Using a Dankoff Solar Powered Water Pump – Part 4, by Tunnel Rabbit

Water pressure per foot of head, in the Dankoff chart indicates 60 PSI static pressure at 140 feet.  When water is pumped, if my gauge is accurate, 60 PSI was reached at about 100 feet. Note that 60psi is the maximum pressure rating of most 1/2″ drip irrigation that is the least expensive black poly pipe. As a quick reference when designing a system, download the PDF of this chart.  To save money, you can use inexpensive black poly pipe rated for 100 or 160 PSI for lifts above 120 feet. To take full advantage of the Dankoff Model 1308’s ability to lift water to a maximum of 400 feet (@173 PSI), Pex, or similarly-rated high-pressure PVC should be used. However, we should not attempt lifts greater than ‘low lift’ without a pump controller, or without using small PV system as a power supply.

When the linear current booster pump controller is used pump production can be increased by up to 40%, or conversely, less wattage can be used.  It functions in great part in the same way a MPPT charge controller converts voltage to current to perform work, in this case, the pumping of water.  Conversely, it can be said that less PV power is required to pump the same amount of water.  My pump controller was purchased in 2008, and is no longer available on the market. Hopefully, it’s replacement will be similar, or better.  It allows the use of 24 volt PV panel arrays to power a 12 VDC pump motor. This is a significant advantage as less expense smaller gauge wire can be used, or one can double the length of the cable run from a tree-shaded water source out to a clearing that offers full sun exposure throughout the day.

With a step down in voltage from 24VDC to 12VDC, there is also a greater differential of one order of magnitude increase in potential relative to a nominal 12VDC array, an improved advantage and ability to produce amperage in low light conditions available early in the morning and nearing dusk. Ask Thad at humboltsolarwaterpumps.com if he can suggest an alternate pump controller.  There is no limit to how much PV wattage can feed a pump controller. A large array can be spit into three sections, one oriented to the east, one to the south, and one to the west, and the pump will produce twice or more the gallons per day that most charts indicate.

To leverage the use of a pump controller to the fullest, that is, to maximize array-direct production without batteries, supply the pump controller with 300 or more watts, and it will start the pump in very low light, and keep it running at a fast rate, even when there is heavy cloud cover, and keep it turning in the waning hours of the day.

There are numerous advantages to using a pump controller.  It not only safely manages the pump’s operation by limiting voltage, but it also is necessary for the function of two key safety devices. These are 1.) the temperature cut off switch, and 2.) float switches that can be installed in the water source, and at the tank being filled.

Without a pump controller currently available, this method is most foolproof, and easiest to employ if we wish to extend the life of the pump. However, the system is limited by the life of its batteries, but even if the batteries are too weak to run it all night, given ample PV wattage, it could still be run during the daytime.

One advantage is the lower supplied voltage of around 12 to 14.5VDC to the pump motor.  It would receive voltage as the charge controller delivers it to a battery bank that should be no more than 14.5 VDC, and at night the voltage supplied by the batteries would be between 12.0 to 12.6 VDC. Relative to Array-Direct, or the voltages the Pump Controler uses, these low voltages will produce less water. There would be a reduction of about 20 percent. However, operation can continue throughout the night. And we can safely operate the pump to its maximum rated lifts in elevation.

Minimum system requirement for 24/7 low lift operation would be around 500 watts charging a 220 Amp-Hour battery bank with a 25 Amp charge controller. I would recommend a high-quality charge controller such as the Morningstar Sun Saver Duo that can charge two battery banks separately. This unusual feature allows two dissimilar batteries to be charged at a 50% rate each, or 90% and 10%, or one battery bank to be charged 100 percent of the time.  When batteries are in short supply, this feature could be used to charge spare lead-acid batteries.  This can supply enough power in low lift conditions to operate the pump 24 hours per day for the life of the batteries. Typically this is three years, and as battery capacity diminishes, during the daytime only.

If the Dankoff Model 1303 is used, and powered by such a solar system, then the daily output could be as much as 2,300 gallons per day with a lift of no more than 60 feet.  If the lift needs to be up to 120 feet, then use the Model #1308 pump, but the output will be about half that of the Model #1303, or about 1,150 GPD (Gallons Per Day) in a 24 hour period.

With the voltages supplied by a PV system, maximum output would be no more than 1 GPM for the Model #1308, and 2 GPM for the Model #1303, less than is produced via the other power source options.

