INTRODUCTION TO SOLAR PUMPING.
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An interesting application of solar energy is the pumping of water from wells or simply the elevation of water to fill a reservoir or other uses. This application is especially interesting in communities living in the rural world where conventional electricity is not available and wasteful hydrocarbon-based generators have to be used.
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The more remote a place is, the more expensive it is to transport fuel for pumping stations, and that is precisely where solar pumps are effective.
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The energy savings that we can achieve by replacing conventional systems is really important. But to maximize energy savings you have to understand some concepts that will help us design the most economical application.
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A small solar pumping station has a price very similar to that of conventional applications, so it is a very advantageous solution since we avoid the consumption of fossil fuels. In remote places it is even more advantageous since the cost of transporting fuel is usually high. The current trend is that the pumping of water using photovoltaic technology is already very profitable compared to other technologies such as kerosene, diesel or gasoline pumps.
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Another of the great advantages of wind pumping is that energy does not need to be stored, so we avoid much of the costs that accompany renewable energy. The water is extracted when the generator has the capacity to drive the pump and the rest of the time it simply stops. Since water, unlike the enegía, water can be stored without appreciable losses, we gain great efficiency compared to other methods.
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CONFIGURATION OF THE SOLAR PUMPING SYSTEM. Power generation with photovoltaic panels can be done with both fixed and mobile installations. In places where it is not prudent for solar panels to remain unattended, transportable supports can be built that can even be incorporated on a vehicle. The use of direct current pumps allows us to start extracting water with powers as small as 96 Wp, which means that with a single solar panel we will have enough energy to start pumping water. Thanks to the fact that we do not lose with the conversion of energy, using direct current pumps we can start pumping 440 liters of water per hour from 6 meters deep with only that energy. But the same system will allow us to work at great depths, although in that case it will only pump 82 liters per hour from 70 meters deep.
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ABOUT THE DESIGN
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STEP 1 - TRY TO CALCULATE THE LOWEST POSSIBLE FLOW RATE
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The sun is a continuous resource, and yet water is usually not required more than at specific times. That is why as a general rule solar pumps will be much more effective if they are used to fill cisterns or tanks and then we dispose of the water when we need it. Solar pumps can work all year round without spending a penny on fuel, and if we decrease the flow the pump will be cheaper and we will need less panel capacity.
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STEP 2 - DETERMINE THE LITERS OF WATER WE NEED TO PUMP PER DAY.
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From this approach of optimizing consumption, we will calculate the amount of water we need to pump per day. This data is what we really need to calculate. If we need the pump for example for an irrigation application, we can fill the raft for months until we need to use the water. This makes it possible to design very economical pumping systems for applications that require a lot of water consumption but in a short time.
If the weather is predictable, with high solar radiation throughout the year, we can use a small pumping system to store water in large quantities, and then use it when needed: filling cisterns, irrigation applications, etc. If, for example, a crop is irrigated once a week, we can calculate the time it will take to fill the reservoir.
An estimate of daily water consumption can be as follows:
Each person consumes between 190 and 300 liters.
Cow about 132 liters.
Horse between 38 and 75 liters.
Sheep about 8 liters.
Pork about 16 liters.
One hundred hens about sixteen liters.
If we are going to use water to irrigate crops, the criteria we can use to calculate the water we need daily (in the irrigation period) are the following:
STEP 3 - DETERMINE STORAGE
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Designing a generation system by solar pumping requires storage that allows water to be stored to meet the needs. For this we must design a storage system according to the needs of each user.
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By storing water we will protect ourselves when the pump does not work due to lack of sun. If the water is used for irrigation, the tanks can be filled with low-flow pumps and the water can be used when the time comes.
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Water storage tanks can be constructed of steel, polyvinyl chloride (PVC), fiberglass, concrete, or masonry. Steel, fiberglass, and PVC water tanks are used in both wind and solar energy systems.
Water tanks can be built underground, elevated above ground, or on the surface. Concrete tanks are recommended for underwater water storage in hot climates as they keep the water cold. Steel deposits can easily rust near the sea, so they must be protected with anti-rust treatment.
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If the water will be used for irrigation a small reservoir can be built, in which case it must be waterproofed to reduce the size of the photovoltaic generator.
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The use of plastic sheets for soil waterproofing and water accumulation has undergone a wide expansion around the world and the process is not yet finished, as it evolves in parallel with the growing shortage of rainfall in growing areas with a mild climate.
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To distribute water to a rural community, it is advisable to first pump it into an elevated tank and then distribute it.
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STEP 4 - CALCULATE THE HEIGHT.
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The next step is a very important factor in the design. It is about estimating the effective pressure with which the pump can operate. This pressure is measured in height that is expressed in feet or meters. To estimate the needs we must add:
Pumping level: It is the level at which the water that we must pump is.
Vertical lift: The elevation between the pump discharge pipe and the point of use. It may be too few meters or it may be necessary to climb the water to a hill.
Friction losses: In most cases, friction losses can be simplified.
