Civilisations have been using wind as a source of energy for more than 100 years. Wind has been used from the very early stages to sail boats right up till today where it is used to provide a source of electricity. Wind energy was first used in the 1880 by P. La'cour to produce electricity.
Wind turbines can be either categorised in to horizontal axis or vertical axis turbines based on the positioning of their rotors. In a horizontal axis turbine the rotor axis are kept horizontal and aligned parallel in the direction of the wind. Where as in the vertical axis turbine the rotor axis is vertical and fixed, and remains perpendicular to the wind stream.(Kothari). Wind turbines are usually fitted with blades, sails or buckets which are connected to a central shaft which rotates. The rotating of this shaft can be used to drive a pump or generate electricity.
Vertical rotor axis turbines
Wind turbines that incorporate rotor concepts with vertical axis are:
- The Savonius rotor
- The Darrieus rotor
- The H rotor
The Savonius rotor was first developed in Finland in 1920 by an engineer called Savonius. This was one of the first vertical rotors which work on the drag principle. It is made up of two half-cylindrical blades placed facing each other. The blades slightly overlap to allow the redirected wind to flow from one blade to the other causing the driving force. The efficiency of the Savonius rotor is slightly better than one of a simple drag device as it also utilises the lift principle. However, the efficiency is much worse than that of a good lift device, reaching maximum power coefficients of the order of 0.25 (Hau, 2000). As with all rotors there is advantages and disadvantages the main advantage of this type of rotor is that they can operate at very low wind speeds though the disadvantage of them is that they have a high material.
The Darrieus rotor was first developed in 1929 by a French engineer called Georges Darrieus.
Technically advanced windmills ground corn or pumped water. In the US thousands of \Testern Mills were used in agriculture, but all these windmills were mechanical systems. Wind generators providing electricity started to enter the market in the early 1980s in Denmark and the US. In Germany wind power had an unexpected boom in the 1.990s, making Germany the largest wind power market in the world. In 2002, the German wind power industry achieved a turnover of nearly 4 billion and created more than 45,000 new jobs. Altogether, 15,797 wind generators with a total capacity of 15,327 MW and an electricity generation potential of 30 TWh/year were installed in Germany by mid-2004 (Ender, 2004). This is equivalent to nearly 6 per cent of Germany's electricity demand. If Germany continues with the same growth rates as in the late 1990s, it will cover more than 10 per cent of its electricity demand in a few years. More recently, Spain started a similar exploration of its wind potential. Although Germany only has limited areas suited for setting up wind farms, the potential is considerable. Excluding conservation areas and allowing for safe distances to settlements due to noise considerations,53.5 Gw could be achieved from installations onshore. This capacity could produce 85 TWh/year, 15 per cent of the electricity demand. The German offshore potential is 23.6 GW, which could produce 79 TWh/year. The potential in other countries is even higher. In the UK, wind power could produce well above 1000TWh/year, which is much more than the total British electricity demand. Also the US could cover its entire electricity demand using wind power. Chapter 5 describes in detail the use of wind power for electricity supply.
The earths wind is developed from the movement of atmospheric air masses as a result of variations in atmospheric pressure, which in turn are the direct result of differences in solar heating across the earth's surface (Boyle, 2004)
The wind energy potential on the Earth has enough, in principle, to meet the worlds energy demand all. This is because every country has virtually enough sites with average wind speeds of more than 5 m/s measured at a height of 10 m (Sesto & Casale, 1998).
(up to about 35- 40% conversion efficiency for existing turbines under design working conditions) (Gipe, 1995) States ‘a wind turbine is a device that is able to tap some of the wind power to generate electricity'. And have the potential of 35 - 40% conversion efficiency under design conditions. Wind turbine capacities range from as low as 1kW to 7MW the largest being the Enercon E-126 (AnonS, 2009).
Horizontal-axis wind turbines with propeller-type rotors are those which have to date been most developed technologically and are most widely available commercially (Gipe, 1995)
Manufacturers are continually developing these machines to increase efficiency and further reduce manufacturing costs. Manufacturers are trying to achieve this by means of using direct drive systems, passive control of blade pitch and operation of the rotor at variable speed and through a static power convertor which allows a constant grid-connection to be maintained at peak aerodynamic efficiency as wind speeds vary (Sesto & Casale, 1998).
With regards to vertical-axis wind turbines, only a few models have been developed in series by the industry. Particularly, the Darrieus machines which have curved blades are currently the only design being developed to the greatest extent. These machines do not have to be yawed into the wind direction and their mechanical and electrical parts are more easily accessible compared to a horizontal axis turbine. (Boyle, 2004).
Wind energy development has both positive and negative impacts on the environment but more so positives. The scale of the future implementation will rely on successfully maximising the positives and keeping the negatives to a minimum.
The Wind Energy market is constantly growing and will become a major generator of electricity throughout the world in the future. Within Europe, the off shore exploitation of wind energy will make a major contribution in reducing carbon dioxide emissions from the electricity sector (Boyle, 2004).