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Water Pumping Systems
These designs are printed in the "Stand-Alone Photovoltaic Systems - a Handbook of Recomended Design Practices" available from Sandia. A description of different pumping systems are described in this section. To see how the design was completed for these applicaations or for additional information select from the menu below.
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Water pumping is an application common around the world. Stand-alone PV systems are being used increasingly for intermediate sized pumping applications - those between hand pumps and large generator powered systems. The advantages of PV powered pumps are
The disadvantages are the high initial cost and the variable water production.
To achieve a reliable pump system the system designer must be familiar with the well, the storage system, the terrain surrounding the well, and manufacturers' data on available pumps. The first requirement is an estimate of the water needed and the amount of water that can be supplied by the source (flowrate). If the water needs vary throughout the year, a monthly profile should be drawn and matched to a monthly profile of the production capability of the water source. It is important to know the worst case conditions, so data on production and demand for the driest months of the year should be available or estimated. If the capability of the water source is limited, the designer must take action. One thing that can be done is to improve the water source or develop other sources. Using a smaller pump is another option but the availability of different size pumps is limited. Another method is to incorporate batteries into the system and distribute the pumping time over a longer period. This is one of two reasons to use batteries in a water pumping system. The other is if the pumping time needs to be controlled - usually to pump at a high flowrate for a short time. An example might be a residential system with storage tanks when you want to pump all the water for the household during times when other loads are not operating. Although using batteries in a system will maximize the pump efficiency - because of the steady operating conditions presented to the pump and motor--most water pumping systems do not contain batteries. It is usually less expensive to store water than to store electricity. If a tank is available, the system can pump all day and the water stored for later use. Gravity-feed or a small pressure pump can then be used to deliver the water to the user.
There are two broad categories of pumps being used in stand-alone PV systems around the world - rotating and positive displacement - and there are many variations on the designs of these two basic types. Examples of the rotating pump type are centrifugal, rotating vane, or screw drive. These pumps move water continuously when power is present at the pump. The output of these pumps is dependent on head, solar radiation (current produced), and operating voltage. They are well suited for pumping from shallow reservoirs or cisterns. They can be tied directly to the PV array output but their performance will be improved by using an electronic controller such as a linear current booster to improve the match between the pump and PV array.
Volumetric or displacement pumps are used for deep wells. These pumps use a piston or diaphragm to move packets of liquid through a sealed chamber. These pumps are used to pump from greater depths as typified by the oil well pump jacks that use a "walking beam" to pull a long rod that operates a piston far below the surface. A small amount of liquid is moved upward during each cycle of the pump. The pumping rate is almost independent of depth but the current demand varies as the pump cycles from lifting water (upstroke) to accepting more (downstroke). A matching device (battery or electronic) is required between the pump and the array. The pump size, operating time, and total power demand can be calculated if the efficiency of the pump and the depth of the water are known. Some typical wire-to-water efficiencies are given in the table.
Pumps are also categorized as surface or submersible. Surface pumps have the obvious advantage of being more accessible for maintenance. When specifying a surface pump you must distinguish between suction and lift. A pump may be installed a few feet above the water level, with a pipe from the pump to the water. The maximum length of the pipe is determined by the suction capability of the pump. The pump may then "lift" the water to a storage tank above the pump. The elevation of the storage tank is determined by the lift capability of the pump. Most submersible pumps have high lift capability. They are sensitive to dirt/sand in the water and should not be run if the water level drops below the pump.
Both rotating and displacement pumps can be driven by ac or dc motors. The choice of motor depends on water volume needed, efficiency, price, reliability, and availability of support. DC motors are an attractive option because of their compatibility with the power source and because their efficiency is usually higher than that of ac motors. However, their initial cost is higher, the selection may be limited in some countries, and the brush type motor requires periodic maintenance. Some brushless dc motors are available and promise improved reliability and decreased maintenance. AC motors require a dc to ac inverter, but their lower price and wider availability are advantages.