If the system storage tank is located near the well, 9 meters or less, and the recommended pipe size is used, a simple standard can be used. In cases where the reservoir is located away from the well, more than 10 meters, more accurate calculations can be used based on the length of the pipe, the number and type of fittings and the flow rate.
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STEP 5 - CONFIGURATION
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In some applications we may need constant water availability, but in others it will not be necessary. All this we must assess to decide whether or not to place a battery that manages to pump also on cloudy days. If we do not place the battery, on cloudy days the system will not work. It will be a matter of seeing each application. If, for example, it is an irrigation application, cloudy days decrease evaporation and therefore it will not matter that we delay irrigation one day. .
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The motor-pump subsystem includes motor, pump, and coupling. Various types of pumps and motors can be used in photovoltaic pumping applications, always depending on the type of application and water demand. .
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Different types of pumps and motors are available in water pumping applications depending on water requirements, pumping height, suction height (for surface pumps) and water resources. The most common types of coupling used to pump water are belts and pulleys, drive screws, direct coupling (rack and pinion or bolts and flanges), and gear transmission. The efficiency of the transmission mechanism depends on the coupling ratio, which is the ratio of engine torque to load torque. In direct coupling of the transmission the losses are only 2%. However, the losses in cases of direct speed reduction are 40%. Power transmission in case of gear transmission, which depends on the design of the gearbox, the gear ratio, and the size of the motor in relation to the speed reduction, can be very high. .
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Let's look at some typical solar water supply configurations used in solar pumping: .
Submerged pump with surface-mounted motor: These pumps were commonly used in remote locations, such as applications in the Sahel (sub-Saharan Africa) in the 70s. The advantage is that it gives easy access to the motor brushes for changeover and other maintenance operations. But it has as a disadvantage the energy losses in the kgb bearings and the high installation costs. In general this configuration is largely replaced by submerged motor and pump groups.
Submerged multi-stage centrifugal pump motor group: this type is probably the most common configuration in solar water supply. The advantages of this configuration are that it is easy to install, flexible pipes were supported, and the submerged pump is far from possible damage. AC or DC pumps can be used. DC motors use brushes that need to be replaced approximately every two years. Other motors suitable for this application are brushless DC that require electronic switching. The most commonly used system consists of an AC pump and an inverter with a photovoltaic configuration of less than 1,500 Wp.
Floating pump-motor group: the versatility of floating units make them ideal for pumping applications in open channels and wells. The pump is easily transportable and there is little risk of the pump working dry. Most of these configurations use single-stage submerged centrifugal pumps. The most common type uses a brushless DC (electronically switched) motor.
Reciprocal positive displacement pump: The reciprocal positive displacement pump (known as jack or nodding donkey) are very suitable for high heights and low flow. The performance is proportional to the speed of the pump. At high altitudes the friction forces are low compared to hydrostatic forces and often makes positive displacement pumps more efficient than centrifugal pumps for this situation. Reciprocal positive displacement pumps create a cyclic load on the motor that, for efficient operations, needs to be balanced.
Suction pump group: This type of pump is not recommended except in applications where the operator may always be attending. Although the use of primary chambers and non-return valves can prevent impulse losses, in practice problems of self-starting and impulsion are experienced.
STEP 6 - DETERMINE THE TOTAL CAPACITY
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With the above sequence of actions we will be able to determine the equipment we need for our application. It is really the flow that will finally determine the size of the final installation, that is, the volume of water we need daily. Pumps are available that can work with heights of up to 200 m and flow rates of up to 250 m3/day. .
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We've talked about some of the basic applications of solar energy to provide pumping solutions, but there are many more. For example, the bombita shown in the image allows you to take water from a pool or similar tank and place an automatic sprinkler irrigation system, and best of all, without any installation.
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STEP 7 – ESTIMATION OF SOLAR RESOURCES
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Solar radiation data is not usually available in many remote areas where solar pumping is required, so we must consider in the calculation that the data will not necessarily be accurate.
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If we work on a large project it is interesting to make some direct measurements and for this several instruments are available.
STEP 8 - LOW-COST TECHNOLOGIES.
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A necessary step in making this technology accessible to remote communities has been the market launch of low-cost solar panels. The only problem with these panels, usually cadmium telluride, the only problem they have is that they require a larger surface, but in remote communities space is not usually a problem. .
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The pumps can be used directly by pumping water and using a solar power system controller and inverter. The pumps used are single-phase or three-phase submersibles. .
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Another way to minimize the cost of the system is by using variable speed pumps. Although these pumps are more expensive, they can vary their operation adapting to the production of the sun and therefore manage to extract more water since they work in periods of less solar radiation. Additionally, energy tracking capabilities allow the controller to constantly adjust to peak solar radiation production. .
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Current controllers can achieve a power conversion efficiency of up to 97%, which is higher than conventional inverter technology. .
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EXAMPLES OF APPLICATION IN POOR COUNTRIES.
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In poor countries, more than 1.1 billion people have inadequate access to clean water, and the worst thing is that their numbers continue to increase because there is no electricity infrastructure available. It is very expensive to make a well and even more expensive to extract water from great depths. Let's look at some examples of solar pumping in projects that have been carried out in places with difficulties.
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Uganda
