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DUBLIN CITY UNIVERSITY

School of Electronic Engineering

EE535

Wind Turbines

Partial Lecture Notes

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1 History2 Resources3 Types of wind turbines

* 3.1 Horizontal axis o 3.1.1 HAWT Subtypes o 3.1.2 HAWT advantages o 3.1.3 HAWT disadvantages * 3.2 Vertical axis o 3.2.1 VAWT subtypes o 3.2.2 VAWT advantages o 3.2.3 VAWT disadvantages

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HistoryWind machines were used in Persia as early as 200 B.C.

This type of machine was introduced into the Roman Empire by 250 A.D. However, the first practical windmills were built in Sistan, Iran, from the 7th century. These were vertical axle windmills, which had long vertical driveshafts with rectangle shaped blades.

Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind corn and draw up water, and were used in the milling and sugarcane industries

By the 14th century, Dutch windmills were in use to drain areas of the Rhine River delta. In Denmark by 1900 there were about 2500 windmills for mechanical loads such as pumps and mills, producing an estimated combined peak power of about 30 MW.

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Dutch Windmill

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The first known electricity generating windmill operated was a battery charging machine installed in 1887 by James Blyth in Scotland.

The first windmill for electricity production in the United States was built in Cleveland, Ohio by Charles F Brush in 1888, and in 1908 there were 72 wind-driven electric generators from 5 kW to 25 kW.The largest machines were on 24 m (79 ft) towers with four-bladed 23 m (75 ft) diameter rotors. Around the time of World War I, American windmill makers were producing 100,000 farm windmills each year, most for water-pumping.[6] By the 1930s windmills for electricity were common on farms, mostly in the United States where distribution systems had not yet been installed. In this period, high-tensile steel was cheap, and windmills were placed atop prefabricated open steel lattice towers.

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Power

E= .5Mv^2

Power in free-flowing windP = .5dAv^3

Power from Turbine:Pt = .5dACv^3

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HAWT

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HAWT Advantages

# Variable blade pitch, which gives the turbine blades the optimum angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.

# The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.

# High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.

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HAWT Disadvantages

needs to be mounted on a high tower with high transport, installation & maintenance costs;

very inefficient at low wind speeds;unusable at high wind speeds, so wasting the most useful high energy winds

[Energy is proportional to wind velocity cubed];environmentally problematic at the best locations;noise & RF interference;transportation costs, esp for export potential;needs to be pointed at wind, which increases costs.

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VAWT

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VAWT sub-typesDarrieus wind turbine "Eggbeater" turbines, or Darrieus turbines,

which were named after the French inventor, Georges Darrieus. They have good efficiency, but produce large torque ripple and cyclic stress on the tower, which contributes to poor reliability. Also, they generally require some external power source, or an additional Savonius rotor, to start turning, because the starting torque is very low.

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VAWT sub-types

Savonius wind turbine These are drag-type devices with two (or

more) scoops that are used in anemometers, Flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops. They sometimes have long helical scoops to give a smooth torque.

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VAWT advantages

 Lower construction cost as compared to HAWTs because there is no need to build a massive tower structure to support the heavy HAWT blades and turbine assembly;

Easier to maintain as the moving parts are located near the ground;c) Lower wind start-up speeds than HAWTs. Typically, they start creating electricity at 6 m.p.h. The effective cut-in speed can be made even lower by clever funnelling;d) Less likely to break in high winds as they usually have a lower Tip-Speed /Wind-Speed ratio. Also, our VAWT can be partially shielded by the movable doors;e) Low height useful at locations where laws do not permit structures to be placed high;f) Hilltops, ridgelines and passes can have faster winds near the ground because the wind is forced up a slope or funnelled into a pass and into the path of VAWTs situated close to the ground;g) Does not need to turn to face the wind if the wind direction changes making them ideal in turbulent wind conditions. Thus yaw drive not needed, lowering cost;h) VAWT blades are easily seen by birds;

i) VAWTs are quieter and interfere less with TV, radio & radar signals;j) VAWT systems can also directly drive pumps which may be used to store wind generated energy.

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VAWT disadvantages

Current VAWTs are less efficient than HAWTs as some blades always oppose wind flow;Do not take advantage of the stronger wind at higher elevation as the HAWTs do. * A VAWT that uses guy-wires to hold it in place puts stress on the bottom bearing as all the

weight of the rotor is on the bearing. Guy wires attached to the top bearing increase downward thrust in wind gusts. Solving this problem requires a superstructure to hold a top bearing in place to eliminate the downward thrusts of gust events in guy wired models.

* The stress in each blade due to wind loading changes sign twice during each revolution as the apparent wind direction moves through 360 degrees. This reversal of the stress increases the likelihood of blade failure by fatigue.

* While VAWTs' parts are located on the ground, they are also located under the weight of the structure above it, which can make changing out parts nearly impossible without dismantling the structure if not designed properly.

* Having rotors located close to the ground where wind speeds are lower due to wind shear, VAWTs may not produce as much energy at a given site as a HAWT with the same footprint or height.

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DCU VAWT

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Wind Funnelling

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Advantages of new WT

May be made in Ireland;Lower installation & transport costs;Efficient;Less visually obtrusive;Possible future link-up with SoI