Using a Dankoff Solar Powered Water Pump – Part 2, by Tunnel Rabbit

There are clear advantages with the addition of electronics, and a battery-based pump system.  However reliable they may, or may not be, both of these options can fail at a future date.  As it is, if there are not the microchips to produce it today, so I would not expect it to be available during or after TEOTWAWKI. If we know how to work around a complex device normally used to run solar pumps, then we can also avoid the expense of either the pump controller or perhaps even a PV system altogether. Simplicity is better than complexity in any plan.

Before jumping into the power supply options, the mechanical and electrical requirements and design limits of the Dankoff slow pump should be understood.  Thereby we can better appreciate the Pros and Cons of each option, and make more wise decisions as to how the pump should be deployed, and powered in various situations.  With such examination, we will also come to appreciate the risks, and take measures to avoid damaging a critical asset.

If the Dankoff slow pump is driven faster than 1,725 Revolutions Per Minute (RPM), the pump head can be damaged. Above 1,725 RPM, the pump does not produce significantly more water, as would normally be correlated with voltage, current, and head. At that RPM, we are at the point of diminishing mechanical return, and increased wear of tight tolerance parts occurs. The pump head is the heart of the system and is vulnerable, so it is a wise investment of time to understand it, and to have a spare pump head or two, especially if one wishes to operate the system for period of 15 to 20 years.

The motor used in the Dankoff slow pump is an over-engineered vintage design,  and therefore is robust, and time-tested. However, every machine has its limits.  A nominal 12 volt, or 24 volt motor can be damaged by excessive voltage over time. The 12-volt motor will last longer if not exposed to voltages greater than 16VDC, and the 24-volt motor would last longer run at voltage less than 32 VDC.  The motor will tend to last longer if the maximum voltage used is closer to 12VDC than 16VDC, and the 24-volt model will last longer if run closer to 24 VDC, than nearer to 32VDC. In addition, at voltages greater than 16VDC (or 32VDC if it is a nominal 24-volt motor), the pump head risks being driven at an excessive speed, greater than it was designed for, namely, 1,725rpm.  The motor is well designed to work with the pump head design, to produce the maximum amount of water with the least amount of electrical power, and to run at very slow speeds and low voltages without damage.

The 24-volt motor has an advantage in that it could be run directly off a nominal 12 volt PV array when the maximum amperage is produced, and when the voltage is typically around 17 to 18 volts.  A 24-volt motor could be run directly off a nominal 12-volt array without regulation.  This is a significant advantage for long-term operation. However, its production will be less than half of that of a finely regulated 12-volt motor operating within safe limits. If we believe that additional water production is necessary and that we have mastered the principles and techniques needed to regulate a 12-volt motor to run safely within limitations on a 12-volt array, then we have the best of both worlds. However, to create a 24-volt panel array to achieve the same high output of the 12-volt pump, we would need twice as many panels and with that, we would also have to double the gauge of the cabling. And if we wish to run the pump using 12-volt alternators or generators, then the output of the 24-volt pump would half as much.

If we choose the 24-volt motor to be powered by a 24-volt array, we can save money by using a smaller gauge power cable, or we can make a run that is twice as long as if we used a 12-volt motor.  If our system design can avoid the expense of heavier cables, we might then be able to afford to purchase an additional 200 watts, or more, to either extend the number of hours that pump will operate per day, or to double the output given a limit insolation window, where trees might shorten the number of hours of full sunlight available to the array at a particular location. For your consideration, one might use a 24 volt 1303 pump directly powered by 12-volt array.  It would produce a volume similar to a Model 1308-12, and would not be subject to the over-voltage issues. This could be a consideration if the #1308 would meet your gallons per day requirement.

Regardless of our choice of motors, even the 48-volt motor, the power supply options and methods of limiting input voltage in Array Direct configurations apply. In the discussion will be used the lowest common denominator that is the 12-volt motor.

