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Essay on Wind Energy
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Essay Contents:
- Essay on the Introduction to Wind Energy
- Essay on the Wind Power Plant
- Essay on the Classification of Wind Power Plants
- Essay on the Principles of Power Generation
- Essay on the Wind Energy in India
- Essay on the Power Co-Efficient of Windmills
- Essay on the Forces on the Blades and Torque of Wind Mill
- Essay on the Wind Turbine Operation and Control
- Essay on the Site Selection for Wind Mills
- Essay on the Off-Shore Wind Farms
- Essay on the Wind Diesel Hybrid Systems
- Essay on the New Developments in the Field of Wind Energy
Essay # 1. Introduction to Wind Energy:
The winds account for a power of 2 × 1013 W. Though this is only a small portion of the incident solar energy when compared, but it is more than the total energy consumption rate of the world. It is estimated that just one percent of potential wind power is equivalent to 2 × 1011 W or approximately three percent of the current world energy consumption rate.
If wind power were used- for the generation of electricity, its thermal equivalent would be as high as 8-9 per cent of total. Technical and economic problems tend to limit the attractiveness of wind energy conversion systems, but the potential extractable wind power is not insignificant.
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Following a better understanding of aerodynamics of rotating aero-lifts, there is a revival of interest in harvesting wind energy. The importance of wind as alternative source of energy is increasing with the depletion of fossil fuels and the need to maintain our ecosystem. The Government of India has formulated and launched a special wind energy programme.
At a conservative assessment, wind power potential in India is around 20,000 MW. At present, an aggregate wind power capacity of about 2002 MW has been established in the country and it is proposed to establish additional capacity in the coming years. Although in many countries and some parts of India, electric power is being commercially produced from windmills; wind energy can be more appropriately utilized for pumping water for irrigation due to high variation of power density of wind over time and location.
In India, windmills have been tried for pumping water for drinking, irrigation for fodder crops, forest nurseries, and agro-forestry plantations. But, so far, wind mills have not been tried among individual farmers and in the irrigation of annually cultivated crops. There is a large potential of ground water, especially to operate sprinkler systems.
Essay # 2. Wind Power Plant:
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The wind energy is the kinetic energy of the airflow. The total power of a wind stream is equal to the rate of the incoming kinetic energy of the stream.
The total power of a wind stream is directly proportional to its density, area and the cube of velocity.
The wind velocity (Vi) increases with the height above the earth surface. If Vi is measured at 10m height where air density is 1.205 kg/m3 (at 20°C and 1.013 bar), the power density will be as follows:
Due to comparatively low power density of air stream, wind power plants are required to be built with a correspondingly large rotor size for large power outputs. Presently, a wind power plant of 2 MW capacities with a rotor diameter of 60 m is in operation in Denmark.
Wind speeds increase with height because of reduction in the drag effect of the earth’s surface. The tower height is usually 1 to 2 times the rotor diameter.
The wind velocity across the turbine decreases since kinetic energy is converted into mechanical work. Two types of forces act on the blade of the wind turbine rotor. The lift force acts perpendicular to the direction of air flow as circumferential force in the direction of the turbine rotation that provides the torque. The drag force acts perpendicular to lift force as axial force in the direction of the wind stream that provides an axial thrust. This must be counterbalanced by proper mechanical design.
The main components of a wind power plant are:
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i. Wind Mill:
Wind Mill of a Suitable Size and Shape. The blades are mounted on the hub of the rotor which has a provision inside to adjust the pitch of the blades to suit the wind velocity.
ii. Wind Direction Follow Up Device:
The horizontal axis wind turbines are rotated about a vertical axis with the help of a motor so that maximum swept area of the rotor faces the wind direction. In small turbines, yaw action is controlled by a tail vane.
iii. Coupling:
The turbine rotor is connected to the generator through a hydraulic transmission system.
iv. Hydraulic Transmission:
The speed of wind turbines is controlled by varying the pitch of the rotor blades. The normal speed of rotation is 40 to 50 rpm. It has to be increased to match the generator speed of 1800 rpm. A mechanical gear box is used for top mounted equipment. For bottom mounted equipment hydraulic transmission is used which gives a high degree of design flexibility and large savings in cost.
v. Electric Generator:
For large systems, synchronous generators are used.
vi. Tower:
Truss tower is widely used for wind mills.
Essay #
3. Classification of Wind Power Plants:
The wind power plants can be classified as per different parameters:
I. Axis of Rotation:
There are vertical axis and horizontal axis wind turbines. They use either lift or drag forces to harness the wind energy. The majority of modern wind turbines are the horizontal axis lift machines. Fig. 4.2. Shows simple diagrams of different types of wind turbines.
