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solar power water pump

 
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I would like to water my lawn using zero energy and water from the grid. For this purpose I've set a rain water harvesting system, which can capture sufficient water as long as there is rain. I now want to run my water pump using a solar charged battery. However I do not know what parameters should I have for my solar system.

I have a spare car battery, which is 12V and it says it can store 60Ah's of energy. I need to buy an inverter and a solar panel. Probably I need to buy a charge regulator as well.

My water pump is rated as 700W.

My questions are:
- 60Ah driven by 12V equates to 720Wh. Does it mean that I can run my pump for 1 hour using this single battery?
- What should be the ratings of my inverter, charge regulator, and the solar panel(s)?

Help is most appreciated as this will be my first solar project installation.
 
pollinator
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You would be better off not getting the inverter and purchasing a 12 volt pump. A single lead acid battery could not run that pump more then a few minutes at a time due to it's high draw. A RV type diaphragm pump would be perfect for the task...
Cheers,   David
 
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Ahmet,

I am going to have to agree with David.  But first I need some basic specs.  Primarily, how much land are you planning on watering?  This going to determine how much water you will need.  How exactly are you watering?  By a little center pivot sprinklers?  The reason I ask is that you need a minimum pressure and volume of water to make those work.

Water is surprisingly difficult to pump.  It involves moving a large mass of water typically up a substantial height.  And one still wants pressure after pumping up to ground level.  

Ahmet, I am not saying that you cannot do this, but I don’t think that singular car battery is going to work.  I would imagine needing a bank of probably 100 ah batteries.  Actually they would likely need to be SLA deep cycle batteries.  Deep cycle batteries give up their charge to almost zero without losing much voltage.  A typical car battery strangely won’t give up its full charge and is not really conducive to running an application for a long period of time.  They are designed to provide a high amperage output for a short period of time.  This is perfect for starting a car (maybe 3-5 seconds of high output run time after which it will recharge).  

It is possible to witness this effect when trying to start a stubborn engine.  At first the battery and starter enthusiastically surge forward for 10-15 seconds.  After that you can probably hear the battery getting tired and the starter no longer has the enthusiasm it did at first.  Keep going for say 30-60 seconds and the starter sounds downright anemic.  All that energy was given up in the first 15 seconds.

Further complicating matters, how long does the irrigation need to run?  20 minutes?  An hour?  This requires a substantial battery reserve.  I just checked the other day and a 125 ah battery costs about $225 on Amazon.  I agree with David that a 12v pump is best in order to avoid power losses through an inverter.  The solar panels and charge controllers add substantially to the ever-growing price tag.

Ahmet, again, I am not trying to discourage you.  If this is something you want to do, then by all means do it.  I am starting to price out a home made “battery generator” which is really a device with a battery that outputs to USB, 12v, and 120v and maybe another spec if I can find one.  The challenges in building these are very similar.  I want to build mine for emergencies.  I do wish you luck and really, if there is anything I can do to help, please don’t hesitate to ask.

Eric
 
pollinator
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You have the theory of runtime calculation correct: Amp-hours*Volts=Watt-hours.

But there are gotchas lurking.

1) Lead acid is not going to last long if discharged below 1/2 capacity.

2) Listed amp-hours is going to be based on a relatively low current. Drawing a higher current will result in disproportionate shortening of runtime. Peukert's law, if you wish to know more.

3) Losses to wiring/fuse resistance and inverter efficiency if applicable will eat a bit more.

With a battery that small the impact will be quite significant; as previous replies have suggested, you're almost certainly going to need more battery and/or less pump.
 
Ahmet Oguz Akyuz
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Thanks so much for these detailed replies. Very informative.

Ok so I understand that my old car battery is not up to the task. I've made a search and found a battery known as gel battery. I think it is a type of deep-cycle battery. These are expensive but assuming that I decide to pay for it, how many such batteries would I need? 12V DC pumps that give high pressure are also expensive so I think I want to go with the inverter solution to use my existing pump. This way I can also use it for powering other appliances if need be.

You have asked about my garden setup. Basically it is a 1000 sq-feet grass lawn. If I water it for 15 minutes daily it usually stays green most of the time

Thanks for the help.
 
Eric Hanson
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Ahmet,

Thanks for giving us the specs.  I think 1000 sqft is a reasonable amount of land to water,  I still have to ask though, how do you plan to irrigate?  Little center pivot sprinklers are the easiest and least expensive option, but they require a healthy pressure and are terribly wasteful of water.  On the other hand, drip irrigation requires less pressure (but make no mistake—they do need a minimum pressure to work) and they are very very efficient in terms of water useage.  I have used drip line irrigation before and it is quite nice,  I simply lay a drip line where I want the water and then I bury with soil, woodchips, straw—basically anything to keep sunlight off the tube to make it last longer (as in 5+years.  Mine are over a decade old and work just fine).  Center pivot can, depending on circumstances, waste up to 50% of the water to evaporation (much while airborne!) and wind drift.  Drip irrigation wastes nothing.  Not one single drop.  I think by now you know my opinion.

