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Essay # 1. Introduction to the Novel Wind Turbines:

Propeller-type wind turbines which work on aero-foil design are the main wind turbines used for wind power generation, there are new designs being investigated which may offer technical and economic advantages in future over the existing designs.

Some of the new turbines are:

A. Magnus rotor wind turbine.

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B. Madaras rotor wind turbine.

C. Darrieus Machine.

A. Magnus Effect:

A uniform non-viscous air stream when flows past a vertical cylinder, stream lines as shown in Fig. 4.15 are generated. The velocity at any point on the surface of the cylinder.

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Vθ = 2 Vi sineθ … (1)

Where,

Vθ = air velocity on the surface of the cylinder at angle θ.

Vi = incoming uniform air velocity, far from the cylinder

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θ = polar angle, measured from the stagnation point.

Steam Lines across a Cylinder

Applying Bernoulli’s equation to the point at angel θ on the surface of the cylinder,

Plotting these values on a cylinder, there is a symmetrical distribution of pressure and there is no resultant force. Therefore, there is no lift or drag on the cylinder for ideal non-viscous flow as shown in Fig. 4.16.

Stream Lines and Pressure Distribution for Nonviscous Flow

If a cylinder is rotated about its axis in still air, a circulatory flow is pro­duced around the cylinder where the velocity is inversely proportional to the distance from cylinder axis.

If a cylinder is rotating in a stream of air, the above two cases will be superimposed in such way that at cylinder top the two velocities will reinforce each other and at the bottom of horizontal cylinder, the two velocities will oppose each other.

The pressure on the cylinder will be higher where the velocities are low.

The Lift Force:

Using Barnaullis equation,

Velocity Stream Lines around a Rotating Cylinder an Air Stream

The lift force can be obtained by integrating the pressure components perpendicular to free stream.

Where FL = lift force on cylinder

H = length of cylinder

FL = CL Aρ Vi2/2

Where CL = 2π Vp/Vi = lift coefficient (dimensionless)

The Drag Force:

The imbalance in pressures results in drag force in the direction of free stream.

FD = CD Aρ Vi2/2

Where,

CD = drag coefficient (dimensionless)

CD is a function of Reynolds number.

The force acting on the cylinder is the resultant of lift and drag forces.

B. The Madaras Rotor Wind Machine:

A number of tall cylinders are rotated about their axes in the presence of wind. The resultant of lift and drag forces propels the cylinders horizontally along a track as shown in Fig. 4.18.

Madaras Rotor

A continuous train of four-wheel flat cars move on a circular train track. Each flat car has 30m high cylinder rotated about its vertical axis at 120 rpm by a small electric motor. The flat cars move on a circular track by the resultant force on the cylinder. The carriage wheels drive electrical generators and the electricity generated is transmitted to the mains by a trolley.

C. The Darrieus Machine:

It is a vertical-axis machine and looks like an egg beater with two or three slender blades looped over the top and joined at top and bottom to the vertical axis see Fig. 4.19.

Darrieus Machine

Darrieus machine can spin in any wind direction. It does not require watch control. It is a vertical axis machine and electrical generator or pump can be installed at ground level.

Essay # 2. Specifications of Wind Turbine:

i. Wind Turbine for Water Pumping:

Specific speed or tip-speed ratio of a windmill rotor is the ratio of its peripheral speed to wind speed. Rotors of low tip-speed ratio are preferred for driving piston pumps, which require a high starting torque and low rotational speed. The rotor of low tip-speed ratio also gives high efficiency and low cost. The other important characteristics of windmill rotors are cut-in speed and cut-out speed.

Wind turbine for water pumping should have the following characteristics:

i. It should be water pumping, low tip-speed ratio wind mill preferably of vertical axis design.

ii. It should be suitable for installation on open wells, canals, river beds, ponds, lakes, etc.

iii. The total head over which water has to be lifted should be 15-20m with suction head 10.5 m, if installed on ground level.

iv. It should be suitable for rated wind speed of 2 m/s with cut-in speed of 1.5 m/s.

v. Discharge at 2m/s of wind speed and total head of 15 m should be 0.5 to 1 litre/s.

vi. The system design should be suitable for operating a sprinkler system for irrigation.

The wind mill WP-2, developed at National Aeronautical Laboratory, Bangalore can pump 2.9 m3/hour against a head of 10m at a wind speed of 3.5 m/s. A sail windmill developed at the same laboratory uses six triangular canvas sails on a rotor of 7m diameter and pumps 5.4 m2/hour of water at a head of 10m at wind speed of 3.5m/s.

The specifications of a propeller type windmill pump-set, which has been standardised for installation in India, are listed below:

Tower height = 7m

No. of propellers = 12

Rotor diameters = 5m

Machine power ratio = 0.36

Water depths = 20m

Maximum daily output = 23m3 at a wind speed of 10m/s.

The total cost of the machine including transportation and erection is Rs. 11750 and a similar expenditure is to be incurred on well boring and construction of storage tank. The Government provides a subsidy of Rs. 7500.

The windmills installed so far are not only of small size (about 5m diameter), but also require high maintenance and support. Research and Development (R and D) efforts are needed so as to reduce the maintenance expenditure of windmills to occasional oiling and changing the pump washer only. Somewhat larger windmills have to be developed than the presently available.

ii. Wind Turbine for Electric Power Generation:

The technical data of wind power plants with an electric power output in the range of 500 kW to 2MW are listed in Table 4.1.

Specifications of Windmills for Power Generation

The annual energy production of a wind power plant depends upon the wind speed and the rotor diameter as shown in Fig. 4.10.

Wind Power Plant Output

Important trend in the development of wind energy technology is the appli­cation of high capacity wind turbines.

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