11 Principle and Applications of Wind Power

Dr. Dhanya M.S M.S

1. Introduction

The wind power is one of the indirect solar energy technologies. The wind is the air in motion resulting from the pressure gradient caused by solar radiation. About two per cent of the solar radiation reaching the earth’s surface is converted to kinetic energy and this is sufficient to meet the energy demands if it is harnessed. The kinetic energy of the wind is utilized directly or converted to mechanical energy or used for electricity generation. Apart from its use for grinding grains and pumping water by wind mills, wind turbines are familiar for electricity generation. The nature of the terrain, degree of cloud cover and angle of the sun plays important role in wind generation of an area. Various factors affect the distribution of wind energy are as follows:

(i) The chain of mountains channelizes the air currents.

(ii) The hills, trees and building act as obstructions and change the direction of airflow.

(iii)The frictional effect of the surface determines the wind speed. This is the reason why wind speed is quite high at seashore as frictional effect is less on smooth surface or sea surface.

(iv) Climatic disturbances resulting from rains affect wind speed.

(v) Topography of an area also affects wind speed, for example, wind can speed up when passing through narrow gap such as mountains gap.

2. Energy available in the wind

The power in the wind is computed by the concept of kinetics. The wind mill works on the principle of converting kinetic energy of the wind into mechanical energy.

Power = Energy/unit time

Energy available = Kinetic energy of the wind

3. Wind Energy Conversion System

The air circulation results from non- uniform heating of earth’s surface by the sun. The expansion of air occurs immediately above a warm area which is forced upwards by cool, denser air which flows in from the adjacent area. The energy in wind is due to its motion

The wind energy is harnessed by wind turbine which is mounted on a tower. The speed of the wind depends directly with height. The tower height equal or more than 30 meters (100 feet) is beneficial as it can use faster and less turbulent wind. The wind speed at a height of 10 m is 20-25 percent higher than the surface and 30-60 percent at a height of 60 m. This is due to reduction in drag effect of the earth’s surface. The average wind speeds in a particular location need to exceed at least 6–8 metres per second (m/s) for a small wind turbine to be economically viable.

Wind energy conversion systems are referred as WECS or aerogenerators or wind turbine generators or wind turbines.

The factors influence the output from a wind energy converter

Wind speed Cross section of wind swept by rotor

Overall conversion efficiency of the rotor, transmission system and generator or pump

The maximum wind energy available is proportional to the cube of the wind speed. So a small increase in wind speed may increase the wind power.

4. Lift and Drag Force

The basis for wind energy conversion is Lift and Drag. The extraction of power from the wind depends on creating certain forces and applying them to rotate a mechanism. There are two primary mechanisms for producing forces from the wind

• – Lift and

• – Drag

The lift forces act perpendicular to the air flow while drag forces act in the direction of flow. These forces are produced by changing the velocity of the air stream flowing over either side of the lifting surface speeding up the air flow causes the pressure to drop, while slowing the air stream down leads to increase in pressure. The change in velocity leads to pressure difference across the lifting surface. This pressure difference produce a force that begins to act on high pressure side and moves towards the low pressure side of the lifting surface is called airfoil.

A good airfoil has a high lift/drag ratio. The lift increases as the angle formed at the junction of airfoil and the air-stream becomes less acute upto the point where the angle of the air flow on the low pressure side becomes excessive.

Due to turbulences the lift decreases and drag force increases substantially, which is known as stalling.For efficient functioning of the wind turbine there should be high lift but less drag force.

Apart from airfoil, two other mechanisms for producing lift are Magnus effect is that caused by spinning a cylinder inn an air stream at a high temperature. The spinning slows down the air speed on the side where the cylinder is moving into wind and increases it on the other side. The air is blown through narrow slots in a cylinder and it emerges tangentially is known as Thwaits slot. This creates rotation of the air flow which in turn generates lift.

