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STUDY

OF

WIND

MACHINES

07 : MG : 26

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PRINCIPLE OF WIND GENERATION: AIR IN MOTION ARISES DUE TO

PRESSURE GRADIENT caused by the non-uniform heating of earth surface by

the sun.

EXAMPLE: WIND FLOW AT THE SEA-SHORE [DUE TO SOLAR RADIATION],

AROUND AND ABOVE THE MOUNTAINS.

IT IS TO BE NOTED THAT THERE EXISTS FRICTIONAL EFFECTS BETWEEN THE

MOVING AIR AND THE EARTHS SURFACE.

AS ALTITUDE INCREASES, THERE IS CONSIDERABLE CHANGE IN AIR

VELICITY.COMPARED TO NEAR THE SURFACE, THERE IS A 20-25% INCREASE IN WIND

SPEEDS AFTER TEN METRES.

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PLANETARY WINDS Solar radiation heats the air near the equator, thus this low density heated air

buoyed up and thus causing winds to flow from the poles towards the equator

by convective circulation.

Due to earths rotation, the net effect is a

large counter-clockwise circulation of air in northern

Hemisphere and clockwise in southern

Hemisphere.

The strength and direction of the

planetary winds change with

seasons as the solar input

varies.

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LOCAL WINDS

LOCAL WINDS are caused due to two mechanisms:

1) Differential heating of land and water2) Hills and mountain sides.

Winds blow more consistently and with greater strength over the watersurface where there is less surface drag.

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POWER IN THE WIND

Any sort of wind energy converter has capability of slowing down the

mass of moving air, extracting part of energy and converting into usefulwork.

FACTORS: 1) Wind speed2) C/S of wind swept by rotor

3) overall conversion efficiency of rotor, transmission systemand

generator of the pump.

At most, only one-third of its free velocity can be extracted.100% efficient aero generator can only convert 60% of kinetic energy

to mechanical energy. Well designed Blades can produce 70% to themaximum.

We know that, Kinetic Energy = 0.5 x Mass x Velocity2

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But, Mass/sec (kg/s) = Velocity (m/s) x Area (m2) x Density (kg/m3)

The above two eqautions when merged gives us the formula for Power of wind:

Power = 0.5 x Swept Area x Air Density x Velocity3

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PITCH CONTROL

Pitch control in a wind machine means to control the pitch or angle of attack thatthat a rotor airfoil presents to the wind stream.

In earlier times, pitch control was done mechanically using a spring system. Higherrotational speeds generates the centrifugal forces which compress a spring. Forcethe weight rotates the blades to vary the blade angle.

Nowadays, electronic controller senses the power outputs at intervals andaccordingly sends a elec. signal to vary the pitch of the blade .

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YAW POWER CONTROL

Yaw control is getting control overrotational speed and power output

by rotating the whole rotor

mechanism out of the wind

direction.

This technique is used for small wind

turbines of 1 KW rated power or less.

It may subject large wind turbines tocyclic stresses that can lead to the

fatigue failure of the entire structure.

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BASIC COMPONENTS OF A WIND MACHINE

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AERO TURBINES convert energy in moving air to rotary mechanical energy.It requires pitch control and yaw control. It transmits power to the generatorthrough mechanical interface consisting of a step up gear and coupling.

Sub-Components: 1) Wind Rotor2) Windmill Head3) Transmission and control4) Supporting Structure

Wind Rotor may be of horizontal axis or vertical axis.Vertical axis rotors operate in all wind directions and

need no yaw adjustments.

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Windmill Head supports the rotor, housing, the rotor bearings and thecontrol mechanisms incorporated.

Transmission. Rotation rate produced by the rotor are very low while therate needed for power generation in very large. Transmission options aremechanical systems involving fixed ratio gears, and chains, singly or incombination or hydraulic systems involving fluid pumps and motors.

Controls needed for1. Startup/cut in of equipment2. Power variations3. Generator output monitoring

4. Shutdown due to high winds5. Auxiliary power

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Towers. Four types of supporting towers deserve consideration:

1. The reinforced concrete tower2. The pole tower3. The built up shell tube tower

4. The truss tower

Minimum tower height should generally be 10 m while the max. practical height is60 m.

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The rotor on an UPWIND turbine is in the front of the unit, positioned similar toa propeller driven airplane. This is the most common type of small turbinesoperating in the U.S. To keep it oriented into the wind, a yaw mechanism suchas a tail is needed.

Its advantage is the reduced tower shading. The air will start to bend aroundthe tower before it passes it so there is some loss of power from theinterference, just not the degree as in the downwind turbine.

But due to repeated changes inwind forces on the tower, it will tendto generate vibrations and mayeventually get damaged.

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The DOWNWIND turbine has its rotor on the back side of the turbine. Thenacelle typically is designed to seek the wind, thus negating the need for aseparate yaw mechanism.

Advantage: Such turbines may be less expensive and can relieve stress onthe tower during high or gusty wind conditions since the flexing allows the windload to act directly on the blades instead of the tower.

Tower shadow is problem with adownwind machine since the rotor bladeactually passed behind the tower. Thiscan cause turbulence and increased

fatigue on the unit.

