Sunday, June 16, 2013

Design feasibility of Wind turbine systems


From Wikipedia, the free encyclopedia
A wind turbine power plant uses the kinetic energy in wind for delivering the mechanical power.Wind power is a renewable source of energy. Since the prehistoric days wind is being used as a natural source of power and is being utilized for performing variety of mechanical work. It is one of the cleanest form of energy for producing power. There is no considerable waste materials from wind turbines after operations as compared to other energy source. The total annual wind energyavailable on earth is approximately about 13*1012 kWh which is equivalent to 1500 power stations each delivering 1000 MW of energy
There are different classes of wind turbines ranging from small to large having corresponding suitable applications. The small one having power production less than 10 kW are used in homes, farms and remote applications whereas intermediate wind turbines (10-250 kW ) are having their utilities in village power, hybrid systems and distributed power. The larger wind turbines (660 kW – 2+MW) are used in central station wind farms, distributed power and community wind
Resources
Wind is natural source for wind turbines. The moving air is known as wind. The wind energy is one of the earliest forms of energy which is used for delivering mechanical power from a renewable source. The working fluid of wind turbine plants is flowing air or wind which is freely available and thus leading to the reduction of fuel costs whereas other expenditures may be involved. As similar to other power plants, the cost per unit energy generated decreases with the increase in size of wind turbines.[1] Energy storage system must be employed in wind turbine plants in order to compete with anisotropic or random nature of wind energy.[3] Wind turbines can be set up anywhere if sufficient wind velocities were available for an adequate time.
Available energy calculation
It is obvious that wind contains some energy. Therefore, the available energy in wind during time T is given by, where;
k=constant (depends on the air density, efficiency, and size of wind turbine, c=instantaneous wind velocity
Now, maximum power that a wind turbine can develop is
Pmax. = 0.593ρAv3 /2, where; v = mean wind velocity, A = propeller diameter.
Therefore, after considering efficiency factor (ἠ = 0.59)
P = 0.000206Av3 kW
Calculations for different design of wind turbines
Horizontal axis
Axial thrust due to static pressure difference across the disc,
Fx=A(P1 - P2) = 1/2ρA(cu2 - cd2) ,
where; P1 - P2 = Pressure difference across the blade
cu = upwind velocity, cd = downwind velocity.
Now, power developed by the wind turbine propeller is Pi = 16/27(1/2ρAcu3), where; Pi = Maximum power ignoring aerodynamic and mechanical losses.
Also, Power of upwind stream(Pu) = 1/2ρAcu3
So the power coefficient is CPmaximum = (Pi) / (Pu)
Vertical Axis Wind Turbine
Maximum power, Pmaximum = 2/27cFρAc3, where; cF = coefficient depends on the type of blade and size of the machine, c = wind velocity
Maximum energy input to the machine is
Emaximum = 2/9cFρAc3
Also, maximum power coefficient is
CPmaximum = 0.333
Components of wind turbine
Rotor - It consists of blades. The number of blades depends on the speed of rotor. The blade required will be more for low speed and less for high speed. The number of blades will also be decided after the calculation of vibration. The blades are often subjected to high and alternating stresses so should have high strength and light weight.
Step-up gear - It is used to obtain required speed between the driving and driven shaft. It consists of gear train, which should be highly efficient and light weight.
Speed regulating mechanism - From the point of view of aerodynamics, a constant blade to wind speed ratio IS required. A propeller type of pump and a hydraulic brakes are good speed regulators for both fixed and variable pitch blades.The mechanism for variable pitch blades which is used to make efficient operation of rotor at variable wind velocities is same as Kaplan hydroturbines.The relationship between power developed by various wind turbines of different propeller diameter at different wind velocitiescan be seen from figure 3.
Electric generator - Both direct and alternating current machines are used as electric generators. Direct current generator can work at a wide range of speed but a. c. generator requires fixed speed.
Orientation mechanism - This mechanism is needed in horizontal axis wind turbine to turn the rotor in the wind stream but vertical axis tubine doesn’t need this because they don’t depend on the wind direction.Some of the most common modern orientation mechanisms are :-
By incorporating a wind vane which is used to orient small windmills in the direction of wind. It is the simplest and widely used method.
