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When solar radiation enters the atmosphere, different regions of the atmosphere are heated to different degrees because of earth's curvature. This heating is higher near the equator and lowest at the poles. Since air tends to flow from warmer to cooler regions, this causes winds. It is these air flows, which are harnessed.
Wind energy has been in use in sailing and grinding and other mechanical applications for centuries. It has emerged as a viable renewable energy option with application in electricity generation, water pumping and battery charging. Generation of electricity has emerged as most important application of wind energy worldwide.
In India, the best winds are available during monsoon from May to September and low wind speeds from November to March. Annual national average wind speed is 5 - 6 m/s. The distribution of wind resources in India is given in the map below:
The technology is simple: flowing wind causes rotation of turbine blades and generation of electricity. The blades and generator (housed in a unit called 'nacelle') are mounted at the top of the tower. Wind turbines generally have three rotor blades, which rotate with wind flow and are coupled to a generator either directly or through a gear box. The rotor blades rotate around a horizontal hub connected to a generator, which is located inside the nacelle. The nacelle also houses other electrical components and the yaw mechanism, which turns the turbine so that it faces the wind. Sensors are used to monitor wind direction and the tower head is turned to line up with the wind. The power produced by the generator is controlled automatically as wind speeds vary. The rotor diameters may vary from 30 m to 90 m, whereas the towers on which the wind electric generators (WEG) are mounted, range in height from 25 to 80 m.
A wind generator will produce lesser power in summer than in winter at the same wind speed as air has lower density in summer than in winter. Similarly, a wind generator will produce lesser power in higher altitudes, as air pressure as well as density is lower than at lower altitudes.
In order for a wind system to be effective, a relatively consistent wind flow is required. The following table shown the usability of wind speed:
|Average wind speed (m/s)||Suitability|
|up to 4||No good|
Obstructions such as trees or hills can interfere with the wind supply to the rotors. To avoid this, rotors are placed on top of towers to take advantage of the strong winds available high above the ground. The towers are generally placed 100 m away from the nearest obstacle. The middle of the rotor is placed 10 m above any obstacle that is within 100 m.
WEG starts at a cut-in wind speed of 3 - 3.5 m/s and generally generates power at 4.4 m/s or more wind speed. The optimal wind speed is 10 -14 m/s for power generation. The unit capacities of WEG's presently range from 225 kW to 2 MW, and they can operate in wind speeds from 2.5 m/s to 25 m/s. The power generated by wind turbines is conditioned properly so as to feed the local grid.
The capacity factor (CF) of a wind turbine is actual energy output for the year divided by energy output if the turbine was operated at its rated output for entire year. A reasonable capacity factor will be in the range 0.25 - 0.30 and a very good capacity factor will be 0.40. It is important to select a site with good capacity factor, as economic viability of wind projects is extremely sensitive to capacity factor. Each kW capacity generated around 3 kW⋅h electricity per day.
Wind speed data of potential locations is compiled for a period of one to two years, to identify locations suitable for WEGs. Then WEGs are installed with appropriate distance between them so that disturbance among them is minimal.
Wind turbines are produced indigenously by 18 manufacturers. Around 42 models in the capacity range of 250 kW to 2.5 MW are available.
Wind Power Projects
National Wind Power Program:The National Wind Power Program was initiated in India towards the end of sixth plan, in 1983-84. The program aims at survey, assessment of wind resources, setting up demo projects and provision of incentives. As a result, wind power has emerged as an option for grid-quality power. Government is supporting program through fiscal and promotional incentives, such as 80 % accelerated depreciation, concession of custom duties, 10 % tax holiday on income and excise duty exemption. India now ranks fifth in the world after China, US, Spain and Germany. Most of the capacity addition has been made by private investors.
Cost: The cost of wind power generation varies from ₹ 6 - 6.5 crore per MW depending on site characteristics. The projects have a pay back period of 5 to 8 years depending on machine, site, loan etc.
Wind off-grid Projects
Water-pumping Wind Mill: A water-pumping wind mill pumps water from wells, ponds and bore-wells for drinking, minor irrigation, salt farming, fish-farming etc. Available wind mills are of two types: direct drive and geared. Such wind mills start lifting water when wind speed approaches 2.5 m/s. Normally, a wind mill is capable of pumping water in the range of 1000 - 8000 LPS, depending on the wind speed, water table depth, and type of wind mill. Wind mills are capable of pumping water from depths up to 60 m.
However, water pumping mills have limitation that they can be operated satisfactorily in medium wind of 12-18 km/h. Further, special care is needed for site selection which should be free from obstacles like building or tree.
Cost: ₹ 45,000 to 1.5 lakhs depending on type. Add ₹ 10 000 to 20 000 for foundation, storage tank. As the system has moving parts, it needs frequent maintenance and AMC cost about ₹ 2 000 p.a.
Aero-Generators: An Aero-Generator is a small wind electric generator having a capacity of up to 50 kW. Aero-Gernerator is suitable for power generation in unelectrified areas having good wind speeds. It produces optimal power at a wind speed of 8 m/s. It consists of rotor of 1 - 10 m diameter having 2 - 3 blades, permanent magnet generator, control devices, yaw mechanisms, tower, storage battery etc.
Aero-Generators cost about ₹ 2.0 lacs per kW. AMC cost is about ₹ 2 000 per kW p.a.
Wind-Solar Hybrid Systems: When an Aero-Generator and an SPV are interfaced, the power generation is mutually supplemented, and the resultant hybrid system offers a reliable and cost-effective electric supply in a decentralized mode. During the period of bright sunshine, solar energy is utilized while wind produces most of the energy during monsoon. The wind-solar hybrid consists of one or two Aero-Generators along with SPV panels of suitable capacity, connected with charge controller, battery bank and inverter to supply AC power. The major advantage of the system is that it meets the basic power requirements of non-electrified areas. The power generated from both is stored in a battery bank for use whenever required. Diesel generator is still required, but its use is minimized.
The cost of the system varies from ₹ 2 - 2.5 lakhs per kW. Repair and maintenance costs are about ₹ 2 000 per kW p.a.
Potential and Achievements: Water-pumping wind mills require only medium wind. Considering the availability of required wind speeds and the level of prevailing water table, potential exists for water mill in almost all states except in hilly and rocky regions. Aero-generators and wind-solar hybrid system required high wind and good solar radiation. Potential exists in AP, Gujarat, Karnataka, Kerala, MP, Maharashtra, Orissa, Rajasthan, TN, Uttrakhand, UP, WB and windy regions of J&K and NE states.