Wind Energy - A Tutorial - Wind, Wind Machines, Wind Energy Conversions, Wind Turbines - Components and Configurations



WIND ENERGY

All renewable energy ultimately comes from the sun, except for geothermal and tidal power. Our earth receives power of 1.74 x 1017W/hr from sun. About 1 or 2% of this sun’s energy can be converted to the wind energy, which is approximately 50-100 times more than which is converted to biomass on earth by all plants.


WHAT IS WIND?

Air in motion is nothing but wind. It is caused by the uneven heating of the Earth’s surface by radiant energy from the sun. Since the Earth’s surface is made of very different types of land and water, it absorbs the sun’s energy at different rates. Water usually does not heat or cool as quickly as land because of its physical properties. An ideal situation for the formation of local wind is an area where land and water meet. During the day, the air above the land heats up more quickly than the air above water. The warm air over the land expands, becomes less dense and rises. The heavier, denser, cool air over the water flows in to take its place, creating wind. In the same way, the atmospheric winds that circle the Earth are created because the land near the equator is heated more by the sun than land near the North and South Poles.Differential heating of the earth’s surface and atmosphere induces vertical and horizontal air currents that are affected by the earth’s rotation and contours of the land, which is nothing but wind.

WIND DIRECTION-A weather vane, or wind vane, is used to show the direction of the wind. A wind vane points toward the source of the wind. Wind direction is reported as the direction from which the wind blows, not the direction toward which the wind moves. A north wind blows from the north toward the south.

WIND SPEED-It is important in many cases to know how fast the wind is blowing. Wind speed can be measured using a wind gauge or anemometer. One type of anemometer is a device with three arms that spin on top of a shaft. Each arm has a cup on its end. The cups catch the wind and spin the shaft. The harder the wind blows, the faster the shaft spins. A device inside counts the number of rotations per minute and converts that figure into

mph—miles per hour. A display on the anemometer shows the speed of the wind.

Also, the strength of wind varies, and an average value for a given location does not alone indicate the amount of energy a wind turbine could produce there. To assess the frequency of wind speeds at a particular location, a probability distribution function is often fit to the observed data. Different locations will have different wind speed distributions. The Weibull model closely mirrors the actual distribution of hourly wind speeds at many locations. The Weibull factor is often close to 2 and therefore a Rayleigh distribution can be used as a less accurate, but simpler model.

Fig.1. Distribution of wind speed (red) and energy (blue) for all of 2002 at the Lee Ranch facility in Colorado. The histogram shows measured data, while the curve is the Rayleigh model distribution for the same average wind speed.

Fact about earth’s atmosphere: Consider the relative thickness of the atmosphere to the diameter of the earth: 31 km/12800 km = 0.0024 → 0.24% A dimensionally accurate analogy would be to take a hair from your head and wrap it over the tip of your thumb. The thickness of your hair is about 0.002 inch. The tip of your thumb to the first joint on your thumb is about 1 inch.

HISTORY OF WIND MACHINES


Wind power technology dates back many centuries. There are historical claims that wind machines which harness the power of the wind date back beyond the time of the ancient Egyptians. Hero of Alexandria used a simple windmill to power an organ whilst the Babylonian emperor, Hammurabi, used windmills for an ambitious irrigation project as early as the 17th century BC. The Persians built windmills in the 7th century AD for milling and irrigation and rustic mills similar to these early vertical axis designs can still be found in the region today. In Europe the first windmills were seen much later, probably having been introduced by the English on their return from the crusades in the middle east or possibly transferred to Southern Europe by the Muslims after their conquest of the Iberian Peninsula. It was in Europe that much of the subsequent technical development took place. By the late part of the 13th century the typical ‘European windmill’ had been developed and this became the norm until further developments were introduced during the 18th century. At the end of the 19th century there were more than 30,000 windmills in Europe, used primarily for the milling of grain and water pumping. In the US, the development of the "water-pumping windmill" was the major factor in allowing the farming and ranching of vast areas otherwise devoid of readily accessible water. Wind pumps contributed to the expansion of rail transport systems throughout the world, by pumping water from water wells for the steam locomotives. The multi-bladed wind turbine atop a lattice tower made of wood or steel was, for many years, a fixture of the landscape throughout rural America. When fitted with generators and battery banks, small wind machines provided electricity to isolated farms. In July 1887, a Scottish academic, Professor James Blyth, undertook wind power experiments that culminated in a UK patent in 1891. In the US, Charles F. Brush produced electricity using a wind powered machine, starting in the winter of 1887-1888, which powered his home and laboratory until about 1900. In the 1890s, the Danish scientist and inventor Poul la Cour constructed wind turbines to generate electricity, which was then used to produce hydrogen. These were the first of what was to become the modern form of wind turbine.

