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INTRODUCTION
Energy is key ingredient for the overall development of an economy. India has been endowed with abundant renewable solar energy resource. India is large country and the rate of electrification has not kept pace with the expanding population, urbanization and industrialization and has resulted in the increasing deficit between demand and supply of electricity. This has not only resulted in under electrification but also put heavy pressure on the government to keep pace with the demand for electricity. People not served by the owner grade have to rely on fossil fuel like kerosene and diesel for their energy needs. Where ever the rural areas have been brought under power grid the erratic and unreliable power supply has not helped the farmers and the need for an uninterrupted power supply especially during the critical farming period has been major area of concern. India receives a solar energy equivalent of 5000 trillion KWh/year, with a daily average solar incidence of 4-7 KW-h/m2. This is considerably more than the total energy consumption of the country. Further most part of the country experience 250-300 sunny days in a year which make solar energy viable option in these areas.
Decentralized renewable energy systems, which rely on locally available resources, could provide the solution to the rural energy problem particularly in remote areas where grid extension is not a viable proposition.
Solar energy with its virtually infinite potential and free availability represents a non-polluting and inexhaustible energy source which can be developed to meet the energy needs of mankind in a major way. To evaluate the energy potential at particular place, detailed information on its availability is essential
1.1 UTILIZATION OF SOLAR ENERGY:
Solar energy can be basically utilized in following ways:
1. Solar Thermal (ST) technologies where the heat produced are used to operate devices for heating, cooling, drying, water purification and power generation. The devices suitable for use by village communities include solar hot water heaters, solar cookers and solar driers.
2. Solar Energy for agriculture: The demand for electrical energy is far out stripping supply, especially in the agricultural and it is becoming increasingly difficult to meet this exponential growth in demand agricultural productivity is closely related to direct and indirect inputs and policies are required to consolidate this relationship to the benefit of farmers.
3. Solar energy based water lifting: Among the solar technologies useful in agriculture are water lifting and pumping with solar photovoltaic system. Water pumping by solar power is a concept which has got widespread interest.
4. Solar Photovoltaic (SPV) systems which convert sunlight into electricity for application such as lighting, pumping, communication and refrigeration.
The Solar Energy Program is prominent among the technology based renewable energy program of the MNES. Areas covered under this program include solar thermal technology, solar photovoltaic technologies as well as information dissemination, marketing, standardization of products and R&D.
Solar energy can be utilized to operate pumps, utilization either the thermal or the light part of thermal radiation. With a solar pump, energy is not available on demand and the daily variation in solar power generation necessitates the storage of a surplus of water pump on sunny days for used on cloudy days.
Decentralized renewable energy systems, which rely on locally available resources, could provide the solution to the rural energy problem particularly in remote areas where grid extension is not a viable proposition.
Solar energy with its virtually infinite potential and free availability, represents a non polluting and inexhaustible energy source which can be developed to meet the energy needs of mankind in a major way. To evaluate the energy potential at particular place, detailed information on its availability is essential.
Photovoltaic (PV) panels are often used for agricultural operations, especially in remote areas or where the use of an alternative energy source is desired. In particular, they have been demonstrated time and time again to reliably produce sufficient electricity directly from solar radiation (sunlight) to power livestock and irrigation watering systems.
A benefit of using solar energy to power agricultural water pump systems is that increased water requirements for livestock and irrigation tend to coincide with the seasonal increase of incoming solar energy. When properly designed, these PV systems can also result in significant long-term cost savings and a smaller environmental footprint compared to conventional power systems.
The volume of water pumped by a solar-powered system in a given interval depends on the total amount of solar energy available in that time period. Specifically, the flow rate of the water pumped is determined by both the intensity of the solar energy available and the size of the PV array used to convert that solar energy into direct current (DC) electricity.
The principle components in a solar-powered water pump system include:
• The PV array and its support structure,
• An electrical controller, and
• An electric-powered pump.