Many smaller systems use direct coupled dc rotary pumps. The most common type, a centrifugal pump, uses an rotating impeller that draws liquid through an intake at the center of the impeller and propels it outward by centrifugal force to an outlet at the perimeter of the impeller housing. A single stage centrifugal pump can be used for water levels (head) of 5-7 meters. A jet centrifugal pump redirects a small portion of the pumped water to the impeller intake. This can increase the suction to over 40 meters but the efficiency drops quickly with increasing head. Another method used to increase the pumping head is to stack impellers so that each pump moves water only from the unit below to the one above. This increases cost of the pump system and the efficiency decreases with the number of stages. For any rotary pump system, the water output is proportional to the current provided to the motor that drives the element, and this current is proportional to the solar irradiance which changes continually. Therefore, the efficiency of these pumps will vary widely during a typical day. Using an electronic matching device such as a linear current booster (LCB) will increase pump system efficiency and flow by better matching the array to the pump.
Typical Wire to Water Efficiency Ranges
Head (m)
Type of Pump
W-W Efficiency (%)
0-5
Rotary
15 - 25
6-20
Centrifugal with jet
10 - 20
Displacement
20 - 30
21-100
30 - 40
Jack Pump
30 - 45
greater than 100
35 - 50
Installation Concerns
Many failures of PV pumping systems are caused by pump problems. The PV power supply has much higher reliability than the pump/motor subsystem. A good installation of the pumping hardware will increase reliability. Some things to watch for are described below.
Ñ Varying Water Levels - The water level in a well may vary seasonally, daily, or even hourly. The water level in some wells in rocky areas has been reported to drop as much as 75 feet during pumping. The pump must be mounted to keep the water inlet below the water level at all times. If the replenishment rate of a well is lower than the maximum possible pumping rate, a level switch or mechanical valve should be included to protect the pump from operating dry. Float switches should be used on storage tanks if the volume of the tank is smaller than the daily pump rate. This will prevent wasted water or worse, pump damage due to overheating. Ñ Protect the Pump Input - Sand is a primary cause of pump failure. If the well is located where dirt and sand may be pulled into the pump, a sand screen should be used. Most pump manufacturers offer this option or they can recommend methods for limiting the risk. Ñ Ground the Equipment - Water pumps attract lightning because of the excellent ground they provide. If possible, do not locate the pump system on high ground. Consider erecting lightning rods on higher terrain around the pump. Ground the pump motor, the array frame, all equipment boxes, and one system conductor to the well casing (if metal) or to a bare conductor running down to the water level. Never use the pipe string to the pump as a ground, because the ground would be interrupted when maintenance was being performed. Use of movistors to protect electronics is recommended in areas prone to lightning. Ñ Avoid Long Pipe Runs - Friction losses can significantly increase the head and thus the size of the PV array. Friction losses depend on the size of the pipe, the length, the flow rate, and the number of bends in the pipe. Because the output of a stand-alone PV system is power-limited and varies throughout the day, it is particularly important to keep friction losses low. Pump system efficiency can drop to near zero if a large friction loss must be overcome. Try to limit the friction loss to less than 10 percent of the head. This can be done by oversizing the pipe, eliminating bends and junctions, and reducing flow rate. Data on pipe size and friction rates are available from pump manufacturers. Ñ Use Steel Pipe - Steel pipe is recommended for use in the well, particularly if submersible pumps are used. Plastic pipe may break. However, plastic pipe provides an inexpensive way to run water from the well to the storage tank or end user. Fiberglass sucker rods may be used in a well with a jack pump. They are lighter than metal, buoyant, and much easier to pull for pump maintenance. The pipe diameter should be larger than the pump cylinder. This will allow the pump leathers to be changed by pulling the sucker rod without pulling the pipe. Ñ Protect the Control Equipment - All electronic control equipment should be housed in weather-resistant boxes. All wires should be approved for outdoor use or installed in conduit. Any cables used for submersible pumps should be appropriate for that application. Pump manufacturers will give recommended wire types for their equipment. Ñ Protect the Well - Use sanitary well seals for all wells. Bury pipes from wellhead to tank at a depth that will insure the pipe will not be broken by traffic or during future trenching or excavation. Mark pipe runs for future reference.
Ñ Varying Water Levels - The water level in a well may vary seasonally, daily, or even hourly. The water level in some wells in rocky areas has been reported to drop as much as 75 feet during pumping. The pump must be mounted to keep the water inlet below the water level at all times. If the replenishment rate of a well is lower than the maximum possible pumping rate, a level switch or mechanical valve should be included to protect the pump from operating dry. Float switches should be used on storage tanks if the volume of the tank is smaller than the daily pump rate. This will prevent wasted water or worse, pump damage due to overheating.