II. Size of Machine:
According to power output, wind power plants are divided into the following classes:
a. Small Wind Power Plants:
The power output of 10 to 50 kW and a rotor diameter of 1 to 16m.
b. Medium Wind Power Plants:
The power output of 50 to 500 kW and a rotor diameter of 16 to 50 m.
c. Large Wind Power Plants:
The power output of 500 to 5000 kW and a rotor diameter of 50 to 130 m.
The tower (hub) height is usually 1 to 2 times the rotor diameter.
III. Applications:
The main applications of wind plants are:
i. Grinding grains,
ii. Drainage,
iii. Pumping,
iv. Saw milling, and
v. Electrical generation.
The wind power plants are also classified as:
(a) Small Machines:
For autonomous power generation for local use, wind farms, homes and rural use.
(b) Large Machines:
Power generation for central grid system.
Essay #
4. Principles of Power Generation:
The wind mill works on the principle of momentum. The work done by the turbine rotor is the difference between the kinetic energies of incoming and outgoing streams through the rotor.
A horizontal-axis, propeller type wind mill is shown in Fig. 4.3. The thickness of wheel is a-b. The pressure and velocity changes are also plotted.
The exit velocity Ve less than Vi because kinetic energy is extracted by turbine. The pressure pe is almost equal to pi. Applying total energy equation and taking air density ρ = constant.
If A is the projected area of windmill perpendicular to the wind stream, the axial force.
Example:
A 10m/wind is at 1 standard atmosphere and 15°C.
Calculate:
1. The total power density in the wind stream.
2. The maximum obtainable power density.
3. A reasonably obtainable power density.
4. Total power produced if the turbine diameter is 120 m.
Solution:
The air density,
Essay #
5. Wind Energy in India:
The Indian wind energy sector has an installed capacity of 11807.00 MW (as on March 31, 2010). In terms of wind power installed capacity, India is ranked 5th in the World. Today India is a major player in the global wind energy market. The potential is far from exhausted.
Indian Wind Energy Association has estimated that with the current levels of technology, the ‘on-shore’ potential for utilization of wind energy for electricity generation is of the order of 65,000 MW. The unexploited resource availability has the potential to sustain the growth of wind energy sector in India in the years to come. In WEA will help the industry in realizing this potential quickly and efficiently.
The Indian wind energy sector has an installed capacity of 11807.00 MW (as on March 31, 2010). In terms of wind power installed capacity, India is ranked 5th in the World. Today India is a major player in the global wind energy market.
The potential is far from exhausted. Indian Wind Energy Association has estimated that with the current level of technology, the ‘on-shore’ potential for utilization of wind energy for electricity generation is of the order of 65,000 MW. The unexploited resource availability has the potential to sustain the growth of wind energy sector in India in the years to come.
The potential sites of wind energy harnessing in India are in the states of Tamilnadu, Gujarat, Andhra Pradesh, Maharashtra, Karnataka, Kerala, Lakshadweep, Rajasthan, MP, Orissa, UP, Andaman and Nicobar. Among these Tamil Nadu has highest potential of wind energy.
Essay #
6. Power Co-Efficient of Windmills:
The wind velocity changes along the length of a blade depending upon the blade inlet angle and blade velocity. The coefficient of performance or power coefficient Cp is defined as the actual power to the maximum wind power.
The ratio of blade tip speed to wind speed,
λ = U/V
Maximum value of Cp is equal to 0.593 and is called Betz limit. The value of Cp for most of the turbine rotors lie between 0.25 to 0.45 and is plotted in Fig. 4.4 for various types of rotors.
Essay #
7. Forces on the Blades and Torque of Wind Mill:
Aero-Foil Design:
The wind turbine rotor has an aero-oil shape. There is a pressure difference on the two surfaces of the blade as air flows causing the force of lift FL. There is also force of drag FD due to obstruction to the air flow. The torque causing the rotation is due to force of lift. The drag force causes the axial thrust on the blade which has to be overcome by the bearing.
The torque causing the rotation of the wind turbine shaft depends upon the turbine shaft depends upon the turbine rated power output and rotor angular velocity.
Essay #
8. Wind Turbine Operation and Control:
The power output of a wind mill is proportional to square of its diameter and cube of the wind velocity.
A small fluctuation in wind velocity will lead to large fluctuations in power For example; a 20% drop in wind velocity would result in the loss of half the power. The maximum machine efficiency occurs over a relatively narrow power range.
1. Severe fluctuation in power due to variations in wind velocity results in:
(a) Power oscillations on the grid.
(b) Severe strains on the windmill hardware.
2. If a wind turbine is designed corresponding to maximum prevailing velocity at a given site, then:
(a) Low powers are produced most of the time, and
(b) Capacity of the turbine and generator are unutilized.
