Originally Posted by stanb999
Well after proposing it years ago and getting good direction. I have finally installed my solar water pump in my spring to irrigate my hoop house. The hoop house is 50 ft above the spring and 800 ft away and needs about 25 PSI to operate properly. These two factors made the build rather difficult due to the relatively high pressure needed at the source.
I have a
30 watt panel
a 115 Amp hour deep cycle with cheap charge controller.
A 12 volt water pump.
This setup should more than cover my needs for my drip irrigation system. The math says I need about 2 hours of pumping a week, really 3 times a week turning off and on for 1 hour and 20 mins running about half the time. The pump uses 9.8 amps at max pressure. Running amps. will be somewhat less than this closer to 8 amps as an average. Because it won't always be at max pressure.
Here is a quick break down.
30 watts an hour 3.5 hours a day. or +/- 100 watts a day.
7 days a week for 700 watts potential.
I plan on using about 240 watts a week approx.
The water pump wont run in the colder low light months, due to freezing conditions.
Does this math sound conservative enough? The math for water usage would be well off the charts for inches of precipitation and would only be that high in the hottest sunniest months. Will there be enough "left over" power to water the garden the same amount? Again this would be only during the sunniest time of the year?
If the pump draws 8 amps and its a 12 volt pump, then it will be using (8 amps)(12 volts) = 96 watts when running.
2 hours run time a week is about 0.3 hours a day.
This would be (8 amp)(0.3 hours) = 2.4 amp-hrs each day
Which would be (12 volts)(2.4 amp-hrs) = 29 watt-hours a day.
The watt-hour rating on your battery if you limit yourself to 20% discharge would be (115 amp-hrs)(12 volts)(0.2) = 276 watt-hours. So, to me, your battery seems large enough for this job with a good margin.
A 30 watt panel with (say) 6 hours of sunlight should be able to provide about
(30 watts)(6 hours) = 180 watt-hours a day.
Some of that gets lost because the PV ratings are optimistic and the battery charging is only about 80% efficient.
But, you only need about 30 watt-hours a day, and the panel should do over 100 watt-hours a day even allowing a lot of inefficiencies. So, the panel seems large enough to me with a good margin.
Watt-hours are an amount of energy (as in your battery stores 276 watt-hours). Watts are a rate of using energy (as in your pump draws 96 watts).
Watts per day does not make any sense -- watts are already a rate.
Its been suggested that the (30 Watts)/(12 volts) = 2.5 amp charging current is too low, and that this will result in low battery life. I was not able to find anything out there on this potential problem?
If you run a total of 800 ft of 3/4 pipe at 2.7 gpm, that's 0.81 psi/100ft or about 7 psi for 800 ft.
Add that to 50 ft of vertical head or 22 psi.
Add that to 25 psi of working pressure for the system.
This all totals to 54 psi. Sounds like that is a little about your pumps maximum rating, so it will settle out at a lower flow, depending on the pump curve for that pump.
Your pump may be running near maximum current all the time, but it seems like you have enough margin to cover that.
I think it would be good to hook the pump up and check and see if it can actually do the job. Do you have the specs for the pump?