feasibility of connecting wind turbines to the electric grid, the French decided in 1964 to discontinue further wind energy research and development.
During the 1950s, Hütter of Germany designed and tested wind turbines that were the most technologically advanced for that time and for the next two decades. The rotors had lightweight fiberglass blades (Figure 1.10) mounted on a teetered hub with pitch control and coning since the rotors were downwind. A 10 kW unit was developed and tested, which culminated in a larger unit, 34 m diameter, that produced 100 kW at 8 m/s . This unit had around 4,000 h of operation from 1957 to 1968; however, the experiments proceeded slowly due to lack of funds and problems with blade vibration.
In Denmark, several hundred systems based on the design by La Cour  were built, with rated power from 5 to 35 kW. The units had rotor diameters around 20 m, four blades, which had a mechanical connection to a generator on the ground. By 1900, there were around 30,000 wind turbines for farms and homes, and in 1918, some 120 local utilities in Denmark had a wind turbine, typically 20-35 kW for a total of 3 MW. At that time, these turbines produced around 3% of the Danish electricity. Danish interest waned in subsequent years, until a crisis of production of electricity during World War II. Since the Danes did not have any fossil fuel resources, they looked at connecting wind turbines into their national grid, and the Danish government started a program to
develop large-scale wind turbines for producing electricity. During World War II, a series of wind turbines in the 45 kW range were developed with direct current (DC) generators. These units produced around 4 million kWh per year during this period.
The Danes had the only successful program, which began in 1947 with a series of investigations on the feasibility of using wind power, and continued until 1968 [9, pp. 229-240]. A prototype wind turbine of 7.5 m diameter was built and remained in operation until 1960, when it was dismantled. A wind turbine at Bogo, originally constructed for DC power in 1942, was reconstructed for alternating current (AC) in 1952. Rotor diameter was 13.5 m with a 45 kW generator. The results of the two experimental wind turbines were encouraging and culminated in the Gedser wind turbine (Figure 1.11). This unit was erected in 1957, and during the period 1958-1967 it produced 2,242 MWh. It was shut down in 1967 when maintenance costs became too high. The rotor was 35 m in diameter and the tower, 26 m height, was prestressed concrete. The rotor was upwind of the tower, and the blades were fixed pitch with tip brakes for overspeed control. The wind turbine had an asynchronous generator (rated power of 200 kW at 15 m/s), which provided stall control, and it had an electromechanical yaw mechanism. Denmark and the United States furnished money to place the Gedser wind turbine in operation for a short time period in 1977-1978 for research, which included tests for aerodynamic performance and structural loads.
The successful program of the Danes was overshadowed by the failure of other large machines. The machines failed due to technical problems, mainly stresses due to vibration and control at high wind speeds. Others were economic failures. Everyone agreed there were no scientific barriers to the use of wind turbines tied to the utility grid. In the 1960s development of wind machines was abandoned since petroleum was easily available and inexpensive.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.