5. Terminologies related to wind turbine

5.1 Swept area: The area perpendicular to the wind direction that a rotor will describe during one complete rotation or the area of imaginary circle formed during the rotation of wind turbine is called swept area.

5.2 Diameter of the intercept area: The maximum power available from the wind also depends on the square of diameter of intercept area. If doubled the diameter of the rotor, the available wind power will increase by four times.

5.3 Different wind velocities:

(i) Wind velocity ( o). The velocity of free air stream at sufficient distance away from wind turbine where there is no disturbance due to rotation of the wind turbine.

(ii) Incident wind velocity ( 1).It is the velocity at which the wind strikes the blade this velocity is always slightly lower than the free air stream velocity( o).

(iii) Blade linear velocity ( ).It is the velocity of the blade due to the rotation of blade. If ω is the angular velocity of the blade and R is the length of the blade,then blade linear velocity υ = ω × R.

(iv) Relative velocity( r). It is the relative velocity of wind with respect to the moving blade. It is the vector sum of incident wind velocity( 1) and blade linear velocity (υ) mathematically, it can be given as:

The specified wind speed at which a wind turbine’s rated power is achieved is known as rated wind speed.

Survival wind speed/extreme wind speed: It is the maximum wind speed that a wind turbine is designed to withstand.

5.4 Angle of attack or angle of incidence ( ): It is the angle between the centerline of the aerofoil (blade cross- section and the relative wind velocity r) as shown in fig. the airflow remain attached to the aerofoil for small angle of attack. The airflow is separated from the aerofoil for large angle of attack.

5.5 Pitch angle or blade setting angle ( ): It is the angle between the centerline of the aerofoil and the direction of linear motion of the blade. The output of a turbine is greatly influenced by blade pitch angle. The blade pitch control is a very effective way to control the output power, speed or torque of the turbine.

.

5.6 Drag force ( FD): When a body (aerofoil) is placed in uniform airflow, there are two force acting on the body, namely, pressure force acting normally to the surface of the body, and shear force can be now resolved in the selection parallel to airflow is called drag force. The drag force always opposes the relative motion between the body and the air. It is given by the following equation:

FD = CD x1/2 ρ 12.A

Where CDis drag coefficient and A is projected area of the body aerofoil perpendicular to the direction of air flow.

5.7 Lift force ( FL):

The component of the total force (pressure force and shear force) on the body in the direction perpendicular to airflow is called lift force. As the name suggests, this force tries to lift the body. It is given by the following:

FL = CL x1/2 ρ 12.A where CL is lift coefficient

5.8 Axial force ( FA)

The total force (pressure force and shear force) can also be resolved along the axis of rotation of blade and perpendicular to it (tangential force on the blade). The component of total force acting on the blade along the axis of rotation of the blade is called the axial force. The axial force does not contribute to the rotation of the blade. It is also called thrust force and has to be balanced by a suitable reaction force generated by any thrust bearing provided on a rotor. The axial force contributes to waste energy which can be given as follows:

FA = FL cos + sin

= +

5.9 Tangential force ( FT):

It is the component of total force on the blade acting tangential to its circular path of rotation this is the force which contributes mainly to the useful energy extracted from the wind energy. It should be as high as possible. It is given by:

FT = FL sin + FDcos

6. Parameters used in rotor design of wind turbine

6.1 Solidity

It is the ratio of projected area of blades to the swept area

Solidity = Projected area of blades

Swept area Example:

Example: A wind turbine has a rotor of 6m with 30 blades. Each blade has width of 0.30 m. Find the solidity?

The greater is the solidity of a rotor, the slower it need to turn to intercept the wind. When wind will has a lesser number of blades, it needs to rotate fast to intercept the wind so that wind should not be lost through the large gap existing between two blade without importing a part of wind power.

6.2 Tip speed ratio ( ): the tip speed ratio is defined as the ratio of the speed of tip of the rotor blade to the speed of coming air. Hence, tip speed ratio, TR is:

For a particular wind speed, there exists an optimum turbine tip speed to produce the maximum output. It is important to match the rotation of wind turbine to the corresponding wind speed.

