Information: solar powered drip irrigation system

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Last seen: 03/08/2018 - 21:05
Joined: 09/23/2011 - 16:27

Information: solar powered drip irrigation system

Hi,

This is information rather than a question. I built a drip irrigation system that operates with a small 12V pump, which is connected to a solar charger and battery. The system has been very troublesome, so I am posting some info about my tribulations in case it is helpful to anybody else.

The system consists of:

* 12V 14.7 litres per minute, 40 psi pump (cheap pump, you can get them online for well under $100)

* 12V gel cel battery (now 26 AH, but started with 12 AH)

* 40 watt solar panel (but started with 8 watt)

* Solar battery charger / controller (now 30A, started with 10A)

* 100 drip points over about 500m total length

* About 6 m rise to highest drip point (but some are about 2 m lower than the pump - it's hilly territory)

The good news is that the system is "just right" when it works, providing good flow at all points around the garden, and pump able to pressurise the full run in about 7 minutes.

Problems I have encountered after one year of use:

* The pump! They are incredibly unreliable. The main problem is the pressure switch - they seem to have a mean time between failure of about 5 hours. There are many online vendors selling what appears to be the same pump, just with different labels. They are all made in China. I have bought three pumps and two additional pressure switches so far. Probably because the systems cycles on and off quite quickly, the mechanical pressure switches just can't take it and fail. The switch can be bypassed so the pump just runs continuously, but that sucks a lot of power and can also burst connections on the irrigation pipe. My summer project is to build my own controller so I don't have to use the pump's own pressure switch.

* Gel cel. They have high "internal resistance", which means the voltage drops quite quickly when they are heavily loaded. They bounce back again, but the temporary low voltage can be seen as "discharged" by the controller and it shuts the system off. I have more than doubled the battery size and that seems to have fixed the problem. The pump has a nominal current of 7.5A but it is much higher when the pressure builds up, especially if I have bypassed the pressure switch.

* Battery charger / controller. Not really a problem, I just chose one that was too light. It had a max load of 10A I found the pump often sucks more current than that. It was sensing an "overload" and shutting down. The new 30A unit is OK. They are really cheap anyway.

* Solar panel. Again  not really a problem, but I have found that I want to run the system more often and for longer than I can manage with the original 8 watt panel. The new battery and solar panel setup should be good for about one hour run time per day, but I'll have more data on that by the end of summer. The pump has never been reliable enough to get a "normal" month of operation.

 

Planned improvements:

* Add a pressure accumulator to even out the pump cycles (slow down the on/off cycling)

* Build my own pump controller!

Last seen: 03/08/2018 - 21:05
Joined: 10/20/2011 - 16:16

Hi Frisbie

Interesting project - Solar power is tricky territory that's for sure.

A few questions

Can you supply a link to one of the pumps? I'd like to see it's specs.

What are the emitters? Are they drippers? What flow rate? Brand? Are they pressure compensating?

I'm surprised you're getting pipes blowing off. Are you using proper lateral pipe connectors complete with ratchet clips? When the right gear is used, the pipe will split before the couplings blow.

A pressure accumulator makes sense. Consider teeing it into the line and where teed in, put in a small brass swing check valve back to front with a small hole in the centre of the swing check. That way the accumulator will take a while to fill up before the pressure switch cutting out. The size of the hole will be by trial and error but I'd start pretty small, say a couple of mm. You might get a pressure tank at a pump repair place for next to nothing as they're being replaced now with electronic flow control gear. I'd imagine any pressure switch within pressure spec would work irrespective of the voltage.

You might also be interested in this unit http://www.rainperfectpump.com

regards

Roger

Last seen: 03/08/2018 - 21:05
Joined: 09/23/2011 - 16:27

Hi,

I was going to load a picture of the pump but I can't figure out how to do it with this forum :( It's asking for a URL - does that mean I have to put the photo on a server?

I am using 2 l/hour drippers. At least I was. I have replaced most of them with adjustable drippers as the irregular height of my orchard meant that the lower-down trees were getitng a lot more water than the higher ones.

The link that you posted seems to be for a much smaller pump - you can see that mine has quite a big motor. Mine is working pretty close to capacity so a smaller one would not cope.