Back to the electrical.  So now we have some useful information but we need some more.  You have a 700 watt pump.  That gives us a basic spec.  Is the pump 120v ac electric?  If so then we need some equipment to convert your 12v dc supply to 120v ac.  Each of those changes will cost you around 10%, so figure at least 20% reduction from your battery supply.  BTW, your choice of an gel battery is an excellent choice.  Those batteries are durable, capable of deep discharge and are generally extremely reliable—everything that a car battery is not.  If we are talking about how many batteries you will need, I would personally recommend starting out with two, wired in parallel.  Again, assuming we are talking about 100ah batteries, this should give you an hour with time/charge to spare.

Another seemingly random question, where are you located?  What is your climate?  This is pertinent to setting up solar panels.  Your latitude determines the most advantageous angle to set your solar panels.  Your climate, specifically heat and really especially your humidity play a role in how much water that you need.  I should also add here that your soil level is equally important.  Clay soils, once soaked, will hold on to water quite well.  Sand on the other hand dries out only slightly less than spraying water into the air.

Back in the day when I really used drip lines, I used a pressure compensating 1/2 inch (12mm) line with water emitters spaced 1’ (30 cm) apart and they used 1/2 gallon per hour.  This worked well for me as I have clay soil.  I would typically run the drip line for 1 hour and my clay soil held on well.  If you go the drip line route, you will have to decide how much drip line you need.  If we assume that 1/2 of your garden is walkways that don’t need water, then we have 500 sqft.  Using the tubings I mentioned above, a 100’ line will water about 100 sqft pretty well, but soil, temperature and humidity all play important roles.  But if we go with this figure, then to water 500 sqft, you will need 500 ft of drip line.  BTW, I am referencing pressure compensating drip line, meaning that ALL the emitters release the exact same amount of water regardless of changes in height, and the first and last emitters are likewise equal.

But again, going back to the above example (you adjust as necessary), 500 sqft could be 5 100 ft long lines.  That is 500 emitters each yielding 1/2 gallon/hour or a total of 250 gallons per hour.  I don’t know the output of your pump, but I bet it can handle this load.

Using this thought experiment, we can then begin to make some basic calculations and estimate your basic electrical loads.  Assuming you use the current pump,  a 700 watt pump running for 1 hour will need 700 watt hours of electricity.  A single 100ah battery, assuming a 20% loss for changing ac to dc and 12v to 120v yields up 800 watt hours which gives you just over an hour of run time.  But I would still go with 2 SLA/gel batteries as even deep cycle batteries will lose some charge as they get deep into their cycle.  Further, 1 hr run time might not be enough and I assume that you want some flexibility.  Also, 2 batteries are a pretty simple setup.

So there you have the battery electrical side of things.  I don’t know what exactly you have in mind for solar panels but this at least gives you a start.  

BTW, I used drip line from a company called dripworks.com.  They have every conceivable type of tube, connection, timer or accessory that you could possibly want.  There are other companies and they might be great too, but I only have 1st hand experience with dripworks and I can highly recommend their products.  They are very high quality at very reasonable prices.

Ahmet, this has been a long post and I hope this is helpful.  If you let me know about your latitude, climate, soil, etc we can further refine this project.

Let me know if this helps,

Eric
 
Eric Hanson
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Ahmet,

I really want to smack my forehead with the palm of my hand right now.  I wrote my entire response assuming you were watering a garden and not a lawn—I feel dumb right now.

At any rate, you can still use some of the previous information but you will need to adjust for the output of your sprinklers.

But first a couple of thoughts.  Watering for 1/2 hour every other day is likely better than 1/4 hour each day.  The reason is that longer watering encourages deeper grass rooting and more infrequent watering also encourages deeper rooting.  And you really want those roots to go deep as shallow roots dry out easier.

This assumes an established lawn.  I have started 2 lawns and I never ever want to try another one.  STARTING a lawn requires long, daily watering and our hot sun undoes all the work I put into the lawn!  Aarrrgghh!

At any rate, good Luck with your lawn,  Dripworks does sell some pretty amazing pop-up sprinklers if you are interested, but I am willing to bet you are satisfied with what you have now.

Eric
 
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May I suggest that you can also create a system that requires no battery or timer, and instead relies on the evaporation rates of your yard? There is some setup involved, and parts to purchase, but it is gravity fed from rain barrels. For just 1000'sq this might work for you, or it might not.



Edit: this person also has a setup that uses a balanced valve, where water evaporates from a pan and causes the valve to open and water everything including that pan, and once it fills enough the heavier pan cuts off the valve:

 
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