Solar energy can be directly converted into electricity for water pumping applications with either thermoelectricity or PV cells. However, thermoelectricity is not efficient and practical for pumping water. PV is the most promising and widely used technology available today, so we emphasize windmills and turbines, along with PV, in this report.
Wind has been a traditional energy source for centuries and is still commonly used in many developing countries. It has been harnessed to separate grain from hay and to sail boats. Since the early 13th century, wind has been used to pump water to dewater polders in the Netherlands. Small wind pumps, made of wood, have been used in France, Portugal, and Spain for pumping seawater to produce salt. Then the American wind pump, made of steel with a multibladed fanlike rotor, became the most popular water pumping technology. It was introduced for domestic water supply and railroads in the late 19th century and later used to water livestock in the early 1900s when millions of cattle were brought to the North American Great Plains. During the last 100 years, more than 8 million windmills have been manufactured in the United States, and the design has proven so successful that it has been copied around the world. Today more than 1 million windmills are in use, mostly in United States, Argentina, and Australia. This type of pump drives a piston (reciprocating) pump linked via reduction gear directly located below in the borehole. But because the blades are not true airfoils and the overall operating efficiency is only about 4%-8%, traditional windmills are much less efficient than modern wind turbines. Figure 5-1 shows a typical traditional wind farm for cattle watering on a ranch in Colorado.
Development of electrical wind turbines that could generate either DC or AC current began in the late 1920s. These turbines are designed to produce electricity from a few watts (for charging batteries) to a few megawatts. The small turbines (microturbines), which produce only few watts, can be erected and taken down by one person, and are mainly used for charging batteries on sailboats and recreational vehicles.
Typical wind machines can be designed to rotate either horizontally or vertically. The horizontal axis types are the most practical. The most common vertical axis types of wind machines are the Panemone differential drag devices, the Savonius rotor (or S-rotor), and the Darrieus wind turbine (see Figure 5-2). One of the main advantages with the vertical axis is it accepts the wind from any direction. Whether a wind machine rotates on a vertical or horizontal axis, the mechanism depends on one of two aerodynamic principles (drag or lift) to derive power from the wind. Drag force works by simply obstructing the wind and creating turbulence and the drag force acts in the same direction as the wind. Drag devices are simple wind machines that use flat, curved, or cup-shaped blades to turn the rotor. Cup anemometers, Panemones, and Savonius rotors are typical of drag devices. Because much of the rotor's area is covered with blades, drag devices can produce high starting torque, but they require more materials than do wind machines operating with lift. Drag devices are ideal for pumping water in low volumes.
Section view of Ancient Persian Panamone drag wind machine
Section view of Ancient Persian Panamone drag wind machine
Figure 5-2. Typical vertical axis wind machines: (a) Panemone, (b) Darrieus, and (c) Savonius
Figure 5-2. Typical vertical axis wind machines: (a) Panemone, (b) Darrieus, and (c) Savonius
Wind machines with lift devices use airfoils to propel the rotor. The lifting force mechanism operates with the blades mounted at a small angle to deflect the wind and produce a bigger force perpendicular to the direction of the wind, with much smaller drag force. The maximum possible power captured from the wind using lift devices is 59% (Bert limit). This makes lift devices more attractive than drag devices for generating electricity on a larger scale. Turbines operating with airfoils can be designed with single, double, or triple blades. Those with one slender blade can capture wind power efficiently, but two blades are often used for static balance. However, two-bladed turbines experience dynamic imbalance when the wind machine changes direction, so the three-bladed wind turbines are better for greater dynamic stability.
Old American windmills have been modified by the Australians, the Dutch, and others to decrease their weight and cost and increase their efficiency. As a result, several options are available, ranging from modern mechanical wind pumps to modern wind-electric pumping systems. The two major developments with modern mechanical wind pumps include the design of a counterbalance on the weight of the sucker rod and the development of variable strokes. These improvements can double the water output from the traditional farm windmill.
Traditional windmills tend to speed up when the sucker begins to go down, and the rotor slows on the upstroke because it lifts the weights of the rod and the water. This speed variation changes the tip-speed ratio of the rotor and its efficiency. The second fundamental problem with the wind pumps is the relationship between the wind speed and the stroke. The power in the wind increases with the cube wind speed; the water discharge rate (pumping rate) increases linearly. This relationship affects the performance because the stroke of the old wind pumps had a fixed position. So if the stroke is adjusted for optimum production at high speed for a given well depth and pump size, the pump performs poorly at low winds, or vice versa. Therefore, adding the counterbalance weight on the sucker rod or springs and using variable-stroke technique are the main factors that have improved the mechanical wind pumps in use today.
