 |
|||||||
| About UNEP | UNEP offices | News centre | Publications | Calendar | Awards | Milestones | UNEP Store |
|
|||||||
|
 |
|
|
|
||
| UNEP>DTIE>IETC | |
|
|
Newsletter and Technical Publications 1.5 Pumping of Groundwater Using Non-conventional Sources of Energy Energy needs are a major factor in the cost estimates of water development projects, even to the point of being the determining factor in regard to the economic feasibility of specific projects. The costs of pumping surface or sub-surface water, for example, is an example of the economic obstacles facing water development projects and the improvement of freshwater supplies. Within this context, the possibility of using non-conventional energy sources in water projects merits consideration. Such energy sources also have the potential benefit of being more environmentally-compatible. 1.5.1 Wind Energy The first wind-powered sail windmill emerged in Persia in approximately 400 B.C. Other sail windmails subsequently emerged in the 11th Century in Europe, particularly in The Netherlands. They emerged in the Mediterranean region in the 14th Century. The first wind energy systems for pumping water from wells appeared in 1840. Shortly thereafter, these systems were used in Syria for pumping water from wells. This technology is considered a mechanical system that transforms wind energy into kinetic energy that can run a piston pump to lift the water in hafirs, ground reservoirs or underground wells up to the ground surface. The technology also transforms the wind energy into kinetic energy that can run an electric engine. In turn, the engine can generate electricity that can be used directly to run motors, or alternatively to be stored in batteries for use when necessary to operate electric pumps for water withdrawal from wells. This technology is useful to address domestic and agricultural energy requirements on the farm level. This includes the ability to pump and collect shallow groundwater into a reservoir, to be used as needed for drinking and domestic purposes, livestock watering and limited irrigation of some agriculutral crops (fruit trees, vegetables). Farmers also can use this technology in remote areas containing shallow groundwater, and where public utilities (water, electricity, oil-product devices) are not available, but which do experience active wind movement. Technology Description Wind-driven water pumping equipment typically consists of four basic components; namely, the tower, rotor, gearbox and pump. There are different types and forms of each of these four basic components, with integrated specifications for facilitating the use of the wind in a given region and for meeting the required water pumping demands. Although many studies have been conducted in the industrial European and American countries on various types of these equipment, they are generally not used widely in the West Asia countries primarily because of their relatively high costs. Locally-manufactured systems of two types are most commonly used in the West Asia region. The conventional type was first produced in Syria by the end of the past century and is still produced. The second type is more advanced, being designed in accordance with international specifications similar to those of advanced European systems. The advanced system also is manufactured in Syria, and generates electricity that can be used to operate the well pumps. Depending on the type and size of the system, it also can be sued for lighting and other applications requiring electricity. Each system has accessories that are installed on the basis of the required work and the nature of its use. The most important accessory is probably the water reservoir, with a water volume proportional to the maximum pumping capacity of the system. The water static and dynamic head are the main factors determining the operation of the various types of wind-driven systems used for pumping water. The capacity of the pumping is inversely proportional to the increase in the dynamic head. The conventional windmill is based on the transfer of motion to operate a piston pump with a submerged suction intake, with an optimal suction depth ranging between 10 m (small systems) to 50 m (large systems). In contrast, the advanced or modern system used electricity-powered submerged pumps, with the suction depth dependent on the pump specifications and the electricity-generating system. The proper equipment in the latter system is selected on the basis of such factors as the groundwater levels in a given region, well capacity, wind-speed change system and the intended water use. Thus, each water project can be developed on the basis of the specific project characteristics, including water needs, nature of the region, magnitude and variability in winds, etc. A summary is presented herein, outlining the main components of wind-energy systems. The Tower The windmill towers are made of concrete, wooden or metallic. Large towers are typically concrete, because of their low costs. However, metal towers are generally better, primarily because of their flexibility. Their tubular or lattice form reduced their surface blockage to wind to the extent that horizontal loads on the tower are too small to create any significant stresses on the tower. The towers of wind energy systems generally comprise two namely a free-standing tower and a guyed tower. The guyed tower consists of either a tubular or lattice small central stand, and is characterized by light weight, mobility and low costs. However, it requires the use of guy cables that can obstruct the use of the surrounding land area, and that also typically requires protective fencing for avoiding accidental collisions with the cables. There are three types of free-standing towers, including four-legged lattice towers, three-legged lattice steel towers, and tubular towers. The four-legged lattice steel tower is firmer, more stable and safer than the other two free-standing towers. It is noted that the towers must be able to withstand wind intensities of up to 50 m/sec. This latter consideration is related to the tower height, whose design depends on wind turbulence vortices and wind ground drag. In considering where to site wind energy systems, both low and high turbulent wind areas must be avoided. Low turbulence hinders the operation of