Sources: Gipe 1993 and Fraenkel 1986.
Sources: Gipe 1993 and Fraenkel 1986.
Unlike the changes in air density, changes in the swept area of the rotor significantly change the power. Doubling the area of the rotor doubles the wind power. However, the most important factor for the amount of available wind energy is the wind speed, because the power in the wind is a cubic function of the wind speed. Doubling the wind speed will increase the power eight times. The term power density is frequently used to explain the intensity of the wind energy per unit of rotor area for a given period of time.
The most confusing concept in wind power estimation is the mean power in the wind. Depending on the wind speed distribution (or the frequency of occurrence of different wind speeds), two identical wind machines located at different sites will not necessarily produce the same power output at the same mean wind speed. The reason is that the mean power in the wind depends on the mean of the cubes of the wind speeds over that time and this is different to the cube of the mean wind speed. The frequency of occurrence of different wind speeds can be approximated by a statistical curve using the Weibull distribution. Depending on the distribution, the actual power available will be greater than that estimated from the mean wind speed by a factor from 1.2 to 4. The most realistic factor acceptable to most sites is 2; the actual mean power will be 2 times the power expected from the mean wind speed for most sites.
The wind resource must be assessed before wind machines are considered as potential power sources. Wind data can be taken from meteorological records, direct site measurements, or local knowledge. However, local knowledge alone is not enough to reliably design wind machines. The more quantitative approach could be to take on-site measure ments for a limited period of time to determine the relationship with the nearest meteorological station, and correlate the nearest long-term data to the site. From these data, the frequency of occurrence of consecutive calm days and maximum wind speed should be noted. This will help to determine the necessary battery storage capacity and the type of wind machine that can stand the maximum gust.
The simplest way to measure mean wind speeds is by using a cup-counter anemometer, which adds the wind speed by noting the time when each reading is taken and dividing the difference between the two readings by the time interval. The next simple instrument used to measure wind speed is the anemometer with a meter, which displays the wind speed instantaneously. The sensor (the anemometer head) generates an electrical signal that is proportional to wind speed. Cup anemometers, where the spinning cups drive either DC or AC alternators with digital displays, are far more common. Anemometers with AC alternators, which measure frequency, are more accurate than those with DC generators. Strip-chart recorders are now obsolete. Instruments that measure wind speed instantaneously are useless for finding the average wind speed unless someone checks them 24 hours a day.
Today, most instruments measure and store wind data and give wind speed instantaneously. They can collect, process, and store average wind speed, elapsed time, pick gust, and power density (in W/m ). Some advanced recorders can even record the amount of time the wind was calm, which is very useful for sizing batteries for stand-alone systems, and how much time the winds were above the cut-in speed of a typical wind machine.
According to World Meteorological Organization recommendations, wind measurements should be made at a height of 10 meters with no obstructions. Unfortunately, in most small, rural meteorological stations anemometers are often set on masts of only about 2 meters and surrounded by trees or buildings. Readings from such a site are almost useless for wind energy prediction purposes. Nevertheless, all agricultural meteorological stations use 2 meters as a standard height for estimating water use from crops. In general, 3 years of recordings are required to obtain reasonably representative averages, as the monthly average wind speeds can vary by 10%-25% from year to year.
All energy forms derive directly or indirectly from solar energy. Ocean thermal energy, hydropower, wind, biomass, and tidal energy are indirect forms of solar energy. Crop drying, solar-thermal electric power generation, solar heat collection, and direct conversion of solar energy into electricity are direct forms. Solar energy can be directly converted into electricity using either thermoelectricity or PV cells. Although thermoelectricity has been understood for some time, the maximum overall efficiency apparently cannot exceed 1% at present with the best thermal conductivity materials and technology. Because PV cells for direct production of electricity from the sun are currently the most promising and widely available technology, PV is emphasized in this report.
Although radiation from the sun's surface is reasonably constant by the time it reaches the Earth's surface, it is highly variable because of absorption and scattering in the Earth's atmosphere. When skies are clear, the maximum radiation strikes the Earth's surface when the sun is directly overhead and sunlight has the shortest path through the atmosphere. When heavy clouds cover the sky at low altitude, half the direct beam radiation is recovered in the form of diffuse radiation, and two-thirds of direct beam radiation can be converted to diffuse radiation from cirrus (wispy and high altitude) clouds.