I'm currently working on a controller that will open the tank valve and turn on the pump at scheduled times, if appropriate for the weather. Having both the pump motor and valve that need controlling makes it more complicated.

cheers

Last seen: 12/16/2013 - 09:45
Joined: 12/31/2012 - 02:56

Here is a piece more information , it should probably help, it's partly i saw the wattage and amperage and battery size and don't see how that matches , although i had a look the water pump the other person recommended.

This is not really that domestic.

I've been looking into solar and have some basic vehicle and radio electronics knowledge.

 / means "divided by"

* means "multiplied by"

First, watts / volts = amps

 volts * amps = watts

 watts / amps = volts

 

Approx...

12v at 1Kw = 80amps ,

240v at 1Kw = 4amps , 

24v at 1Kw = 40amps

Don't much care for the watt ratings of the solar , how it manages to push 30amp of 12v in with only 8 watts defies everything except charging characteristic of a battery by "voltage" , it should take 1000's of hours to charge a 26ah battery with that tiny amperage.

Here's some solar wind hybrid research i have been doing on the subject.

(Requires free Adobe Acrobat Reader - document browser used by government and business for online)

Small 30amp 12v Hybrid system

http://wild-canidae-conservation-forum.netne.net/Off-Grid-Electric-Hot-Water(suburban-domestic)PART2.pdf

 

Large off grid 8Kw Hybrid system

http://wild-canidae-conservation-forum.netne.net/48v-8Kw-hybrid-half-half-power-system.pdf

 

These should help.

Note too, with hydraulics , 1 litre of water weighs 1Kg , so get the lenth in centermeters and the diameter of the pipe you use and you will be able to calculate the weight you are pushing inside the pipe.

Pi(3.142 decimal or 22/7 fractional) * D * Height if i remember correct.

 

Last seen: 03/08/2018 - 21:05
Joined: 09/23/2011 - 16:27

Hi there,

This is to calrify the solar system design requirements.

Solar systems very rarely have panels that can deliver the full instantaneous load current. The panel charges the battery over a (long) time, and the battery drives the load over a shorter time. If you want to drive the load straight off the solar panels, it would (a) cost a fortune in solar panels and (b) be unreliable and intermittent, e.g. won't work in the dark. If you had a such a system it would not need a battery though.

Taking the case of the 8 watt panel. That is 0.7 A. Let's say there is 8 hours of good sunlight in a day, which is realistic in summer. That adds up to 5.6 A/hours per day. So that is how much energy I have to use each day, if the sytem is 100% efficient (which it is not - you never get back all the charging energy, especially with lead-based batteries). Let's say the pump is working at 8 A. It I run it for 30 minutes, that will be 4.0 A/hours. So that equation works OK including an allowance for inefficiencies.

However, I want to run it for one hour or more each day and I also want to have a contingency for cloudy days. A 40 watt panel gives 26.7 A/hours of energy per day. So it can theoretically fully  charge my battery every day. And that should be good for over four hours of run time, although the pumps's thermal cutoff would kick in long before that anyway. So the system is actually in balance with the solar power matching the battery capacity, and having plenty of juice for what I want to do.

As to lifting the water I don't think that is really significant. What matters is keeping the system pressurised and maintaining the flow (100 drippers at 2 litres per hour each, @ 30 psi). The drippers need to have good pressure to deliver the rated flow. It's the combination of high flow and strong pressure that limits the choice of pumps. Fountain-style pumps may have the flow, but not the pressure.

Last seen: 12/16/2013 - 09:45
Joined: 12/31/2012 - 02:56

I took a look at various 40 psi 12v diaphram pumps,

and at 40psi there is just under 3 hours of continuous battery usage total *in theory.

 *26ah / 9.1 amps = 3 hours

However, two points, deep cycle batteries not charged as 14v heavy usage are charged by floating between 13.4 and 13.8v charge.

Second, there are generally only 1500 discharges battery lifespan from 100% down to 70% full  = (30% D.O.D.) 

So to discharge the battery for more than 40 minutes (which is really only allowed to go continuous for 20 minutes in 26ah *** i'll explain this in a moment) means you will only have approximately 300 - 350 discharges from its lifespan.

