WIND ENERGY

WIND ENERGY- AVAILABILITY AND APPLICATIONS

              

                Coal, gas and oil will not last forever. So renewable are expected to play a key role. Wind energy is the fastest growing renewable. Wind turbines are up to the task of producing serious amounts of electricity. Turbines vary in size from small 1 kW structures to large machines rated at 2 MW or more.

                In future, fossil fuels are likely to reach their maximum potential, and their price will become higher than other renewable energy options on account of increasingly constrained production and availability. Therefore, renewable are expected to play a key role in accelerating development and sustainable growth in the second half of the next century, accounting then to 50 to 60% of the total global energy supply.

PRINCIPLES

                 The basic principle of winds is the uneven heating of earth’s surface and rotation. The air gets heated up due to geothermal energy. The hot air rises up and starts moving. The air starts moving with higher velocity due to rotation of earth, which results to wind.

                The winds are classified into various categories based on the velocity. It is basically measured on basis of a scale termed as Beaufort wind force scale. The various types of winds are listed in the table as follows:-

Beaufort No.	Description	Wind Speed	Wave Height	Sea Conditions	Land Conditions
0	CALM	< 1 km/h	0 m	Flat	Calm, Smoke rises vertically
		< 1 mph
		< 1 knot	0 ft
		< 0.3 m/s
1	LIGHT AIR	1.1–5.5 km/h	0–0.2 m	Ripples without crests	Smoke drift indicates wind direction, Leaves and wind vanes are stationary
		1–3 mph
		1–3 knot	0–1 ft
		0.3–1.5 m/s
2	LIGHT BREEZE	5.6–11 km/h	0.2–0.5 m	Small wavelets, Crests of glassy appearance, not breaking	Wind felt on exposed skin, Leaves rustle, Wind vanes begin to move
		4–7 mph
		4–6 knot	1–2 ft
		1.6–3.4 m/s
3	GENTLE BREEZE	12–19 km/h	0.5–1 m	Large wavelets, Crests begin to break; scattered whitecaps	Leaves and small twigs constantly moving, light flags extended
		8–12 mph
		7–10 knot	2–3.5 ft
		3.5–5.4 m/s
4	MODERATE BREEZE	20–28 km/h	1–2 m	Small waves with breaking crests, Fairly frequent whitecaps	Dust and loose paper rose, Small branches begin to move
		13–17 mph
		11–16 knot	3.5–6 ft
		5.5–7.9 m/s
5	FRESH BREEZE	29–38 km/h	2–3 m	Moderate waves of some length, Many whitecaps, Small amounts of spray	Branches of a moderate size move, Small trees in leaf begin to sway
		18–24 mph
		17–21 knot	6–9 ft
		8.0–10.7 m/s
6	STRONG BREEZE	39–49 km/h	3–4 m	Long waves begin to form; White foam crests are very frequent, Some airborne spray is present	Large branches in motion, Whistling heard in overhead wires, Umbrella use becomes difficult, Empty plastic bins tip over
		25–30 mph
		22–27 knot	9–13 ft
		10.8–13.8 m/s
7	HIGH WIND, MODERATE GALE, NEAR GALE	50–61 km/h	4–5.5 m	Sea heaps up, Some foam from breaking waves is blown into streaks along wind direction, Moderate amounts of airborne spray	Whole trees in motion, Effort needed to walk against the wind
		31–38 mph
		28–33 knot	13–19 ft
		13.9–17.1 m/s
8	GALE, FRESH GALE	62–74 km/h	5.5–7.5 m	Moderately high waves with breaking crests forming spindrift, Well-marked streaks of foam are blown along wind direction, Considerable airborne spray	Some twigs broken from trees, Cars veer on road, Progress on foot is seriously impeded
		39–46 mph
		34–40 knot	18–25 ft
		17.2–20.7 m/s
9	STRONG GALE	75–88 km/h	7–10 m	High waves whose crests sometimes roll over, Dense foam is blown along wind direction, Large amounts of airborne spray may begin to reduce visibility	Some branches break off trees, and some small trees blow over, Construction/temporary signs and barricades blow over
		47–54 mph
		41–47 knot	23–32 ft
		20.8–24.4 m/s
10	STORM, WHOLE GALE	89–102 km/h	9–12.5 m	Very high waves with overhanging crests, Large patches of foam from wave crests give the sea a white appearance, Considerable tumbling of waves with heavy impact, Large amounts of airborne spray reduce visibility	Trees are broken off or uprooted, structural damage likely
		55–63 mph
		48–55 knot	29–41 ft
		24.5–28.4 m/s
11	VIOLENT STORM	103–117 km/h	11.5–16 m	Exceptionally high waves, Very large patches of foam, driven before the wind, cover much of the sea surface; Very large amounts of airborne spray severely reduce visibility	Widespread vegetation and structural damage likely
		64–73 mph
		56–63 knot	37–52 ft
		28.5–32.6 m/s
12	HURRICANE FORCE	≥ 118 km/h	≥ 14 m	Huge waves, Sea is completely white with foam and spray, Air is filled with driving spray, greatly reducing visibility	Severe widespread damage to vegetation and structures, Debris and unsecured objects are hurled about
		≥ 74 mph
		≥ 64 knot	≥ 46 ft
		≥ 32.7 m/s