By incorporating an automatic direction finder and orientation mechanism.It is faster mechanism.
By incorporating a fan tail mechanism.This is relatively slower mechanism.
Tower - Rotor is to be mounted at high altitude because of high wind velocity and for this purpose a tower is used. Economic and vibration problem are the major factor in designing the tower
Analysis of Suitable Place
t should be noted that the wind cannot be stored elsewhere or in a way it should be pointed out that wind is available in a highly diluted form.The prediction of characteristic of wind due to the wide variations with time during the year and location is quite difficult and thus proper study of both the long and short range behavior of wind of a given location are required for the economic installation of a wind power plant at the corresponding site.
As the wind power is proportional to the cube of wind velocity,the power output of wind turbine increases rapidly with its height above the ground.
Variation of wind velocity with altitude
Although installation and maintenance cost increases with the height above the ground therefore there should be balance between them.An important factor is the wind direction at that site, the more preferential site is that which has prevailing wind. The wind that blows preferentially in one direction than the other directions are known as prevailing wind.The locations which has prevailing wind does not require any orientation mechanisms.The effect of wind energy on wind energy is shown by Wind rose.
Selection of site
Major considerations for selecting the site are :-
High mean wind velocity.
Open area, not surrounded by mountain, buildings, forest etc. as presence of these retard the wind.
Good land quality for such a huge foundation.
Climatic conditions or topography.
Should be located at a distance from the site of application.
Local labor and building materials should be easily available.
Low temperature problem.
Efficiency of wind turbines
A wind turbine is an exclusive form of turbo machine which often operates at relatively lower speeds. Efficiency of wind power plant is maximum at its rated (design) wind velocity and thus efficiency decreases with the fluctuations in wind. The lowest velocity at which the turbine develops its full power is known as rated wind velocity. 8 kmph is the minimum wind velocity below which no useful power output can be produced from wind turbine. There is limitation on both minimum and maximum wind velocity for the efficient operation of wind turbines and 56 kmph is the upper limit on wind velocity above which wind power plants are uneconomical. Therefore, they should work in these velocity ranges.
Factors affecting efficiency
The efficiency of wind turbines are mostly dependent on five important factors:
Wind power- The amount of power generated by a wind turbine would be greater if the wind blows faster and thus applying stronger force. Wind speed vary with the variation in locations and thus relationship between wind velocity with power production should be properly understood. P=Kvf, where P= wind power, V= wind speed, F=wind force and k is a constant
Altitude- Due to various atmospheric factors, wind speeds are higher at higher altitudes there would be lesser obstructions from the surrounding hills, trees and buildings. Adequate study of relationship between wind velocity and altitude shows that if the distance between turbine and ground is doubled then there would be approximately 12% increase in wind speed.
Obstructions- The various factors such as trees, buildings, hills etc. restrict the free air flow by creation some form of friction which eventually slows down the wind and thus reduces the efficiency.
Blade aerodynamics- The angle of airfoil with the direction of relative wind which is known as angle of attack has a significant effect on amount of lift. Turbulence develops and drag increases when angle of attack becomes too large which results in loss of lift.
Thus in order to get maximum efficiency the shape of blades should be accurately selected.
Air temperature- The cold air has higher density and thus increase power output. It has been observed that power output from a wind turbines increases by approximately 16% when the temperature of air is lowered from +20 deg celsius to -20 deg celsius for any given wind speed.
Advantages and Disadvantages
Advantages of Wind turbines
Wind is a renewable source of power and thus available in plenty.
It is eco friendly therefore, no carbon-dioxide emission thus no greenhouse effect.
Wind turbine plants are easier to set-up with easily available materials in a very short time.
Installation of wind turbine is labour intensive so also good for employment.
Fuel expenditure vanishes and thus saves fossil fuels.
After installation no cost for producing electricity except maintenance.
Wind turbines can also build on the sea.
The wind turbine has a operating life of over 30 years and therefore long lasting.
Disadvantages of Wind turbines
Setting or capital cost for wind turbine plant is high.
It requires large area of land for the set-up of wind turbine plants therefore disturbing the habitations of the local people and thus disturbing the environment.
Limited number of places where there is prevailing wind.
Turbines cannot operate continuously for a large part of the year.