Trouble again is two problems of the physics of batteries,

1. If you commit a high discharge rate using a pump which is exactly the same as the deep cycle types purpose the battery "ah" is only HALF the rating that is given because it cannot keep up with the chemical reaction so your usage calculation should be for 13ah not 26ah.

2. Lead acid (whether gel or otherwise for the electrolyte) batteries start to have lower capacity "ah" under 25 degrees Celsius and at around 20 degrees Celsius are somewhere between a half and two thirds of rated capacity usable, but fear not too much on that at times because using heavy draw heats them quickly so it tends to be 2/3rds unless it winter or raining for some appreciable period.

There are pressure controller switches, maybe you need to get a more efficient version (nb: they consume a small quantity of power as test current alike an inverter).

About the weight of the water, it counts,you said it was undulating over terrain and 6m higher at points, and in effect all you need is 1 diameter of the pipe higher for it to be using its weight back at 100%, so calculate it.

Last seen: 03/08/2018 - 21:05
Joined: 09/23/2011 - 16:27

Hi,

Good point about the de-rating of the batteyr - I'm aware of that. However the equation is somewhat more complicated as the pump does not operate in 100% duty cycle. Due to operation of the adjustable pressure switch it cycles on and off. I cannot say what the average current might be as that would require data logging. However, the battery voltage is 12.20 volts after one hour of run time, which is quite acceptable (approx 50% discharged).

Having bought (so far) three of these pumps they appear to be identical, except for adjustment of the pressure switch, i.e. could be sold as a 20, 30 or 40 psi pump. The motor seems to be the same.

I'm puzzled about the issue with weight of the water. The only thing that matters is the flow from each dripper. Suppose that I calculate weight of the water - what am I going to do with it? The motor can't deliver any more power. The only avenue for increased capacity is to add a second, independent system. That does not seem warranted at this stage. Anyway if I am going to duplicate, it will be for redundancy rather than increased capacity.

As to battery life ... well, it is what it is. Can't change that. When the battery capacity falls off I will replace it. I'm pretty confident it will last a couple of seasons and I'll be happy with that.

Last seen: 12/16/2013 - 09:45
Joined: 12/31/2012 - 02:56

The weight of the water, he you has 500 meters of pipe some as much as 6 meters above the pump unless the down hill behind it is as long as the above before the pipe is above the pump then he you has the problem in physics you he don't get something for nothing although if he you has some below the pump level at the end he you would be partially siphoning. He You must expend energy to overcome inertia and hold that state wile applying pressure also which is why the pump(an electric motor) draws a large quantity of current exactly the same as a deep cycle motor.

Why it is exactly the same ultimately is that unfortunately electric motors have a huge overhead in stop start called "surge current draw" , when they overcome interia or require to labour to get their speed to correct rpm they draw as much current as possible at the start. Batteries themselves can only hold out pushing current into a sudden call for more amperage with voltage drop so a specification called CCA Cold Circuit(sometimes "Cranking") Amps is a "rating" measure of how much for how long in seconds a 12v(any volt) battery can stay above 7.5 volts approximately in short circuit or massive current draw such as electric motors.

So really it is exactly the same as using it as though it were continuosly operating.

Both heating and boiling and change of state in electric circuits are much the same thing.

In a constant state requires some energy, but to change state requires an immense input more energy(inclusive vehicle brakes have their problems in this respect).

The weight of water is generally not important if the system is well designed by leveling , it would be a good idea at six meters to start using a tower to hold the pump and put joiners down to the plants requiring to be lowered to, one of the 40psi 12v pumps i looked at said it could be put 50 meters above the water level and the "hose intake nozzle" no more than 8 meters under it.

Last seen: 03/08/2018 - 21:05
Joined: 09/23/2011 - 16:27

Hi,

This seems to be turning into a debate - I'm not sure that is really helpful to anybody thinking about setting up a solar-powered irrigation system. To recap: the current configuration is able to deliver good flow to all points. Flow is of course uneven but that can be improved through use of adjustable drippers.

Your suggestion of using a header is a good one. I considered it and may yet go that way. However I am also working on customised drippers that will meter the amount of water at each point - I think that is a more useful approach.