The intensity of the solar radiation that reaches a PV array depends on the effect of the sun's angle on the array, the location of the array, the effects of the Earth's orbit around the sun, and the effects of the Earth's daily rotation on its axis. The principal geometric attribute of the PV array is its facing direction, which can be characterized by a line perpendicular (normal) to the array surface. The orientation of the array can be specified by the tilt angle and the azimuth angle. The tilt angle is measured from the horizontal and is generally equal to the latitude of the PV array's location. The azimuth angle, like a compass heading, is a bearing clockwise from the north to the horizontal projection of the array normal.
The amount of solar radiation impinging on the surface of the PV array comes from the angle at which the sun's rays strike that array. As the PV array is inclined away from the sun, the intensity of the radiation on the array decreases. The amount of solar radiation intercepted by the surface varies as the cosine of the incident angle between the sun's rays and the normal to the surface. The incident angle to a horizontal surface is called the zenith angle.
The sun shines at different angles in different places. As latitude increases, the curvature of the Earth lowers the observed sun angle in the sky. The array must be tilted toward the equator to compensate for this effect. A PV array tilted south at an angle P at latitude ^ has the same sun incidence angle 9 as a horizontal PV array at latitude of 9-P.
Because of the complex geometry of the position of the sun in relation to the Earth during various seasons, a system is needed to track the sun for PV arrays and thermal heat collectors. A tracking system can be installed for flat or concentrated PV modules and heat collectors. However, systems based on concentrated sunlight can generally accept only rays spanning a limited range of angles. They usually have to track the sun to use the direct component of sunlight, with the diffused component wasted. This tends to offset the advantage gained by such tracking systems of intercepting maximum power density by always being normal to the sun's rays. Tracking systems for village water supply are not economical and should not be considered.
A nondimensional coefficient called tilted factor is used to calculate the solar radiation on the tilted surface of the PV array at any location from the solar radiation on the horizontal surface. The tilted factor is the ratio of the cosine of the incidence angle to the cosine of the zenith angle or the ratio of solar radiation on a tilted surface to that on a horizontal surface for any season, latitude, and tilt angle. Like the intensity of solar radiation energy, the tilted factor varies depending on location, season, and tilt angle.
The design of PV systems depends heavily on the availability and accuracy of solar radiation data. The availability of solar radiation depends not only on gross geographical features such as latitude, altitude, climate classification, and prevailing vegetation, but also on geographical features. Unfortunately, accurate solar radiation data are rarely available from remote locations where many PV systems are to be installed. However, numerous approaches for estimating solar radiation energy have been developed based on commonly available sunshine hour and satellite cloud cover data or on direct measurements.
There are various instruments for measuring insulation levels. Direct beam solar radiation is usually measured by a pyrheliometer and global solar radiation is usually measured by a pyranometer. Diffuse radiation can be measured if the pyranometer is shaded. Silicon-based sensors are used to measure incidence radiation. These types of instruments measure the intensity of solar radiation directly when the electrical characteristics change in the presence of solar radiation, and they are categorized as photoelectric devices. The other types of instruments are categorized as bolometric devices. These instruments operate on the principle of Angstrom or electrical compensation. The simplest is the heliograph, which measures the bright sunshine hours by using focused light to burn a hole in a rotating chart. Campbell-strokes recorders were the commonly used instruments for many years and are still in use in many developing countries, but it can result in errors greater than +10% and is obsolete.
Silicon-based sensors are recommended for measuring the intensity of solar radiation for PV cells because they accept the same wavelength ranges as the normal pyranometers. Because solar radiation is the main source of energy for PV systems, the use of this technology for different applications, such as pumping water, requires systematically processed weather data. The most commonly used weather data for designing PV systems are solar radiation, ambient air temperature, and wind speed data. PV systems are sensitive to weather changes and their performance changes accordingly. Also, long-term weather conditions cannot be forecast, so systematically processed past weather information is needed to design PV systems. The data should cover as many years as possible. The simplest method may be to use long-term average weather data; however, average data do not reflect the extreme solar radiation days, and the system must consistently meet its intended daily requirements. For this reason, a "typical meteorological year" (TMY) is usually used for designing PV systems. "Typical" refers to the long-term database covering a year. Because no actual year has the same diurnal solar radiation pattern represented by the TMY, the TMY is a fictitious yearly database.