3. Flat Rating:
It is more cost effective to design a windmill to produce rated power at less than the maximum prevailing wind velocity i.e., using a smaller turbine and generator and to maintain a constant output at all wind speed above the rated capacity. This is called flat rating.
4. Cut-In Velocity:
Because of severe loss in efficiency and power at low wind speeds, a wind turbine is designed to come into operation at a minimum wind speed called the cut-in velocity.
5. Cut-Out Velocity:
To protect the turbine wheel against damage at very high wind velocity, it is designed to stop operation (by feathering the blades) at a cut-our velocity.
6. The wind turbine operates with variable load over a narrow range between the cut-in velocity and rated and at constant power between the rated velocity. The turbine ceases to operate below cut-in velocity and above cut-out velocity.
Example:
Performance data of a wind turbine:
i. Type: Three bladed propeller type.
ii. Rate power output: 600 kW.
iii. Rated wind velocity: 14.5 m/s.
iv. Cut-in velocity: 4.5 m/s
v. Cut-out velocity: 25 m/s.
7. Load Curve:
The performance of a wind power plant is characterized by the annual energy production for which annual load curves are plotted as shown in Fig. 4.8.
Fig. 4.8. Annual Load Curve of a Wind Power Plant.
8. Availability Factor:
It is the fraction of time, during a given period, that the turbine is actually on line. For wind power plants, availability factor is usually 90%.
9. Overall Load Factor:
It is the ratio of the total energy generated during a given period of time to the total rated generation capacity during the same period. This factor takes into account operations at less than rated wind velocity, nonoperation below cut-in and above cut-out velocities, and power outages caused by various situations such as repairs, maintenance, etc. The overall load factor is of the order of 30- 40%.
The overall load factor is also called the plant operating factor or plant capacity factor. The wind turbines don’t always operate at rated power because of changing wind velocities, the overall load factor is much lower than the availability factor.
Referring to Fig. 4.8.
X = rated net capacity of plant,
Y = peak load during period, d
Z = average load during period, d.
Plant overall load factor or plant operating factor
= (A + B + C)/X. d
Plant load factor = Z /Y = average load/peak load
Plant availability factor = a + b + c /d
Where d is usually taken as 1 year
Essay #
9. Site Selection for Wind Mills:
Siting of wind mills depends upon its application and the wind characteristics of the location. Small wind turbines of less than 100 kW capacities are usually used for local power generation and water pumping. Large wind turbines of 100 kW capacity or greater are used in wind farms to generate power for distribution in central power grids. The objective is to minimize the cost of power production.
Suitable sites for large wind mills around the world depend upon favourable wind activity. Maps showing annual average wind-power density, W/m2, are available.
Mean Wind and Energy Velocities:
The wind-power potential can be estimated from the mean velocity, V, which is based on measurement over a period of time.
Mean wind velocity,
Power plant sizing would be grossly under estimated if it were rated at the mean wind velocity. The power depends upon the cube of the wind velocity. The power plants are sized on mean energy velocity.
The mean energy velocity,
Example:
If V̅E greater than V̅ by 25%, the power potential would be (1.25)3 or 2 times less if estimated on V̅.
Total Energy Production:
The total energy production of a wind mill depends upon the following factors:
i. Turbine rating.
ii. No. of hours of operation.
iii. Availability factor.
iv. Variation of wind velocity.
The total energy production can be estimated by Weibull Distribution Model given below:
Exceedance or Wind-Speed Distribution Curves:
The wind-distribution curves are used to estimate the annual power output in MWh/year.
Indian Example:
For a wind speed of 10m/s, the effective power obtained is 0.25 kW/m2 of swept area. As wind velocity of 10m/s occurs only for a few hours a year, the mean annual available power is very less. Depending upon local wind conditions, the annual available energy per m2 of swept surface may vary between 10 kWh for calm areas and 500 kWh for very windy areas.
At-least 25 percent of Indian land especially coastal areas are situated in reasonably windy areas. The scope of small water pumping mills is quite significant.
The wind energy data for Veraval situated on the windy west coast of India is as follows:
Max usable energy in wind
For 30m2 swept areas: 20,000 kWh.
Machine power ratio: 0.20
Annual power output: 4,000 kWh
Water depth: 10m
Average daily output: 273m3 of water
Max, daily output: 2428m3 of water
Essay #
10. Off-Shore Wind Farms:
Large capacity wind farms are being set up off the coasts of Northern Europe.
These farms offer many advantages as compared to on-land wind farms:
i. The wind speeds are higher over the open sea surface especially at a height of 60m above the water surface.
ii. There is less concern about the noise emission.
iii. There is no landscape view deterioration.