The information about electrical properties of motors is not entirely correct. From a NASA paper (link http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890010122_1989010122.pdf), the starting current of a perment magnet DC motor is 1.2 times the normal current. You may be referring to "inrush" current but that only lasts for a matter of milliseconds so is of no concern to long-term battery load. DC motors are happy to operate over a wide range of speed, up to their design maxium (I recently bought a bench drill with adjustable speed DC motor - it cost a lot more than the standard fixed-speed AC motor that you normally get on bench drills).

So the question of whether it is better to keep the motor running or let is cycle depends entirely on whether the duty cycle is 1/1.2 = 83% or less. In any case the cycling is primarily to regulate pressure, which is important. I doubt if the water stops flowing during the off-cycle. The drippers don't stop. The pump only stops because the pipes have become pressurised and that pressure keeps the flow going. An accumulator will improve this characteristic.

But all of this is needless academia. Most folks would only care: does it work? With the exception of the ongoing switch weakness, it is a pretty good system and very cheap.

Anyway, all the best for New Year.

Last seen: 12/16/2013 - 09:45
Joined: 12/31/2012 - 02:56

...Turning into a debate...forum...hmmmm...No, not needless academia, however needless to be absolute perfection on parts acquirement and that does count thats not too bad since i saw the PDF from NASA , that's quite true when their is not a shunt-exciter circuit(standard vehicle starter motor system). However, a glorified way of saying it doesn't use a shunt and that's all the electricity it recieves is whatever is there, and that proves my point about inertia being part of any calculation to start something moving (it said 1.2 x as much as rated) to change state, but i do believe you might commit a bit of real attention to whether your battery and "pressure controller" have enough electricity at the time they fail whether the pump operates or not, any vastly uncontrolled electric "motor"(not pressure controller) will pump until voltage and current supply is too low and drained to move the solenoids or spin the rotor soif you don't mind telling,

did you do a proper ammeter a check on your batteries at each failure(would help to simply know it is not the cause)?

And does your pressure controller use electricity for measuring for testing by time based retest or is it isimply part of a switching action mechanically?

At least by proving the system to yourself by "academia" as you call it(i call it not spending a damn thing more and knowing i'll get something from it) you can realise quite quickly if failure is occurring rather than wait until it is blatant, more than that its cheaper and lose less downtime from the television set and couch.

 

ADDED:

....Most folks would only care if it works...

Well, i do believe you do not have much battery power for the motor if it runs or attmpts to for 24hours, My minimum is 100ah battery(about the best economy size $160 - $200), a 20amp charger(allows 200w load max the difficulty for a 12v load controller is exactly how much the solar controller will allow output on the load when programmed). On ebay you can get an 80watt 12v panel from one company for $100, which is quite good the wattage rating for your motor as an aberrative is 120w -140w to be producing enough directly(but is not voltage controlled and peaks at around 17volts), however the battery needs to be charged and remain charged during daylight so the 20 amp charger with 240w(3 x 80w PV) is the most effective. The 100ah would be better two 2 x 70ah (140ah all through a winter night if it switches on and off every 2 minutes or less) batteries.

Wouldn't mind knowing how long it lasts because it seems as if it is be being kicked to beyond existance within 6 months, tell us when the batteries are gone. For batterries to be properly maintained they need to stay 70% charged and above in the float region, so below it requires deep cycle charging which uses a 14.5v charge and is assumed it will be drained to around 30% full, that highe voltage is because no settling of the chemical reaction by "stage charging" will occur so it requires some extra force to directly lay it onto the plates and that severely reduces the life of the battery, BUT, not charging it properly does the same.

 

You're reminding me of my oldest problem in being clever at it, by the end of it , it is very rare to get away with less than the very actual designed and marketed systems  because they are calculated.

Last seen: 05/20/2020 - 08:45
Joined: 05/14/2020 - 04:05

i have a question:
i am working on my final year automated irrigation project.
the question is that i am using 12v (7A) battery with solar panels.
how much time power would require a 0.7A (12v) solenoid valve from 12v battery and i have to run the system minimum 8 hours a day and how many hours the solenoid valve will work if the battery is fully charged?
thanks.

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