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CLASSIFICATION OF WEC SYSTEMS

1. According to selection of AXIS:- Horizontal Axis Machines- Vertical Axis Machines

2. According to SIZE:- Small Scale ( up to 2KW)- Medium Size Machines ( 2-100 KW)- Large Scale (above 100 KW)

3. According to the OUTPUT POWER:- DC output- AC output

4. According to the ROTATIONAL SPEED- Constant Speed- Nearly Constant Speed (with fixed pitch blades)- Variable Speed (with fixed pitch blades)

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Utilization of the output power may be in different forms like

Battery storage

Direct connection to an electromagnetic energy converter

Other forms (thermal potential etc.) of storage

Interconnection with conventional electric utility grids

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HORIZONTAL - AXIAL MACHINES

The common with turbines with a horizontal axis may be simple in principle,but the design of a complete system (esp. a large one) is complex.

Some of the horizontal axis type wind machines are as follows:

1. Horizontal axis using two aerodynamic blades

2. Horizontal axis propeller type using single blade

3. Horizontal axis multibladed type

4. Horizontal axis wind mill Dutch type

5. Sail type

All the above types mentioned are briefly explained in the following slides.

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Horizontal Axis using two aerodynamic blades has a rotor with two bladeswhich drives a generator through a step up gear box.

The blade rotor is usually designed to be oriented downwindof the tower.

The rotor blades are constantly flexed by unsteadyaero-dynamic, gravitational and inertia loads

when the machine is in operation.

Because of high cost of blades of blades, rotors morethan two blades are not recommended. Though, rotorswith more than two blades(say 3 or 4) have a slightlyhigher power coefficient.

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Horizontal Axis Propeller type using single blade has a long blademounted on a rigid hub. The system includes induction generator and(planetary) gear box.

In extremely long blades on rigid hub, large blade root bending momentsmay occur

occur due to tower shadow, gravity and sudden shifts in wind directions.

To reduce rotor cost, use of low cost counterweight is recommended whichbalances long blade centrifugally.

Its simple blade controls, low cost and rugged

structure is opposed by the vibration produceddue to aerodynamic torque, blade bending andits unconventional appearance.

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Horizontal Axis Multibladed type machines have multi-blades made of sheetmetal or aluminium, on rotor.

The rotors have high strength to weight ratios and work steadily even at 60 km/hr

winds.

The wind-engaging end of each blade isarc-shaped and is turned about 120degrees to maximize efficiency.

They have good power coefficient, highstarting torque along with simplicity and

low cost.

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Horizontal Axis wind mill Dutch type has one oftheoldest designs.

The blade surfaces are made from array of woodenscale

which feather at high wind speeds.

Sail type Blades are the recent origin.

The blade surfaces is made from cloth, nylon or

plasticsarranged as mast and pole or sail wings .

Horizontal axis types have better performance and their general applications

include electric power generation and pumping water.

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VETRICAL AXIS MACHINES

Vertical-axis machines so called panemones are the wind energy converters that

predominantly use DRAG forces to rotate the rotors.

They have:

1. High starting torques compared to lift devices

2. Relatively low top-to-wind speeds

3. Lower power coefficients at high TSR

4. Difficult to be controlled in strong winds

Though lower efficient, panemones offer simpler

design and low construction and maintenance

costs.

Moreover, the transmission and generator are the

ground .

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Vertical axis machines react to the wind from any direction so no yawing

equipment required to turn the rotor.

As the blades do not turn end over end, the rotor is not subjected to cyclic

gravity loads; a major benefit in design considerations.

Vertical axis machines are less known compared to horizontal axis machines.

The two types of vertical axis machines are:

1. THE SAVONIUS TYPE ROTOR OR S TYPE

2. DARRIEUS TYPE MACHINE

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SAVONIUS TYPE ROTOR

Savonius Rotor is the simplest of the modern types of wind energy conversion

systems which works like a cup-anemometer.

Two half cylinders facing opposite to each other mounted on vertical axis normal

to wind.

Wind dashes cupped face and reduces pressure on its back sides; both help to

drive the rotation. That is due to the wide slot between inner edges.

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Characteristics of Savonius Rotor:

1. Self starting

2. Low Speed

3. Low efficiency

Advantages of Savonius Rotor:

1. As it is vertical, it eliminates the cost of expensive

transmission system2. Wind flow intensity and Direction not an issue

3. Simple structure

4. Yaw / Pitch controls not needed

5. Over all reduced weight

Disadvantages of Savonius Rotor:

1. Machine becomes too solid with unnecessary extra weight

2. Not much useful for tall installations

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Darrieus Rotor

The Darrieus rotor wind machine was originally invented and patented in 1925 byG.J.M. Darrieus, a French engineer.

The machine makes use of an efficient airfoil, which effectively intercepts large areawind with a small blade area.

Along with the advantages a vertical-axis machine, it also minimizes the bendingstresses in normal operation.

It has 2 or 3 thin, curved blades ofairfoil c/s, and constant chord length.

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Pure tension on the blades due to rotation provides stiffness to help withstand thewind forces.

Characteristics of Darrieus Rotor:

1. Low starting torque2. High Speeds3. High efficiency4. Potentially low capital cost

Darrieus Rotors can also be combinedwith various auxiliary rotors to increasetheir starting torque.

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Advantages of Darrieus Rotor:

1. Accepts wind from any direction

2. It can be operated on ground level

3. It eliminated Yaw control and provides a simple structure

4. Airfoil manufacture costs are quite less than conventional rotor blades. Absenceof pitch control reduces further costs.

Disadvantages of Darrieus Rotor:

1. Requires initial Mechanical Aid for start up

2. Rotor efficiency lesser than conventional horizontal rotors

3. As at the ground level, it experiences lower wind speeds

4. Vibratory stresses encountered due to varied local flow conditions per revolution

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PERFORMANCE CURVES OF WIND MACHINE

C(p) TSR

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C(t) TSR

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