Another crucial development with modern wind pumps is that they use only 6-8 blades of true airfoils, in contrast to traditional windmills, which have 15-18 curved steel plates. Using fewer blades decreases the cost. The rotor diameter of traditional wind pumps is 2-5 meters. The Australian-made Southern Cross machine has an available rotor diameter as long as 8 meters. Modern wind pumps are thus twice as efficient as traditional wind pumps, but they are still bulky and are best suited for light wind regions.
The so-called third generation windmills use a direct drive mechanism rather than a geared transmission. They are designed to produce high torque at low wind speeds and provide rotor speed control at high wind speeds. The main objective of this design is to reduce the starting torque. This is possible because of the counterbalance attached to the actuating pump beam, which is designed to reduce the starting rotor torque to start pumping. A report by the University of Calgary, Canada, shows that a direct drive-type wind pump (similar to an oil-field jack pump) can start pumping at 50% lower rotor torque (or 30% less wind speed) relative to a system with no counterbalance. These types of wind pumps are promising because they do not require gearboxes for power transmission from the rotor to the shaft.
The windmill uses a reciprocating or piston pump, or positive displacement pump. For these pumps to start pumping, the wind pump crank force needs enough force on the pump rod to lift the weight of the pump rods, the piston and the water in the piston, and the friction. The amount of water delivered by the pump for a given pumping head depends on the diameter of the pump and on the wind speed. The bigger the pump diameter, the larger the amount of water delivered. The size of the pump determines the starting wind speed for a given wind pump and pumping head because bigger pumps require larger starting torque. A pump fitted to a windmill should generally be sized to run at about three-quarters of the local mean wind speed. This allows the wind pump to run frequently enough and to achieve better water output at stronger winds.
One of the main disadvantages of a mechanical wind pump is that it must be located directly over the borehole so the pump rod is directly connected with the rising main and the pump. The best water resource location is at lower ground, which is generally a poor location for winds, so mechanical wind pumps are typically limited to flat and arid regions. Efforts have been made to locate the windmills further from the borehole by using remote electrical, pneumatic, hydraulic, and mechanical transmissions. An induction generator to produce electricity, coupled with an induction motor and a pump is a good alternative technology for water pumping, which we discuss in the next section.
Pneumatic transmission wind pumps operate on the principle of compressed air by using a small industrial air compressor to drive an airlift pump or pneumatic displacement pumps. The main advantage of this method is that there is no mechanical transmission from the windmill to the pump, which avoids water hammer and other related dynamic problems. The pump can operate slowly even while the windmill is running rapidly with no dynamic problem. Other advantages are its simplicity and low maintenance. However, this technology is still under development and will require intensive field testing before it can be commercialized.
Power transmission using hydraulic means is another option for water pumping. This type of system can have either one or two pipes, and the fluid can be either water or oil. Although this mechanism has been demonstrated in the field, it is still far from commercialization. Similarly, mechanical transmission of windmills for remote pumping has been tried, but such mechanisms are robust, expensive, and not feasible in the near future.
In general, commercialized mechanical wind pumps are good for low wind speeds because of their high solidity rotors, which limit the piston pump speed to 40-50 strokes per minute. The overall conversion efficiency of mechanical pumps using an average wind speed is 7%-27%.
Electrical wind turbine pumps offer a more promising technology. Modern wind generators can produce AC or DC electrical output and can pump water directly by connecting to AC or DC motors. Because electrical wind turbines are designed for low-solidity rotors, centrifugal pumps are used. This technology:
• Eliminates the need for batteries and inverters by directly coupling the wind turbine with an AC motor, which then drives the centrifugal pump at varying speeds.
• Simplifies the matching of wind turbines with water pumps by varying the load electrically instead of mechanically (similar to varying the stroke in the case of windmills).
• Alleviates the problem of setting wind turbines over water wells because wind is best at the crest of a hill, while water is found at the foot of hills or lower elevations.
Wind turbines can be located where the winds are strongest at the optimum cable length from the well. Figure 5-3 illustrates a typical electrical wind pump.
Unlike traditional windmills, electrical wind turbines require higher starting wind speeds and perform better at high winds than at low winds. They are twice as efficient as traditional windmills; are cost competitive with diesels, PV systems, and traditional windmills; and have fewer moving parts than traditional windmills, which keeps maintenance costs low.
The theoretical maximum conversion efficiency of kinetic energy used by the perfect wind turbine is 59.3% (Betz limit, after the German scientist, Albert Betz). However, in practice, wind turbine rotors convert much less energy. Optimally designed rotors reach slightly above 40%. Electrical wind turbines capture 12%-30% of the energy in the wind. Rotor efficiency is about 40%, transmission is about 90%, generator efficiency is about 90%, and power conditioning, yawing, and gusts efficiency are about 90%. Small electric wind turbines convert 25%-30% of the power in the wind at places with average wind speeds below 5.5 m/s (12 mph) and less than 20% at windier sites. Medium-sized wind turbines perform better than small turbines at high wind sites.