Grid power is a centralized power source that can be generated from hydropower, nuclear, geothermal, diesel generator, coal, biomass, and other renewable energy sources.
Coal, hydropower, geothermal, and nuclear power are most commonly used. Localized grid power can be generated from mini-hydropower systems, diesel generators, renewable energy sources, or hybrids.
A major obstacle for centralized grid power sources in many developing countries is the lack of infrastructure. The cost of extending the grid is very high—$5,000-$ 10,000/km. Localized grid or stand-alone systems are more attractive for generating electricity in such areas.
Because grid power transmissions use high voltages to minimize power losses in wires, step-down transformers are used to reduce voltage. Step-up transformers are used at the power generation for high-voltage power transmission. Grid power sources are either 50 Hz or 60 Hz. Unlike DC power, AC power supply wires are not positive and negative, but are live and neutral. A single-phase power supply has three wires—one wire is live and the rest are neutral and ground wires. A three-phase power supply has three live wires and a neutral fourth wire for protection. The live wires must be protected by fuses and contact breakers. A single-phase supply with a 50-Hz frequency has a voltage range of 220 to 230 volts (or 110-120 volts for 60-Hz power supplies). A three-phase supply with a 50-Hz frequency is 380 or 415 volts (or 240 or 480 volts for 60 Hz). Single- or three-phase induction motors are the most popular for water pumping applications. Single-phase motors are typically used for smaller pumping applications and three-phase motors for higher water demands.
There are two main advantages of using grid power sources to pump water: there is no need for storage batteries, and the power supply can be reliable unless there are transmission or power generation problems. A simple control box with a power breaker controls the motor, the water level, and the pump. Maintenance costs are usually very low as long as the system is designed properly. The motor and the pump should be properly matched and the controller must be the right type to handle voltage fluctuations. An integrated water level control system must be installed to control dry running of the pump. The investment costs of such systems depend on the cost of the grid extension and the size of the transformer used. Usually power from high-voltage grid power transmission lines is not used for small pumping systems because the step-down transformer is expensive. The operating cost depends mainly on the electricity tariffs—high tariffs contribute to high pumping costs.
Another advantage to using the grid power source for water pumping applications is that there is no need for extra space other than a small room for the control box. In general, the grid power source system is safer and more reliable than any other power source.
In principle, there are two types of combustion engines, the external and the internal combustion engine. As the name implies, the external combustion engine burns its fuel externally; the internal combustion engine burns within the cylinder. In an external combustion engine, the fuel is used to heat a gas or vapor in an external chamber. One advantage of this type of engine over the internal combustion engine is that the fuel can be any combustible agricultural residue or waste material, such as coal, peat, or biomass.
There are two basic types of external combustion engines: the steam engine (which operates by expanding steam or vapor to drive a mechanism) and the Sterling engine (which uses hot air or gas). Both types require a heat exchanger or a boiler to produce the heat, and both are useful for water pumping applications. The overall efficiency of steam engines as a prime mover (the basic steam engine and the boiler) is 1.5%-3%. More sophisticated engines have efficiencies of 3%-9%. However, safety is a concern because boilers can explode.
Sterling engines have the potential to be more efficient than steam engines. In particular, the direct-action types of Sterling engine-piston water pumps are more promising for further development. As heat engines, Sterling engines can be designed to work even at fractional horsepower sizes, which makes them especially attractive for small water pumping applications. However, because external combustion engines need further development before they can be commercialized for water pumping applications, we emphasize internal combustion engines in this report.
In contrast to external combustion engines, the success of internal combustion engines results from their compact size, their instant startup capabilities, and their high power-weight ratio. These capabilities make them ideal for powering small isolated machines like cars, boats, lawnmowers, and irrigation pumps. The two main types of internal combustion engine are the compression ignition engine (fueled with diesel) and the spark ignition engine (fueled with gasoline, kerosene, or liquefied petroleum gas [LPG]).
Internal combustion engines will wear prematurely if they run continuously at a rated power. The optimum efficiency—the point where the engine consumes the least amount of fuel—of most engines is achieved at around 70%-80% of the rated power. Engines should therefore be derated around 70%-80%. Further derating is also necessary at higher ambient temperature and altitudes. Usually, derating of 1% for each 5°C temperature is necessary for rises above 16°C, and 10% derating is necessary for every kilometer above sea level. For example, for a 3-kW load requirement at 2,000 meters above sea level, and at 25°C ambient temperature, the engine capacity should be 4.8-5.5 kW.
