Wind Power Plants what are they? How do they work

INTRODUCTION.

HISTORICAL DEVELOPMENT OF WIND POWER PLANTS.

• For Centuries :Sailing ships,Grinding grain,Pumping water,Power Source for factories.
• In 1940:First wind Electric Power plant to generate hydrogen by electrolysis.
• By 1940: Thousands of wind-electric plants were in use to supply power to isolated communities.
• 1940-1970:interest declined as the grid expanded and oil prices were low.

OLD DUTCH WIND MILLS

Often used for grain milling and water pumping,

A VERY FAMILIAR LANDSCAPE IN NETHERLANDS

Now mainly preserved for touristic purposes,

MODERN WIND ELECTRIC GENERATORS A FAMILIAR SIGHT ALL OVER EUROPE

REVIVAL OF WIND POWER

• The oil shock in  1970’s.
• Financial and regulatory incentives.Caused a renewed interest in wind-electric systems in 70’s and early 80’s in USA.
• However,the interest in USA was a short lived as the performance of turbines of turbines was below expectations.As the tax=credits were withdrawn the wind power industry came to a hand still in USA.
• Wind turbine technology continued to thrive in the European countries such as,Denmark ,Germany,Spain.
• Sue to the increased concern on environmental pollution and increased oil prices.the interest in renewable energies in general and wind power in particular, increased tremendously from mid 1990’s.By that time,above countries were ready for the boom and are the current market leaders in wind turbines.

WORLD INSTALLED CAPACITY

TYPES OF WIND TURBINES.

VERTICAL AXIS WIND TURBINES.

• The only vertical axis turbine that had any commercial success ts the “Darrieus Rotor” first developed in 1920’s.
• The main advantages of the darrieus rotor are,
  • No need of Yaw control,i.e .positioning the blades to face wind by turning around the tower.
  • The heavy machinery such as the gearbox,generator can be kept on the ground making it easy to mount and maintain.
  • The tower need not be always be a strong as HAWT as it does not have to bear the weight of heavy machinery.
  • The blades are not subjected to flexing so they can be lighter and inexpensive.
• The main dis advantages of Darrieus rotor are,
  • Wind speed is lower and more turbulent closer to the ground.Therefore,power output is less.
  • Very little starting torque and as the wind speed is low.
  • Cannot reduce the output power as wind speeds as the case of HAWT by pitch control of the blades.

HORIZONTAL AXIS WIND TURBINES

There are two main types as:

Upwind machine: Blades are facing the wind. Wind first meets the blades and then the tower. This is the most commonly used type of turbine.

Downwind machine: Blades are facing away from the wind. Wind first meet tower and then the blades.

DOWNWIND MACHINES

The main advantage is there is no need of yaw control as the wind automatically turns the nacelle and blades in to the right position with respect to the wind direction.  The main disadvantage is the wind-shadowing effect of the tower. That is when a blade swings past the tower it experiences reduced wind speed. This causes the blades to flex which eventually causes fatigue failure of blades. It also reduces the output power and increases the audible noise.

UPWIND MACHINES

• There is no wind-shadowing effect due to tower.
•  Deliver more power than downwind machine and operates with less audible noise.
• Requires somewhat complex yaw control to keep the blades in the correct position with respect to the varying wind direction.
•  Most modern wind turbines are of this type.

NUMBER OF BLADES

•  As the number of blades increases,Higher torque , Lower maximum possible rotational speed and Less efficiency
•  Thus designs such as typical ‘windmill’ with large number of blades are suitable for low-speed operations requiring larger starting and running torques. Ex. Water pumping. Grinding grain.
•  They are not suitable to generate electricity as with lower rotating speeds, the generator size needs to be larger.
• 3-Blade Turbines: Most modern European turbines are having three blades. 2-Blade Turbines: Most American machines have two blades.
• 3-Blade Turbines are smoother and less noisy in operation. Because the variation of wind speed with height is more evenly felt by the blades.
• However, the third blade adds to the cost and the weight of the turbine considerably. It is also more difficult during construction and maintenance.

MAXIMUM THEORETICAL EFFICIENCY OF THE TURBINE

• Betz Efficiency: According to the original derivation by German physicist Albert Betz, the maximum theoretical efficiency of a wind turbine is 59.3%. It happens when the turbine reduces the wind speed to one third of its original speed.

EFFICIENCIES OF PRACTICAL TURBINES

How close can a practical turbine get to Betz efficiency? At best about 80%. That is a turbine efficiency of 45-50%. Turbine efficiency is a function of rotor speed. If the rotor turns too slowly, more wind will ‘pass-through’ the swept area without getting affected by the blades. Therefore the efficiency will be less. If the turbine turns too fast, the effect of one blade on wind is increasingly felt by the other blade that follows. This turbulence reduces the efficiency. Thus the efficiency is maximum at a certain speed. If the number of blades is less, the rotor speed can be increased to a higher level before the turbulence is affected.

Theoretical Power Curve,

POWER CURVE

• Rated Wind Speed (VR): At rated wind speed, the generator is delivering the rated power for which it is designed.
• Cut out (Furling) Wind Speed (VF): It is mechanically unsafe to operate the wind turbine beyond this speed. Therefore, it is stopped. No power is produced beyond this wind speed.
• v < v < vR: The turbine is operated with maximum possible efficiency to generate as much power as possible. The angle of attack of blades with respect to wind is at the optimum value. The rotor speed should be varied to get maximum efficiency. But some turbines are operated at constant speed to reduce costs.
• vR < v < vF: The output power is maintained at the rated value by one of the many possible ways of controlling the angle of attack.