It is important that the components be designed as part of an integrated system to ensure that all the equipment is compatible and that the system operates as intended. It is therefore recommended that all components be obtained from a single supplier to ensure their compatibility.
The following information is required to design a PV-powered pump:
• The site-specific solar energy available (referred to as “solar insolation”).
• The volume of water required in a given period of time for livestock or irrigation purposes, as well as for storage. (A storage volume equal to a three-day water requirement is normally recommended for livestock operations as a backup for the system’s safety features and cloudy days.)
• The total dynamic head (TDH) for the pump.
• The quantity and quality of available water.
• The system’s proposed layout and hydraulic criteria.
The following sections will first provide an introduction to the basic concepts involved in solar-powered pump systems, then descriptions of and design considerations for the previously mentioned, individual system components.
1.2 The Photoelectric Effect
PV systems harness the sun’s energy by converting it into electricity via the photoelectric effect. This occurs when incoming photons interact with a conductive surface, such as a silicon cell or metal film, and electrons in the material become excited and jump from one conductive layer to the other.
In the figure below, the excitation of electrons and their movement from the p-layer to the n-layer results in a voltage differential across the electrical circuit, causing electrons to flow through the rest of the circuit to maintain a charge balance.
The system is designed so that there is an electrical load in the external circuit, permitting the current flow to perform a useful function. In other words, the behavior of electrons in the solar cell creates a voltage that can be utilized to, for example, operate a water pump system.
PV pumping technology
A PVP typically consists of the following main components:
1. Photovoltaic array: An array of photovoltaic modules connected in series and possibly strings of modules connected in parallel.
2. Controller: An electronic device which matches the PV power to the motor and regulates the operation, starting and stopping of the PVP. The controller is mostly installed on the surface although some PVPs have the controller integrated in the submersible motor-pump set:
a. DC controller: usually based on a DC to DC controller with fixed voltage set point operation.
b. AC controller (inverter): converts DC electricity from the array to alternating current electricity often with maximum power point tracking.
3. Electric motor: There are a number of motor types: DC brush, DC brushless, or three phase induction and three phase permanent magnet synchronous motors.
4. Pump: The most common pump types are the helical rotor pump (also referred to as progressive cavity), the diaphragm pump, the piston pump and the centrifugal pump. Some years ago there were PVP models on the market that operated with batteries and a conventional inverter. However it was soon realized that the cost savings on the pump did not make up for the overall substandard efficiency and the higher maintenance cost due to battery replacements. Instead it became clear that it is more economic to rather store water in a reservoir than electricity in a battery bank.
There are currently three pumping configurations commonly utilized in India:
1. DC drive with positive displacement pumps: This consists of four pump technologies:
a. Diaphragm pump driven by brushed DC motor: Submersible motor/pump: Example: Shurflo, DivWatt, All Power Watermax.
b. Helical rotor pump driven by brushless DC motor: Submersible motor/pump. Example: Total Energy TSP 1000.
c. Helical rotor pump driven by surface mounted brushed DC motor: Example: Mono/Orbit pumps with DC motor.
d. Piston pump driven by surface mounted brushed DC motor: Example: Juwa pump.
2. AC drive powering a submersible induction motor/centrifugal pump unit:
Example: Total Energy TSP 2000, 4000 & 6000 range; Grundfos SA 1500 and SA 400 which has been utilized extensively in Namibia but may be phased out in the near future.
3. AC drive powering a three phase permanent magnet synchronous motor:
This category consists of:
a. Positive displacement helical rotor pump: Example: Grundfos SQ Flex, Lorentz HR range.
b. Centrifugal pump: Example: Grundfos SQ Flex, Lorentz C range.
The above technologies have specific features which make them suitable for particular applications:
Array voltage: Some of the pumping systems have high array voltages. This has the advantage that the array may be further from the borehole without significant voltage drop (dependant on cable size and current). Array positioning may be important where there is potential for theft.
DC motors: DC motors reach efficiencies of up to 80% and are therefore significantly more efficient than sub-kW three phase motors which have efficiencies in the region of 60% to65%.