The rotor blades of a wind turbine interact with wind. The slow rotation of the blades allow wind to pass freely through rotor swept area without interaction while for the fast movement rotor swept area acts as barrier deflecting win and cause turbulence thereby losing wind power.

The optimal tip speed ratio for horizontal-axis, three blade turbines, which are predominantly used in onshore and offshore wind farms, is approximately 5:1 to 6:1.

Example:

Find the tip speed ratio if a 6 m diameter rotor has rotation of 20 rpm and the wind speed is 4 m/s?

The wind mill rotating fast has tip speed ratio greater than one. Two or three blade rotors rotate faster and have tip speed ratio of 3 to 10. More bladed rotors rotate slowly having tip speed ratio between 1 and 2.

7. Principle of power generation from wind:

Wind turbine is used to extract useful energy from wind. The energy can be extracted by partially decelerating and expanding the airstream (reduction of pressure) using wind turbine. The rotor of the wind turbine collected wind from the whole area swept by the rotor. The area swept can be considered as airstream tube which is continuously expanding air (Fig 1,2,3). This airstream model is also called Betz model of expanding air. As air mass flow rate should be the same must decrease within the stream tube according to the law of continuity, the wind speed must decrease as air expands. As shown, airstream tube has area of A0 at upstream, area of A1 while passing through rotor blade (aerofoil) and area A2 downstream.

Consider that oand 2 are wind velocities at upstream and downstream. The velocity reduction from oto 2 means that there is reduction in momentum of wind as it passes through wind turbine, resulting in a force being exerted on the blade of rotor, given by following equation

Force , F = m x    o – m x     2 = m (   o –     2) …………… (8)

where ‘m’ is mass flow per unit time through stream tube.

The above force is exerted at a uniform rate when airflow moves over the rotor blade with velocity of 1. Therefore power extracted is equal to the work done by airstream in moving for a distance of 1 against force of F which is given by

Power of turbine, PT = F x   1 …………..(9)

The power coefficient, Cp indicates the portion or fraction of wind power that is extracted by wind turbine. The ideal or maximum, theoretical efficiency, maxof a wind turbine, also known as power co-efficient is the ratio of maximum power obtained from the wind to the total power available in the wind. It is the ratio of actual power to the theoretical power.

8. Theoretical maximum wind power

The air density is proportional to the air temperature and the air pressure, both of which vary with height above sea level. The power in the wind cannot be completely converted to mechanical energy of a wind turbine. The losses due to friction reduce the actual wind power. The theoretical maximum of energy extraction from wind was calculated by Betz in 1926.

The power output from wind turbine is given as follows

As per Betz theory a wind turbine able to utilize only 59 percent of the wind power even if no losses has occurred and so the power is multiplied by a factor of 0.59 . When some unavoidable swirl losses occur the turbine is able to utilize only 42 percent and the value reduces to 0.42.

Almost all large wind turbines are of horizontal axis wind turbine designs having an upwind rotor with three blades, attached to a nacelle on top of a tall tubular tower. So the wind energy is captured by the rotor blades in the turbine. So the kinetic energy is converted by wind turbines to mechanical power. The conversion of kinetic energy into mechanical or electric energy is Pmech = M Ω = M 2 π n

where  M – torque

n – rotational number

Ω – angular velocity

The mechanical power in the rotation of blades is as follows

The mechanical power in the rotor blades is coupled to a transmission or gear box having the transmission or gearbox efficiency.

The optimization of wind turbine designs to suit the different wind conditions and use of a variable pitch control system, yaw control, large sized blades, variable speed drive, tilt control of blades, stall control by shifting of blades, eddy current bracking system to maintain speed, power electronics can increase the efficiency of a wind turbine

9. Selection criteria for wind turbine siting:

The main consideration for selecting a site for wind generator is as follows:

High annual mean wind speed. A basic requirement for a successful use of a windmill of farm is an adequate supply of wind and good wind speeds. The wind power is proportional to the cubic power of wind speed.