Small wind turbines for lighting of isolated rural buildings were widespread in the first part of the 20th century. Larger units intended for connection to a

distribution network were tried at several locations including Balaklava USSR in 1931 and in a 1.25 megawatt (MW) experimental unit in Vermont in 1941. In

the 1970s, U.S. industries teamed with NASA in a research program which created the NASA wind turbines, developing and testing many of the features of modern utility-scale turbines. The modern wind power industry began in 1979 with the serial production of wind turbines by Danish manufacturers Kuriant, Vestas, Nordtank, and Bonus. These early turbines were small by today's standards, with capacities of 20–30 kW each. Since then, they have increased greatly in size, with the Enercon E-126 capable of delivering up to 7 MW, while wind turbine production has expanded to many countries. Today, wind generated energy is the fastest growing source of renewable energy. Wind power is expected to grow worldwide in the twenty-first century.


PRINCIPLES OF WIND ENERGY CONVERSION

There are two primary physical principles by which energy can be extracted from the wind; these are through the creation of either lift or drag force (or through a combination of the two). The difference between drag and lift is illustrated by the difference between using a spinnaker sail, which fills like a parachute and pulls a sailing boat with the wind, and a Bermuda rig, the familiar triangular sail which deflects with wind and allows a sailing boat to travel across the wind or slightly into the wind. Drag forces provide the most obvious means of propulsion, these being the forces felt by a person (or object) exposed to the wind. Lift forces are the most efficient means of propulsion but being more subtle than drag forces are not so well understood.

The basic features that characterise lift and drag are:
• drag is in the direction of air flow
• lift is perpendicular to the direction of air flow
• generation of lift always causes a certain amount of drag to be developed
• with a good aerofoil, the lift produced can be more than thirty times greater than the drag
• lift devices are generally more efficient than drag devices

WIND POWER ESTIMATION

Significant areas of the world have mean annual wind speeds of above 4-5 m/s (metres per second), which makes small-scale wind powered electricity generation an attractive option. It is important to obtain accurate wind speed data for the site in mind before any decision can be made as to its suitability.

The power in the wind is proportional to:
• the area of windmill being swept by the wind
• the cube of the wind speed
• the air density - which varies with altitude


WIND TURBINE

Winds are influenced by the ground surface at altitudes up to 100 meters. It is slowed by the surface roughness and obstacles. When dealing with wind energy, we are concerned with surface winds. A wind turbine obtains its power input by converting the force of the wind into a torque (turning force) acting on the rotor blades. The amount of energy, which the wind transfers to the rotor depends on the density of the air, the rotor area, and the wind speed. The kinetic energy of a moving body is proportional to its mass (or weight). The kinetic energy in the wind thus depends on the density of the air, i.e. its mass per unit of volume. In other words, the "heavier" the air, the more energy is received by the turbine. At 15° Celsius air weighs about 1.225 kg per cubic meter, but the density decreases slightly with increasing humidity. A typical 600 kW wind turbine has a rotor diameter of 43-44 meters, i.e. a rotor area of some 1,500 square meters. The rotor area determines how much energy a wind turbine is able to harvest from the wind. Since the rotor area increases with the square of the rotor diameter, a turbine which is twice as large will receive 22 = 2 x 2 = four times as much energy. To be considered a good location for wind energy, an area needs to have average annual wind speeds of at least 12 miles per hour.


Large turbines are able to deliver electricity at lower cost than smaller turbines, as foundation costs, planning costs, etc. are independent of size and are well-suited for offshore wind plants. In areas where it is difficult to find sites, one large turbine on a tall tower uses the wind extremely efficiently.