Brushless DC motors: This combines the high efficiency of DC motors with low maintenance as opposed to brushed DC motors which require regular brush replacement (approximately every one to two years – head and quality dependent).
Three phase permanent magnet motors: This similarly combines the high efficiency of permanent magnet motors with low maintenance.
Positive displacement vs. centrifugal pump: Positive displacement pumps have a better daily delivery than centrifugal pumps when driven by a solar PV system with its characteristic variable power supply. This is due to the considerable drop in efficiency of the centrifugal pump when operating away from its design speed. This is the case in the morning and the afternoon of a centrifugal pump driven by a PV array, unless that array tracks the sun (which is why centrifugal PVPs effectiveness improves more with a tracking array than a positive displacement PVP). The efficiency curve of a positive displacement pump is flatter over a range of speeds. However the efficiency of positive displacement pumps decreases with the shallowness of the borehole (the constant fixed friction losses become a more significant part of the power it takes to lift water). Therefore it is not surprising that both Grundfos and Lorentz use centrifugal pumps for applications where the lift is less than 20 to 30 metres but switch to positive displacement pumps for deeper wells.
1.4 SOLAR RADIATION, SOLAR IRRADIANCE, AND SOLAR INSOLATION
To design a solar-powered water pump system, you will need to quantify the available solar energy. It is therefore important for you to be familiar with the definitions and distinctions between the three related terms “solar radiation,” “solar irradiance,” and “solar insolation.”
Solar radiation is the energy from the sun that reaches the earth. It is commonly expressed in units of kilowatts per square meter (kW/m2). The earth receives a nearly constant 1.36 KW/m2 of solar radiation at its outer atmosphere. However, by the time this energy reaches the earth’s surface, the total amount of solar radiation is reduced to approximately 1 KW/m2.
The intensity of sunshine (i.e. solar radiation) varies based on geographic location. A good analogy to describe this variation is the different conditions that can be found on the north slope of a mountain versus its south slope.
The intensity of sunlight also varies based on the time of day because the sun’s energy must pass through different amounts of the earth's atmosphere as the incident angle of the sun changes. Solar intensity is greatest when the sun is straight overhead (also known as solar noon) and light is passing through the least amount of atmosphere. Conversely, solar intensity is least during the early morning and late afternoon hours when the sunlight passes through the greatest amount of atmosphere. In most areas, the most productive hours of sunlight (when solar radiation levels approach 1 kW/m2) are from 9:00 a.m. to 3:00 p.m. Outside of this time range, solar power might still be produced, but at much lower levels.
Solar irradiance, on the other hand, is the amount of solar energy received by or projected onto a specific surface. Solar irradiance is also expressed in units of KW/m2 and is measured at the surface of the material. In the case of a PV-powered system, this surface is the solar panel.
Finally, solar insolation is the amount of solar irradiance measured over a given period of time. It is typically quantified in peak sun hours, which are the equivalent number of hours per day when solar irradiance averages 1 kW/m2. It is important to note that although the sun may be above the horizon for 14 hours in a given day, it may only generate energy equivalent to 6 peak sun hours.
PHOTOVOLTAIC (PV) PANELS
PV panels are made up of a series of solar cells, as shown in Figure 5, below. Each solar cell has two or more specially prepared layers of semiconductor material that produce DC electricity when exposed to sunlight. A single, typical solar cell can generate approximately 3 watts of energy in full sunlight.
The semiconductor layers can be either crystalline or thin film. Crystalline solar cells are generally constructed out of silicon and have an efficiency of approximately 15%. Solar cells that are constructed out of thin films, which can consist of a variety of different metals, have efficiencies of approximately 8% to 11%. They are not as durable as silicon solar cells, but they are lighter and considerably less expensive.
1.5.1 PV Panel Electrical Characteristics
PV panels are rated according to their output, which is based on an incoming solar irradiance of 1 kW/m2 at a specified temperature. Panel output data include peak power (Watts [Pw]), voltage (Volts [V]), and current (Amps [A]).