Ñ Protect the Pump Input - Sand is a primary cause of pump failure. If the well is located where dirt and sand may be pulled into the pump, a sand screen should be used. Most pump manufacturers offer this option or they can recommend methods for limiting the risk.
Ñ Ground the Equipment - Water pumps attract lightning because of the excellent ground they provide. If possible, do not locate the pump system on high ground. Consider erecting lightning rods on higher terrain around the pump. Ground the pump motor, the array frame, all equipment boxes, and one system conductor to the well casing (if metal) or to a bare conductor running down to the water level. Never use the pipe string to the pump as a ground, because the ground would be interrupted when maintenance was being performed. Use of movistors to protect electronics is recommended in areas prone to lightning.
Ñ Avoid Long Pipe Runs - Friction losses can significantly increase the head and thus the size of the PV array. Friction losses depend on the size of the pipe, the length, the flow rate, and the number of bends in the pipe. Because the output of a stand-alone PV system is power-limited and varies throughout the day, it is particularly important to keep friction losses low. Pump system efficiency can drop to near zero if a large friction loss must be overcome. Try to limit the friction loss to less than 10 percent of the head. This can be done by oversizing the pipe, eliminating bends and junctions, and reducing flow rate. Data on pipe size and friction rates are available from pump manufacturers.
Ñ Use Steel Pipe - Steel pipe is recommended for use in the well, particularly if submersible pumps are used. Plastic pipe may break. However, plastic pipe provides an inexpensive way to run water from the well to the storage tank or end user. Fiberglass sucker rods may be used in a well with a jack pump. They are lighter than metal, buoyant, and much easier to pull for pump maintenance. The pipe diameter should be larger than the pump cylinder. This will allow the pump leathers to be changed by pulling the sucker rod without pulling the pipe.
Ñ Protect the Control Equipment - All electronic control equipment should be housed in weather-resistant boxes. All wires should be approved for outdoor use or installed in conduit. Any cables used for submersible pumps should be appropriate for that application. Pump manufacturers will give recommended wire types for their equipment.
Ñ Protect the Well - Use sanitary well seals for all wells. Bury pipes from wellhead to tank at a depth that will insure the pipe will not be broken by traffic or during future trenching or excavation. Mark pipe runs for future reference.
Wiring should be sunlight resistant USE or UF type cable with insulation rated for installation in damp conditions. All above ground connections should be in water-tight junction boxes with strain relief connectors. Array wiring should be laced and attached to support structure with wire ties. Use conduit for output wiring to the pump motor (or the controller and batteries.) The array should be grounded using bare copper grounding wire (No. 8 or larger) securely attached to each support structure. Array tracking is recommended for most pumping applications. A fused disconnect or circuit breaker in a rainproof enclosure should be installed at the array. Simple metering of voltage and current is recommended. Because PV powered pumps operate typically at low voltages, the currents will be high and wire size must be appropriate to keep wiring losses to less than 2.5 percent.
Array tracking is recommended for most PV water pumping systems. Passive trackers that support up to 16 modules are available. Average wind velocities must be taken into account when considering the use of a tracking support structure. Wind velocities above 25 mph may prevent tracking if a passive freon-driven tracker is used. Support structures should be aluminum, galvanized or stainless steel designed for maximum anticipated wind velocities. Locate the array as close to the well as practical to keep wire runs to a minimum. Fencing may be required to protect the array from animals in stock watering applications. A good ground is required&emdash;many pump systems are struck by lightning. The ground can be made to the well casing or wellhead. Never use the metal pipe string because the ground would be interrupted anytime the string was pulled for pump maintenance.
In water pumping systems, storage can be achieved by using batteries or by storing the water in tanks. Adding batteries to a system increases cost and decreases reliability. Water storage is better for most applications. However, considerable evaporation losses can occur if the water is stored in open tanks or reservoirs. Closed tanks large enough to store several days water supply can be expensive. In some countries, these tanks are not available or the equipment necessary to handle, move, and install the tanks may not be available. Also, any water storage is susceptible to vandalism and pollution.
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