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The sun?s energy falling on the earth produces large-scale motions of the atmosphere causing winds, which are also influenced by small scale flows caused by local conditions such as nature of terrain, buildings, water bodies, etc. Wind energy is extracted by turbines to convert the energy into electricity.

A small-scale and large-scale wind industry exists globally. The small-scale wind industry caters for urban settings where a wind farm is not feasible and also where there is a need for household electricity generation. The large-scale industry is directed towards contributing to countrywide energy supply.

Wind resource in India

The wind resource assessment in India estimates the total wind potential to be around 45 000 MW (mega watt). This potential is distributed mainly in the states of Tamil Nadu, Andhra Pradesh, Karnataka, Gujarat, Maharashtra, and Rajasthan. The technical potential that is based on the availability of infrastructure, for example the availability of grid, is estimated to be around 13 000 MW. In India, the wind resources fall in the low wind regime, the wind power density being in the range of 250 -450 W/m2. It may be noted that this potential estimation is based on certain assumptions. With ongoing resource assessment efforts, extension of grid, improvement in the wind turbine technology, and sophisticated techniques for the wind farm designing, the gross as well as the technical potential would increase in the future.

Status

Wind power has become one of the prominent power generation technology amongst the renewable energy technologies. By the end of 2005, the total wind power installed globally was about 59 084 MW, a growth of 24% over 2004. The leading countries in wind power installation are Germany (18 428 MW), Spain (10 027 MW), the USA (9 149 MW), India (4 430 MW) and Denmark (3 122 MW). India has overtaken Denmark and is the fourth largest wind market in the world.

Wind energy technology trends

Use of wind energy started long ago when it was used for grinding. The commercial use of wind energy for electrical power generation started in 1970s. Horizontal axis wind turbines are most commonly used for power generation, although some vertical axis wind turbine designs has been developed and tested. The vertical axis turbines have structural as well as aerodynamic limitations and, hence, are not commercially used. The wind power generation is simple conversion of kinetic energy in the wind into electrical energy. However, the mechanism to capture, transmit, and convert the energy into electrical energy involves several stages, components, and controls. The important components/controls of horizontal axis wind turbine are

Ÿ         rotor blades,

Ÿ         generator,

Ÿ         aerodynamic power regulation,

Ÿ         yaw mechanism, and

Ÿ         tower.

The wind turbine technology is being continuously improved worldwide resulting in improved performances, optimal land use, and better grid integration. The areas in which development work is being targeted are large size wind turbines, powerful and larger blades, improved power electronics, and taller towers.

Rotor blades

The rotor blade is the most critical component of the wind turbine. It captures the wind energy and transfers it to torque required to generate power. The aerodynamic design of the blade is important as it determines the energy capture potential. One indicator of effective blade design is the weight/swept area ratio. As the size of the wind turbine increases, the size of blade length increases proportionally which results in capturing more energy. These blades are of higher tensile strength and lower body mass. Commonly used materials for making the blades are composite materials like the glass fibre epoxy, carbon epoxy, fibre-reinforced plastic, etc.

Generator

The kinetic energy captured by the rotor blades is transferred to the generator through the transmission shaft. Wind machines with induction generators come with gear boxes.

Wind machines which have synchronous generators have no gear boxes since they could be designed for continuous variation according to the wind speed. These machines have an added advantage over induction machines because variable speed increases the energy capture. This increases the efficiency of the system on the whole by exactly matching the wind speed to the rotor speed of the generator. Variable speed machines grant flexibility and good power quality but are expensive because of the power electronics involved.

Aerodynamic power regulation

Out of the two basic concepts of aerodynamic controls, the stall and pitch mechanisms, the pitch control is predominantly used especially for the larger size wind turbines. Pitch regulation offers better control on the power regulation with independent pitching of the blades. The latest concept is active pitch or active stall.

Increasing number of larger wind turbines (1 MW and above) are being developed with an active stall control mechanism. At low wind speeds, the machines are usually programmed to pitch their blades much like a pitch-controlled machine. However, when the machine reaches its rated power and the generator is about to be overloaded, the machine will pitch its blades in the opposite direction. This is similar to normal stall power limitation, except that the whole blade can be rotated backwards (in the opposite direction as is the case with pitch control).

One of the advantages of active stall is that one can control the power output more accurately than with stall, so as to avoid overshooting the rated power of the machine at the beginning of a gust of wind. Another advantage is that the machine can be run almost exactly at rated power at all high wind speeds. In active pitch control, the blade pitch angle is continuously adjusted based on the measured parameters to generate the required power output. It has been established that active pitch regulation reduces the wind generator output fluctuations.

Tower

Two most common tower designs are lattice and tubular. Lattice tower is cheaper compared to the tubular tower and being usually a bolted structure is easier to transport. However, since lattice tower consists of many bolted connections, these connections need to be tightened and checked periodically, thereby increasing the operation and maintenance cost. By nature, tubular tower is stiffer than the lattice one. However, the tubular tower allows full internal access to the nacelle.

Larger turbine size

An important improvement in the wind turbine design has lead to increased size and performance. From machines of just 25 kW two decades ago, the commercial range sold today is typically from 600 - 2 500 kW. As such, the largest wind turbine capacity today is 5 MW. With the development of higher size turbines for a required capacity, lower number of turbines are required which has implication on the investment as well as O&M costs.

Off shore wind

As a result of lower resistance, the wind resource at the offshore locations is higher in terms of wind speed. Also, wind resources are uniform having lower variations and turbulence. The higher capacity wind turbines, which are being developed today, focus on the off shore applications. The related foundation technologies are also being developed for the erection of higher capacity wind turbines. In case of India, however, the development for offshore wind is yet to start.

Wind power in India

Wind turbines offered in India range from 250 kW to 2 MW capacities. As of 31 March 2006, the total installed capacity in the country was 5340 MW, which is 46% of the total capacity of renewable resources based power generation. There are 7 manufacturers of wind turbine generators in India.

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