Under conditions of reduced solar radiation, the current produced is decreased accordingly, but the voltage is reduced only slightly.
PV Panel Orientation and Tracking
To be most effective, PV panels need to continuously and directly face incoming sunlight, which requires the use of one or two tracking mechanisms. A single-axis tracking mechanism will rotate a PV panel about its vertical axis to follow the sun throughout the day. A double-axis mechanism will also control the panel tilt angle (the angle of the panel relative to horizontal where 0° is horizontal and 90° is vertical) to adjust for the elevation of the sun in the sky throughout the year.
Single-axis tracking can be very effective for increasing energy production throughout the year, by up to 50% during some months. Passive single trackers, which require no energy input, can be used. They use the heat from the sun to cause Freon or a substitute refrigerant to move between cylinders in the tracker assembly, which causes the panels to shift so that they maintain a constant 90-degree angle to the sun throughout the day. Single-axis trackers tend to be more appropriate for sites between +/- 30 degrees latitude. Also, their benefits at higher altitudes tend to be less during the winter months when the sun is low on the horizon.
In general, though, due to the complexity of tracking mechanisms and their associated controls, most installations for water pumps are stationary and oriented due south to take advantage of the maximum sunlight available in the middle of the day.
The default tilt angle for a PV panel is equal to the latitude of the location. For a fixed array, this default angle will maximize annual energy production.
A tilt angle of +/- 15 degrees from latitude will increase energy production for the winter or summer months, respectively. Most solar panels that are used for water pumping are set to collect the maximum amount of energy in the summer, when water demands are greatest. However, to maximize energy for both summer and winter pumping, it is recommended that the tilt angle be adjusted at the spring and autumn equinoxes (March 21st and September 21st). In other words, the panel array tilt angle should be adjusted as follows:
• Summer tilt angle = latitude – 15° (when the sun is higher in the sky).
• Winter tilt angle = latitude + 15° (when the sun is lower in the sky).
For example, latitudes in Oregon range from 42° to 46° north, so summer tilt angles are expected to range from 27° to 31° while winter tilt angles should range from 57° to 61°.
1.6 SOLAR-POWERED PUMPS
Pumps that use PV systems are normally powered by DC motors. These motors use the DC output from the PV panels directly. Alternating current (AC) motors are sometimes used, but they require more complex control systems. They also result in less total energy availability due to the electrical losses caused when an inverter is used to convert the DC to AC electricity. Because DC motors do not require an inverter, utilize a less complex control system, and result in more total energy availability, they are most commonly paired with solar-powered pumps.
The type of pump configuration and mounting can be either submersible, surface mount, or floating, depending on the water source.
Solar-powered pumps are characterized as either positive displacement pumps (e.g., diaphragm, piston, or helical rotor) or centrifugal pumps. Positive displacement pumps are typically used when the TDH is high and the flow rate (measured in lpm) required is low. Conversely, centrifugal pumps are typically used for low TDH and high flow rates. The TDH and flow rate characteristics for a given pump can be found in the pump manufacturer’s specifications.
Another important consideration when selecting the appropriate pump is the pump’s minimum voltage. Pump manufacturers may provide pumps with similar operating characteristics but different voltages. A higher operating voltage tends to be more efficient since there is less energy loss from the reduced current required to deliver the same power (wattage). This is important when considering the placement of the panels and controller relative to the location of the pump. A general rule of thumb is that if the array consists of four or more panels and is located more than 50 feet away from the pump, the use of a higher voltage pump should be considered.
Pump Selection and System Design
Factors affecting the selection of a solar-powered pump include the following:
• TDH (in feet).
• The water source (surface vs. well).
• The available electrical power (peak power) and energy (total energy, i.e. power x time) produced by the PV panel array.
• The water requirement (flow rate and/or total volume in a given time period, including the storage requirement).
The water quality (including the amount of sediment, organic content, sand, and total dissolved solids [TDS]) may also be a required consideration for selecting a pump, as per the manufacturer’s specifications.
