Sizing Examples for
Photovoltaic Power Water Pumping Systems
DC Pump
Jack Pump
AC Pump
Low Flow Pump
You will find filled-in worksheets that illustrate the calculations needed to design a PV power system for these applications. Blank worksheets and instructions may be accessed and copied by going here.
DC Surface Pump for Livestock Watering
Rural electric cooperatives in the United States are beginning to supply PV power to their customers who need to pump water for livestock. The conventional method of service was to extend the grid line to the isolated well&emdash;even though the income from the customer could never pay for the maintenance of the line, let alone provide a return on the original investment. As these remote lines require replacement, either because of age or storm damage, a number of utilities have discovered that a PV pumping system can be provided for the customer at a fraction of the cost of replacing the line. This example is of a rancher in Nebraska that requires about 500 gallons of water per day for 40 cows. The pasture where the well is located is used in wintertime only There is a 2,500 gallon tank near the well that provides water (gravity fed) to smaller water tanks distributed throughout the pasture. The load is critical as the cattle cannot go without water more than one day in winter. This small pump uses a 12 volt dc two stage diaphragm pump and is designed to pump over 500 gallon per day (~ 2,000 liters per day) from a level of 20 meters. The water level in this well is only 10 meters with a maximum drawdown of 6 meters. A float switch in the main watering tank controls operation of the pump.
Site
Potter, Nebraska
Location/Elevation
40°N - 103° W - 200 meters
Environment
Grassland
Temperature Range (°C)
-10 to 35
Maximum Wind Speed (m/s)
20
Availability Required
near 100 percent (critical)
Days of Storage
4 - Fiberglass Tank
Source
Cased 3-inch Borehole
Dynamic Head (m)
17
Water Required
2000 liters/day (500 gpd) October to March
Installation
The PV modules are mounted on a passively controlled tracking structure and tilted at 55° for maximum winter performance. Sunlight resistant USE wire is used to interconnect the modules. A switch box was attached to the pole and the linear current booster was installed in the box. The pump was attached to 2-inch plastic pipe and installed in the 3-inch bore hole. A float switch was installed near the top of the holding tank and the control cables are protected from damage by conduit. The system was grounded to the well casing using a short length of No. 6 stranded copper wire.
Worksheet 1-Calculate Water Pumping Load
Worksheet 2-Design Current and Array Tilt
Worksheet 4-Calculate System Array Size
Worksheet 2-Pumped Water and Pumping Rate
Controller Specifications
This linear current booster was used between the pump and the PV array to provide a better match between source and load.
Switches and Protection Components
DC Wire Sizing Specifications
The wiring diagram for this system is represented below.
The economic analysis and life cycle cost calculation for this system is given below.
Deep Well Jack Pump for Livestock Water
This system is also used for livestock watering but the water level in southern New Mexico is 110 meters or more. A jack pump with a 75-volt dc motor is used. The array is connected to the motor through the maximum power point tracker. The water demand is 2,200 gallons per day in summer and the water is stored in a 15,000 gallon open metal tank on site. A generator can be connected to the motor if a major failure occurs so the system sizing is considered noncritical. The pump stroke is approximately 14 inches at 30 strokes per minute under full sun. A two-pole fused disconnect switch is mounted on the base of the array tracker support post. Summer thundershowers cause much lightning in the area so movistors or varistors are installed from the leads to ground in the disconnect switch box.
Las Cruces, New Mexico
32°N - 106° W - 1670 meters
High Desert Grassland
-5 to 45
30
90 percent (not critical)
7 - Open Metal Tank
Cased 6-inch Borehole
122
9,000 liters/day (2,200 gpd) June through August
The array was mounted on a passive single-axis tracker tilted at 18° to maximize summertime production. The array tracker pedestal foundation was designed to withstand local windloading conditions. The tracker pedestal was located away from the jack pump and wellhead to allow the access needed for pump maintenance. The pump cylinder was 1 7/8 inches and installed in the well on 21/4 inch galvanized steel pipe. This allows the leathers to be changed without pulling the pipe and cylinder from the well. A fiberglass sucker rod was used to connect the jack pump to the pump cylinder. Wires from the linear current booster (LCB) to the motor mounted on the jack pump were enclosed in conduit and buried. The array frame and tracker were grounded to the well casing using No. 6 copper wire. The grounding conductor was run outside the conduit in the same trench as the power leads. Each series string of photovoltaic modules has a blocking diode to prevent damage to a failed string. An array safety switch was installed on the jackpump housing within easy reach of the motor. The site was fenced to prevent animal access to the array or jack pump.