POWER VS VELOCITY

               The velocity of wind increases with the increase in the height above the surface. If we consider a graph of factor of increment in wind speed with height above surface, then we will get a parabolic curve. For the operation of wind turbine to be commercially feasible, average wind speed should be in the range of 13-30 mph. At 25-30 mph, a turbine operates at full capacity and at higher wind speeds the turbine should be shut down to avoid damage.

LIFT AND DRAG FORCES

         There are two primary physical principles by which energy can be extracted from the wind. These are through the creation of either drag or lift force (or through a combination of the two).

The basic features that characterize lift and drag are:

• Drag is in the direction of airflow.

• Lift is perpendicular to the direction of airflow. Generation of lift always causes a certain amount of drag to be developed. With a good aerofoil, the lift produced can be more than thirty times greater than the drag.

• Lift devices are generally more efficient than drag devices.

SOLIDITY AND TIP SPEED RATIO

            The tip speed ratio is defined as the ratio of the speed of the extremities of a windmill rotor to the speed of the free wind. It is a measure of the 'gearing ratio' of the rotor. Drag devices always have tip speed ratios less than one and hence turn slowly, whereas lift devices can have high tip speed ratios and hence turn quickly relative to the wind.

Tip speed ratio = blade tip speed/wind speed

            Solidity is usually defined as the percentage of the circumference of the rotor which contains material rather than air. High solidity machines carry a lot of material and have coarse blade angles. They are inherently less efficient than low-solidity machines due to generation of higher starting torque. The extra materials also cost more money. However, low-solidity machines need to be made with more precision which leads to little difference in costs.

PERFORMANCE CO-EFFICIENT AND BETZ CRITERION

           The proportion of the power in the wind that the rotor can extract is termed the coefficient of performance (power coefficient or efficiency, Cp) and its variation as a function of tip speed ratio is commonly used to characterize different types of rotor. It is physically impossible to extract all the energy from the wind, without bringing the air behind the rotor to a standstill.

          After performing certain analysis, the maximum coefficient of performance achieved is 59.3% by following BETZ criterion.

GROWTH OF WIND ENERGY

         Wind has huge potential for the future world but it is contributing only 0.4% of total energy. Wind energy is freely available, widely distributed, renewable and also nature-friendly. It is estimated that if all the available wind energy is harnessed, it can contribute about five times the total energy demands of the world at present.

           Wind energy is the fastest growing renewable energy source in the world. The world wide installed capacity is growing at a rapid rate of over 30% per year. The total installed capacity of wind by the world in 2004 was 47.6 GW. Out of this, the whole Asian continent shares 10% only. 

         World wide wind generating capacity is less than 5000 MW in 1995 and is 39000MW in 2003, an increase of nearly eight times. The reasons for this rapid growth are as follows:-

 Ø  Declining cost (4-6 cents kWh)

 Ø  Technological advances

 Ø  Revenue for landowners & tax jurisdictions

 Ø  Consumer demand

The available potential for wind in India is 45000MW, out of which at present we are using only 3500 MW. If we consider the installed capacity of all the available energies in India, wind energy shares only 3% of total energy. The installed capacity for wind is rapidly increasing in India.

                In the current world, we are having a lot of varieties in wind turbines. There are many things which are being considered while designing and constructing wind turbines. Please have a look on the following link, if you are interested to know about technologies involved in wind turbines:-

WIND ENERGY – TECHNOLOGY IN WIND TURBINES