the system rotors, which cannot adapt to sudden fluctuations in the turbulent wind. In contrast, high turbulence accelerates the wearing of the wind energy system components, thereby decreasing its operational lifespan. Experiments in Syria have demonstrated that the optimum solution for relatively flat regions is to use wind energy system towers with heights of approximately 12 m. This type of tower represents the best compromise, from a technical, technological and economic perspective, in using wind energy systems to pump water from wells. Wind energy systems also must be provided with proper lighning-resistant devices to prevent problems arising from their being located in flat regions. The Rotor Horizontal axis wind energy systems are available with two, three or multiple blade rotors. The deciding factor is the maximum diameter of the rotor in regard to meeting the energy needs of its varioius applications, incluidng operating electric pumps. The horizontal axis wind energy systems can generate low voltages with small wind speeds. However, the ideal wind speed usually must be between 3-4.5 m/sec before the system can produce sufficiently high voltage to take care of immediate energy needs while at the same time charging the storage batteries. This means it is not feasible to use wind energy systems to generate electricityi at sites where the average wind speed is less than about 4-4.5 m/sec. Above this minimum wind speed, the power generated by the wind energy system will increase rapidly with increasing wind speed up to about 10-13 m/sec, at which point the system will reach its nominal capacity and start braking the rotor. The number of rotor blades has no relation with the system capacity (which is directly proportional to the square radius of the rotor). Thus, because of its simplicity and low cost, some designers prefer a dual blade rotor. However, because they are better in terms of balance and reduction of stresses on the blades, tower, gearbox and main shaft, the majority of rotor designed prefer triple blade rotors. It is noted that the smaller the number of rotor blades of the wind energy system, the higher the rotation speed needed to transform the wind energy into useful mechanical energy (ACSAD, 1983). Further, the larger the number of rotor blades, the easier it is to rotate the rotor, thereby requiring lower wind speed for rotor operation. These concepts are typically expressed in engineering terms as rotor solidity (ratio of blade area to total area of the rotor). If a lower start-up wind speed is desired without increasing the rotor solidity, the load at the start-up period must be decreased using either (1) special angles of attack for rotor blades at the start-up period, or (2) a zero-load type of pump (known as the swinging vane rotor pump) at the start-up period. Based on these considerations, multiple blades rotors are generally suitable for wind driven energy systems in low wind areas, thereby requiring systems with a low start-up velocity. At the same time, however, a large number of blades can hinder the system at higher wind speeds. Thus, the nominal system velocity must be reduced to lower velocity values. In general, horizontal axis wind energy systems with multiple blade rotors remain the most suitable type for areas exhibiting low and moderate wind speeds. The power-generation ability of wind energy systems varies on the basis of their location. Depending on the location, each square meter of the system’s rotor area can generate power ranging between 200-2,000 kw/year. Some engineers have concluded that a location has good possibilities for wind-energy investments if one square meter of a good system rotor area can generate about 500 kw of energy per year. The wind energy system absorbs 15 - 20% of the wind energy passing through it. However, the wind recovers this energy at a distance equivalent to about ten rotor diameters away from the system. Where many wind energy systems are installed close to each other, therefore, the minimum distance between them should not be less than ten times the rotor’s diameter. In regions with low speed winds that produce only a small quantity of energy, the rotor diameter must be increased. In such cases, a 10 m diameter rotor should be used (rather than a 5 m diameters) to providek the required energy. The Pump The pump used in wind energy systems is a piston pump made of corrosion-resistant bronze. Its axis is connected to a long steel bar composed of many connected and easy to mantle and dismantle parts, and linked at its end to the gearbox at the top of the tower on the rotor axis. The only part of the pump susceptible to damage is the leather seals on the piston disk, whose operational lifetime depends on the purity and degree of salinityh of the pumped water. The costs of the seals is relatively low and they generally last for three years or longer under normal operating conditions. The Gearbox The gearbox consists of a group of gears and cogwheels of various diameters. Most gearboxes contain five sizes of gears and cogwheels, allowing the velocity of the system to change by shifting from a large diameter cogwheel to a smaller diameter cogwheel. The gearbox helps control the pump power, based on the wind intensity and pumping head. With increasing water depth and wind speeds, the gearbox facilitates pump operation at moderate velocity, thereby preventing damage. The principle of gear operation is similar to that of automobiles (Figure 35). The main factor determining the ability to utilize this technology is the wind. Thus, the prevailing wind characteristics where it is to be used require detailed study. Wind speed and direction should be measured in all stations in the region for a relatively long period of time, ideally no less than 5 years. This will allow one to obtain a clear understanding of possible changes in wind properties, including (1) monthly and annual average wind speed (m/s), and (2) monthly and annual maximum wind speed, (3) range of average hourly wind speed, and (4) hourly frequency of each wind speed and average frequency.
Figure 35. Manufacturing workshop for conventional wind energy systems in Al-Nabek city, Syria
|
|
|
|||
Table
of Contents
 |
© United
Nations Environment Programme | privacy
policy | terms
and conditions |contacts|support
UNEP | UNEP
sitemap
|