Conversion of wind power to shaft power

There are two main mechanisms for converting the kinetic energy of the wind into mechanical work; both depend on slowing the wind and thereby extracting kinetic energy. The crudest and least efficient technique is to use drag; drag is developed simply by obstructing the wind and creating turbulence and the drag force acts in the same direction as the wind. Some of the earliest and crudest types of wind machine, known generically as "panamones", depend on exposing a flat area on one side of a rotor to the wind while shielding (or reefing the sails) on the other side; the resulting differential drag force turns the rotor.

The other method, used for all the more efficient types of windmill, is to produce lift. Lift is produced when a sail or a flat surface is mounted at a small angle to the wind; this slightly deflects the wind and produces a large force perpendicular to the direction of the wind with a much smaller drag force. It is this principle by which a sailing ship can tack at speeds greater than the wind. Lift mainly deflects the wind and extracts kinetic energy with little turbulence, so it is therefore a more efficient method of extracting energy from the wind than drag.

It should be noted that the theoretical maximum fraction of the kinetic energy in the wind that could be utilized by a "perfect" wind turbine is approximately 60%. This is because it is impossible to stop the wind completely, which limits the percentage of kinetic energy that can be extracted.

Most wind turbines have upwind rotors that are actively yawed to preserve alignment with wind direction. The three‐bladed rotor is the most popular and, typically, has a separate front bearing with a low speed shaft connected to a gearbox which provides an output speed suitable for a four‐pole generator. Commonly, with the largest wind turbines, the blade pitch will be varied continuously under active control to regulate power at the higher operational wind speeds (furling). Support structures are most commonly tubular steel towers tapering in some way, both in metal wall thickness and in diameter from tower base to tower top. Epoxy based resin systems dominate the market in blade manufacture and carbon fibre reinforcement is increasingly used in big blades. In 2006, the focus of attention is on technology around and above the 2 MW rating and commercial turbines now exist with heights over 100 m and rotor diameters up to 100 m. Designs with variable pitch and variable speed dominate the market while direct drive generators are becoming more prevalent.

Principles of wind energy conversion

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). Drag forces provide the most obvious means of propulsion, these being the forces felt by a person (or object) exposed to the wind. Lift forces are the most efficient means of propulsion but being more subtle than drag forces are not so well understood.

 

Figure – Illustration of drag and lift

The basic features that characterise 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

Energy available in the wind

Wind power in an open air stream is thus proportional to the third power of the wind speed; the available power increases eightfold when the wind speed doubles. Wind turbines for grid electricity therefore need to be especially efficient at greater wind speeds.

Unfortunately, the general availability and reliability of wind speed data is extremely poor in many regions of the world. Large areas of the world appear to have mean annual wind speeds below 3m/s, and are unsuitable for wind power systems, and almost equally large areas have wind speeds in the intermediate range (3-4.5m/s) where wind power may or may not be an attractive option. In addition, significant land areas have mean annual wind speeds exceeding 4.5m/s where wind power would most certainly be economically competitive.

Wind Energy

Wind Energy is generated by harnessing the kinetic energy of atmospheric air. Wind Energy has had been in use for centuries for several other purposes such as sailing, irrigation and for grinding grain. Wind power systems transform kinetic energy of the wind into useful sources of power.

During ancient times, wind power systems were used for both milling and irrigation. It was during the early years of the 20th century that wind power was started to be harnessed for generation of electricity. Windmills have also been used in several countries to pump water.

Wind turbines work by transforming the Wind Energy into mechanical power that can be used for conversion to electricity or for other mechanical purposes like grinding. Wind turbines are used either as stand-alone units or in groups known as Wind Farms. Small-sized wind turbines, known as aero generators are used for charging large-sized batteries.

More than 80% of the global Wind Energy capacity is installed in 5 countries with India at the 5th position. Wind power is the fastest growing source of renewable energy globally with an established capacity in excess of 14,000 MW.