Smaller internal combustion engines are normally started using a hand crank or a pull cord starter. Larger engines require an auxiliary electrical system and a battery with an electric starter. Approximately one-third of the heat produced in the internal combustion engine is dissipated through the walls of the engine cylinder, and air or water cooling is used specially for medium to large engines. Water cooling is much better than air cooling to control the heat and for quieter operation. However, corrosion is the main problem unless special anticorrosion chemicals are used. The engine can be damaged when the cooling water runs out.
The cost of internal combustion engines depends mainly on their size and speed. A higher power-weight ratio is normally achieved by running the engine at high speed. This means that when the engine runs at higher speed, the more air and fuel mixture is burned, producing greater amounts of energy. For the same rated power, smaller higher speed engines are cheaper than heavier lower speed engines. Engines at higher speed wear faster, so there should be tradeoffs between heavy lower speed engines and lightweight, higher speed engines.
Power transmission from the engine to a pump depends on the type of engine and pump design. Power transmission can be coupled using direct mechanical coupling to the pump, gearbox transmission, using belt drives. Transmission losses in direct couplings are generally negligible, but are high for gearbox drives. Pumps can be centrifugal or positive displacement (volumetric) pumps. (See Chapter 4 for further discussion of pumps.)
Combustion ignition engines are also called diesel engines. They ignite fuel by the heating effect that results from mixing highly compressed air with pressurized fuel sprayed into the cylinder at the appropriate time and temperature for ignition. Diesel engines are generally heavy and robust compared to spark ignition engines. The high compression ratio allows a diesel engine to draw in more air per stroke in relation to the combustion space; the fuel injection allows the air-fuel mixture to run more smoothly for ignition than in spark ignition engines. The other advantage with diesel engines is the fuel. According to Fraenkel (1986), diesel fuel has a higher density that makes it 18% richer in energy per liter than gasoline. Diesel engines can operate more hours per day than gasoline or kerosene engines, and they typically have a longer operational life. Larger diesel engines are also more efficient (generally 30%-40%) than spark ignition engines (25%-30%), but smaller diesel engines tend be less efficient (as low as 15%). Several factors— size, type, design quality, and age—contribute to lower efficiency in diesel engines. They can be as low as 10% and as high as 35%.
Diesel engines are categorized as high-speed or low-speed. Low-speed engines operate at 450-1,200 rpm, tend to be heavier and more expensive, and have longer operational lives than high-speed engines. High-speed engines operate at 1,200-2,500 rpm and wear fast, resulting in shorter operational lives. The weight of high-speed engines per rated power is lower, almost by half, than that of the low-speed engines. High-speed engines typically operate no more than 10 hours per day; low-speed engines can operate 24 hours per day.
Spark ignition engines operate by mixing the vaporized fuel (gasoline, kerosene, or LPG) with air, compressing the mixture, and igniting it at the right moment by an electrical spark in the engine cylinder. Spark plugs are used to create the electrical discharge in the cylinder. Typically, spark ignition engines are lighter, more compact, and less expensive than diesel engines. Such engines cannot be designed for a high compression ratio because the fuel-air mixture would ignite prematurely and cause knocking. The caloric values of gasoline, kerosene, and LPG are also quite low compared to those of diesel fuel.
Spark ignition engines are generally designed for small applications (to about 3 kW) and are best for small, lightweight, and portable applications, such as irrigation or lighting a few households. They are affordable, simple to maintain, and ideal for low-head and high-discharge (mainly floating) pumps, even though their operational lives are generally shorter than those of diesel engines. Gasoline engines have shorter daily operational lives (as long as 4 hours) than kerosene engines, which can operate as long as 6 hours a day. Gasoline engines are also most commonly used for cars and light trucks.
Kerosene has approximately 10% more energy per liter than gasoline and is taxed less in many countries. It is easy to store because it is less dangerous than gasoline. Because kerosene does not vaporize quickly or adequately in a cold engine, most kerosene engines are designed to be started with gasoline until the engine warms. Then they switch back to gasoline before the engine stops to make ready for the next start. Most kerosene engines have separate fuel compartments for storing gasoline and taps to switch to kerosene. However, buying and storing both fuels can be inconvenient for users (especially farmers) because two storage tanks are needed to store the fuels.
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