A 40 MW farm comprising 20 × 2 MW turbines with a rotor diameter of 76 m and hub height of 60 m has been installed near Copenhagen, Denmark. This farm generates 89 GWh, electricity annually which is sufficient to feed 25000 households. A 100 MW offshore wind farm is planned for installation near Rostock is Germany. Another 100 MW offshore wind farm is planned off the Netherlands’s coast in the North Sea.
These large capacity off-shore wind farms will be using wind turbines of 2 to 6 MW capacity, rotor diameter of 75 to 100 m and hub height 60 m or more above water surface.
Essay # 11. Wind Diesel Hybrid Systems:
A hybrid system is conceived to compensate for the intermittency of wind energy, not by storage but by using the wind as the source of energy when available and sufficient and diesel engine at other times.
A simple combination of a wind energy plant with a diesel power plant is shown in Fig. 4.14. The wind energy plant is always operated in parallel with the diesel engine. It supervises the grid conditions automatically and switches itself on/off depending upon the wind speed. The WEC works as a normal grid, connected machine, and frequency and voltage control is taken over by the Diesel-Generator set.
The load circuits of lower priority at least should be disconnect-able to make possible a simple load adaption.
Advantages:
i. Simple and robust conception.
ii. No additional electronic control unit.
iii. Easily extendable.
Disadvantages:
i. Operation of the wind energy plant possible when the diesel set is on.
ii. Uneconomical part-load operation of the diesel engines cannot be avoided.
Essay # 12. New Developments in the Field of Wind Energy:
The present windmill technology is inadequate for the low wind speed regions in the plains. Special development projects in the following areas must be taken up so that wind energy can also be used in the low wind speed regions.
i. Artificial Winds:
Generation of artificial winds to drive windmills by heating large surfaces with favourable thermodynamic properties is technically feasible. A project report has been prepared to heat a large surface in which case the resulting current (artificial wind) can drive turbines. The efforts needed to pursue the project in the form of money, manpower and time are huge.
ii. Aero-Electric Plant:
The low wind velocity in the plains can be augmented by the use of diffusers at intake to wind mills. Besides the propellers, Madaras and Darrieus, there has been a plethora of designs for wind machines. One intriguing power plant design, called the aero-electric plant, uses the flow up a tower that looks like a cooling tower as shown in Fig. 4.11. Its walls are heated by solar radiation.
Since the walls are circular, the sun’s rays need not be tracked as it changes position in the sky during the day. The heated walls, in turn, heat the inside air and a flow up the tower is established. This air flow is made to drive a number of air turbines located near the top of the tower. The driving pressure causing air flow is given by the well-known chimney effect.
P. Carlson of California has proposed a slightly modified form in which, the interior air in a very tall tower would be cooled by pumping water to the top. The water evaporates in the low pressure air there, causing a downward flow of cooled air. The driving pressure can be calculated in a manner similar to that for wet cooling towers.
A conceptual design of such a plan called for 2.4 km high, 300 m diameter tower located in a hot desert and 10 wind turbines surrounding the tower periphery at the bottom producing 2500 MW.
iii. Low Wind Speed Turbines:
The turbines available in India and abroad are suitable for a rated wind speed of 3.5m/s or more whereas low wind speed turbines for rated values of 1.5 – 2m/s are needed for plain areas. Special efforts are, therefore, needed to develop cheap and simple rotors, which can cut-in at low wind speeds available in the plains.
The Savonious rotor and American multi-blade type windmills have optimum power coefficients at a very low tip-speed and can therefore be used as starting point to develop windmills suitable for low wind speeds.
iv. Energy Storage Schemes:
A combination of a wind mill and a micro-hydro turbine can be used for storage of energy as potential energy of water as shown in Fig. 4.12.
Water could be pumped to high reservoir during periods of high winds and the potential energy could then be used in a micro hydro power plant during periods of low winds, ensuring a constant output from the wind-water generating system as the basis for sizing of water generator and storage system.
v. Windmill with Hydraulic Transmission:
Hydraulic transmission consisting of a split fluid coupling/torque converter can be used to run a centrifugal pump to lift deep ground water for the operation of a sprinkler system for irrigation as shown in Fig. 4.13.
The windmill is directly coupled to hydraulic pump part of coupling/torque convertor while the hydraulic turbine part, which is directly coupled to the centrifugal water pump, is lowered deep in the open well. The variable speed/torque hydraulic pump sends pressure fluid through a pipe to drive the hydraulic turbine to operate the centrifugal pump.
The use of hydraulic transmission has many advantages such as optimum adoption of rotor speed to guarantee a maximum output; no gear box; no controlling and switching equipment’s as no electrical generator is involved; simple maintenance as no reciprocating pumps and allied crank mechanism; and the centrifugal pump can be installed on ground level or lowered into the deep well to suit the suction lift available.