Small turbines are more suitable for local electrical grids, as they may not be able to handle the large electrical output from a large turbine. These are economical in some areas as foundations for large turbines may cost high.


COMPONENTS OF A WIND TURBINE

Blades: Most wind turbines have three blades, though there are some with two blades. The most important reason is the stability of the turbine. A rotor with an odd number of rotor blades (and at least three blades) can be considered to be similar to a disc when calculating the dynamic properties of the machine. A rotor with an even number of blades will give stability problems for a machine with a stiff structure. The reason is that at the very moment when the uppermost blade bends backwards, because it gets the maximum power from the wind, the lowermost blade passes into the wind shade in front of the tower. Blades are generally 30 to 50 meters (100 to 165 feet) long, with the most common sizes around 40 meters (130 feet).

Controller: There is a controller in the nacelle and one at the base of the turbine. The controller monitors the condition of the turbine and controls the turbine movement.

Gearbox: Many wind turbines have a gearbox that increases the rotational speed of the shaft. A low-speed shaft feeds into the gearbox and a high-speed shaft feeds from the gearbox into the generator. Some turbines use direct drive generators that are capable of producing electricity at a lower rotational speed. These turbines do not require a gearbox.

Generators: Wind turbines typically have a single AC generator that converts the mechanical energy from the wind turbine’s rotation into electrical energy. Clipper Wind power uses a different design that features four DC generators.

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Nacelles: The nacelle houses the main components of the wind turbine, such as the controller, gearbox, generator, and shafts.
Rotor: The rotor includes both the blades and the hub (the component to which the blades are attached).

Towers: Towers are usually tubular steel towers 60 to 80 meters (about 195 to 260 feet) high that consist of three sections of varying heights. (There are some towers with heights around 100 meters (330 feet)).

These parts work together to convert the wind’s energy into electricity.
1. The wind blows and pushes against the blades on top of the tower, making them spin.

2. The turbine blades are connected to a low-speed drive shaft. When the blades spin, the shaft turns. The shaft is connected to a gearbox. The gears in the gearbox increase the speed of the spinning motion on a high-speed drive shaft.

3. The high-speed drive shaft is connected to a generator. As the shaft turns inside the generator, it produces electricity.
4. The electricity is sent through a cable down the turbine tower to a transmission line.


PARAMETERS USED TO CHARACTERIZE WINDMILL ROTOR

The tipspeed ratio is defined as the ratio of the speed of the extremities of a windmill rotor to the speed of the free wind. Drag devices always have tip- speed ratios less than one and hence turn slowly, whereas lift devices can have high tip-speed ratios (up to 13:1) and hence turn quickly relative to the wind.

The proportion of the power in the wind that the rotor can extract is termed the coefficient of performance (or power coefficient or efficiency; symbol Cp) and its variation as a function of tip-speed ratio is commonly used to characterise different types of rotor. As mentioned earlier there is an upper limit of Cp = 59.3%, although in practice real wind rotors have maximum Cp values in the range of 25%-45%.

Solidity is usually defined as the percentage of the area of the rotor, which contains material rather than air. Low-solidity machines run at higher speed and tend to be used for electricity generation. High-solidity machines carry a lot of material and have coarse blade angles. They generate much higher starting torque (torque is the twisting or rotary force produced by the rotor) than low-solidity machines but are inherently less efficient than low-solidity machines. The windpump is generally of this type. High solidity machines will have a low tip-speed ratio and vice versa.

There are various important wind speeds to consider:
• Start-up wind speed - the wind speed that will turn an unloaded rotor
• Cut-in wind speed – the wind speed at which the rotor can be loaded
• Rated wind speed – the windspeed at which the machine is designed to run (this is at optimum tip-speed ratio
• Furling wind speed – the windspeed at which the machine will be turned out of the wind to prevent damage
• Maximum design wind speed – the windspeed above which damage could occur to the machine

WIND TURBINE CONFIGURATIONS

1. Based on Position of axis: There are two main types of wind machines- Vertical axis wind turbine (VAWT) and Horizontal axis wind turbine (HAWT).
Fig.4. HAWT and VAWT
2. Based on resulting force acting by the flow: Lift type or drag type. Most HAWT’s are lift design based.


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