No obstruction. There should not be any high structure to obstruct wind for a distance of 3 km to the windmill.

Open plain. The site should be the open plain such as open sea shoreline when the windmill is located on hill or a ridge with gentle slope.

Height. The wind speed increase with height, which can be obtained when the windmill is located on a hill or a ridge with gentle slope.

Near lake or ocean. Differential heating of water and land generates wind of sufficient speeds.

Topography. Topography such as mountain gap helps to channelize and speed up winds. Favourable land cost. It helps in restriction or reducing the cost of project.

Nearness to load centre. It reduces the cost of transmission of the generated power. Nearness to road or rail link.It helps in installation of windmill.

Availability of wind rose. It helps in designing of windmill as wind data of the site can be determined.

10. Application of wind energy

There are two types of small scale wind turbines utilized in domestic, community and wind energy projects. The power generated by wind turbines are connected to grid (state or central).

The first windmill used for the production of electric power was built in Scotland in July 1887 by Prof James Blyth of Anderson’s College,

10.1 Stand-alone systems- are used for electricity production for battery charging. It is used for running small electrical appliances mainly in remote areas with no power supply. It helps in water heating or pumping, electric livestock fencing, lighting or any kind of small electronic system needed to control or monitor remote equipment.

10.2 Grid-connected Small Wind Turbines- The output from the wind turbine is directly connected to the existing mains electricity supply. This type of system can be used both for individual wind turbines and for wind farms exporting electricity to the electricity network.

10. Wind farms

The group of wind turbines which are located in the same place for the electricity generation is known as wind farm or wind power plants. It varies in its capacity of few Mega watts to hundreds ofMega watts.A large wind farm may consist of several hundred individual wind turbines distributed over an extended area, but the land between the turbines may be used for agricultural or other purposes. Gansu Wind Farm is the largest wind farm in the world. A wind farm may also be located offshore.

11. Benefits of wind energy

The wind energy is a potential energy source in rural areas. It is useful in rural areas where the electricity is not accessible.

The energy production is high from a suitable wind turbine design It has low maintenance cost

Pollution free and so environmental friendly

12 Limitations of wind energy

The wind is

• – erratic,
• – unsteady and
• – not reliable as it varies over the places and time

13. Wind energy in India

India is the fourth largest wind energy producer with installations of 31 GW by March 2017. It accounts for around 6.6 percent share f global market. The world leader is China followed by USA and Germany. The wind energy potential in India is estimated to be 2.5 x 104 MW with installed capacity of about 1870 MW. The government has started wind energy demonstration programmes with installation of wind pumps, wind battery chargers stand alone wind electric generators and wind farms.

WEG of 2 MW capacity is installed in Suzlon (Tamilnadu) is the largest power rating in India. The maximum WEG installations are in Tamilnadu of capacity about 1639MW. And the maximum gross estimated wind potential is in Gujarat of 9675 MW.

Around 233 wind monitoring stations, covering sites in 14 States/UTs, viz. Tamil Nadu, Gujarat, Orissa, Maharashtra, Andhra Pradesh, Rajasthan, Lakshdweep, Karnataka, Kerala, Madhya Pradesh, West Bengal, Andaman &Nicobar, Jammu and Kashmir and Uttaranchal have an annual mean wind power density greater than 200 watts/ m at 50 m height. Centre for Wind Energy Technology (C-WET) of MNRE located in Chennai, Tamil Nadu is responsible for wind resource assessment in the country including R&D, technology upgrading, testing, certification and standardization in close association with the wind turbine industry.

Conclusion

The wind energy in a location depends on wind velocity and swept area of the wind turbine system. The selection criteria of the sites are important in increasing the efficient wind energy production. Wind energy along with solar energy harnessing will help in full utilization of land resources and transmission of power produced from the renewable energy sources. The on-shore and offshore wind energy potential is immense in India.