Dankoff Solar Pumps manufactures several types of low-power, high-performance pumps for use in off-grid DC systems.
The Slowpump can push water up a hill as high as 450 vertical feet. It runs on very low power, with or without batteries, to supply between 200 and 2,600 gallons per day. The rotary vane pump mechanism is housed in forged brass for durability. Because of tight tolerances, the water must be very clean, so fine filtration is required to protect the pump (5 micron filtration is recommended).
The Slowpump chart shows the gallons per minute (GPM) output from the pump and power (Watts) consumed by the pump for various vertical lifts (this is the "total head" or the lift from the water level at the source to the top of the storage tank, in actual elevation change, not pipe distance).
The pump performance in the chart is measured at array-direct voltages. The peak power voltage of a nominal 12 VDC PV module is about 17.5 VDC. A battery-powered pump will have 10-20% lower output and power draw because the battery is operating at a lower voltage. Flow rates are in GPM. To estimate gallons per day delivered, multiply the GPM by 60 (minutes) to get the gallons per hour (GPH) amount. Then multiply by the peak-sun-hours per day at the site location (this data is available from NREL's PVWATTS Solar Calculator) to get the gallons per day (GPD).
For PV-direct operation (without batteries), the PV array nameplate output must exceed the required pump wattage by at least 20%. Over-sizing the array will increase the amount of water delivered per day because a higher wattage array will produce the pump's required wattage in less light, allowing the pump to run at full speed for more hours each day, or in cloudy weather. For example, a 100 W module will produce 50 W in one half the amount of sunlight present at noon. The extra available power will not damage the pump as long as the array voltage is within the voltage range of the pump motor.
For array-direct operation, a linear current booster (LCB) with the required output amperage must be used (see LCBs listed at the end of Water Pumping.) To find the amperage needed, divide the motor voltage by the required pump wattage. AC models use a low-surge permanent magnet motor that greatly reduces starting surge, relieving stress on inverters and generators.
Slowpumps have a suction lift of up to 20' at sea level. Subtract 1' for each 1,000' above sea level. These pumps cannot run dry without damage, so the optional dry-run switch is recommended. Wearing parts are replaceable and typically last 5-10 years, and overall life expectancy is 15-20 years.
Slowpumps are NSF approved for pumping potable water. Slowpumps can also be used to pump saltwater. Dimensions: 5.7"W x 15.5"L. Weight 16 lbs. 1-year warranty.
¼ HP Slowpumps (model series 1300 and 2500) are available with 12 or 24 VDC motors, for use with 12 VDC and 24 VDC array-direct or battery systems. These are also available in 120 VAC versions for inverter, generator, or grid connection. ½" female pipe fittings.
½ HP Slowpumps (model series 1400 and 2600) are available with 24 or 48 VDC motors, for use with 24 or 48 VDC battery systems, or PV direct with a 36 VDC nominal array. Also available in 120 VAC versions for inverter, generator, or grid connection. ¾" male pipe fittings.