Stan was faced with a difficult garden watering problem in which he had to get water from a spring to a greenhouse that was located several hundred feet away and 50 ft uphill from the spring. The flow requirement was relatively small at about 3 gpm for for an hour and half a day about 3 times a week.
Using a conventional AC powered pump would have meant running a lot of wire, and the 800ft distance would have meant a larger than normal wire gage to keep the voltage drop from being excessive.
Using one of the submersible well pumps that are made to run directly from solar PV panels is a nice solution, but the pumps are expensive and they require quite a bit of PV panel area to drive. So, this would have been an expensive solution for the relatively low flow required.
Stan's solution was to use a relatively inexpensive 12 VDC Shurflo pump that is intended for spraying and RV applications. The pump draws about 8 amps, so, to drive it directly with PV panels would have required at least 100 watts of PV array, and perhaps a linear current booster for startup. Instead of direct PV drive, Stan incorporates a deep cycle 12 volt battery to drive the pump, and then uses a small (30 watt) PV panel to charge the battery over the course of the day. This works well because the run time for the pump is not very long and can be handled by the battery, and the PV panel has all day to recharge the battery.
PV panel, pump and battery located at the spring.
Pump and battery with weather cover off.
Lets go through the sizing for Stan's situation -- you can adjust the numbers for your situation as needed.
Flow: About 1 hour every other day at about 2.7 gpm (about 160 gallons per pumping session).
Pump draws about 8 amps when running.
Pv panel is a 30 watt 12 VDC panel.
Pump is 800 ft from the greenhouse -- connected with 3/4 inch black poly pipe.
Energy use: effective run time is half an hour a day with a draw of 8 amps from the 12 VDC battery.
This is (8 amp)(0.5 hrs) = 4 amp-hrs a day
In watt-hours, this is (4 amp-hrs)(12 Volts) = 48 watt-hours per day.
Is the battery size OK?
If the battery discharge is limited to 20% of its full capacity for long life, then the allowable discharge is (115 amp-hrs)(12 volts)(0.2 discharge) = 276 watt-hours per day.
Since the actual energy use on an average day is less than 1/5th of this, the battery should have plenty of margin for cloudy days or occasional longer pumping periods.
Is the PV panel size OK?
On a sunny day, the 30 watt PV panel should be able to provide about (6 hours)(30 watts) = 180 watt-hours a day.
The optimistic ratings on PV panels, battery charging efficiency of about 80%, and limits of simple charge controllers will result in less than this, and cloudy days must be considered.
But, we only need about 48 watt-hours on a typical day, and the PV should be able to provide much more than this even allowing for charging efficiency and some cloudy days, so it seems fine.
Does the pump have sufficient pressure capability?
The pump has to overcome 1) 50 ft of vertical rise, 2) pipe friction in 800 ft of 3/4 inch pipe, and 3) provide enough pressure at the greenhouse end to make a soaker hose work.
The desired flow rate is at least 2.7 gpm.
50 ft of vertical rise is (50 ft)(0.433 psi/ft) = 22 psi
800 ft of 3/4 inch black poly pipe with 2.7 gpm has a pipe friction pressure drop of 0.81 psi per hundred ft, or 7 psi for 800 ft.
So, if the pumps maximum pressure is 45 psi, then there will be (45 - 7 - 22) = 16 psi to drive the water through the soaker hose. Stan has found this to be fine.
This kind of pump has a high pressure switch that shuts the pump off when the pressure the pump sees goes above 45 psi. So, if the pump is seeing quite a bit of resistance, it will cycle off and on as it reaches its high pressure limit. The pump is designed for this, but it will have longer life if the load can be adjusted so that the pump stays on. Stan found that with one soaker hose, the pump would cycle on for 8 seconds and then turn off for a couple seconds and so on. By adding a 2nd soaker hose, the pump stays on continuously.
The costs of the main components are:
30 watt, 12 volt PV panel $70
Shurflo 12 VDC pump $90
115 amp-hr deep cycle battery $80
Charge controller $40
Total for main components $280
Does not include pipe, a little wire, fuse, etc.
Update June 15, 2013 from Stan:
The system is working great. It produces about an hours worth of pumping each day and is maintaining the battery at full power almost always. It pumps the amount of water an hour that was expected with good pressure even at the hoop. The soaker hose didn't work. It got "clogged" from the unfiltered water (the water is crystal clear spring water) . So I added drip nozzles at each plant. This seems to be working ok for now. When I'm outside You can see the pressure drop, then the pump is activated and the pressure returns till you shut it off.
Thanks to Stan for providing this information -- there is more on this on this thread on the Homesteading Today Alternative Energy Forum... A very nice forum for all kinds of Alternative Energy issues.
For more information on solar pumping systems...
Gary April 26, 2013