Electrical wind turbines rated as low as 50 W are commercially available, and generally require high wind speeds. For example, a small wind turbine of about 1.5 kW rated output requires an average wind speed of 4-5 m/s to start pumping, compared to mechanical wind pumps, which can start pumping at about 2.5 to 3.5 m/s. Larger wind turbines require higher wind speeds to start the rotor. They become competitive with windmills above average wind speeds of 5-6 m/s for water pumping applications. Therefore, the pumping location's wind regime determines whether mechanical or electrical wind pumps are good for pumping water. Electrical wind turbines have some potential advantages over mechanical wind turbines. They are versatile—the surplus electrical power can be stored in batteries and used for lighting or other purposes. The wind turbine need not necessarily be located directly over the borehole or near the site where the power is needed. It can be located at the best wind regime location and the power generated from the turbine can be wired to the pumping site.
A solar pump is powered by solar energy, either directly by converting the solar resource into electricity or indirectly by using solar-thermal heat collectors. However, water pumping using this technology is not attractive and is not discussed in this report. A pump powered by directly converting solar energy into electricity is called a PV pump, and is one of the most reliable technologies for pumping water from boreholes, rivers, lakes, shallow wells, and canals. Because of the PV array's modularity, the pumps can be redesigned as the demand increases by changing the motor-pump subsystem as long as the borehole yield is sufficient. Unlike wind pumps, PV pumps can be easily moved with little dismantling and low reinstallation costs. Figure 5-4 shows a typical village water supply using a PV pump; Figure 5-5 shows a PV pump used for irrigation and livestock watering.
Because sunlight is the only source for electricity generation in such a system, the PV array output depends on the intensity of the solar radiation striking the PV array. The amount of water delivered by the PV array depends mainly on the amount of the solar radiation it receives, which depends on the location, the seasonal conditions, the size of the PV array, and the performance of the subsystem. Energy in the form of water produced by PV pumps can be stored and batteries for energy storage may not necessary. However, other load profiles may require battery storage for storing electricity generated during the day for night uses.
A PV-powered water pumping system is simpler than any other pumping system. PV-powered pumping systems have a PV array, a motor and pump set, and a water storage mechanism. The PV array converts the solar energy into DC electricity and the motor and pump converts the electrical output into hydraulic power. Through its distribution system, the storage mechanism delivers water to its point of use. The PV array can be directly coupled to a DC motor, or to an AC motor through an inverter. The inverter converts the DC power into three-phase AC power and the current varies continuously as a function of the solar radiation. The most common components of PV pumping systems are presented in Figure 5-6.
The use of PV water-pumping systems varies widely, depending on the requirements and the conditions under which water is pumped. The volume of water required varies by season, by time of day, and by type of application. For example, water supply for irrigation is seasonal. Domestic water supply requires continuous water production for the entire year. The availability of water from a PV pump over the year also depends on factors such as borehole yield, borehole recovery rate, pumping head, seasonal variability of static water level, and more importantly, the availability of solar radiation, which varies seasonally and by time of day. For all these reasons, a PV pumping system must be properly configured and designed based on the need and the type of application. The most suitable components, configuration, and location must be selected for the system to perform well and be economically viable.
The water pumping subsystems must be matched properly with a PV array for maximum use of the system; however, it is problematic with many PV systems. The main problems of the load matching with a PV array power source are related to the nonlinear solar irradiance and cell temperature-dependent voltage and current characteristics of the PV array generator. In general, volumetric pumps are linear, and can use the energy from the sun with the smart electronic controller. Centrifugal pumps are nonlinear; hence, water production drops when the pump operates away from the designed point.
The use of solar generators (PV arrays) to run water pumps, especially in sunny and developing countries, is very promising. The efficiency of PV modules is about 15%, the motor-pump subsystem efficiency is 40%-60%, and inverters are about 95%. The overall efficiency of a PV pump is 6.5%-9%.
PV cells are made of semiconductor materials that can generate electricity electromagnetically when exposed to sunlight. If a minority electron-hole pair generated by absorption of photons in the semiconductor material (the holes in n-regions, and the electrons in p-region) diffuses into a boundary region in which there is an electric field, the electron will be accelerated into the n-region, and the hole into the p-region. This causes the n-region to accumulate a negative charge and the p-region builds up a positive charge, resulting in a photovoltage. If there is a closed external circuit, a photocurrent and photovoltage can be measured by the external resistance. This process is explained using the simple diagram shown in Figure 5-7.
Was this article helpful?