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Posts: 1
Reply with quote  #1 

Hi.  I'm new to solar want to design a system to keep water moving between some water tanks and two wicking raised beds using a solar powered pump, and would like some feedback from an experienced person.  The idea is that water flows out from the tanks into the raised wick beds, and from their flows into an underground cistern, like a drain.  Once the cistern (65 gallons) fills up, it triggers a float switch that turn on a pump that pumps the water back into the tanks.  I can only guess the rate at which the cistern would fill, but I would think it would be about once every hour.  The pump is a 12 volt 9.5 amp transfer pump that pumps about 320 gals per hour.  So it would take about 20 minutes to pump all the water from the cistern back in the tank where it started from.  I would like to have this thing running whenever it's sunny, but also to be able to pump out water using the charged batteries whenever there's a rainstorm.  I am thinking about buying:

1.  An Expert power, Sealed lead-acide battery 12v, 12ah

2.  A Renology, 50Watt Panel and charge controller

My question is, will the battery charge fast enough (in full sunlight) to maintain a rate of pumping 20 minutes every hour?  Any other feedback about this design is appreciated.


pump system.jpg 


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Posts: 3,004
Reply with quote  #2 
Hi Marco,

Welcome to the forum, I was hoping one of the greenhouse guys would come on but since they haven't...

1 It might, but I think you're going to need a bigger battery and probably a bigger solar panel. You haven't bothered to tell us where you are (IT MATTERS) but unless you live in the desert there is always the chance of several days of crappy weather, and if so that toy battery won't cut it. You'll get flooded.

2 most float switches only have an operating range of a few inches, so it's likely that the pump will come on much more frequently but will run for a shorter period of time. Not that it matters, the total water pumped will be the same.

3 why is the pump located at the highest point in the system? Generally you'd want it at or near the LOWEST point for more efficient pumping.

4 how are you going to control the water flow from the tanks to the plant beds? The "totes" shown hold about 275 gallons EACH. If the pump fails for any reason (clogged, dead battery) the water will overflow the cistern.

Nice idea though, good luck with it!

Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay

Posts: 3
Reply with quote  #3 
The proper approach to determining battery and panel sizes is first to estimate how much water (@ 62.4 lbs/cubic ft) get lifted how high every day, where how high the water gets lifted will require some estimating on your part because it depends on the water level in the cistern (which varies).  That'll give you the foot-lbs of work the pump has to do every day, and from that, once you know how many watt-hrs you can get per sq ft of panel in your location, you can determine how much solar panel you'll need.  Calculating battery size is trickier, but basically involves providing enough watt-hr capacity to even out the (any) mismatch between power coming in from the panel(s) and power going to the pump.  If you've got enough panel area to keep things going all day (including on cloudy days) easiest will be to start with whatever battery you've got and add more in parallel if you find it runs out.  And don't forget you need a charge controller between the panels and the battery or you'll ruin your battery in short order.

I agree with Willie your pump should be below the totes rather than above, and to save energy (reduce panel size req'd) you should put a check valve in the pump outlet (to prevent runback) and have it discharge into the bottom of one of the bottom totes (to reduce the distance the water has to be lifted).  
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