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Report 4 of 4: Winds of Change
- Sri Lanka
Introduction
Wind energy offers the potential to generate substantial amounts of electricity without the pollution problems of most conventional forms of electricity generation. The equipment used for wind energy technology is being continually improved to make it cheaper and more reliable. It is therefore expected that wind energy will become even more economically competitive over the coming decades.
Wind generation for developing countries
Unlike the trend toward large-scale grid connected wind turbines seen in the North, the more immediate demand for rural energy supply in developing countries is for smaller machines in the 5-100 kW range. These can be connected to small, localised micro-grid systems and used in conjunction with diesel generating sets and/or solar photovoltaic systems. Currently, the use of wind power for electricity production in developing countries is limited, the main area of growth being for very small battery charging wind turbines (50-500 Watts). Other applications for small wind machines include water pumping, telecommunications power supply and irrigation.
Power in the wind
Wind is simply the movement of air from one place to another. There are localised wind patterns due to the effects of temperature differences between land and seas, or mountains and valleys. Wind speed generally increases with height above ground because the roughness of ground features, such as vegetation and houses, cause the wind to slow down.
Significant areas of the world have mean annual wind speeds of above 4-5 metres per second which make wind powered electricity generation an attractive option. Wind speed data can be obtained from wind maps or from the meteorology office. Unfortunately, the general availability and reliability of wind speed data is extremely poor in many regions of the world. It is important to obtain accurate wind speed data for the site in mind before any decision can be made as to its suitability.
The power in the wind is proportional to:
- the square of the rotor diameter
- the cube of the wind speed
- the air density - which varies with altitude
The actual power that can be used will depend on several factors, such as the type of generator and rotor used, the sophistication of blade design, friction losses, losses in the pump or other equipment connected to the wind machine. There are also physical limits to the amount of power which can be extracted realistically from the wind. Any windmill can only extract a maximum of 59.3% of the power from the wind (this is known as the Betz limit). In reality, this figure is usually around 45% (maximum) for a large electricity producing turbine and around 30% to 40% for a wind pump.
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 lift or drag force (or through a combination of the two).
The basic features that characterise lift and drag are:
- drag is in the direction of air flow
- lift is perpendicular to the direction of air flow
- 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
The horizontal axis wind generator is the most commonly used type of wind machine and uses lift forces to harness the wind. The choice of rotor is dictated largely by the characteristic of the generator or the end use. For example, for electricity generation, a small number of blades will give the highest rpm needed for most generators.
Small wind generators
A typical small wind generator has a rotor which is directly coupled to a generator which produces electricity either at 120/240 volt alternating current for direct domestic use or at 12/24 volt direct current for battery charging. A tail vane keeps the rotor orientated into the wind. Some wind-machines have a tail vane which is designed for automatic furling (turning the machine out of the wind) at high wind speeds to prevent damage. Larger machines have pitch controlled blades (the angle at which the blades meet the wind is controlled) which achieve the same function. The tower will often be guyed to give it support.
Grid connected or battery charging
Depending on the circumstances, the distribution of electricity from a wind machine can be carried out in one of various ways. Commonly, larger machines are connected to a grid distribution network. This can be the main national network, in which case electricity can be sold to the electricity utility (providing an agreement can be made between the producer and the grid) when an excess is produced and purchased when the wind is low. Using the national grid helps provide flexibility to the system and does away with the need for a back-up system when wind speeds are low.
Micro-grids distribute electricity to smaller areas, typically a village or town. When wind is used for supplying electricity to such a grid, a diesel generator set is often used as a backup for the periods when wind speeds are low. Alternatively, electricity storage can be used but this can be a more expensive option. Hybrid systems use a combination of two or more energy sources to provide electricity to provide electricity in all weather conditions. The capital cost for such a system is high but subsequent running costs will be low compared with a pure diesel system.
In areas where households are widely dispersed or where grid costs are prohibitively expensive, battery charging is an option. For people in rural areas a few tens of watts are sufficient for providing lighting and a source of power for a radio or television. Batteries can be returned to the charging station occasionally for recharging. This reduces the inconvenience of an intermittent supply due to fluctuating wind speeds. 12 and 24 volt direct current wind generators are commercially available which are suitable for battery charging applications. Small wind generators of this size (50-500 Watt) can also be used for individual household connection.
Cost – economics
The cost of producing electricity from the wind is heavily dependent on the local wind regime. As mentioned earlier, the power output from the wind machine is proportional to cube of the wind speed and so a slight increase in wind speed will mean a significant increase in power and a subsequent reduction in costs. For example, if the wind speed doubles, the power in the wind increases by a factor of eight. It is therefore worthwhile finding a site which has a relatively high mean wind speed. It is also worth bearing in mind that a wind machine will only operate at its maximum efficiency for a fraction of the time it is running, due to variations in wind speed.
Capital costs for wind power can be high, but running costs are low and so access to initial funds, subsidies or low interest loans are an obvious advantage when considering a wind-electric system. A careful matching of the load and energy supply options should be made to maximise the use of the power from the wind - a load which accepts a variable input is ideally matched to the intermittent nature of wind power.
Environmental concerns
Wind power is a clean renewable energy source. There are, however, some environmental considerations to keep in mind when planning a wind power scheme.
- Noise - wind rotors, gearboxes and generators create acoustic noise when functioning; this needs to be considered when siting a machine.
- Visual impact - modern wind machines are large objects and have a significant visual impact on their surroundings. Some argue that it is a positive visual impact, others to the contrary.
- Electromagnetic interference - some television frequency bands are susceptible to interference from wind generators.
Local manufacture
| Local production of wind generators can be carried out in many developing countries. The production of small and medium sized machines locally is a great deal cheaper than imported machines and, during the production process, it enables manufacturers to make minor modifications in order to match systems with desired end-uses and to the conditions under which they are expected to operate.
Depending on the availability of materials, rotor blades can be made locally from laminated wood, steel, plastics or combinations of these materials, whilst some of the machinery components can be made by small engineering workshops. Other parts, including special bearings, gear-boxes, generators and other electrical equipment may have to be imported if they are not available in the country of assembly. Towers can be made of welded steel, preferably galvanised, which can be manufactured in many local engineering works, whilst the foundations can be cast from reinforced concrete on site. |
Source: Renewable Energy Technologies, IT Publications, 1991.
For further information, please contact:
Further reading available from ITDG Development Bookshop
Wind Energy Basics: A Guide to Small and Micro Wind Systems
Paul Gipe
Chelsea Green £14.95 1999 ISBN 1890132071
Windpower Workshop: Building your own wind turbine
Hugh Piggott
Windpower Workshop explains very clearly the steps involved in setting up a wind system, including a valuable market survey of wind turbines, suppliers, and support organisations.
Centre For Alternative Technology £10.00 1997 ISBN 1898049203
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This document is an output from a project funded by the UK Department for International Development (DFID) for the benefit of developing countries. The views expressed are not necessarily those of the DFID.
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