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close this bookCERES No. 122 (FAO Ceres, 1988, 50 p.)
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View the documentThe astonishing resources of the physic nut tree
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View the documentSoil erosion data to convince the bank

Winds of change for windmills

In every region of the world, people have devised a host of ways to draw groundwater from its source, among them such traditional devices as buckets attached to ropes or levers, the Archimedean screw, and the shaduf. But whatever the means, until the early nineteenth century, they all had to rely on only four available sources of power - human or animal muscle, the force of running water itself, and the wind.

Attempts to harness the wind have a history that goes back at least 5 000 years to when the Egyptians first used sails to propel boats on the Nile. Fixed windmills are known to have existed in parts of the Middle East by 200 B.C.; by A.D. 1000, they were common around the Mediterranean, and by the twelfth century had reached northern Europe, where they became an important source of energy, primarily for milling grain or, as in the Netherlands, for draining flooded lowlands.

In fact, by the eighteenth century, windmills represented one of the most advanced forms of technology. However, with the invention of the steam engine early in the nineteenth century and later of the internal combustion engine, the value of wind-driven machines for pumping water or other purposes diminished, and windmills seemed relegated to being archaic monuments of a picturesque past. It is therefore something of an irony that the potential of windpumps, after several centuries of neglect, has recently been "rediscovered" and that efforts are being made to develop and promote their use, particularly for remote areas in the developing world. According to the authors of the Windpumping Handbook, there are now between three-quarters of a million and one and a half million windpumps operating around the world, and they are being manufactured and sold in at least 28 countries, 14 of them developing nations.

The UNDP Centre on Small Energy Sources, in its February 1987 Newsletter, cites the renewal of interest in windpumps in a number of Third World countries. In Sri Lanka and Morocco, for example, programmes have been launched to replace tens of thousands of diesel/kerosene pumps with windpumps.

Why this recent interest in an ancient technology? One obvious answer is the realization that the world's supply of fossil fuels is limited, non-renewable, and fast being depleted - the energy crisis that is fueling the incentive to develop all possible sources of alternative energy: nuclear, solar, geothermal, biogas, and wind. But there are other reasons as well. Fossil fuels, which now run most water pumps, must be imported by the majority of developing countries - a significant, often crippling drain on their meagre foreign exchange resources. Internal combustion engines, whether they use petrol, diesel, or kerosene, require frequent and relatively skilled maintenance for which many rural users may be inadequately prepared. Even with proper care, these high-speed engines have a fairly short operating life; repairs require trained technicians and parts are often expensive, in short supply, or simply unavailable. And unless a country is geared to their manufacture, both engines and parts must be purchased abroad with precious foreign exchange.

The windpump, such as the one designed by the Intermediate Technology Development Group, a British charity dedicated to increasing the income-generating capabilities of poor people in rural areas of the developing world, avoids all these problems. It runs on a free energy source. It requires little maintenance and has a life of 20 years or more. It is simple enough in design and materials to be manufactured by small scale industry and thus can be made locally, saving on cost and foreign exchange, as well as providing local employment.

Unfortunately, for all the wind pump's advantages, it has at least one major limitation. It will work only where there is adequate wind - meaning wind with the proper speed, frequency, and timing. Modern wind pumps need a minimum wind speed of about 2.5 metres per second (6 mph or 5 knots) and are designed to furl in winds above 10-12 m/s. The wind must also blow frequently enough to permit the pump to draw sufficient water to meet the needs of humans, animals, or crops, and it must blow during the seasons when rainwater is scarce or absent. Fortunately, these conditions are met in perhaps half the world's land mass.

However, the presence of adequate wind is not in itself a guarantee that windpumps will be feasible. The pumps can be installed only where there is minimal turbulence caused by hills, valleys, trees, and buildings, and, of course, where the water table is near enough to the surface - 200 metres or less. Air density also affects the windpump's efficiency. It is estimated that a well-designed windmill can harness 25-40 per cent of the wind's kinetic energy, but this energy is reduced by factors which lower air density, especially altitude. For example, at 1000 metres, wind at a given speed loses some 11 per cent of its sea-level kinetic energy, although this loss is frequently compensated for by higher wind speeds.

The modern windpump consists of five basic components: wind sails, a tail, a transmission mechanism, a tower, and the pump, together with some type of storage system. A variety of designs have been developed for each of these elements. The most common sail is the multibladed, horizontal axis type. The tail, which projects from the rear of the rotating blades, serves to keep the sail facing into the wind and, in high winds (usually above 10-12 m/s), furls the sails by turning them parallel to the windstream, thus reducing the danger of damage. The transmission converts the rotary motion of the sail's axis into a form suitable for driving the pump. The tower, normally between 10 and 20 metres high, serves two purposes: it raises the sail above ground turbulence and up where wind speeds are higher because not slowed by ground friction. The type of pump will depend on the "pump head", the distance the water must be lifted. For surface or shallow water, a suction pump is normally used, while for deep wells, some form of centrifugal or positive displacement pump is more appropriate. Because the centrifugal type requires high rotation speeds and will work only within a narrow rotation speed range, the positive displacement pump is usually preferred.

While much has changed over the centuries, many things have remained the same. One of these is man's need for fresh water, a need as imperative today as it was thousands of years ago. Another is man's continual search for ways to ease the burden of physical labour by harnessing the forces of nature. Millions of fertile acres in the developing world still yield marginal crops because they are not adequately irrigated; millions of people, most of them women, spend grueling hours a day just hauling water for domestic use. The ancient technology of windmills, updated, is beginnning once again to play a significant role in providing rural folk with the necessary power to obtain this life-giving liquid.

Farhana Haque