Peak sun hours are estimated from El Paso, TX weather data.
The nominal system voltage is set to that required by the d.c. motor.
Worksheet 2- Design Current and Array Tilt
A summer maximum of 8300 L/day (2000 gal/day) is required. One-axis tracking data is used because the array will be mounted on a passive tracker.
This system has no battery so Worksheet #3 is not required.
Worksheet 2-Pumped Water and Pumping rate
The pumping rate does not exceed source capacity.
A linear current booster is used to improve the match between the PV array and the motor. This unit includes peak power tracking.
No fuses are used because the motor can withstand the maximum array short circuit current.
DC Wire Sizing Specification
The wiring diagram for this system is shown in the following drawing.
The economics analysis for this system is given below.
AC Submersible Pump for Community Water Supply
A village on the island of Antigua requires 5,000 gallons per day of water for domestic use. The available well has an adequate water supply but many maintenance problems have occurred with the gasoline generator being used to drive the ac submersible pump. A direct-coupled pumping system was designed to replace the generator and a new pump with a 1,500 watt constant-voltage three-phase inverter as part of the pumping package was installed. The stainless steel multistage submersible pump system came with a three-phase 1.5-hp induction motor, system switch box, and heavy-duty submersible pump cable. The control electronics remain above ground for easy access.
Bendals, Antigua West Indies
17°N - 61° W - 0 meters
Tropical Island
15 to 35
40
95 percent (critical)
N/A
47
20,000 liters/day (5,000 gpd)
The tropical environment and proximity to the ocean were major considerations in the specification of the hardware. The tracking array support structure was made of corrosion resistant steel with anodized aluminum module frames and stainless steel hardware. Hurricane ties were placed on the tracker frame to stabilize it during tropical storms. The tracker pedestals were designed for storm conditions. The inverter and disconnect switch were mounted in an enclosure on the array tracker support post and strain relief connections and drip loops were used to prevent moisture penetration into the box. The pump was attached to 2-inch galvanized steel pipe and installed in the 6-inch borehole. The power cables were secured to the drop pipe to prevent them from abrading on the casing as the pump was lowered into the well. The submersible pump cable was run in conduit between the junction box at the array and the wellhead. The conduit was sized to accommodate three No. 6 wires with insulation jacket. A sanitary well seal was placed on the well casing to seal the drop pipe and conduit entries and prevent water contamination of the well.
Worksheet 1-Calculate the Water Pumping Load
A year-round average of 8 peak sun hours per day is used for this single-axis tracking array.
The input voltage of 105 is the input voltage recommended by the pump/motor manufacturer. A hybrid system is not feasible for this application.
Converter
The dc to ac inverter is integral with the pump/motor.
AC Wire Sizing Specification
The submersible pump cable was supplied by the pump dealer.
The drawing for this system is given below.
The economic analysis for this system is given below.
Low-Flow Pump for Domestic Water
This small pumping system was installed to provide domestic water for residents in a village in Bolivia. Water was pumped to a set of six 2,000 liter storage tanks located on a hill above the village. The water was then gravity fed to a faucet centrally located in the village. Because the replenishment rate of the source was low, it was decided to incorporate batteries into the system and allow pumping 24 hours a day. The pumping rate was decreased accordingly.
Achacachi, Bolivia
17°N - 68° W - 3903 meters
Alto Plano
-12 to 30
75
Cased 6-inch borehole
8000 liters/day (2000 gpd)
A fixed array was specified and ground mounted on concrete piers near the wellhead. The batteries were placed in a fiberglass box and buried near the well. A disconnect switch for the array was installed in a weatherproof enclosure on the back of the array support structure. The conductors between the array and the battery and wellhead were enclosed in conduit. The array was grounded to the well casing and the bare copper ground cable was buried in the same trench as the conduit. A sanitary well seal was used to cover the top of the well casing. The power conductors from the battery to the pump entered the well casing through a conduit connector attached to the well seal. The pump conductors were securely fastened to the drop pipe as it was lowered into the well. A safety rope was attached to the pump to prevent its loss if it became disengaged from the drop pipe during or after installation. The drop pipe was secured with an adaptor that allows the water to exit the well casing below ground level. This helps to protect the water delivery pipes against freezing.
The water source will not supply the required water unless the pump is operated over 15 hours per day. Batteries are used to extend the pumping time.
June is the least insolation desing month in the Southern Hemisphere.
Worksheet 3-Calculate system Battery Size
The modules are only derated 5% because the high altitude site and cool weather are ideal for PV module operation.
The storage tanks have a level switch to shut off the pump if the tanks are full.
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