Solar-Powered Pump Characteristics
A water pump can be selected using pump performance curves that show the operating characteristics for the solar-powered pump, such as those in Figure 1.7 and Figure 1.8 below. The curves in Figure 1.7 are for positive displacement pumps, and those in Figure 1.8 are for centrifugal pumps.
Alternatively, some suppliers have computer programs and web-based utilities for selecting and sizing pumps for specified values of available solar radiation, pump flow rate, and pumping head.
Applications
Solar pumps are used principally for three applications:
• Village water supply
• Livestock watering
• Irrigation
A solar pump for village water supply is shown schematically in Figure 1.9. With village water supply, a constant water demand throughout the year occurs, although there is need to store water for periods of low insolation (low solar radiation). Typically in Sahelian Africa the storage would be 3-5 days of water demand. In environments where rainy seasons occur, rainwater harvesting can offset the reduced output of the solar pump during this period. The majority of the 6000 or more solar pumping systems installed to date are for village water supply or livestock watering.
LITERATURE REVIEW
PREVIOUS WORK
Assessment of the Potential of PV Pumping Systems includes:
Ground water resource assessment.
Analysis of existing borehole installations.
Institutional setup at Rural Water Supply.
Economic analysis of solar PV and diesel water pumps.
Technical, economic and social site selection criteria.
PVP suppliers in INDIA.
Assessment of PVP potential.
2.1 Solar Water Pumping
The sun is the natural source of energy for an independent water supply. Solar pumps operate anywhere that the sun shines, and the longer it shines, the more water they pump. When it’s cloudy, they pump less water, but often you need less water when it is cloudy.
Photovoltaic modules, the power source for solar pumping, have no moving parts, require no maintenance and last for decades. A properly designed solar pumping system will be efficient, simple and reliable. Solar water pumping systems operate on direct current. The output of the solar power system varies throughout the day and with changes in weather conditions. The nature of variable electricity in the form of direct current (DC) is quite different from conventional, steady alternating (AC) current from the utility grid or a generator. To use solar energy economically, the pumping system must utilize the long solar day, drawing a minimum of power. This means pumping more slowly than conventional pumps. Pumping at rates of less than 6 lpm requires different mechanisms from the conventional (centrifugal) pumps. Small solar pumps are unique, both electrically and mechanically.
The most efficient pumps are "positive displacement" pumps. They pump a certain amount of water with each rotation If it is cloudy or early morning, the pump will receive less energy and run more slowly A positive displacement pump will pump approximately half as much water with half as much energy.
Conventional AC pumps are usually centrifugal pumps that spin at a high speed to pump as many gallons per minute as possible. They also consume a large amount of power. If you run a centrifugal pump at half speed, it pumps one quarter the pressure.
Their efficiency is very low at low speeds and when pumping against high pressure. If your water sources are remote from power lines, add up your long-term costs of fuel and repairs on generators, or the cost of utility line extensions. Now consider the savings with a solar pump that needs attention only once every 2 to 20 years depending on the model.
Solar powered pumps can provide an equal volume of water per day without the high and inefficient energy demands of a large capacity AC pump. Instead of pumping a large volume of water in a short time and turning off, the solar pump works slowly and efficiently all day. Often a solar pump will work fine in a well with a recovery rate too slow for a conventional AC pump.
2.2 The Technology
Systems are broadly configured into 5 types as described below:
2.2.1 Submerged multistage centrifugal motor pump set:
This type is probably the most common type of solar pump used for village water supply. The advantages of this configuration are that it is easy to install, often with lay-flat flexible pipe work and the motor pump set is submerged away from potential damage.
Either ac or dc motors can be incorporated into the pump set although an inverter would be needed for ac systems. If a brushed dc motor is used then the equipment will need to be pulled up from the well (approximately every 2 years) to replace brushes. Brushless dc motors would require electronic commutation. The most commonly employed system consists of an ac pump and inverter with a photovoltaic array of less than 1500Wp.