For example, given this level of output, and assuming that 1 acre of land needs 300 gallons of water per day to be adequately irrigated to grow potatoes, then we can irrigate up to 7.6 acres when the sun is shining brightly when the Dankoff Model 1303 is used, or 3.8 arces when the Model 1308 is used. This assumes very hot and dry weather and when crops need to be irrigated daily. Of course, the crop might only need to be irrigated every other day, or less, and the acreage can be doubled. Water storage commensurate with the acreage could supply and lower the minimum required daily production, during anticipated hot and dry conditions. But it is better to have the added capacity as large water storage containers would cost no less than $1 per gallon, if these can be found used.

These figures are only an estimate. If the lift will be 60 feet or less, the Model #1303 is twice as productive, and may not need to be run near as long as the Model #1308 to meet the demand.  The #1303 will pump to 250 feet, but the amperage would be closer to 16 amps, and could only be operated during the day without an increase in the size of the PV system used.  To extend the service life of this 500 watt system, double the battery bank to a minimum of 440Ah capacity for 25 percent depth of discharge, or best yet, triple it to 660Ah to reduce the depth of discharge closer to 10 percent per day. A depth of discharge less than 10 percent does not extend the life of flooded lead-acid batteries. With a 660Ah battery bank, it is possible to keep the pump in operation 24/7 for perhaps as long as years, and longer if operated only during the day when there is ample sunshine to run the pump with discharging the batteries only slightly, if at all.

If the demand is small, only few hundred gallons per day, only a small PV system is needed.  It could use a single standard automotive starting battery and only one 100 watt panel and a 6 amp charge controller. This would be a dedicated system designed to operate the pump only for a few hours, preferable in the morning before noon, then shut down to allow the PV panel to fully charge the battery before nightfall.

For 24 hour operation at maximum lifts for either pump, the amperage required will be closer 16Ah at 12VDC, or 8 Amp-Hours at 24VDC. The example assumes a 12VDC system, and would be comprised of 2,000 watts of PV panels charging 880Ah of lead acid batteries.  Service life would be limited by the batteries that might provide 24/7 operation for 3 to 4 years. Doubling or tripling the battery bank capacity, as in the prior example, would yield similarly longer service.

If for example the pump will only be operated during one or two growing seasons per year, less battery capacity is needed for operation beyond 3 to 4 years.  An additional benefit of an investment in these systems is that they need not be dedicated, but only used during the growing season, and could be utilized to supplement a primary PV system that is operated during the darker, shorter winter daylight hours.

Having spare pump heads, and all other “wear” parts is a good investment for a long-term worst-case scenario. To avoid the high prices of dealers, and to receive better service, I recommend purchasing the pump and replacement brushes directly from Dankoff Solar Pumps. Contact: kenny@dankoffsolarpumps.com.  A new replacement pump head can run as high as $350 from most dealers. Therefore I recommended purchasing them directly from Procon for less than half the retail price. If the model pump head you need is found on sale, these heads can be purchased for as little as $84. I d the sale-pricing opportunities to assemble my own spare slow pump for less than $150.  It works well, but because of the lower quality motor, it is best used in a light-duty low head only application, such as a transfer pump.

The pump head is made from brass and can wear out prematurely for several reasons. The biggest cause of premature failure is due to excessive sediment in the water.  These pumps require a 10 micron filter to remove most of the sediment. Do not use a 5 micron filter as it can become plugged up quickly and cavitation could result. The pump will make a buzzing noise if cavitation (air bubbles) occurs when the intake is blocked.

A pump head can also be damaged if the water level is below the pickup, and the pump is run dry, then overheats, and is damaged. Because we might not use a pump controller, the optional temperature protection switch can not be used. Or, if the intake line develops a leak, it can also be damaged by cavitation that is created by air bubbles in the line. Stock two 10 micron filters per season if the water source is not dirty, or more filters if it is. The length of the growing season should determine how many spare pump heads to have on hand. In addition to the pump head that came on the pump, 2 to 3 spares is a good minimum. If the growing season is year-round, then keeping 4 to 6 spare pump heads would be good.

Brushes are easily installed in these motors, and are a “wear item” that should be replaced every 5 to 10 years. The cost is not excessive if purchased directly from Dankoff Solar Pumps. Therefore it would be best to have 2 or 3 sets of replacement brushes. To wit, these run around $30 per set. Some vendors have attempted to charge outrageous shipping charges, quoted exorbitant prices, or provided very poor service. Therefore, I would stick to the sources recommended.

(To be concluded tomorrow, in Part 5.)