With the Dankoff production chart at hand, we can see that the Dankoff Model #1303 pump is more desirable in terms of its production given a known and limited height.  Where the #1308 would be advantageous is in a situation where future circumstances and unknown production sites are undetermined.  The #1308 is most desirable if it would be used in a variety of locations that require a lift that would exceed the #1303’s ability, if only a Array-Direct configuration used. At head elevations where the #1303 could pump to upwards of 250 feet, or where the #1308 would be required to pump to it’s maximum 400 feet, a pump controller, or a battery-based PV system would be needed.

As someone with minimal financial resources, my first acquisition was the #1308, as required wattage to operate it was also minimal and half that than is necessary the #1303 while providing more than adequate amounts of water for several families. There are other pump heads available that produce less than the #1308, the #1322, and then there is the #1304 that is in between the #1308 and #1303.  For practical purposes, the choices were limited as I could not afford all of these pumps, and had to choose the best two for the widest performance for most likely locations (lift heights) that might be encountered.  If choosing only one for myself, then it would be the Dankoff Model #1308.

The array-direct method of power supply is technically challenging for most, yet it is the most reliable method for the long term, if the technical challenges can be met.  Regardless, it is best to understand the challenge and recognize our ability to meet it, or not. Knowing this, we might use another pump or a different method of supplying power to this pump.  At the very least, if it is easier to use the other power supply options until a time when they can no longer be supported  This is to say, that if all else fails, we can resort to the Array-Direct method.

In low-lift situations, as I define it, that is less than 120 feet for the Model 1308, and 60 feet for the Model 1303, these Dankoff Slow Pumps can irrigate land adjacent to a surface water source, a cistern, pond, creek, or any water source that is no more than the specified lift, or slightly higher in elevation, with ease.  In low-lift situations, the pump can be used without a pump controller that includes a linear current booster (LCB). The technical reasons are many, but if the lift is higher than the specified, that is defined here as a ‘low lift’ situation, a pump controller (LCB 16 amp) is needed to safely supply enough current, yet limit the voltage so that the motor is not damaged and does not turn the pump head at a rate faster than it is designed for, or we must use a smaller PV system.

Because we may operate the pump directly off a PV array (array-direct), a measurement in gallons per minute is necessary. To realize or exceed the GPD, two 100 watt PV panels, one facing the East, as opposed to a second panel at for example, a 30 to 45 degree difference in azimuth, orientated more toward the West, will capture the most insolation (solar power) throughout the day and maximize production for a particular location with the least number of cost-efficient and common 100 watt 12VDC PV panels. During overcast conditions, both panels can be faced toward the South, or in the direction of the sun during the course of a day, to produce the necessary amperage.  To accomplish this goal, one of the techniques should be used to limit the voltage applied to the motor.

Using the Dankoff output chart, if the pump is producing only slightly more than it is rated at a given lift, then the pump head will have a shorter life span.  Either pump head Model 1308 or Model 1303 can handle low lifts without an expensive pump controller if it draws no more than 2-8 amps at less than 16 VDC. The best way to limit the voltage, and pump output is to provide only enough watts directly to the pump.  For example, if the Model 1308 pump is rated for 1.25 gpm with a lift of no more than 20 feet, then the minimum watts of a PV panel that should be connected to the pump should between 40 to 50 watts.

The Dankoff Model 1308 can produce 380 gallons on average 5-hour per day solar day of most latitudes, using power from only 40 watts (2.2 amps @ 12VDC) given a 20-foot head, and the model 1303 can produce on average, 750 gallons per day with only 65 watts (3.5 amps at 12VDC), also given a 20 foot lift, if a pump controller is used.  However, if powered array-direct, roughly 30 percent more wattage will be needed. For example, if the chart indicates that 40 watts is needed to lift water 20 feet at the maximum output, the rated wattage of a PV panel should around 52 watts in ideal solar conditions.

(To be continued tomorrow, in Part 3.)